Swept-source optical coherence tomography with enhanced signal detection

Abstract

1. A swept-source optical coherence tomography (SS-OCT) system for imaging a sample treated with a material treatment beam, the system comprising: an interferometer used in combination with a tunable light source to generate an imaging optical signal having a time-varying wavenumber k and a sweep rate in the range of 1 kHz to 20 MHz; a photodetector configured to detect a combined optical signal from the interferometer to generate at least one interferometer output signal; and a processing unit configured to detect distortions in the at least one interferometer output signal caused by time-varying differences in optical path lengths of the interferometer, and to apply one or more corrections to the at least one interferometer output signal accordingly, for the purpose of determining at least one characteristic of a treatment region on the sample.

Description

[Technical Field]

[0001] Related Applications This application claims the benefit of U.S. Provisional Application No. 63 / 440,486, filed January 23, 2023, entitled "SWEPT-SOURCE OPTICAL COHERENCE TOMOGRAPHY WITH ENHANCED SIGNAL DETECTION," the contents of which are incorporated herein by reference in their entirety.

[0002] 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. [Background technology]

[0003] Optical Coherence Tomography (OCT) is an imaging technique that splits light output from a light source into an imaging beam and a reference beam, detects the interference signal resulting from the superposition of the imaging beam and the reference beam reflected from the surface of an object or sample, and creates 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 by using information about how the beam changes upon reflection.

[0004] A common OCT technique is Fourier domain OCT (FD-OCT), which generally comes in two types: Spectral-Domain OCT (SD-OCT) and Swept-Source OCT (SS-OCT). However, these two systems differ in the type of light source each utilizes and how the interference signal is detected.

[0005] SD-OCT systems use a broadband light source and a spectrally resolved detector system to determine different spectral components in a single axial scan (A-scan) of a sample. Therefore, SD-OCT systems typically decode the spectral components of an interference signal through spatial separation. As a result, the detector system is typically complex because it must simultaneously detect all wavelengths of optical signals within the scan range and then convert them into corresponding interference data sets. This impacts the speed and performance of the SD-OCT system.

[0006] In contrast, SS-OCT systems encode spectral components in time rather than spatial separation. SS-OCT systems typically utilize an instantaneous narrowband imaging light source with an emission wavelength that varies over time, sometimes referred to as a "swept source" or "swept-source." SS-OCT systems acquire A-lines by using such a light source with time-domain optical detection. A spectrogram of the interference light, often referred to as an interferogram, corresponds to the A-line. The spectrogram of the interference light is acquired by sequentially detecting the interference light over time as the wavelength of the light source is changed. The interference signal is typically detected by a non-spectrally resolving detector, one non-limiting example of which includes a balanced-detector photodiode detection system.

[0007] Compared to SD-OCT techniques, SS-OCT is less susceptible to sensitivity degradation at longer imaging depths, offers faster scan speeds and improved signal-to-noise ratios ("SNR"), and reduces detector system complexity. Summary of the Invention

[0008] Aspects and embodiments are directed to methods and systems for using SS-OCT in material modification processes.

[0009] According to one embodiment, there is provided a swept-source optical coherence tomography (SS-OCT) system for performing imaging of a sample treated by a material treatment beam, the material treatment beam interacting with material of the sample at a treatment region on the sample, the SS-OCT system comprising: an interferometer having at least one reference arm and at least one sample arm configured to direct an imaging optical signal to the treatment region; and a tunable light source for generating an imaging optical signal, the imaging optical signal having at least one time-varying wavenumber k and a sweep rate in the range of 1 kilohertz (kHz) to 20 megahertz (MHz), inclusive; the interferometer directing the imaging optical signal to the at least one reference arm and the at least one sample arm and combining the optical signals returning from the at least one reference arm and the at least one sample arm. 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 treatment region, detect a distortion in the at least one interferometer output signal, the distortion being caused by a time-varying difference in optical path lengths 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 the at least one characteristic of the treatment region.

[0010] In one example, the at least one feature includes depth information of the treatment region. In a further example, the depth information includes a range including at least 1 mm. In a further example, the depth information includes a range including at least 5 mm. In a further example, the depth information includes a range including at least 21 mm. In a further example, the depth information includes a range including at least 50 mm. In another example, the material treatment beam forms a phase change region (PCR) in the treatment region, and the depth information includes a keyhole depth of the PCR. In another example, the system further includes at least one directing element that directs the imaging light signal to one or more selected locations within and / or near the PCR.

[0011] In one example, the processing unit is further configured to control at least one processing parameter of a material modification process performed on the specimen by the material processing beam based on at least one characteristic of the processing region.

[0012] In one example, the processing unit is further configured to determine a sample position based on the at least one corrected interferometer output signal.

[0013] In one example, the processing unit is further configured to determine a velocity of a material of the specimen 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 offsets in alignment 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 light signal.

[0014] In one example, the processing unit is configured to control the tunable light source so that a time rate of change (adjustment rate dk / dt) of at least one wavenumber k of the imaging optical signal includes at least two adjustment rates dk / dt associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections, wherein calculating the one or more corrections includes identifying a distortion in at least one of the one or more interferometer output signals associated with at least one of the at least two adjustment rates and performing an evaluation of the distortion, and the one or more corrections for the at least one interferometer output signal are based on the evaluation.

[0015] In one example, the at least two adjustment rates dk / dt include at least one positive adjustment rate dk / dt and at least one negative adjustment rate dk / dt. In a further example, performing the evaluation includes comparing a distortion in the at least one interferometer output signal associated with the positive adjustment rate dk / dt to a distortion in the at least one interferometer output signal associated with the negative adjustment rate dk / dt.

[0016] In one example, performing the evaluation includes comparing distortions in at least two interferometer output signals. In one example, the distortions correspond to distortions 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, center of mass, amplitude, height-to-width ratio, and geometric area under a curve. In one example, performing the evaluation includes comparing one or more geometric aspects encoded in the at least two interferometer output signals.

[0017] 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 physical properties.

[0018] In one example, the processing unit is configured to perform the evaluation by comparing the one or more geometric aspects to at least one of the 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.

[0019] 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.

[0020] 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 assessment of the distortion, hi a further example, the mathematical model is further configured to generate an estimate of a magnitude and / or direction of a velocity of motion of the sample based on the assessment of the distortion.

[0021] In one example, the material modification process performed on the specimen by the material treatment beam is a deposition process, the material treatment beam forms a phase change region (PCR) in the treatment region, and the processing unit is configured to generate one or more corrections using the evaluation and calculate measurements of one or more features moving in the PCR using the one or more generated corrections. In one example, the one or more features moving in the PCR are moving as a result of the material treatment process.

[0022] In one example, the material modification process performed on the sample by the material treatment beam is a deposition process, the material treatment beam forms a phase change region (PCR) in the treatment region, and the processing unit is configured to use the evaluation to generate an estimate of the rate at which the material is processed within the PCR.

[0023] In one example, applying one or more corrections to the at least one interferometer output signal includes discarding at least one of the one or more interferometer output signals, weighting at least one of the one or more interferometer output signals, promoting 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 for later discarding or use.

[0024] In one example, the interferometer is a first interferometer, and the system further includes at least one additional interferometer, wherein the sample arm of the first interferometer and the sample arm of the at least one additional interferometer are configured to share at least one optical element, wherein a first imaging optical signal configured with one of the at least two tuning ratios dk / dt is directed to the at least one reference arm and the at least one sample arm of the first interferometer, and a second imaging optical signal configured with another of the at least two tuning ratios 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 strain is identified based on one or more interferometer output signals of the first interferometer and the at least one additional interferometer. In a further example, the first and second imaging optical signals are directed simultaneously to the processing region.

[0025] In one example, the time-varying difference in optical path length is caused by sample motion. In one example, the sample motion speed is greater than 10 mm / s. In a further example, the sample motion speed is greater than 100 mm / s. In a further example, the sample motion speed is greater than 500 mm / s. In a further example, the sample motion speed is greater than 1000 mm / s. In a further example, the sample motion speed is greater than 10,000 mm / s.

[0026] In one example, the processing unit is configured to derive tracking data from the at least one interferometer output signal, and one or more corrections are applied to the tracking data.

[0027] In one example, the system further includes at least one k-clock module that generates a k-clock signal indicating when a wavenumber k of the imaging optical signal is substantially changed by one or more increments. In one example, a rate of change with time in at least one wavenumber k of the imaging optical signal (adjustment rate dk / dt) is non-uniform, and the at least one k-clock module is configured to uniformly increment the wavenumber k to trigger acquisition of the interferometer output signal. In one example, the processing unit is configured to process the at least one interferometer output signal based on the interferometer output signal sampled uniformly in wavenumbers. In one example, a rate of change with time in at least one wavenumber k of the imaging optical signal (adjustment rate dk / dt) is uniform, and the at least one k-clock module is configured to uniformly increment the wavenumber k to trigger acquisition of the interferometer output signal. In one example, the processing unit is configured to acquire the k-clock signal simultaneously with acquisition of 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 of the interferometer output signal at uniform intervals in 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 applied directly to an interferometer output signal that is not uniformly sampled in k. In one example, the processing unit is configured to acquire the k-clock signals to be time-gated with respect to acquisition of the interferometer output signal, and apply the time-gated k-clock signals to processing of the subsequently acquired interferometer output signal. 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 of the interferometer output signal at uniform intervals in 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 signal to define a discrete Fourier transform method that can be applied directly to the interferometer output signal that is not uniformly sampled at k. In one example, at least one of the interferometer sample arm and the interferometer reference arm is configured with one or more optical elements used to generate the k-clock signal. In one example, the at least one k-clock module is configured with multiple optical paths used to generate the k-clock signal. In a further example, the at least one k-clock module is configured to simultaneously generate multiple optical paths by splitting an optical signal. In a further example, the at least one k-clock module is configured such that multiple optical paths are available for selection.

[0028] In one example, the processing unit is further configured to simulate a k-clock signal indicating when at least one wavenumber k of the imaging optical signal substantially changes by one or more increments 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 one example, the processing unit is configured to use the simulated k-clock signal to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals in 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 interferometer output signals that are not uniformly sampled in k.

[0029] In one example, the processing unit is further configured to generate a mathematical model of k(t) and / or the 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 a further example, the processing unit is configured to use the mathematical model to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals in wavenumber k. In a further 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 interferometer output signals that are not uniformly sampled in k.

[0030] 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 performed on the sample by the material processing beam.

[0031] In one example, the time-varying difference in optical path length is caused by intrinsic sample motion that is not caused by a material modification process performed on the sample by the material processing beam.

[0032] In one example, the processing unit is configured to control the tunable light source such that the time rate of change (tuning rate dk / dt) of at least one wavenumber k of the imaging light signal includes at least two tuning rates dk / dt. In a further 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 such that the imaging light signal includes a superposition of at least two tuning rates dk / dt. In a further example, the superposition of at least two tuning rates dk / dt includes at least one negative dk / dt and at least one positive dk / dt.

[0033] In one example, the tunable light source is a first tunable light source and the system further comprises at least one other tunable light source. In one example, the interferometer is a first interferometer and the system further comprises at least one additional interferometer, the first interferometer configured with the first tunable light source and the at least one additional interferometer configured with the at least one other tunable light source, and the first interferometer and the at least one additional interferometer 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 time rate of change of wavenumber k of at least one of the imaging optical signals is an adjustment 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 adjustment rate dk / dt and to control at least one other tunable light source such that an imaging optical signal generated by the at least one other tunable light source has a second adjustment rate dk / dt different from the first adjustment rate dk / dt. In one 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 other tunable light source are transmitted simultaneously. In one example, the first and second adjustment rates are associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections, wherein calculating the one or more corrections includes identifying a distortion in at least one of the one or more interferometer output signals associated with at least one of the first and second adjustment rates and performing an evaluation of the distortion, and the one or more corrections for the at least one interferometer output signal are based on the evaluation. In one example, the first adjustment rate dk / dt is a positive dk / dt and the second adjustment rate dk / dt is a negative dk / dt.

[0034] 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 such that an output of a first arm of the at least two arms is 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 such that a time 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. In one example, the at least two tuning rates dk / dt include a positive tuning rate dk / dt associated with either 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 a further example, the first arm is configured to be directed to at least one of different reference arms, different sample arms, overlapping reference arms, and 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 the first interferometer and the 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 simultaneously directed to 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 of the first interferometer or the at least one additional interferometer.In one example, the adjustment rates dk / dt of at least two of the first and second arms are associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections, where calculating the one or more corrections includes identifying a distortion in at least one of the one or more interferometer output signals associated with at least one of the positive and negative adjustment rates and performing an evaluation of the distortion, and the one or more corrections for the at least one interferometer output signal are based on the evaluation.

[0035] In one example, the optical frequency of the imaging optical signal varies at a rate within the range of 8 PHz / s to 2 Z Hz / s inclusive.

[0036] 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.

[0037] In one example, the processing unit is further configured to generate OCT data from the processed interferometer output signal and / or the corrected interferometer output signal, and transmit the OCT data to an external device.

[0038] 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 amplifier has an output power of at least 20 milliwatts (mW). In a further example, the amplifier has an output power of at least 30 mW. In a further example, the amplifier has an output power of at least 50 mW. In a further example, the amplifier has an output power of at least 100 mW. In a further example, the amplifier has an output power of at least 500 mW. In a further example, the amplifier has an output power of at least 1 watt (W). In a further example, the amplifier has an output power of 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.

[0039] In one example, the SS-OCT system has a sensitivity of at least 105 dB.

[0040] 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.

[0041] 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.

[0042] In one example, the system further includes a record generator that generates a record of a material modification process performed on the specimen by the material treatment beam based on the at least one interferometer output signal at the plurality of times. In a further example, the processing unit is further configured to evaluate a quality of a weld produced by the material modification process performed on the specimen by the material treatment beam based at least in part on the record.

[0043] In one example, the system further includes a notification generator that generates a notification related to a material modification process performed on the specimen by the material processing beam based on the at least one interferometer output signal at the multiple times.

[0044] In one example, the system further includes at least one directing element that directs the imaging light signal. hi a further example, the at least one directing element is configured such that the imaging light signal is within 50 nm of a focal point of the material processing beam at the processing region.

[0045] In one example, the system further includes an auxiliary measurement system configured to measure process radiation.

[0046] In one example, the system is configured to image a sequence of multiple material modification processes performed on a specimen by a material processing beam.

[0047] In one example, the system further includes a safety interlock device integrated into the tunable light source. hi another example, the safety interlock device integrated into the tunable light source and configured to enable an eye-safe mode of operation for the tunable light source, the eye-safe mode of operation characterized by having a reduced imaging light emission power.

[0048] In one example, the system further includes at least one of a material processing energy source generating the material processing beam and a beam delivery system for the material processing beam and the imaging optical signal. In a further example, the system further includes 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 generating the material processing beam and the beam delivery system based on at least one characteristic of the treatment region.

[0049] In one example, the material processing system includes a 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 a further example, the beam delivery system is configured with a dichroic optical system configured to combine the imaging optical signal and the material processing beam into a combined optical path. In one example, the dichroic optical system is configured with a transmission spectrum having a first band edge and a reflection spectrum having a second band edge, the first band edge and the second band edge having 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 system over a range of angles of incidence.

[0050] According to another exemplary embodiment, a swept-source optical coherence tomography (SS-OCT) method for imaging a treatment region on a sample being treated by a material treatment beam is provided, the method comprising: providing an interferometer including at least one sample arm, at least one reference arm, and a tunable wavelength light source configured to generate an imaging optical signal having at least one wavenumber 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; and receiving optical signals returning from the at least one reference arm and the at least one sample arm. generating a combined optical signal from the sample arms; generating at least one interferometer output signal from the combined optical signal; processing the at least one interferometer output signal to determine at least one characteristic of the treatment region; detecting a distortion in the at least one interferometer output signal, the distortion caused by 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 treatment region.

[0051] In one example, the at least one feature includes depth information of the treatment region. In a further example, the depth information includes a range including at least 1 mm. In a further example, the depth information includes a range including at least 5 mm. In a further example, the depth information includes a range including at least 21 mm. In one example, the depth information includes a range including at least 50 mm. In one example, the material treatment beam forms a phase change region (PCR) in the treatment region, and the depth information includes a keyhole depth of the PCR. In a further example, the imaging light signal is directed to one or more selected locations within and / or near the PCR.

[0052] In one example, the SS-OCT method further includes controlling at least one processing parameter of a material modification process performed on the specimen by the material processing beam based on at least one characteristic of the treatment region.

[0053] In one example, the SS-OCT method further includes determining a sample position based on the at least one corrected interferometer output signal.

[0054] In one example, the SS-OCT method further includes determining a velocity of material of the specimen based on the at least one corrected interferometer output signal.

[0055] In one example, the SS-OCT method further includes determining one or more alignments and / or one or more offsets in alignment 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.

[0056] In one example, the SS-OCT method further includes controlling the tunable light source so that a time rate of change (adjustment rate dk / dt) in at least one wavenumber k of the imaging optical signal includes at least two adjustment rates dk / dt associated with the one or more interference signals, and 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 adjustment rates and performing an evaluation of the distortion, and the one or more corrections for the at least one interferometer output signal are based on the evaluation.

[0057] In one example, the at least two adjustment rates dk / dt include at least one positive adjustment rate dk / dt and at least one negative adjustment rate dk / dt, and performing the evaluation includes comparing a distortion of the at least one interferometer output signal associated with the positive adjustment rate dk / dt to a distortion of the at least one interferometer output signal associated with the negative adjustment rate dk / dt.

[0058] In one example, performing the evaluation includes comparing distortions in at least two interferometer output signals. In one example, the distortions correspond to distortions in one or more geometric aspects encoded in the interferometer output signals. In a further example, the one or more geometric aspects include at least one of position, symmetry, peak width, centroid, geometric second moment, center of mass, amplitude, height-to-width ratio, and geometric area under a curve. In a further example, performing the evaluation includes comparing one or more geometric aspects encoded in the at least two interferometer output signals.

[0059] 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 one or more predetermined thresholds and / or baselines, where 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 underlying physical properties.

[0060] In one example, performing the evaluation includes comparing the one or more geometric aspects against one or more geometric aspects encoded in at least one other of the interferometer output signals and against at least one of one or more predetermined thresholds and / or baselines associated with the one or more geometric aspects.

[0061] In one example, performing the evaluation includes applying a predetermined threshold to the distortion and determining whether the distortion exceeds the predetermined threshold.

[0062] In one example, performing the evaluation includes determining whether a relative difference between two or more distortions exceeds a predetermined threshold.

[0063] 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 assessment of the distortion. In a further example, generating the mathematical model includes generating an estimate of the magnitude and / or direction of the velocity of motion of the sample based on the assessment of the distortion.

[0064] In one example, the material modification process performed on the specimen by the material treatment beam is a deposition process, the material treatment beam forms a phase change region (PCR) in the treatment region, and the SS-OCT method further includes generating one or more corrections using the evaluation and calculating measurements of one or more features moving in the PCR using the one or more corrections generated by the evaluation. In a further example, the one or more features moving in the PCR are moving as a direct result of the material modification process.

[0065] In one example, the material modification process performed on the sample by the material treatment beam is a deposition process, the material treatment beam forms a phase change region (PCR) in the treatment region, and the SS-OCT method further includes using the evaluation to generate a velocity measurement of the material being treated in the PCR.

[0066] In one example, applying one or more corrections to the at least one interferometer output signal includes discarding, weighting, promoting, or using the at least one interferometer output signal, or selecting the at least one interferometer output signal for later discarding.

[0067] In one example, the interferometer is a first interferometer, and the SS-OCT method further includes providing at least one additional interferometer, configuring the first interferometer and the at least one additional interferometer 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, directing a first imaging optical signal configured with one of the at least two tuning ratios 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 two tuning ratios dk / dt to the at least one reference arm and the at least one sample arm of the at least one additional interferometer, and identifying the distortion based on the one or more interferometer output signals of the first interferometer and the at least one additional interferometer. In a further example, the SS-OCT method further includes simultaneously directing the first and second imaging optical signals to the processing region.

[0068] In one example, the time-varying difference in optical path length is caused by sample motion. In one example, the sample motion speed is greater than 10 mm / s. In a further example, the sample motion speed is greater than 100 mm / s. In a further example, the sample motion speed is greater than 500 mm / s. In a further example, the sample motion speed is greater than 1000 mm / s. In a further example, the sample motion speed is greater than 10,000 mm / s.

[0069] 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.

[0070] In one example, the SS-OCT method further includes providing at least one k-clock module configured to generate a k-clock signal indicating when a wavenumber k of the imaging optical signal changes substantially by one or more increments. In one example, a rate of change with time (adjustment rate dk / dt) in at least one wavenumber k of the imaging optical signal is non-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 in wavenumber k. In another example, the SS-OCT method further includes processing the at least one interferometer output signal based on the interferometer output signal sampled uniformly in wavenumber. In one example, a rate of change with time (adjustment rate dk / dt) in at least one wavenumber k of the imaging optical signal 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 in wavenumber k.

[0071] In one example, the k-clock module provided is configured with one or more optical elements present in at least one of the sample arm and reference arm of the interferometer.

[0072] In one example, the k-clock signals are acquired simultaneously with the acquisition of the interferometer output signals, and the SS-OCT further includes at least one of using the acquired k-clock signals to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signals at uniform intervals at wavenumber k, using the acquired k-clock signals to calculate at least one correction for one or more distortions in the interferometer output signals, and using the acquired k-clock signals to define a discrete Fourier transform method that can be directly applied to interferometer output signals that are not uniformly sampled at k.

[0073] In one example, the k-clock signals are acquired in a time-gated manner relative to the acquisition of the interferometer output signal, and the SS-OCT method further includes at least one of using the acquired k-clock signals to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals at wavenumber k, using the acquired k-clock signals to calculate at least one correction for one or more distortions in the interferometer output signal, and using the acquired k-clock signals to define a discrete Fourier transform method that can be directly applied to the interferometer output signal that is not uniformly sampled at k.

[0074] In one example, the SS-OCT method further includes simulating a k-clock signal indicating when at least one wavenumber k of the imaging optical signal changes substantially by one or more increments based at least in part on one or more characteristics of the SS-OCT system and one or more characteristics of the tunable wavelength light source, and the SS-OCT method further includes at least one of using the simulated k-clock signal to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals in wavenumber k; using the simulated k-clock signal to calculate at least one correction for one or more distortions in the interferometer output signal; and using the simulated k-clock signal to define a discrete Fourier transform method that can be directly applied to interferometer output signals that are not uniformly sampled in k.

[0075] In one example, the SS-OCT method further includes generating a mathematical model of k(t) and / or the 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, interpolating, and / or estimating the interferometer output signal at uniform intervals in wavenumber k; using the mathematical model to calculate at least one correction for 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 not uniformly sampled in k.

[0076] In one example, the SS-OCT method further includes controlling the tunable light source such that a time rate of change (tuning rate dk / dt) at least one wavenumber k of the imaging optical signal includes at least two tuning rates dk / dt associated with the one or more interference 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 the SS-OCT system including the interferometer and the photodetector. In another example, the SS-OCT method further includes using the mathematical model to associate estimates of at least one of the value of k and the tuning rate dk / dt with sampled data from the interferometer output signal and / or the corrected interferometer output signal, using the associated measurements of the wavenumber k to estimate the value of k at the sampled interferometer output signal values, and using the associated measurements of the wavenumber k to perform at least one of sampling, resampling, interpolating, and estimating the interferometer output signal at uniform intervals in k. In another example, the SS-OCT method further includes calculating at least one correction for one or more distortions in the interferometer output signal using a mathematical model.

[0077] 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 performed on the sample by the material processing beam. In one example, the time-varying difference in optical path length is caused by intrinsic sample motion that is not caused by a material modification process performed on the sample by the material processing beam.

[0078] In one example, the SS-OCT method further includes controlling the tunable light source such that a time 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. In a further 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 at least two tuning rates dk / dt. In a further 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.

[0079] In one example, the tunable light source is a first tunable light source, and the SS-OCT method further includes providing at least one other tunable light source. In one example, the interferometer is a first interferometer, and providing the at least one other tunable light source further includes providing at least one additional interferometer, the first interferometer configured with the first tunable light source, the at least one additional interferometer configured with the at least one other tunable light source, and the first interferometer and the at least one additional interferometer configured to share at least one optical element. In another example, a time rate of change of at least one wavenumber k of the imaging optical signal is an adjustment 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 the first adjustment rate dk / dt, and controlling at least one other tunable light source such that an imaging optical signal generated by the at least one other tunable light source has a second adjustment rate dk / dt that is different from the first adjustment rate dk / dt. In a further 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 other tunable light source are transmitted simultaneously. In a further example, the first and second adjustment rates are associated with one or more interferometer output signals, and the SS-OCT method further includes 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 first and second adjustment rates and performing an evaluation of the distortion, and the one or more corrections for the at least one interferometer output signal are based on the evaluation. In another example, the first adjustment rate dk / dt is a positive dk / dt and the second adjustment rate dk / dt is a negative dk / dt.

