Signal mixing in optical coherence tomography for use in material processing

Abstract

An imaging system for performing imaging of a sample treated by a material processing beam, where the material processing beam interacts with material of the sample at a processing region on the sample, the imaging system including an interferometer configured to produce multiple interferometric output signals generated from interference between a measurement beam that is a light beam radiated toward a measurement target on the sample and reflected from the measurement target and a reference beam that is reflected from an optical reference, and a processing unit configured to: receive the multiple interferometric output signals, apply at least one signal mixing operation between one or more signals of the multiple interferometric output signals, produce an interferometric output based at least in part on the at least one signal mixing operation, and process the interferometric output to determine at least one feature of the processing region.

Description

[0001] SIGNAL MIXING IN OPTICAL COHERENCE TOMOGRAPHY FOR USE IN MATERIAL PROCESSING

[0002] RELATED APPLICATIONS

[0003] The present application claims priority to U.S. Provisional Application No. 63 / 671,538, titled “SIGNAL MIXING IN OPTICAL COHERENCE TOMOGRAPHY FOR USE IN MATERIAL PROCESSING / ’ filed July 15, 2024, the content of which is hereby incorporated by reference in its entirety.

[0004] This application is related to PCT Application No. PCT / US24 / 12552, titled “SWEPT- SOURCE OPTICAL COHERENCE TOMOGRAPHY WITH ENHANCED SIGNAL DETECTION,” filed on January 23. 2024, referred to herein as the “12552 application,” which is owned by Applicant and is herein incorporated by reference in its entirety.

[0005] BACKGROUND

[0006] Technical Field

[0007] The technical field relates generally to Optical Coherence Tomography (OCT), and more specifically to an improved algorithm for analyzing interferometric measurements obtained from Swept-Source OCT (SS-OCT) systems.

[0008] Background Discussion

[0009] Optical coherence tomography (OCT) is an imaging technology employing low- coherence interferometry (LCI) that involves dividing the light output from a light source into an imaging beam and a reference beam, detecting interference signals obtained by superimposing the imaging beam reflected from an object or sample surface with the reference beam, and forming a tomographic image of the object based on the detection result. The tomographic image may comprise two-dimensional cross sections or three-dimensional volume renderings of the object or sample by using information on how the beams are changed upon reflection. Additionally , successive one-dimensional LCI measurements may be obtained over time to measure temporal changes in sample geometry .

[0010] Swept-Source OCT systems (SS-OCT) typically utilize instantaneously' narrowband imaging sources with an emitted wavelength that varies over time. These systems can be used for monitoring manufacturing processes including those that process materials using laser beams. As discussed in the 12552 application, motion artifacts (created by the workpiece sample and / or environment and / or other causes) decrease the image quality and can lead to inaccurate measurement values. An OCT imaging system that features advanced motion compensation techniques is desirable.

[0011] SUMMARY

[0012] Aspects and embodiments are directed to a method and system for imaging a workpiece.

[0013] In accordance with at least one embodiment, there is provided an imaging system for performing imaging of a sample treated by a material processing beam, the material processing beam interacting with material of the sample at a processing region on the sample, the imaging system comprising: an interferometer configured to produce multiple interferometric output signals generated from interference between a measurement beam that is a light beam radiated toward a measurement target on the sample and reflected from the measurement target and a reference beam that is reflected from an optical reference, and a processing unit configured to: receive the multiple interferometnc output signals, apply at least one signal mixing operation between one or more signals of the multiple interferometric output signals, produce an interferometric output based at least in part on the at least one signal mixing operation, and process the interferometric output to determine at least one feature of the processing region.

[0014] In one example, the imaging system is configured such that a tuning rate dk / dt of the measurement beam changes between two or more different tuning rates. In another example, the two or more different tuning rates include at least one positive tuning rate and at least one negative tuning rate. In one example, one tuning rate has a magnitude of zero and another tuning rate has a non-zero magnitude.

[0015] In one example, the measurement beam is associated with a signal, and the processing unit is configured to apply the at least one signal mixing operation between different time windows within the signal. In a further example, each time window is associated with a different tuning rate. In yet a further example, the processing unit is configured to Hilbert transform each portion of the signal associated with each time window and the at least one signal mixing operation is applied to tw o or more Hilbert transformed portions of the signal.

[0016] In one example, the measurement beam of the interferometer includes a first measurement beam and the interferometer further comprises a second measurement beam, the first measurement beam having a first tuning rate that is different than a second tuning rate of the second measurement beam. In a further example, the first and second tuning rates are different for at least one portion of time.

[0017] In one example, the imaging system is configured such that the multiple interferometric output signals of the first and second measurement beams are received simultaneously by the processing unit.

[0018] In one example, the processing unit is configured to apply a Hilbert transform to each of the interferometric output signals of the first and second measurement beams.

[0019] In one example, the at least one signal mixing operation is applied to the Hilbert transformed interferometric output signals of the first and second measurement beams.

[0020] In one example, the processing unit is configured to apply a Fourier transform to the Hilbert transformed interferometric output signals after the signal mixing operation.

[0021] In one example, the interferometric output corrects for one or more distortions in the interferometric output signals. In one example, the one or more distortions are created by a time-vary ing difference in optical path lengths included in the interferometric output signals and the time-varying difference in optical path lengths is caused by at least one of sample motion relative to an axis of the measurement beam, a material modification process implemented by the material processing beam on the sample, and intrinsic sample motion not caused by a material modification process implemented by the material processing beam on the sample.

[0022] In one example, the first tuning rate is a positive tuning rate and the second tuning rate is a negative tuning rate.

[0023] In one example, the imaging system further comprises at least one directing element that is configured to direct the first and second measurement beams to the processing region simultaneously.

[0024] In one example, the interferometric output signals are segmented prior to applying the at least one signal mixing operation. In another example, the at least one signal mixing operation is applied to the one or more signals of each segmented interferometric output signal. In a further example, the segmented interferometric output signals are further recombined via summation.

[0025] In one example, the interferometer further comprises at least one tunable light source configured to generate the measurement beam.

[0026] In one example, the imaging system further comprises an amplifier for amplifying the tunable light source. In one example, the interferometer is configured as one of a Michelson or Mach Zehnder interferometer.

[0027] In one example, the at least one feature includes depth information of the processing region.

[0028] In one example, the processing unit is further configured to determine a sample position and / or a velocity of the material of the sample based on the interferometric output.

[0029] In one example, the imaging system further comprises a material processing beam source configured to generate the material processing beam.

[0030] In accordance with one embodiment, a material processing system is provided that comprises the imaging system, a material processing beam source that generates the material processing beam, and a beam delivery system for the material processing beam and the measurement beam.

[0031] In accordance with at least one embodiment, an imaging method for imaging a processing region on a sample being treated by a material processing beam is provided, the method comprising: provide an interferometer configured to produce multiple interferometric output signals generated from interference between a measurement beam that is a light beam radiated toward a measurement target on the sample and reflected from the measurement target and a reference beam that is reflected from an optical reference, apply at least one signal mixing operation between one or more signals of the multiple interferometric output signals, produce an interferometric output based at least in part on the at least one signal mixing operation, and process the interferometric output to determine at least one feature of the processing region.

[0032] In one example, the imaging method further comprises changing a tuning rate dk / dt of the measurement beam between two or more different tuning rates. In one example, the two or more different tuning rates include at least one positive tuning rate and at least one negative tuning rate.

[0033] In one example, the measurement beam is associated with a signal, and the at least one signal mixing operation is applied to different time windows within the signal. In one example, each time window is associated with a different tuning rate.

[0034] In one example, the imaging method further comprises Hilbert transforming each portion of the signal associated with each time window, and applying the at least one signal mixing operation to two or more Hilbert transformed portions of the signal. In one example, one tuning rate has a magnitude of zero and another tuning rate has a non-zero magnitude.

