Swept source optical coherence tomography with enhanced signal detection

Abstract

A swept source optical coherence tomography (SS-OCT) system for performing imaging of a sample processed by a material processing beam includes an interferometer for use in combination with a tunable light source that generates an imaging optical signal having a temporally variable wavenumber k and a sweep rate in the range of 1 kHz to 20 MHz, comprising end values; an optical detector configured to detect a combined optical signal from the interferometer and to generate at least one interferometer output signal; and a processing unit configured to detect a distortion in the at least one interferometer output signal, the distortion resulting from a time-varying difference in the optical path length of the interferometer, and, in response, detect a distortion in the at least one interferometer output signal, one or more corrections are applied to the at least one interferometer output signal in order to determine at least one characteristic of a processing region on the sample.

Description

[0001] Related applications

[0002] This application claims priority to U.S. Provisional Application Serial No. 63 / 440,486, filed on January 23, 2023, entitled “SWEPT—SOURCE OPTICAL COHERENCETOMOGRAPHY WITH ENHANCED SIGNAL DETECTION,” the contents of which are hereby incorporated by reference in their entirety. Background Art Technical Field

[0003] The technical field relates generally to swept-source optical coherence tomography (SS-OCT), and more particularly to SS-OCT systems and methods configured with improved imaging capabilities.

[0004] Background Discussion

[0005] Optical coherence tomography (OCT) is an imaging technique that involves splitting light output from a light source into an imaging beam and a reference beam, detecting an interference signal obtained by superimposing the imaging beam reflected from the surface of an object or sample with the reference beam, and forming a tomographic image of the object based on the detection results. The tomographic image can include a two-dimensional cross-section or a three-dimensional volume rendering of the object or sample obtained by using information about how the light beam changes upon reflection.

[0006] A common OCT technique is Fourier-domain OCT (FD-OCT), of which there are generally two types: spectral-domain OCT (SD-OCT) and swept-source OCT (SS-OCT). However, these two systems differ in the type of light source they each utilize and how they detect the interference signal.

[0007] SD-OCT systems use a broadband light source and a spectrally resolved detector system to determine the different spectral components within a single axial scan (A-scan) of a specimen. Consequently, SD-OCT systems typically decode the spectral components of the interferometric signal by spatially separating them. Consequently, the detector system is often complex, as it must simultaneously detect all wavelengths of the optical signal within the scan range and then convert them into a corresponding interferometric dataset. This impacts the speed and performance of SD-OCT systems.

[0008] In contrast, SS-OCT systems encode spectral components temporally rather than by spatial separation. SS-OCT systems typically utilize an instantaneous narrowband imaging source whose emission wavelength varies with time, sometimes referred to as a "Swept Source" or "Swept-Source." SS-OCT systems acquire A-lines by using such a light source and time-domain optical detection. The spectrum of the interference light corresponding to the A-line is acquired by sequentially detecting the interference light over time as the wavelength of the light source varies, typically referred to as an interferogram. The interference signal is typically detected by a non-spectrally resolving detector, a non-limiting example of which includes a balanced detector photodiode detection system.

[0009] Compared to SD-OCT technology, SS-OCT is less susceptible to sensitivity drop at longer imaging depths, offers faster scanning speeds and improved signal-to-noise ratio ("SNR"), and reduces detector system complexity. Summary of the Invention

[0010] Various aspects and embodiments are directed to methods and systems for using SS-OCT in material modification processes.

[0011] According to one embodiment, a swept source optical coherence tomography (SS-OCT) system for imaging a sample processed by a material processing beam that interacts with material of the sample at a processing region on the sample is provided, the SS-OCT system comprising: an interferometer having at least one reference arm, at least one sample arm configured to direct an imaging optical signal to the processing region, and a tunable light source for generating the imaging optical signal, the imaging optical signal having at least one temporally variable wave number k and a sweep rate in the range of 1 kilohertz (kHz) to 20 megahertz (MHz), inclusive, the interferometer being configured to direct the imaging optical signal to the at least one reference arm and the at least one sample arm, and combining the imaging optical signal from the at least one reference arm and the at least one sample arm. an optical signal returned by the at least one sample arm and the at least one reference arm to generate a combined optical signal; an optical detector configured to detect the combined optical signal and generate at least one interferometer output signal; and a processing unit configured to: receive the at least one interferometer output signal, process the at least one interferometer output signal to determine at least one characteristic of the processing region, detect distortion in the at least one interferometer output signal resulting from a time-varying difference in optical path length between the at least one sample arm and the at least one reference arm, apply one or more corrections to the at least one interferometer output signal in response to detecting the distortion to generate a corresponding corrected interferometer output signal, and process the at least one corrected interferometer output signal to determine at least one characteristic of the processing region.

[0012] In one example, at least one feature includes depth information of the processing area. In another example, the depth information includes a range of at least 1 mm, inclusive. In another example, the depth information includes a range of at least 5 mm, inclusive. In another example, the depth information includes a range of at least 21 mm, inclusive. In another example, the depth information includes a range of at least 50 mm, inclusive. In another example, the material processing beam produces a phase change region (PCR) at the processing area, and the depth information includes a keyhole depth of the PCR. In another example, the system further includes at least one guiding element that guides the imaging optical signal to one or more selected locations in and / or near the PCR.

[0013] In one example, the processing unit is further configured to control at least one processing parameter of a material modification process implemented on the sample by the material processing beam based on the at least one characteristic of the processing region.

[0014] In one example, the processing unit is further configured to determine the sample position based on the at least one corrected interferometer output signal.

[0015] In one example, the processing unit is further configured to determine a velocity of a material of the sample based on the at least one corrected interferometer output signal. In one example, the processing unit is further configured to determine one or more alignments and / or one or more alignment offsets between a coordinate system of a beam delivery system for the material processing beam and a coordinate system of a delivery system for the imaging optical signal.

[0016] In one example, a processing unit is configured to control a tunable light source such that a rate of change in time of at least one wave number k of the imaging optical signal (tuning rate dk / dt) includes at least two tuning rates dk / dt associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections, and calculating the one or more corrections includes: identifying a distortion in at least one interferometer output signal of the one or more interferometer output signals associated with at least one tuning rate of the at least two tuning rates; performing an evaluation of the distortion,

[0017] One or more corrections to at least one interferometer output signal are based on the evaluation.

[0018] In one example, the at least two tuning rates dk / dt include at least one positive tuning rate dk / dt and at least one negative tuning rate dk / dt. In another example, performing the evaluation includes comparing distortion in at least one interferometer output signal associated with the positive tuning rate dk / dt and distortion in at least one interferometer output signal associated with the negative tuning rate dk / dt.

[0019] In one example, performing the evaluation includes comparing distortion in at least two interferometer output signals. In one example, the distortion corresponds to distortion in one or more geometric aspects encoded in the interferometer output signals. In one example, the one or more geometric aspects include at least one of position, symmetry, peak width, centroid, geometric second moment, centroid, amplitude, aspect ratio, and geometric area under the curve. In one example, performing the evaluation includes comparing the one or more geometric aspects encoded in the at least two interferometer output signals.

[0020] In one example, performing the evaluation includes comparing the one or more geometric aspects to one or more predetermined thresholds and / or baselines associated with the one or more geometric aspects. In one example, the one or more predetermined thresholds and / or baselines are established based on at least one of: system and / or component requirements, one or more application requirements, one or more calibrations, one or more models, hardware and / or software limitations, one or more algorithms, and underlying physics.

[0021] In one example, the processing unit is configured to perform the evaluation by comparing the one or more geometric aspects relative to at least one of: one or more geometric aspects encoded in at least one other of the one or more interferometer output signals, and one or more predetermined thresholds and / or baselines associated with the one or more geometric aspects.

[0022] In one example, performing the evaluation includes applying a predetermined threshold to the distortion and determining whether the distortion exceeds the predetermined threshold. In another example, performing the evaluation includes determining whether a relative difference between two or more distortions exceeds a predetermined threshold.

[0023] In one example, the processing unit is further configured to generate a mathematical model based at least in part on one or more characteristics of the SS-OCT system and the evaluation of the distortion. In another example, the mathematical model is further configured to generate an estimate of the magnitude and / or direction of the sample's motion velocity based on the evaluation of the distortion.

[0024] In one example, the material modification process achieved by the material processing beam on the sample is a welding process, and the material processing beam generates a phase change region (PCR) at the processed region, and the processing unit is configured to use the evaluation to generate one or more corrections, and use the one or more generated corrections to calculate a measurement of one or more features in motion in the PCR. In one example, the one or more features in motion in the PCR are in motion as a result of the material processing process.

[0025] In one example, the material modification process effected by the material processing beam on the sample is a welding process, and the material processing beam produces a phase change region (PCR) at the processing region, and the processing unit is configured to use the evaluation to generate an estimate of the speed at which the material is processed in the PCR.

[0026] In one example, applying one or more corrections to at least one interferometer output signal includes discarding, weighting, boosting, or using at least one of the one or more interferometer output signals, or selecting at least one of the one or more interferometer output signals to discard or use at a later time.

[0027] In one example, the interferometer is a first interferometer, and the system further includes at least one additional interferometer and is configured such that a sample arm of the first interferometer and a sample arm of the at least one additional interferometer share at least one optical element, a first imaging optical signal configured with one of at least two tuning rates dk / dt is directed to at least one reference arm and at least one sample arm of the first interferometer, a second imaging optical signal configured with another of the at least two tuning rates dk / dt is directed to at least one reference arm and at least one sample arm of the at least one additional interferometer, and the distortion is identified based on one or more interferometer output signals of the first interferometer and the at least one additional interferometer. In another example, the first imaging optical signal and the second imaging optical signal are simultaneously directed to a processing region.

[0028] In one example, the time-varying difference in optical path length is caused by sample motion. In one example, the sample moves at a speed greater than 10 mm / s. In another example, the sample moves at a speed greater than 100 mm / s. In another example, the sample moves at a speed greater than 500 mm / s. In another example, the sample moves at a speed greater than 1000 mm / s. In another example, the sample moves at a speed greater than 10,000 mm / s.

[0029] In one example, the processing unit is configured to derive tracking data from the at least one interferometer output signal and to apply one or more corrections to the tracking data.

[0030] In one example, the system further includes at least one k-clock module that generates a k-clock signal that indicates when a wavenumber k of the imaging optical signal changes substantially by one or more increments. In one example, the rate of change of at least one wavenumber k of the imaging optical signal over time (tuning rate dk / dt) is non-uniform, and the at least one k-clock module is configured to trigger acquisition of an interferometer output signal at uniform increments of the wavenumber k. In one example, the processing unit is configured to process the at least one interferometer output signal based on the uniformly sampled interferometer output signal across the wavenumbers. In one example, the rate of change of at least one wavenumber k of the imaging optical signal over time (tuning rate dk / dt) is uniform, and the at least one k-clock module is configured to trigger acquisition of the interferometer output signal at uniform increments of the wavenumber k. In one example, the processing unit is configured to acquire the k-clock signal while acquiring the interferometer output signal. In one example, the processing unit is configured to use the acquired k-clock signal to perform at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of the wavenumber k. In one example, a processing unit is configured to use the acquired k clock signals to calculate at least one correction for one or more distortions in an interferometer output signal. In one example, the processing unit is configured to use the acquired k clock signals to define a discrete Fourier transform method that can be directly applied to an interferometer output signal that is unevenly sampled across k. In one example, the processing unit is configured to acquire the k clock signals in a time-gated manner relative to acquisition of the interferometer output signals, and to apply the time-gated k clock signals to processing subsequently acquired interferometer output signals. In one example, the processing unit is configured to use the acquired k clock signals to perform at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signals at uniform intervals of wavenumber k. In one example, the processing unit is configured to use the acquired k clock signals to calculate at least one correction for one or more distortions in the interferometer output signal. In one example, the processing unit is configured to use the acquired k clock signals to define a discrete Fourier transform method that can be directly applied to an interferometer output signal that is unevenly sampled across k. In one example, at least one of the interferometer sample arm and the reference arm is configured with one or more optical elements for generating a k-clock signal. In one example, at least one k-clock module is configured with multiple optical paths for generating the k-clock signal. In another example, at least one k-clock module is configured to simultaneously generate multiple optical paths by splitting the optical signal. In another example, at least one k-clock module is configured to make multiple optical paths available for selection.

[0031] In one example, the processing unit is further configured to simulate a k-clock signal based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source, the k-clock signal indicating when at least one wavenumber k of the imaging optical signal changes substantially by one or more increments. In one example, the processing unit is configured to use the simulated k-clock signal to perform at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wavenumber k. In one example, the processing unit is configured to use the simulated k-clock signal to calculate at least one correction for one or more distortions in the interferometer output signal. In one example, the processing unit is configured to use the simulated k-clock signal to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is unevenly sampled across k.

[0032] In one example, the processing unit is further configured to generate a mathematical model k(t) and / or a tuning rate dk / dt of the imaging optical signal based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source. In another example, the processing unit is configured to use the mathematical model to perform at least one of sampling, resampling, interpolation, and / or estimation of the interferometer output signal at uniform intervals of wave number k. In another example, the processing unit is configured to use the mathematical model to calculate at least one correction for one or more distortions in the interferometer output signal. In one example, the processing unit is configured to use the mathematical model to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is unevenly sampled across k.

[0033] In one example, the time-varying difference in optical path length is caused by motion of the sample relative to the axis of the imaging optical signal. In one example, the time-varying difference in optical path length is caused by a material modification process on the sample effected by the material processing beam.

[0034] In one example, the time-varying differences in optical path length are caused by intrinsic sample motion rather than by a material modification process achieved on the sample by the material processing beam.

[0035] In one example, the processing unit is configured to control the tunable light source so that the temporal rate of change of at least one wave number k of the imaging optical signal (tuning rate dk / dt) includes at least two tuning rates dk / dt. In another example, the at least two tuning rates dk / dt include at least one negative dk / dt and at least one positive dk / dt. In one example, the tunable light source is configured so that the imaging optical signal includes a superposition of the at least two tuning rates dk / dt. In another example, the superposition of the at least two tuning rates dk / dt includes at least one negative dk / dt and at least one positive dk / dt.

[0036] In one example, the tunable light source is a first tunable light source, and the system further includes at least one additional tunable light source. In one example, the interferometer is a first interferometer, and the system further includes at least one additional interferometer, the first interferometer being configured with the first tunable light source, the at least one additional interferometer being configured with the at least one additional tunable light source, the first interferometer and the at least one additional interferometer being configured such that a sample arm of the first interferometer and a sample arm of the at least one additional interferometer share at least one optical element. In one example, a temporal rate of change of at least one wavenumber k of the imaging optical signal is a tuning rate dk / dt, and the processing unit is configured to control the first tunable light source such that a first imaging optical signal generated by the first tunable light source has a first tuning rate dk / dt, and to control the at least one additional tunable light source such that an imaging optical signal generated by the at least one additional tunable light source has a second tuning rate dk / dt that is different from the first tuning rate dk / dt. In one example, at least a portion of the first imaging optical signal and at least a portion of an imaging optical signal generated by the at least one additional imaging optical signal are transmitted simultaneously. In one example, a first tuning rate and a second tuning rate are associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections, and calculating the one or more corrections includes: identifying distortion in at least one interferometer output signal of the one or more interferometer output signals associated with at least one of the first tuning rate and the second tuning rate; and evaluating the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation. In one example, the first tuning rate dk / dt is positive dk / dt, and the second tuning rate dk / dt is negative dk / dt.

[0037] In one example, the system further includes: a splitter for splitting the imaging optical signal into at least two arms; and an optical delay element configured to cause an output of a first arm of the at least two arms to be delayed in time relative to an output of a second arm of the at least two arms. In one example, the processing unit is configured to control the tunable light source so that a temporal rate of change (tuning rate dk / dt) of at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt. In one example, the at least two tuning rates dk / dt include a positive tuning rate dk / dt associated with the first arm or the second arm, and a negative tuning rate dk / dt associated with the other of the first arm or the second arm. In one example, the interferometer is a first interferometer, and the system further includes at least one additional interferometer. In another example, the first arm is configured to be directed to at least one of the following arms: different reference arms, different sample arms, partially overlapping reference arms, and partially overlapping sample arms of the first interferometer and the at least one additional interferometer. In another example, the first arm and the second arm are configured to be directed to at least one sample arm and at least one reference arm of a first interferometer and at least one additional interferometer. In another example, the first interferometer and the at least one additional interferometer are configured such that the sample arm of the first interferometer and the sample arm of the at least one additional interferometer share at least one optical element, and the first arm and the second arm of the imaging optical signal are configured to be directed to both the first interferometer and the at least one additional interferometer. In another example, the first arm is configured to be directed to the first interferometer or the at least one additional interferometer, and the second arm is configured to be directed to the other interferometer of the first interferometer or the at least one additional interferometer. In one example, at least two tuning rates dk / dt of the first arm and the second arm are associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections, and calculating the one or more corrections includes: identifying distortion in at least one interferometer output signal of the one or more interferometer output signals associated with at least one of a positive tuning rate and a negative tuning rate; and performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

[0038] In one example, the optical frequency of the imaging optical signal varies at a rate in the range of 8 PHz / s to 2 ZHz / s, inclusive.

[0039] In one example, the processing unit is configured to generate an OCT image based on the at least one interferometer output signal and / or the at least one corrected interferometer output signal, and transmit the generated OCT image to a display device.

[0040] In one example, the processing unit is further configured to generate OCT data based on the processed interferometer output signal and / or the corrected interferometer output signal, and transmit the OCT data to an external device.

[0041] In one example, the tunable light source is a tunable vertical cavity surface emitting laser (VCSEL). In another example, the system further includes an amplifier for amplifying the VCSEL. In one example, the amplifier is configured as a fiber amplifier. In one example, the output power of the amplifier is at least 20 milliwatts (mW). In another example, the output power of the amplifier is at least 30 mW. In another example, the output power of the amplifier is at least 50 mW. In another example, the output power of the amplifier is at least 100 mW. In another example, the output power of the amplifier is at least 500 mW. In another example, the output power of the amplifier is at least 1 watt (W). In another example, the output power of the amplifier is at least 5 W. In one example, the amplifier is configured to have a peak gain at a wavelength between 1010 nm and 1050 nm. In one example, the amplifier is configured to have a peak gain at a wavelength between 1050 nm and 1090 nm. In one example, the amplifier is configured with one, two, or three amplification stages.

[0042] In one example, the SS-OCT system has a sensitivity of at least 105 dB.

[0043] In one example, the processing unit is further configured to modulate or demodulate the at least one interferometer output signal using a predetermined carrier frequency.

[0044] In one example, the system further includes a digitizer configured to digitize the at least one interferometer output signal and generate a corresponding digital signal.

[0045] In one example, the system further includes a record generator that generates a record of a material modification process implemented on the sample by the material processing beam based on the at least one interferometer output signal at a plurality of times. In another example, the processing unit is further configured to evaluate a quality of a weld produced by the material modification process implemented on the sample by the material processing beam based at least in part on the record.

[0046] In one example, the system further includes a notification generator that generates notifications related to a material modification process achieved on the sample by the material processing beam based on the at least one interferometer output signal at a plurality of times.

[0047] In one example, the system further comprises at least one guiding element that guides the imaging optical signal.In another example, the at least one guiding element is configured such that the imaging optical signal is within 50 nm of a focal point of the material processing beam at the processing region.

[0048] In one example, the system also includes an auxiliary measurement system configured to measure process radiation.

[0049] In one example, the system is configured to image a sequence of multiple material modification processes effected on a sample by a material processing beam.

[0050] In one example, the system further includes a safety interlock device integrated into the tunable light source. In another example, a safety interlock device integrated into the tunable light source is configured to enable an eye-safe operating mode of the tunable light source, the eye-safe operating mode being characterized by having reduced imaging optical emission power.

[0051] In one example, the system further comprises at least one of: a material processing energy source that generates the material processing beam, and a beam delivery system for the material processing beam and the imaging optical signal. In another example, the system further comprises a laser head coupled to the material processing energy source and housing the beam delivery system. In one example, the processing unit is further configured to control at least one of the material processing energy source that generates the material processing beam and the beam delivery system based on at least one characteristic of the processing region.

[0052] In one example, a material processing system includes an SS-OCT system, a material processing energy source that generates a material processing beam, and a beam delivery system for the material processing beam and an imaging optical signal. In another example, the beam delivery system is configured with a dichroic optical device that is configured to combine the imaging optical signal and the material processing beam into a combined optical path. In one example, the dichroic optical device is configured to have a transmission spectrum having a first band edge, a reflection spectrum having a second band edge, and the first band edge and the second band edge have a maximum wavelength separation of 25 nm. In one example, the beam delivery system is configured to impinge the imaging optical signal on the dichroic optical device over a range of incident angles.

[0053] According to another exemplary embodiment, a swept source optical coherence tomography (SS-OCT) method for imaging a processing region on a sample processed by a material processing beam is provided, the method comprising: providing an interferometer having at least one sample arm, at least one reference arm, and a tunable light source, the tunable light source configured to generate an imaging optical signal having at least one wave number k that is substantially variable in time and a sweep rate in the range of 1 kilohertz (kHz) to 20 megahertz (MHz), inclusive; directing the imaging optical signal to at least one reference arm and at least one sample arm of the interferometer, the at least one sample arm configured to direct the imaging optical signal to the at least one reference arm and the ..., the at least one sample arm configured to direct the imaging optical signal to the at least one reference arm and the at least one sample arm, the at least one sample arm configured to direct the imaging optical signal to the at least one sample arm. an optical signal directed to a processing region; generating a combined optical signal based on optical signals returned from at least one reference arm and at least one sample arm; generating at least one interferometer output signal based on the combined optical signal; processing the at least one interferometer output signal to determine at least one characteristic of the processing region; detecting distortion in the at least one interferometer output signal, the distortion resulting from a time-varying difference in optical path length between the at least one sample arm and the at least one reference arm; applying one or more corrections to the at least one interferometer output signal in response to detecting the distortion to generate a corresponding corrected interferometer output signal; and processing the at least one corrected interferometer output signal to determine at least one characteristic of the processing region.

[0054] In one example, at least one feature includes depth information of the treatment area. In another example, the depth information includes a range of at least 1 mm, inclusive. In another example, the depth information includes a range of at least 5 mm, inclusive. In another example, the depth information includes a range of at least 21 mm, inclusive. In one example, the depth information includes a range of at least 50 mm, inclusive. In one example, the material processing beam creates a phase change region (PCR) at the treatment area, and the depth information includes a keyhole depth of the PCR. In another example, the imaging optical signal is directed to one or more selected locations in and / or near the PCR.

[0055] In one example, the SS-OCT method further includes controlling at least one processing parameter of a material modification process implemented on the sample by the material processing beam based on at least one characteristic of the processing region.

[0056] In one example, the SS-OCT method further includes determining a sample position based on the at least one corrected interferometer output signal.

[0057] In one example, the SS-OCT method further includes determining a velocity of a material of the sample based on the at least one corrected interferometer output signal.

[0058] In one example, the SS-OCT method further includes determining one or more alignments and / or one or more alignment offsets between a coordinate system of a beam delivery system for the material processing beam and a coordinate system of a delivery system for the imaging optical signal.

[0059] In one example, the SS-OCT method further includes: controlling a tunable light source so that a temporal rate of change (tuning rate dk / dt) of at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt associated with one or more interferometer measurement signals; calculating one or more corrections, wherein calculating the one or more corrections includes: identifying a distortion in at least one of the interferometer output signals associated with at least one of the at least two tuning rates; and performing an evaluation of the distortion, wherein one or more corrections to the at least one interferometer output signal are based on the evaluation.

[0060] In one example, the at least two tuning rates dk / dt include at least one positive tuning rate dk / dt and at least one negative tuning rate dk / dt. In one example, performing the evaluating includes comparing distortion in at least one interferometer output signal associated with the positive tuning rate dk / dt to distortion in at least one interferometer output signal associated with the negative tuning rate dk / dt.

[0061] In one example, performing the evaluation includes comparing distortion in at least two interferometer output signals. In one example, the distortion corresponds to distortion in one or more geometric aspects encoded in the interferometer output signals. In another example, the one or more geometric aspects include at least one of position, symmetry, peak width, centroid, geometric second moment, centroid, amplitude, aspect ratio, and geometric area under a curve. In another example, performing the evaluation includes comparing the one or more geometric aspects encoded in at least two interferometer output signals.

[0062] In another example, performing the evaluation includes comparing the one or more geometric aspects to one or more predetermined thresholds and / or baselines associated with the one or more geometric aspects. In one example, the SS-OCT method further includes establishing the one or more predetermined thresholds and / or baselines, and the establishing is performed based on at least one of the following: system and / or component requirements, one or more application requirements, one or more calibrations, one or more models, hardware and / or software limitations, one or more algorithms, and basic physics.

[0063] In one example, performing the evaluation includes comparing the one or more geometric aspects relative to at least one of: one or more geometric aspects encoded in at least one other of the interferometer output signals, and one or more predetermined thresholds and / or baselines associated with the one or more geometric aspects.

[0064] In one example, performing the evaluation includes applying a predetermined threshold to the distortion and determining whether the distortion exceeds the predetermined threshold.

[0065] In one example, performing the evaluation includes determining whether a relative difference between two or more distortions exceeds a predetermined threshold.

[0066] In one example, the SS-OCT method further includes generating a mathematical model based at least in part on one or more characteristics of the SS-OCT system and the evaluation of the distortion. In another example, generating the mathematical model includes generating an estimate of the magnitude and / or direction of the sample's motion velocity based on the evaluation of the distortion.

[0067] In one example, the material modification process effected on the sample by the material processing beam is a welding process, and the material processing beam generates a phase change region (PCR) at the processed region, and the SS-OCT method further includes using an evaluation to generate one or more corrections, and using the one or more corrections generated by the evaluation to calculate a measurement of one or more features in motion in the PCR. In another example, the one or more features in motion in the PCR are in motion as a direct result of the material modification process.

[0068] In one example, the material modification process achieved by the material processing beam on the sample is a welding process, and the material processing beam produces a phase change region (PCR) at the processing area, and the SS-OCT method also includes using the evaluation to generate an estimate of the speed at which the material is processed in the PCR.

[0069] In one example, applying one or more corrections to the at least one interferometer output signal includes discarding, weighting, boosting, or using the at least one interferometer output signal, or selecting the at least one interferometer output signal to be discarded at a later time.

[0070] In one example, the interferometer is a first interferometer, and the SS-OCT method further includes: providing at least one additional interferometer, and configuring the first interferometer and the at least one additional interferometer such that a sample arm of the first interferometer and a sample arm of the at least one additional interferometer share at least one optical element; directing a first imaging optical signal configured with one of at least two tuning rates dk / dt to at least one reference arm and at least one sample arm of the first interferometer; directing a second imaging optical signal configured with another of the at least two tuning rates dk / dt to at least one reference arm and at least one sample arm of the at least one additional interferometer; and identifying the distortion based on one or more interferometer output signals of the first interferometer and the at least one additional interferometer. In another example, the SS-OCT method further includes simultaneously directing the first imaging optical signal and the second imaging optical signal to a processing region.

[0071] In one example, the time-varying difference in optical path length is caused by sample motion. In one example, the sample moves at a speed greater than 10 mm / s. In another example, the sample moves at a speed greater than 100 mm / s. In another example, the sample moves at a speed greater than 500 mm / s. In another example, the sample moves at a speed greater than 1000 mm / s. In another example, the sample moves at a speed greater than 10,000 mm / s.

[0072] In one example, the SS-OCT method further includes deriving tracking data from the at least one interferometer output signal, and applying one or more corrections to the tracking data.

[0073] In one example, the SS-OCT method further includes providing at least one k-clock module, the at least one k-clock module configured to generate a k-clock signal that indicates when a wave number k of the imaging optical signal changes substantially by one or more increments. In one example, the rate of change of at least one wave number k of the imaging optical signal over time (tuning rate dk / dt) is non-uniform, and the SS-OCT method further includes configuring the at least one k-clock module to trigger acquisition of an interferometer output signal at uniform increments of the wave number k. In another example, the SS-OCT method further includes processing the at least one interferometer output signal based on the interferometer output signal uniformly sampled across the wave number. In one example, the rate of change of at least one wave number k of the imaging optical signal over time (tuning rate dk / dt) is uniform, and the SS-OCT method further includes configuring the at least one k-clock module to trigger acquisition of the interferometer output signal at uniform increments of the wave number k.

[0074] In one example, a k-clock module is provided that is configured with one or more optical elements present in at least one of a sample arm and a reference arm of an interferometer.

[0075] In one example, a k-clock signal is acquired while acquiring the interferometer output signal, and the SS-OCT further comprises at least one of: performing at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wave number k using the acquired k-clock signal; computing at least one correction for one or more distortions in the interferometer output signal using the acquired k-clock signal; and defining a discrete Fourier transform method using the acquired k-clock signal, which discrete Fourier transform method can be directly applied to the interferometer output signal that is unevenly sampled on k.

[0076] In one example, a k-clock signal is acquired in a time-gated manner relative to acquisition of an interferometer output signal, and the SS-OCT method further comprises at least one of: performing at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wavenumber k using the acquired k-clock signal; computing at least one correction to one or more distortions in the interferometer output signal using the acquired k-clock signal; and defining a discrete Fourier transform method using the acquired k-clock signal that can be directly applied to the interferometer output signal that is non-uniformly sampled across k.

[0077] In one example, the SS-OCT method further includes simulating a k-clock signal based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source, the k-clock signal indicating when at least one wavenumber k of the imaging optical signal substantially changes by one or more increments, and the SS-OCT method further includes at least one of: performing at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wavenumber k using the simulated k-clock signal; calculating at least one correction to one or more distortions in the interferometer output signal using the simulated k-clock signal; and defining a discrete Fourier transform method using the simulated k-clock signal that can be directly applied to the interferometer output signal that is non-uniformly sampled on k.

[0078] In one example, the SS-OCT method further includes generating a mathematical model k(t) and / or a tuning rate dk / dt of the imaging optical signal based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source, and the SS-OCT method further includes at least one of: using the mathematical model to perform at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wave number k; using the mathematical model to calculate at least one correction to one or more distortions in the interferometer output signal; and using the mathematical model to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is unevenly sampled on k.

[0079] In one example, the SS-OCT method further includes: controlling a tunable light source such that a temporal rate of change (tuning rate dk / dt) of at least one wavenumber k of the imaging optical signal includes at least two tuning rates dk / dt associated with one or more interferometry signals; and generating a mathematical model of the at least two tuning rates based at least in part on one or more characteristics of the tunable light source and one or more characteristics of an SS-OCT system including an interferometer and an optical detector. In another example, the SS-OCT method further includes: correlating an estimate of at least one of the value of k and the tuning rate dk / dt with data sampled from an interferometer output signal and / or a corrected interferometer output signal using the mathematical model; estimating the value of k at the sampled interferometer output signal value using the associated measured value of the wavenumber k; and performing at least one of sampling, resampling, interpolating, and estimating the interferometer output signal at uniform intervals of k using the associated measured value of the wavenumber k. In another example, the SS-OCT method further includes using the mathematical model to calculate at least one correction for one or more distortions in the interferometer output signal.

[0080] In one example, the time-varying difference in optical path length is caused by sample motion relative to the axis of the imaging optical signal. In one example, the time-varying difference in optical path length is caused by a material modification process on the sample effected by the material processing beam. In one example, the time-varying difference in optical path length is caused by intrinsic sample motion rather than by a material modification process on the sample effected by the material processing beam.