[0080] 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 such that an output of a first arm of the at least two arms is 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 the tunable light source such that a time 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. In a further example, the at least two tuning rates dk / dt include a positive tuning rate dk / dt associated with either 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 different reference arms, different sample arms, overlapping reference arms, and 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 the first interferometer and the 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 simultaneously directed to the first interferometer and the at least one additional interferometer. In a further 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 of the first interferometer or the at least one additional interferometer.In another example, at least two adjustment ratios of the first and second arms are associated with one or more interferometer output signals, and the SS-OCT method further includes calculating one or more corrections, wherein calculating the one or more corrections includes identifying a distortion in at least one of the one or more interferometer output signals associated with at least one of the positive and negative adjustment ratios and performing an evaluation of the distortion, and the one or more corrections for the at least one interferometer output signal are based on the evaluation.

[0081] In one example, the tunable light source is configured such that the optical frequency of the imaging optical signal varies at a rate within the range of 8 PHz / s to 2 ZHz / s, inclusive.

[0082] 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 from the processed interferometer output signal and / or the at least one corrected interferometer output signal and transmitting the OCT data to an external device.

[0083] 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.

[0084] In one example, the SS-OCT method further includes directing the imaging light signal with a directing element. hi a further example, the imaging light signal is directed to be within 50 nm of the material treatment beam at the treatment region.

[0085] In one example, the SS-OCT method includes processing at least one interferometer output signal to detect distortion; Further comprising providing a processing unit configured to apply the one or more corrections and to process the at least one corrected interferometer output signal.

[0086] In one example, the SS-OCT method further includes providing a material treatment source configured to generate a material treatment beam. In another example, the SS-OCT method further includes controlling at least one processing parameter of a material modification process performed on the specimen by the material treatment beam based on at least one characteristic of the treatment region.

[0087] In one example, the SS-OCT method further includes generating at least one interferometer output signal using a photodetector.

[0088] Further aspects, embodiments, and advantages of these exemplary aspects and embodiments are discussed in detail below. Furthermore, 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 may be combined with other embodiments, and references to "an embodiment," "an example," "some embodiments," "some examples," "alternative embodiments," "various embodiments," "one embodiment," "at least one embodiment," "this embodiment and other embodiments," "a particular embodiment," etc. are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. Appearances of such terms herein do not necessarily all refer to the same embodiment.

[0089] 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 a further understanding of various aspects and embodiments, and are incorporated into and constitute a part of this specification, but are not intended as a definition of the limitations of any particular embodiment. The drawings, together with the remainder of the specification, serve to explain the principles and operation of the described and claimed aspects and embodiments. In the figures, each identical or nearly identical component shown in various figures is represented by a like numeral. For clarity, not every component is labeled in every figure. The drawings are as follows: [Brief explanation of the drawings]

[0090] [Figure 1] 1 is a schematic diagram of an example of an SS-OCT system according to an embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram of a balanced photodetector configuration for light detection according to an aspect of the present invention. [Figure 3] FIG. 10 is a schematic diagram of another example of an SS-OCT system showing alternative locations for image beam amplification in accordance with an embodiment of the present invention. [Figure 4] FIG. 1 is a schematic diagram of a beam delivery system according to an aspect of the present invention. [Figure 5A] FIG. 1 is a schematic diagram of an SS-OCT system incorporating auxiliary sensing capabilities according to an embodiment of the present invention. [Figure 5B] FIG. 1 is a schematic diagram of a beam delivery system incorporating auxiliary sensing capabilities in accordance with an aspect of the present invention. [Figure 6] FIG. 2 is a block diagram illustrating logical modules of a multiplexing system according to an aspect of the present invention. [Figure 7A] 1 is a schematic diagram of an example interferometer according to an aspect of the present invention; [Figure 7B] 1 is a schematic diagram of an example interferometer according to an aspect of the present invention; [Figure 8]1 is a comparison of M-mode data from an SD-OCT system (top graph) and an SS-OCT system (bottom graph), according to an embodiment of the present invention. [Figure 9A] 1 is a first example showing both uncorrected weld image signal data and weld image signal data corrected for distortion caused by specimen movement, according to an embodiment of the present invention. [Figure 9B] 10 is a second example showing both uncorrected weld image signal data and weld image signal data corrected for distortion caused by specimen movement. [Figure 10A] FIG. 10 illustrates interferometry signal data according to an aspect of the present invention. [Figure 10B] FIG. 10 illustrates interferometry signal data according to an aspect of the present invention. [Figure 11A] 10 shows uncorrected A-line depth versus amplitude data in accordance with an embodiment of the present invention. [Figure 11B] 10 illustrates A-line depth versus amplitude data with motion compensation applied, in accordance with an embodiment of the present invention. [Figure 12A] 1A-1C show uncorrected and corrected M-mode OCT images, respectively, according to an embodiment of the present invention. [Figure 12B] 1A-1C show uncorrected and corrected M-mode OCT images, respectively, according to an embodiment of the present invention. [Figure 13A] 10A-10C show OCT image data from uncorrected and corrected welding processes, respectively, in accordance with an embodiment of the present invention. [Figure 13B] 10A-10C show OCT image data from uncorrected and corrected welding processes, respectively, in accordance with an embodiment of the present invention. [Figure 14A] 10 shows raw M-mode image data from a welding process according to an embodiment of the present invention. [Figure 14B] 14B shows motion artifact corrected track image data corresponding to the raw image data from FIG. 14A. FIG. [Figure 14C] 14C shows an image of a cross section of a weld made in the welding process of FIGS. 14A and 14B. [Figure 14D] 14C shows an image of a cross section of a weld made in the welding process of FIGS. 14A and 14B. [Figure 15] FIG. 2 is a block diagram of an exemplary computer control system for implementing control and processing in an SS-OCT system, according to an embodiment of the present invention. [Figure 16A] 10A-10C show M-mode OCT images captured during the deposition process on mild steel, aluminum, and copper substrates, respectively, according to an embodiment of the present invention. [Figure 16B] 10A-10C show M-mode OCT images captured during the deposition process on mild steel, aluminum, and copper substrates, respectively, according to an embodiment of the present invention. [Figure 16C] 10A-10C show M-mode OCT images captured during the deposition process on mild steel, aluminum, and copper substrates, respectively, according to an embodiment of the present invention. [Figure 17A] 1 shows exemplary M-mode OCT images captured on the SD-OCT system during the welding process. [Figure 17B] 17B shows an exemplary M-mode OCT image captured on an SS-OCT system, captured simultaneously with the SD-OCT capture of FIG. 17A during the same welding process. [Figure 17C] 17B shows welding image signal data derived from the M-mode OCT image data of FIG. 17A. [Figure 17D] 17C shows fusion image signal data derived from the M-mode data of FIG. 17B. [Figure 18A] FIG. 1 is a schematic diagram of an example SS-OCT system showing a dual interferometer configuration with overlapping interferometers in the sample arm and with separate light sources, according to an embodiment of the present invention. [Figure 18B] FIG. 1 is a schematic diagram of an example SS-OCT system showing a dual interferometer configuration with interferometers overlapping in the sample arm and featuring buffered light sources, according to an embodiment of the present invention. [Figure 18C]FIG. 10 is a schematic diagram of another example of an SS-OCT system having a dual interferometer configuration with interferometers overlapping in the sample arm and featuring buffered light sources, in accordance with an embodiment of the present invention. [Figure 18D] 18B is a schematic diagram of another example of an SS-OCT system having a dual interferometer configuration with separate light sources, according to an embodiment of the present invention, displaying a variation of the interferometer topology shown in FIG. 18A. [Figure 18E] FIG. 10 is a schematic diagram of another example SS-OCT system having a dual interferometer configuration, featuring a buffered light source and overlapping interferometers in the sample arm, in accordance with an embodiment of the present invention. [Figure 19] FIG. 1 is a schematic diagram of an example SS-OCT system showing an interferometer configuration including a buffered light source and generating multiple swept-source optical signals, according to an embodiment of the present invention. [Figure 20A] 1 is a graph illustrating an example of quality degradation over time in a process, according to an aspect of the present invention. [Figure 20B] 1 is a graph illustrating an example of process quality degradation over time corrected via AI / ML monitoring techniques, according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0091] overview Disclosed herein are enhanced SS-OCT devices and methods for monitoring manufacturing processes, particularly for monitoring and controlling the application of lasers and other energy beams to treat 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 devices include a light source, an amplifier (particularly an amplified, fast-tunable light source with a long instantaneous coherence length), an interferometer, a k-clock, scanning optics, integration of the interferometer into the energy beam delivery system (i.e., beam delivery head), detection electronics, signal processing electronics, control electronics, and software and feedback loops (e.g., electronic feedback loops and / or software-enabled feedback loops) through which some aspects of the system control other aspects. These aspects are integrated to perform (in some embodiments, in real time) manufacturing process measurements, including but not limited to, in-line coherent imaging, which is one of several known methods for directly measuring laser weld penetration depth in real time, and perhaps the only known method widely applicable to serial production in many markets. According to at least one embodiment, the SS-OCT system is based on a fast tunable light source, which offers significant advantages in speed, sensitivity, flexibility, and instantaneous imaging range over other conventional OCT systems. In some embodiments, the tunable light source is a high-power light source. Furthermore, commercial implementation of such a light source and processing its signal 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 may be associated with a material modification process, and the characteristic information may correspond to a position or location within and / or near a phase change region (PCR) at times before, during, and / or after the material modification process.According to certain embodiments, the disclosed SS-OCT system is configured to determine accurate position information and direct measurement of the sample's motion velocity (e.g., in the axis of the imaging beam) through evaluation and compensation of motion artifacts caused by interference pattern modulation. In one embodiment, the sample motion is inherent to a material modification process.

[0092] According to various embodiments, the average return signal intensity seen across a given weld is higher with the disclosed SS-OCT system than with an SD-OCT system. Part of the difference in signal level can be attributed to the higher imaging power available from the disclosed SS-OCT system. At least part of the difference in signal level between the two system configurations can be attributed to the higher imaging sensitivity achievable with the disclosed SS-OCT system compared to state-of-the-art SD-OCT systems. However, another contradiction arises from the manifestation of motion artifacts in each of these different approaches to OCT. In the SD-OCT approach, motion artifacts manifest as fringe washout, which reduces useful signal levels and eliminates the ability to acquire and decode this data. In the disclosed SS-OCT approach, motion artifacts manifest as distortions in the interferogram data, resulting in a bright, but potentially distorted, signal. An example of distorted A-lines generated based on distorted interferogram output data is shown in FIG. 11A. FIG. 11B shows corrected A-lines generated based on the same interferometer output data after one or more corrections to the interferometer output data have been applied based on the distortion. These figures are discussed in more detail below. This artifact often appears during imaging of weld keyholes because these structures are known to contain fast-moving metal. Disclosed herein is the implementation of one or more algorithms that allow for correction of this motion artifact.

[0093] Overall system description One non-limiting example of an SS-OCT system 100 according to at least one embodiment is shown in FIG. 1 . The system 100 includes an imaging light source (which may be considered separately or as a component of an interferometer, as described in more detail below), which in this example is a tunable light source 105 that provides coherent narrowband light with a time-varying wavelength, 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 amplified light from an amplifier based on the tunable light source as a seed source may be referred to herein as swept light signal, imaging light, imaging light signal, imaging beam, ICI beam, and / or interferometer light, as appropriate, depending on the context.

[0094] 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 needed to control and communicate with one or more components of system 100.

[0095] For example, the processing unit 150 includes the necessary control electronics for the tunable light source 105 and the amplifier 106, and / or other system components. In some embodiments, a light source control module 109 may be included in the controller 150 that controls the tunable light source 105 and the amplifier 106.

[0096] 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 swept optical signal power; a splitting element 126 that splits the imaging light (imaging optical signal) into two or more paths within the system; an adjustable delay line that functions as a reference arm 122; a sample arm 124 that is configured to direct the swept optical signal to a processing region (i.e., on the workpiece or sample 102) or other region otherwise associated with the material modification process; and a combining element 128 that combines the imaging light after the imaging light has traversed its paths. 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 returning 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 may be implemented. Certain embodiments having these configurations are described in further detail below. The interferometer embodiment shown in system 100 of Figure 1 is a Mach-Zehnder interferometer (MZI) topology, which exhibits improved photon economy compared to other interferometer configurations and the ability to perform balanced detection. The Mach-Zehnder interferometer also includes fiber optic circulators 121, 123 and a polarization controller 127, which are also included in the illustrated embodiment. However, it should be understood that other interferometer topologies, such as a Michelson interferometer, are within the scope of this disclosure.

[0097] The system 100 also includes a material processing beam light source 110 and a respective beam delivery system 115 (which may also be referred to as a beam delivery module and may be housed within the laser head) whereby the imaging light is at least partially combined with the material processing beam using a dichroic 117 and then applied to the workpiece 102 (also referred to herein as a "sample"). The workpiece 102 or sample may be any object or surface undergoing a material modification process (e.g., welding) or otherwise being treated by a material processing beam 112 (also referred to herein simply as a processing beam or process beam). The material processing beam 112 interacts with the material of the sample 102 at a processing region 103 on the sample 102. For example, in some embodiments, imaging may be performed on a sample that is about to undergo or has just undergone a material modification process. Within the beam delivery system 115, there may be beam directing elements, such as galvanometers, that deflect or otherwise steer the imaging beam 108, the material processing beam 112, or both (e.g., scanners 116, 118). In some embodiments, the material processing beam 112 forms a phase change region (PCR) in the processing region 103. In one embodiment, at least one directing element directs the imaging beam to one or more selected locations within and / or near the PCR. The system 100 also includes a photodetector 130 (also simply referred to as a detector) or multiple detectors that receive imaging light, such as a combined optical signal from the reference and sample arms of the interferometer 120. The detector 130 is configured to generate at least one interferometer output signal from the detected combined optical signal. One non-limiting example of the detector 130 is a balanced photodetector (BPD), an example of which is shown schematically in FIG. 2 , which, in this embodiment, enhances system performance by reducing common-mode noise. According to one embodiment, detector 130 is configured as a BPD with an integrated transimpedance amplifier.

[0098] SS-OCT often requires a laser light source (for imaging) with an instantaneous narrowband wavelength that can be rapidly tuned. According to at least one embodiment, the tunable light source 105 generates an imaging optical signal having at least one time-varying wavenumber k and a sweep rate ranging from 1 kilohertz (kHz) to 20 megahertz (MHz), inclusive. The term sweep rate, as used herein, refers to the rate of repetition in the time-varying spectral output of the imaging optical signal. It is the reciprocal of the sweep interval / sweep period. It can also be considered as the reciprocal of the time it takes to acquire one A-line. According to certain aspects, the imaging optical signal substantially has at least one time-varying wavenumber k. The following are examples of imaging optical signals encompassed by this definition: A) an imaging optical signal having an instantaneous coherence length of 1 mm or greater, comprising at least one spectral feature and / or peak, the feature and / or peak comprising multiple wavenumbers, each feature and / or peak having a characteristic wavenumber k (e.g., a characteristic wavenumber k corresponding to the center, centroid, highest intensity point, intensity-weighted centroid of the instantaneous spectrum, etc.) of the instantaneous spectrum, and wherein the characteristic wavenumber k of the feature varies with time and / or varies with time; and B) The imaging light signal as outlined in A, additionally including other spectral features (e.g., secondary peaks, amplified spontaneous emission), which may or may not partially or completely overlap with the peak at any point during operation of the imaging light signal.

[0099] As used herein, the term "modulation rate" refers to the time rate of change of at least one wavenumber k in the imaging light signal. The term "modulation rate" may be referred to interchangeably as "dk / dt" or "modulation rate dk / dt."

[0100] The imaging light signal may include any other light signal that would be suitable for SS-OCT, as would be recognized by one skilled in the art.

[0101] For a linear tuning profile, different beat frequencies in the combined optical signal (referred to as, or otherwise known as, an "interferogram") correspond to different delays or reflections from different depths within the sample. In practice, most lasers do not exhibit an ideal linear relationship between wavenumber k and time. For at least this reason, system 100 may include at least one k-clock module 145 (also referred to herein as k-clocks). In some embodiments, system 100 includes multiple k-clock modules 145. k-clock module 145 provides a k-clock output, the frequency of which is proportional to the wavelength output by light source 105. When this clock is used as a sampling clock, the analog-to-digital output is sampled substantially linearly 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 tunable light source 105, and is implemented via a low-power tap (e.g., about 1%, in some embodiments less) of tunable light source 105. The k-clock, if included, requires its own detector 147.

[0102] System 100 also includes a digitizer 135 that captures detector signals. In certain embodiments, the digitizer is configured to digitize at least one interferometer output signal and generate a corresponding digital signal. Processing unit 150 may incorporate a signal processor that receives the digitized signals from digitizer 135 and calculates data and / or other related information derived from these signals. Modules associated with processing unit 150 generally include, but are not limited to, data processing modules, communication modules, safety modules, and feedback and control modules. As used herein, the term "module" refers to a logical grouping of design functionality, including, but not limited to, optical, electronic, simulated, analytical, and / or computational implementations of such functionality. "Module" does not necessarily imply that modules are nominally physically separable from one another. While embodiment options for multiplexing one or more components are discussed in more detail below, it should be understood that, according to certain embodiments, modules may be shared with other modules and thus may not always be employed in a 1:1 ratio.

[0103] According to at least one embodiment, the tunable light source 105 comprises a microelectromechanical systems (MEMS) tunable vertical-cavity surface-emitting laser (VCSEL) amplified by a rare-earth doped optical fiber amplifier 106 (also referred to herein as a "fiber amplifier"). In one embodiment, the fiber amplifier 106 has a peak gain at wavelengths between 1010 nm and 1050 nm. In another embodiment, the fiber amplifier 106 has a peak gain at wavelengths between 1050 nm and 1090 nm. Common dopants include erbium and ytterbium. According to certain embodiments, the fiber amplifier 106 has one, two, or three amplification stages. In some embodiments, the fiber amplifier architecture may include optical isolation between amplifier stages in the form of compact isolators and / or semi-compact isolators and / or bulk isolators, where applicable. According to 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 traditionally designed to image biological tissues that would be destroyed by such high power levels. Laser material processing is one of the very few application spaces where managing high-energy laser hazards is commercially routine. Fiber amplifiers are an attractive option because of their high power scalability, as well as their high reliability and redundancy. Additionally, with sufficient economies of scale, fiber amplifiers can be less expensive to manufacture than semiconductor optical amplifiers (SOAs) with comparable power and / or reliability. As used herein with respect to amplifiers, reliability refers to the stability of the amplifier's power output over time.Fiber amplifiers can be extremely reliable when combined with redundant pump diodes that can be held in reserve and brought online if other pump diodes begin to degrade in power or otherwise fail. In certain embodiments, a safety module (not explicitly shown in FIG. 1 ) controls the energy ultimately provided to one or more amplifier stages via redundant means so that the device complies with IEC 60825-1.

[0104] According to certain aspects, the higher emitted power provided by the implementation of the fiber amplifier 106 functions, at least in part, to increase the system's sensitivity. It should be understood that other factors, such as improved photon economy, balanced detection, and detector resolution, also contribute to system sensitivity. Sensitivity, as used herein, 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, which 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 can provide a fused image depth of at least 21 mm. In some embodiments, the disclosed SS-OCT system can provide a fused image depth of at least 17 mm, including at least 20 mm, including at least 30 mm, including at least 40 mm, and / or including at least 50 mm. All of these depths are greater than those provided by conventional coherent imaging systems used in material processing applications.

[0105] In a preferred embodiment, the interferometer 120 of the system 100 includes an imaging light source 105 and a corresponding fiber amplifier 106, each configured as a Mach-Zehnder interferometer with a 90:10 split ratio between the sample arm 124 and the reference arm 122, although it should be understood that other split ratios are within the scope of this disclosure as needed to enable higher sensitivity. The reference arm 122 may be configured with an adjustable delay line. The beam delivery system 115 may be configured as or otherwise implemented in a laser welding head, such as the FLW-D50 welding head available from IPG Photonics, Oxford, MA, USA. The beam delivery system 115 may include a beam steering device, such as a galvanometer scanner 116 for the imaging beam. In some embodiments, two or more galvanometers may be used.

[0106] According to one embodiment, the material processing beam source 110 is configured as a Yb: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 performed by the material processing beam 112.

[0107] In one embodiment, the k-clock module 145 is implemented by tapping the imaging light source 105 (at the beam splitter 104) after the amplification stage to illuminate an additional MZI with a slightly mismatched optical path length, coupled to a second photodetector 147, which in one embodiment is configured as a balanced photodetector. As used herein, k refers to optical wavenumber, which is the inverse representation of the optical wavelength. The k-clock generates an interferometry signal that significantly aids in accurate processing of the primary interferometry signal when the light source modulation function is not linear in time (t), i.e., k(t) is not linearly proportional to t. One use of the k-clock signal is to use it to regulate the sampling rate of the digitizer 135 of the processing module 150 so that signal samples are collected at uniform intervals in wavenumber (k).

[0108] The digitizer 135 (also referred to as an analog-to-digital conversion module) receives the interferometer output signal from the photodetector 130 and generates a corresponding digital signal. In some embodiments, the digitizer 135 receives a signal from the k-clock detector 147, digitizes the signal, and / or uses the signal to regulate the sampling rate. The processing module 150 then processes the at least one interferometer output signal to determine at least one characteristic of the processing region and detect distortions in the at least one interferometer output signal. According to various embodiments, the distortion is caused by sample motion, but more broadly, the distortion is caused by a 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; i.e., the distortion is generated by the time-varying optical path length of the sample arm and cannot be caused by intrinsic motion of the sample itself, although the optical path length may also be altered by other changing external factors, such as the environment. The time-varying optical path length of the sample arm can also be caused by the inherent motion of the sample. The time-varying optical path length of the sample arm can be caused by a combination of inherent motion and environmental factors. In response to detecting the distortion, the processing module 150 applies one or more corrections to the interferometer output signals to generate corresponding corrected interferometer output signals and processes the at least one corrected interferometer output signal to determine at least one characteristic of the processing region. For example, in some embodiments, the processing module 150 receives the digital data, optionally performs spectral shaping and background subtraction operations, and calculates frequency analysis and noise floor equalization of the resulting signal to obtain a function of workpiece reflectivity versus optical path length, known to those skilled in the art as an axial line or "A-line." Other operations can be added to this 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 optical dispersion on a known flat surface.In some embodiments, additional operations may be added to this 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 to be substantially linear in k, additional operations may be added to this processing chain to digitally resample the data digitized by digitizer 135 to be substantially linear in k. Such operations may employ digitized captures of the k clock signal, models of the light source and / or system, including mathematical models, or other information to aid in such resampling.

[0109] The beam delivery system 115 of the system 100 includes at least one directing element, such as a scanner 116, configured to adjust the position of the imaging beam 108 relative to the processing beam 112. According to one embodiment, the at least one directing element directs a swept optical signal (the imaging beam 108). In another embodiment, the at least one directing element is configured so that the imaging optical signal (the focal point) is within 50 nm or less of the focal point 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 subsequent A-line acquisition at various locations in and around phase change regions (PCRs). During processes such as laser welding (an example of a material modification process), PCRs are formed in which material confined to the bond region dynamically changes from a solid to a liquid and / or gaseous state and back to a solid state upon completion of welding. By acquiring subsequent A-lines in or around the PCR formed by the material processing beam 112, various measurements of the processing region / workpiece geometry (i.e., features, including geometric features) can be made to guide, influence, and / or monitor the quality of the process and the resulting workpiece.

[0110] Various System Components and Features Light source There are several possible configurations for the imaging light source 105 of the SS-OCT system disclosed herein. A typical configuration of the light source includes at least one seed light source that generates a wavelength-swept signal (also referred to herein as an imaging optical signal or a swept optical signal), which may be referred to as a swept light source, may be based on some input (referred to as a drive signal), and may include any number of amplifiers, optical buffers, feedback mechanisms, and other elements. Certain requirements that the light source must meet include the availability of wavelength sweeps, a sufficiently narrow instantaneous linewidth, a sufficiently broad sweep range, a sufficiently fast sweep speed, sufficient phase stability, and power requirements, as well as wavelength requirements and various other requirements per system parameter. Overall, the exact selection of the light source depends largely on the application requirements. Non-limiting examples of suitable seed swept light sources include solid-state MEMS (microelectromechanical systems) swept-source VCSELs (vertical-cavity surface-emitting lasers), among other possible configurations. One non-limiting example of a suitable MEMS VCSEL light source includes the SL10280 available from Thorlabs, Inc., Newton, NJ, USA. According to at least one embodiment, the tunable laser source 105 is a tunable VCSEL. The selection and / or design of the optical amplifier 106 for embodiments of the present invention incorporating an amplifier must take into account the characteristics of the seed light 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.

[0111] As mentioned above, according to at least one embodiment, the tunable laser source 105 is a high-power light source. According to various embodiments, the tunable laser 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 source 105 has a power in the range of 30 μW to 500 μW, inclusive.

[0112] Optical amplifier As mentioned above, some embodiments of the present invention include an amplifier 106 to increase the available power to a level higher than would be possible using the seed swept source 105 alone. In some embodiments, the amplifier for amplifying the VCSEL is configured as a fiber amplifier (as used in the examples herein), although it should be understood that other types of amplifiers, such as semiconductor optical amplifiers (SOAs) or solid-state amplifiers, are also within the scope of this disclosure. Some embodiments utilize a constant gain, while other embodiments use a variable gain, for example, to implement feedback control of the amplifier 106 output power. More advanced embodiments may incorporate manual or software-based feedback control incorporating, but not limited to, the detector and amplifier, for example, to prevent detector saturation and effectively increase the dynamic range of the system.