[0035] In one example, the interferometer is provided such that the measurement beam of the interferometer includes a first measurement beam and the interferometer further comprises a second measurement beam, the first measurement beam having a first tuning rate that is different than a second tuning rate of the second measurement beam. In a further example, the multiple interferometric output signals of the first and second measurement beams are received simultaneously.

[0036] In one example, the imaging method further comprises Hilbert transforming each of the interferometric output signals of the first and second measurement beams. In a further example, the at least one signal mixing operation is applied to the Hilbert transformed interferometric output signals of the first and second measurement beams. In yet a further example, the imaging method further comprises applying a Fourier transform to the Hilbert transformed interferometric output signals after the signal mixing operation.

[0037] In one example, the imaging method further comprises directing the first and second measurement beams to the processing region simultaneously.

[0038] In one example, the imaging method further includes segmenting the interferometric output signals prior to applying the at least one signal mixing operation. In another example, the imaging method further includes applying the at least one signal mixing operation to the one or more signals of each segmented interferometric output signal. In another example, the imaging method further includes recombining the segmented interferometric output signals via summation.

[0039] In one example, the interferometer is provided with at least one tunable light source configured to generate the measurement beam. In another example, the imaging method further comprises providing an amplifier for amplifying the tunable light source.

[0040] In one example, the imaging method further comprises determining a sample position and / or a velocity of the material of the sample based on the interferometric output.

[0041] In one example, the imaging method further comprises providing a material processing beam source configured to generate the material processing beam.

[0042] In one example, the imaging method further comprises providing a processing unit configured to apply the at least one signal mixing operation, produce the interferometric output, and process the interferometric output. In one example, the imaging method further comprises controlling at least one processing parameter of a material modification process implemented by the material processing beam on the sample based on the at least one feature of the processing region.

[0043] Still other aspects, embodiments, and advantages of these example aspects and embodiments, are discussed in detail below. Moreover, it is to be understood that both 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 character of the claimed aspects and embodiments. Embodiments disclosed herein may be combined with other embodiments, and references to “an embodiment / ’ “an example,” “some embodiments,” “some examples,” “an alternate embodiment,” “various embodiments,” “one embodiment,” “at least one embodiment,” “this and other embodiments,” “certain embodiments,” or the like 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. The appearances of such terms herein are not necessarily all referring to the same embodiment.

[0044] BRIEF DESCRIPTION OF DRAWINGS

[0045] Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide an illustration and a further understanding of the various aspects and embodiments and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of any particular embodiment. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not even' component may be labeled in every figure. In the figures:

[0046] FIG. 1 shows uncorrected A-line depth versus amplitude data in accordance with aspects of the invention;

[0047] FIG. 2 shows a signal mixing analysis in accordance with aspects of the invention;

[0048] FIG. 3 shows two graphs of depth versus time interferogram data obtained from an SS-OCT system, one graph showing raw OCT data, and the other graph showing the same data where a signal mixing operation was applied in accordance with aspects of the invention; FIG. 4 shows the impact of the signal mixing operation on the width of a selected A- line in accordance with aspects of the disclosure;

[0049] FIG. 5 shows three graphs of depth versus time interferogram data obtained from an SS-OCT system, two showing raw interferometric data, and the third showing the raw data corrected using a signal mixing operation in accordance with aspects of the invention;

[0050] FIG. 6A is a graph showing a of brightest pixel tracking using A-line data generated from an interferometer and compares two correction algorithms in accordance with aspects of the invention;

[0051] FIG. 6B includes two graphs showing depth versus time date using simultaneous k- sweeps in accordance with aspects of the invention;

[0052] FIG. 7A is a schematic representation of one example of an imaging system showing a double interferometer configuration having interferometers that overlap at a sample arm and have separate light sources in accordance with aspects of the invention;

[0053] FIG. 7B is a schematic representation of one example of an imaging system showing a double interferometer configuration having interferometers that overlap at a sample arm and features a buffered light source in accordance with aspects of the invention;

[0054] FIG. 7C is a schematic representation of another example of an imaging system having a double interferometer configuration having interferometers that overlap at a sample arm and features a buffered light source in accordance with aspects of the invention;

[0055] FIG. 7D is a schematic representation of another example of an imaging system having a double interferometer configuration with separate light sources and displays a variant of the interferometer topology show n in FIG. 7A, in accordance with aspects of the invention;

[0056] FIG. 7E is a schematic representation of another example of an imaging system having a double interferometer configuration that features a buffered light source and interferometers that overlap at a sample arms in accordance with aspects of the invention;

[0057] FIG. 8A is a schematic representation of an interferometer with a Michelson configuration in accordance with aspects of the invention;

[0058] FIG. 8B is a schematic representation of an interferometer with a modified Michelson configuration in accordance with aspects of the invention;

[0059] FIG. 8C is a schematic representation of an interferometer with a Mach-Zehnder configuration in accordance with aspects of the invention; FIG. 8D is a schematic representation of an interferometer with a modified Mach- Zehnder configuration in accordance with aspects of the invention;

[0060] FIGS. 9A-9E are schematics used in reference to describe a first algorithm that incorporates signal mixing in accordance with aspects of the invention;

[0061] FIGS. 10A-10C are schematics used in reference to describe a second algorithm that incorporates signal mixing in accordance with aspects of the invention;

[0062] FIG. 11 A is a schematic representation of one example of an imaging system having a single interferometer configuration with alternate locations for the imaging beam amplification in accordance with aspects of the invention;

[0063] FIG. 1 IB is a schematic representation of another example of an imaging system having a single interferometer configuration in accordance with aspects of the invention;

[0064] FIG. 11C is a schematic representation of yet another example of an imaging system having a single interferometer configuration in accordance with aspects of the invention;

[0065] FIG. 1 ID is a schematic representation of one example of an imaging system showing an interferometer configuration that includes a buffered light source and that produces multiple swept source light signals in accordance with aspects of the invention;

[0066] FIG. 12 is a graph showing an example of overlapped z-windows in accordance with aspects of the invention; and

[0067] FIG. 13 is a flowchart showing a signal mixing process in accordance with aspects of the invention.

[0068] DETAILED DESCRIPTION

[0069] Disclosed herein is an optical coherence tomography (OCT) imaging system configured to determine correct position information and direct measurement of sample motion velocity by applying an algorithm that is based at least in part on signal mixing. In some embodiments the signal mixing operation includes applying the Hibert transform. The sample is associated with a material modification process and the OCT system is configured to determine sample position and / or sample feature information. Non-limiting examples of material modification processes include welding, cutting, drilling, ablation, brazing, surface texturing, annealing, and additive manufacturing. As explained in the 12552 application, a known physical effect which impacts all SS-OCT systems is distortion of A-lines when the imaging target moves (sample motion) during the acquisition time of a single A-line. This effect is known to those of ordinary skill in the art as motion artifact. In accordance with various aspects, the distortion is created by the motion of the sample, but more broadly the distortion is created by a time-varying difference in optical path lengths between the sample arm and the reference arm of the interferometer. In accordance with certain aspects, sample motion refers to a time-varying change in the optical path length of the sample arm, i.e., the distortion is created by a time-varying optical path length of the sample arm and may not be caused by intrinsic motion of the sample itself, but the optical path length may also be changed by other external factors that change, e.g.. the environment. In some embodiments, the time-varying optical path length of the sample arm may also be caused by intrinsic motion of the sample and in other embodiments it may be caused by a combination of intrinsic motion and environmental factors.