[0081] In one example, the SS-OCT method further includes controlling a tunable light source such that a temporal rate of change of at least one wave number k of the imaging optical signal (tuning rate dk / dt) includes at least two tuning rates dk / dt. In another example, the at least two tuning rates include at least one negative dk / dt and at least one positive dk / dt. In one example, the tunable light source is controlled such that the imaging optical signal includes a superposition of the at least two tuning rates dk / dt. In another example, the superposition of the at least two tuning rates dk / dt includes at least one negative dk / dt and at least one positive dk / dt.

[0082] In one example, the tunable light source is a first tunable light source, and the SS-OCT method further includes providing at least one additional tunable light source. In one example, the interferometer is a first interferometer, and providing the at least one additional tunable light source further includes providing at least one additional interferometer, the first interferometer being configured with the first tunable light source, the at least one additional interferometer being configured with the at least one additional tunable light source, and the first interferometer and the at least one additional interferometer being configured such that they share at least one optical element. In another example, the temporal rate of change of at least one wavenumber k of the imaging optical signal is a tuning rate dk / dt, and the SS-OCT method further includes: controlling the first tunable light source such that a first imaging optical signal generated by the first tunable light source has a first tuning rate dk / dt; and controlling the at least one additional tunable light source such that the imaging optical signal generated by the at least one additional tunable light source has a second tuning rate dk / dt that is different from the first tuning rate dk / dt. In another example, at least a portion of the first imaging optical signal and at least a portion of the imaging optical signal generated by the at least one additional imaging optical signal are transmitted simultaneously. In another example, a first tuning rate and a second tuning rate are associated with one or more interferometer output signals, and the SS-OCT method further comprises: calculating one or more corrections, wherein calculating the one or more corrections comprises: identifying distortion in at least one of the interferometer output signals associated with at least one of the first tuning rate and the second tuning rate; and evaluating the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation. In another example, the first tuning rate dk / dt is positive dk / dt, and the second tuning rate dk / dt is negative dk / dt.

[0083] In one example, the SS-OCT method further includes providing: a splitter for splitting the imaging optical signal into at least two arms; and an optical delay element configured to cause an output of a first arm of the at least two arms to be delayed in time relative to an output of a second arm of the at least two arms. In another example, the SS-OCT method further includes controlling a tunable light source such that a temporal rate of change (tuning rate dk / dt) of at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt. In another example, the at least two tuning rates dk / dt include a positive tuning rate dk / dt associated with the first arm or the second arm, and a negative tuning rate dk / dt associated with the other of the first arm or the second arm. In one example, the interferometer is a first interferometer, and the SS-OCT method further includes providing at least one additional interferometer. In one example, the first arm is configured to be directed to at least one of the following arms: different reference arms, different sample arms, partially overlapping reference arms, and partially overlapping sample arms of the first interferometer and the at least one additional interferometer. In another example, the first arm and the second arm are configured to be directed to at least one sample arm and at least one reference arm of a first interferometer and at least one additional interferometer. In another example, the first interferometer and the at least one additional interferometer are configured such that the sample arm of the first interferometer and the sample arm of the at least one additional interferometer share at least one optical element, and the first arm and the second arm of the imaging optical signal are configured to be directed to the first interferometer and the at least one additional interferometer simultaneously. In another example, the first arm is configured to be directed to the first interferometer or the at least one additional interferometer, and the second arm is configured to be directed to the other interferometer of the first interferometer or the at least one additional interferometer. In another example, at least two tuning rates of the first arm and the second arm are associated with one or more interferometer output signals, and the SS-OCT method further includes calculating one or more corrections, and calculating the one or more corrections includes: identifying distortion in at least one interferometer output signal of the one or more interferometer output signals associated with at least one of the positive tuning rate and the negative tuning rate; performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

[0084] In one example, the tunable light source is configured such that the optical frequency of the imaging optical signal varies at a rate in the range of 8 PHz / s to 2 ZHz / s, inclusive.

[0085] In one example, the SS-OCT method further includes: generating an OCT image based on the at least one interferometer output signal and / or the at least one corrected interferometer output signal; and transmitting the generated OCT image to a display device. In another example, the SS-OCT method further includes: generating OCT data based on the processed interferometer output signal and / or the at least one corrected interferometer output signal; and transmitting the OCT data to an external device.

[0086] In another example, the SS-OCT method further includes providing the tunable light source as a tunable vertical cavity surface emitting laser (VCSEL). In one example, the SS-OCT method further includes providing an amplifier for amplifying the VCSEL. In one example, the amplifier is configured as a fiber amplifier.

[0087] In one example, the SS-OCT method further includes guiding the imaging optical signal with a guiding element. In another example, the imaging optical signal is guided to within 50 nm of the material processing beam at the processing region.

[0088] In one example, the SS-OCT method further includes providing a processing unit configured to: process at least one interferometer output signal; detect the distortion;

[0089] applying one or more corrections; and processing at least one corrected interferometer output signal.

[0090] In one example, the SS-OCT method further includes providing a material processing source configured to generate a material processing beam. In another example, the SS-OCT method further includes controlling at least one processing parameter of a material modification process implemented on the sample by the material processing beam based on at least one characteristic of the processing region.

[0091] In one example, the SS-OCT method further includes generating at least one interferometer output signal using an optical detector.

[0092] Other aspects, embodiments and advantages of these example aspects and embodiments are discussed in detail below. In addition, it should be understood that the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and characteristics of the claimed aspects and embodiments. The embodiments disclosed herein can be combined with other embodiments, and references to "embodiment," "example," "some embodiments," "some examples," "alternative embodiments," "various embodiments," "one embodiment," "at least one embodiment," "the embodiment and other embodiments," "certain embodiments," etc. are not necessarily mutually exclusive and are intended to indicate that the specific features, structures, or characteristics described may be included in at least one embodiment. The appearance of such terms herein does not necessarily all refer to the same embodiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] Various aspects of at least one embodiment are discussed below with reference to the accompanying drawings, which are not intended to be drawn to scale. The drawings are included to provide illustration and further understanding of the various aspects and embodiments, and are incorporated into and constitute a part of this specification, but are not intended to be a definition of limitations on any particular embodiment. The drawings, together with the rest of the specification, serve to explain the principles and operation of the described and claimed aspects and embodiments. In the drawings, each identical or nearly identical component illustrated in various figures is represented by the same number. For clarity, not every component may be labeled in every figure. In the drawings:

[0094] Figure 1 is a schematic representation of an example of an SS-OCT system according to aspects of the present invention;

[0095] Figure 2 is a schematic representation of a balanced photodetector configuration for optical detection according to aspects of the present invention;

[0096] Figure 3 is a schematic representation of another example of an SS-OCT system according to aspects of the present invention, showing an alternative location for image beam magnification;

[0097] Figure 4 is a schematic representation of a beam delivery system according to aspects of the present invention;

[0098] Figure 5A is a schematic representation of an SS-OCT system incorporating auxiliary sensing capabilities according to aspects of the present invention;

[0099] Figure 5B is a schematic representation of a beam delivery system incorporating auxiliary sensing capabilities according to aspects of the present invention;

[0100] Figure 6 is a block diagram representing the logical modules of a multiplexing system according to aspects of the present invention;

[0101] Figure 7A is a schematic representation of an example of an interferometer according to aspects of the present invention;

[0102] Figure 7B is a schematic representation of an example of an interferometer according to aspects of the present invention;

[0103] Figure 8 is a comparison of M-mode data from an SD-OCT system (top graph) and an SS-OCT system (bottom graph) according to aspects of the present invention;

[0104] Figure 9A is a first example showing both uncorrected weld image signal data and weld image signal data in which distortion resulting from sample motion has been corrected according to aspects of the present invention;

[0105] Figure 9B is a second example showing both uncorrected weld seam image signal data and weld seam image signal data that has been corrected for distortion caused by sample motion;

[0106] Figure 10A and Figure 10B Interferometry signal data according to aspects of the present invention are shown;

[0107] Figure 11A shows uncorrected A-line depth versus amplitude data according to aspects of the present invention;

[0108] Figure 11B shows the relationship between A-line depth and amplitude data with motion compensation applied according to aspects of the present invention;

[0109] Figure 12A and Figure 12B showing uncorrected and corrected M-mode OCT images according to aspects of the present invention, respectively;

[0110] Figure 1 3A and Figure 13B showing uncorrected and corrected OCT image data from a welding process, respectively, according to aspects of the present invention;

[0111] Figure 14A shows raw M-mode image data from a welding process according to aspects of the present invention;

[0112] Figure 14B Shown with Figure 14Atracking image data corrected for motion artifacts corresponding to the original image data;

[0113] Figure 14C and Figure 14D Shown in Figure 14A and Figure 14B An image of a cross section of a preformed weld during welding;

[0114] Figure 15 is a block diagram of an exemplary computer control system for performing control and processing in an SS-OCT system according to aspects of the present invention;

[0115] Figure 16A 、 Figure 16B and Figure 16C showing M-mode OCT images captured during welding processes on mild steel, aluminum, and copper substrates, respectively, according to aspects of the present invention;

[0116] Figure 17A shows an exemplary M-mode OCT image captured on an SD-OCT system during a welding process;

[0117] Figure 17B An exemplary M-mode OCT image captured on a SS-OCT system is shown, which was taken during the same welding process as Figure 17A SD-OCT captures captured concurrently;

[0118] Figure 17C Shown from Figure 17A Weld image signal data derived from M-mode OCT image data;

[0119] Figure 17D Shown from Figure 17B Weld image signal data derived from M-mode data;

[0120] Figure 18A is a schematic representation of one example of a SS-OCT system showing a dual interferometer configuration having interferometers overlapping at the sample arm with separate light sources in accordance with aspects of the present invention;

[0121] Figure 18B is a schematic representation of one example of a SS-OCT system showing a dual interferometer configuration having interferometers overlapping at a sample arm and featuring a buffered light source in accordance with aspects of the present invention;

[0122] Figure 18C is a schematic representation of another example of an SS-OCT system having a dual interferometer configuration with interferometers overlapping at a sample arm and featuring a buffered light source in accordance with aspects of the present invention;

[0123] Figure 18D is a schematic representation of another example of a SS-OCT system having a dual interferometer configuration with separate light sources according to aspects of the present invention, and shows Figure 18A Variations of the interferometer topology shown;

[0124] Figure 18E is a schematic diagram of another example of an SS-OCT system having a dual interferometer configuration featuring a buffered light source and overlapping interferometers at a sample arm according to aspects of the present invention;

[0125] Figure 19 is a schematic representation of one example of an SS-OCT system showing an interferometer configuration including a buffered light source and generating a plurality of swept source light signals according to aspects of the present invention;

[0126] Figure 20A is a graph illustrating one example of quality degradation over time in a process according to aspects of the present invention; and

[0127] Figure 20B is a graph illustrating an example of quality degradation over time during correction via AI / ML monitoring techniques according to aspects of the present invention. DETAILED DESCRIPTION

[0128] Overview

[0129] Disclosed herein is an enhanced SS-OCT apparatus and method for monitoring manufacturing processes, particularly monitoring and controlling the application of laser beams and other energy beams used to process materials in industrial material modification processes. Non-limiting examples of material modification processes include welding, cutting, drilling, ablation, brazing, surface texturing, annealing, and additive manufacturing. According to one or more embodiments, certain aspects of the disclosed apparatus include a light source, an amplifier (particularly an amplified, rapidly tunable light source with a long instantaneous coherence length), an interferometer, a k-clock, scanning optics, integration of the interferometer into an energy beam delivery system (i.e., a beam delivery head), detection electronics, signal processing electronics, control electronics, and software and feedback loops (e.g., electronically and / or software-enabled feedback loops), through which some aspects of the system control other aspects. These aspects are integrated to perform measurements of the manufacturing process (and in some embodiments, real-time measurements), including but not limited to inline coherent imaging, which is one of several known ways to directly measure laser weld penetration depth in real time, and may be the only known way that is widely applicable to series production in many markets. According to at least one embodiment, the SS-OCT system is based on a rapidly tunable light source that provides significant advantages over other conventional OCT systems in terms of speed, sensitivity, flexibility, and instantaneous imaging range. In some embodiments, the tunable light source is a high-power light source. In addition, implementing such a light source and processing its signals in commercial practice requires special attention to safety systems and dynamic range management. According to certain embodiments, the disclosed SS-OCT system is configured to determine sample position information and / or sample characteristic information, including geometric information. The sample can be associated with a material modification process, and the characteristic information can correspond to a position or location in and / or near a phase change region (PCR) at a time point before, during, and / or after the material modification process. According to certain embodiments, the disclosed SS-OCT system is configured to determine correct position information and a direct measurement of the sample motion speed (e.g., on the axis of the imaging beam) by evaluating and compensating for motion artifacts caused by the modulation of the interference fringes. In one embodiment, the motion of the sample is a motion inherent to the material modification process.

[0130] According to various aspects, the average return signal intensity seen throughout a given weld is higher on the disclosed SS-OCT system than on the SD-OCT system. Some of the difference in signal levels can be attributed to the higher imaging power available from the disclosed SS-OCT system. At least some of the difference in signal levels between the two system configurations can be attributed to the higher imaging sensitivity achievable with the disclosed SS-OCT system compared to the prior art SD-OCT system. However, another difference arises in the manifestation of motion artifacts in each of these different OCT methods. In the SD-OCT method, motion artifacts appear as fringe washouts, which reduce useful signal levels and eliminate the ability to acquire and decode that data. In the disclosed SS-OCT method, motion artifacts appear as distortions in the interferogram data, resulting in a bright but potentially distorted signal. Figure 11A An example of a distorted A-line generated based on the distorted interferogram output data is shown in FIG. Figure 11B Corrected A-lines generated based on the interferometer output data after one or more corrections have been applied to the interferometer output data based on the distortion are shown. These figures are discussed in further detail below. This artifact often occurs during imaging of weld keyholes, as these structures are known to contain rapidly moving metal. Specific implementations of one or more algorithms capable of correcting for this motion artifact are disclosed herein.

[0131] Overall system description

[0132] According to at least one embodiment, Figure 1 A non-limiting example of an SS-OCT system 100 is shown in FIG. The system 100 includes an imaging light source (which may be considered alone or as a component of an interferometer, as described in further detail below), which in this example is a tunable light source 105 that provides coherent narrowband light having a wavelength that varies with time, such as an electronically tunable swept source MEMS VCSEL. One or more optical amplifiers 106 may be used to amplify the tunable light source 105. Non-limiting examples of suitable amplifiers include ytterbium fiber amplifiers available from IPG Photonics, Marlborough, MA, USA. Light emitted from the tunable light source and / or light amplified by the amplifier based on the tunable light source as a seed source may be referred to herein as swept optical signal, imaging light, imaging optical signal, imaging beam, ICI beam, and / or interferometer light, as appropriate in the context.

[0133] The system 100 also includes a controller 150 (also referred to herein as a control module, processing module, or processing unit) that includes the necessary hardware and software required to control and communicate with one or more components of the system 100 .

[0134] For example, the processing unit 150 includes the necessary control electronics for the tunable light source 105 and amplifier 106 and / or other system components. In some embodiments, the light source control module 109 can be included in the controller 150 that controls the tunable light source 105 and amplifier 106.

[0135] The system 100 also includes an interferometer 120, which in this example includes a tunable light source 105 that generates a swept optical signal, an amplifier 106 that amplifies the power of the swept optical signal, a branching element 126 that branches the imaging light (imaging optical signal) into two or more paths within the system, an adjustable delay line that serves as a reference arm 122, a sample arm 124 that is configured to direct the swept optical signal to a processing area (i.e., on the workpiece or sample 102) or otherwise associated with the material modification process, and a combining element 128 that combines the imaging light after it traverses its path. The interferometer 120 is configured to direct the swept optical signal to the reference arm 122 and the sample arm 124, and to combine the optical signals returned from the reference arm and the sample arm to generate a combined optical signal. In some embodiments, multiple (two or more) sample arms and / or reference arms can be implemented. Certain embodiments having these configurations are described in further detail below. Figure 1 The interferometer embodiment shown in the system 100 is a Mach-Zehnder interferometer (MZI) topology, which exhibits improved photon economy compared to other interferometer configurations and enables balanced detection. The Mach-Zehnder interferometer also includes fiber circulators 121, 123 and a polarization controller 127, which are also included in the embodiment shown. However, it should be understood that other interferometer topologies (such as a Michelson interferometer) are also within the scope of the present disclosure.

[0136] System 100 also includes a material processing beam source 110 and a corresponding beam delivery system 115 (which may also be referred to as a beam delivery module and may be housed in a laser head), whereby imaging light is at least partially combined with the material processing beam using a dichroic mirror 117 for subsequent application to a workpiece 102 (also referred to herein as a "sample"). Workpiece 102 or sample can be any object or surface undergoing a material modification process (e.g., welding) or otherwise processed by material processing beam 112 (also referred to herein simply as a processing beam or process beam). Material processing beam 112 interacts with the material of sample 102 at a processing region 103 on sample 102. For example, in some embodiments, imaging can be performed on a sample that is about to undergo or has just undergone a material modification process. Beam delivery system 115 may include beam steering elements, such as galvanometers, that deflect or otherwise steer imaging beam 108, material processing beam 112, or both (e.g., scanners 116, 118). In some embodiments, material processing beam 112 generates a phase change region (PCR) at processing region 103. In one embodiment, at least one guiding element directs the imaging light beam to one or more selected locations in and / or near the PCR. The system 100 also includes an optical detector 130 (also referred to simply as a detector) or multiple detectors that receive the imaging light, such as a combined optical signal from the reference arm and the sample arm of the interferometer 120. The detector 130 is configured to generate at least one interferometer output signal based on the detected combined optical signal. A non-limiting example of the detector 130 is a balanced photodetector (BPD), an example of which is shown in FIG. Figure 2 , which in this embodiment enhances system performance by reducing common mode noise. According to one embodiment, the detector 130 is configured as a BPD with an integrated transimpedance amplifier.

[0137] For SS-OCT, a laser source (for imaging) with an instantaneous narrowband wavelength that can be tuned at high speed is often required. According to at least one embodiment, the tunable light source 105 generates an imaging optical signal having at least one wave number k that is variable in time and a sweep rate in the range of 1 kilohertz (kHz) to 20 megahertz (MHz), including end values. As used herein, the term sweep rate refers to the repetition rate in the time-varying spectral output of the imaging optical signal. It is the inverse of the sweep interval / sweep period. This can also be considered as the inverse of the amount of time it takes to collect an A line. According to certain aspects, the imaging optical signal essentially has at least one wave number k that is variable in time. The following cases are examples of imaging optical signals encompassed within this definition:

[0138] A) an imaging optical signal having an instantaneous coherence length of 1 mm or longer, comprising at least one spectral feature and / or peak, the feature and / or peak comprising a plurality of wavenumbers, each feature and / or peak having a characteristic wavenumber k (e.g., corresponding to the center, centroid, highest intensity point, intensity-weighted centroid of the instantaneous spectrum, etc.), the characteristic wavenumber k of the feature being temporally variable; and

[0139] B) An imaging optical signal as outlined in A, further comprising other spectral features (e.g., secondary peaks, amplified spontaneous emission), which may or may not partially or completely overlap with the peak at any time during operation of the imaging optical signal.

[0140] As used herein, the term tuning rate refers to the rate of change in time of at least one wave number k of the imaging optical signal. The term tuning rate may be interchangeably referred to as "dk / dt" or "tuning rate dk / dt."

[0141] The imaging optical signal may include any other optical signal recognized by one of ordinary skill in the art to be suitable for SS-OCT.

[0142] For a linear tuning profile, different beat frequencies (sometimes referred to or otherwise as "interference patterns") in the combined optical signal correspond to different delays or reflections from different depths in the sample. In practice, most lasers do not exhibit a perfectly linear relationship between wavenumber k and time. For at least this reason, the system 100 may also include at least one k-clock module 145 (also referred to herein as a k-clock). In some embodiments, the system 100 includes multiple k-clock modules 145. The k-clock module 145 provides a k-clock output whose frequency is proportional to the wavelength of the light source 105 output. If this clock is used as a sampling clock, the analog-to-digital output is sampled in a substantially linear manner in k-space. In one embodiment, the k-clock module is configured as an optical interferometer designed to generate a reference beat frequency corresponding to a known delay based on the tuning rate of the tunable light source 105 and is implemented by a low-power tap of the tunable light source 105 (e.g., approximately 1%, and in some embodiments, less). The k-clock, if included, requires its own detector 147.

[0143] The system 100 also includes a digitizer 135 that captures the detector signals. In certain embodiments, the digitizer is configured to digitize at least one interferometer output signal and generate a corresponding digital signal. The processing unit 150 may incorporate a signal processor that receives the digitized signals from the digitizer 135 and calculates data and / or other relevant information derived from these signals. Generally speaking, modules associated with the processing unit 150 include (but are not limited to) data processing modules, communication modules, security modules, and feedback and control modules. As used herein, the term "module" refers to a logical grouping of design functions, including but not limited to optical, electronic, simulation, analysis, and / or computational implementations of such functions. "Module" does not necessarily mean that the modules are nominally physically separable from each other. Further discussed in detail below are implementation options for reusing one or more components, but it should be understood that, according to certain embodiments, modules can be shared with other modules and therefore are not always used in a 1:1 ratio.

[0144] In accordance with at least one embodiment, the tunable light source 105 is comprised of a microelectromechanical system (MEMS) tunable vertical cavity surface emitting laser (VCSEL) amplified by a rare earth doped fiber amplifier 106 (also referred to herein as a "fiber amplifier"). In one embodiment, the fiber amplifier 106 has a peak gain at a wavelength between 1010 nm and 1050 nm. In another embodiment, the fiber amplifier 106 has a peak gain at a wavelength between 1050 nm and 1090 nm. Common dopants include erbium and ytterbium. In accordance with certain embodiments, the fiber amplifier 106 has one, two, or three amplification stages. In some embodiments, the fiber amplifier architecture can include optical isolation between amplifier stages, where applicable, in the form of compact isolators and / or semi-compact isolators and / or bulk isolators. In accordance with at least one embodiment, the output power of the fiber amplifier is at least 30 milliwatts (mW). In some embodiments, the output power is at least 20 mW, at least 50 mW, at least 100 mW, at least 500 mW, at least 1 watt (W), and / or at least 5 W. These higher powers are considered novel because most coherent imaging and OCT systems are typically designed for imaging living tissue, which can be damaged by such high power levels. Laser material processing is one of the few application spaces where the management of high energy laser hazards is a commercial practice. Fiber amplifiers are an attractive option because, in addition to their high power scalability, they also have high reliability and redundancy. Furthermore, with sufficient economies of scale, fiber amplifiers can be manufactured at a lower cost than semiconductor optical amplifiers (SOAs) of equivalent output and / or reliability. As used herein with respect to amplifiers, reliability refers to the stability of the power output of the amplifier over an extended period of time. Fiber amplifiers can become very reliable when combined with redundant pump diodes that can be kept as backup and brought online when the output of the other pump diodes begins to decline or otherwise fail. In certain embodiments, the safety module ( Figure 1 The energy ultimately supplied to one or more amplifier stages is controlled by redundant means (not explicitly shown) such that the apparatus complies with IEC 60825-1.

[0145] According to certain aspects, the higher launch power provided by the specific implementation of the fiber amplifier 106 contributes, at least in part, to the increased sensitivity of the system. It should be understood that other factors, such as improved photon economy, balanced detection, detector resolution, etc., also contribute to system sensitivity. As used herein, sensitivity refers to the weakest reflection from the sample or workpiece 102 that can be resolved above the system's noise floor. According to at least one embodiment, the SS-OCT system disclosed herein has a sensitivity of at least 105 dB. This is several orders of magnitude greater than conventional OCT systems used in materials processing. According to at least one embodiment, the disclosed SS-OCT system is capable of providing weld image depths of at least 21 mm, inclusive. In some embodiments, the disclosed SS-OCT system is capable of providing weld image depths of: at least 17 mm; at least 20 mm, inclusive; at least 30 mm, inclusive; at least 40 mm, inclusive; and / or at least 50 mm, inclusive. All of these depths are greater than those provided by conventional coherent imaging systems used in materials processing applications.

[0146] In a preferred embodiment, the interferometer 120 of the system 100 includes an imaging light source 105 and a corresponding fiber amplifier 106, and is configured as a Mach-Zehnder interferometer with a 90:10 splitting ratio between the sample arm 124 and the reference arm 122, respectively, but it should be understood that other splitting ratios required to achieve higher sensitivity are also within the scope of the present disclosure. The reference arm 122 can be configured with an adjustable delay line. The beam delivery system 115 can be configured as or otherwise implemented as a laser welding head, such as the FLW-D50 welding head available from IPG Photonics, Oxford, MA, USA. The beam delivery system 115 can include a beam steering device, such as a galvanometer scanner 116 for the imaging beam. In some embodiments, more than one galvanometer can be used.

[0147] According to one embodiment, the material processing beam source 110 is configured as a Yb-doped fiber laser. Non-limiting examples of such fiber lasers include the IPG YLS-2000 / 4000-SM-AMB and / or other YLS series fiber lasers available from IPG Photonics. The material processing beam source 110 generates a material processing beam 112 that interacts with the material of the sample 102 at a processing region on the sample 102. The material modification processes discussed herein are enabled by the material processing beam 112.

[0148] The k-clock module 145 is implemented in one embodiment by tapping (at the beam splitter 104) the imaging light source 105 after the amplifier stage to illuminate an additional MZI with a slightly mismatched optical path length, which is coupled to a second optical detector 147, which in one embodiment is configured as a balanced photodetector. As used herein, k refers to the optical wave number, which is the inverse of the optical wavelength. In cases where the tuning function of the light source is not linear in time (t), that is, k(t) is not linearly proportional to t, the interference signal generated by the k-clock significantly aids in the accurate processing of the primary interferometry signal. One application of the k-clock signal is to use it to dictate the sampling rate of the digitizer 135 of the processing module 150 so that signal samples are collected at uniform intervals of wave number (k).

[0149] The digitizer 135 (also known as the analog-to-digital conversion module) receives the interferometer output signal from the optical detector 130 and generates a corresponding digital signal. In some embodiments, the digitizer 135 also receives a signal from the k-clock detector 147 and digitizes it and / or uses it to determine the sampling rate. The processing module 150 then processes at least one interferometer output signal to determine at least one characteristic of the processing region and detect distortion in at least one interferometer output signal. According to various embodiments, the distortion is caused by the motion of the sample, but more generally, the distortion is caused by the time-varying difference in optical path length between at least one sample arm and at least one reference arm. According to various aspects, the time-varying difference in optical path length is caused by sample motion. According to certain aspects, sample motion refers to a time-varying change in the optical path length of the sample arm, that is, the distortion is caused by the time-varying optical path length of the sample arm and may not be caused by the intrinsic motion of the sample itself, but the optical path length may also be changed by other external factors (e.g., the environment). The time-varying optical path length of the sample arm may also be caused by the intrinsic motion of the sample. The time-varying optical path length of the sample arm may be caused by a combination of intrinsic motion and environmental factors. In response to detecting the distortion, processing module 150 applies one or more corrections to the interferometer output signals to generate corresponding corrected interferometer output signals, and processes at least one corrected interferometer output signal to determine at least one characteristic of the processed region. For example, in some embodiments, processing module 150 receives digital data, optionally performs spectral shaping and background subtraction operations, and calculates frequency analysis and noise floor equalization of the resulting signal to produce a function of workpiece reflectivity versus optical path length, referred to by those skilled in the art as an axial line or "A-line." Other operations may also be added to the processing chain to compensate for optical dispersion or other distortions present in the interferogram. In some embodiments, the system is configured to automatically compensate for optical dispersion by first training the system on a known flat surface. In some embodiments, additional operations may be added to the processing chain to compensate for image artifacts introduced by rapid motion of the imaging target. In some embodiments, if digitizer 135 is not configured to digitize the signal in a manner that is substantially linear in k, additional operations may be added to the processing chain to digitally resample the data digitized by digitizer 135 to be substantially linear in k. Such operations may employ a digital capture of the k-clock signal, a model of the light source and / or system including a mathematical model, or other information to assist in such resampling.

[0150] The beam delivery system 115 of system 100 includes at least one guiding element, such as a scanner 116, configured to adjust the position of imaging beam 108 relative to processing beam 112. According to one embodiment, the at least one guiding element guides the swept optical signal (imaging beam 108). In another embodiment, the at least one guiding element is configured so that the imaging optical signal (focus) is within 50 nm or less of the (focus) of the material processing beam 112 at the processing region. According to some embodiments, the ability to adjust the position of the imaging beam relative to the processing beam enables the acquisition of subsequent A-lines at various locations in and around the phase change region (PCR). During processes such as laser welding (an example of a material modification process), PCRs are generated where the material located at the bonding region dynamically changes from a solid to a liquid and / or gaseous state and back to a solid again upon completion of the weld. By acquiring subsequent A-lines in or around the PCRs generated by material processing beam 112, various measurements of the processing region / workpiece geometry (i.e., features, including geometrical characteristics) can be made to guide, influence, and / or monitor the quality of the process and the resulting workpiece.

[0151] Various system components and features

[0152] Imaging light source

[0153] There are many possible configurations for the imaging light source 105 of the SS-OCT system disclosed herein. The general configuration of the light source includes at least a seed source that generates a wavelength swept signal (also referred to herein as an imaging optical signal or a swept optical signal), which can be referred to as a swept source, can be based on some input (referred to as a drive signal), and can include any number of amplifiers, optical buffers, feedback mechanisms, and other components. Certain requirements that the light source must meet include the availability of wavelength sweeping, a sufficiently narrow instantaneous linewidth, a sufficiently wide sweep range, a sufficiently fast sweep rate, sufficient phase stability and power requirements, as well as wavelength requirements, and various other requirements based on system parameters. In general, the precise choice of light source is highly dependent on the requirements of the application. Non-limiting examples of suitable seed swept sources include solid-state MEMS (micro-electromechanical systems) swept source VCSELs (vertical cavity surface emitting lasers), as well as other possible configurations. A non-limiting example of a suitable MEMS VCSEL light source includes the SL 10280 available from Thorlabs, Inc., Newton, NJ, USA. According to at least one embodiment, the tunable laser light source 105 is a tunable VCSEL. For amplifier-integrated embodiments of the present invention, the selection and / or design of the optical amplifier 106 must take into account the characteristics of the seed source, including seed / output power, wavelength sweep range, instantaneous linewidth, response time, safety requirements, and other requirements. Non-limiting examples of suitable amplifiers 106 include doped fiber amplifiers, such as diode-pumped erbium (doped) or ytterbium (doped) fiber amplifiers.

[0154] As previously mentioned, according to at least one embodiment, the tunable laser light source 105 is a high-power light source. According to various embodiments, the tunable laser light source 105 has a power of at least 1 microwatt (μW), at least 30 μW, at least 100 μW, at least 500 μW, at least 1 mW, at least 5 mW, at least 10 mW, at least 20 mW, at least 50 mW, at least 100 mW, at least 500 mW, and / or at least 1 W. In one embodiment, the tunable laser light source 105 has a power in the range of 30 μW to 500 μW, inclusive.

[0155] Optical amplifier

[0156] As mentioned above, some embodiments of the present invention include an amplifier 106 to increase the available power to a level greater than that possible using the seed swept source 105 alone. In some embodiments, the amplifier used to amplify the VCSEL is configured as a fiber amplifier (as used in the examples herein), but it should be understood that other types of amplifiers are also within the scope of the present disclosure, such as semiconductor optical amplifiers (SOAs) or solid-state amplifiers. Some embodiments utilize constant gain, while other embodiments use variable gain, for example to achieve feedback control of the output power of the amplifier 106. More advanced embodiments may incorporate manual or software-based feedback control in conjunction with (but not limited to) the detector and amplifier, for example to prevent detector saturation, thereby effectively increasing the system dynamic range.

