Static and dynamic calibration for coherence imaging measurement systems and methods

Abstract

To provide a device and a method for improving the accuracy of an alignment procedure, improving the precision of the alignment procedure, reducing the time requirement of the alignment procedure, automating the alignment procedure (partially or completely), or reducing the requirement of minimum knowledge and experience for executing the alignment procedure.SOLUTION: Systems and methods for static and dynamic calibration can be used to provide alignment of a measurement beam from a coherence imaging (CI) measurement system to a processing beam from a material processing system. In these systems and methods, a calibration measurement output can be obtained from a CI measurement system and / or an auxiliary sensor. Further measurements performed by the CI measurement system may be modified based at least in part on the calibration measurement output.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Related Applications This application claims the benefit of U.S. Provisional Patent Application No. 63 / 011,235, filed April 16, 2020, which is incorporated by reference herein in its entirety.

[0002] The present disclosure relates to coherence imaging measurement systems and methods for monitoring high-energy beam processing applications, and more particularly to static and dynamic calibration techniques for positioning a measurement beam in coherence imaging measurement systems and methods. [Background technology]

[0003] The use of Coherence Imaging (CI) measurement systems (e.g., IPG Photonics LDD700 systems) to monitor laser processing applications (e.g., laser welding, laser additive manufacturing, laser marking, laser material removal, laser cleaning, etc.) provides significant benefits to the results of the process. The benefits of CI systems are primarily realized in their spatial and temporal relationships to the laser process itself. For example, a more obvious relationship is the spatiotemporal relationship of the CI measurement beam to the processing laser beam spot on the workpiece undergoing processing. A typical single measurement of a CI system, referred to herein as an A-line, is given meaning based on the context in which it is obtained.

[0004] This meaning is an assigned meaning based, in part, on where the measurement is made: in space relative to the workpiece (e.g., subcomponent A and subcomponent B in a bonding application to form component AB), in space relative to the processing cell (e.g., relative to the workpiece fixture, relative to the cell coordinate frame, relative to the tool coordinate frame, etc.), in space relative to the processing (before the processing path vector), in time relative to the processing (e.g., 100 ms after processing is completed), in space relative to the processing laser focal spot (e.g., 0.1 mm behind the processing beam), in time relative to the processing laser power modulation (e.g., 0.5 ms after each laser pulse in a series of laser pulses), and in time relative to the processing laser beam modulation pattern (e.g., 0.05 ms after the start phase of a circular laser wobble pattern).

[0005] In the example of laser welding, CI measurements can be taken during the laser process at a specific distance far enough in front of the process laser focal spot, but close enough to the focal spot, so that the measurements are not affected by the process itself, so that accurate measurements of the part geometry (e.g., the seam line between two components being joined by the welding process) can be used to determine whether the focal spot is sufficiently aligned with the seam line. In some cases, these measurements can be further used to dynamically correct for misalignment. Similarly, CI measurements performed in the phase change region during laser welding can be used to measure the keyhole depth of the weld, a critical metric for many laser welding processes.

[0006] For many laser processing applications, the alignment of the CI measurement to the laser focal spot and temporal power modulation profile must be accurate and precise enough for the resulting measurements to be useful in process monitoring and control contexts. Specific requirements may vary depending on the application and the type of CI measurement being performed for the application. For example, in multimode laser welding applications, it is often sufficient to spatially align the CI measurement to the process beam at the workpiece surface within a distance on the order of tens to hundreds of microns. In single-mode laser welding applications, alignment distances on the order of tens to a few microns may be desirable.

[0007] Similarly, temporal alignment to the temporal power profile of the processing laser on the order of tens to hundreds of microseconds may be desired for keyhole depth measurements in pulse welding applications. Such alignment may be desirable to ensure that CI measurements are performed while the keyhole vapor passage is open.

[0008] The above examples serve to illustrate the importance of spatiotemporal alignment of the CI system to the processing laser in some laser processing applications. However, the overall importance extends far beyond the limited examples provided above. Such alignment is useful in altering the behavior of the measurement system to obtain more useful measurements and to determine the usefulness of the measurements (e.g., by quantifying lack of alignment) and alter measurement data processing accordingly.