[0070] The motion artifact can manifest as a shift of the A-line, which may be greater than the actual displacement of the imaging target during the imaging time. The motion artifact can additionally manifest as a broadening, blur, or distortion of the A-line in addition to or instead of the shift. The correct position information and measurement of the sample motion velocity’ can be determined by evaluating and compensating for motion artifacts caused by modulation of the interference fringes. The motion artifact may present as a distortion of the interferogram data, which results in a bright, distorted signal. An example of distorted A- lines generated based on distorted interferogram output data is shown in FIG. 1 , where one A-line generated using a positive (forward) tuning rate and another A-line is generated using a negative (reverse) tuning rate and the distortion is visible as a displacement between the positive and negative A-line peaks, as well as an asymmetry and distorted shape of each peak.

[0071] In accordance with certain embodiments, one or more algorithms are used to address and correct for the effect of the motion artifact (distortion). In some embodiments, a working principle is that the motion artifact is related to the change in the source (imaging source of interferometer) wavenumber over time (tuning rate dk / dt). The source is swept such that interferometric output signals corresponding to portions of the source sweep at two or more (at least two) tuning rates dk / dt are captured. For example, in some embodiments one tuning rate has a magnitude of zero and another tuning rate has a non-zero magnitude. The different tuning rates enable the capture of data that itself provides pertinent information regarding the motion artifact and can be used to analyze and / or correct for the motion artifact.

[0072] As noted herein, information from measurements taken with differing dk / dt values can be applied to compute corrections which reduce or otherwise eliminate the effect of the motion artifact. A further concept described herein allows for the capability to acquire forward (positive) and reverse (negative) sweeps (dk / dt) simultaneously. Several concepts related to this capability are outlined below, with physical embodiments and further details also included below. Overarching concepts include the use of a single imaging light source or multiple imaging light sources that are used to illuminate multiple optical interferometers simultaneously, interferometers with at least partially overlapping sample arms (multiple interferometers “sharing” a common sample arm), the implementation of delay lines, especially delay lines incorporated into a source module which enable the output of delayed imaging signals simultaneously with non-delayed imaging signals from a source module, and an imaging light source configured such that it simultaneously generates distinct (i.e., substantially different) swept light signals.

[0073] According to certain embodiments, the distortion corresponds to one or more geometric aspects encoded in the interferometric output signal (such as the geometric characteristics of the A-line, which are encoded within the frequency of the interferometric output signal) as discussed in the 12552 application. However, unlike the 12552 application, embodiments herein do not require detection of the geometric characteristics per se, rather the algorithms, including the signal mixing operation(s), disclosed herein can be viewed as agnostic to the geometric characteristics. Adequate information can be derived based on information contained within an un-corrected interferometric output signal when sample settings of the system are set appropriately to capture interferometric output signals corresponding to swept signals at two or more tuning rates dk / dt. In some embodiments, to address the motion artifact of an un-corrected interferometric output signal, the signal capture must include at least two separable segments of data for which the imaging source has different dk / dt. Different dk / dt may comprise different rates of tuning of the imaging source, and / or may comprise positive and negative dk / dt. Distortion of the interferometric output signal during the time of the acquisition can be addressed, and this distortion information can be related to motion and / or velocity of the imaging sample. As noted herein, certain embodiments make use of multiple interferometers, and in additional embodiments the first and second imaging signals are directed to the processing region simultaneously.

[0074] • The forward and the reverse sweeps may not be perfect mirror images of one another in time, due to differences in how the light source may be driven forw ard and backward, and / or differences in how the light source responds to the driving signal forwards and backwards, and / or differing optical media that the light propagates through for forward and reverse sweeps.

[0075] • That is to say, sweep parameters vs. time, including absolute sweep rate (dk / dt), intensity7, wavenumber k.

[0076] • In some embodiments, the light source and / or the light source drive signal may be designed to reduce these differences, but it may not be practically achievable to eliminate them.

[0077] • To the extent that they exist, their effect may be further reduced through signal processing in a variety of means known to those of ordinary skill in the art o e.g. scaling, resampling, calibration and lookup tables, and signal transforms

[0078] • Such effects may also be present between different portions of interest of the sweep which are not necessarily forward and reverse (e.g. between two different subsections of a forward sweep). The methods above may also be used to compensate for differences between these portions of the sweep.

[0079] The geometric aspects which are distorted by the motion artifact can include aspects known by those of ordinary' skill in the art which correspond to features, including geometric features, of the phase change region (and more generally the processing region) and more generally to features, including geometric features, of the sample. As such, correction of the motion artifact is crucial to obtaining an accurate measurement of one or more features, including geometric features, of interest in the sample w hen the sample is in motion. A pertinent example of features or geometric aspects to be measured when the sample is in motion is obtaining an accurate measurement of keyhole geometry while the molten metal at the bottom and side walls of the keyhole is in motion. Based on the algorithm described herein, which includes a signal mixing operation, a correction to the distortion is applied to produce a corrected interferometric output.

[0080] According to at least one embodiment, one or more interferometric output signals are mixed that each correspond to portions of the source that are swept at two or more (at least two) different tuning rates dk / dt. In some embodiments of the system, the interferometer may be configured such that two or more interferometric output signals corresponding to tw o or more tuning rates dk / dt are acquired simultaneously. In some embodiments of the system, different time windows (each with a different tuning rate dk / dt) of the same signal may also be mixed. Examples of signal mixing include but are not limited to addition, multiplication, subtraction, division, and / or convolution, in the time domain and / or in the frequency domain. It is to be understood that signals may be combined according to several signal mixing steps. A scaling factor, which may comprise a constant, an integrated signal power, a signal time and / or amplitude characteristic, a calibrated value, a programmed value, some other quantity useful to scaling the mixed signal, and / or any combination thereof may be applied before, during or after the signal mixing process. In some embodiments, after the operation of signal mixing, the mixed signal is Fourier transformed.

[0081] The purpose of mixing the signals w ith different tuning rates dk / dt is that through the signal mixing, acquisitions of the sample with different characteristics may be corrected, emphasized, de-emphasized, or identified. For example,

[0082] • A method of mixing the signal may emphasize or aid in the identification of sample points with high velocity motion

[0083] • A method of mixing the signal may aid in the identification of points not impacted by motion artifacts

[0084] • A method of mixing the signal may enable the correction of motion artifact inherently (the signal mixing may produce an interferogram which is corrected)

[0085] • A method of mixing the signal may suppress the amplitude of points impacted by motion artifacts

[0086] • A method of mixing the signal may enable the measurement of the velocity of motion at the time of the signal

[0087] • A method of mixing the signal may enable the filtering or separation of the signal based on the observed motion artifact

[0088] According to at least one embodiment, the Hilbert transform is applied to at least two (or different time windows of one) interferometric output signals (digital signal) to compute the (complex) analytic signal corresponding to the interferometric output signal, (it is to be appreciated that an analytic signal is known as a complex-valued signal that has no negative frequency components). In accordance with at least one embodiment, the (complex) analytic signal is generated by summing the original signal and the imaginary unit “i” times the Hilbert transform of the original signal. A signal mixing operation as described above can be applied to the analytic signals generated by applying the Hilbert transform e.g. to remove the motion artifact component from the interferogram. In other words, mixing (e.g.. multiplying) the analytic signal resulting from the two interferograms forward sweep and backw ard sweep) acts on (e.g., reduces, eliminates) the motion artifact. This generates a motion- corrected interferogram, which may then be transformed to produce A-lines, B-scans, etc. As mentioned previously, in some embodiments imaging signals with two different tuning rates dk / dt are employed, one being a forward / positive tuning rate, and the other being a reverse / negative tuning rate, and the Hilbert transform is applied to each of the forward and reverse sweeps. In some embodiments, the opposing sweeps are performed simultaneously.