[0113] In preferred embodiments of the system, doped fiber amplifiers are used, and 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 informed by the requirements of the application, including, for example, the desired wavelength, time-dynamic characteristics, and gain requirements.

[0114] Certain embodiments may use custom population inversion or time-dynamic pumping to optimize the amplifier gain ratio at each point in the swept-source sweep. In embodiments where the seed swept source is known to vary its output power over time, power level feedback can be used to compensate for long-term source degradation. In embodiments where the seed source's emission power varies predictably and regularly based on the instantaneous emission wavelength, parameters including length and population inversion can be used to optimize the amplifier configuration to partially or fully compensate for power variability. Amplifier pumping can also be performed in a time-dynamic manner synchronized with the adjustment signal to compensate for output power fluctuations across the swept spectrum.

[0115] In some embodiments of the system, feedback control (e.g., via processing unit 150) can be implemented to maximize amplifier power output stability. In some embodiments, feedback control can be implemented in the amplifier system to adjust amplifier gain based on return signals, such as from the primary detector 130 or other sensors throughout the system, thereby adjusting imaging power to increase the effective dynamic range of the system, for example, by increasing imaging power when imaging low-reflectivity surfaces or decreasing imaging power when the detector is saturated. Other embodiments of the system can incorporate variable gain without feedback, for example, by designing a variable gain signal to drive the amplifier based on the imaging requirements being designed for a given process. Feedback control can be implemented, for example, to compensate for degradation of optical transmission or performance due to system aging.

[0116] Some embodiments of the system may incorporate two or more amplifiers 106. The amplifiers may be arranged in series to increase the overall gain available. According to certain embodiments, the amplifiers may be located in various locations within the interferometer itself, as appropriate, based on specific design requirements. One non-limiting example of an SS-OCT system with alternative amplifier arrangements is shown in system 400 of FIG. 3. One or more of the amplifiers 406 as shown in FIG. 3 may be implemented in addition to an amplifier immediately after the seed light source 405 (not shown in FIG. 3), or without such an amplifier. Examples of amplifier placement include placing an amplifier in the sample arm of the system immediately after it splits from the reference arm 422 to deliver increased power to the sample (e.g., amplifier 406a), or placing an amplifier at the end of the sample arm to increase the amplitude of the return signal for imaging (e.g., 406b). Some embodiments of the system may incorporate two or more amplifiers. For example, multiple amplifiers may be located at different locations within the interferometer itself.

[0117] In system embodiments with amplifiers, the electrical configuration can incorporate interlocking features to allow the system to switch between different laser safety classes, e.g., Class 2 vs. Class 3b, for different processes such as manual alignment vs. cell operation.

[0118] delay line All embodiments incorporate one or more delay lines into the interferometer topology to match the path length between the sample arm 124 and the reference arm 122. In some embodiments, the delay lines are designed to allow adjustment of the optical path length, for example, by incorporating precision mechanical components and motors to move mirrors or other optical components. In certain embodiments, the delay lines are manually adjustable, and in certain embodiments, automatic adjustment can be performed, for example, using an electronically controlled device. 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 system's sample arms. Such delay line adjustments can be performed based on pre-programmed algorithms that account for expected variability in sample height within the field of view. Alternatively, such delay line adjustments can be performed in response to system conditions, for example, via feedback control.

[0119] Certain embodiments incorporate an adjustable delay line on the reference arm of the system. This topology allows for adjustment of the depth field of view of the OCT system, thereby allowing imaging targets of interest to be positioned within the field of view without having to change the mechanical configuration of the target, greatly simplifying configuration and increasing flexibility.

[0120] Some embodiments of the system incorporate an adjustable delay line in the k-clock interferometer 145. Adjusting the path length in the k-clock allows the k-clock frequency to be adjusted, enabling k-clock optimization of a single system at several different seed source sweep rates. For example, if the seed source sweep rate is slowed down, the k-clock path length separation can be increased, resulting in a larger available imaging depth and more available points for acquisition within the sweep while maintaining appropriate frequency characteristics of the sampling and digitization system (because the seed source sweeps slower and therefore dk / dt is slower). Alternatively, a similar effect can be achieved by making several discrete paths of different lengths available to the k-clock using techniques (e.g., shutters, mechanical devices) to generate and / or switch between multiple different optical paths.

[0121] Feedback Control In some embodiments, active feedback control (through functionality of processing unit 150) is implemented within and between various system components and / or modules, including, but not limited to, optical swept source 105, amplifier 106, delay lines (such as reference arm 122), and photodetector 130. In some embodiments, feedback control or feedback loops may include implementation of electronic feedback loops and / or software-enabled feedback loops.

[0122] According to at least one embodiment, various feedback controls of the optical amplifier 106 are implemented. Feedback controls can be implemented within the amplifier module to maintain a stable power output over varying operating conditions. The amplifier module controls also receive input from the system controller 150, which can, for example, enable automatic increases in beam power to compensate for low-reflectivity imaging target materials and enable the OCT system to achieve higher sensitivity. Output power control in all embodiments is subject to necessary interlocks and other safety measures to comply with relevant laser safety product standards.

[0123] To utilize optimization for various situations, feedback control may also be applied to system components including, but not limited to, k-clock arm length, reference arm length, source sweep rate, and source sweep span.

[0124] According to further embodiments (and as discussed elsewhere in this disclosure), feedback control is implemented to control the material processing laser source 110 and / or the beam delivery system 115 using information obtained from the SS-OCT system 100 (and / or other components, such as the auxiliary measurement system 160) and processed by the controller 150. For example, the controller 150 may use processed output obtained from the interferometer 120 (and / or associated components, such as the photodetector 130, digitizer 135, etc.) to control one or more processing parameters (non-limiting examples of which are provided below) of the beam delivery system 115 and / or the material processing beam source 110. The processed output may include, for example, at least one characteristic of a phase change region.

[0125] detection SS-OCT and time-domain imaging approaches typically do not use detectors that can substantially distinguish between different frequencies of light. This means that such detectors are susceptible to being overloaded by inconsistent process emissions and / or high-power correction energy. 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 line (e.g., fiber Bragg gratings), or at the detector, to separate the imaging light from unwanted signals can be employed for the material processing applications described herein. Balanced detection is another method of removing these unwanted signals and can be applied in addition to or instead of blocking filters.

[0126] According to at least one embodiment, the photodetector 130 is configured as a balanced photodetector (BPD). BPD circuitry is employed to convert differential interferometry signals from the optical domain to the electronic domain for both OCT data and k-clock data. Using a BPD to detect differential signals can eliminate common-mode noise and DC components from interferogram data collected during OCT capture and improve the amplitude of the acquired signal relative to unbalanced detection methods. A schematic example of a photodetector 130 configured as a balanced photodetector is shown in FIG. 2.

[0127] According to one or more embodiments, the analog electronic gain of the BPD signal is adjustable and / or selectable to enable ideal signal levels for digitization. In some embodiments, feedback control is implemented to automatically optimize the BPD gain level. In other embodiments, gain control is implemented by a user or some combination of user control and feedback control.

[0128] In certain embodiments, an electronic amplifier is included in the BPD circuit, which has characteristics of noise reduction, adequate bandwidth, and common-mode and DC signal rejection. The design of the detection module of one or more embodiments also takes into account analog bandwidth filtering for the specific purpose of anti-aliasing. Additional amplifier optimizations can be implemented in other system embodiments, as needed. The amplifier topology is designed using circuit techniques known to those skilled in the art.

[0129] According to other embodiments, alternative optical signal detection methods may be used, including photodiodes and other detector technologies known to those skilled in the art. These embodiments may also include variable gain, which may or may not be feedback controlled, and amplifier topologies optimized for particular applications. Embodiments employing different optical detection methods, for example, different detection methods in the k-clock and OCT signal portions of the system, are also possible.

[0130] In some embodiments, different portions of the interferometer output are split to multiple detectors. In this way, even if one detector channel saturates because the reflected signal is too bright, another detector channel can read the signal. In this way, the dynamic range of the overall system can be substantially increased. In certain embodiments, additional attenuation in the optical and / or electronic domains can be included at different levels for different detectors to enhance this effect and provide an even greater dynamic range.

[0131] K Clocking Method Swept-source OCT (SS-OCT) systems may use a signal that indicates when the wavenumber k of an imaging optical signal or signal spectrum changes substantially by one or more increments, known to those skilled in the art as a k-clock. Some embodiments of the present invention may use a sampling clock that samples at uniform time intervals. Some embodiments of the present invention may use a sampling clock in which the wavenumber k is uniform, 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 light source adjustment method and other system considerations.

[0132] According to at least one embodiment, the imaging light source generates a swept light signal with a variable rate of change of wavenumber k with respect to time (referred to as the modulation rate dk / dt). In some embodiments, the modulation 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 ZHz / s to 2 ZHz / s, inclusive. Most practical implementations of SS-OCT systems require resampling or k-clocking of the interferometer output signal sampling to compensate for variations, including instability in the sweep phase, variability in dk / dt throughout the sweep, and / or nonlinearities in the modulation of the swept light source (such as a nonlinear relationship between wavenumber k and time t). The use of a signal providing some measure of the relative or absolute wavenumber of the imaging light source light with respect to time, referred to herein as a “k-clock signal,” to aid 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 may be caused by asymmetric behavior of the light source due to, for example, intentional driving, fundamental characteristics of the light source, or an unavoidable consequence of manufacturing the light source.

[0133] According to some embodiments, at least one of the interferometer sample arm and the interferometer reference arm is configured with one or more optical elements that are used to generate the k-clock signal. In some embodiments, these optical elements are already present, while in other embodiments, these one or more optical elements are added and used to generate the k-clock signal.

[0134] According to at least one embodiment, the SS-OCT system requires applying a Fourier transform to the interferometer data to generate an OCT image. To generate an OCT image that is appropriately scaled in meaningful units of physical space, the Fourier transform must be performed on a signal uniformly sampled in k, or a method (e.g., a homodyne matrix DFT) must be employed that otherwise corrects 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 simulated methods) must be incorporated to properly process or sample the interferometer output signal for the Fourier transform and / or to properly construct the Fourier transform for the interferometer output signal.

[0135] The k-clock, in some embodiments, may be generated by an optical k-clock module (e.g., k-clock module 145) that generates a signal indicating each time the swept source 105 is adjusted through a predetermined k (inverse frequency) increment of the scan band. The k-clock, in some embodiments, may be generated by an electronic k-clock module that estimates the time-domain behavior of the wavenumber k of the swept-source output based on the adjustment signal applied to the swept source. The k-clock may also, in some embodiments, be simulated based on a model of the system. In some embodiments, a combination of the described approaches may be used. The k-clock is used to correct for nonlinearities in the time domain of the frequency sweep of the swept source 105.

[0136] In one embodiment, the k-clock module 145 is configured to trigger sampling of the interferometer output signal by the photodetector 130 at uniform intervals in wavenumber k. The rate of change of wavenumber k may or may not be uniform in time. This embodiment represents a preferred implementation of k-clocking in an SS-OCT system in which the dk / dt of the imaging swept source 105 is non-uniform over time. In system embodiments implemented with sampling at uniform intervals in wavenumber, the processing unit may be configured to process the interferometer output signal sampled uniformly in wavenumber, which may provide advantages, for example, in terms of measurement accuracy or computational efficiency. In another embodiment, the controller 150 is configured to acquire the k-clock signal simultaneously with acquisition of the interferometer output signal. In some embodiments, the controller 150 (processing unit) is configured to acquire the k-clock signal simultaneously with acquisition of the interferometer output signal, with both signals being acquired simultaneously at uniform time increments. In one or more of these embodiments, the k-clock measurement signal and any derivatives thereof may be used by processing unit 150 to sample, resample, interpolate, and / or estimate the interferometer output signal at uniform intervals in wavenumber k using digital processing approaches. The k-clock measurement signal may 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 may be used by processing unit 150 to define a discrete Fourier transform method that can be directly applied to interferometer output signals that may not be uniformly sampled in k. One example of such a method is using the k-clock signal as an input to calculate an appropriate matrix for implementing a matrix-DFT that can be applied to interferometer output data that is uniformly sampled in time but not uniformly sampled in k. In one embodiment, k-clock module 145 is temperature stabilized.

[0137] The use of a k-clock as the sample clock results in evenly spaced interference data in the optical wavenumber domain, or k-space, providing maximum SNR and axial imaging resolution for subsequent Fourier transform-based signal processing of the acquired interferometer output signal. The Fourier transform provides A-scan information, or an axial scan depth profile within the sample. The OCT system may also digitally resample or interpolate the interference data set using wavenumber information acquired via sampling the k-clock signal in the time domain to achieve uniform k-space sample spacing.

[0138] According to at least one embodiment, a k-clock module is used. The k-clock signal is generated to assist the system in processing OCT interferometer data. Fourier transform processing of the OCT interference signal is simplified when discrete signal samples are evenly spaced in units of wavenumbers (k). Tunable laser sources, such as the tunable light source 105, are often not tuned so that their wavenumbers are linearly proportional to time. The k-clock generates a signal that oscillates in time at equal intervals of wavenumbers. By detecting the k-clock oscillations (typically defined by the signal's "zero crossings" after low-frequency components have been removed), a relationship between the time domain and a domain that is substantially linear in k can be established. According to one or more aspects, the k-clock measurement signal and any derivatives thereof may be used to sample, resample, interpolate, or estimate the OCT interferometer output signal at uniform intervals in wavenumber k. The sampling, resampling, interpolation, or estimation may be performed using digital processing approaches, analog continuous-time signal processing, in either the electrical or optical domain, or alternatively, before, during, or after digitization of the signal.

[0139] In some embodiments, the k-clock signal is generated using an optical path that is independent of the optical path used for OCT measurements. The independent k-clock design can incorporate devices, including, but not limited to, free-space and fiber-based optical devices, in fixed and / or adjustable configurations known to those skilled in the art. For example, the k-clock can include a delay line and / or an adjustable delay line. In some examples, multiple k-clocks that can be selected or hot-swapped can be used. In some embodiments, the k-clock includes multiple reflectors designed to be tuned to various desired frequencies.

[0140] In other embodiments, the k-clock signal is generated using the same optical path 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., physical surfaces) or dynamic (e.g., galvanometers), and the k-clock optical path may include any combination of such elements, as well as others. 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 skilled in the art. In other embodiments, features already present in the OCT system (i.e., not added for the express purpose of forming the k-clock path) may comprise some or all of the elements in the k-clock optical path.

[0141] In some embodiments, the k-clock signal may be acquired (e.g., by processing unit 150) so as to be time-gated with respect to the interferometer signal, such as during a calibration phase, and stored for application to at least one subsequently acquired interferometer capture. Such an approach may be implemented using optical features within the OCT interferometer and may obviate the need for a separate k-clock optical path. Applying time gating may also obviate the need for a separate detection module for the k-clock signal.

[0142] In some embodiments, various k-clock paths are available, and the k-clock path in use can be switched during operation or idle times. Multiple k-clocks can be generated by splitting a swept optical signal into multiple simultaneous paths; in other embodiments, the k-clock path can include actuating 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, with selection made via electronic control of the data acquisition and sampling system.

[0143] In some embodiments, k-clock module 145 comprises at least one optical component. In certain embodiments, k-clock module 145 comprises at least one electronic component. In some embodiments, k-clock module 145 comprises at least one computational and / or simulated component. According to certain embodiments, k-clock module 145 comprises at least two of at least one optical component, at least one electronic component, and at least one computational and / or simulation component.

[0144] According to at least one embodiment, non-limiting examples of optics that may be included in K-clock module 145 may include fiber-based optics and / or free-space optics, and / or some combination thereof. For example, in embodiments in which the k-clock signal is generated using an optical path that is separate from the optical path used for OCT measurements, this optical path may include a beam splitter in the form of a fused fiber coupler or a beam splitter cube to split the light entering the K-clock device along multiple paths. As another example, in embodiments in which the K-clock signal is generated using the same optical path as used for OCT measurements, partially reflective optical elements, such as optics with dichroic coatings, may be used.

[0145] The electronic components in some embodiments may perform a variety of functions in a given K-clock embodiment, including electronic photodetectors and associated electronics for signal processing and / or digitization of the K-clock signal. Electronic components may also be used, for example, in simulation, timing synchronization, and / or control of opto-mechanical components, applying techniques known to those skilled in the art.

[0146] As used herein, the terms "simulated" or "simulation" generally refer to the solution of a model by numerical or analytical methods. According to some embodiments, a simulated component may 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, a simulated component may take measured values ​​based on a physical system as inputs to the simulation inputs. Such values ​​may include values ​​measured by the system itself during operation. Such values ​​may include values ​​measured prior to operation using at least one of the system components and / or other instrumentation, as well as inputs to the system (e.g., calibration data). For example, a simulated K-clock component may be synchronized via an electronic signal to a line trigger of an SS-OCT system and generate a simulated time-domain trace of the K-clock corresponding to the desired behavior of the K-clock trace after such trigger based on programmed, calibrated, and / or detected conditions.

[0147] In some embodiments, 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. This mathematical model can be applied to associate an estimate of at least one of the k value or adjustment rate dk / dt with data sampled from the interferometer output signal. In some embodiments, processing unit 150 is configured to simulate a k-clock signal indicative of when the wavenumber k of the imaging optical signal or signal spectrum is substantially changed by one or more increments. The simulated k-clock and / or mathematical model of k (also referred to as k(t)), or the adjustment rate dk / dt thus obtained, can be used to facilitate any of the operations described above as operations facilitated by physical, optical, or electronic embodiments of the k-clock, including, as appropriate, triggering acquisition of the interferometer output signal at uniform intervals in wavenumber k. This model and / or simulated k-clock may also be used to facilitate resampling, interpolating, or estimating values ​​of an interferometer output signal sampled at uniform intervals at wavenumber k when the original sampling clock was not uniform at wavenumber k. This model and / or simulated k-clock may also be used to calculate at least one correction for one or more distortions in the interferometer output signal. This model and / or simulated k-clock may also be used to calculate a discrete Fourier transform method that can be directly applied to an interferometer output signal that may not be uniformly sampled at k. In some embodiments of the system, the simulated k-clock and / or mathematical model of dk / dt uses the conditioning signal provided to the swept light source as input, and the model may be capable of modeling dk / dt based on a light source conditioning waveform of arbitrary complexity.

[0148] According to at least one embodiment, the processing unit is configured to control the tunable light source such that the time rate of change (tuning rate dk / dt) of at least one wavenumber k of the imaging light signal 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 are interferometer output signals with time-varying Dk / dt, the at least two tuning rates dk / dt are associated with the one or more interferometer output signals corresponding to different points in time.

[0149] According to at least one embodiment, the sweep rate of the light source is varied (typically at the expense of the imaging frequency) to adjust the range / depth field of view of the system. As previously mentioned, the term sweep rate refers to the rate of repetition in the time-varying spectral output of the imaging light signal. It is the inverse of the sweep interval / sweep period. It can also be thought of as the inverse of the time it takes to acquire one A-line. When the sweep rate of the system is reduced, the frequency of the signal provided by the fixed k-clock is also reduced. In some embodiments where the sample rate is independent of the k-clock frequency, the number of samples acquired by the primary detector between k-clock ticks can be increased (e.g., to increase the axial resolution of the A-lines), or the sample rate can be adjusted to maintain the original ratio of the sampling clock to the k-clock. In some embodiments where the sample rate is dependent on the k-clock frequency, no adjustment of the sampling rate is necessary when the sweep rate is adjusted. In some embodiments where the sample rate depends on the k-clock frequency, the k-clock may be adjusted through mechanical, optical, digital, or other means in coordination with the imaging frequency to enable other imaging capabilities, e.g., to increase the imaging distance without increasing the frequency, to extend the k-clock delay while slowing down the adjustment of the light source.

[0150] 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 a particular digitizer (e.g., digitizer 135) configuration (which may generally incorporate a maximum sampling rate) and / or any subsequent interpolation techniques (e.g., interpolated sweep), thus making these respective approaches more effective.

[0151] According to certain embodiments, the k-clock frequency is monitored relative to another electronic clock within the processing electronics. By monitoring / measuring the rate of change of the light source k (i.e., dk / dt in Leibniz notation, or k-dot, or light source wavenumber sweep rate / adjustment rate), important information can be gleaned for compensation of distortions caused by workpiece motion during the waveform sweep, known to those skilled in the art as motion artifacts. In some embodiments, in addition to using the k-clock signal to drive the OCT interferometer sample clock, time-domain sampling of the k-clock can be performed, which can be achieved using electronic design techniques known to those skilled in the art. Such time-domain measurement of the k-signal is motivated by the usefulness of this signal in some embodiments of motion artifact compensation. Further information regarding the concept of motion artifacts and the consequences of these artifacts on the system, and more specifically, on the design and performance of the k-clock, is discussed below in the section on correcting motion artifacts.

[0152] According to some embodiments, a line start signal is generated at a specific wavelength for each sweep to provide additional information (e.g., for system synchronization). This signal may 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 may be generated in-line with a selected k-clock or OCT optical path embodiment, or it may be generated separately from other optical paths using a tap on the optical source.

[0153] In certain embodiments, signal processing operations are performed on the k-clock signal to enhance 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 light source, using methods known to those skilled in the art. In one embodiment, the value of k at at least one point in the sweep is measured separately and called upon 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 are possible and within the scope of the present disclosure to enhance the data collected from the system.

[0154] Optimized swept source drive The SS-OCT system 100 includes an imaging light source 105 configured as a swept light source that can be driven by a drive signal (e.g., an electrical signal transmitted by a controller 150). The appropriate selection of a drive source and pattern for generating the drive signal is believed to be important to the function of the device, as the drive has a strong effect on the interferogram generated by the optical module of the device. A nonlinear relationship may exist between the applied drive pattern and the spectral emission of the swept light source.

[0155] Certain embodiments may drive the swept source using standard waveforms such as ramp functions, sawtooth functions, or sinusoidal waves. Other embodiments may incorporate more complex drive waveforms, such as drive signals specifically designed to generate a swept laser signal that is linear in wavenumber k or designed to maximize device lifetime. Some embodiments may incorporate various swept drive waveforms to allow adjustment of sweep speed, adjustment rate, span, or waveform shape to monitor different aspects of the laser process, such as (for example) keyhole measurement during laser welding or scanning of a completed weld. Adjustments to the swept drive waveform of the swept source, including, by way of non-limiting example, adjustment of the sweep drive frequency, the rate of change (slope) of the swept drive waveform, 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 (suppress in some embodiments, but enhance in some embodiments) 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.

[0156] To increase the effective sweep speed, certain embodiments may incorporate optical buffering or interleaving of the swept source light. These embodiments of the present invention may incorporate a combination of optical paths, shutters, couplers, and other optical elements to achieve an effective sweep speed greater than the physical sweep speed of the light source. Optical buffering may be implemented at any point in the system, including, for example, either before or after amplifier 106, as appropriate for performance for a particular application.

[0157] In some embodiments, the swept source drive rate can be varied between tasks, or in some cases, during a task, to allow for optimization of the source sweep. Varying the source sweep rate has been demonstrated in experiments to change the performance of the system.

[0158] Certain embodiments may use drive waveforms that are specifically designed to account for different operating modes of the swept source. For example, the tuning mechanism of a MEMS VCSEL may be different for sweeping from a low wavelength to a high wavelength compared to sweeping from a high wavelength to a low wavelength. Thus, the tuning waveform may be designed to tune one sweep direction differently from the other, thereby optimizing the sweep behavior for each sweep direction.

[0159] Wavelength Selection According to various embodiments, the swept source is available in several wavelengths so that different wavelength OCT systems can be implemented, with each wavelength option exhibiting a different intensity.

[0160] 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 welding monitoring using a 1070 nm processing laser source 110, a swept source centered at approximately 1030 nm is advantageous due to its proximity to the wavelength of the material processing beam while also exhibiting suitable resolution for welding imaging. An additional advantage is the commercial availability of solid-state swept sources at this wavelength. This wavelength's proximity to the typical process beam wavelength of 1070 nm reduces undesirable effects (such as chromatic aberrations) arising from the process beam optics, but presents the challenge of developing suitable dichroic coatings to separate the imaging beam from the process beam.

[0161] In some embodiments, the imaging system spectral range is designed to take advantage of low-cost spectral ranges, such as telecommunications. Light sources centered at 1550 nm utilize common telecommunications wavelengths that allow the use of less expensive, commercially available, off-the-shelf components in the optical system, but these wavelengths offer a coarser axial resolution for a given sweep bandwidth (in nm) compared to the axial resolution available from systems centered at shorter wavelengths, such as 1030 nm, with comparable sweep bandwidths (in nm).

[0162] In some embodiments, the spectral range of the imaging system is designed to maximize resolution: System embodiments based on light sources centered around shorter wavelengths, such as 800 nm, may provide better axial resolution of OCT data in practical implementations.

[0163] In some embodiments, the spectral range of the imaging system is designed to match the reflectance (or possibly the transmittance) of at least a portion of the material undergoing laser processing. For example, in dissimilar plastic material lap welding applications, the spectral range may be designed so that the top material is at least partially transparent to the imaging system.

[0164] The selection of wavelengths implemented by the disclosed systems and methods depends on the context in which the wavelength is applied, including, but not limited to, resolution requirements, commercial availability of components, material properties of the material being imaged, and other processes occurring simultaneously 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. Thus, possible embodiments can be considered to exist theoretically at any wavelength, and practical embodiments can be considered to exist at all wavelengths for which suitable technology exists.