[0157] In preferred embodiments of the system, doped fiber amplifiers are used. Examples of such amplifiers include erbium- and ytterbium-doped fiber amplifiers. In some embodiments of the system, solid-state amplifiers may be used. In other embodiments of the system, SOAs may be used. The selection and / or design of an appropriate amplifier may be based on the requirements of the application, including, for example, the desired wavelength, temporal dynamics, and gain requirements.

[0158] Certain embodiments may use customized population inversion or time-dynamic pumping to optimize the gain ratio of the amplifier at each point in the swept source sweep. In embodiments where the output power of a known seed swept source varies with time, power level feedback may be used to compensate for long-term source degradation. In embodiments where the emission power of the seed source varies predictably and regularly based on the instantaneous emission wavelength, parameters including length and population inversion may be used to optimize the amplifier configuration to partially or fully compensate for power variability. Amplifier pumping may also be implemented in a time-dynamic manner synchronized with the tuning signal to compensate for output power variability across the entire swept spectrum.

[0159] In some embodiments of the system, feedback control can be implemented (e.g., via processing unit 150) to maximize amplifier power output stability. In some embodiments, feedback control can also be implemented in the amplifier system to increase the effective dynamic range of the system by adjusting the amplifier gain based on a return signal, such as from the primary detector 130 or other sensors throughout the system, thereby adjusting the imaging power, for example to increase the imaging power when imaging a low reflectivity surface, or to reduce the imaging power in the event of detector saturation. Other embodiments of the system can incorporate variable gain without feedback, for example by designing a variable gain signal for the amplifier drive based on the imaging requirements designed for a given process. Feedback control can also be implemented to compensate for reduced optical transmission or performance, such as degradation due to system aging.

[0160] Some embodiments of the system may incorporate more than one amplifier 106. The amplifiers may be arranged in series to increase the overall gain available. According to certain embodiments, amplifiers may also be placed at various locations within the interferometer itself, where appropriate, based on specific design requirements. Figure 3 A non-limiting example of an SS-OCT system with alternative amplifier placement is shown in system 400. Figure 3 One or more amplifiers are shown in amplifier 406, with or without an amplifier directly following seed source 405 ( Figure 3 (not shown in FIG. 1 ). Examples of amplifier placement include placing an amplifier in the sample arm immediately after the system's sample arm branches off from the reference arm 422 to deliver increased power to the sample (e.g., amplifier 406a), or placing an amplifier at the final phase of the sample arm to increase the amplitude of the return signal for imaging (e.g., 406b). Some embodiments of the system may incorporate more than one amplifier. For example, multiple amplifiers may be placed at different locations within the interferometer itself.

[0161] In system embodiments with amplifiers, the electrical configuration may incorporate an interlock feature to allow the system to be switched between different laser safety levels (eg, Class 2 and Class 3b) for use in different processes such as manual alignment and cell operation.

[0162] Delay Line

[0163] All embodiments incorporate one or more delay lines within the interferometer topology to match the path length between the sample arm 124 and the reference arm 122. In some embodiments, the delay line is designed to be able to adjust the optical path length, for example by incorporating precision mechanical components and motors to move mirrors or other optical components. In some embodiments, the delay line is manually adjustable, while in some embodiments, automatic adjustment can be achieved using, for example, electronic control devices. In some embodiments, automatic delay line movement can be used to effectively extend the imaging range by adjusting the delay line in response to different optical path lengths present in the sample arm of the system. Such delay line adjustment can be implemented based on a pre-programmed algorithm that takes into account the expected variability of sample height within the field of view. Alternatively, such delay line adjustment can be implemented in response to system conditions (e.g., via feedback control).

[0164] Certain embodiments incorporate an adjustable delay line on the reference arm of the system. This topology enables adjustment of the depth field of view of the OCT system, allowing the imaging target of interest to be positioned within the field of view without requiring changes to the mechanical setup of the target, greatly simplifying the setup and increasing flexibility.

[0165] Some embodiments of the system incorporate an adjustable delay line in the k-clock interferometer 145. Adjustment of the path length in the k-clock in turn enables adjustment of the k-clock frequency, thereby allowing k-clock optimization for a single system at multiple different seed source sweep rates. For example, if the seed source sweep rate is slowed, the length of the k-clock path length interval can be increased, thereby obtaining a greater usable imaging depth and more usable acquisition points within a single sweep, while maintaining appropriate frequency characteristics for the sampling and digitizing systems (because the seed source sweep rate is slower, and therefore dk / dt is also slower). Alternatively, techniques for generating and / or switching between multiple different optical paths (e.g., shutters, mechanical devices) can be used to achieve a similar effect by making multiple discrete paths of different lengths available for the k-clock.

[0166] Feedback control

[0167] In some embodiments, active feedback control is implemented (depending on the functionality of processing unit 150) within and between various system components and / or modules, including but not limited to optical swept source 105, amplifier 106, delay lines (e.g., in reference arm 122), and optical detector 130. In some embodiments, feedback control or feedback loops may include electronically and / or software-enabled implementations of feedback loops.

[0168] According to at least one embodiment, various feedback controls for the optical amplifier 106 are implemented. Within the amplifier module, feedback control can be implemented to maintain stable power output across varying operating conditions. The amplifier module control can also receive input from the system controller 150, for example to allow for automatic increases in beam power to compensate for less reflective imaging target materials, thereby enabling the OCT system to achieve higher sensitivity. Output power control in all embodiments will be subject to required interlocks and other safety measures in accordance with relevant laser safety product standards.

[0169] Feedback control can also be applied to system components in order to take advantage of optimization for various conditions, including but not limited to k clock arm length, reference arm length, source sweep rate, and source sweep span.

[0170] According to further embodiments (and as discussed elsewhere in this disclosure), feedback control is implemented using information collected from the SS-OCT system 100 (and / or other components, such as the auxiliary measurement system 160) and processed by the controller 150 to control the material processing laser source 110 and / or the beam delivery system 115. For example, the controller 150 can use processed output obtained from the interferometer 120 (and / or associated components, such as the optical detector 130, the digitizer 135, etc.) to control one or more processing parameters (non-limiting examples are given below) of the beam delivery system 115 and / or the material processing beam source 110. The processed output can include, for example, at least one characteristic of a phase change region.

[0171] Detection

[0172] SS-OCT and time-domain imaging methods typically do not use detectors that can substantially distinguish between different frequencies of light. This means they are more easily overloaded by incoherent emissions from the process and / or high-power correction energies. Adding blocking filters at various locations in the interferometer (such as in the material processing beam delivery head (e.g., dichroic optics), inside the fiber optic line (e.g., fiber Bragg gratings, etc.), or at the detector) to isolate the imaging light from unwanted signals can be used for the material processing applications described herein. Balanced detection is another method of suppressing these unwanted signals, which can be applied in addition to or instead of blocking filters.

[0173] In accordance with at least one embodiment, optical detector 130 is configured as a balanced photodetector (BPD). The BPD circuit is used to convert the differential interferometry signal of the OCT data and k-clock data from the optical domain to the electrical domain. Using the BPD to detect the differential signal can eliminate common-mode noise and DC components from the interferogram data collected during the OCT capture and improve the amplitude of the acquired signal relative to unbalanced detection methods. Figure 2 A schematic example of an optical detector 130 configured as a balanced photodetection device is shown in FIG.

[0174] According to one or more embodiments, the analog electronic gain of the BPD signal is adjustable and / or selectable to achieve the desired signal level for digitization. In some embodiments, feedback control is implemented to automatically optimize the BPD gain level. In other embodiments, gain control is implemented by the user, or some combination of user control and feedback control.

[0175] In certain embodiments, an electronic amplifier is included in the BPD circuit and features noise reduction, appropriate bandwidth, and common-mode and DC signal rejection. Analog bandwidth filtering for the specific purpose of anti-aliasing is also considered in the design of the detection module of one or more embodiments. Additional amplifier optimizations may be incorporated into other system embodiments as needed. Amplifier topologies are designed using circuit techniques known to those skilled in the art.

[0176] According to other embodiments, alternative optical signal detection methods can be used, including photodiodes and other detector technologies known to those skilled in the art. These embodiments can also include variable gain (with or without feedback control) and amplifier topologies optimized for specific applications. Embodiments employing multiple photodetection methods (e.g., different detection methods in the k-clock and OCT signal portions of the system) are also possible.

[0177] In some embodiments, different portions of the interferometer's output are routed to multiple detectors. This allows one detector channel to saturate if the reflected signal becomes too bright, while another detector channel can still read the signal. This can substantially increase the dynamic range of the overall system. In certain embodiments, additional attenuation in the optical and / or electronic domains can be included at varying levels for different detectors to enhance this effect and provide an even greater dynamic range.

[0178] K clock control method

[0179] Swept-source OCT (SS-OCT) systems may utilize a signal indicating when the wavenumber k of the imaging optical signal or signal spectrum changes substantially by one or more increments, referred to by those skilled in the art as a k-clock. Some embodiments of the present invention may utilize a sampling clock that samples at uniform time intervals. Some embodiments of the present invention may utilize a sampling clock that is uniform across wavenumber k, which may be based on a k-clock signal. The relationship between time and wavenumber k may or may not be linear, depending on the imaging source tuning method and other system considerations.

[0180] According to at least one embodiment, the imaging source generates a swept optical signal for which the rate of change of the wave number k over time (referred to as the tuning rate dk / dt) is variable. In some embodiments, the tuning rate dk / dt can be equivalently expressed in terms of the rate of change of optical frequency and is in the range of 1 PHz / s to 2 ZHz / s (inclusive), and in some embodiments, in the range of 8 PHz / s to 2 ZHz / s (inclusive). Most practical implementations of SS-OCT systems require resampling or k-clocking of the interferometer output signal samples to compensate for variations, including instabilities in the swept phase, variability in dk / dt across the sweep, and / or nonlinearities in the swept source tuning (such as the nonlinear relationship between the wave number k and time t). Using a signal that provides some reference for the relative or absolute wave number of the imaging source light over time (referred to herein as the "k-clock signal") to assist in sampling, signal processing (including interpolation and resampling), and / or analysis of the OCT interferometer output signal is referred to herein as "k-clocking." Variations in dk / dt can be caused by asymmetric behavior of the light source, e.g. due to intentional driving, fundamental properties of the light source or unavoidable consequences of light source manufacturing.

[0181] According to some embodiments, at least one of the interferometer sample arm and the reference arm is configured with one or more optical elements for generating a k-clock signal. In some embodiments, these optical elements already exist, and in other embodiments, these one or more optical elements are added and used to generate the k-clock signal.

[0182] According to at least one embodiment, an SS-OCT system requires applying a Fourier transform to interferometer data to generate an OCT image. To generate an OCT image properly scaled with meaningful units of physical space, a Fourier transform must be performed on a signal uniformly sampled in k, or a method (e.g., homodyne matrix DFT) must be employed to correct for known non-uniformities in sampling in k-space. In such embodiments where dk / dt is variable, k-clocking (including, but not limited to, optical, electronic, or simulation methods) is required to properly process or sample the interferometer output signal for the Fourier transform and / or to properly construct a Fourier transform for the interferometer output signal.

[0183] In some embodiments, the k-clock can be generated by an optical k-clock module (e.g., k-clock module 145) that generates a signal indicating the predetermined k (inverse frequency) increment each time the swept source 105 is tuned through the swept frequency band. In some embodiments, the k-clock can be generated by an electronic k-clock module that estimates the time-domain behavior of the wavenumber k output by the swept source based on the tuning signal applied to the swept source. In some embodiments, the k-clock can also be simulated based on a model of the system. In some embodiments, a combination of the described methods can be used. The k-clock is used to correct for nonlinearities in the time domain of the frequency sweep of the swept source 105.

[0184] In one embodiment, the k-clock module 145 is configured to trigger the optical detector 130 to sample the interferometer output signal at uniform intervals of wavenumber k. The rate of change of the wavenumber k can be uniform or non-uniform over time. This embodiment represents a preferred specific implementation of k-clock control in an SS-OCT system, wherein the dk / dt of the imaging swept source 105 is non-uniform over time. In an embodiment of the system implemented using sampling at uniform intervals of wavenumbers, the processing unit can be configured to process the interferometer output signal that is uniformly sampled over the wavenumber, which can provide advantages, for example, in terms of measurement accuracy or computational efficiency. In another embodiment, the controller 150 is configured to collect the k-clock signal while collecting the interferometer output signal. In some embodiments, the controller 150 (processing unit) is configured to collect the k-clock signal while collecting the interferometer output signal, wherein the two signals are collected simultaneously at uniform time increments. In one or more of these embodiments, the k-clock measurement signal and any derivatives derived therefrom can be used by processing unit 150 to sample, resample, interpolate, and / or estimate the interferometer output signal at uniform intervals of wavenumber k using digital processing methods. The k-clock measurement signal can also be used by processing unit 150 to calculate at least one correction for one or more distortions in the interferometer output signal. The k-clock signal can also be used by processing unit 150 to define a discrete Fourier transform method that can be directly applied to the interferometer output signal, which may be non-uniformly sampled across k. An example of such a method would be to use the k-clock signal as input to calculate an appropriate matrix for a specific implementation of a matrix DFT that can be applied to interferometer output data that is uniformly sampled in time but non-uniformly sampled across k. In one embodiment, k-clock module 145 is temperature stabilized.

[0185] Using the k-clock as the sampling clock produces uniformly spaced interferometric data in the optical wavenumber domain, or k-space. This maximizes signal-to-noise ratio (SNR) and axial imaging resolution for subsequent Fourier transform-based signal processing of the acquired interferometer output signals. The Fourier transform provides A-scan information, or the axial scan depth distribution within the sample. The OCT system can also use the wavenumber information obtained by sampling the k-clock signal in the time domain to digitally resample or interpolate the interferometric data set to achieve uniform k-space sample spacing.

[0186] According to at least one embodiment, a k-clock module is used. A k-clock signal is generated to assist in processing the OCT interferometer data of the system. When the discrete signal samples are evenly spaced in units of wavenumber (k), Fourier transform processing of the OCT interferometer signal is simplified. Tunable laser sources (such as tunable light source 105) are typically not tuned in a manner in which their wavenumber is linearly proportional to time. The k-clock generates a signal that oscillates at equal wavenumber intervals in time. By detecting the oscillations of the k-clock (typically defined by the "zero crossings" of the signal after removing low-frequency components), a relationship can be established between the time domain and a domain that is substantially linear in k. According to one or more aspects, the k-clock measurement signal and any derivatives derived from the signal can be used to sample, resample, interpolate, or estimate the OCT interferometer output signal at evenly spaced wavenumbers k. The sampling, resampling, interpolation, or estimation can be performed in the electrical or optical domain using digital processing methods, analog continuous-time signal processing, and can alternatively be performed before, during, or after signal digitization.

[0187] In some embodiments, a k-clock signal is generated using an optical path independent of the optical path used for OCT measurements. The design of an independent k-clock can incorporate fixed and / or tunable configurations known to those skilled in the art, including but not limited to free-space and fiber-based optical devices. For example, the k-clock can include a delay line and / or an adjustable delay line. In some cases, multiple k-clocks can be used that can be selected or hot-swapped. In some embodiments, the k-clock includes multiple reflectors designed to be tuned to a variety of desired frequencies.

[0188] In other embodiments, the k-clock signal is generated using the same optical path as that used for OCT measurements. One or more mechanical and / or optical features may be included within the OCT system to facilitate k-clock signal generation. Mechanical features may be fixed (e.g., a physical surface) or dynamic (e.g., a galvanometer), and the k-clock optical path may include any combination of such elements and other elements. Optical features may include, but are not limited to, specular and diffuse reflective elements, partially transparent or partially reflective elements (e.g., a protective cover glass for a processing head), and other elements known to those of ordinary skill in the art. In other embodiments, features already present in the OCT system (i.e., not added for the express purpose of creating a k-clock path) may include some or all of the elements in the k-clock optical path.

[0189] In some embodiments, the k-clock signal can be acquired (e.g., by processing unit 150) in a time-gated manner relative to the interferometer signal, such as during a calibration phase, and stored for use in at least one subsequently acquired interferometer capture. This approach can be implemented using optical features in the OCT interferometer and eliminates the need for a separate k-clock optical path. The application of time gating can also eliminate the need for a separate detection module for the k-clock signal.

[0190] In some embodiments, various k-clock paths are available, and the k-clock path in use can be switched during operation or during idle time. Multiple k-clocks can be generated by splitting a swept optical signal into multiple simultaneous paths, and in other embodiments, the k-clock path can include actuated elements that modify a single path, and in still further embodiments, the k-clock path can include multiple fixed or active paths that can be selected using optical devices such as shutters or switches. The k-clock in use can be selected using optical means, and multiple k-clocks can be active simultaneously, and selection can be performed via electronic control of the data acquisition and sampling system.

[0191] In some embodiments, the K-clock module 145 includes at least one optical component. In certain embodiments, the K-clock module includes at least one electronic component. In some embodiments, the K-clock module 145 includes at least one computational and / or simulation component. According to certain embodiments, the K-clock module 145 includes at least two of the following: at least one optical component, at least one electronic component, and at least one computational and / or simulation component.

[0192] According to at least one embodiment, non-limiting examples of optical components that may be included in the K-clock module 145 may include fiber-based and / or free-space optical components and / or some combination thereof. For example, in embodiments where a separate optical path from the optical path used for OCT measurements is used to generate the K-clock signal, the optical path may include a fused fiber coupler or a beam splitter in the form of a beam splitter cube to split the light entering the K-clock device along multiple paths. As another example, in embodiments where the K-clock is generated using the same optical path as that used for OCT measurements, partially reflective optical elements, such as optics with dichroic coatings, may be used.

[0193] In some embodiments, electronic components can implement various functions in a given K clock implementation, including electronic photodetectors and associated electronics for signal processing and / or digitization of the K clock signal. The electronic components can also be used, for example, to simulate, synchronize, and / or control optomechanical components using techniques known to those skilled in the art.

[0194] As used herein, the term "simulated" or "simulation" generally refers to the solution of a model by numerical or analytical methods. According to some embodiments, a simulation component can be implemented in a system computer or processor based on various electronic inputs, programmable inputs, and mathematical and / or analytical and / or numerical computer models that model the behavior of system components or calculate various other useful outputs. According to at least one embodiment, the simulation component can use input values ​​measured based on the physical system as inputs to the simulation. Such values ​​can include values ​​measured by the system itself during operation. Such values ​​can include values ​​measured before operation using at least one of the system components and / or other instruments and inputs to the system (e.g., calibration data). For example, a simulated K clock component can be synchronized with a line trigger of the SS-OCT system via an electronic signal and generate a simulated time domain trace of the K clock based on programmed, calibrated, and / or detected conditions, which corresponds to the desired behavior of the K clock trace after such a trigger.

[0195] In some embodiments, the processing unit 150 is further configured to generate a mathematical model of dk / dt based, at least in part, on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source. The mathematical model can be applied to correlate an estimate of at least one of the value of k or the tuning rate dk / dt with data sampled from the interferometer output signal. In some embodiments, the processing unit 150 is configured to simulate a k-clock signal that indicates when the wavenumber k of the imaging optical signal or signal spectrum changes substantially by one or more increments. The simulated k-clock and / or the mathematical model of k (also referred to as k(t)) or the tuning rate dk / dt thus obtained can be used to facilitate any of the operations described above as facilitated by the physical, optical, or electronic embodiments of the k-clock, including appropriately triggering the acquisition of the interferometer output signal at uniform intervals of wavenumber k. The model and / or the simulated k-clock can also be used to facilitate resampling, interpolation, or estimation of values ​​of the sampled interferometer output signal at uniform intervals of wavenumber k, where the original sampling clock is non-uniform across wavenumber k. The model and / or simulated k-clock can also be used to calculate at least one correction for one or more distortions in the interferometer output signal. The model and / or simulated k-clock can also be used to calculate a discrete Fourier transform method that can be directly applied to the interferometer output signal, which may be unevenly sampled across k. In some embodiments of the system, the simulated k-clock and / or dk / dt mathematical model uses as input the tuning signal provided to the swept source, and the model can be capable of modeling dk / dt based on a source tuning waveform of arbitrary complexity.

[0196] According to at least one embodiment, the processing unit is configured to control the tunable light source so that the rate of change of at least one wave number k of the imaging optical signal over time (tuning rate dk / dt) includes at least two tuning rates dk / dt. In some embodiments, the at least two tuning rates dk / dt are associated with one or more interferometer output signals. In some embodiments, the at least two tuning rates include at least one positive tuning rate dk / dt and at least one negative tuning rate dk / dt. In some embodiments where there is an interferometer output signal in which dk / dt varies with time, the at least two tuning rates dk / dt are associated with one or more interferometer output signals corresponding to different points in time.

[0197] In accordance with at least one embodiment, the sweep rate of the light source is varied to adjust the range / depth of view of the system (typically at the expense of imaging frequency). As previously discussed, the term sweep rate refers to the repetition rate in the time-varying spectral output of the imaging optical signal. It is the inverse of the sweep interval / sweep period. This can also be thought of as the inverse of the amount of time it takes to acquire a single A-line. As the sweep rate of the system decreases, the frequency of the signal provided by the fixed k-clock also decreases. In some embodiments where the sampling rate is independent of the k-clock frequency, the number of samples collected by the primary detector between ticks of the k-clock can be increased (e.g., to increase the axial resolution of the A-lines), or the sampling rate can be adjusted to maintain the original ratio of the sampling clock to the k-clock. In some embodiments where the sampling rate depends on the k-clock frequency, the sampling does not need to be adjusted when adjusting the sweep rate. In some embodiments where the sampling rate depends on the k-clock frequency, the k-clock can be adjusted in conjunction with the imaging frequency by mechanical, optical, digital, or other means to achieve other imaging performance, such as extending the k-clock delay while slowing the tuning of the source to increase the imaging distance without increasing the frequency.

[0198] As previously mentioned, in some embodiments, the tunable light source 105 has a sweep rate in the range of 1 kHz to 20 MHz, inclusive. According to at least one aspect, this range is selected to accommodate certain digitizer (e.g., digitizer 135) configurations (which may generally be combined with a maximum sampling rate) and / or any subsequent interpolation techniques (e.g., interpolated sweeps), thereby making these respective approaches more efficient.

[0199] In accordance with certain embodiments, the k-clock frequency is monitored relative to another electronic clock in the processing electronics. By monitoring / measuring the rate of change of k of the light source (i.e., dk / dt in Leibniz notation, or the k-point, or the light source wavenumber sweep speed / tuning rate), important information can be gathered to compensate for distortion caused by artifact motion during the waveform sweep, which is referred to as motion artifact by those of ordinary skill in the art. In some embodiments, in addition to using the k-clock signal to drive the sampling clock of the OCT interferometer, time domain sampling of the k-clock can also be implemented, which can be achieved using electronic design techniques known to those of ordinary skill in the art. This time domain measurement of the k-signal is motivated by the utility of this signal in some embodiments of motion artifact compensation. Further information on the concept of motion artifacts and the impact of these artifacts on the design and performance of the system (more specifically, the k-clock) is discussed below in the "Motion Artifact Correction" section.

[0200] According to some embodiments, a line start signal is generated at a specific wavelength in each frequency sweep to provide additional information (e.g., for system synchronization). This signal can be generated by an optical bandpass device, examples of which include, but are not limited to, a Bragg grating or a fiber Bragg grating. This signal can be generated inline with the selected k-clock or OCT optical path embodiment, or it can be generated using a tap from a source separate from the other optical paths.

[0201] In certain embodiments, signal processing operations are performed on the k-clock signal to enhance the system data. One example of such an operation is computing a k-space uniform FFT of the k-clock signal, which can provide a reference point for depth calibration of the A-line signal and compensate for phase variability in the source using methods known to those skilled in the art. In one embodiment, the k-value is measured separately at at least one point in the sweep and recalled as needed as part of the calibration procedure. As will be appreciated by those skilled in the art, many other k-clock signal processing operations can be performed to enhance the data collected from the system and are within the scope of the present disclosure.

[0202] Optimized sweep source driver

[0203] The SS-OCT system 100 includes an imaging light source 105 configured as a swept source that can be driven by a drive signal (e.g., an electrical signal sent by a controller 150). The proper selection of the drive source and pattern used to generate the drive signal is considered important for the function of the device because the drive has a strong influence on the interferogram generated by the optical module of the device. There may be a nonlinear relationship between the applied drive pattern and the spectral emission of the swept source.

[0204] Certain embodiments may use a standard waveform (such as a ramp function, a sawtooth function, or a sine wave) to drive the swept source. Other embodiments may incorporate more complex drive waveforms, such as drive signals specifically designed to produce a swept laser signal that is linear in wave number k, or drive signals designed to maximize device life. Some embodiments may incorporate multiple swept drive waveforms to allow adjustment of the sweep rate, tuning rate, span, or waveform shape to monitor different aspects of the laser process, such as keyhole measurement during laser welding or finished weld scanning (for example). Adjustment of the swept drive waveform of the swept source (including, as non-limiting examples, adjustment of the swept drive frequency, the rate of change of the swept drive waveform (slope), and / or the repetition rate of the swept drive waveform) may also be used to adjust the imaging range or sensitivity of the system, or to manage (in some embodiments, suppress, but in some embodiments, enhance) motion artifacts. Some embodiments of the system may employ a slower swept source drive mode relative to other available modes for the specific purpose of increasing the available imaging range in scenarios where fringe frequency, rather than coherence length, is the limiting factor.

[0205] To increase the effective sweep rate, certain embodiments may incorporate optical buffering or interleaving of the swept source light. These embodiments of the present invention may incorporate combinations of optical paths, shutters, couplers, and other optical components to achieve an effective sweep rate greater than the physical sweep rate of the source. Optical buffering can be implemented at any point in the system, including, for example, before or after amplifier 106, as appropriate for the application's specific performance.

[0206] In some embodiments, the rate at which the swept source is driven can be varied between tasks, or in some cases, during a task, to optimize the source sweep. Experiments have shown that varying the source sweep rate can alter the performance of the system.

[0207] Certain embodiments can use drive waveforms specifically designed to account for the different operating modes of a swept source. For example, the tuning mechanism of a MEMS VCSEL may be different for sweeping from low to high wavelengths than for sweeping from high to low wavelengths. Therefore, the tuning waveform can be designed so that the tuning for one sweep direction is different from the tuning for another sweep direction, thereby optimizing the sweep behavior for each sweep direction.

[0208] Wavelength selection

[0209] According to various embodiments, OCT systems at different wavelengths can be implemented because a swept source is available at multiple wavelengths. Each wavelength option exhibits a different intensity.

[0210] In some embodiments, the wavelength of the imaging system is matched to the wavelength of the material processing laser beam. For example, in the specific application of laser weld monitoring utilizing a 1070 nm process laser source 110, swept sources centered around 1030 nm are advantageous because they are close to the wavelength of the material processing beam while also exhibiting suitable resolution for weld imaging. Another advantage is the commercial availability of solid-state swept sources at this wavelength. This wavelength close to the typical process beam wavelength of 1070 nm reduces undesirable effects (such as chromatic aberration) caused by process beam optics, but introduces the challenge of developing appropriate dichroic coatings to separate the imaging beam from the process beam.

[0211] In some embodiments, the imaging system spectral range is designed to utilize a low-cost spectral range, such as telecommunications. Sources centered at 1550 nm utilize common telecommunications wavelengths, which allows the use of less expensive commercial off-the-shelf components in the optical system. However, these wavelengths exhibit a coarser axial resolution for a given swept bandwidth (in nm) than can be obtained from a system centered at a shorter wavelength (such as 1030 nm) with an equivalent swept bandwidth (in nm).

[0212] In some embodiments, the imaging system spectral range is designed to maximize resolution.System embodiments based on sources centered around shorter wavelengths (such as 800 nm) may provide better axial resolution of OCT data in practical implementations.

[0213] In some embodiments, the imaging system spectral range is designed to match the reflectivity (or in some cases, transmittance) of at least a portion of the material being processed or treated by the laser. For example, in various plastic material lap welding applications, the spectral range can be designed so that the top material is at least partially transparent to the imaging system.

[0214] The choice of wavelength implemented by the disclosed systems and methods depends on the context in which they are applied, including but not limited to resolution requirements, market availability of components, material properties of the material being imaged, and other processes occurring concurrently with imaging. The operating principles and general topology of the SS-OCT systems and methods described herein are the same for SS-OCT systems at any wavelength. Therefore, possible implementations exist theoretically at any wavelength, and practical implementations can be considered to exist at all wavelengths for which suitable technology exists.

[0215] Data processing

[0216] For practical application of embodiments of the system, the capture and processing of interferogram data is necessary. This can be accomplished by processing unit 150. In general, a given embodiment of the system will be designed to ensure sufficient signal levels, data rates, timing characteristics, data accuracy, and other parameters so that sufficient data integrity is preserved from the optical signal to the OCT image and any derived quantities (if applicable). Many of the required design practices are known to those of ordinary skill in the art, but certain key elements that present unique aspects in system design are discussed in further detail below.

[0217] Photoelectric signal conversion

[0218] Embodiments disclosed herein include systems for converting optical OCT signals into electronic signals. In some embodiments, this level of signal processing can be achieved by using a balanced photodetector configuration, thereby reducing or eliminating the DC component of the OCT signal and amplifying the resulting electronic signal to a level optimized for digitization systems. Figure 2 A high-level schematic outlining an example balanced photodetector configuration with an electronic amplifier is shown in Other optical detection methods known to those of ordinary skill in the art are also suitable for optical-to-electrical signal conversion.

[0219] Some embodiments may incorporate feedback electronic amplifiers to maintain consistent output signal amplitude and maximize the SNR of the interferogram, and other embodiments may have multiplexed or multi-stage amplifiers to achieve a higher dynamic range for the system. Some embodiments may use analog electronic filter networks to filter or otherwise enhance the signal generated by the photodetector. Embodiments of the system without balanced detection and embodiments with alternative detection techniques are also within the scope of the present disclosure.

[0220] In embodiments of systems that include a k-clock, both the k-clock signal and the interferogram signal are converted from the optical interferogram into analog electronic signals. The circuitry used to detect, amplify, and process these different signals is configured based on the properties of each signal, and in some embodiments, different methods are employed for each respective signal.

[0221] Digitalization

[0222] According to at least one embodiment, the disclosed SS-OCT systems and methods employ the use of a digitizer 135. The digitizer 135 is configured to digitize at least one interferometer output signal, or a modified interferometer output signal, or a corrected interferometer output signal, or an amplified interferometer output signal using appropriately applied analog electronic signal processing and generate a corresponding digital signal.

[0223] Digitization of the detected interferogram enables further processing and storage of the OCT data. The resolution, range, and sampling rate of the digitization depend on the specific parameters of the physical system, including the seed source tuning rate, optical power and clocking requirements, and other physical parameters.

[0224] In some embodiments, dedicated digitization circuitry is implemented to allow the interferogram signal to be sampled in a domain that is linearly proportional to the wave number k of the OCT swept source 105, and not necessarily uniformly sampled in time. These implementations may involve any combination of optical and / or analog electronics and / or simulated digital and / or simulated analog k-clocking signals to achieve appropriate sampling.