[0089] According to at least one embodiment:

[0090] • The system used to capture the interferometric output signals comprises a dual interferometer system (e.g., see FIGS. 7A-7E))

[0091] • The dual interferometer system is configured such that at the sample arm, two or more distinct measurement beams of imaging light both interact with the sample. o Each imaging beam may correspond to its own reference arm o Some number of imaging beams may also share reference arms o During at least a portion of time, a tuning rate dk / dt of one (or more) measurement beam of imaging light is different than a tuning rate dk / dt of another (or more) measurement beam of imaging light

[0092] • The two or more interferometric output signals corresponding to portions of a source sweep at two or more (at least two) tuning rates dk / dt are acquired simultaneously

[0093] • The two or more simultaneously acquired signals are mixed, e.g. using the Hilbert augmented mixing methods above o The signals may be mixed in the time domain and / or the K-domain in some embodiments.

[0094] ■ In certain embodiments the signals may be mixed in the frequency domain o For acquisitions in the time domain, the timescale of correction resultant from mixing in the time / frequency domain can be shorter than the timescale of a single A-line o As a result, it is possible with the signal mixing method to acquire correction of motion on timescales faster than are required to acquire one A-line o As will be appreciated, the timescale of motion correction is predicated on acquiring a sufficient number of fringes to measure the desired geometric characteristics (e.g., weld depth) with sufficient resolution • The mixed signals produce a corrected interferometric output signal, which may- then be transformed (e.g., Fourier transformed) to produce an A-line.

[0095] In accordance with one or more aspects, it may be stated that the Hilbert transform is applied to at least one dataset derived from the interferometric output signals of one or more measurement beams. This may generate an analytic signal, and the signal mixing operation is applied to the analytic signal. A Fourier transform may then be applied after the signal mixing operation.

[0096] FIG. 2 shows an overview in accordance with one embodiment where the Hilbert transform is implemented. In FIG. 2, Ii and H are interferograms taken with forw ard and reverse k-sweeps (i.e., different sweep rates), respectively. As used herein, the term(s) forward sweep / k-sweep may be understood to refer to a portion of a swept source tuning sweep which corresponds to a positive tuning rate dk / dt, and a reverse sweep / k-sweep corresponds to a portion corresponding to a negative tuning rate dk / dt. The top expression in FIG. 2 shows the interferogram correction equation. The middle expression in FIG. 2 is the Hilbert Transform and the bottom expression is the Motion Offset Cancellation. These processes are described in further detail below- in the Examples section.

[0097] FIG. 3 shows depth versus time interferometric data obtained from an SS-OCT imaging system. The top graph includes uncorrected forward and reverse sweep data, and the bottom graph shows the same data after application of the Hilbert transform, as well as application of the signal mixing operation. As can be seen, the corrected data shows a single, sharper interface as compared to the multiple, broader, and less bright interface of the uncorrected data. FIG. 4 show s the impact of the Hilbert transform and the signal mixing operation on the width of a selected A-line. The inset graph show s intensity versus depth data taken at the 5 ms time point for the simultaneously obtained uncorrected positive (upsweep, showing a peak at about 8.95 mm) and negative (down-sweep, showing a peak at about 9. 15 mm) tuning rates (it is to be appreciated that the positive (or up-sw eep) and negative (or down-sweep) pertain to different tuning rates dk / dt, e.g., positive (also referred to herein as forward) tuning rate dk / dt, negative (also referred to herein as reverse) tuning rate dk / dt), and the corrected data generated by applying the signal mixing operation, which show s a peak at about 9.05 mm and reflects the '‘true” depth. On average, the Hilbert transform and the signal mixing operation resulted in a factor of two improvement in the fullwidth at half maximum (FWHM) compared to the uncorrected data. FIG. 5 show s similar depth versus time interferometric data as FIG. 3. The top graph in FIG. 5 is a forward uncorrected sweep, the middle graph is a reverse uncorrected sweep, and the bottom graph is the corrected sweep data obtained from applying the Hilbert transform, as well as application of the signal mixing operation, to the forward and reverse uncorrected sweeps. Again, the signal-to-noise ratio (SNR) is much improved (higher) for the corrected data, and a more accurate trend line can be established. FIG. 6A is a graph of depth versus time of (corrected) brightest pixel tracking using A-line data generated from an SS-OCT interferogram where the two k-sweeps were collected non-simultaneously. and shows a comparison between a first correction algorithm using a two-sweep average (i.e., average of forward and reverse sweep value) and a second correction algorithm where the Hilbert transform and signal mixing operation were applied. The bottom two graphs in FIG. 6A show a side-by-side comparison between the two correction approaches at the 11.50-13.50 ms time interval. FIG. 6B shows two graphs of depth versus time data as a comparison between non-corrected depth data (top) and corrected data (bottom) for the case of simultaneously collected forw ard and reverse sweeps.

[0098] In accordance with certain embodiments, applying a Hilbert transform to interferometric output signals may require that the Hilbert transform is applied on a signal which is sampled uniformly in time.

[0099] As will be appreciated, applying a Hilbert transform to interferometric output signals may require that the Hilbert transform is applied either on a signal which is sampled uniformly in k. or otherwise employs a method which corrects for known non-uniformity in sampling in k-space. This is for the purpose of producing an OCT image that is appropriately scaled in meaningful units of physical space. As such, the incorporation of k-clocking (including but not limited to optical methods, electronic methods, simulated methods) is required to appropriately process or sample the interferometric output signal for the Hilbert transform and / or appropriately construct a Hilbert transform for the interferometric output signal.

[0100] In accordance with various embodiments, data analysis employed in processing the generated A-lines from interferograms and subsequent images includes pre-processing, signal mixing, transforming, and post-processing of the data. Each of these processes may be handled by a specifically coded FPGA for real-time processing, as will be appreciated by those of skill in the art.

[0101] Pre-processing may include concepts and actions as discussed in the 12552 application. For example, in some embodiments pre-processing includes all digital operations which may be performed prior to the application of a transform to enhance the interferogram data and improve the final result. This may include windowing acquisitions with analytical or arbitrary envelope functions. Digital filtering may also be applied to the data, as well as DC signal subtraction. Many additional pre-processing methods have been developed for OCT applications, and may be deployed as a component of the system as they are known to those of ordinary skill in the art. In some embodiments pre-processing may include (but is not limited to) windowing a digital signal representing an interferogram in two or more different ways to extract data captured at portions of the sweep with different dk / dt.

[0102] Since the signal mixing process multiplies all signals present, including noise, there will always be a term that is the mixture of the brightest signal and noise. The effect of this is that the noise floor appears to rise by the brightest signal, causing lower intensity interfaces to drop below the noise floor. Since each A-line is re-normalized to maintain consistency, what is observed is that when multiple interfaces of differing magnitude are present in the same A- line, the lower intensity' interfaces appear suppressed after signal mixing. In welding, this situation appears most commonly when there is reflection from both the keyhole and top surface.

[0103] Signal pre-processing can also mitigate the effect of signal mixing artifacts that appear after motion correction (caused by the presence of multiple interfaces as described above). These can be prevented by stopping different interfaces from being mixed bybreaking up the interferogram into component parts before signal mixing. An interferogram can be broken into multiple segments in z-space (or pseudo z-space) before the signal mixing operation. The signal mixing operation is repeated for each / -window separately. After this, they can be recombined into a single interferogram via summation. This prevents crosstalk between interfaces that are far away in z, such as top surface and keyhole reflections.

[0104] In accordance with at least one embodiment, the interferometric output signals are segmented prior to applying the at least one signal mixing operation. In a further embodiment, the at least one signal mixing operation is applied to one or more signals of each segmented interferometric output signal, and in another embodiment the segmented interferometric output signals are further recombined via summation.