[0009] In some applications, strict alignment requirements require trained professionals to set up the CI system on-site at the laser processing station. In some applications, the start-up procedure may need to be performed repeatedly until specific requirements are achieved. In these cases, the alignment requirements imposed by the application are very time-consuming. The minimal knowledge and time required to perform such procedures can be a significant barrier to future modifications to the laser processing station, as such modifications may involve the redevelopment of trained professionals, extensive remote assistance from trained professionals, or a significant amount of prior training for the laser processing station operators.

[0010] In many applications, alignment procedures and calibrations of CI systems are specific to the hardware configuration (e.g., specific to the laser processing station) and need to be performed for each deployed CI system. Engineering and manufacturing tolerances can require calibrations to be performed for each laser processing station, even if the stations are nominally the same. Similarly, in some applications, additional calibration procedures are desired to perform alignment procedures that are not only specific to the laser processing station, but also specific to the process itself. As a result, CI systems used to monitor various laser processing applications, or different types of the same laser processing application, can involve many calibrations.

[0011] When a CI system has some resonant components, jitter characteristics or other characteristics where the phase of the A-line acquisition timing cycle is difficult to control and / or predict the spatiotemporal relationship between the A-lines and the rest of the processing sometimes need to be determined retrospectively. However, even in these cases, the techniques and apparatus described herein provide benefits of comparable value when compared to implementations where A-lines can be triggered "on demand."

[0012] Manufacturing tolerances combined with very tight alignment requirements can result in CI system recalibration following relatively infrequent, but common, laser processing station procedures. These procedures may include changing the laser head's protective cover slide, changing the focal length of the laser head focusing optics, replacing the laser delivery fiber, replacing the laser head with a spare, and adjusting the CI system delivery optics. In such applications, the end user of the CI system may need to monitor the system's alignment throughout its use or may need to perform multiple calibrations throughout its operational life.

[0013] External environmental influences (e.g., changes in temperature, vibration, humidity) and maintenance (replacement of process fiber, cleaning of system) can potentially change the alignment between the process and imaging optics. Because these influences cannot always be isolated to a subset of system components, a means for directly co-aligning the various imaging and energy beam systems that considers the net effect of all of these influences on the co-alignment is desirable. In many cases, the best way to do this is to use an imaging system to make observations of the process beam's interaction with the material. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] U.S. Patent No. 8,822,875 [Patent Document 2] U.S. Patent No. 9,757,817 [Patent Document 3] U.S. Patent No. 10,124,410 [Patent Document 4] U.S. Patent Application Publication No. 2020 / 0023461 [Patent Document 5] U.S. Patent Application No. 16 / 721,306 [Patent Document 6] U.S. Patent Application Publication No. 2020 / 0198050 Summary of the Invention [Problem to be solved by the invention]

[0015] Thus, there is a need for new apparatus and methods that improve the accuracy of the alignment procedure, improve the precision of the alignment procedure, reduce the time requirements of the alignment procedure, automate (partially or fully) the alignment procedure, reduce the minimum knowledge and experience requirements for performing the alignment procedure, or some combination thereof. [Means for solving the problem]

[0016] Consistent with an aspect of the present disclosure, a system includes a material processing system, a coherence imaging (CI) measurement system, and one or more controllers. The material processing system includes a processing beam source for generating a processing beam and a processing beam head for delivering the processing beam to a target. The coherence imaging (CI) measurement system includes a coherence imaging (CI) core unit for generating a measurement beam and a CI scanning module for delivering the measurement beam to the target, and the CI measurement system generates a CI measurement output. The one or more controllers are configured to receive the CI measurement output from the CI core unit for monitoring and / or controlling the material processing system. The one or more controllers are also configured to receive the CI measurement output from the CI core unit for monitoring and / or controlling the material processing system. The controller is also configured to receive a calibration measurement output and to control the CI measurement system based at least in part on the calibration measurement output, such that the controller is configured to control the CI measurement system to correct future measurements made by the CI measurement system for alignment adjustment between the CI measurement system and the processing beam.

[0017] Consistent with other aspects of the present disclosure, a method is provided for calibrating a coherence imaging (CI) measurement system. The method includes providing a material processing system configured to generate and deliver a process beam to a target; providing a coherence imaging (CI) measurement system configured to generate and deliver a measurement beam to the target and configured to provide a CI measurement system output for controlling and / or monitoring the material processing system; obtaining a calibration measurement output from the CI measurement system and / or an auxiliary sensor; and automatically controlling the CI measurement system based at least in part on the calibration measurement output, wherein the CI measurement system is controlled to correct future measurements made by the CI measurement system due to alignment between the CI measurement system and the process beam. Correcting future measurements made by the CI measurement system may include performing a system-level calibration, a process-level correction, and / or a process-by-process correction.