[0225] This implementation presents an advantage because it eliminates computationally expensive interpolation operations from the data processing.Digitization methods that scale linearly with the wave number k were discussed above in the "K-Clocked Method" section.

[0226] Other embodiments may involve digital sampling that is uniform in time. In some embodiments involving digital sampling that is uniform in time, the imaging light source sweep is implemented in a certain way so that the wave number k is substantially linearly related to time in the region of interest. In a preferred embodiment in which the digital sampling of the OCT interferometer output signal is uniform in time, a sampling method that is also uniform in time is used to digitize the K clock signal. The K clock signal can be collected based on the method discussed in the "K Clocking Method" section above. In such cases, the K clock signal can be used to extract information that can be used to relate the uniform sampling in the time domain to the k domain. This relationship is important for the application of computational methods (e.g., Fourier transforms, homodyne matrices) that enable the extraction of depth information from the interferometer output signal.

[0227] In some embodiments, the controller 150 can be configured to calculate a resampled digital signal based on one or more digitally sampled interferogram signals. In some embodiments, the K-clock interferogram signal can be digitized on the same clock as the OCT interferogram signal. This can be achieved using electronic design methods known to those skilled in the art, such as by implementing a dual-channel ADC.

[0228] K clock acquisition

[0229] One or more embodiments of the system include an interferometer that acts as a physical optical k-clock 145 along with the light tapped from the imaging source 105. Such embodiments also include the necessary electronic circuitry to detect the k-clock signal, such as detector 147. In some embodiments, the k-clock signal (after appropriate conditioning) can be used to directly drive the sampling clock on the OCT signal digitizer, resulting in a digitization that is not necessarily linear in time but is substantially linear in k. This embodiment presents the advantage of eliminating the computationally intensive resampling step while still allowing the generation of interferograms with uniform k-spacing, which enables the use of FFT methods during the processing phase. Another advantage of this approach is that using an optical k-clock directly associated with the imaging swept source automatically synchronizes the acquisition rate to any sweep rate that meets the specifications. In some embodiments, the k-clock signal (after appropriate conditioning) can be digitized on some other clock, such as a clock that is uniform in the time domain. In some embodiments, digital signal processing of the digitized k-clock signal is used to determine a basis that is substantially linear in k, and other signals in the system are resampled to this basis.

[0230] In other embodiments of the system, a k-clock can be used to calculate dk / dt using a direct optical clocking method sampled on a uniform electronic clock, or by determining a piecewise function using at least one fiber Bragg grating and / or at least one electronic oscillator. Simulation methods for k-clocking are also within the scope of the present disclosure. Included herein are specific implementations of embodiments in which dk / dt is determined by measurements along a reference configuration of a primary interferometer, and dk / dt can be saved or modeled electronically and / or computationally to aid in the development of appropriate transformations and processing methods. A more detailed discussion of the k-clocking method can be found in the "K-Clocking Method" section above, including details on different methods for processing OCT data based on the k-clocking method applied.

[0231] Treatment method

[0232] The general data processing method employed in generating A-lines from the interferogram and subsequent images (M-mode (A-lines acquired at fixed lateral positions over time) or B-scans (two-dimensional cross-sectional views)) is related to another aspect of the present disclosure. An overview of the key steps in the data flow according to one embodiment includes pre-processing, Fourier transforms, and post-processing.

[0233] In some embodiments, preprocessing includes all digital operations that can be performed before the transformation is applied to enhance the interferogram data and improve the final result. This can include windowing the acquisition using analytical or arbitrary envelope functions. Digital filtering and DC signal subtraction can also be applied to the data. Many additional preprocessing methods have been developed for OCT applications and can be deployed as components of the system as they are known to those of ordinary skill in the art. In the specific case of compensating for motion artifacts, which is extensively discussed below, preprocessing can include (but is not limited to) windowing the digital signal representing the interferogram in two or more different ways to extract data captured at portions of the sweep with different dk / dt.

[0234] In accordance with at least one embodiment, the OCT data may not be uniformly sampled in K-space (e.g., a signal that is uniformly sampled in time in a swept frequency method where K and time are not linearly related). In such embodiments, the acquired digital data may be resampled to generate an OCT data set that is uniformly sampled in K-space. The relationship between time and K-space may be mapped to assist in resampling using a variety of methods, including acquiring an optical K-clock signal, implementing a calibration method to characterize the source sweep in the time domain and / or K-domain, simulating the K-clock signal based on system and light source parameters, and / or some combination of methods. Based on the relationship between time and K-space, digital resampling methods may be applied to time-domain OCT data capture. Digital resampling methods include those known to those of ordinary skill in the art, such as application of linear interpolation, spline interpolation, and / or sliding window signal processing functions. Mismatches in optical dispersion between the arms of the interferometer may distort the relationship between time and K-space, as detailed in the "Dispersion Compensation" section (discussed in further detail below). In some embodiments, the distortion can be estimated and calculated a priori, and compensation for the distortion can be incorporated into the resampling of the interferometer output signal. In such embodiments, dispersion correction can be applied with minimal additional real-time computational load.

[0235] There are a variety of methods to perform the transformation step from the interferogram data in k-space to data in the depth domain (referred to as axial lines or A-lines). In some embodiments, the transformation method can be selected based on what is best suited for a given application of the system. For example, depending on the sampling method, an FFT can be employed (for embodiments that sample uniformly over k), a homodyne matrix DFT can be used (for embodiments that sample unevenly over k, such as those that sample uniformly in time, where k varies in a nonlinear manner over time), or a fractional FFT can be used (also for embodiments that sample unevenly over k). Factors including the stability of the tuning source, the tuning rate, and the available processing power can be considered when selecting the transformation processing method. Some transformation processing methods can incorporate compensation for various physical and optical effects that affect the accuracy and resolution of the data, which will be discussed in more detail below. Non-limiting examples of such effects are motion artifacts or optical dispersion effects discussed below.

[0236] Post-processing according to some embodiments includes operations that track geometrical properties (features) of a single A-line, as well as operations that track features in many A-lines. The features / geometric aspects that are tracked are encoded in the interferometer output signal used to generate the A-lines.

[0237] Non-limiting examples of features (e.g., geometric properties) that can be tracked for a single A-line include position, symmetry, peak width, centroid, geometric second moment, center of mass, amplitude, aspect ratio, and geometric area under the curve. According to certain embodiments of the present invention, certain A-lines can be represented based on a set of parameters, features, measurements (including geometric parameters and features) or other parameters (which can be derived from the properties of the A-line) extracted from the imaging data. Such representations of the A-line can be referred to as "tracked A-line data" or simply "tracked data" and / or "tracking data". Depending on the requirements of the application, in some embodiments, the processing system can be configured to output only tracking data, or to output tracking data in addition to the raw A-line data. Incorporating tracking data into embodiments of the system can provide insight into key features / geometric aspects of the sample that are measured by SS-OCT at a much lower data density than the raw data, which can make this approach advantageous from a computational perspective.

[0238] In at least one embodiment, post-processing operations are designed to extract quantitative parameters from composite imaging data derived from multiple A-lines (such as M-mode, B-scans, or 3D volume scans). In some embodiments, this includes evaluating groups of A-lines based on analysis of shared features. Some embodiments include developing metrics that quantify the correlation between subsequent A-lines. Filtering of a group of A-lines can be performed by applying filtering methods known to those skilled in the art, such as Kalman filtering. In some embodiments, B-scan or M-mode data can be treated similarly to images, and in such embodiments, appropriate image processing algorithms and methods can be applied to extract data or manipulate the data to aid further interpretation. Representations of A-lines within an OCT capture, and more generally, representations of features (e.g., geometry or geometric features) derived based on parameters extracted from composite imaging data, can also be referred to as "tracked data" or "tracking data." Tracking data based on only one A-line can be more specifically referred to as "A-line tracking (tracked) data" and / or "tracking (tracked) A-line data." Tracking data based on more than one A-line can be more specifically referred to as "batch tracking (tracked) data" and / or "tracking (tracked) batch data." In some embodiments, operations including statistical analysis, mathematical analysis, geometric analysis, and / or signal processing can be applied to a set of more than one A-line tracking data corresponding to more than one A-line to generate batch tracking data. In some embodiments, such operations as described can be applied to a set of more than one A-line to generate batch tracking data.

[0239] Additional methods of extracting simplified data from A-line or B-scans, M-mode, etc. may also be employed, including those known to those of ordinary skill in the art of signal processing and / or artificial intelligence (AI) and / or machine learning (ML), and are considered to be within the scope of the present disclosure.

[0240] Signal processing methods / methods known to those skilled in the art can be used as part of the processing method for pre-processing (e.g., pre-processing of interferometer output signals in the analog or digital time and / or K-domain) or post-processing (e.g., post-processing of A-lines or groups of A-lines). Digital or analog high-pass or low-pass filtering and Kalman filtering are examples of such methods. Further examples of such methods include smoothing filters such as median and percentile filters. Image processing methods (e.g., those developed for analysis and feature extraction) can also be applied to the data and may be particularly useful when dealing with image-like outputs produced when combining multiple A-lines (such as B-scans, M-modes, or volume scans). Examples of image processing methods include signal processing operations designed for processing 2-dimensional or 3-dimensional digital imaging data, such as convolution filters, frequency domain masking (including methods based on the 2-D Fourier domain), and other methods known to those skilled in the art. Signal and image processing methods can be applied at any level of the data—e.g., in post-processing, a filter can be applied to each individual A-line, while a separate filter can be applied to the entire B-scan. The filters can be applied to extract the same information from the image, or each filter can be applied to extract different information from the image, for example, one filter can be developed and optimized to extract features of PCR from B-scans, while another filter can be developed to suppress noise on B-scans.

[0241] The application of ML and / or AI methods to data processing can be applied at various levels of the data. For example, ML training techniques known to those skilled in the art (such as backpropagation and logistic regression) can be applied to train algorithms (such as commonly used algorithms, such as neural networks, CNNs, regression algorithms, self-organizing maps, k-nearest neighbors) that are applied to SS-OCT data to extract useful information, such as the evolution of weld keyholes and / or other characteristics of the processed area / workpiece, including the geometry of the workpiece. Algorithms trained using ML techniques can be used in conjunction with algorithms that do not utilize ML. Algorithms that include at least one component developed using ML methods are referred to herein as ML-trained and / or ML-enhanced algorithms (used interchangeably).

[0242] In some embodiments, the ML-trained algorithm may take as input raw SS-OCT data, which may include interferometer output signals, A-lines, B-scans, and / or M-modes. In some embodiments, the ML-trained algorithm may take as input one or more features / geometric properties of the signals and / or A-lines and / or groups of A-lines (such as B-scans or M-modes), for example, location, shape, symmetry, width of one or more peaks, centroid, geometric second moment, center of mass, amplitude, aspect ratio, geometric area under one or more curves. In some embodiments, the ML-trained algorithm may take as input some combination of these data types and / or other data types. In some embodiments, the ML-trained algorithm may take as input SS-OCT data and data from other detectors within the system, examples of which may include photodiode data or acoustic data (other detectors are discussed below in the section entitled "Auxiliary Detectors"). ML- or AI-enhanced algorithms may be used to extract tracking data from the A-lines and / or groups of A-lines. ML- or AI-enhanced algorithms may be used to extract features of interest from the data and / or tracking data. In some cases, the ML-enhanced processing paradigm may involve applying a pre-trained algorithm as part of the data processing. When developing the algorithm, care must be taken to ensure that the training data used is appropriate for the conditions under which the ML-enhanced algorithm is being applied. For example, an ML algorithm can be trained on SS-OCT weld keyhole data, using longitudinal cross-sections of the imaged weld as ground truth to employ a supervised learning approach. This algorithm can then be applied to post-processing of the keyhole data to estimate the keyhole depth based on the imaging signal. In some cases, it may be desirable to further narrow the scope of the training set to apply ML techniques to produce an algorithm with greater specificity for a given problem. As an example, ML techniques can be applied to train the algorithm only on data from single-mode copper welds. This can allow the ML training method to extract features specific to single-mode laser welding dynamics and copper material properties (and the interaction between the two). When features (e.g., geometric features) of the PCR are extracted from the copper weld, the resulting ML-enhanced post-processing algorithm can have improved performance compared to algorithms trained on data derived from many materials or processes. In the context of laser material processing, many sources of ground truth are available for supervised learning methods applied to ML-trained SS-OCT algorithms. Examples of training data sources include datasets concurrently captured using another instrument during a material process (e.g., X-ray synchrotron data, SS-OCT data), datasets based on analysis of finished welds (e.g., metallographic analysis, X-ray computed tomography), or synthetic / simulated datasets. In some embodiments, unsupervised learning methods can be applied to develop algorithms for ML training. In some embodiments, ML and / or AI algorithms in SS-OCT systems can use a combination of supervised and unsupervised learning methods, and / or other appropriate algorithm development methods.

[0243] ML and / or AI algorithms can be applied to identify, evaluate, and / or correct artifacts (including motion artifacts) in OCT signals. Motion artifacts and the application of AI / ML algorithms are further discussed below in the section entitled "ML / AI Algorithms and Motion Artifacts."

[0244] In some embodiments, it may be appropriate to employ ML / AI methods on the output of an SS-OCT system over a broader trend-based approach. For example, an AI-based monitor may monitor quality metrics, such as the QA output of one or more SS-OCT systems within a factory and, based on the evolution of the SS-OCT data over time, provide timely updates to help human operators proactively manage issues. In this example, an AI program may take data from seam tracking or keyhole depth measurements and identify general trends in part positioning and welding over a process spanning many hours, days, or longer. It is common for process parameters to drift slightly over time, for example due to wear on fixtures or accumulation of contaminants (such as weld fume) on system components, and this process drift typically affects the quality metrics of the process product. As process parameters drift, the SS-OCT system generates batch data indicating this shift in quality metrics, which can be analyzed comprehensively to understand the shift in the average process parameter closer to the boundaries of an acceptable process. As process quality metrics drift from nominal values, real-time feedback provided by SS-OCT can help keep the process within set boundaries. However, if the process drifts beyond a certain point, intervention may be required to reestablish nominal performance, such as through maintenance or calibration. AI programs can generate alerts for human operators and identify cells that may require maintenance based on SS-OCT data trends before the process falls far out of specification and begins producing faulty parts. Figure 20A and Figure 20B An illustration of process drift over time is included. Figure 20A Typical statistical properties of the distribution of process parameters and quality metrics are illustrated, the quality metrics being derived at least in part from SS-OCT data associated with the process, and the figure shows the gradual degradation of the quality metrics over time caused by process drift. Figure 20B The impact of AI / ML supervisors or monitors on process supervision is illustrated: AI / ML monitors identify statistical trends where quality metrics begin to deviate from specifications, allowing corrective action to be taken before a part is marked as out of bounds for the quality metric. This AI supervision, coupled with a network of QA and feedback systems, can proactively manage production assets to ensure timely maintenance and limit the number of failures throughout the production facility and / or supply chain.

[0245] In some embodiments, AI and / or ML methods can be employed on interferograms, A-lines, or groups of such data types. In some embodiments, AI and / or ML methods can be employed to identify and / or measure and / or characterize specific features of the PCR, including weld depth. As previously mentioned, filtering methods known to those skilled in the art of signal processing, such as percentile filters or Kalman filters, can be applied as additional methods for generating simplified tracking data.

[0246] According to some embodiments of the system, the application of tracking data plays a key role, primarily in reducing the data footprint required to store meaningful measurement information and enabling the development of quality assurance (QA) algorithms. In embodiments of the system where the SS-OCT system is integrated with a laser material processing system, QA metrics can be developed based on material processing goals, and the SS-OCT data (either as tracking data or otherwise) can be applied to provide feedback about the material processing. A related example of the application of tracking data is an embodiment of a system for capturing at least some interferometer data, for which at least some interferometer data, the imaging beam is aligned with the keyhole of the laser weld so that at least some data representing OCT returned from the bottom of the keyhole is collected. Features, including geometric features derived from such A-lines, can be processed to generate tracking data representing the keyhole bottom, which can be used to help track the keyhole depth of the PCR, for example, throughout the welding process. In this disclosure, as understood by those skilled in the art, "keyhole depth" refers to the maximum depth of the vapor capillary.

[0247] Processing hardware and hardware implications

[0248] In some embodiments, data is pipelined into a specially coded FPGA for real-time processing, which may include any or all of the pre-processing, transformation, or post-processing steps. Such embodiments enable the possibility of real-time output and the potential use of OCT data as a feedback mechanism in the process. Further processing methods can also be implemented on the FPGA, including but not limited to depth tracking or motion artifact correction, which will be explained extensively below. Tracking methods that combine data from multiple A-lines, such as percentile tracking or averaging, can be developed to reduce the effects of transient chaotic variability, particularly when tracking keyhole (welding) processes. In some embodiments, the digitized data is processed using programs on one or more CPUs and / or GPUs. In some embodiments, the digitized data is processed using an ASIC (application-specific integrated circuit). In some embodiments, the digitized data is processed using some combination of processing hardware, which, as non-limiting examples, may include FPGAs, CPUs, GPUs, ASICs, microcontrollers, and other processing hardware known to those of ordinary skill in the art. In some embodiments, when the processing is configured to enable the calculation of weld metrics (such as keyhole depth or finished weld surface height), the calculation can be performed in real time. In some embodiments, quality assurance (QA) metrics may also be developed to provide rapid feedback on the suitability of imaged parts for industrial applications.

[0249] Motion artifact correction

[0250] General problem description

[0251] A known physical effect that affects all swept-source OCT systems is the distortion of the A-lines when the imaging target moves during the acquisition time of a single A-line (sample motion). Those skilled in the art refer to this effect as motion artifact. Motion artifact can manifest as an offset that can be greater than the actual displacement of the imaging target during the imaging time. In addition to or in lieu of this offset, motion artifact can additionally manifest as a widening, blurring, or distortion of the A-lines. In laser weld monitoring applications, motion is known to exist in many of the desired imaging targets, particularly at the bottom and sidewalls of keyholes. Therefore, understanding and compensating for this motion artifact is important for the purpose of generating accurate weld monitoring information, such as keyhole depth.

[0252] In some embodiments, the time-varying difference in optical path length (between at least one sample arm and at least one reference arm of the interferometer) is caused by motion of the sample relative to the axis of the imaging optical signal. In some embodiments, the time-varying difference in optical path length is caused by a material modification process achieved on the sample by a material processing beam. In some embodiments, the time-varying difference in optical path length is caused by intrinsic sample motion rather than by the material modification process. Motion artifacts cause modulations in the interference spectrum, resulting in incorrect depths being recorded due to reflector movement when processed using conventional means. The magnitude of the motion artifact depends on the dk / dt of the imaging source, so careful analysis of changes in dk / dt is useful in compensating for the offset.

[0253] According to certain embodiments, one or more algorithms are used to identify, resolve and correct the effects of motion artifacts (distortions). According to certain embodiments, the processing unit is further configured to determine the sample position based on at least one corrected interferometer output signal. In some embodiments, the operating principle is that the detected motion artifact is related to the change in source wave number over time (tuning rate dk / dt). The source is swept so that the interferometer output signal corresponding to the portion of the source sweep at two or more (at least two) tuning rates dk / dt is captured. This makes it possible to identify distortions that can be attributed to motion artifacts. Based on these distortions, corrections are calculated and applied to produce motion-artifact-corrected A-lines or M-modes or other motion-artifact-corrected data. In some embodiments, correction of motion artifacts includes correction of higher-order distortions that affect the shape and characteristics of the complete original A-line. Other embodiments of motion artifact correction only include correction of key image features (such as tracked A-line peaks). In order to deal with the noise that still exists after motion artifact correction, in some embodiments, smoothing and tracking algorithms are applied.

[0254] The selection of an appropriate method for motion artifact compensation depends on the specific requirements of the application, including accuracy, resolution, data rate, computing power, speed of operation, output data requirements, etc. Details of various general methods are disclosed below, however, specific steps in these methods can sometimes be interchanged with other steps disclosed below or with operations of similar mathematical utility and effect known to those of ordinary skill in the art where appropriate (for example, the use of median, average, and mode can be interchanged where averaging is required, depending on the performance of the application).

[0255] Differences from medical OCT

[0256] Research on medical OCT has involved work on motion artifact correction. However, there are many differences between the applications of OCT in medicine and in materials processing, and therefore, there are substantial differences in the size and nature of the motion artifacts observed and the requirements for motion artifact correction.

[0257] For example, in the case of medical OCT, consideration must be given within the system to operating at wavelengths and powers that are harmless to the biological tissue and / or material being imaged, whereas in materials processing, relatively high powers can be applied to the OCT without damaging the sample. Additionally, medical OCT typically involves imaging materials that are at least partially transparent to the OCT wavelength, where subsurface structures are of interest. This type of imaging requires signal-to-noise characteristics that make it possible to clearly distinguish low-brightness features within the sample volume. Common materials processing applications employ sample materials, including metals, that are opaque to typical OCT wavelengths. Consequently, surface features are of greater interest, particularly at the interface between the solid processing material and the surrounding gas. Additionally, OCT is unable to acquire features within the sample volume for sample materials that are opaque to typical OCT wavelengths.

[0258] Additionally, motion observed in medical OCT typically consists of slow, overall sample motion during acquisition, with typical values ​​reported in the range of less than 1 mm / s. Motion observed in OCT applications for materials processing can include relatively high-speed overall sample motion, intrinsic sample motion caused by material modification processes, temporally and spatially non-uniform motion within or relative to the sample volume or portion of the sample volume, and generally arbitrarily complex and rapid motion. Certain material modification processes induce intrinsic motion at the PCR and / or other regions of interest in the sample, which can vary significantly and chaotically over the timescale of SS-OCT acquisition. Motion can also be intrinsic sample motion not caused by the material modification process. This motion can also include high-speed components, where the imaged sample moves at speeds greater than 10 mm / s, greater than 100 mm / s, greater than 500 mm / s, greater than 1000 mm / s, greater than 5000 mm / s, or even greater than 10,000 mm / s. In some cases, this sample motion speed can be inherent to the material modification process. Of particular interest is the movement of material within the keyhole of a laser weld, which has been shown under certain conditions to exhibit some of the above-mentioned properties. The techniques known to those skilled in the art of medical SS-OCT are insufficient to address such situations.

[0259] Medical OCT is typically applied in settings where a qualified human operator oversees the images and can repeat them if overall motion is perceived to degrade the images to an unacceptable level. Unlike medical OCT, materials processing OCT is typically used in automated processes where the images must be processed using algorithms to produce, for example, quality assurance results. In many materials processing situations, an OCT image may also be acquired only once, for example where repetition is impractical or impossible because it is associated with a single material modification process (such as welding) and must be acquired during the processing time. These differences mean that the development of motion artifact correction algorithms specific to material modification process monitoring OCT presents new challenges compared to medical OCT motion artifact management and / or correction. The challenges are further exacerbated by the need for automation and the stringent quality assurance and reliability requirements in the manufacturing space. Outlined below are motion artifact identification and correction algorithms that have been specifically developed to address the capabilities and challenges of applying OCT to material modification processes.

[0260] Identification of motion artifact distortion in SS-OCT data

[0261] In embodiments of a system requiring motion artifact compensation or correction, the first step is to identify and characterize motion artifact distortion. According to certain embodiments, the distortion corresponding to motion artifact corresponds to one or more geometric aspects encoded in the interferometer output signal (such as the geometric properties of the A-line, which are encoded within the frequency of the interferometer output signal). When the system's sampling settings are appropriately configured to capture interferometer output signals corresponding to swept optical signals at two or more tuning rates dk / dt, sufficient information can be derived based on the information contained in the uncorrected interferometer output signal. In some embodiments, to extract motion artifact characteristics of the uncorrected interferometer output signal, the signal capture must include at least two separable data segments where the imaging sources have different dk / dt. The different dk / dt can include different tuning rates of the imaging sources and / or can include positive and negative dk / dt. By evaluating the frequency components seen in at least two interferometer output signals associated with the at least two imaging source dk / dts, information about the distortion of the interferometer output signal during the acquisition time can be extracted, and this distortion information can be correlated to the motion and / or velocity of the imaged sample. The evaluation of the distortion may include, but is not limited to, measuring the amplitude of individual frequency components or groups of frequency components within the uncorrected interferogram signal segment, evaluating frequency components observed at different dk / dt, evaluating one or more geometric aspects or characteristics encoded in the interferometer output signal, including but not limited to the position, shape, symmetry, width of one or more peaks, centroid, geometric second moment, center of mass, amplitude, aspect ratio, and geometric area under the curve of the signal and / or A-line and / or A-line group (such as B-scan or M-mode) corresponding to the uncorrected data. According to certain embodiments, performing the evaluation of the distortion includes comparing the distortion in at least one of the at least two interferometer output signals relative to the distortion in at least another of the at least two interferometer output signals. In some embodiments, performing the evaluation includes comparing the distortion in at least one interferometer output signal associated with a positive tuning rate dk / dt with the distortion in at least one interferometer output signal associated with a negative tuning rate dk / dt. In certain embodiments having multiple interferometers (systems configured with multiple interferometers are discussed in more detail below), a sample arm of a first interferometer and a sample arm of at least one additional interferometer share at least one optical element, and a first imaging signal configured with one of at least two tuning rates dk / dt is directed to at least one reference arm and at least one sample arm of the first interferometer, a second imaging signal configured with another of the at least two tuning rates dk / dt is directed to at least one reference arm and at least one sample arm of the at least one additional interferometer, and the distortion is identified based on one or more interferometer output signals of the first interferometer and the at least one additional interferometer.In additional embodiments, the first imaging optical signal and the second imaging optical signal are directed to the treatment region simultaneously.

[0262] According to certain embodiments, the distortion corresponds to one or more geometric aspects of the interferometer output signals and / or the A-line. The one or more geometric aspects may include at least one of the following: position, symmetry, shape of the signal, shape of the envelope function, width of one or more peaks, displacement of one or more peaks, amplitude, centroid, aspect ratio, center of mass, geometric area under the curve, and geometric second moment. According to certain embodiments, evaluating the distortion includes comparing the one or more geometric aspects encoded in at least one of the at least two interferometer output signals relative to one or more geometric aspects encoded in at least another of the at least two interferometer output signals. According to some embodiments, evaluating the distortion includes comparing the one or more geometric aspects encoded in at least one of the at least two interferometer output signals to one or more predetermined thresholds and / or baselines associated with the one or more geometric aspects. Such predetermined baselines and / or thresholds may be established based on one or more of the following: system requirements, component requirements, one or more application requirements, one or more calibrations, one or more models, one or more algorithms, fundamental physics, hardware limitations, and / or software limitations.

[0263] The geometric aspects distorted by motion artifacts can include aspects known to those of ordinary skill in the art as corresponding to features (including geometric features) of the phase change region (and more generally, the processed region) and, more generally, features (including geometric features) of the sample. Therefore, the identification and subsequent correction of motion artifacts is crucial for obtaining accurate measurements of one or more features (including geometric features) of interest in the sample while the sample is in motion. A relevant example of a feature or geometric aspect to be measured while the sample is in motion is obtaining accurate measurements of the keyhole geometry while the molten metal at the bottom and sidewalls of the keyhole is in motion.

[0264] It should be understood that there are situations in which the controller 150 does not detect distortion in the interferometer output signal, and such scenarios are encompassed by one or more embodiments disclosed herein. Therefore, the controller 150 processes the interferometer output signal to determine at least one characteristic of the processing region (e.g., depth information). However, when the controller 150 detects distortion in at least one interferometer output signal, in response, the controller 150 applies one or more corrections to the at least one interferometer output signal, as discussed herein.

[0265] Imaging target velocity estimation and compensation

[0266] A theoretical framework and corresponding mathematical model can be developed, based at least in part on one or more characteristics of an SS-OCT system, to relate the distortion of at least one of at least two interferometer output signals at different dk / dt to the magnitude and / or direction of the sample's motion velocity, particularly when the sample's motion is relative to the axis of the imaging optical signal. This enables the development of correction factors (also referred to simply as "corrections") that can be applied to the interferogram to correct for the effects of motion artifacts. An appropriate correction for calculating a given corrected A-line can be calculated based on a combination of this theoretical model and information extracted from analysis of geometric aspects encoded within the interferometer output data (examples of which are described above), the interferometer output data comprising at least two segments or portions of a source sweep with different dk / dt. In other words, the calculation of a given corrected A-line can be based at least in part on the theoretical model. The theoretical model can be constructed and / or updated based at least in part on interferometer output data, K-clock data, analytical and / or numerical models of the SS-OCT system and / or its components, or results derived from some combination thereof. The theoretical model can be combined with information extracted from analysis of geometric aspects encoded within the interferometer output data, which contains at least two segments or parts of a source frequency sweep with different dk / dt. The information extracted from the frequency analysis of the uncorrected interferogram, combined with the theoretical framework and knowledge of the key physical parameters of the OCT imaging system, can be used to calculate an estimate of the sample velocity at the time of A-line capture, which is affected by observed motion artifacts. Therefore, based on this compensation, position data as well as velocity data can be extracted from the OCT system scan.

[0267] According to some embodiments, similar to dispersion compensation, the calculated correction factors are applied in the transform phase of OCT imaging by means of corrections factored into each element of the DFT matrix. Thus, the controller 150 is configured to transform the time-domain interferometer signal (interferometer output signal) into a corresponding frequency-domain signal by calculating a corrected DFT matrix, which is applied to the time-domain interferometer signal by multiplication.

[0268] According to some embodiments, time domain measurements of k and / or dk / dt provided by the K-clock signal and / or a simulated / modeled k-clock signal are used to calculate at least one correction to one or more distortions in the interferometer output signal.

[0269] Motion artifacts distort the position and other geometric aspects of the signal (e.g., symmetry, full width at half maximum, amplitude above the noise floor). In some embodiments of the system, implementation of appropriate correction factors includes the corrections needed to compensate for the higher-order effects of motion artifacts, and the corrected A-lines exhibit improved symmetry and / or improved amplitude and / or reduced full width at half maximum when compared to uncorrected A-lines.

[0270] In embodiments of the system where variable motion of the imaging target is expected, a number of DFT matrices with velocity compensation included as needed can be pre-calculated and stored in a lookup table to facilitate rapid processing of data sets with variable velocity. Other embodiments with less variable velocity can sometimes be calibrated based on their characteristic velocity and designed to compensate for the characteristic velocity of each A-line based on calibration of the singular common transformation matrix. Depending on the specific needs of the process, including computational speed, sampling rate, source sweep rate, and accuracy requirements, a variety of approaches can be applied to develop an optimal velocity compensation method to meet the needs of the application. An example of an embodiment of a system where variable motion of the imaging target is expected is one in which the SS-OCT system is configured to image a welding process, and the material processing beam generates a phase change region (PCR) at the processing area. In such an embodiment, the imaging beam can be directed toward a region of the process in motion, such as a weld keyhole, i.e., an imaging target known to move at varying velocities as the process evolves over time and / or space. In such an embodiment, an evaluation of the distortion corresponding to motion artifacts can be used to generate corrected A-lines corresponding to the geometry of the PCR and / or to generate an estimate of the velocity of the material being processed in the PCR.

[0271] Tracking data and motion artifact correction

[0272] For application-specific reasons, in some embodiments of the present invention, it may be preferred to derive simplified tracking data from the interferometer output signal or its corresponding A-line, B-scan, M-mode, etc., referred to herein as tracking data. The concept of tracking data has been discussed previously. Tracking data can be established in a manner specific to SS-OCT applications and can include an expression of relevant information related to the geometry of the sample, possibly including phase change regions (PCRs) and / or processing regions. For example, A-lines can be used to identify and / or measure and / or characterize specific features of the PCR, particularly weld depth. Some non-limiting examples of tracking data include signal position, symmetry, peak width, centroid, geometric second moment, centroid, amplitude, aspect ratio, and geometric area under the curve.