[0165] Data Processing For practical application of system embodiments, capture and processing of interferogram data is required. This can be performed by processing unit 150. Overall, a given embodiment of the system is designed to ensure appropriate signal levels, data rates, timing characteristics, data accuracy, and other parameters such that sufficient data integrity is preserved from the optical signal to the OCT image and any applicable derived quantities. While many of the necessary design practices are known to those skilled in the art, certain key elements of the system design that present unique aspects are discussed in more detail below.

[0166] Optical-electrical signal conversion Embodiments disclosed herein include a system for converting optical OCT signals to electronic signals. In some embodiments, this signal processing step can be performed by using a balanced photodetector configuration, which reduces or eliminates the DC component of the OCT signal and amplifies the generated electronic signal to a level optimized for the digitization system. A high-level schematic outlining an exemplary balanced photodetector configuration with an electronic amplifier is shown in FIG. 2. Other optical detection methods known to those skilled in the art are also suitable for optical-to-electrical signal conversion.

[0167] Some embodiments may incorporate a feedback electronic amplifier to maintain a consistent output signal amplitude and maximize the SNR of the interferogram, while 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. System embodiments that do not perform balanced detection are also within the scope of this disclosure, as are embodiments that use alternative detection techniques.

[0168] In embodiments of the system that include a k-clock, both the k-clock signal and the interferogram signal are converted from the optical interferogram to analog electronic signals. Circuits for detection, amplification, and processing of these separate signals are configured based on the properties of each signal, and in some embodiments, different methods will be employed for each respective signal.

[0169] Digitalization According to at least one embodiment, the disclosed SS-OCT system and method employs the use of a digitizer 135. The digitizer 135 is configured to digitize at least one interferometer output signal, or modified interferometer output signal, or corrected interferometer output signal, or amplified interferometer output signal using analog electronic signal processing as appropriate, to generate a corresponding digital signal.

[0170] Digitization of the detected interferogram allows for further processing and storage of the OCT data, with the digitization resolution, range, and sampling rate depending on the specific parameters of the physical system, including the seed light source tuning rate, optical power, and clocking requirements, as well as other physical parameters.

[0171] In some embodiments, dedicated digitization circuitry is implemented to allow sampling of the interferogram signal in a region that is linearly proportional to the wavenumber k of the OCT swept source 105 and that is not necessarily uniformly sampled in time. These implementations may include any combination of optical and / or analog electronic and / or simulated digital and / or simulated analog k-clock signals to achieve the appropriate sampling.

[0172] This implementation offers advantages because it eliminates computationally expensive interpolation operations from the data processing. Digitization approaches that are linearly proportional to wavenumber k are discussed above in the "K Clocking Method" section.

[0173] Other embodiments can include temporally uniform digitized sampling. In some embodiments including temporally uniform digitized sampling, the imaging source sweep is performed such that the wavenumber k is substantially linearly related to time in the region of interest. In preferred embodiments in which the digitized sampling of the OCT interferometer output signal is temporally uniform, the K-clock signal is digitized using a sampling method that is also temporally uniform. The K-clock signal can be obtained based on the methods discussed in the "K-Clocking Method" section above. The K-clock signal in such cases 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 applying computational methods (e.g., Fourier transforms, homodyne matrices) that allow for the extraction of depth information from the interferometer output signal.

[0174] In some embodiments, the controller 150 may 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 may be digitized with the same clock as the OCT interferogram signal. This may be accomplished using electronic design approaches known to those skilled in the art, for example, by implementing a dual-channel ADC.

[0175] K Clock Acquisition One or more embodiments of the system include an interferometer that functions as a physical optical k-clock 145, supplied with tapped light from the imaging light source 105. Such embodiments also include the electronic circuitry, e.g., detector 147, necessary to detect the k-clock signal. In some embodiments, the k-clock signal (after appropriate adjustments) can be used to directly drive a sampling clock on an OCT signal digitizer, resulting in digitization that is substantially linear in k, though not necessarily linear in time. This embodiment offers the advantage of eliminating a computationally intensive resampling step while still allowing the generation of interferograms with uniform k-spacing, which may enable the use of FFT methods during the processing stage. A further advantage of this method is that the use of an optical k-clock directly related to the imaging swept light source can automatically synchronize the acquisition rate to any sweep rate within specifications. In some embodiments, the k-clock signal (after appropriate adjustments) 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 resample other signals in the system into such a basis.

[0176] In other embodiments of the system, the k-clock can function to calculate dk / dt using a direct optical clocking method sampled with a uniform electronic clock, or by employing at least one fiber Bragg grating and / or at least one electronic oscillator to determine a piecewise function. Simulated approaches to k-clocking are also within the scope of this disclosure. Implementations of embodiments in which dk / dt is determined through measurements along a reference configuration of a first-order interferometer are included herein, and dk / dt can be stored or modeled electronically and / or computationally to aid in the development of appropriate conversion and processing methods. A more detailed discussion of k-clocking methods can be found in the "K-Clocking Methods" section above, including details regarding different approaches for processing OCT data based on the applied k-clocking methodology.

[0177] Processing Approach The general data processing approach employed for the generation of A-lines from interferograms and subsequent images (M-mode (A-lines acquired over time at a fixed transverse position) or B-scans (two-dimensional cross-sectional images)) is relevant to another aspect of this disclosure. An overview of the major steps in the data flow according to one embodiment includes pre-processing, Fourier transformation, and post-processing.

[0178] In some embodiments, preprocessing includes any digital operations that may be performed prior to the application of a transform to enhance the interferogram data and improve the final result. This may include windowing with an analytical envelope function or any arbitrary envelope function. Digital filtering may also be applied to the data, as well as DC signal subtraction. Many additional preprocessing methods have been developed for OCT applications and may be developed as components of the system, as known to those skilled in the art. In the specific case of motion artifact compensation, which will be developed extensively below, preprocessing may include (but is not limited to) windowing the digital signal representing the interferogram in two or more different ways to extract data captured in portions of the sweep having different dk / dt.

[0179] According to at least one embodiment, OCT data may not be uniformly sampled in K-space (e.g., in a sweeping method where K and time are not linearly related, the signal is uniformly sampled in time). In such embodiments, the acquired digital data is resampled to generate an OCT data set that is uniformly sampled in K-space. The relationship between time and K-space can be mapped to assist in the resampling using various methods, including acquiring an optical K-clock signal, performing a calibration method to characterize the light 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, a digital resampling method can be applied to the time-domain OCT data capture. Digital resampling methods include those known to those skilled in the art, such as linear interpolation, spline interpolation, and / or applying a sliding window signal processing function. Mismatches in optical dispersion between the arms of an interferometer can distort the relationship between time and K-space, as detailed in the "Dispersion Compensation" section (discussed in more detail below). In some embodiments, this distortion can be estimated and calculated in advance, and compensation for this 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.

[0180] There are various methods for performing the step of transforming interferogram data in k-space to data in a depth region known as an axial line or A-line. In some embodiments, the transform method can be selected based on the most appropriate one for a given application of the system. For example, depending on the sampling method, an FFT can be employed (for embodiments uniformly sampled in k), a homodyne matrix DFT can also be used (for embodiments not uniformly sampled in k, e.g., for embodiments uniformly sampled in time where k varies nonlinearly in time), or a fractional FFT can be used (also for embodiments not uniformly sampled in k). When selecting a transform processing approach, factors can be considered, including the stability of the modulated light source, the rate of modulation, and the available processing power. Some transform processing approaches can incorporate compensation for various physical and optical effects that affect the accuracy and resolution of the data, which are discussed in more detail below. A non-limiting example of such an effect is the effect of motion artifacts, i.e., light dispersion, discussed below.

[0181] Post-processing according to some embodiments includes tracking geometric characteristics of a single A-line as well as tracking characteristics across many A-lines, where the tracked characteristics / geometric aspects are encoded in the interferometer output signals used to generate the A-lines.

[0182] Non-limiting examples of features (e.g., geometric properties) that may be tracked for a single A-line include position, symmetry, peak width, centroid, geometric second moment, center of mass, amplitude, height-to-width ratio, and geometric area under the curve. According to certain embodiments of the present invention, a particular A-line may be represented based on some set of parameters, features, measurements, or other parameters extracted from imaging data that may be derived from the properties of that A-line, including geometric parameters and features. Such representations of the A-line may 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 may be configured to output tracking data alone or in addition to raw A-line data. Incorporation of tracking data into system embodiments can provide insight into important features / geometric aspects of the sample being measured via SS-OCT at a data density significantly lower than that of raw data, which may make this approach advantageous from a computational perspective.

[0183] 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 involves evaluating groups of A-lines based on analysis of shared features. Some embodiments involve developing metrics to quantify correlation between subsequent A-lines. Filtering of groups of A-lines may be performed by applying filter methods known to those skilled in the art, such as Kalman filtering. In some embodiments, B-scan or M-mode data may be treated similarly to images, and in such embodiments, appropriate image processing algorithms and approaches may be applied to extract or manipulate data to aid further interpretation. Representations of A-lines, and more generally, representations of features (e.g., geometric features), within OCT captures derived based on parameter extraction from composite imaging data may also be referred to as “tracking data” or “tracking data.” Tracking data based on only one A-line may be more specifically referred to as “A-line tracking data” and / or “tracking A-line data.” Tracking data based on two or more A-lines may be more specifically referred to as "bulk tracking data" and / or "tracking bulk data." In some embodiments, operations including statistical analysis, mathematical analysis, geometric analysis, and / or signal processing may be applied to a set of two or more A-line tracking data corresponding to two or more A-lines to generate the bulk tracking data. In some embodiments, operations as described may be applied to a set of two or more A-lines to generate the bulk tracking data.

[0184] Additional methods of extracting simplified data from A-lines or B-scans, M-modes, etc., including those known to those skilled in the art of signal processing and / or artificial intelligence (AI) and / or machine learning (ML), may also be employed and are considered within the scope of this disclosure.

[0185] Signal processing approaches / methods known to those skilled in the art can be employed as part of the processing approach, either in pre-processing (e.g., of interferometer output signals in analog or digital time and / or K-domain) or post-processing (e.g., of A-lines or groups of A-lines). Digital or analog high-pass or low-pass filtering and Kalman filtering are examples of such approaches. Further examples of such methods include smoothing filters such as median and percentile filters. Image processing approaches, e.g., those developed for analysis and feature extraction, can also be applied to the data and can be particularly meaningful when dealing with image-like output generated when combining multiple A-lines (e.g., B-scans, M-mode, or volumetric scans). Examples of image processing approaches include signal processing operations designed to operate on two-dimensional or three-dimensional digital imaging data, such as convolution filters, frequency-domain masking (including 2D Fourier-domain-based methods), and others known to those skilled in the art. Signal and image processing approaches can be applied at any level of the data, for example, in post-processing, one filter can be applied to individual A-lines while a separate filter can be applied to the complete B-scan. 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 PCR features from B-scans while another filter can be developed to suppress noise on the B-scans.

[0186] 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 (commonly applied algorithms, e.g., neural networks, CNNs, regression algorithms, self-organizing maps, k-nearest neighbors, etc.) operating on SS-OCT data to extract useful information such as the evolution of the weld keyhole and / or other features of the processing area / workpiece, including the workpiece geometry. Algorithms trained using ML techniques can be used in conjunction with algorithms that do not use ML, and algorithms that include at least one component developed using ML methods are referred to herein as ML-trained algorithms and / or ML-augmented algorithms (used interchangeably).

[0187] In some embodiments, the ML training algorithm may take as input raw SS-OCT data, possibly including interferometer output signals, A-lines, B-scans, and / or M-modes. In some embodiments, the ML training algorithm may take as input one or more features / geometric characteristics of the signal and / or A-lines and / or groups of A-lines (such as B-scans or M-modes), such as position, shape, symmetry, width of one or more peaks, center of gravity, geometric second moments, center of mass, amplitude, height-to-width ratio, or geometric area under one or more curves. In some embodiments, the ML training algorithm may take as input some combination of these and / or other data types. In some embodiments, the ML training algorithm may take as input SS-OCT data and data from other detectors in the system; examples may include photodiode data or acoustic data (other detectors are discussed below in the section entitled "Auxiliary Detectors"). ML or AI augmentation algorithms can be used to extract tracking data from A-lines and / or groups of A-lines. ML or AI-augmented algorithms may be used to extract features of interest from the data and / or tracking data. In some cases, the ML-augmented processing paradigm may include a pre-trained algorithm applied as part of the data processing. When developing this algorithm, care must be taken to ensure that the training data used is appropriate for the conditions to which the ML-augmented algorithm will be applied. For example, an ML algorithm may be trained on SS-OCT weld keyhole data using imaged longitudinal cross-sections of the weld as ground truth to employ a supervised learning approach. Such an algorithm may then be applied to post-processing the keyhole data to estimate the keyhole depth based on the imaged signal. In some cases, it may be desirable to further narrow the scope of the training set in order to apply ML techniques to generate an algorithm with greater specificity for a given problem. As an example, ML techniques may be applied to train an algorithm on only data from a single-mode copper weld.This may enable ML training methods to extract features specific to single-mode laser weld dynamics and copper material properties (and the interaction between the two). The resulting ML-enhanced post-processing algorithms may have improved performance when extracting PCR features (e.g., geometric features) from copper welds compared to algorithms trained on data derived from multiple materials or processes. In the context of laser material processing, numerous sources of ground truth are available for application to supervised learning methods for ML-trained SS-OCT algorithms. Examples of training data sources include datasets captured simultaneously during material processing using another instrument (e.g., X-ray synchrotron data, SS-OCT data), datasets based on analysis of completed welds (e.g., metallographic analysis, X-ray computed tomography), or synthetic / simulated datasets. In some embodiments, unsupervised learning methods may be applied to develop ML training algorithms. In some embodiments, ML and / or AI algorithms in SS-OCT systems may use a combination of supervised and unsupervised learning methods and / or other algorithm development methods as needed.

[0188] ML and / or AI algorithms may be applied to identify, assess, and / or correct artifacts in OCT signals, including motion artifacts. The application of motion artifacts and AI / ML algorithms is discussed further below in the section entitled "ML / AI Algorithms and Motion Artifacts."

[0189] In some embodiments, it may be appropriate to employ ML / AI methods for a broader trend-based approach to the output of SS-OCT systems. For example, an AI-based monitor could monitor quality metrics, such as the QA output of one or more SS-OCT systems in a plant, providing timely updates based on the evolution of the SS-OCT data over time to help human operators proactively manage issues. In this example, an AI program could ingest data from seam tracking or keyhole depth measurements and identify general trends in part positioning and welding over many hours, days, or even longer. It is common for process parameters to drift slightly over time, for example, due to fixture wear or the accumulation of contaminants such as welding fumes on system components, and this process drift typically impacts the quality metrics of the process product. As process parameters drift, the SS-OCT system generates bulk data indicative of this shift in quality metrics, which can be analyzed as a whole to understand the shift in average process parameters closer to the boundaries of acceptable processes. As process quality metrics shift from nominal, real-time feedback provided by SS-OCT can help maintain the process within established boundaries, but at certain process levels, drift intervention may be required to re-establish nominal performance, for example, through maintenance or calibration operations. Based on trends in SS-OCT data, AI programs can generate alerts to human operators and identify cells that may require maintenance before the process deviates too far from specification and begins producing rejected parts. Diagrams of process drift over time are included in Figures 20A and 20B. Figure 20A illustrates typical statistical properties of process parameter and quality metric distributions, where the quality metrics are derived at least in part from SS-OCT data associated with the process, demonstrating the gradual degradation of the quality metrics over time resulting from process drift.20B illustrates the impact of process monitoring by an AI / ML teacher or monitor that identifies statistical trends where quality metrics begin to deviate from specifications, enabling corrective action before parts are flagged outside the boundaries of the quality metric. Such an AI teacher, aided by a network of QA and feedback systems, may be able to proactively manage production assets to ensure timely maintenance and limit the number of breakdowns across an entire production facility and / or supply chain.

[0190] In some embodiments, AI and / or ML methods may be employed on interferograms, A-lines, or groups of such data types. In some embodiments, AI and / or ML methods may be employed to identify and / or measure and / or characterize specific features of 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, may be applied as additional methods for generating simplified tracking data.

[0191] The application of tracking data plays an important role in some system embodiments, primarily reducing the data footprint required to store meaningful measurement information and enabling the development of quality assurance (QA) algorithms. In system embodiments in which an SS-OCT system is integrated with a laser material processing system, QA metrics can be developed according to material processing objectives, and tracking data, or other SS-OCT data, can be applied to provide feedback regarding the material processing. A suitable example of the application of tracking data is for system embodiments in which an imaging beam captures at least some interferometry data aligned with the keyhole of a laser weld so that at least some data representative of the OCT return from the keyhole bottom is collected. Features, including geometric features derived from such A-lines, can be processed to generate tracking data representative of the keyhole bottom, which can be used, for example, to help track the keyhole depth of a PCR throughout the welding process. In this disclosure, "keyhole depth" refers to the maximum depth of the vapor capillary, as understood by those skilled in the art.

[0192] Processing Hardware and Hardware Implications In some embodiments, data is pipelined to a specially coded FPGA for real-time processing, which may include any or all of the pre-processing, conversion, or post-processing steps. Such embodiments enable the possibility of real-time output, with the potential to use OCT data as a feedback mechanism within the process. Additional processing methods, including but not limited to depth tracking or motion artifact correction, as described extensively below, may also be implemented on the FPGA. To reduce the effects of instantaneous chaotic variability, particularly when tracking the keyhole (welding) process, tracking methods incorporating data from multiple A-lines, such as percentile tracking or averaging, may be developed. In some embodiments, the digitized data is processed using a program 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 may include, by way of non-limiting examples, FPGAs, CPUs, GPUs, ASICs, microcontrollers, and other processing hardware known to those skilled in the art. In some embodiments, calculation of weld metrics such as keyhole depth or finished weld surface height can be performed in real time when the processing is configured to allow real time. In some embodiments, quality assurance (QA) metrics can also be developed to provide rapid feedback regarding the suitability of the imaged part for industrial applications.

[0193] Motion artifact correction General Problem Description A known physical effect that affects all swept-source OCT systems is distortion of the A-line when the imaging target moves (sample motion) during the acquisition time of a single A-line. This effect is known to those skilled in the art as a motion artifact. The motion artifact can appear as a shift that may be larger than the actual displacement of the imaging target during the imaging time. The motion artifact can additionally appear as broadening, blurring, or distortion of the A-line in addition to or instead of a shift. In laser weld monitoring applications, it is known that motion exists in many desirable imaging targets, particularly the bottom and sidewalls of the keyhole. Therefore, understanding and compensating for this motion artifact is important for the purpose of generating accurate weld monitoring information, such as keyhole depth.

[0194] 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 sample motion 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 performed by a material processing beam on the sample. In some embodiments, the time-varying difference in optical path length is caused by intrinsic sample motion that is not caused by a material modification process. The motion artifact causes modulation of the interference spectrum that registers an incorrect depth for a moving reflector when processing is performed using conventional means. Because the magnitude of the motion artifact depends on the dk / dt of the imaging light source, careful analysis of the dk / dt variation is useful to compensate for the shift.

[0195] According to certain embodiments, one or more algorithms are used to identify, address, 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 detected motion artifacts are related to the change in source wavenumber with time (adjustment rate dk / dt). The light source is swept so that interferometer output signals corresponding to portions of the source sweep at two or more (at least two) adjustment rates dk / dt are captured. This allows for the identification of distortions that may be due to motion artifacts. Based on these distortions, corrections are calculated and applied to generate motion artifact-corrected A-lines or M-modes or other motion artifact-corrected data. In some embodiments, motion artifact correction includes correction of higher-order distortions that affect the shape and characteristics of the complete raw A-lines. Other embodiments of motion artifact correction include only correction of important image features, such as tracked A-line peaks. To address noise still present after motion artifact correction, some embodiments apply smoothing and tracking algorithms.

[0196] Selection of an appropriate motion artifact compensation method will depend on the particular requirements dictated by the application, including requirements for accuracy, resolution, data rate, computational power, operating speed, output data requirements, etc. Details of various general approaches are disclosed below, although certain steps within these approaches may be exchanged, where appropriate, for other steps disclosed below or for operations of similar mathematical utility and effect known to those skilled in the art (e.g., exchanging the use of median, mean, and mode when averaging is required for application performance).

[0197] Distinction from medical OCT Research on the topic of medical OCT has led to research on motion artifact correction. However, several differences exist between the application of OCT to medical fields and to materials processing, and therefore there are important differences in the magnitude and nature of the motion artifacts observed, as well as the requirements for motion artifact correction.

[0198] For example, in medical OCT, care must be taken within the system to operate at wavelengths and powers that are not harmful to the biological tissue and / or material being imaged, whereas in material processing, significantly higher powers can be applied to OCT without damaging the sample. Additionally, medical OCT often involves imaging materials that are at least partially transparent to the OCT wavelength, where subsurface structures are of interest. Such imaging requires signal-to-noise characteristics that allow low-intensity features within the sample volume to be clearly distinguished. Typical material processing applications employ sample materials that are opaque to typical OCT wavelengths, including metals. Therefore, surface features, especially the interface between the solid processing material and the surrounding gas, are more important. Furthermore, for sample materials that are opaque to typical OCT wavelengths, features within the sample volume cannot be accessed by OCT.

[0199] Additionally, motion observed in medical OCT typically consists of slow bulk motion of the sample during acquisition, with typical values ​​reported in the range of less than 1 mm / s. Motion observed in OCT applied to material processing can include fairly fast bulk sample motion, intrinsic sample motion caused by material modification processes, motion that is non-uniform in time and space 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 in PCR and / or in other regions of interest within the sample, which can be quite chaotically variable over the time scale of SS-OCT acquisition. Motion can also be intrinsic sample motion not caused by the material modification process. Such motion may also include a high-velocity component, with the motion speed of the sample being imaged exceeding 10 mm / s, 100 mm / s, 500 mm / s, 1000 mm / s, 5000 mm / s, or even 10,000 mm / s. In some cases, this sample motion speed may be inherent to a material modification process. Of particular interest is the movement of material within a laser welding keyhole, which has been demonstrated under several conditions to exhibit some of the above-mentioned properties. Techniques known to those skilled in the art of medical SS-OCT are not sufficient in such cases.

[0200] Medical OCT is typically applied in situations where a qualified human operator monitors the image and may repeat the image if bulk motion is perceived to degrade the image to an unacceptable degree. Unlike medical OCT, material processing OCT is often employed in automated processes where images must be processed using algorithms to generate quality assurance results, for example. For many material processing applications, OCT images are associated with a single material modification process, such as welding, and must be acquired during the processing time, so they can only be acquired once, in situations where repetition is impractical or impossible. These differences mean that developing motion artifact correction algorithms specific to material modification process monitoring OCT presents novel challenges compared to medical OCT motion artifact management and / or correction. This challenge is further heightened by the demands of automation in the manufacturing space and strict quality assurance and reliability requirements. Below, we outline motion artifact identification and correction algorithms specifically developed to address the capabilities and challenges of applying OCT to material modification processes.

[0201] Identifying motion artifact distortion in SS-OCT data In system embodiments where motion artifact compensation or correction is required, the first step is to identify and characterize the motion artifact distortion. According to certain embodiments, the distortion corresponding to the motion artifact corresponds to one or more geometric aspects encoded in the interferometer output signal (such as the geometric characteristics of the A-line encoded in the frequency of the interferometer output signal). When the system's sample setup is properly configured to capture the interferometer output signal corresponding to the swept optical signal at two or more adjustment rates dk / dt, appropriate information can be derived based on the information contained in the uncorrected interferometer output signal. In some embodiments, to extract the motion artifact characteristics of the uncorrected interferometer output signal, the signal capture must include at least two separable data segments in which the imaging light source has different dk / dt. The different dk / dt may include different adjustment rates of the imaging light source and / or may include a positive dk / dt and a negative dk / dt. By evaluating frequency components found in at least two interferometer output signals associated with at least two imaging light sources dk / dt, information regarding distortion of the interferometer output signals during acquisition time can be extracted, and this distortion information can be related to the motion and / or velocity of the imaging sample. Distortion evaluation can include, but is not limited to, measuring the amplitude of individual frequency components or groups of frequency components within an uncorrected interferogram signal segment, evaluating frequency components observed at different dk / dt, and evaluating one or more geometric aspects or characteristics encoded within the interferometer output signals, including, but not limited to, the position, shape, symmetry, width of one or more peaks, center of gravity, geometric second moments, center of mass, amplitude, height-to-width ratio, and geometric area under one or more curves of signals and / or A-lines and / or groups of A-lines (e.g., B-scans or M-modes) corresponding to the uncorrected data. According to certain embodiments, performing distortion evaluation includes comparing the distortion in at least one of the at least two interferometer output signals to the distortion in at least one other of the at least two interferometer output signals.In some embodiments, performing the evaluation includes comparing a distortion in at least one interferometer output signal associated with a positive tuning rate dk / dt to a 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), the sample arm of a first interferometer and the sample arm of at least one additional interferometer share at least one optical element, a first imaging signal configured with one 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 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 the one or more interferometer output signals of the first interferometer and the at least one additional interferometer. In additional embodiments, the first and second imaging optical signals are simultaneously directed to the processing region.