[0018] In one embodiment of the method, the CI measurement system is automatically controlled to correct future measurements made by the CI measurement system such that a static calibration is performed to align the CI measurement system with the process beam. In another embodiment of the method, the CI measurement system is automatically controlled to correct future measurements made by the CI measurement system such that a dynamic calibration is performed to align the CI measurement system with the process beam.

[0019] Consistent with further aspects of the present disclosure, a system includes a material processing system, at least one material processing system controller, a coherence imaging (CI) measurement system, and at least one CI system controller. The material processing system includes a laser for generating a laser processing beam and a processing beam head for delivering the laser processing beam to a target. The laser has an M of substantially less than 2.0. 2The material processing system controller is configured to cause the material processing system to generate and direct a laser processing beam to a target for the calibration measurement.

[0020] In some embodiments of this system, the material processing system controller may be configured to cause the material processing system to generate and direct a laser processing beam toward a target to generate a physical modification at the target. In these embodiments, the calibration measurement output may be based on the physical modification, and the CI system controller may be configured to control the CI measurement system to align the CI measurement system with respect to the physical modification. In other embodiments, the material processing system controller may be configured to cause the material processing system to generate and direct a laser processing beam toward a target to generate localized processing radiation at the target. In these embodiments, the calibration measurement output may be based on the localized processing radiation, and the CI system controller may be configured to control the CI measurement system to align the CI measurement system with respect to the localized processing radiation.

[0021] The CI measurement system includes a coherence imaging (CI) core unit for generating a measurement beam and a CI scanning module for delivering the measurement beam to a target, where the CI measurement system generates a CI measurement output. A CI system controller is configured to receive the CI measurement output from the CI core unit to monitor and / or control the material processing system. The CI system controller is also configured to receive a calibration measurement output and control the CI measurement system based at least in part on the calibration measurement output. The CI system controller is configured to control the CI measurement system to correct future measurements made by the CI measurement system for alignment adjustment between the CI measurement system and the processing beam. In some embodiments of this system, the CI measurement system may include an inline coherent imaging (ICI) measurement system, where the CI measurement system may be configured to generate the calibration measurement output. In other embodiments, an auxiliary sensor may be configured to generate the calibration measurement output.

[0022] Consistent with yet other aspects of the present disclosure, a method is provided for calibrating a coherence imaging (CI) measurement system. The method includes providing a material processing system including a laser configured to generate and deliver a laser processing beam to a target; providing a coherence imaging (CI) measurement system configured to generate and deliver a measurement beam to the target and configured to provide a CI measurement system output for controlling and / or monitoring the material processing system; generating and directing the laser processing beam to the target for a calibration measurement; obtaining a calibration measurement output from the CI measurement system and / or an auxiliary sensor; and automatically controlling the CI measurement system based at least in part on the calibration measurement output, the CI measurement system being controlled to correct future measurements made by the CI measurement system for alignment adjustments between the CI measurement system and the processing beam. In one embodiment of the method, the laser has an M 2 The signal may be configured to generate a single spatial mode output with a value.

[0023] In some embodiments of the method, the laser processing beam generates a physical modification at a target, the target corresponding to the position of the processing beam. In these embodiments, the calibration measurement output may be based on the physical modification, and the CI system control system may be controlled to align the CI measurement system with the physical modification. Obtaining the calibration measurement output may include, for example, measuring the physical modification with the CI measurement system. In other embodiments, the laser processing beam generates local processing radiation at a target, the target representing the position of the processing beam. In these embodiments, the calibration measurement output may be based on the local processing radiation, and the CI measurement system may be controlled to align the CI measurement system with the local processing radiation. Obtaining the calibration measurement output may include, for example, detecting the local processing radiation with the CI measurement system.