[0105] The spacing betw een and size of the z-windows can be arbitrary, and may be chosen to be either regularly spaced intervals, or based on known geometry-. For example, windows may be used to separate top surface and keyhole reflections. An example of regularly spaced (uniformly spaced) windows is given in FIG. 12. In this non-limiting example, the windows sum to unity inside the boundary covered by the windows.

[0106] In accordance with various aspects, the windows may either be applied as a digital time-domain filter or as a window function in Fourier space. Fourier space allows for greater control over the shape of the filter, allowing for a set of filters to be designed such that signal power is conserved when the interferograms are recombined (windows are overlapped and summed to unity). An example flowchart for the process is shown in FIG. 13 and includes both forward and reverse interferograms that explains the signal mixing process with z- windowing added in. The end result is a motion-compensated interferogram that may be treated as the normal output of the signal mixing process.

[0107] Post-processing in accordance with some embodiments includes operations which track one or more features of a singular A-line, as w ell as operations which track properties across many A-lines. The features / geometric aspects which are tracked are encoded in the interferometric output signal.

[0108] Transforming the data may in accordance with certain embodiments include the application of a Fourier transform, a Discrete Fourier transform, or other Fourier transform methods familiar to those of ordinary skill in the art. As mentioned previously, the Fourier transform may be applied after the signal mixing operation.

[0109] As also discussed in the 12552 application, non-limiting examples of features (e.g.. geometric properties) which may be tracked for a singular A-line include a position, a symmetry, a width of a peak, a centroid, a geometric second moment, a center of mass, an amplitude, a height to width ratio, and a geometric area under a curve. In accordance with certain embodiments, certain A-lines may be represented based on some set of parameters, features, measurements, including geometric parameters and features, or other parameters extracted from the imaging data, which may be derived from the properties of that A-line. Such representations of A-lines can be referred to as “tracked A-line data” or simply “tracked data” and / or “track data.” In accordance with the requirements of the application, in some embodiments the processing system may be configured to output only track data, or track data in addition to raw A-line data. The incorporation of track data into embodiments of the system can offer insight into the key features / geometric aspects of the sample which are being measured via SS-OCT at a considerably lower data density than that of raw data, which can make this approach advantageous from a computational standpoint. System Description

[0110] As will be appreciated, there are several non-limiting possible configurations for an imaging system capable of performing the algorithms described herein. FIGS. 7A-7E and 11A-1 ID are non-limiting examples of imaging systems in accordance with several embodiments. All systems include at least one interferometer and a processing unit (also referred to herein as a controller).

[0111] The interferometer comprises a measurement beam capable of producing multiple interferometric output signals (which may be acquired subsequently or simultaneously in time) generated from interference between retroreflection of a measurement beam with an optical reference. All the imaging systems are described in reference to a material processing beam 112 that interacts with material of a sample (workpiece 102) at a processing region on the sample. The material processing beam 112 is generated from a material processing beam source 110 that according to some embodiments is configured as a fiber laser. Non-limiting examples of fiber lasers include ytterbium fiber lasers, including those available from IPG Photonics (Marlborough, MA). The material processing beam 112 performs material modification processes (e.g.. welding, cutting, drilling, ablation, etc.) on the sample 102. A beam delivery device 115 (also referred to herein as a laser head) contains at least one directing element (e.g., mirrors, galvanometers) that is configured to direct the imaging beam 108 and / or material processing beam 112 to the workpiece. The laser head 115 may also include beam shaping optics, e.g., lenses as will be appreciated by those of skill in the art.

[0112] A first non-limiting example of an imaging system 100a is shown in FIG. 7A in accordance with at least one embodiment. The example imaging systems shown in FIGS. 7A-7E are all configured as ‘‘double” interferometer systems, but it is to be appreciated that imaging systems with a single interferometer configuration are also within the scope of this disclosure, and non-limiting examples of single interferometer configurations are shown in FIGS. 11 A-l ID. For example, in some embodiments where signal mixing is employed between two different portions of the same signal at different time windows, a single interferometer configuration may be used.

[0113] System 100a comprises an interferometer (not explicitly labeled in the figure but components of w hich are described herein) that produces multiple interferometric output signals that are generated from interference between the measurement beam 124 (sample arm) that is a light beam radiated toward a measurement target on the sample and reflected from the measurement target and a reference beam 122 (reference arm) that is reflected from an optical reference 123. For example, interference(s) between reflection from a sample or measurement arm 124 (reflected off the measurement target of the sample / workpiece being processed, which may include the one or more portions of the processing region) and a reference arm 122 (reference beam reflected from an optical reference 123 (e.g., reflective mirror) create interferometric measurement signal(s). Generally speaking, and as will be readily appreciated by those of skill in the art, a tunable light source 105 (imaging light source, measurement light source) generates a measurement beam that is split and directed into a reference arm 122 (which reflects from an optical reference 123 (e.g., a mirror) and a sample arm 124 (which reflects from workpiece 102).

[0114] As indicated in FIGS. 7A-7E, the imaging system also comprises at least one optical detector 130 that receives the interferometric output or signal from the interferometer and sends it to the processing unit 150. In some embodiments the optical detector 130 is configured as a balanced photodetector.

[0115] Processing unit 150 includes the requisite hardware and software needed to process, control, and communicate with one or more components of system 100a, and is present in all imaging systems discussed herein. Although not explicitly shown in the figures, processing unit 150 may be in communication with or otherwise coupled to optical detector 130, imaging light source 105, material processing beam source 110, beam delivery7system 115, and / or k-clock module 145. In addition, although one processing unit 150 is shown, according to some embodiments multiple processing units may be used.

[0116] Processing unit 150 is configured to receive and process the interferometric output signals from the interferometer (e.g., via optical detector 130). The processing unit 150 is configured to apply at least one signal mixing operation between one or more signals of the multiple interferometric output signals produced by the interferometer and produce an interferometric output based at least in part on the at least one signal mixing operation. The processing unit 150 also processes the interferometric output to determine at least one feature of the processing region (e.g., weld depth).

[0117] The processing unit 150 is configured to perform one or more algorithms, including at least one signal mixing operation that, as previously discussed, processes the interferometric output signal in a manner which corrects for the motion artifact. As also mentioned above, the one or more algorithms may include such operations as a Hilbert transform to produce the interferometric output. In accordance with at least one embodiment, this interferometric output can be considered a “corrected"’ interferometric output since the distortion is no longer altering the signal to a degree that inaccurate measurements are obtained.

[0118] The processing unit 150 processes the interferometric output to determine at least one feature of the processing region. In some embodiments, the at least one feature includes depth information of the processing region (e.g., weld depth). For example, in some embodiments the material modification process is a welding process, and the material processing beam creates a phase change region (PCR) at the processing region, where a weld depth can be measured. Depth information includes keyhole depth of the PCR, seam tracking, finished weld surface height, workpiece height, and other measurements related to the workpiece geometry at time points that occur during, and both pre- and post- processing. In some embodiments, the processing unit 150 is configured to determine a sample position and / or a velocity of the material of the sample based on the interferometric output.

[0119] In some instances, processing unit 150 is also configured to control at least one processing parameter of the material modification process based on the at least one feature of the processing region. In some embodiments, the at least one processing parameter comprises at least one property of the material processing beam and / or material processing beam source, non-limiting examples of which include on / off state, average power, pulse duration, peak intensity7or peak power, energy' density, fluence, wavelength, pulse repetition rate, pulse energy, pulse shape, scan speed, focal diameter, focal position, spatial pattern (on the sample). Other non-limiting examples of processing parameters include material feed rate, cooling media flow rate, cover / assist gas flow rate, cover / assist gas pressure, cover / assist gas blend, and additive material feed rate.