[0202] According to certain embodiments, the distortion corresponds to one or more geometric aspects of the interferometer output signals and / or the A-lines. The one or more geometric aspects may include at least one of position(s), symmetry, signal shape, envelope function shape, width(s) of one or more peaks, displacement(s) of one or more peaks, amplitude, center of gravity, height-to-width ratio, center of mass, geometric area under the curve, and geometric second moments. According to certain embodiments, performing the distortion assessment includes comparing one or more geometric aspects encoded in at least one of the at least two interferometer output signals to one or more geometric aspects encoded in at least one other of the at least two interferometer output signals. According to some embodiments, performing the distortion assessment includes comparing 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 system requirements, component requirements, one or more application requirements, one or more calibrations, one or more models, one or more algorithms, underlying physics, hardware limitations, and / or software limitations.

[0203] The geometric aspects distorted by motion artifacts can include aspects known to those skilled in the art as corresponding features, including geometric features of the phase change region (and more generally, the processing region), and more generally, including geometric features of the sample. Therefore, the identification and subsequent correction of motion artifacts is crucial to obtaining accurate measurements of one or more features, including geometric features of interest, in a sample when the sample is moving. A suitable example of a feature or geometric aspect to be measured when the sample is moving is obtaining accurate measurements of a keyhole geometry while the molten metal at the bottom and sidewalls of the keyhole is moving.

[0204] It should be understood that there may be cases where no distortion is detected in the interferometer output signals by controller 150, and this scenario is encompassed by one or more embodiments disclosed herein. Accordingly, controller 150 processes the interferometer output signals to determine at least one characteristic (e.g., depth information) of the processing region. However, if controller 150 detects distortion in the at least one interferometer output signal, then in response, controller 150 applies one or more corrections to the at least one interferometer output signal, as discussed herein.

[0205] Imaged target velocity estimation and compensation. A theoretical framework and corresponding mathematical model can be developed based at least in part on one or more characteristics of the SS-OCT system, particularly when the sample motion is relative to the axis of the imaging optical signal, to relate distortions of at least one of the at least two interferometer output signals at different dk / dt to the magnitude and / or direction of the sample motion velocity. This allows for the development of correction factors (also simply referred to as "corrections") that can be applied to the interferogram to correct for the effects of motion artifacts. Appropriate corrections for calculating a given corrected A-line can be calculated based on this theoretical model combined with information extracted from an analysis of geometric aspects encoded in the interferometer output data, examples of which are described above, where the interferometer output data includes at least two segments or portions of the 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 results derived from the interferometer output data, K-clock data, analytical and / or numerical models of the SS-OCT system and / or its components, or some combination thereof. The theoretical model can be combined with information extracted from an analysis of the geometric aspects encoded in the interferometer output data, which includes at least two segments or portions of the source sweep with different dk / dt. Information extracted from the frequency analysis of the uncorrected interferogram can be used, combined with a theoretical framework and knowledge of the key physical parameters of the OCT imaging system, to calculate an estimate of the sample's velocity at the time of A-line capture, as affected by the observed motion artifact. Based on this compensation, it is then possible to extract positional and velocity data from the OCT system scan.

[0206] According to some embodiments, the calculated correction coefficients are applied in the transformation stage of OCT imaging, with the correction factored into each element of the DFT matrix, similar to dispersion compensation. Thus, the controller 150 is configured to transform the time-domain interference signals (interferometer output signals) into their respective frequency-domain signals by calculating the corrected DFT matrices that are applied by multiplication to the time-domain interference signals.

[0207] According to some embodiments, the time domain measurements of k and / or dk / dt provided by the K clock signal and / or the simulated / modeled k clock signal are used to calculate at least one correction for one or more distortions in the interferometer output signal.

[0208] Motion artifacts distort both 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 corrections necessary to compensate for higher-order effects of motion artifacts, such that corrected A-lines exhibit improved symmetry and / or improved amplitude and / or reduced full width at half maximum when compared to uncorrected A-lines.

[0209] In system embodiments where a variety of motion conditions are expected for the imaging target, multiple DFT matrices containing velocity compensation can be precalculated and stored in a lookup table, as needed, for rapid processing of datasets with variable velocities. Other embodiments with less velocity variation can be calibrated from time to time based on their characteristic velocities and designed to compensate all A-lines for their characteristic velocities based on the calibration of a single common transformation matrix. Depending on the specific needs of the process, including computation speed, sample rate, source sweep rate, and accuracy requirements, various approaches can be applied to develop an optimal velocity compensation method that meets the needs of the application. In one example of a system embodiment where a variety of motion conditions are expected for the imaging target, the SS-OCT system is configured to image a welding process, where a material processing beam forms a phase change region (PCR) in the processing area. In such an embodiment, the imaging beam can be directed toward a region of the moving process, such as a welding keyhole, or toward an imaging target known to be moving at a velocity that varies as the process evolves over time and / or space. Estimation of distortion corresponding to motion artifacts in such embodiments can be used to generate corrected A-lines corresponding to the geometry of the PCR and / or used to generate estimates of the velocity of material processed within the PCR.

[0210] Tracking Data and Motion Artifact Correction For application-specific reasons, in some embodiments of the present invention, it may be preferable to derive simplified tracking data, referred to herein as tracking data, from the interferometer output signal or its corresponding A-lines, B-scans, M-modes, etc. The concept of tracking data has been discussed above. Tracking data may be developed in a manner specific to SS-OCT applications and may include representations of relevant information about the geometry of the sample, including, in some cases, phase change regions (PCRs) and / or processing regions. For example, A-lines may be employed to identify and / or measure and / or characterize specific features of PCRs, particularly weld depth. Some non-limiting examples of tracking data include signal position, symmetry, peak width, centroid, geometric second moment, center of mass, amplitude, height-to-width ratio, and geometric area under the curve.

[0211] Additional methods of extracting simplified data from A-lines or B-scans, M-modes, etc., including those known to those skilled in the art of image processing and / or artificial intelligence (AI) and / or machine learning (ML), may also be employed and are considered within the scope of the present disclosure. For example, in some embodiments, AI and / or ML methods may be employed on an interferogram, A-line, or group of interferograms. In some embodiments, AI and / or ML methods may be employed to identify and / or measure and / or characterize specific features of 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, may be applied as additional methods for generating simplified tracking data.

[0212] 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 important information from raw OCT data is an integral part of the approach. Tracking methods can be used on data prior to correction or compensation to establish and quantify observed distortions, generally characterize the uncorrected signal, and for other purposes. Tracking data from two or more interferometer output signals captured at two or more adjustment rates dk / dt can be employed to perform the evaluations necessary to establish the presence of absolute or relative distortions used as the basis for motion artifact compensation.

[0213] In some embodiments of the invention, at least one correction for motion artifacts is applied to tracking data or simplified data extracted from the uncompensated interferometer output signal. This corrected tracking data is understood to be correction data. In some embodiments of the invention, at least one correction for motion artifacts is calculated based on tracking data or simplified data derived from the interferometer output.

[0214] All embodiments of the present invention in which the distortion components, corrections, and corrected data, as well as 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.

[0215] Geometric compensation of motion artifacts without velocity estimation. In some embodiments of the system, it may be appropriate to apply a simpler methodology for calculating corrections for detected motion artifacts that does not involve incorporating theoretical models or velocity estimation. The motivations for simplifying the compensation method are varied, but typically timing and computational power (especially in, for example, real-time measurement systems) are limited or a concern. In such cases, it may be preferable to forgo the full complexity of motion compensation methods and instead apply a simpler approach. It should be understood that this may involve a fundamental sacrifice in data density or accuracy, but may be preferable for many applications.

[0216] Compensating for motion artifacts without velocity modeling and estimation still requires frequency analysis of an interferogram signal capture that includes at least two segments of the light source sweep with different dk / dt. Based on this analysis, a correction can be calculated for direct application to at least one of the at least two uncorrected A-lines. This correction may include shifting the A-line, applying an amplification or attenuation factor, a filter, or other signal processing operations or methods known to those skilled in the art. In some cases, it may be appropriate to first track key features of the at least two uncorrected A-lines and apply the correction only to the tracked features. Such tracked features may be geometric features of interest within or on the sample, as discussed above.

[0217] Artifact size filtering 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 both the position and other geometric aspects of the signal (e.g., position, symmetry, peak width, centroid, geometric second moment, center of mass, amplitude, height-to-width ratio, geometric area under the curve, and / or amplitude above the noise floor). Signals with smaller motion artifact magnitudes are likely to be less distorted in these other respects as well. Therefore, the magnitude of the motion artifact can be used as an indicator of the relative quality and / or accuracy of A-lines generated from a given interferometer output signal. Selecting the A-line with the smallest motion artifact can reduce noise, improve tracking accuracy, and more accurately visualize process features of interest. 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, the magnitude of distortion of some geometric aspect of one or more distorted interferograms may be considered in absolute terms and / or relative to a predetermined threshold when assessing motion artifacts. In some embodiments, applying a correction to at least one interferometer output signal according to the assessment of the motion artifact includes discarding, weighting, promoting, or using the interferometer output signal based on the assessment of the motion artifact, or evaluating the signal for later discarding and / or processing.

[0218] According to at least one embodiment, one or more corrections applied by the controller 150 to at least one interferometer output signal include discarding, weighting, or using the interferometer output signal or any derivative values ​​and signals from this signal. For example, an analysis performed by the controller 150 between at least two segments of a source sweep having different dk / dt can result in a relative difference between the at least two segments being of a value or magnitude such that the data is unusable and therefore discarded. The decision to discard, not discard, expedite, modify, or leave data unmodified constitutes a correction as the term is used herein. The motivation for including discarding, leaving unmodified, and modifying as options is that noise in the system may be sufficient to wash out A-line information under certain motion artifact conditions; therefore, in certain cases, discarding data results in no information loss and noise reduction and increased efficiency. This method is therefore an appropriate correction. To manage the variable quality of returned data as a component of motion artifact compensation, it is also possible to flag data for further processing based on whether it exceeds a critical threshold.

[0219] This phenomenon can be used to filter uncompensated A-lines, for example, by selecting only those A-lines that fall below a given motion artifact magnitude threshold. This has the effect of selecting A-lines for which motion compensation provides the highest quality and / or most accurate resulting A-lines and applying motion compensation only to those A-lines. Such methods have been shown to improve the tracking accuracy of certain analytical methods for some processes. Downsampling based on motion artifact magnitude can also effectively reduce noise seen in M-mode and tracking data, creating a clearer view of the imaged material modification process and its key features, enabling more accurate analysis.

[0220] It should be understood that this filtering can be applied in addition to the geometric correction or motion compensation outlined above.

[0221] Whether corrected and / or uncorrected interferometer output signals are used, the controller 150 can process these signals to determine at least one characteristic of the processing region, such as depth information of the processing region. Depth information includes PCR keyhole depth, seam tracking, finished weld surface height, workpiece height, and other measurements related to the workpiece geometry at times occurring during pre-processing and post-processing. In some examples, the controller 150 is also configured to control at least one processing parameter of the material modification process based on the at least one characteristic. In some embodiments, the 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, fluence, wavelength, pulse repetition rate, pulse energy, pulse shape, scan speed, focal spot diameter, focal spot position, and spatial pattern (on the sample). Other non-limiting examples of process parameters include material feed rate, coolant flow rate, cover / assist gas flow rate, cover / assist gas pressure, cover / assist gas blend, and additive material feed rate.

[0222] ML AI Algorithms and Motion Artifacts 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. One particularly exciting possibility in SS-OCT derives from SS-OCT's ability to capture data from a target even when the target is moving. This capability, and approaches for addressing the resulting artifacts, are discussed herein. ML / AI techniques offer the potential for extracting information from motion artifacts. For example, a keyhole collapse event generates rapid motion in the phase-change region that washes out the imaging signal in the SD-OCT context. However, in SS-OCT, this event generates a data signature, albeit distorted due to the occurring motion. ML algorithms can be developed to identify such disruptions in SS-OCT imaging data based on the motion artifact signatures they generate. ML algorithms can also be developed to identify SS-OCT data signatures resulting from motion in material processing region characteristics of events such as pore formation, weld blowout, or other process anomalies and / or fluctuations. Detection of such events provides new opportunities to ensure the quality and integrity of the weld.

[0223] Additional examples of ML applications include using ML algorithms to identify and / or correct distortions in data resulting from motion artifacts, identify the velocity of a target or other characteristics of its motion resulting from motion artifacts, etc. For example, an ML algorithm may be trained on SS-OCT A-line data retrieved with one or more known sample motion patterns in the data. Once trained, such an algorithm may be applied toward identifying and / or correcting motion artifacts, depending on implementation details. For example, AI or ML algorithms may be implemented to extract velocity information from SS-OCT A-line data, identify distortions in the interferometer output signal resulting from sample motion, correct distortions in the interferometer output signal resulting from sample motion, reconstruct distorted signals, and / or achieve any of the other corrections for motion artifacts as outlined above, including filtering and / or selection operations (as appropriate).

[0224] Example of imaging target velocity estimation and compensation As referred to herein, an interferogram or interference pattern may 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 may 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). 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, which generates at least one interferometer output signal. The signal generated in the optical detector 130 can be converted from an analog signal to a digital signal (by the digitizer) and processed by the controller 150. The controller 150 is configured to enable the information generated in the optical detector 130 to be sampled by the digital converter 135. In some embodiments, the analog electronic signal is sampled at uniform intervals in k, the k clock 145 is detected using a detector 147, and the resulting signal is passed to a digital converter 135 where it is used to time these uniform intervals.

[0225] 10A is a non-limiting example of a captured interferometry signal that is in the time domain prior to capture and ultimately represents one or more A-lines. It should be understood that many of the interferometry signals shown in FIG. 10A need to be captured to generate a cross-sectional image (B-scan) or repeated A-lines (M-mode) acquired over time at a fixed lateral position. For example, according to one embodiment, an M-mode OCT image includes a set of subsequent A-lines shown adjacent to each other in intensity contrast for the purpose of capturing the evolution of a target (e.g., a weld keyhole) over time. Once captured, the interferometry signal may contain information at various dk / dt sweep states.

[0226] According to at least one embodiment, the tunable light source 105 is controlled such that the tuning rate dk / dt of the imaging optical signal includes at least two tuning rates dk / dt associated with one or more interferometer output signals, respectively. In some embodiments, the processing unit 150 is configured to control the tunable light source such 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, both positive and negative tuning rates are captured, as shown in the interferometry signal of FIG. 10B. An appropriate Fourier transform is applied to each of the positive / forward and negative / reverse interferogram portions to generate two A-lines. The A-lines are scaled in units corresponding to depth based on one or more characteristics of the light and / or the system. Thus, the peak of such an A-line corresponds to the depth at which the reflective sample interface is found (relative to the depth of the reference interface). If the sample interface is moving during interferogram capture, distortions will be present in the resulting A-line. Such distortions are an example of "motion artifacts," as discussed above. Motion artifacts distort both forward (positive) and reverse (negative) dk / dt A-lines. The distortions have varying effects based on the dk / dt rate present in the interferogram used to generate the A-line. An example of this distortion is shown in the A-line depth versus amplitude data in FIG. 11A, which shows one A-line generated using a portion of the positive dk / dt interferogram with no motion compensation or correction applied, referred to as the "positive A-line," and one A-line generated using a portion of the negative dk / dt interferogram with no motion compensation or correction applied, referred to as the "negative A-line." The distortion is visible as a displacement between the forward and reverse A-line peaks, as well as an asymmetry and distorted shape of each peak, which is an example of a distortion in at least two of the two or more interferometer output signals and is identified by controller 150.The controller 150 performs an estimate of the distortion in each of the at least two interferometer output signals and calculates and applies a correction to the at least one interferometer output signal based on the estimate.

[0227] According to certain embodiments, the distortion corresponds to one or more geometric aspects of the interferometer output signals, as specified above (section "Identifying Motion Artifact Distortion in SS-OCT Data"). Calculating the correction includes analyzing the frequency components of two or more interferometer output signals and evaluating the frequency components of the two or more signals. Following this evaluation, various correction methods can be applied to the A-lines. Non-limiting examples of possible correction methods include developing a full motion compensation matrix as outlined above, applying a geometric correction as informed by the evaluation as outlined above, or determining whether the relative difference between the frequency components of at least two interferometer output signals exceeds a certain threshold.

[0228] Returning to the example of Figure 11A, the necessary corrections can be calculated based on an estimate of the distortion of the A-lines produced by the forward and reverse interferogram directions. In some embodiments, the estimate of the distortion is also used to calculate an estimate of the sample velocity during the acquisition time. In some embodiments, an estimate of the distortion component of each of the at least two interferometer output signals is used by controller 150 to generate an estimate of the velocity of the material being processed in PCR.

[0229] According to one embodiment, the corrections are calculated based on a physical model that includes a set of functions that describe the relationships between different variables, including the movement within the keyhole, the behavior of the swept light source adjustment, 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 an evaluation of one or more characteristics and distortions of the SS-OCT system. In some embodiments, the controller 150 is configured to generate a mathematical model of the adjustment 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, a second model is configured to provide an estimate of the movement of the sample on the k-clock time scale. In some embodiments, these two models are used to estimate the time elapsed between k-clock samples and / or to estimate the displacement of the target (sample) on these time scales, and / or in some embodiments, to calculate the appropriate corrections.

[0230] Once the corrections are calculated (e.g., by processing unit 150), in some embodiments, the corrections are then incorporated into the construction and modification of a new, corrected DFT matrix. In the example shown in FIG. 11A , a transformation matrix incorporating the corrections is calculated for each of the positive and negative dk / dt interferogram portions, and the new matrix is ​​applied to the respective portions of the interferogram to generate corrected A-lines that display the corrected depth of the interface. FIG. 11B shows an example of the corrected A-line depth versus amplitude data of FIG. 11A . In other embodiments, the corrections are calculated and / or applied based solely on geometric factors.

[0231] According to at least one embodiment, the corrected A-lines can be tracked, filtered, and analyzed to identify material-modifying process features, such as keyhole depth in a welding process. Additionally, if multiple interferograms (e.g., M-mode or B-scan images) have distortions from the same motion speed, one (corrected) modified DFT matrix can be used to correct all of the distortions. However, if multiple interferograms have distortions from different or varying speeds, each interferogram requires a modified DFT matrix corresponding to its speed. An example of an uncorrected M-mode OCT image is shown in Figure 12A. This M-mode image was obtained from a benchtop experiment and provides a view of a sample target in uniform motion. Figure 12A clearly displays the gap between the positive and negative dk / dt sweep portions, which indicates motion artifacts. Figure 12B shows the same M-mode OCT image with motion artifact correction applied.

[0232] FIGS. 13A and 13B show uncorrected and corrected OCT image M-mode data, respectively, of a bead-on-plate keyhole welding process. OCT image data was captured inline with the welding process, and the data represent the evolution of the weld keyhole throughout the process. The bead-on-plate process was performed on a copper substrate, the process speed was 150 mm / s, and the processing laser, an IPG Photonics YLS-6000, was configured with a welding head to emit a beam with 6 kW power and a 200 μm focal spot size. The corrections applied to the data to produce the corrected image in FIG. 13B include the methods described in the sections "Identifying Motion Artifact Distortion in SS-OCT Data" and "Imaging Target Velocity Estimation and Compensation," discussed above.

[0233] Head integration Integration into laser material processing heads To achieve inline coherent imaging (ICI) for laser material processing applications, according to at least one embodiment, one or more components of the system are integrated into a laser processing head. A schematic illustrating one possible configuration for such integration, including elements for steering the imaging beam and elements for steering the combined imaging and processing beams, can be seen in the non-limiting example of laser head 715 in FIG. 4. Some embodiments of the system will employ the use of dichroic mirrors (7.8) to combine the material processing beam and the imaging beam and separate them depending on the reflection from the process. When the imaging beam and the process beam have similar wavelengths, the optical design of the dichroic is critical.

[0234] Various embodiments of the present invention can be incorporated into different types of laser material processing heads, including, but not limited to, scanner heads where active beam steering is performed on the process beam (e.g., using galvanometers 7.9 and 7.10), wobble heads where the process beam is steered 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 tasks, such as cutting, cleaning, and additive manufacturing, can be developed based on the same general principles of process beam and imaging beam integration.

[0235] 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 achieved 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 a subspectral range of the light source), or using electromechanical optics. Some embodiments may direct light to 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 scaling or calibration benchmarks. Such benchmark paths may be achieved by splitting the imaging beam using partially reflective optics or using electromechanical optics.

[0236] In some embodiments, multiple sample arm beam paths can be introduced for the purpose of imaging multiple positions (e.g., aligned, front, back, left, right, etc.) relative to the processing beam 112. Measurements can be performed simultaneously or gated in time. Such measurements can be used, for example, in laser welding to direct the imaging beam to a keyhole (along the processing beam or immediately behind the processing beam), the surface of the material, the solidified weld surface (following the processing beam), or a region of interest on the welded material (e.g., seam location). Optical paths can be designed with intentional delays so that measurements from each path can be distinguished based on their position within the system's full Z field of view. Optical paths can be designed with intentional attenuation to compensate for differences in reflectivity on the sample (e.g., keyhole vs. surface). Optical paths can be configured to be spectrally selectable by controlling the swept source wavelength and sweep parameters.

[0237] In some embodiments of the system, the OCT sample arm imaging beam path includes at least one directing 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 in multiple positions relative to the processing beam (e.g., aligned, front, back, left, right, etc.). In certain embodiments, the steering element operates at a sufficiently high speed so that multiple positions relative to the beam can be repeatedly imaged within a single process. In certain embodiments, the imaging beam steering element can be used to assist in alignment or to align the imaging beam to compensate for optical phenomena affecting the imaging beam, such as chromatic aberration caused by the material processing beam optics.

[0238] In some embodiments, a common-mode interferometry configuration may 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, a coating on the laser head's cover glass (7.12) may be designed to partially reflect the imaging wavelength band, creating a common-mode reference arm path that terminates at the output of the laser head. Such a path may be used with a long-Z FOV system to reduce the effects of thermal drift on absolute OCT measurements.

[0239] Dichroic Mirror As pertinent to embodiments involving laser material processing, optics must be designed to combine the beams to partially or fully combine the imaging beam with the material processing beam (e.g., into a combined optical path). This optics must also be able to separate the imaging beam from other beams (e.g., process beams) and / or emissions (e.g., blackbody thermal radiation from the process) following reflection and / or backscattering of the imaging beam from the workpiece. A typical optical device that may 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 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, along with other design requirements as needed.

[0240] According to at least one embodiment, a beam delivery system (e.g., beam delivery system 115) is configured with a dichroic optic 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 optic over a range of angles of incidence. According to one embodiment, the dichroic optic is configured to have a transmission spectrum having a first band edge and a reflection spectrum having a second band edge, the first band edge and the second band edge having a maximum wavelength separation of 25 nm.

[0241] While the examples of laser processing heads described herein include the use of dichroic mirrors, it should be understood that other configurations are within the scope of this disclosure, such as off-axis integration of the imaging beam into the material processing beam by methods known to those skilled in the art.

[0242] Beam Steering and Alignment In at least one embodiment, the imaging beam path includes steering / deflection elements (eg, galvanometers, resonant mirrors, polygonal mirrors, acousto-optical devices, electro-optical devices) that specifically align its position with respect to the process beam.

[0243] These elements allow for more precise alignment of the imaging beam to different process regions, such as keyholes or seams, which may be of interest for metrology purposes. In some embodiments, steering elements may be used to steer the beam in a pattern that allows for collection of data about multiple process regions within the weld throughout the process (e.g., keyhole depth, seam tracking, finished weld surface height). In some embodiments, steering elements may 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.

[0244] According to at least one embodiment, the control for scanning the imaging beam is synchronized to the adjustment cycle of the imaging light source. In some embodiments, the imaging light source adjustment cycle is synchronized to the control for scanning the imaging beam. The synchronization serves to reduce position jitter in the obtained geometric measurements of the workpiece. Through precise synchronization of the imaging beam scan and the imaging light source adjustment, a lateral resolution of 20 microns or better can be achieved on the OCT system.

[0245] In some embodiments, alignment between the imaging beam and the material processing beam must be maintained, and in some embodiments the design of the laser material processing head may be sufficient to maintain this alignment, while in other embodiments active correction of the alignment using, for example, imaging beam path steering elements, is also appropriate.

[0246] Purpose Examples of uses The disclosed SS-OCT systems and methods can be employed for a variety of applications. Various embodiments of the present invention can be optimized for imaging and quality assurance of several applications, including, but not limited to, laser material processing (e.g., welding, cutting, marking, brazing, cleaning, scribing, sintering, powder bed additive manufacturing, and wire-fed additive manufacturing). In particular for laser material processing applications, an imaging beam can be incorporated into the material processing beam for accurate imaging of the process during the process, also known as inline coherent imaging (ICI). Such imaging can provide information about characteristics of the processed region. One example of a characteristic that can be characterized by such imaging is depth information of the processed region. In some applications, depth information of the processed region can include a range that includes at least 1 mm, a range that includes at least 5 mm, a range that includes at least 21 mm, and / or a range that includes at least 50 mm.

[0247] scan Some embodiments of the system incorporate the imaging beam into a beam delivery head, such as a scanner head, that uses optical or opto-mechanical elements (e.g., galvanometers) to move the material processing beam. Non-limiting examples of such heads include two-dimensional scanner heads such as the IPG Photonics 2D High-Power Scanner, three-dimensional scanner heads such as the IPG Photonics 3D High-Power Scanner (e.g., heads in which the focal length and position within the focal plane of the material processing beam can be adjusted by the beam delivery optics), and advanced scanner configurations such as polygon scanner systems.