[0024] These and other features and advantages will be better understood from a reading of the following detailed description taken in conjunction with the drawings. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic block diagram of a laser processing system and a coherence imaging (CI) system capable of calibration measurements of the CI system consistent with an embodiment of the present disclosure. [Figure 2] FIG. 10 is a schematic block diagram of a laser processing system and a coherence imaging (CI) system with a 2D in-line camera for calibration measurements of the CI system consistent with other embodiments of the present disclosure. [Figure 3] FIG. 10 is a schematic block diagram of a laser processing system and a coherence imaging (CI) system with a double-clad fiber and a photodetector for calibration measurements of the CI system consistent with a further embodiment of the present disclosure. [Figure 4] FIG. 10 is a schematic block diagram of a laser processing system and a coherence imaging (CI) system with a 2D camera below the laser head for calibration measurements of the CI system consistent with a still further embodiment of the present disclosure; [Figure 5] 10 is a series of images showing an example processing beam alignment calibration performed using interferometric output measurements acquired during a raster scan of a marked calibration target and processed using a series of image processing algorithms. [Figure 6] FIG. 10 is a block diagram illustrating an example of an interferometric data processing algorithm employed for processing beam alignment calibration. [Figure 7] FIG. 1 is a schematic diagram of an example CI system scanning module measurement acquisition scan pattern employed for feature detection in a calibration measurement of the CI system. [Figure 8] 10 is a graph illustrating an example of a temporal gating strategy used to distinguish between back-reflected processing beam light and processing radiation emanating from the laser interaction zone. [Figure 9]2 is a plot showing an example process radiation line scan performed with the system shown in FIG. 1 at two positions within the laser scan head scan range. [Figure 10] FIG. 10 is a schematic diagram of an example of alignment of a treatment beam using an apparatus with a 2D in-line camera. [Figure 11] FIG. 1 is a schematic diagram of a shared optical path between the scanning module and the in-line camera of a CI system. [Figure 12] 3 is a graph illustrating an example temporal gating scheme used to distinguish between proxy measurements of the processing beam and the imaging beam using a 2D camera in the system of FIG. 2; [Figure 13] FIG. 1 is a schematic diagram of a calibration for aligning the reference frame of the CI system to the process beam / workpiece reference frame. [Figure 14] 2 is a plot showing an example of processed radiometric measurements recorded by the system shown in FIG. 1 (left) and data processing operations such as noise removal and curve fitting (right) that can be used to find the centroid of the data. [Figure 15] 10 is a schematic diagram highlighting an example of the difference in commanded imaging beam position and actual measurement beam position with accurate and inaccurate tracking errors when scanning in opposite directions. [Figure 16] 1 is a graph showing example signal measurements obtained while scanning over a static feature in two opposite directions using accurate tracking error (top) and inaccurate tracking error (bottom). [Figure 17] 10 is a schematic diagram of an example of the type of correction that needs to be applied to the scanning module position commands of the imaging system as a function of position in the scan field to correct for optical distortions in the laser scanning head. [Figure 18] FIG. 10 is a schematic diagram of an example of path-specific imaging and processing beam alignment measurements for an apparatus with a laser scanning head. [Figure 19]4 is an image showing a calibration measurement performed by the system of FIG. 3 employing a double-clad fiber in combination with an auxiliary photodiode, used to determine the process beam center position (1) and path direction (2) during laser processing. [Figure 20] 3 is an image showing a calibration measurement performed by the system of FIG. 2 employing a 2D in-line camera used to determine the processing beam center position and path direction during laser processing. [Figure 21] 10A and 10B are images and graphs illustrating an example calibration measurement technique implemented by a system that uses point-based measurement acquisition (e.g., interferometric output, or single-point process radiometric measurement in combination with a scanning module of a CI system) to identify process direction. [Figure 22] FIG. 10 is a schematic diagram of an example of how changes to a keyhole laser welding process, such as changes in process speed, can change the location of the vapor path relative to the process beam focus position. [Figure 23] FIG. 10 is a schematic diagram of how the use of measurement lines transverse to the nominal laser processing path direction can be used to identify differences between the nominal and actual processing paths. [Figure 24] FIG. 1 is a schematic diagram of a local surface tilt calibration measurement process. [Figure 25] 1 is an image showing an area of ​​a workpiece captured by a CI system, including a laser spot diameter determined using imaging data collected by a CI system scan. DETAILED DESCRIPTION OF THE INVENTION

[0026] Systems and methods for static and dynamic calibration consistent with this disclosure can be used to provide alignment of a measurement beam from a coherence imaging (CI) measurement system (also referred to as a CI system) relative to a process beam from a material processing system. Calibration can be performed on the system before a process is performed (i.e., system-level calibration), during a process (i.e., process-level calibration), and / or between processes (i.e., process-level calibration). While the illustrated embodiment shows a laser processing system, the systems and methods described herein may be used in any high-energy beam processing application (e.g., electron beam) other than laser processing. Examples of CI measurement systems include inline coherent imaging (ICI), such as those disclosed in U.S. Patent Nos. 6,279,949; 6,279,949; and 6,279,949; and U.S. Patent No. 6,279,949 (now U.S. Patent No. 6,279,949), all of which are incorporated herein by reference.