[0120] Several options exist for the imaging light source 105 of the imaging systems disclosed herein. The general construction of the light source contains at least a seed source that produces the wavelength swept signal, which can be referred to as a swept source. The light source 105 is in communication with and configured to be controlled by processing unit 150. For example, the output of light source 105 is based on an input (referred to as the driving signal) sent by processing unit 150. In accordance with various aspects, the imaging light source 105 may be considered to be a component of the interferometer and / or the imaging system. Non-limiting examples of an appropriate seed swept source include solid- state MEMS (Micro-Electro-Mechanical System) swept-source VCSELs (Vertical Cavity Surface Emitting Lasers) among other possible configurations. According to at least one embodiment, the tunable laser light source 105 is a tunable VCSEL. One non-limiting example of a suitable MEMS VCSEL light source includes the SL 10280 available from Thorlabs. Inc., Newton, NJ, USA. According to at least one embodiment, the imaging light source 105 has a power within a range of at least 1 microwatt (pW) to 1 watt inclusive, with some embodiments having a range of 30 - 500 pW inclusive.

[0121] As mentioned above, some embodiments include an amplifier 106 to increase the power output of the imaging light source 105. In some embodiments (and as shown in FIGS. 7A-7E) the amplifier 106 for amplifying the imaging light source 105 is configured as a fiber amplifier, but other types of amplifiers are also within the scope of this disclosure (e.g., SIOA or solid-state). A doped fiber amplifier is also used in some embodiments, such as erbium and ytterbium-doped fiber amplifiers. Some embodiments may incorporate the use of more than one amplifier 106, and multiple locations for the amplifier are possible, including in the sample arm.

[0122] As mentioned previously, in certain embodiments the OCT imaging signal needs to be sampled uniformly in k or otherwise correct for non-uniformity in sampling in k-space. This includes the incorporation of k-clocking that is implemented by a k-clock. The k-clock may in some embodiments be generated by an optical k-clock module (e.g.. k-clock module 145) that generates a signal (k-clock signal) indicating every time the swept source 105 tunes through a predetermined k (inverse of frequency) increment of the scan band. The k-clock may in some embodiments be generated by an electronic k-clock module which estimates the time-domain behavior of the wavenumber k of the swept optical source output based on the tuning signal applied to the swept source. The k-clock may also in some embodiments be simulated based on a model of the system. A combination of the described approaches may be used in some embodiments. The k-clock is used to correct for non-linearity in the time domain of the frequency sweeping of the swept source 105.

[0123] In accordance with at least one embodiment, a k-clocking method employs the following steps:

[0124] 1. Hilbert transform the k-clock signal to obtain an analytic signal

[0125] 2. Compute the argument (phase) of the analytic signal

[0126] 3. "Unw rap” the phase (remove discontinuities greater than pi or in some instances 2 pi)

[0127] 4. Divide by the k-clock equivalent distance to obtain wavenumber (k) as a function of time

[0128] 5. Use k(t) to resample the OCT signal with linear or cubic interpolation This is just one example of a k-clocking method and it is to be appreciated that other approaches are within the scope of this disclosure.

[0129] As noted previously, imaging systems that comprise “single” interferometer configurations are also within the scope of this disclosure, and non-limiting examples are shown in FIGS. 11 A-l IB where feature numbers are repeated that are similar to those discussed above in reference to FIGS. 7A-7E (e g., reference arm 122 is 222 and sample arm 124 is 224). FIG. 1 IB shows an example of an imaging system 200b configured with a Michelson interferometer (discussed in more detail below), and the imaging system 200c of FIG. 11C is configured with a Mach-Zender interferometer (also discussed below). The interferometer of imaging system 100a of FIG. 11 A is one non-limiting example of where the optical amplifiers 206 are positioned in locations (e.g., the sample arm) other than directly following the tunable light source 205. Imaging system 200d of FIG. 1 ID is an example of a configuration where an optical buffer implemented as a delay line fiber 225 is included in the light source to produce light that is substantially distinct from the non-delayed light.

[0130] In some embodiments, the interferometer is configured as a Michelson interferometer. Other interferometer configurations are also within the scope of this disclosure, including Mach-Zehnder configurations and modified versions of interferometer topologies. At least one embodiment comprises an interferometer configured as a Mach-Zehner interferometer. FIGS. 8A-8D show four different interferometer configurations in accordance with various embodiments. FIG. 8A is a schematic of a Michelson interferometer, configured with a 50:50 splitter that directs the incoming imaging beam from the light source into two separate sample and reference arms, and the retroreflections from each of these arms are directed to a detector. A modified Michelson configuration is shown in FIG. 8B, which includes the addition of a circulator and a balanced photodetector. The circulator is introduced prior to the splitter, which directs light reflected from the sample and reference arms which has coupled back into the splitter arm corresponding to the imaging source to a balanced photodetector. FIG. 8C is a schematic representation of a Mach-Zehnder configuration where one splitter is configured to direct the incoming imaging signal to two circulators that in turn direct and receive retroreflections from the sample and reference arms, and a balanced photodetector is positioned downstream from a second splitter. A modified Mach-Zehnder configuration is shown in FIG. 8D, where two splitters replace the circulators shown in FIG. 8C. In accordance with various aspects, replacing the circulators with the beam splitter configuration as exemplified in the interferometer of FIG. 8D offers the advantage of having a lower cost, but also offers lower photon efficiency. However, this latter aspect is not an issue if enough signal (power) is implemented with the imaging signal.

[0131] FIGS. 7B-7E are schematics of four other non-limiting examples of imaging systems lOOb-lOOe, respectively, that can also be described as “double” interferometer configurations. Components of imaging systems lOOb-lOOe are similar to that of 100a in FIG. 7A. For all double interferometer configurations, multiple (e.g.. first and second) measurement beams are implemented (which can be generated using at least one tunable light source 105), and the processing unit 150 applies the signal mixing operation to the interferometric output signals of the first and second measurement beams. Although the examples here refer to first and second measurement beams, it is to be appreciated that some embodiments comprise a first measurement beam and at least one other measurement beam. In some embodiments, a first tuning rate of the first measurement beam is different than a second tuning rate of the second measurement beam. For example, the first tuning rate may be a positive tuning rate and the second tuning rate may be a negative tuning rate (or vice versa). The first and second tuning rates are different for at least one portion of time. In some embodiments, the first measurement beam and the second (or at least one other) measurement beam are spatially superimposed. For instance, the beams may be combined in fiber and when emitted from the fiber, the beams fully overlap one another.

[0132] The multiple interferometric output signals of the first and second measurement beams are received simultaneously by the processing unit 150. In some embodiments, the first and second measurement beams are directed to the processing region by the at least one directing element of beam delivery system / laser head 115. In some embodiments, the first and second measurement beams are directed to the processing region simultaneously. This allows for the processing unit 150 to process the interferometric output signals associated with each measurement beam simultaneously.

[0133] FIGS. 7A and 7D are configured such that a first interferometer is configured with a first tunable light source and at least one additional interferometer is configured with (at least one) other tunable light source. More than one separate light source may be used to create distinct imaging light used for the measurement beam. FIG. 7A is a non-limiting example where the SS-OCT topology has separate light sources and a shared sample arm. In accordance with at least one embodiment, 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. For example, in FIGS. 7A and 7D. sample arms of the first interferometer and the at least one additional interferometer share at least one optical element, e.g., fiber of the overlapped sample arm prior to introduction to the beam delivery head 115. It is to be appreciated that 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 that are shared between the interferometer sample arm and processing beam (e.g., scanning / wobble galvos, processing beam lenses, other lenses, cover glass), 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).