[0248] In some embodiments, the wavelengths of the imaging beam and the material processing beam are selected to be spectrally close to each other to minimize chromatic aberrations that occur when the combined imaging and processing beams are steered toward the edge of the scanner head optics. For example, to minimize chromatic aberrations in embodiments 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 near the 1000-1050 nm wavelength range. Alternatively, if the material processing laser operates near 1030 nm (e.g., a disk laser), the imaging system can be centered closer to the 1040-1070 nm wavelength range. Some embodiments of the system can correct for chromatic aberrations using software and / or opto-mechanical methods, for example, by using beam steering corrections on the imaging beam in the operating region where chromatic aberrations are observed and realigning it as desired.

[0249] In system embodiments in which the material processing beam and the imaging beam are actively steered / scanned using elements in the beam delivery head, the optical path length to the workpiece can vary substantially throughout the scanning range of the beam delivery configuration. In embodiments of the present invention incorporating a 3D scanner head, the optical path length, specifically 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 can also exist in many other beam delivery head configurations, such as 2D scanners. In some embodiments, system parameters can be set such that the imaging range is appropriate to capture the full range of varying optical path lengths across the beam delivery head field of view without delay line adjustment. The substantial (>50 mm) coherence length available from certain SS-OCT implementations makes this approach possible. 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 throughout the scanning range.

[0250] In system embodiments incorporating 3D scanning, the imaging beam focusing and collimation apparatus may comprise electronic, optomechanical, or optoelectronic devices that allow for active adjustment of the focal point of the imaging beam, whereby the focal plane of the imaging beam may 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 may be used to provide active feedback regarding, for example, workpiece height and / or keyhole penetration to guide and verify 3D scanning performance.

[0251] In some embodiments of the system, calibrations may be performed to characterize the optical path length at various locations within the available field of view of the beam delivery head (e.g., the scan field of view of the scanner head). In some embodiments of the system, calibrations may be performed to characterize the chromatic aberration (CA) of the imaging beam relative to the process beam at various locations within the available field of view of the beam delivery head (e.g., the scan field of the scanner head). Either or both of these calibrations may form the basis for applying compensation methods to correct for variability in these parameters. Compensation methods may include computational corrections (e.g., algorithms) and / or physical corrections (e.g., imaging beam steering, reference arm adjustments). Calibrations may also be performed to characterize the field of view of the imaging beam steering optics, either independently of or simultaneously with the material processing beam steering optics.

[0252] Additional applications of system embodiments are possible using various active scanner and / or beam steering configurations implemented in the beam delivery head. For example, some embodiments of the system may incorporate a polygon scanner in the beam delivery head, which may be applied to welding, cutting, surface treatment, coating removal, patterning, and web processing.

[0253] High power welding Certain embodiments of the system may be designed for high-power and ultra-high-power weld monitoring (>20 kW). The imaging depth field of view of some tested embodiments of the disclosed SS-OCT system is large compared to conventional SD-OCT systems (e.g., 12 mm for conventional SD-OCT systems, but over 50 mm for certain embodiments of the SS-OCT system disclosed herein). The greater imaging depth than is available through SS-OCT opens the possibility of imaging welds at much greater depths and monitoring additional metrics, such as the workpiece surface and the finished weld surface. System embodiments designed for high-power welding may require that the design and integration of the material processing head be suitable to handle the energy and heat levels generated in such processes.

[0254] Reflectance measurement In some embodiments, reflectance measurements are performed in addition to, instead of, 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 reflectance and coherent measurements. In some embodiments, amplifier 106 may be used to increase the DC signal level for improved measurement signal-to-noise ratio (SNR). In such cases, a feedback signal sampled from the output of amplifier 106 may be used to control amplifier stabilization for more accurate reflectance measurements. Additionally, back-reflected measurement beam light from the sample may also be used as an amplifier control signal to normalize measurements from the surface of the part (workpiece).

[0255] Sequence Mode In some embodiments, the system may be designed to capture multiple sets of OCT captures, including A-line, B-scan, M-mode, volume capture, or other imaging formats known to those skilled in the art, corresponding to one or more subsequent tasks and / or material modification processes. Such sets of captures are referred to as a sequence of imaging tasks. Such sequences may include any combination of measurements of various specimen targets, such as fixed parts, seams, weld keyholes, and finished surfaces after processing. In some embodiments, imaging tasks within a sequence may be simultaneous with material processing. In some embodiments, imaging tasks within a sequence may occur before and / or after material processing or may correspond to metrology tasks or captures. In some embodiments, some or all of these types of imaging tasks are incorporated. Sequential imaging may, in some embodiments, include repeated measurements of aspects of the same process, or, in other embodiments, include sets of measurements of different subsequent processes.

[0256] An imaging process in sequence mode may incorporate one or more processing elements described in this disclosure, non-limiting examples of which include dispersion compensation, motion artifact compensation, or tracking data generation. According to some embodiments, imaging tasks in a sequence may be used to develop quality assurance (QA) metrics that may correspond to the quality of the process corresponding to the current imaging task, or to extract parameters for feedback control of either the current processing task or a subsequent processing task in the sequence. In some embodiments, QA metrics may be developed based on the results of multiple imaging tasks in the sequence, up to all tasks in the sequence.

[0257] 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 alarms to generate alerts and / or notifications when quality assurance metrics are not met for a task within the sequence, or for the entire sequence itself. Such embodiments can be integrated with other methods of generating notifications based on the results of the sequence.

[0258] Characterization of the workpiece before welding In certain embodiments of the system, the imaging system may be used to characterize a material-modified sample or workpiece at times other than during the material modification process. In such cases, the imaging beam may still be integrated into the beam delivery apparatus for the material processing beam to enable imaging of the workpiece along or proximate the location of the material processing beam. In some such configurations, the imaging beam may be steered independently from the material processing beam using a separate beam steering element in the imaging beam path. In some such configurations, the imaging beam may 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.

[0259] In some system embodiments, the SS-OCT system may be configured to measure the distance between the workpiece and the focal plane of the material modification beam optics. In material modification optics, such as a scanner head, where the material modification beam is actively steered, the SS-OCT system may be further configured to characterize the entire focal space or focal plane through which the material modification beam may travel. Such characterization may be completed as a calibration prior to the material modification processing step. In system embodiments integrated with material modification optics where the focal length may be actively adjusted, the SS-OCT system may be used to provide feedback regarding the characteristics / geometry of the specimen and / or workpiece, such as the distance between the focal plane of the material modification beam optics and the workpiece surface, or measurements of the position and geometry of fixture components for mounting evaluation. In such systems, workpiece geometry measurements may be used in active feedback control of the material processing beam head, for example, to provide “autofocus” or weld path correction capabilities.

[0260] Top surface reference point measurement (TSRP) In some embodiments, one or more imaging beam positions can be used to measure a top surface reference point (TSRP), and further embodiments include the ability to simultaneously measure the TSRP and weld depth using multiple imaging beams. The TSRP is a reference position established using a point on the sample surface; if the sample is substantially flat, at least one TSRP can be used to define a top surface reference plane. While discussed herein in the context of setting the TSRP before 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 accomplished by obtaining baseline depth measurement(s) at a location on the sample that is not affected by the welding process. In some examples, the TSRP can be determined by obtaining one or more measurements of the material immediately before welding begins. If the material is sufficiently flat for weld movement, this initial measurement can define the TSRP for the remainder of the weld. Other techniques for measuring the TSRP are known in the art.

[0261] Seam Tracking In certain embodiments of systems including laser welding, an imaging system may be used to obtain measurements taken at a specific distance along the path of the material processing beam during processing, far enough forward from the processing beam focal point so that the measurements are not affected by the process itself, but close enough to the focal point so that accurate measurements of the part's mounting and / or geometry (e.g., the seam line between two components being joined by the welding process) can be used to determine whether the focal point is sufficiently aligned with the seam line. In certain embodiments, the imaging system may be used to obtain measurements of the part's mounting and / or geometry, for example, as a prescan, before the start of material processing, or after material processing. In one example, a scanner may be used in combination with the imaging beam to identify one or more surface features (e.g., grooves) that can be used to provide measurements of the seam location and / or geometry before the material processing beam reaches a given location in the process. In certain examples, these measurements may further be used to dynamically correct for misalignment, for example, by providing real-time feedback to a material processing beam steering device.

[0262] 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. Similar to seam tracking, in some embodiments, the imaging beam can be configured to scan the desired feature before the start of the material modification process. In system embodiments that enable material pre-processing scanning, the processor 150 can be configured to provide real-time feedback based on imaging of the desired feature for purposes of closed-loop feedback control of the material modification process.

[0263] In some embodiments, measurements of seam location and / or geometry, or other measured pre-weld characteristics of interest, may be used in the development of quality assurance (QA) metrics.

[0264] Post-weld characterization In certain embodiments of systems involving laser welding, an imaging system may be used to obtain measurements taken at a specific distance along the path (geometry / orientation) of the material processing beam during processing, far enough back from the processing beam focus so that the measurements are not affected by the process itself, but close enough to the focus so that accurate measurements of the finished weld surface can be used to determine aspects related to the quality of the weld.

[0265] In some embodiments, finished weld surface measurements, or other notable post-weld geometric aspects, may be used in the development of QA metrics.

[0266] 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 following the welding process. For example, measurements can be used to inspect the seam surface following welding to identify the presence of geometric features and / or physical defects (e.g., blowouts). In some embodiments, the imaging beam can be configured to scan the desired features following completion of the material modification process.

[0267] Additive Manufacturing In certain embodiments, material modification may be part of an additive manufacturing process, a subtractive manufacturing process, or some combination thereof. Additive manufacturing processes may 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, which may be broadly categorized as additive and / or subtractive, may include features that provide feedback of particular interest to such processes, some non-exclusive examples of which are detailed below.

[0268] Embodiments of the present invention designed to work with additive manufacturing processes involving the supply of additive manufacturing precursor materials (e.g., powders, wires) can be configured to generate measurement outputs that characterize relevant metrics of precursor supply and / or quality. One example of such a metric is the flatness of a layer of powder deposited during a powder bed fusion additive manufacturing process. In some embodiments, the additive manufacturing setup can be configured to make corrections in response to measurement information acquired by the SS-OCT system. For example, selective laser ablation and / or remelting can be applied to correct part flatness based on OCT measurements taken during or after deposition. Additional measurements, such as measurements of keyhole features / geometry, measurements of deposited tracking surfaces that follow the keyhole, and measurements of the geometry of the part being built, can also be performed. In some embodiments, the SS-OCT system can be used to track measurements of manufactured parts against a model (e.g., a CAD model). Some embodiments of the system can be configured to provide real-time feedback to update and modify build parameters of the additive process to maintain desired properties.

[0269] General Feedback Control Using real-time feedback provided by the imaging system, process variables can be modified to compensate for feedstock variations (e.g., poor attachment) and instabilities (e.g., weld depth variations) in the process as the process accelerates to higher speeds and / or penetrates to greater depths. This can extend the usefulness of the laser welding system to include lower-cost input materials, higher processing speeds, and / or deeper penetration, while maintaining acceptable quality. Real-time feedback can also be applied to enable 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 the power of the laser can be increased via a feedback mechanism when the keyhole depth as measured by the system deviates from a target value. There are many factors that can cause such deviations in weld keyhole depth in a typical material processing system, including degradation of the laser module's output power over time, contamination or obstructions in the beam path (e.g., smoke or weld spatter on the beam delivery optics), etc. Changes occurring within the time span of one material processing process, such as mismatches in material parameters of processed specimens, or changes occurring over the course of many material processing processes, such as changes in ambient conditions, can sometimes contribute to changes in the output quality of the process, such that key process metrics are no longer within target specifications. Real-time feedback provided by the imaging system can sometimes be applied to active feedback control of process parameters to keep the deposition process within target metrics. In some cases, real-time feedback methods may implement control loops and / or theories known to those skilled in the art, including proportional control and / or proportional-integral-derivative control. Feedback control implementations may be based on feedback derived from measurements and / or signals provided by the SS-OCT system and may additionally incorporate one or more of signals and / or signals provided by auxiliary sensors within the SS-OCT system, measurements and / or signals of other sensors within the material processing system, and models of various components of the material processing beam system.

[0270] Transient effects during the initiation or termination of a welding procedure can have adverse effects on the outcome of the weld (e.g., inconsistent seam depth, underfill). For example, quantifying the amount of underfill along the surface of the seam is important information for determining the strength of the weld, its corrosion resistance, and compatibility with subsequent coating processes, such as priming and painting. According to at least one embodiment, feedback provided by the imaging system can be used to reduce these defects by controlling one or more process parameters to compensate for the transient behavior of the weld at the initiation, termination, or both of the weld.

[0271] Additional System Capabilities and Features Auxiliary Measurement System According to at least one embodiment, the SS-OCT system disclosed herein may also include an auxiliary measurement system 160, which is in communication with the controller 150 and includes auxiliary sensors, such as visible and / or IR sensitive photodiodes, and / or cameras and / or spectrometers, which may, in some examples, be coupled to the welding head by 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.

[0272] Note that, unlike biological applications of coherent imaging, material processing applications, as the primary focus of this disclosure, are often characterized by the presence of incoherent light sources that are incoherent with the imaging light in the form of blackbody radiation and the material processing beam light itself. In spectrally differentiated SD-OCT approaches, much of this light is automatically discarded by the spectrometer. According to at least one embodiment, additional filtering elements (FBGs, WDMs, and other forms of filters known to those skilled in the art) are added at one or more locations within the sample arm of the SS-OCT system to ensure that these incoherent light signals do not reach the main detector (e.g., detector 130 in FIG. 1). However, it is potentially useful to divert these signals to an auxiliary detector. Several possible arrangements of such auxiliary detectors within the sample arm of the interferometer are shown schematically in FIG. 5A. One possible splitting method is shown in the example beam delivery head 315 in FIG. 5B, which shows a schematic diagram of the material processing beam head 315 with an additional dichroic (3.4) to split the blackbody radiation signal to a detector (3.5) in the imaging beam portion of the head. Splitting of these samples can also be achieved in the fiber portion of the interferometer. The blackbody radiation signal is back-coupled into the sample arm fiber and then split from the OCT signal using a wavelength division multiplexer, such as WDM 614 in FIG. 5A, to enable detection with a dedicated detector, such as auxiliary detector 611 in FIG. 5A, thereby allowing simultaneous detection of blackbody radiation and SS-OCT data. To enhance back-coupling of broadband blackbody radiation, some embodiments employ a double-clad fiber, so that the imaging beam light reflected from the workpiece is coupled into the single-mode core portion of the fiber, while the blackbody radiation is coupled into the multimode inner cladding portion. 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 US Pat. No. 10,898,969, which is incorporated herein by reference.

[0273] One non-limiting example of an implementation including auxiliary detection is to position one or more photodiode sensors for blackbody radiation detection within the beam steering module for the imaging beam, as in the case of auxiliary sensor 613 in FIG. 5A. One possible configuration for such an auxiliary detector is shown in laser head 315 in FIG. 5B, and is described in more detail below. Note that to enable blackbody detection simultaneously with OCT imaging, it may be necessary to include an additional detector (e.g., blackbody sensor 3.5 in FIG. 5B) and ensure that the desired blackbody wavelength can pass through one or more dichroic optics (e.g., dichroic mirror 3.8 in FIG. 5B, which splits the imaging beam and material processing beam as they reflect from the process) while being properly separated from the OCT beam by using appropriate optics (e.g., blackbody signal dichroic 3.4 in FIG. 5B).

[0274] One non-limiting example of an implementation involving auxiliary detection involves including a double-clad fiber containing a single-mode core with an additional inner cladding designed to allow the propagation of multimode light. This configuration is represented by the double-clad fiber 610 and auxiliary detector 612 in FIG. 5A. Details of the double-clad fiber (610A) are shown in FIG. 5A, showing the multimode inner cladding (610B) and single-mode core (610C). An advantage of implementing a double-clad fiber for the detection of blackbody signals is that the larger surface area of ​​the multimode inner cladding allows for greater back-coupling of broadband and incoherent light generated by blackbody and / or thermal radiation, enabling the detection of a stronger signal in the auxiliary detector (e.g., auxiliary detector 612 in FIG. 5A). To enable the detection of blackbody radiation, the multimode light captured in the inner cladding of the double-clad fiber can be coupled into a separate multimode fiber.

[0275] Coupling an additional optical signal, such as blackbody radiation or a back-reflected processing beam, into the sample arm is generally referred to as wavelength multiplexing 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 to aid in aligning the coordinate systems of the process beam and imaging system. The latter approach is implemented by using one or more directing elements to scan the imaging system around the area where the material processing beam source is impinging on the workpiece, calibration target, and / or material test coupon. While this is happening, the material processing beam source is operated (preferably at low intensity) to a) generate reflected material processing beam energy, b) generate blackbody radiation, or c) do both. As the directing elements scan this area, the primary and / or secondary detectors receive 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 through means including peak finding, maximum finding, Gaussian curve fitting, super-Gaussian curve fitting, and other fitting functions used in beam characterization known to those skilled in the art. By selecting which wavelengths of radiation are allowed into the detector, this approach advantageously allows for compensating and mapping chromatic aberrations in the beam delivery system. This mapping can be stored in memory and used by the control module and / or directing element control electronics to enable the directing element to compensate for chromatic aberrations. PCT Patent Application No. PCT / US2021 / 027672, owned by the applicant and incorporated herein by reference, discusses and outlines this process in more detail.

[0276] Capturing the energy spot width of the blackbody radiation or the back-reflected processing beam and using it as a proxy 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 the back-reflected process beam light and blackbody radiation from the OCT optical detection elements. However, in some embodiments, the OCT optical detection elements and the back-reflected processing beam and blackbody radiation collection elements 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 signals via different photodiode detection schemes (e.g., balanced vs. unbalanced), through digital processing (e.g., low-pass, band-pass, or high-pass filtering), through time gating (e.g., collecting OCT signals from auxiliary radiation at different times), or some combination thereof. Time gating can be achieved by turning off the OCT system light source, by reducing the OCT system light source in an amplifier, or through additional filtering elements.

[0277] In some embodiments, the detected process blackbody radiation may be used to calculate an estimate of the temperature of the process or location within the process. This metric may be used to track the stability of the process temperature over time and record absolute or relative values. When involving beam steering of the imaging beam, such measurements may also be used to estimate absolute or relative temperatures throughout the process in space.

[0278] In some embodiments, a portion of the sample arm includes a double-clad MM / SM mode-stripping fiber component, such as a Castor Optics DC1060LE (Montreal, QC, Canada). This enables a variety of functions, including integrated time-of-flight (TOF) / frequency-modulated continuous wave (FMCW) light detection and ranging (LIDAR) detection, and, optionally, with the aid of additional filtering elements, the ability to detect larger amounts of incoherent radiation (for higher SNR-assisted sensing). The multimode channel can also be used to collect additional reflected imaging light, which is transmitted to an auxiliary photodetector to implement closed-loop feedback of the imaging light source's emitted power. The multimode channel can also be used to collect back-reflected process beam light, blackbody radiation, or a combination thereof.

[0279] Interferometer Features In some embodiments, the interferometer has at least two reference arm paths of similar or different lengths. The reference arm can also be configured with at least one actuator to select or deselect one or more of the paths. The selection or deselection means can include adding attenuation to the beam path. Path attenuation can be induced by moving a lens, closing a shutter, activating an optical switch, adding absorbing or reflective optics to the path, changing the polarization of the light, rotating polarization optics, and / or changing the alignment of mirrors in the beam path, including end mirrors. Path selection can also be spectrally selective, using spectrally selective optics (e.g., a dichroic mirror) in conjunction with splitting the swept-source spectral band into two regions (one dedicated to each reference arm). The reference arm is designed so that all paths can be simultaneously illuminated and deselected by the control module. Interestingly, illuminating more than one reference path generates a common-path interference signal that is suppressed by the balanced detection scheme shown in the SS-OCT system 100 of FIG. 1. However, the common path interference signal can nevertheless be recovered by monitoring the single-ended (i.e., unbalanced) detection output from the BPD.

[0280] This is a means of simultaneously measuring the K-clock signal from a single interferometer, with the added benefit of being able to simultaneously compare two known paths to confirm the axial calibration of the system. Finally, it must be emphasized that while simultaneous illumination of multiple reference paths is sometimes considered a waste of optical power by those skilled in the art, the light source described herein generates an extraordinary abundance of imaging light, which means that such a more powerful / practical approach, while less efficient, can be considered without significant loss to image quality.

[0281] According to at least one embodiment, the amount of optical power returning from at least one reference arm is adjusted to shift the dynamic range of the imaging system. In situations where strong reflections return from the sample, it may be desirable to attenuate the reference arm so as not to saturate the detector. This can be implemented as a closed-loop control using the main BPD signal, the single-ended BPD signal, or an auxiliary photodiode serving as the input, and any of the means for attenuating the reference path described in the previous paragraph serving as a variable attenuator and / or actuator. Proportional (P), proportional-integral (PI), and proportional-integral-differential (PID) control schemes are all possible via the system's control and / or feedback module. Another way reference power and dynamic range can be achieved is by modulating the light source power, as described herein.

[0282] According to certain embodiments, a visible light “guide” or “pointing” beam is injected into the imaging beam, for example, via the sample arm, and transmitted through the head to the workpiece. This can be done by a WDM device or other similar components known to those skilled in the art. Adding a guide beam to the imaging system offers various advantages. The core diameter of the imaging fiber is often much smaller than that of the process fiber, creating a sharper point on the focal plane and less modal interference within the fiber. This makes it easier to accurately view the point. By steering the guide beam with a directing element, a visible pattern can be written onto the surface of the workpiece to assist in teaching and aligning the material modification process as well as the imaging system itself. Showing a visible beam representing the imaging system scan also aids in the interpretation of the imaging system data. In further optimization of this approach, the timing of the imaging system acquisition, the scanning path of the directing element, and the emission of the guide beam can be adjusted to compensate for transverse chromatic aberrations in the beam delivery system so that the true imaging path is represented by the guide beam.

[0283] In some embodiments, as shown in FIG. 3, one or more booster amplifiers can be added to the sample arm of the interferometer to increase the sample arm optical power. For example, amplifier 406b is configured to amplify the returned imaging light while other optical signals are blocked. The dispersion and optical path length introduced by the introduction of such an amplifier can be compensated for 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 weak signals are amplified more than strong signals in relative terms. This has the additional benefit of effectively compressing the dynamic range of the system. A control module and / or feedback module (not explicitly shown in FIG. 3 but similar to processing unit 150 in FIG. 1) controls the gain on this amplifier by modulating the pump current of its diode.

[0284] According to some embodiments, a high population inversion is employed to shift the amplifier gain to shorter wavelengths, thereby adjusting the imaging system's operating spectrum further away from the spectra of high-power lasers used for material processing such as welding. For example, a high population inversion YDFA may be employed to produce an amplifier gain peak in the 1020-1050 nm region, thereby keeping the imaging system's operating spectrum away from the typical 1070 nm operating wavelength of selected ytterbium-doped fiber lasers.

[0285] Because stress differences or temperature changes in the interferometer fiber can cause path length variations in the reference or sample arms of the interferometer, in some embodiments mechanisms can be included in the sample arm for compensation, including adjustable mandrels, 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.

[0286] According to at least one embodiment, e.g., 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, allowing the fiber to be used to compensate for the air path length and reducing the dispersion effect on the point spread function of the imaging system.

[0287] According to certain embodiments, one or more portions of the interferometer are constructed using polarization-maintaining (PM) fiber. In this configuration, it is advantageous to illuminate the interferometer with light of a single polarization to reduce path length degeneracy due to fiber birefringence. An advantage of using PM fiber is that manipulation of the sample and reference arm fibers causes less fluctuation 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 the light it emits with the preferred polarization of the interferometer.

[0288] multiplexing According to at least one embodiment, the imaging light source module is shared / multiplexed among multiple interferometers, which may be associated with multiple beam delivery systems, material processing beam sources, detectors, etc. An exemplary configuration is shown in FIG. 6. When using swept lasers in a multiplexed configuration, special care must be taken when selecting the lengths of the fiber and signal cables so that any phase shift between the interferometers is minimized or compensated for. 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.

[0289] In some embodiments, a single k-clock positioned in or before one of the interferometers is used as the k-clock for multiple interferometers when one light source is shared among multiple imaging systems. This configuration means that the time delays between the various multiplexed interferometers / detectors must be taken into account when applying the k-clock signal to the signal processing chain. However, in other embodiments, the k-clock is implemented by sampling the reference arm signal instead.

[0290] In some embodiments, the imaging system scan control is synchronized to the imaging light source adjustment cycle to reduce position jitter in the resulting geometric measurements of the workpiece. In at least one embodiment, the phase and frequency of the light source adjustment cycle are measured and used to calculate a position offset that is added to the geometric measurements to compensate for jitter / variation in the starting phase of the light source adjustment cycle. Ideally, the clocks that drive the deflection system and the light source adjustment signals are themselves synchronized to a common electronic clock.

[0291] According to certain embodiments, the interferometer unit further comprises a shutter or other beam blocking device operated by a safety module to ensure optical safety. One way this can be achieved is by monitoring the position of one or more directing elements and using safety logic to ensure that the light source (in combination with the directing elements that allow the beam to exit the beam delivery system) cannot operate at a potentially hazardous power unless a safety interlock is satisfied.