[0027] 1-4 , a system 100, 100′, 100″, 100′″ consistent with embodiments of the present disclosure generally includes a material processing system, a coherence imaging (CI) measurement system, and a controller / processor for controlling both systems. The material processing system includes at least one material processing beam source (e.g., laser 110) that delivers a processing beam 111 to a workpiece 102 via an energy beam delivery system (e.g., laser head 112) to perform a material modification process. The CI measurement system includes a CI core unit 120 that provides a measurement beam 121 to the workpiece 102 and generates an interference output from at least components of the measurement beam 121 directed toward and reflected from the workpiece 102. One or more of the controller / processors (e.g., laser controller 154 and CI controller 156) can be used as a feedback controller to monitor at least one process parameter of the material modification process and to adjust the behavior of the coherent imaging system based on at least one calibration measurement.

[0028] The coherent imaging system (i.e., CI core unit 120) includes an imaging light source (not shown) that generates imaging light (i.e., measurement beam 121) and an optical interferometer (not shown) that generates an interference output using at least a component of the imaging light delivered to the workpiece 102. The coherent imaging system also includes a scanning module 122 that directs at least a component of the imaging light toward a material processing beam source (e.g., laser 110). The CI controller 156 may include a processing unit that performs data processing and analysis on the interference output and a controller that coordinates measurement acquisition, directs the scanning module position, and communicates with external elements. The CI controller 156 operates as a feedback controller that uses at least one of the interference output measurements to calibrate the behavior of the imaging system via its measurement acquisition and scanning module positioning. In certain embodiments, the measurements used to calibrate the behavior of the imaging system are additionally or alternatively obtained by one or more auxiliary sensors or detection devices.

[0029] The workpiece 102 undergoing material processing may be a component or subcomponent with specific functionality, or may be a calibration target intended to enable calibration measurements of a coherent imaging system.

[0030] The material processing beam source can be a laser 110 or an electron beam source. The laser beam 111 can be a single mode beam or a multimode beam. A single mode beam has an M of 2 or less, more specifically 1.5 or less. 2 The laser beam source 110 may include, without limitation, a fiber laser, a disk laser, a solid-state laser, a diode laser, or a carbon dioxide laser. The laser beam 111 may have spectral content in the ultraviolet, visible, or infrared regions.

[0031] In the case of a laser beam, the energy beam delivery system (e.g., laser head 112) may include one or more of a fixed optical head, a wobble head, a pre-targeting scanning head, or a post-targeting scanning head. The laser head 112 includes an instrumentation port for interacting with the coherent imaging system and introducing the measurement beam 121 substantially coaxially with the laser beam 111. If auxiliary sensors are included in the system, the laser head 112 may include additional instrumentation ports, such as an in-line camera (see, e.g., FIG. 2). The additional auxiliary sensors may include an in-line camera (e.g., 2D in-line camera 230 in FIG. 2), a photodiode (e.g., photodetector 332 in FIG. 3), and a spectrometer. The auxiliary sensors may have their own sensor control modules to analyze and process the measurements. Output from the auxiliary sensors is fed back to the CI controller 156 and used for calibration of the coherent imaging system.

[0032] Material modification processes performed using the systems described herein may include, without limitation, one or more of the following: welding, drilling, cutting, prototyping, drilling, brazing, sintering, surface treatment, additive manufacturing, and subtractive manufacturing. An example of using ICI to monitor and control a wobble process is disclosed in more detail in U.S. Patent Application Publication No. 2007 / 0122990, which is incorporated herein by reference in its entirety.

[0033] The apparatus may include other processing equipment such as a motion control stage (eg, motion stage 104), a robotic arm, a processing atmosphere system, an air knife, a shielding gas, and processing controls.

[0034] The CI measurement system may be one of many common CI measurement system variants, or a variant of a variant, including time-domain optical coherence tomography (TD-OCT), Fourier-domain optical coherence tomography (FD-OCT), spectral-domain optical coherence tomography (SD-OCT), swept-source optical coherence tomography (SS-OCT), low-coherence interferometry (LCI), and inline coherent imaging (ICI).