[0273] Additional methods of extracting simplified data from A-lines or B-scans, M-modes, and the like may also be employed, including those methods known to those of ordinary skill in the art of image processing and / or artificial intelligence (AI) and / or machine learning (ML), and such methods are considered to be within the scope of the present disclosure. For example, in some embodiments, AI and / or ML methods may be employed on interferograms, A-lines, or groups of interferograms. In some embodiments, AI and / or ML methods may be employed to identify and / or measure and / or characterize specific features of the PCR, including weld depth. As previously mentioned, filtering methods known to those of ordinary skill in the art of signal processing, such as percentile filters or Kalman filters, may be applied as additional methods for generating simplified tracking data.

[0274] In the context of identifying and compensating for SS-OCT motion artifacts, it should be understood that, in some embodiments, the application of tracking methods to extract key information from raw OCT data is an integral part of the method. Tracking methods can be applied to the data prior to correction or compensation to establish and quantify observed distortions and to roughly characterize the uncorrected signal, among other purposes. Tracking data from more than one interferometer output signal captured at more than one tuning rate dk / dt can be employed to perform the necessary evaluations to establish the presence of absolute or relative distortions that serve as the basis for motion artifact compensation.

[0275] In some embodiments of the present invention, at least one correction for motion artifacts is applied to tracking or reduced data extracted from the uncompensated interferometer output signal. The resulting corrected tracking data is understood to be corrected data. In some embodiments of the present invention, at least one correction for motion artifacts is calculated based on tracking or reduced data derived from the interferometer output.

[0276] Embodiments of the invention in which the distortion components, corrections and corrected data, and any intermediate data or other related quantities are derived based on the full interferometer output signal or the tracked and simplified data, or some combination of the two, are considered to be within the scope of this disclosure.

[0277] Geometric compensation of motion artifacts without velocity estimation

[0278] In some embodiments of the system, it may be appropriate to apply simpler methods to calculate corrections for detected motion artifacts that do not involve incorporating theoretical models or velocity estimates. The motivations for simplifying compensation methods vary, but often timing and computational power (especially in, for example, real-time metrology systems) are limited and of concern. In such cases, it may be advantageous to forgo fully complex motion compensation methods and instead apply a simpler approach. While this may come at a substantial cost in data density or accuracy, it must be understood that this can be suitable for many applications.

[0279] Compensation for motion artifacts without velocity modeling and estimation still requires frequency analysis of the interferogram signal capture, which includes at least two segments of source sweeps with different dk / dt. Based on this analysis, a correction can be calculated to be applied directly to at least one of the at least two uncorrected A-lines. The correction can include the application of an offset, amplification or attenuation factor, a filter, or other signal processing manipulation or method known to those of ordinary skill in the art for the A-line. In some cases, it is appropriate to first track the key features of the at least two uncorrected A-lines and then apply the correction only to the tracked features. As previously discussed, such tracked features can be geometrically interesting features in or on the sample.

[0280] Filter by artifact size

[0281] In some embodiments of motion artifact correction, data from at least two different dk / dt portions or subsets of a given interferogram signal are analyzed to identify the magnitude of the motion artifact and / or the corresponding distortion in the interferometer output signal. Motion artifacts distort the position and other geometric aspects of the signal (e.g., position, symmetry, peak width, centroid, geometric second moment, center of mass, amplitude, aspect ratio, geometric area under the curve, and / or amplitude above the noise floor). Signals with smaller motion artifact magnitudes are also likely to have smaller distortions in these other aspects. Thus, motion artifact magnitude can be used as an indicator of the relative quality and / or accuracy of the A-lines generated from a given interferometer output signal. By selecting the A-line with the smallest motion artifact, noise can be reduced, tracking accuracy can be improved, and features of the target process can be viewed with greater accuracy. In some embodiments, performing an evaluation of the distortion components (including two or more distortions) includes determining whether the relative difference between the distortion components exceeds a predetermined threshold. In some embodiments, when evaluating motion artifacts, the magnitude of the distortion of some geometrical aspect of one or more distorted interferograms can be considered in an absolute sense and / or relative to a predetermined threshold. In some embodiments, applying a correction to at least one interferometer output signal based on the evaluation of motion artifacts includes discarding, weighting, boosting, or using the interferometer output signal based on the evaluation of motion artifacts, or evaluating the signal for later handling and / or processing.

[0282] According to at least one embodiment, one or more corrections applied by controller 150 to at least one interferometer output signal include discarding, weighting, or using the interferometer output signal, or any derivatives thereof. For example, analysis performed by controller 150 between at least two segments of a source sweep having different dk / dt values ​​may result in the relative difference between the at least two segments having a value or magnitude that renders the data unusable and therefore discarded. As used herein, the decision to discard, not discard, promote, modify, or retain data constitutes a correction. The inclusion of discard, retain, and modify as options is motivated by the fact that noise in the system can be sufficient to erode A-line information under certain motion artifact conditions, so that in some cases, discarding data does not lose information and can result in a reduction in noise and an increase in efficiency. Therefore, this approach is a relevant correction. Data can be flagged based on whether it passes a critical threshold to provide information for later processing, which can also be used to manage the variable quality of return data as part of motion artifact compensation.

[0283] This phenomenon can be used to filter uncorrected A-lines by, for example, selecting only those A-lines below a given (predetermined) motion artifact size threshold. The effect of this is to select those A-lines that, after motion compensation, would provide the highest quality and / or most accurate resulting A-lines and apply motion compensation only to those A-lines. Such methods have been shown to improve the tracking accuracy of certain processes for certain analytical methods. Downsampling based on motion artifact size can also effectively reduce the noise seen in M-mode and tracking data, thereby creating a clearer view of the material modification process and its key features being imaged, and allowing for more accurate analysis.

[0284] It will be appreciated that this filtering may be applied in addition to the geometric correction or motion compensation outlined above.

[0285] Whether using corrected and / or uncorrected interferometer output signals, the controller 150 can process these signals to determine at least one feature of the processing area, such as depth information of the processing area. The depth information includes keyhole depth of the PCR, seam tracking, finished weld surface height, workpiece height, and other measurements related to the workpiece geometry at time points occurring during pre-processing and post-processing. In some cases, the controller 150 is also configured to control at least one processing parameter of the material modification process based on at least one feature. In some embodiments, at least one processing parameter includes at least one characteristic of the material processing beam and / or material processing laser source, non-limiting examples of which include on / off state, average power, pulse duration, peak intensity, energy density, flux, wavelength, pulse repetition rate, pulse energy, pulse shape, scan speed, focal diameter, focal position, spatial mode (on the sample). Other non-limiting examples of processing parameters include material feed rate, cooling medium flow rate, cover / auxiliary gas flow rate, cover / auxiliary gas pressure, cover / auxiliary gas blend, and additive feed rate.

[0286] ML AI algorithms and motion artifacts

[0287] The application of ML and AI algorithms to SS-OCT need not be limited by the capabilities of currently available industrial OCT systems, which are primarily based on SD-OCT. Some of the exciting possibilities in SS-OCT stem from its ability to capture data from a target even when it is in motion. This article discusses this capability and methods for addressing the resulting artifacts. ML / AI techniques offer the potential to extract information from motion artifacts. For example, in the context of SD-OCT, a keyhole collapse event will produce rapid motion in the phase change region, thereby washing out the imaging signal. However, in SS-OCT, this event will produce a data signature, albeit one distorted by the ongoing motion. ML algorithms can be developed to identify such collapses in SS-OCT imaging data based on the motion artifact signature they produce. ML algorithms can also be developed to identify SS-OCT data signatures generated by motion in material processing areas, characterized by events such as porosity formation, weld blowout, or other process anomalies and / or fluctuations. Detecting such events offers new opportunities for ensuring weld quality and integrity.

[0288] Additional examples of applications of ML include using ML algorithms to identify and / or correct distortion in data resulting from motion artifacts, identify the velocity of an object or other characteristics of its motion resulting from motion artifacts, and the like. For example, an ML algorithm can be trained on acquired SS-OCT A-line data having one or more known sample motion patterns. Once trained, such an algorithm can be applied to the identification and / or correction of motion artifacts, depending on the specific implementation details. For example, an AI or ML algorithm can be implemented to extract velocity information from SS-OCT A-line data, identify distortion in an interferometer output signal resulting from sample motion, correct distortion in an interferometer output signal resulting from sample motion, reconstruct a distorted signal, and / or implement any other correction for motion artifacts as described above (as appropriate), including filtering and / or selection operations.

[0289] Example of Imaging Target Velocity Estimation and Compensation

[0290] As referred to herein, an interferogram or interference pattern can be a pattern formed by the interference of light waves from the reference arm 122 and the sample arm 124 of the SS-OCT system 100. The interferogram can also be considered a time-varying signal that can be converted into a digital electronic signal using an optical detector 130 and an analog-to-digital converter (digitizer 135). The light reflected from the reference arm 122 and the sample arm 124 is combined into a combined optical signal and directed to the optical detector 130, thereby generating at least one interferometer output signal. The signal generated at the optical detector 130 can be converted from an analog signal to a digital signal (via a digitizer) and processed by the controller 150. The controller 150 is configured to enable the information generated at the optical detector 130 to be sampled by the digitizer 135. In some embodiments, the analog electronic signal is sampled at uniform intervals of k, and the k clock 145 is detected using a detector 147, and the resulting signal is passed to the digitizer 135, where it is used to time these uniform intervals.

[0291] Figure 10A is a non-limiting example of a captured interferometry signal that is in the time domain prior to capture and represents what will eventually become one or more A-lines. It will be understood that in order to produce a cross-sectional image (B-scan) or repeated A-lines acquired over time at a fixed transverse position (M-mode), it is necessary to capture Figure 10A Many interferometry signals are shown. For example, according to one embodiment, an M-mode OCT image includes a set of subsequent A-lines that are shown adjacent to each other with intensity contrast to capture the evolution of a target (e.g., a weld keyhole) over time. The interferometry signal can contain information under various dk / dt sweep conditions as it is captured.

[0292] According to at least one embodiment, the tunable light source 105 is controlled so that the tuning rate dk / dt of the imaging optical signal includes at least two tuning rates dk / dt that generate corresponding interferometer output signals or are otherwise associated with corresponding interferometer output signals. In some embodiments, the processing unit 150 is configured to control the tunable light source so that the imaging optical signal includes at least two tuning rates dk / dt associated with one or more interferometer output signals. In some embodiments, the at least two tuning rates dk / dt include at least one negative dk / dt and at least one positive dk / dt. In one embodiment, as Figure 10BAs shown in the interferometric signal of FIG, both positive and negative tuning rates are captured. An appropriate Fourier transform method is applied to each of the positive / forward and negative / reverse interferogram portions to produce two A-lines. The A-lines are scaled to units corresponding to depth based on one or more characteristics of the light and / or system. Thus, the peak in such an A-line corresponds to the depth at which the reflecting sample interface is found (with respect to the reference to the depth of the reference interface). In the case where the sample interface is in motion during the interferogram capture, there is distortion in the resulting A-line. Such distortion is an example of a "motion artifact" as described above. Motion artifacts cause both the forward (positive) and reverse (negative) dk / dt A-lines to be distorted. The distortion has the effect of varying based on the dk / dt rates present within the interferogram used to generate the A-lines. An example of such distortion is Figure 11A The A-line depth versus amplitude data is shown in FIG. , which shows an A-line generated using a positive dk / dt interferogram portion without motion compensation or correction, referred to as a "positive A-line," and an A-line generated using a negative dk / dt interferogram portion without motion compensation or correction, referred to as a "negative A-line." Distortion manifests as a shift between the positive and negative A-line peaks, as well as an asymmetry and distorted shape of each peak. This is just one example of distortion in at least two of the two or more interferometer output signals, and is identified by controller 150. Controller 150 evaluates the distortion for each of the at least two interferometer output signals and, based on the evaluation, calculates and applies a correction to at least one of the interferometer output signals.

[0293] According to certain embodiments, the distortion corresponds to one or more geometric aspects of the interferometer output signals, as specified above (in the "Identification of Motion Artifact Distortion in SS-OCT Data" section). Computing the correction includes analyzing the frequency content of two or more interferometer output signals and evaluating the frequency content of the two or more signals. Following this evaluation, various correction methods can be applied to the A-line. Non-limiting examples of possible correction methods include: developing a full motion compensation matrix as described above, applying a geometric correction with reference to the evaluation as described above, or determining whether the relative difference between the frequency content of at least two interferometer output signals exceeds a certain threshold.

[0294] Return to Figure 11A For example, based on an evaluation of the distortion of the A-lines generated by the forward and reverse interferogram directions, the required correction can be calculated. In some embodiments, the evaluation of the distortion is also used to calculate an estimate of the sample velocity during the acquisition time. In some embodiments, the controller 150 uses the evaluation of the distortion component of each of the at least two interferometer output signals to generate an estimate of the velocity of the material being processed in the PCR.

[0295] According to one embodiment, the correction is calculated based on a mathematical model that includes a set of functions that describe the relationship between different variables, including the motion in the keyhole, the behavior of the swept source tuning, and the timing characteristics of the physical system. In one embodiment, the controller 150 is configured to generate the mathematical model based at least in part on one or more characteristics of the SS-OCT system and an assessment of the distortion. In some embodiments, the controller 150 is configured to generate a mathematical model of the tuning rate dk / dt based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source 105. In another embodiment, the second model is configured to provide an estimate of the motion of the sample on a k-clock time scale. In some embodiments, the two models are used to estimate the time elapsed between k-clock samples and / or estimate the displacement of the target (sample) on these time scales and / or to calculate appropriate corrections in some embodiments.

[0296] Once the corrections are calculated (e.g., by processing unit 150), in some embodiments, they are incorporated into the construction and modification of the new modified DFT matrix. Figure 11A In the example shown, a transformation matrix incorporating this correction is calculated for each of the positive and negative dk / dt interferogram portions, and the new matrices are applied to the corresponding portions of the interferogram to produce corrected A-lines showing the corrected depth of the interface. Figure 11B Shown Figure 11A An example of the relationship of the corrected A-line depth to amplitude data. In other embodiments, the correction is calculated and / or applied based solely on geometric factors.

[0297] According to at least one embodiment, the corrected A-lines can be tracked, filtered, and analyzed to identify material modification process characteristics, such as keyhole depth during welding. Furthermore, if multiple interferograms (e.g., M-mode or B-scan images) have distortions from the same motion speed, a single (corrected) correction DFT matrix can be used to correct all of the interferograms. However, if multiple interferograms have distortions from different or varying speeds, each interferogram requires a correction DFT matrix corresponding to its speed. Figure 12A An example of an uncorrected M-mode OCT image is shown in . This M-mode image was acquired from a benchtop experiment and provides a view of the sample target in uniform motion. Figure 12A The gap between the positive and negative dk / dt sweep portions is clearly shown, indicating motion artifacts. Figure 12B Shown are the same M-mode OCT images with motion artifact correction applied.

[0298] Figure 13A and Figure 13BUncorrected and corrected OCT image M-mode data of a keyhole weld process on a plate are shown, respectively. The OCT image data was captured inline with the welding process, and the data represents the evolution of the weld keyhole throughout the process. The plate weld process was performed on a copper substrate at a process speed of 150 mm / s, and the process laser IPGPhotonics YLS-6000 was configured with a welding head to emit a beam with 6 kW power and a 200 μm focal size. To generate the corrected Figure 1 3B The corrections applied to the data include the methods described in "Identification of motion artifact distortion in SS-OCT data" and "Imaging target velocity estimation and compensation" discussed above.

[0299] Head Integration

[0300] Integration into laser material processing heads

[0301] To enable inline coherent imaging (ICI) for laser material processing applications, according to at least one embodiment, one or more components of the system are integrated with the laser processing head. Figure 4 A schematic diagram showing one possible configuration for such integration can be seen in the non-limiting example of a laser head 715, which includes elements for steering the imaging beam and steering the combined imaging + processing beam. Some embodiments of the system will employ a dichroic mirror (7.8) to combine the material processing beam and the imaging beam and separate them when they are reflected from the process. In cases where the imaging beam and the process beam have similar wavelengths, the optical design of the dichroic mirror is critical.

[0302] Various embodiments of the present invention can be integrated into different types of laser material processing heads, including but not limited to: scanner heads, where active beam steering occurs on the process beam (e.g., using galvanometers 7.9 and 7.10); oscillating heads, where the process beam is manipulated in a repeating xy pattern to enhance material processing; and fixed optics. Applications of the disclosed SS-OCT systems and methods to material processing heads designed for other laser processing operations, such as cutting, cleaning, and additive manufacturing, can be developed based on the same general principles of process and imaging beam integration.

[0303] In some embodiments, the laser processing head may include special considerations for the imaging system. Such embodiments may include dedicated optical paths and targets for the purpose of compensating for thermal drift in the OCT system. Some embodiments may include a dedicated k-clock optical path within the head. Some embodiments may include one or more reference arm paths within the head. Such optical paths may be implemented by beam splitting the imaging beam path (e.g., using a 50:50 or 99:1 non-polarizing beam splitter), spectrally splitting the imaging beam (e.g., using a dichroic mirror and sub-spectral regions of the light source), or using electromechanical optics. Some embodiments may direct light into other paths based on light intensity (controlled by an amplifier). Some embodiments may include one or more imaging beam targets within the head that provide a scaling or calibration reference. Such a reference path may be implemented by splitting the imaging beam using partially reflective optics or using electromechanical optics.

[0304] In some embodiments, multiple sample arm beam paths can be introduced with the goal of imaging multiple positions relative to the processing beam 112 (e.g., aligned, in front of, behind, to the left, to the right, etc.). The measurements can be performed simultaneously or can be gated in time. Such measurements can be used, for example, in laser welding to direct the imaging beam to a keyhole (in line with or just behind the processing beam), the surface of the material, the solidified weld surface (trailing the processing beam), or an area of ​​interest on the weld material (e.g., the seam location). The optical paths can be designed to have intentional delays so that measurements from each path can be distinguished based on the position within the total Z field of view of the system. The optical paths can be designed to have intentional attenuation to compensate for reflectivity differences on the sample (e.g., keyhole vs. surface). The optical paths can be configured so that they can be spectrally selected by controlling the swept source wavelength and the sweep parameters.

[0305] In some embodiments of the system, the OCT sample arm imaging beam optical path will include at least one guiding element, such as a galvanometer or other beam steering element (7.3, 7.4), which can be used to position the sample arm beam path at multiple positions relative to the processing beam (e.g., aligned, in front of, behind, to the left, to the right, etc.). In some embodiments, the steering element will operate at a sufficiently high speed that multiple positions relative to the beam can be repeatedly imaged in a single process. In some embodiments, the imaging beam steering element can be used to assist in alignment, or to align the imaging beam in order to compensate for optical phenomena that affect the imaging beam, such as chromatic aberration caused by the material processing beam optics.

[0306] In some embodiments, a common-mode interferometry setup can be employed in which the reference arm path shares at least a portion of the imaging system's beam delivery path to the sample (i.e., the sample arm path). For example, the coating of the laser head's cover glass (7.12) can be designed to partially reflect the imaging wavelength band, thereby creating a common-mode reference arm path that terminates at the laser head's output. Such a path can be used in conjunction with a long ZFOV system to reduce the effects of thermal drift on absolute OCT measurements.

[0307] Dichroic Mirrors

[0308] In order to partially or completely combine the imaging beam with the material processing beam (e.g., into a combined optical path), as this relates to embodiments involving laser material processing, optics must be designed to combine the beams. The optics must also be capable of separating the imaging beam from other beams (e.g., process beams) and / or emissions (e.g., blackbody thermal radiation from the process) after the imaging beam is reflected and / or backscattered from the workpiece. A typical optical device that can be implemented in this function is a dichroic mirror, which has different reflection / transmission characteristics at different wavelengths. Such mirrors can be simulated and designed for specific wavelengths, and different embodiments of the present invention will incorporate different dichroic mirrors as needed based on design parameters including the wavelength of the imaging beam, the wavelength of the material processing beam, and the respective power of each beam, as well as other desired design requirements.

[0309] In accordance with at least one embodiment, a beam delivery system (e.g., beam delivery system 115) is configured with a dichroic optical device that is configured to combine the imaging optical signal and the material processing beam into a combined optical path. In some embodiments, the beam delivery system is configured to impinge the imaging optical signal on the dichroic optical device over a range of incident angles. In one embodiment, the dichroic optical device is configured to have a transmission spectrum having a first bandedge, a reflection spectrum having a second bandedge, and the first bandedge and the second bandedge have a maximum wavelength separation of 25 nm.

[0310] While the laser processing head examples described herein include the use of dichroic mirrors, it should be understood that other configurations are also within the scope of the present disclosure. For example, off-axis integration of the imaging beam into the material processing beam by methods known to those skilled in the art is also within the scope of the present disclosure.

[0311] Beam steering and alignment

[0312] In at least one embodiment, the imaging beam path will contain a steering / deflecting element (e.g., a galvanometer, resonant mirror, polygonal mirror, acousto-optic device, electro-optic device) that is specifically aligned with respect to its position relative to the process beam.

[0313] These elements enable the imaging beam to be more precisely aimed at different process areas, such as keyholes or seams, which may be of particular interest for measurement purposes. In some embodiments, the steering element can be used to steer the beam in a pattern that enables data to be collected on multiple process areas within the weld throughout the process (e.g., keyhole depth, seam tracking, finished weld surface height). In some embodiments, the steering element can be used to scan the imaging beam across the workpiece. In embodiments that implement imaging beam steering, the steering is typically applied before the imaging beam is combined with the process beam.

[0314] According to at least one embodiment, control of the imaging beam scanning is synchronized with the imaging light source tuning cycle. In some embodiments, the imaging light source tuning cycle is synchronized with control of the imaging beam scanning. Synchronization serves to reduce positional jitter in the resulting geometric measurements of the workpiece. Through precise synchronization of imaging beam scanning and imaging light source tuning, a lateral resolution of 20 microns or better can be achieved on an OCT system.

[0315] In some embodiments, alignment between the imaging beam and the material processing beam must be maintained. In some embodiments, the design of the laser material processing head may be sufficient to maintain such alignment, and in other embodiments, active correction of the alignment using, for example, an imaging beam path steering element may be appropriate.

[0316] application

[0317] Application Examples

[0318] The disclosed SS-OCT systems and methods can be used in a variety of applications. Various embodiments of the present invention can be optimized for imaging and quality assurance for many applications, including but not limited to laser material processing (e.g., welding, cutting, marking, brazing, cleaning, scribing, sintering, powder bed additive manufacturing, wire feeding additive manufacturing). In particular, in laser material processing applications, the imaging beam can be integrated into the material processing beam to accurately image the process during the process, also known as inline coherence imaging (ICI). Such imaging can provide information about the characteristics of the processing area. An example of a feature that can be characterized by such imaging is depth information of the processing area. In some applications, the depth information about the processing area can include a range of at least 1 mm, including the end value; a range of at least 5 mm, including the end value; a range of at least 21 mm, including the end value; and / or a range of at least 50 mm, including the end value.

[0319] scanning

[0320] Some embodiments of the system incorporate the imaging beam into a beam delivery head (where optical or optomechanical elements (e.g., galvanometers) are used to move the material processing beam), such as a scanner head. Non-limiting examples of such heads include: two-dimensional scanner heads, such as the IPG Photonics 2D High Power Scanner; three-dimensional scanner heads (e.g., heads where the focal length and position within the focal plane of the material processing beam can be adjusted by the beam delivery optics), such as the IPG Photonics 3D High Power Scanner; and advanced scanner configurations, such as multi-faceted scanner systems.

[0321] In some embodiments, the wavelengths of the imaging beam and the material processing beam are selected to be closely spaced in the spectrum in order to minimize chromatic aberration caused when the combined image and process beams are steered toward the edge of the scan head optics. For example, to minimize chromatic aberration in an embodiment configured to operate with a 1070 nm fiber laser in the scanner head, some embodiments of the system can be designed using a swept source in the vicinity of the 1000 to 1050 nm wavelength range. Alternatively, if the operating wavelength of the material processing laser is closer to 1030 nm (e.g., a disk laser), the center wavelength of the imaging system can be closer to the 1040 nm to 1070 nm wavelength range. In some embodiments of the system, software and / or optomechanical methods can be used to correct for chromatic aberration, for example using beam steering corrections to the imaging beam to realign as needed in the operating region where chromatic aberration is observed.

[0322] In embodiments of the system where elements in a beam delivery head are used to actively steer / scan the material processing and imaging beams, the optical path length to the workpiece can be substantially varied over the entire scan range of the beam delivery arrangement. In embodiments of the invention incorporating a 3D scanner head, the optical path length, and in particular the distance between the beam delivery head and the workpiece, can be substantially and intentionally variable. However, variability in the optical path length to the workpiece may also be present in many other beam delivery head configurations, such as 2D scanners. In some embodiments, the parameters of the system can be set so that the imaging range is sufficient to capture the full range of optical path lengths within the field of view of the beam delivery head without adjusting the delay line. The relatively large (>50mm) coherence lengths available from certain SS-OCT implementations enable this approach. In some embodiments incorporating scanning, an active delay line can be included to adjust the imaging range in response to variations in the optical path length to the workpiece over the entire scan range.

[0323] In embodiments of the system incorporating 3D scanning, the focusing and collimation mechanism of the imaging beam can be equipped with electronic, optomechanical, or optoelectronic devices capable of actively adjusting the focus of the imaging beam, such that the focal plane of the imaging beam can be actively adjusted to partially or completely coincide with the focal plane of the material processing beam. In 3D scanning applications where the focal length of the material processing beam varies throughout the weld, the imaging system can be used to provide active feedback, such as regarding workpiece height and / or keyhole penetration, to guide and verify 3D scanning performance.

[0324] In some embodiments of the system, calibrations can be performed to characterize the optical path length at various locations in the available field of view of the beam delivery head (e.g., the scanning field of view of the scanner head). In some embodiments of the system, calibrations can be performed to characterize the chromatic aberration (CA) of the imaging beam relative to the process beam at various locations in the available field of view of the beam delivery head (e.g., the scanning field of view of the scanner head). Either or both of these calibrations can form the basis for applying compensation methods to correct for variability in these parameters. The compensation methods can involve computational corrections (e.g., algorithms) and / or physical corrections (e.g., imaging beam steering, reference arm adjustment). Calibrations can also be performed independently or concurrently with the material processing beam steering optics to characterize the field of view of the imaging beam steering optics.

[0325] Additional applications of embodiments of the system are possible by implementing various active scanners and / or beam steering configurations in the beam delivery head. For example, some embodiments of the system may incorporate a multi-faceted scanner in the beam delivery head, which may be applied to welding, cutting, surface treatment, coating removal, patterning, and web processing.

[0326] High power welding

[0327] Certain embodiments of the system may be designed for high power and extremely high power weld monitoring (>20kW). Some tested embodiments of the disclosed SS-OCT system have a greater imaging depth of field of view than conventional SD-OCT systems (e.g., 12 mm for conventional SD-OCT systems versus greater than 50 mm for certain embodiments of the SS-OCT system disclosed herein). The greater imaging depth achievable with SS-OCT unlocks the possibility of imaging welds at greater depths while still monitoring additional metrics, such as the workpiece surface and the finished weld surface. Embodiments of the system designed for high power welding may require material handling head design and integration suitable for handling the energy and heat levels generated in such processes.

[0328] Reflectivity measurement

[0329] In some embodiments, reflectivity measurement is performed as a supplement to, as an alternative to, or between coherent measurements to monitor additional aspects of the process. In one embodiment, the same light source (imaging light source 105) is used for both reflectivity measurement and coherent measurement. In some embodiments, amplifier 106 can be used to increase the DC signal level to improve the measurement signal-to-noise ratio (SNR). In such cases, a feedback signal sampled from the output of amplifier 106 can be used to control amplifier stability to achieve more accurate reflectivity measurement. In addition, the back-reflected measurement beam light from the sample can also be used as an amplifier control signal to normalize the measurement value from the part (workpiece) surface.

[0330] Sequence Mode

[0331] In some embodiments, the system can be designed to capture a set of multiple OCT captures corresponding to one or more subsequent tasks and / or material modification processes, including A-lines, B-scans, M-modes, volumetric captures, or other imaging formats known to those of ordinary skill in the art. Such a set of captures is referred to as an imaging task sequence. Such a sequence can include any combination of measurements of various sample targets, such as fixed parts, seams, weld keyholes, finished product surfaces after processing, etc. In some embodiments, imaging tasks within a sequence can be performed concurrently with material processing. In some embodiments, imaging tasks within a sequence can occur before and / or after material processing, or can correspond to metrology tasks or captures. In some embodiments, some or all of these types of imaging tasks are combined. Sequence imaging can include repeated measurements of various aspects of the same process in some embodiments, or, in other embodiments, include a set of measurements of subsequent different processes.

[0332] The imaging process within the sequence mode can incorporate one or more processing elements described in the present disclosure, non-limiting examples of which include dispersion compensation, motion artifact compensation, or generation of tracking data. According to some embodiments, the imaging tasks within the sequence can be used to develop quality assurance (QA) metrics, which can correspond to the quality of the process corresponding to the current imaging task, or can be used to extract parameters for feedback control of the current processing task or subsequent processing tasks within the sequence. In some embodiments, QA metrics can be developed based on the results of multiple imaging tasks within the sequence, up to all tasks within the sequence.

[0333] In some embodiments of the system, a programmed sequence of imaging tasks can continuously monitor a given process in an unsupervised, automated manner. Such embodiments can be integrated with quality assurance alerts to generate alerts and / or notifications when tasks within a sequence or the entire sequence itself do not meet quality assurance metrics. Such embodiments can be integrated with another method of generating notifications based on the results of the sequence.

[0334] Workpiece characterization before welding

[0335] In certain embodiments of the system, the imaging system can be used to characterize a material modification sample or workpiece at times other than during the material modification process. In such cases, the imaging beam can still be integrated into the beam delivery device for the material processing beam to enable imaging of the workpiece in line with or in close proximity to the material processing beam position. In some such configurations, the imaging beam can be steered independently of the material processing beam using a separate beam steering element in the imaging beam path. In some such configurations, the imaging beam can be steered by a beam steering element in the material processing beam path. In some such configurations, the imaging beam may not be actively steered.

[0336] In some embodiments of the system, the SS-OCT system can be configured to measure the distance between the workpiece and the focal plane of the material modification beam optics. In material modification optics that actively steer the material modification beam (e.g., a scanner head), the SS-OCT system can be additionally configured to characterize the entire focal space or focal plane that the material modification beam may pass through. This characterization can be done as a calibration prior to the material modification processing step. In embodiments of the system integrated with material modification optics that can actively adjust the focus, the SS-OCT system can be used to provide feedback about sample and / or workpiece features / geometry, such as the distance between the focal plane of the material modification beam optics and the workpiece surface, or measurement of the position and geometry of fixed parts for evaluation of assembly. In such systems, workpiece geometry measurements can be used for active feedback control of the material processing beam head, such as providing "auto focus" or weld path correction capabilities.