[0134] In accordance with at least one embodiment, the imaging system further includes a splitter to split the imaging signal (imaging beam) into at least two arms, and an optical delay element. Non-limiting examples of such a configuration are shown in FIGS. 7B. 7C, and 7E (e.g.. optical delay element 125 in FIGS. 7B, 7C, and 7E). The optical delay element (e.g., 125) is configured such that one arm of one interferometer is delayed in time relative to the (equivalent) arm of the second interferometer. As will be appreciated, this is one method for creating or otherwise generating different tuning rates dk / dt as described herein, besides controlling the swept source 105 (e.g.. by varying the time rate of change of voltage applied to the light source to generate different tuning rates dk / dt).

[0135] In accordance with various embodiments, a first arm is configured to be directed to at least one of different reference arms, different sample arms, partially overlapped reference arms, and partially overlapped sample arms of a first interferometer and at least one additional interferometer. This is shown in FIGS. 7B, 7C, and 7E where the delayed arm (passing through optical delay element 125) is directed to sample arms of the first interferometer and the at least one additional interferometer share at least one optical element. It is to be appreciated that this is just one non-limiting example and other embodiments exist where the delayed arm is directed to different reference arms of multiple interferometers, different sample arms of different interferometers, and / or partially overlapped reference arms of multiple interferometers.

[0136] In some embodiments, two or more interferometers include configurations where there is at least partial overlapping at the sample (or ‘"sample arm") so that each interferometer may be used to image the same sample. Three examples of topologies where two distinct interferometers are at least partially overlapped are shown in FIGS. 7A-7E. It is to be understood that variants of such interferometers are also possible and considered to be within the scope of this disclosure, for example a version of the interferometer in FIG. 7B with K-Clock module 1, where K-clocking for the buffered light signal is handled byapplying a time-delayed copy of the signal from K-clock Module 1. FIG. 7A illustrates an embodiment with two distinct partially overlapping interferometers that are configured such that each interferometer has a separate tunable light source. FIGS. 7B, 7C, and 7E illustrate embodiments w here an optical delay line 125 is implemented to produce distinct swept signals for input to each of the partially overlapping interferometers. FIG. 7D is another example of a configuration with two separate tunable light sources, which illustrates an interferometer topology which can be constructed without the use of optical circulators, and FIG 7E illustrates this concept of an interferometer without circulators with a buffered source providing distinct swept signals for input to each of the partially overlapping interferometers. Variants of double interferometer topologies based on the interferometer features, topologies, and functions described in this document are also considered within the scope of this disclosure.

[0137] EXAMPLES OF HILBERT TRANSFORM APPLICATION

[0138] The imaging systems and techniques disclosed herein may be more fully understood based on the examples described below. The following examples are intended to illustrate various aspects of the disclosed imaging systems and methods but are not intended to fully exemplify the full scope thereof.

[0139] Example 1 - K-domain Hilbert signal mixing

[0140] The first detailed non-limiting example is capable of being implemented on an imaging system with either a single or dual interferometer configuration and is described below- in reference to FIGS. 9A-9E. In addition, for the described operations computation of the analytic signal using the Hilbert transform, H(fn), an example algorithm is described as follows, where H is the operation of computing the analytic signal of function fh. which is itself a function or signal with real values:

[0141] H(fn) = x + i(F-1(F(x)2U)) = x + iy where:

[0142] F = the Fourier transform U = the unit step function x = the real-valued signal of the function fn y = the Hilbert transform of x

[0143] The first step involves acquiring forward and reverse sweep k-clock and OCT traces. This includes acquiring four signals as conceptually shown in FIG. 9 A: (1) OCT (imaging signal) forward (positive), (2) OCT signal reverse (negative), (3) k-clock signal forward, and (4) k-clock signal reverse. In some embodiments, this may be implemented using four acquisition channels. Ideally, all the signals are digitized using the same clock.

[0144] The next step involves using the forward and reverse k-clock traces to complete a k- versus-time function, a non-limiting example of which is shown in FIG. 9B. This is followed by using the k-versus-time function to assign a k-value to each OCT sample in each digitized OCT signal, as shown in FIG. 9C, where the following applies:

[0145] • at point t = ta(example point shown with arrow in OCT FWD(t) line of FIG. 9C), I(ta) = OCT FWD (ta) and KFwD(ta) = K

[0146] • at point t = ta(example point shown with arrow in OCT REV(t) line of FIG. 9C), I(ta)= OCT REV (ta) and KREV(ta) = K

[0147] Next, the assigned k-values at each time point are used to generate a function of OCT (FWD / REV) as a function of K. An example of such a graph is shown in FIG. 9D. and interpolation is used to resample this function to a uniform spacing in K. A Hilbert transform is applied to the (real) measured signal to generate a new signal that is 90 degrees out of phase from the original. The signals are then mixed through multiplication. The depth (z) values are divided by two (2) to compensate for the multiplication operation. One example of this operation is shown in FIG. 9E. A Fourier transform is then applied to ICOrreoted (k) such that A-line = F(I00rreoted(k)).

[0148] Example 2 - Time domain Hilbert signal mixing

[0149] The second detailed example is also capable of being implemented on an imaging system with either a single or dual interferometer configuration and is described below in reference to FIGS. 10A-10C.

[0150] The first step involves acquiring the four signals associated with the forward and reverse sweep k-clock and OCT traces as described previously in reference to FIG. 9A.

[0151] A Hilbert transform is applied to the (real) measured signal to generate a new signal that is 90 degrees out of phase from the original. The time-domain signals are then mixed through multiplication, as shown in FIG. 10A and represents a complex valued signal. The next step involves obtaining the real component of the signal:

[0152] OCTcorrected(t) = Re(OCTmixed(t))

[0153] The next step involves using the forward and reverse k-clock traces to complete k- versus-time (KFWD(I), KREV(I)) functions in a similar manner as was described above in reference to FIG. 9B. These are then summed to obtain KSUM(I):

[0154] KFWD(t)+K.REv(t) = KsUM(t)

[0155] A K-value is then assigned to each OCTCOrrected(t) sample using KSUM(I), which is conceptually shown in FIG. 10B, where at point t = ta(example point shown with arrow in FIG. 10B):

[0156] I—OCTcorrected(ta) and

[0157] K = KsUXl(ta)

[0158] This I-K mapping is then used to generate a function for OCTcorrcctcd as a function of k (an example of which is shown in FIG. IOC) and interpolation is used to resample this function to a uniform spacing in k. A Fourier transform is then applied to OCTCorreoted (k) such that A- line = F(OCToorrected(k)).

[0159] The aspects disclosed herein in accordance with the present invention, are not limited in their application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. These aspects are capable of assuming other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, components, elements, and features discussed in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiments.

[0160] Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Any references to examples, embodiments, components, elements or acts of the systems and methods herein referred to in the singular may also embrace embodiments including a plurality, and any references in plural to any embodiment, component, element or act herein may also embrace embodiments including only a singularity. References in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. The use herein of "‘including,” “comprising.” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to ”or” may be construed as inclusive so that any terms described using ■‘of’ may indicate any of a single, more than one, and all of the described terms. In addition, in the event of inconsistent usages of terms between this document and documents incorporated herein by reference, the term usage in the incorporated reference is supplementary to that of this document; for irreconcilable inconsistencies, the term usage in this document controls. Moreover, titles or subtitles may be used in the specification for the convenience of a reader, which shall have no influence on the scope of the present invention.

[0161] Having thus described several aspects of at least one example, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. For instance, examples disclosed herein may also be used in other contexts. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the scope of the examples discussed herein. Accordingly, the foregoing description and drawings are by w ay of example only.