[0035] In SD-OCT and its variants, a low-coherence light source generates light in a finite spectral band, e.g., 830-850 nm. This light is injected into an interferometer and ultimately measured using a spectrometer, which may include a diffraction grating and a line-scan camera. Examples of light sources used in SD-OCT systems include superluminescent diodes (SLDs) and optical frequency combs.

[0036] In SS-OCT and its variations, a tunable narrowband light source generates narrowband light that is injected into an interferometer. The center wavelength of the injected light is tuned across a spectral band to generate a single interferometric output. The light is typically measured by a photodiode or balanced photodetector that is synchronized to the source spectral sweep. Examples of light sources used in SS-OCT include Fourier-domain mode-locked (FDML) sources and vertical-cavity surface-emitting lasers (VCSELs).

[0037] In general, variations on CI measurement systems share the following features: a beam splitter (e.g., a fiber 50:50 evanescent mode coupler) that splits light from the light source into a sample arm that delivers the imaging beam light to the workpiece being measured, and a reference arm that contains the reference optical path of the interferometer and can be static or adjustable. Back-reflected light from the sample arm and reference arm is recombined at the beam splitter and transmitted to a detector in the imaging system. CI measurement systems also include a signal processor for interpreting measurements coming from the detector.

[0038] In the systems described herein, a beam scanning module 122 (e.g., a galvanometer scanner) is used to dynamically control the position of the measurement beam 121 (part of the sample arm path) relative to the processing beam 111. The CI controller 156 can apply system calibrations to affect how the imaging or measurement beam 121 is positioned relative to the processing beam 111 when measurements are performed and / or how the measurements are processed and analyzed. The CI scanning module can comprise any device and / or optics capable of moving or deflecting one or more measurement beams, including, without limitation, a galvanometer scanner, one or more moving lenses, a piezoelectric scanner, a MEMS scanner, a KTN scanner, an electro-optic deflector (EOD), and an acousto-optic deflector (AOD).

[0039] The system further includes a feedback controller or processor (e.g., CI controller 156) to use the coherent measurements to make decisions about the quality of the process. The decisions may be Boolean OK / NOK decisions or may involve more complex process analysis. The feedback controller or processor may utilize the measurement data to adjust process parameters to improve future aspects of the process or future processes.

[0040] The system may include controllers for each of the major subsystems (e.g., laser controller 154, laser head controller 152, and motion stage controller 158). The laser source 110 may have its own controller 154 to control the output power profile, timing, monitor laser health, and communicate with other sub-modules or process masters. The motion control equipment and process support subsystems may have their own controllers 158 to control operation and communicate with other equipment. The laser head 112 may have its own controller 152 to monitor laser head health, drive opto-mechanical components, and communicate with other equipment. A process master controller 150 (e.g., a programmable logic controller) may be used to control and coordinate all of the various subsystems and their controllers. In some cases, one of the subsystem controllers (e.g., laser head controller 152) may assume the role of process master.

[0041] System and Contrast Mechanism Embodiments According to various embodiments, different systems and contrast mechanisms can be used as inputs to provide calibration measurements to a feedback controller. As used herein, "contrast mechanism" refers to a physical characteristic and / or measurement principle that allows one or more aspects of the workpiece and / or phase change region to be distinguished from one another. Embodiments of these systems and contrast mechanisms are described below.

[0042] System 100 using coherent imaging contrast mechanisms 1 can use the interferometric output measurements to provide calibration measurements to a feedback controller (e.g., CI controller 156). In this embodiment, system 100 can still incorporate auxiliary sensors, but their measurements are not used as calibration inputs for the CI measurement system.

[0043] In this system 100, the measurement system hardware responsible for generating the interferometric output measurements of the laser process is the same as that used to generate the calibration measurements of the coherent imaging system. In most cases, this hardware maintains a similar configuration when performing the calibration measurements as when performing the process measurements. However, for some calibrations, it may be desirable to modify the configuration, behavior, and / or parameters of the hardware to improve the quality of the calibration measurement signal.

[0044] For example, in an SD-OCT-type scheme, the integration time of the detector may be increased to improve the SNR when measuring a particular calibration feature. In a typical interferometer setup, the reference optical path may be modified to better match the optical path length associated with the calibration artefact. Similarly, the power level of the imaging light source may be similarly modified to produce a level of back-reflected imaging beam that is more favorable for measuring the calibration target.