[0337] Top Surface Reference Point Measurement (TSRP)

[0338] In some embodiments, one or more imaging beam positions can be used to measure a top surface reference point (TSRP), with further embodiments including the ability to simultaneously measure the TSRP and weld depth using multiple imaging beams. The TSRP is a reference position established using points on the sample surface, and where the sample is substantially flat, at least one TSRP can be used to define a top surface reference plane. Although reference is made here in the context of setting the TSRP prior to the welding process, it should be understood that the TSRP can be set, measured, or calibrated before, during, or after the welding process. This can be achieved by taking one or more baseline depth measurements at locations on the sample that are not affected by the welding process. In some cases, the TSRP can be determined by taking one or more measurements of the material immediately before welding begins. If the material is sufficiently flat relative to the weld motion, this initial measurement can define the TSRP for the remainder of the weld. Other techniques for measuring TSRP are known in the art.

[0339] Seam tracking

[0340] In certain embodiments of systems involving laser welding, an imaging system can be used to obtain measurements taken along the path of a material processing beam during processing at a specific distance far enough in front of the processing beam focal point that the measurements are not affected by the process itself, but close enough to the focal point so that accurate measurements of the part fit and / or geometry (e.g., a seam line between two parts joined by a welding process) can be used to determine whether the focal point is adequately aligned with the seam line. In certain embodiments, the imaging system can be used to obtain measurements of the part fit and / or geometry before material processing begins (e.g., as a pre-scan) or after material processing. In one example, a scanner can be used in combination with the imaging beam to locate one or more surface features (e.g., grooves) that can be used to provide measurements of the position and / or geometry of the seam before the material processing beam reaches a given position in the process. In some cases, these measurements can also be used to dynamically correct for misalignment, such as by providing real-time feedback to a material processing beam steering device.

[0341] In addition to seam tracking, the scope of the present disclosure also includes other desired features (e.g., workpiece height) that can be tracked by the imaging beam before or during the material modification process. As with seam tracking, in some embodiments, the imaging beam can be configured to scan the desired feature before the material modification process begins. In embodiments of the system that implements pre-processing scanning of the material, the processor 150 can be configured to provide real-time feedback based on the imaging of the desired feature for the purpose of closed-loop feedback control of the material modification process.

[0342] In some embodiments, measurements of the location and / or geometry of the seam, or other significant pre-weld characteristics measured, may be used to develop quality assurance (QA) metrics.

[0343] Post-weld characterization

[0344] In certain embodiments of systems involving laser welding, an imaging system can be used to obtain measurements taken along the path (geometry / direction) of the material processing beam during processing at a specific distance far enough behind the focal point of the processing beam that the measurement is not affected by the process itself, but close enough to the focal point that accurate measurements of the finished weld surface can be used to determine aspects related to the quality of the weld.

[0345] In some embodiments, finished weld surface measurements or other significant post-weld geometric aspects may be used to develop QA metrics.

[0346] According to at least one embodiment, the scope of the present disclosure also includes monitoring desired features that can be tracked by an imaging beam after the welding process. For example, the measurements can be used to inspect the surface of the joint after welding and identify the presence of geometric features and / or physical defects (e.g., splatter). In some embodiments, the imaging beam can be configured to scan the desired feature after the material modification process is completed.

[0347] Additive Manufacturing

[0348] In certain embodiments, material modification can be part of an additive manufacturing process, a subtractive manufacturing process, or some combination thereof. Additive manufacturing processes can include powder bed fusion, directed energy deposition, wire-fed additive manufacturing, laser sintering, or variations and / or combinations thereof. Embodiments of the present invention applied to material modification processes that can be broadly categorized as additive and / or subtractive can include features that provide feedback of particular interest to such processes, some non-exclusive examples of which are detailed below.

[0349] Embodiments of the present invention designed to work with additive manufacturing processes involving the delivery of additive manufacturing precursor materials (e.g., powders, wires) can be configured to produce measurement outputs that characterize relevant metrics of precursor delivery and / or quality. An example of such a metric is the flatness of a powder layer deposited during a powder bed fusion additive manufacturing process. In some embodiments, the additive manufacturing setting can be set to perform corrections in response to measurement information collected by the SS-OCT system. For example, selective laser ablation and / or remelting can be applied to correct the flatness of a part based on OCT measurements taken during or after deposition. Additional measurements can also be performed, such as measurements of the features / geometric properties of a keyhole, measurements of the deposition tracking surface trailing the keyhole, and measurements of the geometry of the part being constructed. In some embodiments, the SS-OCT system can be used to track measurements of a manufactured part relative to a model (e.g., a CAD model). Some embodiments of the system can be configured to provide real-time feedback to update and modify the build parameters of the additive process to maintain desired properties.

[0350] General feedback control

[0351] The real-time feedback provided by the imaging system can be used to change process variables in a manner that compensates for material variations (e.g., poor fitment) and instabilities (e.g., variable weld depth) in the process as the process is accelerated to higher speeds and / or pushed to greater depths. This can expand the usability of laser welding systems to include lower-cost input materials, higher processing speeds, and / or deeper weld penetrations while maintaining acceptable quality. Real-time feedback can also be applied to achieve greater consistency in process output over time. For example, real-time feedback can be used to control the power of a diode laser used for material processing, and when the keyhole depth, as measured by the system, deviates from a target value, the laser power can be increased via a feedback mechanism. In a typical material processing system, a number of factors can cause such deviations in weld keyhole depth, including a decrease in the output power of the laser module over time, contamination or obstruction of the beam path (e.g., smoke or weld spatter on the beam delivery optics), and so on. Changes that occur over the time span of a material processing process (such as inconsistencies in the material parameters of the sample being processed) or across many material processing processes (such as changes in environmental conditions) can, in some cases, cause the output quality of the process to vary, causing key process metrics to no longer be within target specifications. In some cases, the real-time feedback provided by the imaging system can be applied to active feedback control of process parameters to keep the welding process within target metrics. In some cases, the real-time feedback method can implement control loops and / or theories known to those skilled in the art, including proportional control and / or proportional-integral-derivative control. The specific implementation of the feedback control can be based on feedback derived from the measurements and / or signals provided by the SS-OCT system, and can additionally incorporate one or more of the following: these signals and / or signals provided by auxiliary sensors in the SS-OCT system, measurements and / or signals from other sensors within the material processing system, and models of various components of the material processing beam system.

[0352] Transient effects during the start or end of a welding procedure can negatively impact the outcome of the weld (e.g., inconsistent joint depth, underfill). For example, quantifying the amount of underfill along the surface of a joint is important information for determining weld strength, its corrosion resistance, and its compatibility with subsequent coating processes such as priming and painting. According to at least one embodiment, feedback provided by an imaging system can be used to compensate for transient behavior of the weld at its start, at its end, or both, by controlling one or more process parameters, thereby reducing these defects.

[0353] Additional system capabilities and features

[0354] Auxiliary measurement system

[0355] According to at least one embodiment, the SS-OCT system as disclosed herein may further include an auxiliary measurement system 160 in communication with the controller 150 and including auxiliary sensors, such as visible and / or IR sensitive photodiodes and / or cameras and / or spectrometers, which in some cases may be coupled to the welding head via optical fibers. The auxiliary measurement system 160 may be configured to measure process radiation, for example, within a spectral band between 100 nm and 20 nm.

[0356] It should be noted that unlike biological applications of coherent imaging, material processing applications (which are the primary focus of this disclosure) are typically characterized by the presence of incoherent light sources that are incoherent with the imaging light and the material processing beam light itself in the form of blackbody radiation. In spectrally discriminated SD-OCT methods, a significant amount of this light is automatically discarded by the spectrometer. According to at least one embodiment, additional filtering elements (in the form of FBGs, WDMs, and other filters known to those of ordinary skill in the art) are added at one or more locations in the sample arm of the SS-OCT system to ensure that these incoherent optical signals do not reach the primary detector (e.g., Figure 1 However, it may be useful to transfer these signals to an auxiliary detector. Figure 5A Some possible placements of such auxiliary detectors within the sample arm of the interferometer are outlined in . Figure 5B One possible transfer method is shown in the example of a beam delivery head 315 in FIG, wherein a schematic diagram of a material processing beam head 315 is shown, which includes an additional dichroic mirror (3.4) to separate the blackbody radiation signal to a detector (3.5) in the imaging beam part of the head. The transfer of these samples can also be achieved in the fiber section of the interferometer. The blackbody radiation signal can be coupled back into the sample arm fiber and then used, for example, using a wavelength division multiplexer (such as Figure 5A The WD)M 614 in the CMOS is split from the OCT signal to enable detection on a dedicated detector such as Figure 5A Detection is performed on an auxiliary detector 611 in the sample arm, thereby enabling concurrent detection of blackbody radiation and SS-OCT data. To enhance the reverse coupling of broadband blackbody radiation, in some embodiments, a double-clad fiber can be employed such that the blackbody radiation is coupled into the multimode inner cladding portion, while the imaging beam reflected from the workpiece is coupled into the single-mode core portion of the fiber. An example of incorporating blackbody detection into an OCT system via a double-clad fiber in the sample arm is described in more detail in U.S. Patent No. 10,898,969, which is hereby incorporated by reference.

[0357] One non-limiting example of an implementation that includes auxiliary detection is to position one or more photodiode sensors for blackbody radiation detection in a beam steering module for the imaging beam, such as Figure 5A The situation of the auxiliary sensor 613 in . Figure 5B One possible configuration of such an auxiliary detector is shown in the laser head 315 of FIG. 1 and will be described in more detail below. Note that in order to be able to perform blackbody detection simultaneously with OCT imaging, it may be necessary to include additional detectors (e.g., Figure 5B blackbody sensor 3.5) and ensure that the desired blackbody wavelength can pass through one or more dichroic optical devices (e.g., Figure 5B dichroic mirror 3.8, which splits the imaging beam and the material processing beam when they are reflected from the process), while at the same time by using appropriate optical devices (e.g. Figure 5B The blackbody signal dichroic mirror 3.4) is correctly separated from the OCT beam.

[0358] One non-limiting example of an implementation including assisted detection involves a double-clad optical fiber comprising a single-mode core with an additional inner cladding designed to propagate multimode light. Figure 5A The double-clad optical fiber 610 and the auxiliary detector 612 are shown in FIG. Figure 5A Detail of a double-clad fiber (610A) showing the multimode inner cladding (610B) and the single-mode core (610C) is shown in FIG. The advantage of implementing a double-clad fiber for detecting blackbody signals is that the larger surface area of ​​the multimode inner cladding enables greater back-coupling of broadband and incoherent light generated by the blackbody and / or thermal radiation, thereby enabling the detection of signals at auxiliary detectors (e.g., Figure 5A A stronger signal is detected at the auxiliary detector 612 in the . To enable detection of blackbody radiation, the multimode light trapped in the inner cladding of the double-clad fiber can be coupled out into a separate multimode fiber.

[0359] The practice of coupling additional optical signals such as blackbody radiation or a back-reflected processing beam into the sample arm is generally referred to as wavelength multiplexing of the sample arm return signal. In some embodiments, this multiplexed return signal can be detected and used as an additional metric for process quality control and also to aid in alignment of the coordinate systems of the process beam and the imaging system. The latter approach is performed by using one or more guiding elements to cause the imaging system to scan around the area where the material processing beam source illuminates the workpiece, calibration target, and / or material test specimen. While this is happening, the material processing beam source is operated (preferably at low intensity) so as to a) produce reflected material processing beam energy, b) produce blackbody radiation, or c) both. As the guiding element scans over the area, the primary detector and / or auxiliary detector receives radiation that can be correlated to the scan position. In this way, the energy distribution and position of the material processing beam source can be determined by means including peak finding, maximum finding, Gaussian curve fitting, super-Gaussian curve fitting, and other fitting functions known to those of ordinary skill in the art for beam characterization. By selecting which wavelengths of radiation are allowed to reach the detector, this approach conveniently allows for compensation and mapping of chromatic aberrations of the beam delivery system. This mapping can be stored in memory and used by the control module and / or guide element control electronics to allow the guide element to compensate for chromatic aberration. PCT Patent Application No. PCT / US2021 / 027672, owned by the applicant and incorporated herein by reference, reviews and outlines this process in more detail.

[0360] Capturing the energy spot width of the blackbody radiation or back-reflected processing beam and using it as a substitute for the material processing beam can be used to generate additional beam characterization measurements, such as beam caustics and focal plane determination. In some embodiments, spectral filtering can be used to intentionally remove back-reflected process beam light and blackbody radiation from the OCT optical detection element. However, in some embodiments, the OCT optical detection element and the back-reflected processing beam and blackbody radiation collection element can be shared (i.e., use a common optical path) to reduce optical complexity and cost. In such cases, the OCT signal can be distinguished from the back-reflected process beam and blackbody radiation signal by different photodiode detection schemes (e.g., balanced versus unbalanced), by digital processing (e.g., low-pass, band-pass, or high-pass filtering), by time gating (e.g., collecting the OCT signal from the auxiliary radiation at different times), or some combination thereof. Time gating can be achieved by turning off the OCT system light source, reducing the OCT system light source at the amplifier, or by additional filtering elements.

[0361] In some embodiments, the detected process blackbody radiation can be used to calculate an estimate of the process temperature or position within the process. This metric can be used to track the stability of the process temperature over time and record absolute or relative values. When accompanied by beam steering of the imaging beam, this measurement can also be used to estimate the absolute or relative temperature of the entire process in space.

[0362] In some embodiments, a portion of the sample arm includes a double-clad MM / SM mode stripper fiber component, such as the Castor Optics DC1060LE (Montreal, QC, Canada). This allows for multiple functions, including integrated time-of-flight (TOF) / frequency-modulated continuous wave (FMCW) light detection and ranging (LIDAR) detection, and optionally with the help of further filtering elements, the ability to detect larger amounts of incoherent emissions (for auxiliary sensing with higher SNR). The multimode channel can also be used to collect additional reflected imaging light, which is transmitted to an auxiliary photodetector to perform closed-loop feedback of the imaging light source emission power. The multimode channel can also be used to collect back-reflected process beam light, blackbody radiation, or a combination thereof.

[0363] Interferometer characteristics

[0364] In some embodiments, the interferometer has at least two reference arm paths of similar or dissimilar lengths. The reference arm can also be configured with at least one actuator that causes one or more of the paths to be selected or deselected. The means of selecting or deselecting can include adding attenuation to the beam path. Attenuation of the path can be caused by moving a lens, closing a shutter, actuating an optical switch, adding absorptive or reflective optics to the path, changing the polarization of the light, rotating the polarization optics, and / or changing the alignment of mirrors (including end mirrors) in the beam path. Path selection can also be spectrally selective, where spectrally selective optical elements (e.g., dichroic mirrors) can be used in conjunction with dividing the swept source spectral band into two regions (one region dedicated to each reference arm). The reference arm is designed so that all paths can be illuminated simultaneously and deselected by the control module. Interestingly, illuminating two or more reference paths will produce a common path interference signal that will be Figure 1 However, it can still be recovered by monitoring the single-ended (ie, unbalanced) detection output from the BPD.

[0365] This is a means of simultaneously measuring the K-clock signal from a single interferometer, and has the added benefit of being able to compare two known paths simultaneously in order to confirm the axial alignment of the system. Finally, while one of ordinary skill might view illuminating multiple reference paths simultaneously as a waste of optical power, it should be emphasized that the light source described herein produces an exceptionally rich imaging light, which means that less efficient but more powerful / practical approaches like this one can be considered without significant loss of image quality.

[0366] In accordance with at least one embodiment, the amount of optical power returned from at least one reference arm is adjusted in order to shift the dynamic range of the imaging system. In the event that strong reflections are returning from the sample, it may be desirable to attenuate the reference arm so as not to saturate the detector. This can be performed as a closed-loop control, where the main BPD signal, the single-ended BPD signal, or the auxiliary photodiode are used as inputs, and a variable attenuator and / or any of the devices described in the previous paragraph for attenuating the reference path are used as actuators. Proportional (P), proportional-integral (PI), and proportional-integral-derivative (PID) control schemes can all be implemented via the control and / or feedback modules of the system. Another way in which the reference power and dynamic range can be affected is through modulation of the light source power, as described herein.

[0367] According to certain embodiments, a visible light "guide" or "pointing" beam is injected into the imaging beam, for example, via a sample arm, and transmitted through the head to the workpiece. This can be accomplished by a WDM device or other similar components known to those of ordinary skill in the art. Adding a guide beam to the imaging system provides several benefits. Since the imaging fiber core diameter is typically much smaller than the process fiber, it produces a sharper point in the focal plane and less modal interference in the fiber. This makes it easier to see the point accurately. By steering the guide beam with a guiding element, a visible pattern can be drawn on the surface of the workpiece to aid in the teaching and alignment of the material modification process as well as the imaging system itself. Showing a visible beam representing the scan of the imaging system also helps in interpreting the imaging system data. In further optimization of the method, the timing of the imaging system acquisition, the scanning path of the guiding element, and the emission of the guide beam can be tuned to compensate for lateral chromatic aberration in the beam delivery system so that the true imaging path is represented by the guide beam.

[0368] In some embodiments, one or more boost amplifiers can be added to the sample arm of the interferometer to increase the sample arm optical power, such as Figure 3As shown. For example, amplifier 406b is configured to amplify the returned imaging light because other optical signals have been blocked. The dispersion and optical path length introduced by the introduction of such an amplifier can be compensated by adding appropriate compensation elements to the reference arm. By selecting a short amplifier with a low amount of stored energy and pump power, spontaneous emission is reduced, and from a relative perspective, weak signals are amplified more than strong signals. This has the additional benefit of effectively compressing the dynamic range of the system. The control module and / or feedback module ( Figure 3 Not explicitly shown, but similar to Figure 1 The processing unit 150) controls the gain on the amplifier by modulating the pump current of its diode.

[0369] According to some embodiments, high population inversion is employed to shift the amplifier gain to shorter wavelengths, thereby tuning the operating spectrum of the imaging system further away from the spectrum of high-power lasers used for material processing, such as welding. For example, a high population inversion YDFA can be employed to generate an amplifier gain peak in the 1020 nm to 1050 nm region, thereby shifting the operating spectrum of the imaging system away from the typical 1070 nm operating wavelength of a selected ytterbium-doped fiber laser.

[0370] Because differential stress or temperature changes in the interferometer's optical fibers can cause path length changes in the reference or sample arms of the interferometer, in some embodiments, mechanisms for compensation can be included in the sample arm, including adjustable spindles, motorized stages containing optical components, adjustable mirrors, and other optical mechanisms known to those skilled in the art. In some embodiments, dedicated correction paths can be included in the form of mechanical or optical features in the sample or reference arms, which can be used for automatic or manual compensation.

[0371] In accordance with at least one embodiment, for example, an embodiment employing an imaging light source having a wavelength near 1550 nm, the system is configured with dispersion-shifted fiber to reduce chromatic dispersion in the interferometer, thereby allowing compensation of air path length with the fiber and reducing the dispersion effects on the point spread function of the imaging system.

[0372] According to certain embodiments, one or more portions of the interferometer are configured with polarization-maintaining (PM) fiber. In this configuration, it is advantageous to illuminate the interferometer with a single polarization to reduce path length degeneracy due to birefringence of the fiber. A benefit of using PM fiber is that manipulation of the sample and reference arm fibers reduces variations in the interference signal received at the BPD. To achieve this, in some embodiments, polarization optics are added to the light source module to align the polarization of its emitted light with the preferred polarization of the interferometer.

[0373] Reuse

[0374] In accordance with at least one embodiment, imaging light source modules are shared / multiplexed among multiple interferometers, which may be associated with multiple beam delivery systems, material processing beam sources, detectors, and the like. Figure 6 An example configuration is shown in Figure 1. Using swept lasers in a multiplexed configuration requires careful selection of fiber and signal cable lengths so that any phase shifts between the interferometers are minimized or compensated. In some embodiments, the fiber lengths between the light source module and each multiplexed interferometer unit are matched to a propagation time of less than 1 ns. In other embodiments, the match is less than 10 ns.

[0375] In some embodiments, when a single light source is shared between multiple imaging systems, a single k-clock located in or before one of the interferometers serves as the k-clock for multiple interferometers. This configuration means that when the k-clock signal is applied to the signal processing chain, the time delays between the various multiplexed interferometers / detectors must be accounted for. However, in other embodiments, the k-clock is implemented by sampling the reference arm signal.

[0376] In some embodiments, the imaging system scan control is synchronized with the imaging light source tuning cycle to reduce positional jitter in the resulting geometric measurements of the workpiece. In at least one embodiment, the light source tuning cycle phase and frequency are measured and used to calculate a positional offset, which is added to the geometric measurements to compensate for jitter / variation in the starting phase of the light source tuning cycle. Ideally, the clocks running the deflection system and light source tuning signals are themselves synchronized to a common electronic clock.

[0377] According to certain embodiments, the interferometer unit is additionally equipped with a shutter or other beam blocking device that is operated by the safety module to ensure optical safety. One way this can be achieved is to monitor the position of one or more of these guiding elements and use safety logic to ensure that the light source cannot be operated at potentially dangerous powers (in combination with the guiding elements that allow the beam to leave the beam delivery system) unless a safety interlock is met.

[0378] Autofocus for applications with considerable imaging depth variability

[0379] Certain embodiments may take advantage of the available deep imaging field of view to the extent that the imaging depth of view is greater than the depth range over which the focus of the imaging beam is suitable. In such applications, optical, mechanical and / or electronic components that allow for a focus depth shift may be integrated into the system. An example of a focus depth shift that can be achieved within the field of view is a focus-adjustable lens, an electro-optical technology. Some embodiments of the system may utilize feedback control between a focusing element and return power or image data to implement an "auto focus" operation. Possible optimizations of focus include, but are not limited to, optimizing the focus of the imaging beam to align with the workpiece surface, or with the bottom of the keyhole, or with the surface of the finished weld.

[0380] Sweeping the imaging beam's focal depth profile opens up the potential for many useful measurement algorithms. The backreflection intensity at the fitted wavelength can be used to detect the focal plane of the system. Sweeping the focus over the entire Z range to measure the peak possible amplitude for all interfaces in the imaging frame can help identify the true brightest interface by algorithmically correcting for the effects of defocus. The focus can also be tuned to track variable interface depths, for example, by using pre-weld scan imaging data to estimate the upcoming interface depth and adjusting the focus in response to the upcoming depth.

[0381] Interferometer topology

[0382] According to various embodiments, the interferometer is implemented using fiber-based optical path components or free-space optical path components or some combination of fiber-based optical path components and free-space optical path components. In some embodiments of the system including fiber-based interferometer components, polarization-maintaining fiber can be used. In some embodiments of the system including fiber-based interferometer components, single-mode fiber can be used. In some embodiments, the dispersion present in the sample arm and reference arm of the interferometer can be designed to match as well as possible to improve imaging performance. The specific implementation of the interferometer in a particular embodiment of the present invention can vary based on the requirements of that embodiment and can be combined with interferometer components and methods known to those of ordinary skill in the art.

[0383] According to some embodiments, a Mach-Zehnder interferometer (examples of which are shown in Figure 1 ) is preferred because it allows for easy adjustment of both the initial split ratio from the imaging light source and balanced detection. However, a typical implementation of this topology requires two Faraday devices, which increases cost and complexity. In a Mach-Zehnder interferometer configuration, care must be taken in the interferometer design to ensure that the two paths to BPD 130 are approximately matched, so that the positive and negative interference signals arrive at the BPD simultaneously.

[0384] Since the power of the amplifier light source can (for practical intents and purposes) be high enough not to be a limiting factor in the optical design, in some embodiments the Michelson interferometer can be used with (e.g. Figure 1 of the detector 130) or not with (e.g., Figure 7A The detector 230) is used together with balanced detection with little overall impact on the signal-to-noise ratio (SNR) / sensitivity. Figure 7A is a schematic representation of an SS-OCT system 200 configured with a Michelson interferometer and shows how this can be achieved based on the interferometer topology. In some embodiments where the power of the amplifier light source is not considered a limiting factor, the amplifier light source can be split by a 50:50 splitter ( Figure 7B 521, 523) replace the circulator ( Figure 1 121, 123) to construct an interferometer similar to the Mach-Zehnder interferometer, an example of which is shown in Figure 7B This configuration can provide cost advantages compared to implementations that include a circulator. Although such an interferometer will have a poorer photon economy than a more standard Mach-Zehnder implementation, it should be noted that in some embodiments the amplifier can provide exceptionally abundant power, allowing for the use of a circulator even in applications such as Figure 7B High return interferometer power can also be achieved in such interferometers as shown. In either interferometer configuration, care needs to be taken in the interferometer design to ensure that the dispersion mismatch is minimized. Although not explicitly shown, Figure 1 One or more imaging source amplifiers similar to amplifier 106 may also be included in system 200 and / or system 500 .

[0385] exist Figure 1 In the system 100, a 90:10 split ratio is used between the sample and reference arms. This has the benefit of greater photon economy between the source, workpiece, and detector. It will be appreciated that various other split ratios (e.g., 99:1, 95:5) can be used to shift the dynamic range and further optimize the photon economy of the system.

[0386] In some embodiments, the interferometer topology is a demodulating interferometer.

[0387] According to certain embodiments, the reference path includes elements capable of controlling the power of the reference signal. This can be achieved by constructing and selecting different reference paths using branching methods and optical elements known to those skilled in the art. This can alternatively be achieved by introducing attenuation into a single reference path as needed. Irradiating the reference paths with different output powers or modifying the output power of a given reference path can be used to match the power levels delivered to the BPD by the sample arm and the reference arm, respectively. This can be applied to enhance the dynamic range of the system by allowing a full BPD detection range under different sample power return conditions and preventing saturation. In embodiments applied to processes in which multiple reflectivity conditions exist or may exist on the sample arm, this configuration of the reference path enables optimization of the return power for various sample powers, thereby improving the dynamic range of the system under each individual return power condition, thereby effectively increasing the balanced dynamic range of the system. Irradiating reference paths of different lengths or modifying the length of a single reference path (such as according to the methods described above in "Interferometer Characteristics"), combined with adjusting any system timing as needed, can, for example, modify the imaging range of the system or the axial resolution of a given system during operation.

[0388] Dual interferometer / dual or buffered source SS-OCT topology

[0389] As described herein, information from measurements taken at different dk / dt values ​​can be applied to compute corrections that reduce or otherwise eliminate the effects of motion artifacts. Another concept described herein allows the ability to simultaneously acquire forward (positive) and reverse (negative) frequency sweeps (dk / dt). Several concepts related to this capability are outlined below, with physical implementations and further details also included below. The overall concepts include: the use of a single imaging light source or multiple imaging light sources for simultaneously illuminating multiple optical interferometers; interferometers having sample arms that share at least one optical element (e.g., interferometers having at least partially overlapping sample arms (multiple interferometers "sharing" a common sample arm)); a specific implementation of a delay line, particularly a delay line incorporated into a source module that enables a delayed imaging optical signal to be output simultaneously with a non-delayed imaging optical signal from the source module; and an imaging light source that is configured such that it simultaneously generates different (i.e., substantially different) frequency swept optical signals.

[0390] In some embodiments, the processing unit 150 is configured to control the tunable light source 105 so that the rate of change (tuning rate dk / dt) of at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt. In some embodiments, the at least two tuning rates dk / dt include at least one negative dk / dt and at least one positive dk / dt. In a certain embodiment, the tunable light source 105 is configured so that the imaging optical signal includes a superposition of multiple optical signals. In some embodiments, the multiple optical signals superimposed to form the imaging optical signal have different wave numbers k and / or different tuning rates dk / dt. In such embodiments, the different tuning rates dk / dt can overlap with each other, including, for example, sharing one or more optical elements. In some embodiments, the tunable light source 105 is configured to generate multiple different imaging optical signal outputs. In some embodiments, the different outputs produce imaging optical signals that are different at the output (e.g., different in optical power, wave number k, and / or tuning rate dk / dt). In some embodiments, the tunable light source 105 is configured such that the imaging optical signal comprises a superposition of at least two tuning rates dk / dt. In other embodiments, the superposition of different tuning rates dk / dt comprises at least one negative dk / dt and at least one positive dk / dt.

[0391] In accordance with at least one embodiment, a SS-OCT system includes a first tunable light source and at least one other tunable light source. Figure 18A and Figure 18D A non-limiting example of such a configuration is shown in FIG (e.g., tunable light sources 805a and 805b). The first interferometer is configured with a first tunable light source (e.g., Figure 18A and Figure 18D 805a), and at least one additional interferometer is configured with at least one other tunable light source (e.g., Figure 18A and Figure 18D 805b). According to at least one embodiment, the first interferometer and the at least one additional interferometer are configured such that the sample arm of the first interferometer and the sample arm of the at least one additional interferometer share at least one optical element. Figure 18A and Figure 18D, sample arm 824a (of a first interferometer) and sample arm 824b (of at least one additional interferometer) share at least one optical element, such as an optical fiber, of the overlapping sample arms prior to introduction into the beam delivery head 815. It should be understood that optical fiber is only one non-limiting example, and that other optical elements that may be shared include collimators, free-space beam steering elements (e.g., beam steering elements that position the imaging beam relative to the process beam), dichroic optics (e.g., dichroic optics configured to combine the dual interferometer sample arms with the beam path of the material processing beam), any other beam delivery optics shared between the interferometer sample arms and the processing beam (e.g., scanning / wobbling galvanometers, processing beam lenses, other lenses, cover glass), and / or any other fiber-based elements (e.g., circulators, isolators, fused fiber couplers, or other fiber-optic elements specific to a particular design or topology). In another embodiment, the processing unit 150 is configured to control the first tunable light source such that the first imaging optical signal generated by the first tunable light source has a first tuning rate dk / dt, and to control at least one other tunable light source such that the imaging optical signal generated by the at least one other tunable light source has a second tuning rate dk / dt that is different from the first tuning rate dk / dt. In another embodiment, the first tuning rate dk / dt is positive, and the second tuning rate dk / dt is negative. In another embodiment, at least a portion of the first imaging optical signal and at least a portion of the imaging optical signal generated by the at least one other imaging optical signal are transmitted simultaneously. In another embodiment, the first tuning rate and the second tuning rate are associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections (which are applied in response to detecting the distortion), which may include: identifying distortion in at least one interferometer output signal of the one or more interferometer output signals associated with at least one of the first tuning rate and the second tuning rate; and performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

[0392] In some embodiments, the SS-OCT system further includes a splitter for splitting the imaging optical signal into at least two arms, and an optical delay element. Figure 1 8B, Figure 1 8C and Figure 1 A non-limiting example of such a configuration is shown in FIG8E (eg, Figure 1 8B and Figure 18C and Figure 18EThe optical delay element (e.g., 825) is configured to delay the output of a first arm of the at least two arms in time relative to the output of a second arm of the at least two arms. In another embodiment, the processing unit 150 is configured to control the tunable light source (e.g., Figure 18B 、 Figure 18C 、 Figure 18E 805), such that the rate of change in time of at least one wave number k of the imaging optical signal (tuning rate dk / dt) includes at least two tuning rates dk / dt. In some embodiments, the at least two tuning rates dk / dt include a positive tuning rate dk / dt associated with the first arm or the second arm, and a negative tuning rate dk / dt associated with the other arm of the first arm or the second arm. According to various embodiments, the first arm is configured to be directed to at least one of the following arms: different reference arms, different sample arms, partially overlapping reference arms, and partially overlapping sample arms of the first interferometer and at least one additional interferometer. This is Figure 18B 、 Figure 18C and Figure 18E For example, in Figure 18B 、 Figure 18C and Figure 18E In the example, the delay arm (passing through the optical delay element 825) is directed to the partially overlapping sample arms of the first interferometer and at least one additional interferometer (e.g., at Figure 18B 、 Figure 18C and Figure 18E , the sample arm 824a (of the first interferometer) and the sample arm 824b (of at least one additional interferometer) share at least one optical element of the overlapping sample arms, such as an optical fiber, before being introduced into the beam delivery head 815. It should be understood that this is merely a non-limiting example and that other embodiments exist in which the delay arms are directed to different reference arms of multiple interferometers, different sample arms of different interferometers, and / or partially overlapping reference arms of multiple interferometers. In some embodiments, the first arm and the second arm are configured to be directed to at least one sample arm and at least one reference arm of the first interferometer and at least one additional interferometer. In some embodiments, the first interferometer and the at least one additional interferometer are configured such that the sample arm of the first interferometer and the sample arm of the at least one additional interferometer share at least one optical element. For example, in Figure 18B 、 Figure 18C and Figure 18DIn the embodiment, the sample arm 824a (of the first interferometer) and the sample arm 824b (of the at least one additional interferometer) share at least one optical element, such as an optical fiber, of the overlapping sample arms before being introduced into the beam delivery head 815. Other examples of optical elements that can be shared are discussed above. In another embodiment, the first arm and the second arm of the imaging optical signal are configured to be directed to the first interferometer and the at least one additional interferometer simultaneously. For example, the delayed arm and the "non-delayed" arm of the imaging optical signal can be directed to one or more sample arms and / or reference arms of the first interferometer and the at least one additional interferometer simultaneously. In another embodiment, the first arm is configured to be directed to the first interferometer or the at least one additional interferometer, and the second arm is configured to be directed to another interferometer of the first interferometer or the at least one additional interferometer. For example, the delayed arm can be directed to the sample arm and / or reference arm of the first interferometer, and the second (non-delayed) arm can be directed to the sample arm and / or reference arm of the at least one additional interferometer (or vice versa). In another embodiment, at least two tuning rates dk / dt of the first arm and the second arm are associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections (which are applied in response to detecting the distortion), and calculating the one or more corrections includes: identifying distortion in at least one interferometer output signal of the one or more interferometer output signals associated with at least one of the positive tuning rates and the negative tuning rates; and performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

[0393] In accordance with at least one embodiment, a SS-OCT system can be configured such that a sample is illuminated using more than one substantially different swept-frequency sources of imaging light.