Claims

CLAIMSWhat is claimed is:

1. An imaging system for performing imaging of a sample treated by a material processing beam, the material processing beam interacting with material of the sample at a processing region on the sample, the imaging system comprising: an interferometer configured to produce multiple interferometric output signals generated from interference between a measurement beam that is a light beam radiated toward a measurement target on the sample and reflected from the measurement target and a reference beam that is reflected from an optical reference; and a processing unit configured to: receive the multiple interferometric output signals; apply at least one signal mixing operation between one or more signals of the multiple interferometric output signals; produce an interferometric output based at least in part on the at least one signal mixing operation; and process the interferometric output to determine at least one feature of the processing region.

2. The imaging system of claim 1, configured such that a tuning rate dk / dt of the measurement beam changes between two or more different tuning rates.

3. The imaging system of claim 2, wherein the two or more different tuning rates include at least one positive tuning rate and at least one negative tuning rate.

4. The imaging system of claim 2, wherein the measurement beam is associated with a signal, and the processing unit is configured to apply the at least one signal mixing operation between different time windows within the signal.

5. The imaging system of claim 4, wherein each time window is associated with a different tuning rate.

6. The imaging system of claim 5, wherein the processing unit is configured to Hilbert transform each portion of the signal associated with each time window and the at least one signal mixing operation is applied to two or more Hilbert transformed portions of the signal.

7. The imaging system of claim 2, wherein the measurement beam of the interferometer includes a first measurement beam and the interferometer further comprises a second measurement beam, the first measurement beam having a first tuning rate that is different than a second tuning rate of the second measurement beam.

8. The imaging system of claim 7, wherein the first and second tuning rates are different for at least one portion of time.

9. The imaging system of claim 7, configured such that the multiple interferometric output signals of the first and second measurement beams are received simultaneously by the processing unit.

10. The imaging system of claim 7, wherein the processing unit is configured to apply a Hilbert transform to each of the interferometric output signals of the first and second measurement beams.1 1 . The imaging system of claim 10, wherein the at least one signal mixing operation is applied to the Hilbert transformed interferometric output signals of the first and second measurement beams.

12. The imaging system of claim 11, wherein the processing unit is configured to apply a Fourier transform to the Hilbert transformed interferometric output signals after the signal mixing operation.

13. The imaging system of claims 6 and 11, wherein the interferometric output corrects for one or more distortions in the interferometric output signals.

14. The imaging system of claim 13. wherein the one or more distortions are created by a time-varying difference in optical path lengths included in the interferometric output signals and the time-varying difference in optical path lengths is caused by at least one of sample motion relative to an axis of the measurement beam, a material modification process implemented by the material processing beam on the sample, and intrinsic sample motion not caused by a material modification process implemented by the material processing beam on the sample.

15. The imaging system of claim 7, wherein the first tuning rate is a positive tuning rate and the second tuning rate is a negative tuning rate.

16. The imaging system of claim 7, further comprising at least one directing element that is configured to direct the first and second measurement beams to the processing region simultaneously.

17. The imaging system of claim 1, wherein the interferometric output signals are segmented prior to applying the at least one signal mixing operation.

18. The imaging system of claim 17. wherein the at least one signal mixing operation is applied to the one or more signals of each segmented interferometric output signal.

19. The imaging system of claim 18. wherein the segmented interferometric output signals are further recombined via summation.

20. The imaging system of claim 1, wherein the interferometer further comprises at least one tunable light source configured to generate the measurement beam.

21. The imaging system of claim 20. further comprising an amplifier for amplifying the tunable light source.

22. The imaging system of claim 1, wherein the interferometer is configured as one of a Michelson or Mach Zehnder interferometer.

23. The imaging system of claim 1, wherein the at least one feature includes depth information of the processing region.

24. The imaging system of claim 1, wherein the processing unit is further configured to determine a sample position and / or a velocity of the material of the sample based on the interferometric output.

25. The imaging system of claim 1, further comprising a material processing beam source configured to generate the material processing beam.

26. A material processing system, comprising: the imaging system of claim 1; a material processing beam source that generates the material processing beam; and a beam deliver)’ system for the material processing beam and the measurement beam.

27. An imaging method for imaging a processing region on a sample being treated by a material processing beam, the method comprising: provide an interferometer configured to produce multiple interferometric output signals generated from interference between a measurement beam that is a light beam radiated toward a measurement target on the sample and reflected from the measurement target and a reference beam that is reflected from an optical reference; apply at least one signal mixing operation between one or more signals of the multiple interferometric output signals; produce an interferometric output based at least in part on the at least one signal mixing operation; and process the interferometric output to determine at least one feature of the processing region.

28. The imaging method of claim 27, further comprising changing a tuning rate dk / dt of the measurement beam between two or more different tuning rates.

29. The imaging method of claim 28, wherein the two or more different tuning rates include at least one positive tuning rate and at least one negative tuning rate.

30. The imaging method of claim 28, wherein the measurement beam is associated with a signal, and the at least one signal mixing operation is applied to different time windows within the signal.

31. The imaging method of claim 30, wherein each time window is associated with a different tuning rate.

32. The imaging method of claim 31, further comprising Hilbert transforming each portion of the signal associated with each time window, and applying the at least one signal mixing operation to two or more Hilbert transformed portions of the signal.

33. The imaging method claim 28. wherein the interferometer is provided such that the measurement beam of the interferometer includes a first measurement beam and the interferometer further comprises a second measurement beam, the first measurement beam having a first tuning rate that is different than a second tuning rate of the second measurement beam.

34. The imaging method of claim 33, wherein the multiple interferometric output signals of the first and second measurement beams are received simultaneously.

35. The imaging method of claim 33, further comprising Hilbert transforming each of the interferometric output signals of the first and second measurement beams.

36. The imaging method of claim 35, wherein the at least one signal mixing operation is applied to the Hilbert transformed interferometric output signals of the first and second measurement beams.

37. The imaging method of claim 36, further comprising applying a Fourier transform to the Hilbert transformed interferometric output signals after the signal mixing operation.

38. The imaging method of claims 32 and 36, wherein the interferometric output corrects for one or more distortions in the interferometric output signals.

39. The imaging method of claim 38, wherein the one or more distortions are created by a time-vary ing difference in optical path lengths included in the interferometric output signals and the time-varying difference in optical path lengths is caused by at least one of sample motion relative to an axis of the measurement beam. a material modification process implemented by the material processing beam on the sample, and intrinsic sample motion not caused by a material modification process implemented by the material processing beam on the sample.

40. The imaging method of claim 34, wherein the first tuning rate is a positive tuning rate and the second tuning rate is a negative tuning rate.

41. The imaging method of claim 34, further comprising directing the first and second measurement beams to the processing region simultaneously.

42. The imaging method of claim 27, further comprising segmenting the interferometric output signals prior to applying the at least one signal mixing operation.

43. The imaging method of claim 42, further comprising applying the at least one signal mixing operation to the one or more signals of each segmented interferometric output signal.

44. The imaging system of claim 43, further comprising recombining the segmented interferometric output signals via summation.

45. The imaging method of claim 27, wherein the interferometer is provided with at least one tunable light source configured to generate the measurement beam.

46. The imaging method of claim 45, further comprising providing an amplifier for amplifying the tunable light source.

47. The imaging method of claim 27, wherein the interferometer is configured as one of a Michelson or Mach Zehnder interferometer.

48. The imaging method of claim 27, wherein the at least one feature includes depth information of the processing region.

49. The imaging method of claim 27, further comprising determining a sample position and / or a velocity of the material of the sample based on the interferometric output.

50. The imaging method of claim 27, further comprising providing a material processing beam source configured to generate the material processing beam.

51. The imaging method of claim 27, further comprising providing a processing unit configured to apply the at least one signal mixing operation, produce the interferometric output, and process the interferometric output.

52. The imaging method of claim 27, further comprising controlling at least one processing parameter of a material modification process implemented by the material processing beam on the sample based on the at least one feature of the processing region.

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