[0292] Autofocus for applications with significant imaging depth variation Certain embodiments may take advantage of the available deep imaging field of view to the extent that the imaging depth field of view is greater than the depth range for which the imaging beam is adequately focused. In such applications, optical, mechanical, and / or electronic elements that enable a shift in the depth of focus may be integrated into the system. One example of enabling a shift in the depth of focus within the field of view is a variable-focus lens, an electro-optical technology. Some embodiments of the system may utilize feedback control between the focusing element and the return power or image data to enable "auto-focus" operation. Possible optimizations in focus include, but are not limited to, optimizing the focus of the imaging beam to align with the workpiece surface, or the bottom of the keyhole, or the surface of the completed weld.

[0293] Sweeping the focal depth plane of the imaging beam offers the potential for many useful measurement algorithms. A fit of the backreflected intensity to wavelength can be used to detect the focal plane of the system. Measuring the peak possible amplitude of all interfaces in an imaging frame by sweeping the focus across the entire Z range can help identify the true brightest interface by algorithmically correcting for the effects of defocus. The focus can also be adjusted to track variable interface depths, for example, by using pre-weld scan imaging data to estimate the next interface depth and adjusting the focus in response to the next depth.

[0294] Interferometer Topology According to various embodiments, the interferometer is implemented using fiber-based light path components, or free-space light path components, or some combination of fiber-based and free-space light path components. In some embodiments of systems including fiber-based interferometer components, polarization-maintaining fiber may be used. In some embodiments of systems including fiber-based interferometer components, single-mode fiber may be used. The dispersion present in the sample and reference arms of the interferometer may, in some embodiments, be designed to be as well matched as possible to improve imaging performance. The implementation of the interferometer in a particular embodiment of the present invention may vary based on the requirements of the embodiment and may incorporate interferometer components and methods known to those skilled in the art.

[0295] According to some embodiments, a Mach-Zehnder interferometer (an example of which is shown in FIG. 1) is preferred because it simultaneously allows for easy adjustment of the initial splitting ratio from the imaging light source and balanced detection. However, typical implementations of this topology require two Faraday devices, which can increase cost and complexity. In a Mach-Zehnder interferometer configuration, care must be taken in the interferometer design to ensure that the two paths to the BPD 130 are nearly coincident so that the positive and negative interference signals arrive at the BPD simultaneously.

[0296] Because the power of the amplifier source can be high enough so that it is not (for practical intents and purposes) a limiting factor in the optical design, in some embodiments, a Michelson interferometer can be used with (e.g., detector 130 in FIG. 1 ) or without (e.g., detector 230 in FIG. 7A ) balanced detection with little overall penalty to signal-to-noise ratio (SNR) / sensitivity. FIG. 7A is a schematic diagram of an SS-OCT system 200 configured with a Michelson interferometer, illustrating how this can be achieved with respect to interferometer topology. In some embodiments where the power of the amplifier source is not considered a limiting factor, a Mach-Zehnder-like interferometer can be constructed by replacing the circulators (121, 123 in FIG. 1 ) with 50:50 splitters (521, 523 in FIG. 7B ); an example of such an interferometer is shown in FIG. 7B . This configuration may offer a cost advantage over implementations that include circulators. While such interferometers are less photon-economical than more standard Mach-Zehnder embodiments, it should be noted that in some embodiments, amplifiers can provide exceptionally high power, enabling high return interferometer power even in interferometers such as those shown in FIG. 7B. In any interferometer configuration, care must be taken in the interferometer design to ensure that dispersion mismatch is minimized. Although not explicitly shown, one or more imaging source amplifiers similar to amplifier 106 of FIG. 1 may also be included in system 200 and / or system 500.

[0297] 1 uses a 90:10 split ratio between the sample arm and the reference arm, which has the advantage of greater photon economy between the light source, workpiece, and detector. It is recognized 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.

[0298] In some embodiments, the interferometer topology is a demodulating interferometer.

[0299] According to certain embodiments, the reference path includes elements that allow control of the reference signal power. This can be achieved through the construction and selection of different reference paths using splitting methods and optical elements known to those skilled in the art. Alternatively, this can be achieved by introducing attenuation into a single reference path as needed. Illuminating 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 enabling a full BPD detection range at different sample power return conditions and preventing saturation. In embodiments applied to processes where multiple reflectivity conditions exist or may exist on the sample arm, this configuration of the reference path allows optimization of the return power for various sample powers, improving the system's dynamic range at each individual return power condition, thereby effectively increasing the system's equilibrium dynamic range. Illuminating reference paths of different lengths or varying the length of a single reference path (e.g., according to the methods described above under "Interferometer Features"), along with adjusting any system timing as needed, can, for example, modify the system's imaging range or the axial resolution of a given system during operation.

[0300] Dual interferometer / dual or buffered light source SS-OCT topology As described herein, information from measurements taken using different dk / dt values ​​can be applied to calculate corrections that reduce or otherwise eliminate the effects of motion artifacts. A further concept described herein enables the ability to simultaneously acquire forward (positive) and reverse (negative) sweeps (dk / dt). Some concepts related to this capability are outlined below, with physical embodiments and further details also included below. Generic concepts include the use of a single imaging light source or multiple imaging light sources used to simultaneously illuminate 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 "share" a common sample arm)), delay lines, particularly implementations of delay lines integrated into the light source module that enable output of a delayed imaging optical signal simultaneously with a non-delayed imaging optical signal from the light source module, and imaging light sources configured to simultaneously generate separate (i.e., substantially different) swept optical signals.

[0301] 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 wavenumber k of the imaging light 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 some embodiments, the tunable light source 105 is configured so that the imaging light signal includes a superposition of multiple optical signals. In some embodiments, the multiple optical signals that are superposed to create the imaging light signal have different wavenumbers k and / or different tuning rates dk / dt. In such embodiments, the separate tuning rates dk / dt may overlap with each other, including, for example, by sharing one or more optical elements. In some embodiments, the tunable light source 105 is configured to generate multiple separate imaging light signal outputs. In some embodiments, the separate outputs generate imaging light signals that differ in output, for example, optical power, wavenumber k, and / or tuning rate dk / dt. In some embodiments, the tunable light source 105 is configured such that the imaging light signal includes a superposition of at least two tuning rates dk / dt. In further embodiments, the superposition of different tuning rates dk / dt includes at least one negative dk / dt and at least one positive dk / dt.

[0302] According to at least one embodiment, the SS-OCT system includes a first tunable light source and at least one other tunable light source. Non-limiting examples of such configurations are shown in Figures 18A and 18D (e.g., tunable light sources 805a and 805b). A first interferometer is configured with the first tunable light source (e.g., 805a in Figures 18A and 18D), and at least one additional interferometer is configured with at least one other tunable light source (e.g., 805b in Figures 18A and 18D). 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. 18A and 18D , sample arm 824a (of the first interferometer) and 824b (of at least one additional interferometer) share at least one optical element, e.g., the fiber of the overlapping sample arms, prior to introduction into beam delivery head 815. It should be understood that an optical fiber is just one non-limiting example, and other optical elements that may be shared include collimators, free-space beam directing elements (e.g., beam directing 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., scan / wobble galvos, process beam lenses, other lenses, cover glasses), and / or any other fiber-based elements (e.g., circulators, isolators, fused fiber couplers, or other fiber elements specific to a particular design or topology). In a further 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 the 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 adjustment rate dk / dt is positive dk / dt and the second adjustment rate dk / dt is negative dk / dt. In a further 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 a further embodiment, the first and second adjustment rates are associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections (applied in response to detecting the distortion), wherein calculating the corrections may include identifying a distortion in at least one of the one or more interferometer output signals associated with at least one of the first and second adjustment rates and performing an evaluation of the distortion, and wherein the one or more corrections for the at least one interferometer output signal are based on the evaluation.

[0303] 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. Non-limiting examples of such configurations are shown in FIGS. 18B, 18C, and 18E (e.g., optical delay element 825 in FIGS. 18B, 18C, and 18E). The optical delay element (e.g., 825) is configured such that an output of a first arm of the at least two arms is delayed in time relative to an output of a second arm of the at least two arms. In further embodiments, the processing unit 150 is configured to control the tunable light source (e.g., 805 in FIGS. 18B, 18C, and 18E) such that a time rate of change (adjustment rate dk / dt) of at least one wavenumber k of the imaging optical signal includes at least two adjustment rates dk / dt. In some embodiments, the at least two tuning ratios dk / dt include a positive tuning ratio dk / dt associated with either the first arm or the second arm and a negative tuning ratio dk / dt associated with the other of the first arm or the second arm. According to various embodiments, the first arm is configured to be oriented toward 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. This is shown in Figures 18B, 18C, and 18E. 18B, 18C, and 18E, the delay arm (passing through optical delay element 825) is directed to overlapping sample arms of a first interferometer and at least one additional interferometer (e.g., in FIGS. 18B, 18C, and 18E, sample arm 824a (of the first interferometer) and sample arm 824b (of at least one additional interferometer) share at least one optical element, e.g., the fiber of the overlapping sample arms, prior to introduction to beam delivery head 815). It should be understood that this is just one non-limiting example, and that other embodiments exist in which delay arms are directed to different reference arms of multiple interferometers, different sample arms of different interferometers, and / or overlapping reference arms of multiple interferometers.In some embodiments, the first and second arms 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 and 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 Figures 18B, 18C, and 18D, 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, e.g., a fiber of the overlapping sample arms, prior to introduction into the beam delivery head 815. Other examples of optical elements that may be shared are discussed above. In further embodiments, the first and second arms of the imaging optical signal are configured to be simultaneously directed to the first interferometer and at least one additional interferometer. For example, the delay arm and the "non-delay" arm of the imaging optical signal can be simultaneously directed to one or more sample and / or reference arms of the first interferometer and at least one additional interferometer. In further embodiments, 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 of the first interferometer or the at least one additional interferometer. For example, the delay arm can be directed to the sample and / or reference arm of the first interferometer, and the second (non-delay) arm can be directed to the sample and / or reference arm of the at least one additional interferometer (or vice versa). In a further embodiment, the at least two adjustment rates dk / dt of the first and second arms are associated with one or more interferometer output signals, and the processing unit is configured to calculate one or more corrections (to be applied in response to detecting the distortion), wherein calculating the one or more corrections includes identifying a distortion in at least one of the one or more interferometer output signals associated with at least one of the positive and negative adjustment rates and performing an evaluation of the distortion, and the one or more corrections for the at least one interferometer output signal are based on the evaluation.

[0304] According to at least one embodiment, the SS-OCT system may be configured to illuminate the sample using two or more substantially separate swept sources of imaging light.

[0305] In some embodiments, the SS-OCT system may be configured to simultaneously illuminate two or more optical interferometers using one or more substantially separate swept sources of imaging light. Some embodiments may include 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.

[0306] According to at least one embodiment, the SS-OCT system can be configured so that the sample is illuminated using two or more substantially separate swept sources 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, such as 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. The substantially different swept sources of imaging light can be derived from two or more light sources. Alternatively, the substantially different sources of imaging light can be derived from light emitted from a single light source. For example, the optical signal can be split, and at least one of the split optical signals can be modified or delayed by methods known to those skilled in the art (such as a delay line, optical buffering, etc.) before arriving at the sample arm so that, at the time each separate optical signal arrives at the sample arm, the imaging light is substantially different from the other imaging light at the sample arm.

[0307] Numerous topologies exist for generating two or more substantially distinct sources of imaging light. For example, two or more separate light sources may be used to generate the substantially distinct imaging light. Such light sources may be synchronized or coordinated using various electronic signals to enable the functionality of the optical system. A schematic diagram of one non-limiting example of such a topology, incorporated into an interferometric SS-OCT topology with separate light sources and a shared sample arm, is shown in FIG. 18A.

[0308] Another example incorporates the concept that two or more substantially separate sources of imaging light may be generated by splitting light from a single light source along two paths, with one of these paths being directed over a substantially different distance than the other paths. At the output of the paths, the light from one path is substantially delayed compared to the light from another path. Directing light into paths of selected optical path lengths for the purpose of introducing a delay into the propagation of the light is commonly referred to by those skilled in the art as implementing an optical delay line. A schematic example of such a topology using a single light source and an optical delay line to generate two substantially separate light beams that are incorporated into a dual interferometer SS-OCT topology is shown in FIG. 18B.

[0309] The optical delay line can be designed according to one or more desired characteristics of the imaging light source to be generated, for example, to provide a specific phase or timing delay. The wavelength tuning of light delayed by the delay line can be, for example, 180 degrees out of phase with the wavelength tuning of the undelayed light when it reaches the sample arm due to the selected design parameters of the delay line. The optical paths can be designed so that the difference in optical path length is greater than the coherence length of the light source, which means that the temporal coherence is also reduced after the light passes through the optical delay line. The reduction in the phase relationship (temporal coherence) of light passing through one path relative to the other path results in a reduction or elimination of interference between the two paths when or if the two paths overlap.

[0310] A non-limiting example of an embodiment utilizing a single light source and delay line is as follows: For a swept source emitting light at 1060±30 nm in a fiber with a refractive index of 1.4, constructing an optical delay line for 180 degrees of delay at a sweep frequency of 100 kHz requires a fiber path length difference of approximately 1071.4 m between the separate paths to introduce the desired delay.

[0311] According to at least one embodiment, the above methods for generating two or more light sources may be combined, for example, two or more separate light sources may be split and delayed as many times as necessary to generate the required light source.

[0312] According to at least one embodiment, the SS-OCT systems disclosed herein are configured so that one or more substantially separate swept sources of imaging light are used to simultaneously illuminate two or more optical interferometers. A preferred embodiment may include two or more interferometers that at least partially overlap at the sample (or “sample arm”), such that each interferometer may be used to image the same sample. Three examples of topologies in which two separate interferometers at least partially overlap are shown in Figures 18A, 18B, 18C, 18D, and 18E. Variations on such interferometers are also possible and are considered within the scope of the present disclosure; for example, it should be understood that the version of the interferometer in Figure 18B includes K-clock Module 1 as an optical K-clock but does not include K-clock Module 2, and K-clocking of the buffered optical signal is addressed by applying a time-delayed copy of the signal from K-clock Module 1. Figure 18A illustrates an embodiment with two separate, partially overlapping interferometers, each configured with a separate tunable light source. 18B and 18C show embodiments in which an optical delay line 825 is implemented, such as by applying the techniques described in the "Multiple Light Sources" section above, to generate separate swept optical signals for input to each of the overlapping interferometers. FIG. 18D is another example of a configuration with two separate tunable light sources, illustrating an interferometer topology that can be constructed without the use of an optical circulator, and FIG. 18E illustrates this concept of a circulator-less interferometer with a buffered light source that provides separate swept optical signals for input to each of the overlapping interferometers. Variations on dual interferometer topologies based on the interferometer features, topologies, and functionality described herein are also considered within the scope of this disclosure.

[0313] According to at least one embodiment, the multiple interferometers are configured such that each interferometer is illuminated by only one swept light source, so that the light in each interferometer is substantially indistinguishable. For example, an SS-OCT system can be configured with only one light source, but with multiple interferometers that all overlap in the sample arm and have separate reference arms. By configuring each reference arm differently according to the system requirements, this method allows for simultaneous capture of multiple different depth ranges within the field of view.

[0314] According to at least one embodiment, the multiple interferometers are configured to be illuminated by substantially separate imaging light sources. For example, each different interferometer can be configured with a swept-source light source corresponding to a different tuning ratio. If 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, causing light from one light source to interfere with light from another light source corresponding to a different interferometer. If the substantially separate light sources are generated by a single light source, a splitter, and a delay line, optical paths that are delayed relative to each other by more than the coherence length of the light source also lack sufficient spatial and temporal coherence to substantially interfere with each other. In this way, as long as the relative coherence of light in any pair of multiple interferometers is negligible, the interferometers and their respective interferometer output signals can be considered independently for analysis and processing of the interferogram, despite overlapping in the sample arm.

[0315] According to at least one embodiment, the two interferometers are illuminated with substantially different light sources, such that the instantaneous rate of change of the wavenumber k of the light source corresponding to one interferometer with respect to time (dk / dt) is substantially different from the instantaneous rate of change of the wavenumber k of the light source corresponding to the other interferometer with respect to time (dk / dt) (for at least some portion of the source sweep duration).

[0316] An example application of such an embodiment is when at least two interferometers are illuminated with substantially separate swept light sources, such that the swept light source of one interferometer has a substantially different dk / dt at a given time (or series of times) compared to the dk / dt of at least one other interferometer. The difference in dk / dt may include a difference in the sign of dk / dt. All of these interferometers are combined such that they share a sample arm, and all of the substantially separate swept light sources simultaneously illuminate this sample arm. As discussed above, 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. In this case, when at least two interferometers are illuminated with light sources having substantially different dk / dt at a given time, interferometer output signals associated with at least two different dk / dt values ​​may be simultaneously generated. This allows for assessment of motion artifact effects on time scales shorter than those required to acquire data to generate a complete A-line for a given system (the sub-A-line time scale), as well as greater time-domain precision regarding the position and velocity characteristics of the workpiece. The observed differences between the interferometer output signals can be used to assess motion artifacts.

[0317] In certain embodiments of at least two interferometers where the dk / dt at a given time are illuminated with substantially different light sources, the symmetry between the sweep patterns of the different interferometer light sources and the dk / dt can be exploited. For example, this symmetry between the positive and negative dk / dt sweeps can make available additional information that would not be present if data were captured only from either the positive or negative dk / dt.

[0318] In certain embodiments of at least two interferometers illuminated at a given time with light sources having substantially different dk / dt, where the interferometers share a common sample arm, the signals recovered from the different interferometers / different light sources may 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 stationary interfaces will typically be at matching depths in each signal, while the interferometer output signals corresponding to moving interfaces will be present with different distortions due to the different dk / dt.

[0319] Interferometer embodiments capable of simultaneously outputting two or more interferometer output signals corresponding to two or more dk / dt are particularly interesting due to the possibilities such embodiments offer for identifying, characterizing, and correcting motion artifacts in SS-OCT. As explained in the "Correcting Motion Artifacts" section, information corresponding to swept light signals at two or more adjustment rates dk / dt can be used to identify, characterize, and / or correct SS-OCT motion artifacts. In an interferometer embodiment for SS-OCT having at least two interferometers overlapping in the sample arm and illuminated with light sources having substantially different dk / dt at a given time, swept light signals corresponding to two or more adjustment rates dk / dt can be collected simultaneously. Therefore, the methods for processing motion artifacts developed in the above sections can be applied using these simultaneously collected signals. This can offer several advantages, including the possibility of processing motion artifacts on much shorter time scales, 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 involving motion, particularly motions that vary on time scales approaching or shorter than the duration of a single A-line acquisition. This approach can also provide enhanced information about instantaneous sample position and velocity relative to methods that rely on capturing signals corresponding to different dk / dt in a substantially time-gated manner due to the simultaneous capture of signals corresponding to the same sample but different dk / dt. This is because, with appropriate overlap of the sample arms, simultaneously captured signals are based on identical sample conditions, while 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 are considered applicable / adaptable to interferometer topologies capable of simultaneously outputting interferometer output signals corresponding to two or more dk / dt, including methods that utilize advanced signal processing methods and / or ML / AI.

[0320] In some embodiments, the output from the fiber amplifier may be split using a beam splitter, which may be configured to achieve a balance of the power (imaging beams) impinging on the workpiece from each optical path and / or apply other signal balancing techniques as known to those skilled in the art.

[0321] According to at least one embodiment, a light source is configured such that the emitted light includes two or more substantially distinct swept optical signals, and this light source is used to illuminate an interferometer for SS-OCT. The distinct swept optical signals may differ in instantaneous wavelength or wavenumber, dk / dt, sweep rate, or other relevant characteristics. In a preferred embodiment, the distinct swept optical signals are not coherent with one another. An example configuration, shown in FIG. 19, illustrates an embodiment in which an optical buffer implemented as a delay line fiber is included in the light source to generate output light that is substantially different from undelayed light. According to at least one embodiment, as shown in FIG. 19, the system may include a light source that simultaneously sweeps in two substantially different ways. The delayed and undelayed light may have substantially different instantaneous wavenumbers k and / or tuning rates dk / dt, and if the length of the optical fiber buffer is longer than the coherence length of the light source, the light from the buffered arm will not be coherent with the light from the unbuffered arm (and vice versa). In the example shown in FIG. 19, the substantially separate swept lights from the optically buffered and non-optically buffered paths are recombined and used to illuminate the balance of the SS-OCT interferometer, including the K clock.

[0322] Similar to the dual interferometer topology described above, this interferometer topology allows for simultaneous illumination of a sample by two or more light sources having two or more dk / dts. 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 simultaneously swept across the sample for at least a portion of the duration of the imaging process. Interferometer output signals derived from such a configuration can be used to estimate sample position in addition to sample velocity based on the relationship between dk / dt and scanning motion artifacts. Additionally, these interferometer output signals, including information from multiple dk / dts, can be used to extract information necessary to identify and / or correct one or more distortions in the signal resulting from motion artifacts, and more generally, to support or implement the approach described in the previous section entitled "Correcting Motion Artifacts." According to at least one aspect, combining information from different dk / dt values ​​to match in both time and space allows for more effective correction for motion artifacts. An example of the increased effectiveness of motion artifact correction enabled by the topology is similar to the example described in the "Dual Interferometer Topology" section above.

[0323] Dispersion compensation In some embodiments of the system, optical elements in the sample arm or the reference arm may cause optical dispersion mismatch between the arms of the interferometer. In some embodiments, optical elements may be added to the reference arm and / or the sample arm of the interferometer to substantially reduce and / or eliminate the observed effects of optical dispersion mismatch. For example, in some embodiments, the reference arm includes optics (not shown) that enable dispersion control. For example, optics may 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 may be applied to numerically compensate for dispersion mismatch in the measured interferometer output signal. Dispersion compensation methods known to those skilled in the art may be applied to this system.

[0324] In some embodiments, the optical components in the sample arm and the reference arm are matched with the goal of reducing any dispersion mismatch between the two arms (in certain embodiments, group delay and higher-order dispersion terms). This may improve axial imaging resolution. It may also be beneficial to modify 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 may be used in both the reference arm path and the sample arm path (including air / vacuum).

[0325] According to one embodiment, at least one of the pre-calculated interferogram and the measured interferogram is shaped to compensate for dispersion mismatch. Compensation can be achieved, for example, through controlled modulation of the complex phase and amplitude of individual elements of the combined interferogram. The amount of modulation can be determined from at least one of experimental calibration of the device, mathematical modeling of light propagation, theoretical analysis of the system response, and combinations of the above. For example, for a fixed dispersion element, a relative phase lag / lead for each wavelength resulting from the dispersion term of the material can be added to each element in the combined interferogram.

[0326] Optical dispersion induced by the sample being measured can adversely affect the axial resolution of the coherent image. In some embodiments, the sample can induce a wavelength-dependent phase shift on the interference pattern, which can depend on the depth to which the light has propagated within the sample. For example, complex phase and amplitude modulation algorithms such as those described above can be used to compensate for these effects. The dispersion coefficient of the material in the sample can be calculated a priori in some embodiments, or iteratively determined in other embodiments. Initially, the phase shift induced by the sample can be assumed to increase linearly with increasing penetration into the sample. In this way, each sample acquired at wavenumber "k" can have a certain phase shift, indicated by the instantaneous wavenumber "k" of the imaging optical beam and the depth from which the signal returns within 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 in advance (e.g., wavenumber "k" or a related related quantity can be known based on the k-clocking method described above), this distortion can be estimated and calculated in advance, 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 may also provide dispersion mismatch information used 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.

[0327] Swept-source OCT with enhanced frequency modulation and demodulation In some embodiments of the SS-OCT system, the time-domain interference signal (interferometer output signal) is captured by a photodetector and converted to an analog electronic signal or an RF analog electronic signal. This signal may have variable frequency and / or instantaneous frequency components. In some system embodiments, it may be advantageous to define a carrier frequency for the signal and use this carrier frequency to demodulate the interferogram using signal processing techniques known to those skilled in the art, such as techniques typically associated with FM demodulation. The signal demodulation process may generate a signal at a lower frequency than the original analog electronic signal. In system embodiments in which the frequency of the analog electronic signal associated with the output of one or more SS-OCT system photodetectors is higher than can be accurately digitized by an ADC, demodulation can preserve the information carried in the signal while reducing the frequency of the signal, thereby allowing the signal to be accurately digitized with limited information loss. In some system embodiments, demodulation may be motivated by design constraints involved in analog signal transmission, as signals with lower frequency components may have improved transmission characteristics and simplified design requirements compared to signals with higher frequency components.

[0328] In some embodiments, the analog electronic signal may instead be modulated by a selected carrier frequency.

[0329] According to 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 an interferometer output signal by several different carrier frequencies, either serially or in parallel, based on system characteristics and requirements. In some embodiments, a signal may be modulated and / or demodulated by one carrier frequency selected from a set of possible carrier frequencies, which may additionally include the option of not modulating or demodulating 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, modulation may be adjusted based on detected changes in sample depth or based on feedback of the reference arm position.