[0394] In some embodiments, the SS-OCT system can be configured so that two or more optical interferometers are simultaneously illuminated using one or more substantially different swept-frequency sources of imaging light. Some embodiments can involve two or more interferometers that (at least partially) overlap at the sample (or share a common "sample arm"). Details of such embodiments are discussed below.

[0395] In accordance with at least one embodiment, the SS-OCT system can be configured such that the sample is illuminated with more than one substantially different swept source of imaging light. In this case, substantially different sources of imaging light refer to swept sources that differ from each other in at least one characteristic, examples of characteristics including the wavelength of the emitted light, the frequency of the source sweep, the phase of the source sweep, polarization, or coherence with a given reference. Substantially different swept sources of imaging light can be derived from more than one light source. Alternatively, substantially different sources of imaging light can be derived from light emitted from one light source. For example, the light signal can be branched, and at least one of the branched light signals can be modified or delayed by methods known to those of ordinary skill in the art (delay lines, optical buffering, etc.) before reaching the sample arm, so that when each different light signal reaches the sample arm, the imaging light is substantially different from the other imaging lights at the sample arm.

[0396] There are many topologies for generating more than one substantially different source of imaging light. For example, more than one separate light source can be used to generate substantially different imaging light. Various electronic signals can be used to synchronize or coordinate such light sources to achieve the functionality of the optical system. Figure 18A A schematic diagram of a non-limiting example of such a topology is shown in , which is incorporated into an interferometer SS-OCT topology with separate light sources and a shared sample arm.

[0397] Another example incorporates the concept that by splitting light from a single source along two paths, more than one substantially distinct source of imaging light can be generated, and one of these paths can be directed to a substantially different distance than the other. At the output of the path, the light from one path will be substantially delayed compared to the light from the other path. Directing light to a path of selected optical path length in order to introduce delay into its propagation is generally referred to by those skilled in the art as implementing an optical delay line. Figure 18B A schematic example of such a topology is shown in , where a single light source and an optical delay line are used to generate two substantially different lights combined into a dual-interferometer SS-OCT topology.

[0398] The optical delay line can be designed based on one or more desired characteristics of the generated imaging light source, for example to provide a specific phase or timing delay. Due to the selected design parameters of the delay line, when the light reaches the sample arm, the wavelength tuning of the light that has been delayed by the delay line can be, for example, 180 degrees out of phase with respect to the wavelength tuning of the light that has not yet been delayed. The optical path can be designed in a certain way so that the difference in path length is greater than the coherence length of the source, which means that after the light passes through the optical delay line, it will also be less temporally coherent. If or when the two paths overlap, the degradation of the phase relationship (temporal coherence) of the light passing through one path relative to the light passing through the other path will result in a reduction or elimination of interference between the two paths.

[0399] A non-limiting example of an embodiment utilizing a single light source and delay line is as follows: In the case of a swept light source emitting 1060 + / - 30 nm light in an optical fiber with a refractive index of 1.4, to construct an optical delay line for a 180 degree delay at a swept frequency of 100 kHz, a fiber path length difference of approximately 1071.4 m between the individual paths would be required to introduce the desired delay.

[0400] According to at least one embodiment, the above methods for generating two or more light sources can be combined, for example, more than one individual light source can be branched and delayed as many times as needed to generate the desired light source.

[0401] According to at least one embodiment, an SS-OCT system as disclosed herein should be configured such that two or more optical interferometers are simultaneously illuminated using one or more substantially different swept sources of imaging light. Preferred embodiments may involve two or more interferometers that at least partially overlap at the sample (or "sample arm") so that each interferometer can be used to image the same sample. Figure 18A 、 Figure 18B 、 Figure 18C 、 Figure 18D and Figure 18E Three examples of topologies where two different interferometers at least partially overlap are shown in FIG. It should be understood that variations of such interferometers are possible and are considered within the scope of the present disclosure, for example, Figure 18B A version of the interferometer in which K-clock module 1 is included as an optical K-clock but without K-clock module 2, wherein K-clocking of the buffered optical signal is handled by applying a time-delayed replica of the signal from K-clock module 1. Figure 18A An embodiment is illustrated having two different partially overlapping interferometers configured such that each interferometer has a separate tunable light source. Figure 1 8B and Figure 18C illustrates an embodiment of implementing an optical delay line 825, such as by applying the techniques described above in the "Multiple Light Sources" section to generate different swept optical signals for input to each of the partially overlapping interferometers. Figure 18D is another example of a configuration with two individually tunable light sources, illustrating an interferometer topology that can be constructed without the use of an optical circulator, and Figure 18E This concept is illustrated for an interferometer without a circulator, where a buffered source provides a different swept frequency optical signal to input to each partially overlapping interferometer. Variations on the dual interferometer topology based on the interferometer features, topologies, and functionality described in this document are also considered within the scope of this disclosure.

[0402] In accordance with at least one embodiment, multiple interferometers are configured such that they are illuminated by only one swept source, so that the light in each interferometer is not substantially different. For example, an SS-OCT system can be configured with only one light source, but with multiple interferometers that all overlap at the sample arm, each having a different reference arm. By configuring each reference arm differently depending on the requirements of the system, this approach enables simultaneous capture of multiple different depth ranges within the field of view.

[0403] In accordance with at least one embodiment, a plurality of interferometers are configured such that the plurality of interferometers are illuminated by substantially different sources of imaging light. For example, each different interferometer may be configured with swept source light corresponding to a different tuning rate. Where the light in each interferometer is emitted by a different light source, there may not be sufficient spatial and temporal coherence between the two light sources such that light from one light source will interfere with light from another light source corresponding to a different interferometer. Where the substantially different light sources are generated by a single light source, splitter, and delay line, then light paths that are delayed relative to each other by more than the source coherence length will also lack sufficient spatial and temporal coherence to substantially interfere with each other. In this way, as long as the relative coherence of the light in any pair of the plurality of interferometers is negligible, those interferometers and their corresponding interferometer output signals can be considered independently for analysis and processing of the interference pattern, notwithstanding their overlap at the sample arm.

[0404] In accordance with at least one embodiment, two interferometers are illuminated with substantially different light sources such that the instantaneous rate of change of wave number k with respect to time (dk / dt) of the light source corresponding to one interferometer is substantially different (for at least some portion of the duration of the light source sweep) from the instantaneous rate of change of wave number k with respect to time (dk / dt) of the light source corresponding to the other interferometer.

[0405] An example of an application of this embodiment is where at least two interferometers are illuminated with substantially different swept sources, such that for the swept sources in one interferometer, the dk / dt is substantially different compared to the dk / dt of at least one other interferometer at a given point in time (or a series of points in time). The difference in dk / dt may include a difference in the sign of dk / dt. All of the interferometers are combined so that they share a sample arm, and all of the substantially different swept sources illuminate the sample arm simultaneously. As previously discussed, information related to motion artifact distortion can be extracted by evaluating frequency components in the interferometer output signals associated with at least two different values ​​of dk / dt. For this case, when at a given point in time at least two interferometers are illuminated with sources having substantially different dk / dt, interferometer output signals associated with at least two different dk / dt values ​​can be generated simultaneously. This enables the evaluation of motion artifact effects on a time scale shorter than the time scale required to acquire data to generate a full A-line for a given system (a sub-A-line time scale), as well as improving the time domain accuracy of position and velocity characteristics of a workpiece. The observed differences between the interferometer output signals can be used to evaluate motion artifacts.

[0406] In certain embodiments where at least two interferometers are illuminated at a given point in time by sources having substantially different dk / dt, the symmetry between the sweep patterns and dk / dt of the different interferometer sources can be exploited. For example, this symmetry between a positive dk / dt sweep and a negative dk / dt sweep can provide additional information that would not be present if data were captured only from positive or negative dk / dt.

[0407] In certain embodiments where at least two interferometers are illuminated at a given point in time by sources with substantially different dk / dt, where the interferometers share a common sample arm, the signals recovered from the different interferometers / different sources can all correspond to the same sample and show the same interface. Importantly, for interferometers with substantially different dk / dt, the interferometer output signals corresponding to a static interface will typically appear at matching depths in each signal, while the interferometer output signals corresponding to a moving interface will exhibit different distortions based on the difference in dk / dt.

[0408] Interferometer embodiments capable of simultaneously outputting more than one interferometer output signal corresponding to more than one dk / dt are of particular interest because such embodiments offer potential for identifying, characterizing, and correcting motion artifacts in SS-OCT. As described in the "Motion Artifact Correction" section, information corresponding to swept optical signals at two or more tuning rates dk / dt can be used to identify, characterize, and / or correct SS-OCT motion artifacts. In embodiments of interferometers for SS-OCT, at least two interferometers overlap at the sample arm and are illuminated at a given point in time with sources having substantially different dk / dts, so that swept optical signals corresponding to two or more tuning rates dk / dt can be collected simultaneously. Consequently, these simultaneously collected signals can be used to apply the methods for handling motion artifacts developed in the above section. This can present a number of advantages, including the possibility of dealing with motion artifacts on much shorter timescales, the ability to exploit certain symmetries in motion artifact-related distortions, and the ability to more accurately characterize and compensate for fast temporal dynamics in samples containing motion (especially motion that varies on timescales with durations close to or shorter than the duration of the acquisition of one A-line). Relative to methods that rely on capturing signals corresponding to different dk / dt in a substantially time-gated manner, this method can also provide enhanced information about instantaneous sample position and velocity due to the simultaneous capture of signals corresponding to the same sample but different dk / dt. This is because, if the sample arms are appropriately overlapped, the simultaneously captured signals are based on the same sample conditions, whereas the time-gated signals may reflect slightly different conditions due to the evolution of the sample over time. In general, any of the methods developed in the "Motion Artifact Correction" section, including those utilizing advanced signal processing methods and / or ML / AI, are considered applicable / adaptable to interferometer topologies that are capable of simultaneously outputting interferometer output signals corresponding to more than one dk / dt.

[0409] In some embodiments, a beam splitter can be used to split the output from the fiber amplifier. The beam splitter can be configured to balance the power (imaging beam) that illuminates the workpiece from each optical path and / or apply other signal balancing techniques known to those skilled in the art.

[0410] In accordance with at least one embodiment, a light source is configured such that the emitted light comprises two or more substantially different swept optical signals, and the light source is used to illuminate an interferometer for SS-OCT. The different swept optical signals can differ in instantaneous wavelength or wavenumber, dk / dt, sweep rate, or other relevant characteristics. In preferred embodiments, the different swept optical signals are incoherent with each other. Figure 19An example of the configuration shown illustrates an embodiment in which an optical buffer implemented as a delay line fiber is included in a light source to produce output light that is substantially different from non-delayed light. According to at least one embodiment, and as Figure 19 As illustrated, the system can include a light source that is swept simultaneously in two substantially different ways. The delayed light and the non-delayed light can differ substantially in instantaneous wave number k and / or tuning rate dk / dt, and if the length of the fiber buffer is longer than the coherence length of the light source, the light from the buffered arm will not coherent with the light from the non-buffered arm (and vice versa). Figure 19 In the illustrated example, substantially different swept-frequency light from the optically buffered and non-optically buffered paths is recombined and used to illuminate the balance of the SS-OCT interferometer, including the K clock.

[0411] Similar to the dual interferometer topology described above, this interferometer topology enables a sample to be simultaneously illuminated by more than one light source having more than one dk / dt. For example, in at least one embodiment, the light sources are configured such that a swept optical signal having a negative dk / dt and a swept optical signal having a positive dk / dt are swept simultaneously, thereby illuminating the sample for at least a portion of the duration of the imaging process. In addition to evaluating sample velocity based on the relationship between dk / dt and motion artifacts of the sweep, the interferometer output signals derived from this configuration can also be used to evaluate sample position. Additionally, these interferometer output signals, which contain information from multiple dk / dt values, are used to extract information needed to identify and / or correct one or more distortions in the signal resulting from motion artifacts, and more generally support or implement the methods described in the previous section entitled "Motion Artifact Correction." According to at least one aspect, combining information from different dk / dt values ​​in a temporally and spatially consistent manner allows for more efficient correction of motion artifacts. Examples of the increased effectiveness of motion artifact correction achieved by this topology are similar to those described above for the "Dual Interferometer Topology" section.

[0412] dispersion compensation

[0413] In some embodiments of the system, optical elements in the sample arm or the reference arm may cause an optical dispersion mismatch between the arms of the interferometer. In some embodiments, optical elements can be added to the reference arm and / or the sample arm of the interferometer to substantially reduce and / or eliminate the observed optical dispersion mismatch effects. For example, in some embodiments, the reference arm contains optical devices (not shown in the figures) that allow dispersion control. For example, optical devices can be placed in the free space portion of the reference arm to match the dispersion introduced by the free space optics in the sample arm. In some embodiments, computational methods can be applied to numerically compensate for the dispersion mismatch in the measured interferometer output signal. Dispersion compensation methods known to those of ordinary skill in the art can be applied to the system.

[0414] In some embodiments, the optical components in the sample and reference arms are matched (in some embodiments, group delay and higher-order dispersion terms) to reduce any dispersion mismatch between the two arms. This can improve axial imaging resolution. It may also be beneficial to vary this dispersion compensation in the reference arm to match the additional dispersion caused by materials present in the sample. In some embodiments, approximately equal amounts of each optical material can be used in the reference arm path and the sample arm path (including air / vacuum).

[0415] According to one embodiment, at least one of the pre-calculated interferogram and the measured interferogram is shaped to compensate for the dispersion mismatch. For example, compensation can be achieved by controlled modulation of the complex phase and amplitude of the individual elements of the resulting interferogram. The amount of modulation can be determined based on at least one of experimental calibration of the device, mathematical modeling of light propagation, theoretical analysis of the system response, or a combination thereof. For example, for a fixed dispersive element, the relative phase lag / advance per wavelength resulting from the material's dispersion term can be added to each element of the resulting interferogram.

[0416] Optical dispersion caused by the sample being measured can adversely affect the axial resolution of the coherence image. In some embodiments, the sample can induce a wavelength-dependent phase shift in the interference pattern, which can depend on the depth of light propagating within the sample. For example, as described above, complex phase and amplitude modulation algorithms can be used to compensate for these effects. In some embodiments, the dispersion coefficient of the material in the sample can be calculated a priori, or in other embodiments, it can be determined iteratively. It can be assumed that the phase shift caused by the sample increases linearly with increasing penetration into the sample. Thus, each sample acquired at wavenumber "k" can have a specific phase shift determined by the instantaneous wavenumber "k" of the imaging beam and the depth at which the signal returns from the sample. If both the wavenumber k associated with each interferometer output signal and the depth associated with each interferometer output signal can be known a priori (e.g., the wavenumber k or a related quantity can be determined based on the k-clocking method described above), this distortion can be estimated and calculated a priori, and compensation for this distortion can be incorporated into the processing of the interferometer output signals. Alternatively, measurements of the optical signal propagating through the system can also provide dispersion mismatch information for compensation. A dispersion compensation lookup table can be prepared before imaging is performed. In such an embodiment, dispersion correction can be applied with zero additional real-time computational load.

[0417] Frequency modulation and frequency demodulation enhance swept-source OCT

[0418] In some embodiments of the SS-OCT system, a time-domain interference signal (interferometer output signal) is captured by a photodetector and converted into an analog electronic signal or an RF analog electronic signal. The signal can have a variable frequency content and / or a variable instantaneous frequency content. In some embodiments of the system, it may be advantageous to use signal processing techniques known to those skilled in the art (such as those typically associated with FM demodulation) to define the carrier frequency of the signal and use this carrier frequency to demodulate the interferogram. The demodulation process of the signal can produce a signal with a lower frequency than the original analog electronic signal. In embodiments of the system where the frequency of the analog electronic signal associated with the output of one or more SS-OCT system photodetectors is higher than a frequency that can be accurately digitized by an ADC, demodulation can preserve the information carried in the signal while reducing the frequency of the signal, thereby enabling the signal to be accurately digitized while limiting information loss. In some embodiments of the system, demodulation may be driven by design constraints involved in analog signal transmission, as signals with lower frequency content can have improved transmission characteristics and simplified design requirements compared to signals with higher frequency content.

[0419] In some embodiments, the analog electronic signal may instead be modulated by a selected carrier frequency.

[0420] In accordance with at least one embodiment, the processing unit 150 is configured to modulate or demodulate at least one interferometer output signal using a predetermined carrier frequency. For example, in some embodiments, it may be desirable to modulate and / or demodulate an analog electronic signal corresponding to the interferometer output signal in series or in parallel with a number of different carrier frequencies, depending on the characteristics and requirements of the system. In some embodiments, the signal can be demodulated and / or modulated by a carrier frequency selected from a set of possible carrier frequencies, such a set may additionally include an option to not modulate or demodulate the original signal. The carrier frequency selected for modulation or demodulation may be selected and / or adjusted based in whole or in part on a feedback mechanism within the system. For example, the modulation may be adjusted based on a detected change in sample depth or based on feedback of the reference arm position.

[0421] In some embodiments, the modulation or demodulation frequency can be selected in response to some feedback about the state of the system. In an SS-OCT system, for a given light source setting, faster interferogram fringes are typically produced when the path length difference between the sample arm and the reference arm of the interferometer is larger. Therefore, as described above, a demodulation method can be used to reduce the frequency content of the interferometer output signal associated with the larger path length difference in SS-OCT capture, thereby enabling the effective capture of data for, for example, a larger range of sample arm depths without exceeding the Nyquist frequency of the ADC. In some embodiments, it may be preferable to tune the swept light source at different frequencies for different imaging functions. The frequency (time-frequency) of the interferometer output signal will change based on the light source tuning frequency. The demodulation or modulation settings can be changed in response to changes in the light source tuning frequency to allow a system with a given ADC sampling frequency to retain the imaging range for higher light source tuning frequencies.

[0422] In a preferred embodiment, the digitized demodulated signal may be processed in a manner that restores the signal to a state comparable to the original modulation state. In a preferred embodiment, the digitized modulated signal may be processed in a manner that restores the signal to a state comparable to the original demodulated state.

[0423] A key practical benefit of modulation and demodulation methods applied to swept-source OCT systems is that they can help extend the imaging range without requiring digitization to arbitrarily high frequencies. In some cases where the analog bandwidth of the hardware is limited, this can improve signal accuracy or the imaging rate and / or range of the system.

[0424] Use motion signatures to align with material handling beams or process areas

[0425] According to certain embodiments, the identified motion artifact distortion can be used to help align various coordinate systems, including but not limited to the coordinate system of a material processing beam delivery system and the coordinate system of an imaging system. In some embodiments, the processing unit is configured to determine one or more alignments and / or one or more alignment offsets between the coordinate system of the beam delivery system for the material processing beam and the coordinate system of the delivery system for the imaging optical signal. Examples of material processing beam delivery systems include fixed optical heads, 2D and / or 3D scanner heads, an example of a suitable scanner head is a 2D high power scanner system available from IPG Photonics. The material processing beam delivery system may also include, for example, a robotic element configured to move the entire beam delivery head. The coordinates of such a robotic system are included in the range of coordinate systems that can be aligned using motion artifact distortion. In some embodiments, the determination of one or more alignments and / or one or more alignment offsets is based at least in part on the spatial distribution of the distortion in the interferometer output signal.

[0426] In certain embodiments, alignment of the imaging system coordinate system and the material processing beam coordinate system is performed by using one or more guiding elements to scan the imaging beam around the area where the material processing beam source illuminates the workpiece, calibration target, and / or material test specimen.

[0427] When this occurs, the material processing beam is manipulated to cause material modification, thereby causing movement of the sample. According to various embodiments, a beam delivery system for an imaging optical signal includes one or more guiding elements configured to adjust or otherwise control a position of the imaging optical signal relative to a position of the material processing beam, and controlling the guiding elements during acquisition of one or more interferometer output signals such that interferometer output signals associated with different positions of the imaging optical signal relative to the position of the material processing beam are obtained.

[0428] In a preferred embodiment, the imaging beam scanning pattern is based on a coordinate system defined relative to the material modification beam. Embodiments in which the imaging beam scanning pattern is based on other coordinate systems (such as those defined in an absolute sense or relative to other elements of the material modification system) are also possible. As the guiding element scans over the area, SS-OCT data is captured, which can include A-lines, which can then be combined with information derived from the guiding element to form a 2D data capture (such as a B-scan and / or M-mode) or a 3D volumetric data capture (such as a point cloud). By identifying motion artifact distortion in each A-line of the 2D or 3D data, the motion artifact distortion can be correlated with the imaging beam position. In this way, the location of the maximum motion artifact distortion can be determined. A non-exhaustive list of methods that can be applied to 2D or 3D data to assist in this determination includes peak finding, maximum finding, and curve fitting. In some embodiments, the motion artifact distortion on the B-scan can be processed before determining the location of the maximum motion artifact distortion by means including integration, outlier removal, and / or other data filtering methods known to those of ordinary skill in the art.

[0429] When considering this alignment method, it is important to note that the motion of a sample undergoing material modification (and therefore the motion artifact distortion when imaging the process with SS-OCT) can vary based on the characteristics of the material modification process. For example, in a material modification process known to those skilled in the art as keyhole welding, the motion of the sample in, for example, the heat-affected zone, the weld pool, and the keyhole is known to be different due to the material being in, for example, a molten state versus a solidified state, thereby allowing different motion artifact distortion to be observed in each of these regions.

[0430] In some embodiments, motion artifact distortion can be used to help locate the origin of the material processing beam coordinate system relative to the origin of the imaging beam coordinate system, and to align the origins of the two coordinate systems.

[0431] In some embodiments, motion artifact distortion can be used to help characterize optical phenomena that affect the position of the imaging beam relative to the material processing beam, and to align the imaging beam to compensate for such phenomena, such as chromatic aberration caused by the material processing beam optics. For exampl...

Claims

1. A swept source optical coherence tomography (SS-OCT) system for imaging a sample processed by a material processing beam that interacts with material of the sample at a processing region on the sample, the SS-OCT system comprising: An interferometer, the interferometer having: at least one reference arm, at least one sample arm configured to direct imaging optical signals to the processing region, and a tunable light source for generating the imaging optical signal having at least one temporally variable wave number k and a sweep rate in the range of 1 kilohertz (kHz) to 20 megahertz (MHz), inclusive, the interferometer being configured to direct the imaging optical signal to the at least one reference arm and the at least one sample arm, and to combine optical signals returning from the at least one reference arm and the at least one sample arm to generate a combined optical signal; an optical detector configured to detect the combined optical signal and generate at least one interferometer output signal; and A processing unit, wherein the processing unit is configured to: receiving the at least one interferometer output signal; processing the at least one interferometer output signal to determine at least one characteristic of the processing region; detecting distortion in the at least one interferometer output signal, the distortion resulting from a time-varying difference in optical path length between the at least one sample arm and the at least one reference arm; applying one or more corrections to the at least one interferometer output signal in response to detecting the distortion to produce a corresponding corrected interferometer output signal; and At least one corrected interferometer output signal is processed to determine the at least one characteristic of the processing region.

2. The SS-OCT system of claim 1, wherein the at least one characteristic comprises depth information of the treatment region.

3. The SS-OCT system of claim 2, wherein the depth information comprises a range of at least 1 mm, inclusive.

4. The SS-OCT system of claim 3, wherein the depth information comprises a range of at least 5 mm, inclusive.

5. The SS-OCT system of claim 4, wherein the depth information comprises a range of at least 21 mm, inclusive.

6. The SS-OCT system of claim 5, wherein the depth information comprises a range of at least 50 mm, inclusive.

7. The SS-OCT system of claim 2, wherein the material processing beam generates a phase change region (PCR) at the processing region, and the depth information comprises a keyhole depth of the PCR.

8. The SS-OCT system of claim 7, further comprising at least one guiding element that guides the imaging optical signal to one or more selected locations in and / or near the PCR.

9. The SS-OCT system of claim 1 , wherein the processing unit is further configured to control at least one processing parameter of a material modification process implemented by the material processing beam on the sample based on the at least one characteristic of the processing region.

10. The SS-OCT system of claim 1, wherein the processing unit is further configured to determine a sample position based on the at least one corrected interferometer output signal.

11. The SS-OCT system of claim 1, wherein the processing unit is further configured to determine a velocity of the material of the sample based on the at least one corrected interferometer output signal.

12. The SS-OCT system of claim 1 , wherein the processing unit is further configured to determine one or more alignments and / or one or more alignment offsets between a coordinate system of a beam delivery system for the material processing beam and a coordinate system of a delivery system for the imaging optical signal.

13. The SS-OCT system according to claim 1, wherein The processing unit is configured to control the tunable light source so that the rate of change in time (tuning rate dk / dt) of the at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt associated with one or more interferometer output signals, and The processing unit is configured to calculate the one or more corrections, and calculating the one or more corrections comprises: identifying distortion in at least one of the one or more interferometer output signals associated with at least one of the at least two tuning rates; performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

14. The SS-OCT system of claim 13, wherein the at least two tuning rates dk / dt include at least one positive tuning rate dk / dt and at least one negative tuning rate dk / dt.

15. The SS-OCT system of claim 14, wherein performing the evaluation comprises comparing distortion in at least one interferometer output signal associated with a positive tuning rate dk / dt and distortion in at least one interferometer output signal associated with a negative tuning rate dk / dt.

16. The SS-OCT system of claim 13, wherein performing the evaluation comprises comparing distortion in at least two interferometer output signals.

17. The SS-OCT system of claim 13, wherein the distortion corresponds to distortion in one or more geometric aspects encoded in the interferometer output signal.

18. The SS-OCT system of claim 17, wherein the one or more geometric aspects include at least one of position, symmetry, peak width, centroid, geometric second moment, center of mass, amplitude, aspect ratio, and geometric area under the curve.

19. The SS-OCT system of claim 17, wherein performing the evaluation comprises comparing one or more geometric aspects encoded in at least two interferometer output signals.

20. The SS-OCT system of claim 17, wherein performing the evaluation comprises comparing one or more geometric aspects to one or more predetermined thresholds and / or baselines associated with the one or more geometric aspects.

21. The SS-OCT system of claim 20, wherein the one or more predetermined thresholds and / or baselines are established based on at least one of: System and / or component requirements, One or more application requirements, One or more calibrations, One or more models, Hardware and / or software limitations, one or more algorithms, and Basic physics.

22. The SS-OCT system of claim 17, wherein the processing unit is configured to perform the evaluation by comparing the one or more geometric aspects with respect to at least one of: one or more geometrical aspects encoded in at least one other of the one or more interferometer output signals, and One or more predetermined thresholds and / or baselines associated with the one or more geometric aspects.

23. The SS-OCT system of claim 13, wherein performing the evaluation comprises applying a predetermined threshold to the distortion and determining whether the distortion exceeds the predetermined threshold.

24. The SS-OCT system of claim 13, wherein performing the evaluation comprises determining whether a relative difference between two or more distortions exceeds a predetermined threshold.

25. The SS-OCT system of claim 13, wherein the processing unit is further configured to generate a mathematical model based at least in part on one or more characteristics of the SS-OCT system and the assessment of the distortion.

26. The SS-OCT system of claim 25, wherein the mathematical model is further configured to generate an estimate of the magnitude and / or direction of the speed of motion of the sample based on the evaluation of the distortion.

27. The SS-OCT system of claim 13 , wherein the material modification process achieved by the material processing beam on the sample is a welding process, and the material processing beam generates a phase change region (PCR) at the processing region, and the processing unit is configured to: using the evaluation to generate one or more corrections, and One or more generated corrections are used to calculate measurements of one or more features in motion in the PCR.

28. The SS-OCT system of claim 27, wherein the one or more features in motion in the PCR are in motion as a result of the material processing process.

29. The SS-OCT system of claim 13 , wherein the material modification process achieved by the material processing beam on the sample is a welding process, and the material processing beam generates a phase change region (PCR) at the processing region, and the processing unit is configured to: The evaluation is used to generate an estimate of the rate at which the material is processed in the PCR.

30. The SS-OCT system of claim 13, wherein applying the one or more corrections to the at least one interferometer output signal comprises: discarding, weighting, boosting or using said at least one interferometer output signal of said one or more interferometer output signals, or The at least one interferometer output signal of the one or more interferometer output signals is selected to be discarded or used at a later time.

31. The SS-OCT system of claim 13, wherein the interferometer is a first interferometer, and the system further comprises at least one additional interferometer and is configured such that: the sample arm of the first interferometer and the sample arm of the at least one additional interferometer share at least one optical element, a first imaging optical signal configured with one of the at least two tuning rates dk / dt being directed to the at least one reference arm and the at least one sample arm of the first interferometer, A second imaging optical signal configured with another tuning rate of the at least two tuning rates dk / dt is directed to the at least one reference arm and the at least one sample arm of the at least one additional interferometer, and The distortion is identified based on the one or more interferometer output signals of the first interferometer and the at least one additional interferometer.

32. The SS-OCT system of claim 31, wherein the first imaging optical signal and the second imaging optical signal are directed to the treatment region simultaneously.

33. The SS-OCT system of claim 1, wherein the time-varying difference in optical path length is caused by sample motion. The SS-OCT system of claim 33 , wherein the sample moves at a speed greater than 10 mm / s.

35. The SS-OCT system of claim 34, wherein the movement speed of the sample is greater than 100 mm / s.

36. The SS-OCT system of claim 35, wherein the movement speed of the sample is greater than 500 mm / s.

37. The SS-OCT system of claim 36, wherein the movement speed of the sample is greater than 1000 mm / s.

38. The SS-OCT system of claim 37, wherein the movement speed of the sample is greater than 10,000 mm / s.

39. The SS-OCT system of claim 1, wherein the processing unit is configured to derive tracking data from the at least one interferometer output signal, and the one or more corrections are applied to the tracking data.

40. The SS-OCT system of claim 1, further comprising at least one k-clock module that generates a k-clock signal that indicates when a wave number k of the imaging optical signal changes substantially by one or more increments.

41. The SS-OCT system of claim 40, wherein the rate of change of the at least one wavenumber k of the imaging optical signal over time (tuning rate dk / dt) is non-uniform, and the at least one k-clock module is configured to trigger acquisition of the interferometer output signal at uniform increments of the wavenumber k.

42. The SS-OCT system of claim 41, wherein the processing unit is configured to process the at least one interferometer output signal based on interferometer output signals uniformly sampled across wavenumbers.

43. The SS-OCT system of claim 40, wherein the rate of change of the at least one wavenumber k of the imaging optical signal over time (tuning rate dk / dt) is uniform, and the at least one k-clock module is configured to trigger acquisition of the interferometer output signal at uniform increments of the wavenumber k.

44. The SS-OCT system of claim 40, wherein the processing unit is configured to acquire the k-clock signal simultaneously with acquiring the interferometer output signal.

45. The SS-OCT system of claim 44, wherein the processing unit is configured to perform at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wave number k using the acquired k clock signals.

46. ​​The SS-OCT system of claim 44, wherein the processing unit is configured to use the acquired k-clock signal to compute at least one correction for one or more distortions in the interferometer output signal.

47. The SS-OCT system of claim 44, wherein the processing unit is configured to use the acquired k clock signals to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is non-uniformly sampled over k.

48. The SS-OCT system of claim 40, wherein the processing unit is configured to acquire the k-clock signal in a time-gated manner relative to acquisition of the interferometer output signal, and to apply the time-gated k-clock signal to processing of subsequently acquired interferometer output signals.

49. The SS-OCT system of claim 48, wherein the processing unit is configured to perform at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wave number k using the acquired k clock signals.

50. The SS-OCT system of claim 48, wherein the processing unit is configured to use the acquired k-clock signal to compute at least one correction for one or more distortions in the interferometer output signal.

51. The SS-OCT system of claim 48, wherein the processing unit is configured to use the acquired k clock signals to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is non-uniformly sampled over k.

52. The SS-OCT system of claim 48, wherein at least one of the interferometer sample arm and reference arm is configured with one or more optical elements for generating the k-clock signal.

53. The SS-OCT system of claim 40, wherein the at least one k-clock module is configured with a plurality of optical paths for generating the k-clock signal.

54. The SS-OCT system of claim 53, wherein the at least one k-clock module is configured to simultaneously generate the plurality of optical paths by branching the optical signal.

55. The SS-OCT system of claim 54, wherein the at least one k-clock module is configured to make the plurality of optical paths available for selection.

56. The SS-OCT system of claim 1 , wherein the processing unit is further configured to simulate a k-clock signal based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source, the k-clock signal indicating when the at least one wavenumber k of the imaging optical signal changes substantially by one or more increments.

57. The SS-OCT system of claim 56, wherein the processing unit is configured to perform at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wave number k using the simulated k-clock signal.

58. The SS-OCT system of claim 56, wherein the processing unit is configured to use the simulated k-clock signal to compute at least one correction for one or more distortions in the interferometer output signal.

59. The SS-OCT system of claim 56, wherein the processing unit is configured to use the simulated k clock signal to define a discrete Fourier transform method that can be directly applied to an interferometer output signal that is non-uniformly sampled over k.

60. The SS-OCT system of claim 1 , wherein the processing unit is further configured to generate a mathematical model k(t) and / or a tuning rate dk / dt of the imaging optical signal based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source.

61. The SS-OCT system of claim 60, wherein the processing unit is configured to perform at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wave number k using the mathematical model.

62. The SS-OCT system of claim 60, wherein the processing unit is configured to use the mathematical model to calculate at least one correction for one or more distortions in the interferometer output signal.

63. The SS-OCT system of claim 60, wherein the processing unit is configured to use the mathematical model to define a discrete Fourier transform method that can be directly applied to interferometer output signals that are non-uniformly sampled in k.

64. The SS-OCT system of claim 1, wherein the time-varying differences in optical path length are caused by sample motion relative to an axis of the imaging optical signal.

65. The SS-OCT system of claim 1, wherein the time-varying difference in optical path length is caused by a material modification process effected on the sample by the material processing beam.

66. The SS-OCT system of claim 1, wherein the time-varying differences in optical path length are caused by intrinsic sample motion rather than by a material modification process effected on the sample by the material processing beam.

67. The SS-OCT system of claim 1 , wherein the processing unit is configured to control the tunable light source so that the temporal rate of change (tuning rate dk / dt) of the at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt.

68. The SS-OCT system of claim 67, wherein the at least two tuning rates dk / dt include at least one negative dk / dt and at least one positive dk / dt.

69. The SS-OCT system of claim 67, wherein the tunable light source is configured such that the imaging optical signal comprises a superposition of the at least two tuning rates dk / dt.

70. The SS-OCT system of claim 69, wherein the superposition of the at least two tuning rates dk / dt comprises at least one negative dk / dt and at least one positive dk / dt.

71. The SS-OCT system of claim 1, wherein the tunable light source is a first tunable light source and the system further comprises at least one other tunable light source.

72. The SS-OCT system of claim 71 , wherein the interferometer is a first interferometer and the system further comprises at least one additional interferometer, The first interferometer is configured with the first tunable light source, said at least one additional interferometer being configured with said at least one other tunable light source, The first interferometer and the at least one additional interferometer are configured such that the sample arm of the first interferometer and the sample arm of the at least one additional interferometer share at least one optical element.

73. The SS-OCT system of claim 72, wherein the rate of change in time of the at least one wave number k of the imaging optical signal is a tuning rate dk / dt, and the processing unit is configured to: controlling the first tunable light source so that a first imaging optical signal generated by the first tunable light source has a first tuning rate dk / dt, and The at least one other tunable light source is controlled such that an imaging optical signal generated by the at least one other tunable light source has a second tuning rate dk / dt that is different from the first tuning rate dk / dt.

74. The SS-OCT system of claim 73, wherein at least a portion of the first imaging optical signal and at least a portion of the imaging optical signal generated from the at least one other imaging optical signal are transmitted simultaneously.

75. The SS-OCT system of claim 74, wherein The first tuning rate and the second tuning rate are associated with one or more interferometer output signals, and The processing unit is configured to calculate the one or more corrections, and calculating the one or more corrections comprises: identifying distortion in at least one of the one or more interferometer output signals associated with at least one of the first tuning rate and the second tuning rate, performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

76. The SS-OCT system of claim 75, wherein the first tuning rate dk / dt is a positive dk / dt and the second tuning rate dk / dt is a negative dk / dt.

77. The SS-OCT system of claim 1 , further comprising: a splitter, configured to split the imaging optical signal into at least two arms; and An optical delay element is configured to delay in time an output of a first arm of the at least two arms relative to an output of a second arm of the at least two arms.

78. The SS-OCT system of claim 77, wherein the processing unit is configured to control the tunable light source so that the temporal rate of change (tuning rate dk / dt) of the at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt.

79. The SS-OCT system of claim 78, wherein the at least two tuning rates dk / dt include a positive tuning rate dk / dt associated with the first arm or the second arm, and a negative tuning rate dk / dt associated with the other of the first arm or the second arm.

80. The SS-OCT system of claim 79, wherein the interferometer is a first interferometer and the system further comprises at least one additional interferometer.

81. The SS-OCT system of claim 80, wherein the first arm is configured to be guided to at least one of: a different reference arm, a different sample arm, a partially overlapping reference arm, and a partially overlapping sample arm of the first interferometer and the at least one additional interferometer.

82. The SS-OCT system of claim 80, wherein the first arm and the second arm are configured to be directed to at least one sample arm and at least one reference arm of the first interferometer and the at least one additional interferometer.

83. The SS-OCT system of claim 80, wherein The first interferometer and the at least one additional interferometer are configured such that the sample arm of the first interferometer and the sample arm of the at least one additional interferometer share at least one optical element, and The first and second arms of the imaging optical signal are configured to be directed simultaneously to the first interferometer and the at least one additional interferometer.

84. The SS-OCT system of claim 83, wherein The first arm is configured to be guided to the first interferometer or the at least one additional interferometer, and The second arm is configured to be guided to the first interferometer or another interferometer of the at least one additional interferometer.

85. The SS-OCT system of claim 83, wherein The at least two tuning rates dk / dt of the first arm and the second arm are associated with one or more interferometer output signals, and The processing unit is configured to calculate the one or more corrections, and Calculating the one or more corrections comprises: identifying distortion in at least one of the one or more interferometer output signals associated with at least one of the positive tuning rate and the negative tuning rate; performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

86. The SS-OCT system of claim 1, wherein the optical frequency of the imaging optical signal varies at a rate in the range of 8 PHz / s to 2 ZHz / s, inclusive.

87. The SS-OCT system according to claim 1, wherein the processing unit is configured to generate an OCT image based on the at least one interferometer output signal and / or the at least one corrected interferometer output signal, and transmit the generated OCT image to a display device.

88. The SS-OCT system according to claim 1, wherein the processing unit is further configured to generate OCT data based on the processed interferometer output signal and / or the corrected interferometer output signal, and transmit the OCT data to an external device.

89. The SS-OCT system of claim 1, wherein the tunable light source is a tunable vertical cavity surface emitting laser (VCSEL).

90. The SS-OCT system of claim 89, further comprising an amplifier for amplifying the VCSEL.

91. The SS-OCT system of claim 90, wherein the amplifier is configured as a fiber amplifier.

92. The SS-OCT system of claim 90, wherein the amplifier has an output power of at least 20 milliwatts (mW).

93. The SS-OCT system of claim 92, wherein the output power of the amplifier is at least 30 mW.

94. The SS-OCT system of claim 93, wherein the output power of the amplifier is at least 50 mW.

95. The SS-OCT system of claim 94, wherein the output power of the amplifier is at least 100 mW.

96. The SS-OCT system of claim 95, wherein the output power of the amplifier is at least 500 mW.

97. The SS-OCT system of claim 96, wherein the output power of the amplifier is at least 1 Watt (W).

98. The SS-OCT system of claim 97, wherein the output power of the amplifier is at least 5 W.

99. The SS-OCT system of claim 90, wherein the amplifier is configured to have a peak gain at a wavelength between 1010 nm and 1050 nm.

100. The SS-OCT system of claim 90, wherein the amplifier is configured to have a peak gain at a wavelength between 1050 nm and 1090 nm.

101. The SS-OCT system of claim 90, wherein the amplifier is configured with one, two, or three amplification stages.

102. The SS-OCT system of claim 90, having a sensitivity of at least 105 dB.

103. The SS-OCT system of claim 1, wherein the processing unit is further configured to modulate or demodulate the at least one interferometer output signal using a predetermined carrier frequency.

104. The SS-OCT system of claim 1 , further comprising: A digitizer is configured to digitize the at least one interferometer output signal and generate a corresponding digital signal.

105. The SS-OCT system of claim 1 , further comprising a record generator that generates a record of a material modification process achieved by the material processing beam on the sample based on the at least one interferometer output signal at multiple times.

106. The SS-OCT system of claim 105, wherein the processing unit is further configured to evaluate the quality of a weld produced by a material modification process implemented by the material processing beam on the sample based at least in part on the record.

107. The SS-OCT system of claim 1 , further comprising a notification generator for generating notifications related to a material modification process achieved by the material processing beam on the sample based on the at least one interferometer output signal at multiple times.

108. The SS-OCT system of claim 1, further comprising at least one guiding element that guides the imaging optical signal.

109. The SS-OCT system of claim 108, wherein the at least one guiding element is configured such that the imaging optical signal is within 50 nm of a focus of the material processing beam at the processing region.

110. The SS-OCT system of claim 1, further comprising an auxiliary measurement system configured to measure process radiation.

111. The SS-OCT system of claim 1 , further configured to image a sequence of a plurality of material modification processes effected on the sample by the material processing beam.

112. The SS-OCT system of claim 1, further comprising a safety interlock device integrated into the tunable light source.

113. The SS-OCT system of claim 1 , further comprising a safety interlock device integrated into the tunable light source, the safety interlock device configured to enable an eye-safe operating mode of the tunable light source, the eye-safe operating mode being characterized by having a reduced imaging optical emission power.

114. The SS-OCT system of claim 1, further comprising at least one of: a material processing energy source that generates the material processing beam, and a beam delivery system for the material processing beam and the imaging optical signal.

115. The SS-OCT system of claim 114, further comprising a laser head coupled to the material processing energy source and housing the beam delivery system.

116. The SS-OCT system of claim 115, wherein the processing unit is further configured to control at least one of a material processing energy source that generates the material processing beam and the beam delivery system based on the at least one characteristic of the processing region.

117. A material processing system, comprising: The SS-OCT system according to claim 1; a material processing energy source for generating said material processing light beam; as well as A beam delivery system for the material processing beam and the imaging optical signal.

118. The material processing system of claim 117, wherein the beam delivery system is configured with a dichroic optical device configured to combine the imaging optical signal and the material processing optical beam into a combined optical path.

119. The material processing system of claim 118, wherein the dichroic optical device is configured with a transmission spectrum having a first bandedge, a reflection spectrum having a second bandedge, and the first bandedge and the second bandedge have a maximum wavelength separation of 25 nm.

120. The material processing system of claim 118, wherein the beam delivery system is configured to impinge the imaging optical signal on the dichroic optical device over a range of incident angles.

121. A swept source optical coherence tomography (SS-OCT) method for imaging a processing region on a sample processed by a material processing beam, the method comprising: providing an interferometer having at least one sample arm, at least one reference arm, and a tunable light source configured to generate an imaging optical signal having at least one wave number k that is substantially variable in time and a sweep rate in the range of 1 kilohertz (kHz) to 20 megahertz (MHz), inclusive; directing the imaging optical signal to the at least one reference arm and the at least one sample arm of the interferometer, the at least one sample arm being configured to direct the imaging optical signal to the processing region; generating a combined optical signal based on optical signals returned from the at least one reference arm and the at least one sample arm; generating at least one interferometer output signal based on the combined optical signal; processing the at least one interferometer output signal to determine at least one characteristic of the processing region; detecting distortion in the at least one interferometer output signal, the distortion resulting from a time-varying difference in optical path length between the at least one sample arm and the at least one reference arm; applying one or more corrections to the at least one interferometer output signal in response to detecting the distortion to produce a corresponding corrected interferometer output signal; and At least one corrected interferometer output signal is processed to determine the at least one characteristic of the processing region.

122. The SS-OCT method of claim 121, wherein the at least one feature comprises depth information of the treatment region.

123. The SS-OCT method of claim 122, wherein the depth information comprises a range of at least 1 mm, inclusive.

124. The SS-OCT method of claim 123, wherein the depth information comprises a range of at least 5 mm, inclusive.

125. The SS-OCT method of claim 124, wherein the depth information comprises a range of at least 21 mm, inclusive.

126. The SS-OCT method of claim 125, wherein the depth information comprises a range of at least 50 mm, inclusive.

127. The SS-OCT method of claim 122, wherein the material processing beam generates a phase change region (PCR) at the processing region, and the depth information comprises a keyhole depth of the PCR.

128. The SS-OCT method of claim 127, further comprising directing the imaging optical signal to one or more selected locations in and / or near the PCR.

129. The SS-OCT method of claim 121, further comprising controlling at least one processing parameter of a material modification process implemented by the material processing beam on the sample based on the at least one characteristic of the processing region.

130. The SS-OCT method of claim 121, further comprising determining a sample position based on the at least one corrected interferometer output signal.

131. The SS-OCT method of claim 121, further comprising determining a velocity of a material of the sample based on the at least one corrected interferometer output signal.

132. The SS-OCT method of claim 121 , further comprising determining one or more alignments and / or one or more alignment offsets between a coordinate system of a beam delivery system for the material processing beam and a coordinate system of a delivery system for the imaging optical signal.

133. The SS-OCT method of claim 121, further comprising: controlling the tunable light source such that the rate of change in time (tuning rate dk / dt) of the at least one wave number k of the imaging optical signal comprises at least two tuning rates dk / dt associated with one or more interferometry signals; Calculating one or more corrections, wherein calculating the one or more corrections comprises: identifying distortion in at least one of the interferometer output signals associated with at least one of the at least two tuning rates; as well as An evaluation is performed of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

134. The SS-OCT method of claim 133, wherein the at least two tuning rates dk / dt include at least one positive tuning rate dk / dt and at least one negative tuning rate dk / dt.

135. The SS-OCT method of claim 134, wherein performing the evaluation comprises comparing distortion in at least one interferometer output signal associated with a positive tuning rate dk / dt and distortion in at least one interferometer output signal associated with a negative tuning rate dk / dt.

136. The SS-OCT method of claim 133, wherein performing the evaluation comprises comparing distortion in at least two interferometer output signals.

137. The SS-OCT method of claim 133, wherein the distortion corresponds to distortion in one or more geometric aspects encoded in the interferometer output signal.

138. The SS-OCT method of claim 137, wherein the one or more geometric aspects include at least one of position, symmetry, peak width, centroid, geometric second moment, center of mass, amplitude, aspect ratio, and geometric area under the curve.

139. The SS-OCT method of claim 137, wherein performing the evaluation comprises comparing one or more geometric aspects encoded in at least two interferometer output signals.

140. The SS-OCT method of claim 137, wherein performing the evaluating comprises comparing one or more geometric aspects to one or more predetermined thresholds and / or baselines associated with the one or more geometric aspects.

141. The SS-OCT method of claim 140, further comprising establishing the one or more predetermined thresholds and / or baselines, and the establishing is performed based on at least one of: System and / or component requirements, One or more application requirements, One or more calibrations, One or more models, Hardware and / or software limitations, one or more algorithms, and Basic physics.

142. The SS-OCT method of claim 137, wherein performing the evaluation comprises comparing the one or more geometric aspects relative to at least one of: one or more geometrical aspects encoded in at least one other of said interferometer output signals, and One or more predetermined thresholds and / or baselines associated with the one or more geometric aspects.

143. The SS-OCT method of claim 133, wherein performing the evaluating comprises applying a predetermined threshold to the distortion and determining whether the distortion exceeds the predetermined threshold.

144. The SS-OCT method of claim 133, wherein performing the evaluation comprises determining whether a relative difference between two or more distortions exceeds a predetermined threshold.

145. The SS-OCT method of claim 133, further comprising generating a mathematical model based at least in part on one or more characteristics of a SS-OCT system and the evaluation of the distortion.

146. The SS-OCT method of claim 145, wherein generating the mathematical model comprises generating an estimate of the magnitude and / or direction of the velocity of motion of the sample based on the evaluation of the distortion.

147. The SS-OCT method of claim 133, wherein the material modification process achieved by the material processing beam on the sample is a welding process, and the material processing beam generates a phase change region (PCR) at the processed region, and the method further comprises: using the evaluation to generate one or more corrections, and The one or more corrections generated by the evaluation are used to calculate measurements of one or more features in motion in the PCR.

148. The SS-OCT method of claim 147, wherein the one or more features that are in motion during the PCR are in motion as a direct result of the material modification process.

149. The SS-OCT method of claim 133, wherein the material modification process achieved by the material processing beam on the sample is a welding process, and the material processing beam generates a phase change region (PCR) at the processed region, and The method also includes using the evaluation to generate an estimate of the rate at which material is processed in the PCR.

150. The SS-OCT method of claim 133, wherein applying the one or more corrections to the at least one interferometer output signal comprises: The at least one interferometer output signal is discarded, weighted, boosted, used, or selected for discarding at a later time.

151. The SS-OCT method of claim 133, wherein the interferometer is a first interferometer, and the method further comprises: providing at least one additional interferometer, and configuring the first interferometer and the at least one additional interferometer such that a sample arm of the first interferometer and a sample arm of the at least one additional interferometer share at least one optical element; directing a first imaging optical signal configured with one of the at least two tuning rates dk / dt to the at least one reference arm and the at least one sample arm of the first interferometer; directing a second imaging optical signal configured with another of the at least two tuning rates dk / dt to the at least one reference arm and the at least one sample arm of the at least one additional interferometer; as well as The distortion is identified based on the one or more interferometer output signals of the first interferometer and the at least one additional interferometer.

152. The SS-OCT method of claim 151, further comprising simultaneously directing the first imaging optical signal and the second imaging optical signal to the treatment region.

153. The SS-OCT method of claim 121, wherein the time-varying difference in optical path length is caused by sample motion.

154. The SS-OCT method of claim 153, wherein the sample moves at a speed greater than 10 mm / s.

155. The SS-OCT method of claim 154, wherein the movement speed of the sample is greater than 100 mm / s.

156. The SS-OCT method of claim 155, wherein the movement speed of the sample is greater than 500 mm / s.

157. The SS-OCT method of claim 156, wherein the movement speed of the sample is greater than 1000 mm / s.

158. The SS-OCT method of claim 157, wherein the movement speed of the sample is greater than 10,000 mm / s.

159. The SS-OCT method of claim 121, further comprising deriving tracking data from the at least one interferometer output signal and applying the one or more corrections to the tracking data.

160. The SS-OCT method of claim 121, further comprising providing at least one k-clock module configured to generate a k-clock signal indicating when a wave number k of the imaging optical signal substantially changes by one or more increments.

161. The SS-OCT method of claim 160, wherein the rate of change of the at least one wavenumber k of the imaging optical signal over time (tuning rate dk / dt) is non-uniform, and the method further comprises configuring the at least one k clock module to trigger acquisition of the interferometer output signal at uniform increments of the wavenumber k.

162. The SS-OCT method of claim 161, wherein the method further comprises processing the at least one interferometer output signal based on interferometer output signals uniformly sampled across wavenumbers.

163. The SS-OCT method of claim 160, wherein the rate of change of the at least one wavenumber k of the imaging optical signal over time (tuning rate dk / dt) is uniform, and the method further comprises configuring the at least one k clock module to trigger acquisition of the interferometer output signal at uniform increments of the wavenumber k.

164. The SS-OCT method of claim 160, wherein a k-clock module is provided configured with one or more optical elements present in at least one of the sample arm and the reference arm of the interferometer.

165. The SS-OCT method of claim 160, wherein the k-clock signal is acquired simultaneously with the interferometer output signal, and the method further comprises at least one of: performing at least one of sampling, resampling, interpolation, and / or estimation on an interferometer output signal at uniform intervals of wave number k using the acquired k clock signals; using the acquired k-clock signals to compute at least one correction for one or more distortions in the interferometer output signal; and The acquired k clock signals are used to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is non-uniformly sampled over k.

166. The SS-OCT method of claim 160, wherein the k-clock signal is acquired in a time-gated manner relative to acquisition of the interferometer output signal, and further comprising at least one of: performing at least one of sampling, resampling, interpolation, and / or estimation on an interferometer output signal at uniform intervals of wave number k using the acquired k clock signals; using the acquired k-clock signals to compute at least one correction for one or more distortions in the interferometer output signal; and The acquired k clock signals are used to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is non-uniformly sampled over k.

167. The SS-OCT method of claim 121 , wherein the method further comprises simulating a k-clock signal based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source, the k-clock signal indicating when the at least one wavenumber k of the imaging optical signal changes substantially by one or more increments, and further comprising at least one of the following: performing at least one of sampling, resampling, interpolation, and / or estimation on the interferometer output signal at uniform intervals of wave number k using the simulated k clock signal; using the simulated k-clock signal to calculate at least one correction to one or more distortions in the interferometer output signal; and The simulated k clock signals are used to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is unevenly sampled over k.

168. The SS-OCT method of claim 121 , wherein the method further comprises generating a mathematical model k(t) and / or a tuning rate dk / dt of the imaging optical signal based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable light source, and the method further comprises at least one of the following: performing at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals of wave number k using the mathematical model; using the mathematical model to calculate at least one correction for one or more distortions in the interferometer output signal; and The mathematical model is used to define a discrete Fourier transform method that can be directly applied to interferometer output signals that are non-uniformly sampled over k.

169. The SS-OCT method of claim 121, further comprising: controlling the tunable light source such that the rate of change in time (tuning rate dk / dt) of the at least one wave number k of the imaging optical signal comprises at least two tuning rates dk / dt associated with one or more interferometry signals; as well as A mathematical model of the at least two tuning rates is generated based at least in part on one or more characteristics of the tunable light source and one or more characteristics of an SS-OCT system including the interferometer and the optical detector.

170. The SS-OCT method of claim 169, further comprising: correlating an estimate of at least one of a value of k and a tuning rate dk / dt with data sampled from the interferometer output signal and / or a corrected interferometer output signal using the mathematical model; using the associated measured value of said wave number k to estimate the value of k at the sampled interferometer output signal value; and At least one of sampling, resampling, interpolation, and estimation is performed on the interferometer output signal at uniform intervals of k using the associated measurement values ​​of the wave number k.

171. The SS-OCT method of claim 169, further comprising using the mathematical model to calculate at least one correction for one or more distortions in the interferometer output signal.

172. The SS-OCT method of claim 121, wherein the time-varying difference in optical path length is caused by sample motion relative to the axis of the imaging optical signal.

173. The SS-OCT method of claim 121, wherein the time-varying difference in optical path length is caused by a material modification process effected on the sample by the material processing beam.

174. The SS-OCT method of claim 121, wherein the time-varying differences in optical path length are caused by intrinsic sample motion rather than by a material modification process effected on the sample by the material processing beam.

175. The SS-OCT method according to claim 121 further comprises controlling the tunable light source so that the temporal rate of change (tuning rate dk / dt) of the at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt.

176. The SS-OCT method of claim 175, wherein the at least two tuning rates include at least one negative dk / dt and at least one positive dk / dt.

177. The SS-OCT method of claim 175, wherein the tunable light source is controlled such that the imaging optical signal comprises a superposition of the at least two tuning rates dk / dt.

178. The SS-OCT method of claim 177, wherein the superposition of the at least two tuning rates dk / dt comprises at least one negative dk / dt and at least one positive dk / dt.

179. The SS-OCT method of claim 121, wherein the tunable light source is a first tunable light source, and the method further comprises providing at least one other tunable light source.

180. The SS-OCT method of claim 179, wherein the interferometer is a first interferometer and providing the at least one other tunable light source further comprises providing at least one additional interferometer, the first interferometer being configured with the first tunable light source, the at least one additional interferometer being configured with the at least one other tunable light source, and the first interferometer and the at least one additional interferometer being configured such that they share at least one optical element.

181. The SS-OCT method of claim 180, wherein the rate of change in time of the at least one wave number k of the imaging optical signal is a tuning rate dk / dt, and further comprising: controlling the first tunable light source so that a first imaging optical signal generated by the first tunable light source has a first tuning rate dk / dt, and The at least one other tunable light source is controlled such that an imaging optical signal generated by the at least one other tunable light source has a second tuning rate dk / dt that is different from the first tuning rate dk / dt.

182. The SS-OCT method of claim 181, wherein at least a portion of the first imaging optical signal and at least a portion of the imaging optical signal generated from the at least one other imaging optical signal are transmitted simultaneously.

183. The SS-OCT method of claim 182, wherein the first tuning rate and the second tuning rate are associated with one or more interferometer output signals, and the method further comprises: Calculating one or more corrections, wherein calculating the one or more corrections comprises: identifying distortion in at least one of the interferometer output signals associated with at least one of the first tuning rate and the second tuning rate; performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

184. The SS-OCT method of claim 183, wherein the first tuning rate dk / dt is a positive dk / dt and the second tuning rate dk / dt is a negative dk / dt.

185. The SS-OCT method of claim 121 , further comprising providing: A splitter for splitting the imaging optical signal into at least two arms; and An optical delay element is configured to delay in time an output of a first arm of the at least two arms relative to an output of a second arm of the at least two arms.

186. The SS-OCT method according to claim 185 further comprises controlling the tunable light source so that the temporal rate of change (tuning rate dk / dt) of the at least one wave number k of the imaging optical signal includes at least two tuning rates dk / dt.

187. The SS-OCT method of claim 186, wherein the at least two tuning rates dk / dt include a positive tuning rate dk / dt associated with the first arm or the second arm, and a negative tuning rate dk / dt associated with the other of the first arm or the second arm.

188. The SS-OCT method of claim 187, wherein the interferometer is a first interferometer and the method further comprises providing at least one additional interferometer.

189. The SS-OCT method of claim 188, wherein the first arm is configured to be guided to at least one of: different reference arms, different sample arms, partially overlapping reference arms, and partially overlapping sample arms of the first interferometer and the at least one additional interferometer.

190. The SS-OCT method of claim 188, wherein the first arm and the second arm are configured to be directed to at least one sample arm and at least one reference arm of the first interferometer and the at least one additional interferometer.

191. The SS-OCT method of claim 188, wherein The first interferometer and the at least one additional interferometer are configured such that the sample arm of the first interferometer and the sample arm of the at least one additional interferometer share at least one optical element, and The first and second arms of the imaging optical signal are configured to be directed simultaneously to the first interferometer and the at least one additional interferometer.

192. The SS-OCT method of claim 191, wherein The first arm is configured to be guided to the first interferometer or the at least one additional interferometer, and The second arm is configured to be guided to the first interferometer or another interferometer of the at least one additional interferometer.

193. The SS-OCT method of claim 191 , wherein the at least two tuning rates of the first arm and the second arm are associated with one or more interferometer output signals, and the method further comprises: Calculating the one or more corrections, wherein calculating the one or more corrections comprises: identifying distortion in at least one of the one or more interferometer output signals associated with at least one of the positive tuning rate and the negative tuning rate; performing an evaluation of the distortion, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

194. The SS-OCT method of claim 121, wherein the tunable light source is configured such that the optical frequency of the imaging optical signal varies at a rate in the range of 8 PHz / s to 2 ZHz / s, inclusive.

195. The SS-OCT method of claim 121, further comprising: generating an OCT image based on the at least one interferometer output signal and / or the at least one corrected interferometer output signal, and The generated OCT image is transmitted to a display device.

196. The SS-OCT method of claim 121, further comprising: generating OCT data based on the processed interferometer output signal and / or the at least one corrected interferometer output signal; as well as The OCT data is transmitted to an external device.

197. The SS-OCT method of claim 121 further comprising providing the tunable light source as a tunable vertical cavity surface emitting laser (VCSEL).

198. The SS-OCT method of claim 197, further comprising providing an amplifier for amplifying the VCSEL.

199. The SS-OCT method of claim 198, wherein the amplifier is configured as a fiber amplifier.

200. The SS-OCT method of claim 121, further comprising guiding the imaging optical signal using a guiding element.

201. The SS-OCT method of claim 200, wherein the imaging optical signal is directed to within 50 nm of the material processing beam at the processing region.

202. The SS-OCT method of claim 121, further comprising providing a processing unit configured to: process the at least one interferometer output signal; detect the distortion; apply the one or more corrections; and process the at least one corrected interferometer output signal.

203. The SS-OCT method of claim 121, further comprising providing a material processing source configured to generate the material processing beam.

204. The SS-OCT method of claim 203, further comprising controlling at least one processing parameter of a material modification process implemented by the material processing beam on the sample based on the at least one characteristic of the processing region.

205. The SS-OCT method of claim 121, further comprising using an optical detector to generate the at least one interferometer output signal.

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