[0330] In some embodiments, the modulation or demodulation frequency may be selected in response to some feedback regarding the system state. In SS-OCT systems, for a given light source setting, faster interference fringes are typically generated when the path length difference between the sample arm and reference arm of the interferometer is larger. Therefore, as described above, a demodulation approach may be useful for reducing frequency components in the interferometer output signal associated with larger path length differences than in SS-OCT capture, enabling effective capture of data over a larger range of sample arm depths without exceeding the Nyquist frequency of the ADC, for example. In some embodiments, it may be preferable to tune the swept light source at different frequencies for different imaging functions. The frequency (time) of the interferometer output signal varies based on the light source tuning frequency. To allow a system with a given ADC sample frequency to preserve imaging range for a higher light source tuning frequency, the demodulation or modulation settings may be changed in response to changes in the light source tuning frequency.

[0331] In a preferred embodiment, the digitized demodulated signal may be processed to restore the signal to the equivalent of its original modulation state.In a preferred embodiment, the digitized modulated signal may be processed to restore the signal to the equivalent of its original demodulation state.

[0332] An important practical implication of modulation and demodulation approaches applied to swept-source OCT systems is that applying such techniques can help extend the imaging range without the need for arbitrarily high digital frequencies. This can increase the signal accuracy or the system's imaging rate and / or range in some cases where the hardware's analog bandwidth is limited.

[0333] Alignment to a material processing beam or process region using motion signatures - Patents.com According to certain embodiments, the identified motion artifact distortions may be used to assist in the alignment of various coordinate systems, including, but not limited to, a coordinate system of a material processing beam delivery system and a 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 offsets in alignment 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. 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 the 2D high-power scanner system available from IPG Photonics. The material processing beam delivery system may include, for example, a robotic element configured to move the entire beam delivery head. The coordinates of such a robotic system fall within a range of coordinate systems that can be aligned using the motion artifact distortions. In some embodiments, the determination of the one or more alignments and / or one or more offsets in alignment is based at least in part on the spatial distribution of distortions in the interferometer output signal.

[0334] In certain embodiments, alignment of the imaging system coordinate system with the material processing beam coordinate system is performed by using one or more directing elements to scan the imaging beam around the area where the material processing beam source is impinging on the workpiece, calibration target, and / or material test coupon.

[0335] While this is occurring, the material processing beam is manipulated to cause material modification, resulting in movement of the sample. According to various embodiments, the beam delivery system for the imaging light signal includes one or more directing elements configured to adjust or otherwise control a position of the imaging light signal relative to a position of the material processing beam, and the directing elements are controlled during acquisition of the one or more interferometer output signals such that interferometer output signals associated with different positions of the imaging light signal relative to the position of the material processing beam are acquired.

[0336] In a preferred embodiment, the imaging beam scan pattern is based on a coordinate system defined relative to the material modification beam. Embodiments in which the imaging beam scan pattern is based on other coordinate systems, such as those defined absolutely or relative to other elements of the material modification system, are also possible. SS-OCT data is captured as the directing element scans across this region, and the SS-OCT data may include A-lines, in which case the A-lines can be combined with information derived from the directing 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 the motion artifact distortion in each A-line of this 2D or 3D data, the motion artifact distortion can be correlated with the location of the imaging beam. In this manner, the location of the maximum motion artifact distortion can be determined. A non-exhaustive list of methods that can be applied to the 2D or 3D data to aid in this determination includes peak finding, maximum finding, and curve fitting. In some embodiments, the motion artifact distortion across the B-scan can be processed before determining the location of the maximum motion artifact distortion through means including integration, outlier removal, and / or other data filtering methods known to those skilled in the art.

[0337] 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 this process with SS-OCT) can vary based on the characteristics of the material modification process. For example, in the material modification process known to those skilled in the art as keyhole welding, the motion of the sample in the heat-affected zone, melt pool, and keyhole is known to be different due to, for example, the material being in a molten state versus a solidified state, allowing for the observation of distinct motion artifact distortion for each of these regions.

[0338] In some embodiments, motion artifact distortion can be used to identify the location of the origin of the material processing beam coordinate system relative to the origin of the imaging beam coordinate system and to help align the origins of the two coordinate systems.

[0339] 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 position of the material processing beam, and the imaging beam can be aligned to compensate for such phenomena, such as chromatic aberrations caused by the material processing beam optics. For example, when a material processing beam and an imaging beam of different wavelengths are both directed using a (pre-objective) scanner, each beam experiences a different refractive index within the scanner lens optics due to their different wavelengths, which results in the beams having different propagation directions at the output of the scanner lens and different positions on the workpiece. This type of chromatic aberration depends on the angle of incidence of the beam on the scanner lens and is different when the beams are directed to different positions within the scanner field of view. Therefore, one beam can be actively steered relative to the other beam to compensate for changes in the relative positions of the beams on the workpiece based on the relative difference in chromatic aberration between the two wavelengths.

[0340] In some embodiments, motion artifact distortion can be used to align the imaging beam to different process regions, such as keyholes or seams, which may be of particular interest for measurement purposes.

[0341] In some embodiments, the material processing beam source is manipulated such that the location of maximum motion artifact distortion correlates with the location of the material processing beam source at the workpiece, and data obtained from the imaging beam scan during such movement can enable the establishment of a relative mapping between the location of the material processing beam at the workpiece and the imaging beam at the workpiece.

[0342] In some embodiments, the material processing beam source operates such that the location of the maximum motion artifact distortion correlates with the location of the processing region. For example, this method can assist in identifying keyhole processing regions by using one or more directing elements to direct the material processing beam and the imaging beam along the same processing path and / or to the same processing location, while using one or more additional directing elements to independently direct the imaging beam around the material processing beam. When combined with information from the directing elements, the motion artifact distortion can be correlated with the location of the imaging beam relative to the material processing beam. The location of the maximum motion artifact relative to the material processing beam, which can be correlated to the location of the keyhole processing region, can then be identified throughout the entire processing path or a segment thereof. Keyhole processing regions are of particular interest for metrology and / or quality assurance purposes, and it is known to those skilled in the art that the location of a keyhole processing region relative to the material processing beam is highly dependent on processing parameters, including ...

Claims

1. 1. A swept-source optical coherence tomography (SS-OCT) system for performing imaging of a sample treated by a material treatment beam, the material treatment beam interacting with material of the sample at a treatment region on the sample, the SS-OCT system comprising:

1. An interferometer comprising: at least one reference arm; at least one sample arm configured to direct an imaging light signal to the processing region; and an adjustable light source for generating an imaging light signal, the imaging light signal having at least one wavenumber k that is variable in time and a sweep rate ranging from 1 kilohertz (kHz) to 20 megahertz (MHz); and the interferometer configured to direct the imaging light signal into at least one reference arm and at least one sample arm and to combine returning light signals from the at least one reference arm and the at least one sample arm to generate a combined light signal; an interferometer having a photodetector configured to detect the combined optical signal and generate at least one interferometer output signal; and A processing unit comprising: receiving at least one interferometer output signal; processing the at least one interferometer output signal to determine at least one characteristic of the treatment region; detecting a distortion in at least one interferometer output signal, the distortion being produced by a time-varying difference in optical path length between at least one sample arm and at least one reference arm; In response to detecting the distortion, applying one or more corrections to at least one interferometer output signal 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 treatment region; a processing unit configured to perform the operation; An SS-OCT system comprising:

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

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

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

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

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

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

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

9. 10. 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 performed by the material processing beam on the specimen based on 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 material of the sample based on the at least one corrected interferometer output signal.

12. 2. 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 of the material processing beam and a coordinate system of a delivery system of the imaging optical signal.

13. the processing unit is configured to control the tunable light source such that a time rate of change of at least one wavenumber k of the imaging light signal (tuning rate dk / dt) comprises at least two tuning rates dk / dt associated with the one or more interferometer output signals; 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 of the one or more interferometer output signals associated with at least one of the at least two tuning rates; Conducting an assessment of distortion; Including, The SS-OCT system of claim 1 , wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

14. 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. 15. The SS-OCT system of claim 14, wherein performing the evaluation includes comparing a distortion of the at least one interferometer output signal associated with a positive tuning rate dk / dt to a distortion of the 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 distortions of 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. 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, height-to-width 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. 18. The SS-OCT system of claim 17, wherein performing the evaluation includes 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 one or more predetermined thresholds and / or baselines may be: 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, The SS-OCT system of claim 20, wherein the SS-OCT system is established based on at least one of the following:

22. SS-OCT system according to claim 17, wherein the processing unit is configured to compare one or more geometric aspects with at least one to perform the evaluation. one or more geometric aspects encoded in at least one of the one or more interferometer output signals; and One or more predetermined thresholds and / or baselines associated with one or more geometric aspects.

23. The SS-OCT system of claim 13, wherein performing the evaluation includes 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 includes determining whether a relative difference between two or more strains 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 the evaluation of one or more characteristics and distortions of the SS-OCT system.

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 sample's velocity based on the strain assessment.

27. The material modification process performed by the material treatment beam on the specimen is a welding process, the material treatment beam creates a phase change region (PCR) in the treatment region, and the treatment unit Using the evaluation to generate one or more modifications; The SS-OCT system of claim 13 configured to use the generated one or more corrections to calculate measurements of one or more moving features within the PCR.

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

29. 14. The SS-OCT system of claim 13, wherein the material modification process performed by the material processing beam on the specimen is a welding process, the material processing beam creating a phase change region (PCR) in the treatment region, and the processing unit uses the evaluation to generate an estimate of the rate at which the material is processed in the PCR.

30. applying one or more corrections to at least one interferometer output signal; discarding, weighting, promoting, 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 for discarding or later use; The SS-OCT system of claim 13, comprising:

31. the interferometer is a first interferometer, and the system further comprises at least one additional interferometer; 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 at least two adjustment rates dk / dt is directed to at least one reference arm and at least one sample arm of a first interferometer; a second imaging optical signal configured at 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 at least one additional interferometer; The SS-OCT system of claim 13, wherein the distortion is identified based on one or more interferometer output signals of the first interferometer and the at least one additional interferometer.

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

33. The SS-OCT system of claim 1 , wherein the time-varying difference in optical path length is caused by sample motion.

34. 34. The SS-OCT system of claim 33, wherein the sample moving speed is greater than 10 mm / sec.

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

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

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

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

39. The SS-OCT system of claim 1 , wherein the processing unit is configured to derive track data from the at least one interferometer output signal, and wherein one or more corrections are applied to the track 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 wavenumber k of the imaging light signal has substantially changed by one or more increments.

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

42. 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 an interferometer output signal uniformly sampled in wavenumber.

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

44. 41. 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. 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 of the interferometer output signal at uniform intervals of wavenumber k using the obtained k-clock signal.

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

47. 45. 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 interferometer output signals that are not uniformly sampled in k.

48. 41. The SS-OCT system of claim 40, wherein 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 the processing of subsequently acquired interferometer output signals.

49. 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 of the interferometer output signal at uniform intervals in wavenumber k using the obtained k-clock signal.

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

51. 49. The SS-OCT system of claim 48, wherein the processing unit is configured to use the obtained k clock signals to define a discrete Fourier transform method that can be directly applied to interferometer output signals that are not uniformly sampled in k.

52. 49. 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 used to generate the k-clock signal.

53. 41. The SS-OCT system of claim 40, wherein at least one k-clock module is configured with multiple optical paths used to generate the k-clock signal.

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

55. 55. The SS-OCT system of claim 54, wherein at least one k-clock module is configured to allow multiple optical paths to be selected.

56. 10. The SS-OCT system of claim 1, wherein the processing unit is further configured to simulate a k-clock signal indicating times at which at least one wavenumber k of the imaging optical signal changes substantially by one or more increments 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.

57. 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 of the interferometer output signal at uniform intervals in wavenumber k using the simulated k-clock signal.

58. 57. The SS-OCT system of claim 56, wherein 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.

59. 57. 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 applied directly to an interferometer output signal that is not uniformly sampled in k.

60. 10. The SS-OCT system of claim 1, wherein the processing unit is further configured to generate a mathematical model of k(t) of the imaging optical signal and / or 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.

61. 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 of the interferometer output signal at uniform intervals of wavenumber k using a mathematical model.

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

63. 61. The SS-OCT system of claim 60, wherein the processing unit is configured to use a mathematical model to define a discrete Fourier transform method that can be applied directly to interferometer output signals that are not uniformly sampled in k.

64. The SS-OCT system of claim 1 , wherein the time-varying difference in optical path length is caused by motion of the sample relative to the 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 performed on the sample by the material processing beam.

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

67. 2. The SS-OCT system of claim 1, wherein the processing unit is configured to control the tunable light source such that a time rate of change (tuning rate dk / dt) of at least one wavenumber k of the imaging optical signal comprises at least two tuning rates dk / dt.

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

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

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

72. the interferometer is a first interferometer, and the system further comprises at least one additional interferometer; the first interferometer comprises a first tunable light source; the at least one additional interferometer is configured with at least one other tunable light source; 72. The SS-OCT system of claim 71, wherein the first interferometer and the at least one additional interferometer are 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.

73. a time rate of change of at least one wave number k of the imaging light signal is an adjustment rate dk / dt, and the processing unit controlling the first adjustable light source such that a first imaging light signal generated by the first adjustable light source has a first adjustment rate dk / dt; 73. The SS-OCT system of claim 72, further comprising controlling the at least one other adjustable light source such that an imaging light signal generated by the at least one other adjustable light source has a second adjustment rate dk / dt different from the first adjustment rate dk / dt.

74. 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 by the at least one other imaging optical signal are transmitted simultaneously.

75. the first and second adjustment rates are associated with one or more interferometer output signals; 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 of the one or more interferometer output signals associated with one of the at least first and second tuning rates; To assess distortion; Including, 75. The SS-OCT system of claim 74, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

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

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

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

79. 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 either 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. 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. 81. The SS-OCT system of claim 80, wherein the first arm is configured to be directed 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 at least one additional interferometer.

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

83. the first interferometer and the at least one additional interferometer are 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; 81. The SS-OCT system of claim 80, wherein the first and second arms of the imaging optical signal are configured to be simultaneously directed to the first interferometer and the at least one additional interferometer.

84. the first arm is configured to be directed toward the first interferometer or the at least one additional interferometer; 84. The SS-OCT system of claim 83, wherein the second arm is configured to be directed towards the other of the first interferometer or the at least one additional interferometer.

85. at least two adjustment rates dk / dt of the first and second arms are associated with one or more interferometer output signals; 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 of the one or more interferometer output signals associated with at least one positive tuning rate and at least one negative tuning rate; To assess distortion; Including, 84. The SS-OCT system of claim 83, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

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

87. 2. The SS-OCT system of 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 to transmit the generated OCT image to a display device.

88. 2. The SS-OCT system of claim 1, wherein the processing unit is further configured to generate OCT data from 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. 90. The SS-OCT system of claim 89, further comprising an amplifier for amplifying the VCSEL.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

107. 10. The SS-OCT system of claim 1, further comprising: a notification generator configured to generate, at multiple times, a notification regarding a material modification process performed by the material processing beam on the sample based on the at least one interferometer output signal.

108. The SS-OCT system of claim 1 , further comprising at least one directing element for directing the imaging light signal.

109. 109. The SS-OCT system of claim 108, wherein the at least one directing element is configured such that the imaging light signal is within 50 nm of a focal point of the material treatment beam at the treatment region.

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

111. 10. The SS-OCT system of claim 1 configured to image a sequence of multiple material modification processes performed on a sample by a material processing beam.

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

113. 10. The SS-OCT system of claim 1, further comprising a safety interlock device integrated into the adjustable light source and configured to enable an eye-safe mode of operation of the adjustable light source, the eye-safe mode of operation reducing imaging light 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, a beam delivery system for the material processing beam, and an imaging optical signal.

115. 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. 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 and a beam delivery system that generate the material processing beam based on at least one characteristic of the processing region.

117. 1. A material processing system comprising: The SS-OCT system according to claim 1; a material processing energy source that generates a material processing beam; a beam delivery system for the material processing beam and the imaging optical signal; A material processing system comprising:

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

119. 119. The material processing system of claim 118, wherein the dichroic optical system comprises a transmission spectrum having a first band edge and a reflection spectrum having a second band edge, the maximum wavelength separation of the first band edge and the second band edge being 25 nm.

120. 119. The material processing system of claim 118, wherein the beam delivery system is configured to illuminate the dichroic optics with the imaging light signal over a range of angles of incidence.

121. 1. A swept-source optical coherence tomography (SS-OCT) method for imaging a treatment area on a sample treated by a material treatment beam, 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 light signal having at least one wavenumber k that is substantially variable in time and a sweep rate in the range of 1 kilohertz (kHz) to 20 megahertz (MHz); transmitting the imaging optical signal to at least one reference arm and at least one sample arm of an interferometer, the at least one sample arm configured to transmit the imaging optical signal to a processing region; generating a combined optical signal from optical signals returning from at least one reference arm and at least one sample arm; generating at least one interferometer output signal from the combined optical signal; processing the at least one interferometer output signal to determine at least one characteristic of the treatment region; detecting a distortion in at least one interferometer output signal, the distortion being produced by a time-varying difference in optical path length between at least one sample arm and at least one reference arm; In response to detecting the distortion, applying one or more corrections to at least one interferometer output signal 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 treatment region; SS-OCT method comprising:

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

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

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

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

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

127. 123. The SS-OCT method of claim 122, wherein the material treatment beam creates a phase change region (PCR) in the treatment region, and the depth information comprises a keyhole depth of the PCR.

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

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

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

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

132. 122. 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 of the material processing beam and a coordinate system of a delivery system of the imaging optical signal.

133. controlling the tunable light source such that the time rate of change of at least one wavenumber k (tuning rate dk / dt) includes at least two tuning rates dk / dt associated with one or more interference signals; and Calculating one or more corrections: identifying a distortion in at least one of the interferometer output signals associated with at least one of the at least two tuning rates; performing an estimate of the distortion and basing one or more corrections to the at least one interferometer output signal on the estimate; calculating one or more corrections, including: The SS-OCT method of claim 121, further comprising:

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

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

137. 134. 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. 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, height-to-width ratio, and geometric area under the curve.

139. 138. 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. 138. The SS-OCT method of claim 137, 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.

141. further including establishing one or more predetermined thresholds and / or baselines, which include: 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, The SS-OCT method according to claim 140, wherein the method is performed based on at least one of the following:

142. Performing the evaluation involves evaluating one or more geometric aspects: one or more geometric aspects encoded in at least one other interferometer output signal; and SS-OCT according to claim 137, comprising comparing with at least one of one or more predetermined thresholds and / or baselines associated with one or more geometric aspects.

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

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

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

146. 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 sample's movement velocity based on the strain assessment.

147. 134. The SS-OCT method of claim 133, wherein the material modification process performed on the sample by the material treatment beam is a welding process, and the material treatment beam creates a phase change region (PCR) in the treatment region; and Using the evaluation to generate one or more modifications; One or more corrections generated by the evaluation are used to calculate measurements of one or more features moving within the PCR.

148. SS-OCT method according to claim 147, characterized in that one or more features moving within the PCR are moving as a direct result of a material modification process.

149. 134. The SS-OCT method of claim 133, wherein the material modification process performed by the material processing beam on the sample is a welding process, the material processing beam creating a phase change region (PCR) in the treatment region, and further comprising using the evaluation to generate an estimate of the velocity of the material being processed in the PCR.

150. applying one or more corrections to at least one interferometer output signal; discarding, weighting, promoting, or using at least one interferometer output signal; or selecting and subsequently discarding at least one interferometer output signal; 134. The SS-OCT method of claim 133, comprising:

151. The interferometer is a first interferometer and the method is a second interferometer. 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 a first interferometer; directing a second imaging optical signal configured at another of the two tuning rates dk / dt to at least one reference arm and at least one sample arm of 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; The SS-OCT method of claim 133, further comprising:

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

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

154. SS-OCT method according to claim 153, wherein the sample movement speed is greater than 10 mm / s.

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

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

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

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

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

160. 122. 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 wavenumber k of the imaging light signal has substantially changed by one or more increments.

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

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

163. 161. The SS-OCT method of claim 160, wherein the time rate of change (tuning rate dk / dt) of at least one wavenumber k of the imaging optical signal 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 wavenumber k.

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

165. A k clock signal is acquired simultaneously with the acquisition of the interferometer output signal, and further: using the obtained k clock signals to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals of wavenumber k; calculating at least one correction for one or more distortions in the interferometer output signal using the obtained k-clock signals; and 161. The SS-OCT method of claim 160, comprising at least one of: using the acquired k clock signals to define a discrete Fourier transform method that can be directly applied to interferometer output signals that are not uniformly sampled in k.

166. The k clock signal is acquired in a time-gated manner relative to the acquisition of the interferometer output signal, and further: using the obtained k clock signals to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals of wavenumber k; calculating at least one correction for one or more distortions in the interferometer output signal using the obtained k-clock signals; and 161. The SS-OCT method of claim 160, comprising at least one of: using the obtained k clock signals to define a discrete Fourier transform method that can be directly applied to interferometer output signals that are not uniformly sampled in k.

167. The method further includes simulating a k-clock signal indicating when at least one wavenumber k of the imaging light signal is substantially changed by one or more increments based at least in part on one or more characteristics of an SS-OCT system and one or more characteristics of the tunable light source, and further comprising: using the simulated k-clock signal to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals of wavenumber k; calculating at least one correction for one or more distortions in the interferometer output signal using the simulated k-clock signal; and 122. The SS-OCT method of claim 121, comprising at least one of: using the simulated k-clock signals to define a discrete Fourier transform method that can be directly applied to interferometer output signals that are not uniformly sampled in k.

168. generating a mathematical model of k(t) and / or the tuning rate dk / dt of the imaging light 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, further comprising: using the mathematical model to perform at least one of sampling, resampling, interpolating, and / or estimating the interferometer output signal at uniform intervals of wavenumber k; calculating at least one correction for one or more distortions of the interferometer output signal using the mathematical model; and 122. The SS-OCT of claim 121, comprising at least one of: using a mathematical model to define a discrete Fourier transform method that can be directly applied to an interferometer output signal that is not uniformly sampled in k.

169. controlling the tunable light source such that the time rate of change of at least one wavenumber k (tuning rate dk / dt) comprises at least two tuning rates dk / dt associated with the one or more interference signals; generating a mathematical model of at least two tuning speeds based at least in part on one or more characteristics of the tunable light source and one or more characteristics of the SS-OCT system including the interferometer and the photodetector; The SS-OCT method of claim 121, further comprising:

170. The SS-OCT method according to claim 169, further comprising: A mathematical model is used to relate estimates of at least one of the value of k and the adjustment rate dk / dt to data sampled from the interferometer output signal and / or the corrected interferometer output signal. using the measurement of the associated wavenumber k to estimate the value of k in the sampled interferometer output signal values; The associated measurements of wavenumber k are used to perform at least one of sampling, resampling, interpolating, and estimating the interferometer output signal at uniform intervals within k.

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

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

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

174. 122. The SS-OCT method of claim 121, wherein the time-varying difference in optical path length is caused by an intrinsic motion of the sample rather than by a material modification process carried out by the material processing beam on the sample.

175. 122. The SS-OCT method of claim 121, further comprising controlling the tunable light source such that a time rate of change (tuning rate dk / dt) of at least one wavenumber k of the imaging light signal comprises at least two tuning rates dk / dt.

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

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

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

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

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

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

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

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

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

185. The SS-OCT method according to claim 121, further comprising: a splitter that splits the imaging optical signal into at least two arms; An optical delay element configured such that an output of a first of the at least two arms is delayed in time relative to an output of a second of the at least two arms.

186. 186. The SS-OCT method of claim 185, further comprising controlling the tunable light source such that the time rate of change (tuning rate dk / dt) of at least one wavenumber k of the imaging light signal comprises at least two tuning rates dk / dt.

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

189. 189. The SS-OCT method of claim 188, wherein the first arm is configured to be directed 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 at least one additional interferometer.

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

191. the first interferometer and the at least one additional interferometer are 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; 189. The SS-OCT method of claim 188, wherein the first and second arms of the imaging optical signal are configured to be simultaneously directed to the first interferometer and the at least one additional interferometer.

192. the first arm is configured to be directed toward the first interferometer or the at least one additional interferometer; 192. The SS-OCT method of claim 191, wherein the second arm is configured to be directed towards the other of the first interferometer or the at least one additional interferometer.

193. at least two adjustment rates of the first and second arms are associated with one or more interferometer output signals; and Calculating one or more corrections, the calculating the one or more corrections comprising: identifying a distortion in at least one of the one or more interferometer output signals associated with at least one positive tuning rate and at least one negative tuning rate; Conducting an assessment of distortion; Including, 192. The SS-OCT method of claim 191, wherein the one or more corrections to the at least one interferometer output signal are based on the evaluation.

194. 122. The SS-OCT method of claim 121, wherein the tunable light source is configured to vary the optical frequency of the imaging optical signal at a rate ranging from 8 PHz / s to 2 ZHz / s.

195. 122. 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 transmitting the generated OCT image to a display device.

196. 122. The SS-OCT method of claim 121, further comprising generating OCT data from the processed interferometer output signal and / or the at least one corrected interferometer output signal, and transmitting the OCT data to an external device.

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

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

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

200. 122. The SS-OCT method of claim 121, further comprising guiding the imaging light signal with a guiding element.

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

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

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

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

205. 122. The SS-OCT method of claim 121, further comprising generating at least one interferometer output signal using a photodetector.

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