[0045] An example calibration measurement routine, processing beam alignment, enabled by an embodiment of system 100 using a coherent imaging contrast mechanism is described below. CI system alignment with respect to processing beam 111 is achieved by analyzing CI system measurements taken when the CI system's scanning module 122 is oriented at a specific position within the scanning module reference frame. Such reference frames may include mechanical mirror deflection angles in the case of galvanometer-based scanning modules, analog drive voltage signal levels in the case of analog galvanometers, digital drive voltage signal levels in the case of digital galvanometers, piezoelectric drive voltages, and a scanning module standard reference frame that may or may not be head-independent.

[0046] While the exact position of the processing beam 111 within the reference frame is unknown and is the target of a calibration routine, mechanical positioning of the opto-mechanical components generally ensures that the processing beam 111 is within the scanning range of the scanning module. In such a system, processing beam alignment is achieved by analyzing coherent measurements of the workpiece 102, which has undergone local physical modifications as a result of the processing beam 111. The physical modifications on the workpiece 102 are used as a proxy for the processing beam spot. Because all of the beam delivery elements in the system are used to make this mark and deliver / recover the imaging beam 121, virtually all anomalous effects that affect the ability of the CI measurement system to remain accurately and precisely co-aligned with the processing beam 111 can be measured and offset at once.

[0047] A typical example of a contrast mechanism involves marking a small spot on the workpiece 102 with the treatment beam 111 such that the spot differs in height and / or reflectivity compared to untreated material. The optical path length change (i.e., height) aspect of the coherent measurement can be used to provide path length-based contrast to identify spots in the material. The back-reflected intensity aspect of the coherent measurement can be used to provide reflectivity-based (and to some extent shape-based) contrast to identify spots in the material. A combination of height-based and intensity-based contrast mechanisms may also be used to identify the location of the treatment beam.

[0048] By performing coherent measurements of the process-induced spot at various points within the scanning module's frame of reference, signal processing and statistical algorithms may be used to identify a region within the scanning module's frame of reference as the process beam (proxy) center. For example, the processed spot may form a small void in the workpiece 102. Measurements based on the void height may exhibit a behavior in which the surface height decreases from the unmodified material surface toward the void center (a proxy for the process beam center location). In a simple implementation, this location may be identified by taking the position (within the scanning module frame) of the coherent measurement that exhibits the lowest measured height. In more complex implementations, image processing algorithms or fitting operations may be deployed to identify the centroid region.

[0049] In addition to identifying centroids, the systems described herein can be used to determine boundary regions, centers of mass, centers of power, or other such definitions for positioning a treatment beam.

[0050] A similar procedure may be implemented by instead using backscattered intensity levels rather than measured height. In other procedures, a combination of the two measurements (e.g., an intensity-weighted height measurement, a height-weighted intensity measurement, etc.) may be used as the signal for processing.

[0051] As shown in the schematic of the spot processing algorithm shown in Figure 5, processing beam alignment calibration can be performed using power measurements acquired during a raster scan of a marked calibration target and processed using a series of image processing algorithms. Figure 6 shows a more detailed example of an interferometric data processing algorithm that can be used for processing beam alignment calibration.

[0052] In addition to variations in the type of signal that can be used to identify the processing beam proxy, different scanning module search strategies can be deployed to accelerate the acquisition process, improve detection accuracy, simplify data processing, or some combination thereof. Scanning strategies (within the scanning module's reference frame) can include a 2D raster pattern across the proxy, an orthogonal scanning vector with repeatedly updated processing beam center position (e.g., a process similar to "moving the beam" to achieve centroid determination for a Gaussian profile by adjusting two adjacent orthogonal axes), a spiral scanning pattern, discrete point measurements, feedback-based random walking scans, etc. As shown by the example scan patterns in FIG. 7, different scanning module measurement acquisition scan patterns can be used for feature detection in CI system calibration measurements, including square sweep patterns, cross patterns, and spiral patterns.

[0053] Averaging of detector measurements at the same CI scanning module position can be performed to increase the SNR and eliminate variations due to processing non-idealities. Differential measurements may also be used to increase the SNR and reduce the effect of characteristic speckle patterns common in CI images. In this situation, the differential measurement may involve measuring the material surface before it is modified and then measuring it again with substantially similar acquisition parameters after it has been modified by the processing beam. The two images are then compared using one or more difference calculations known to those skilled in the art.

[0054] System 100 using native process emission contrast mechanism According to other embodiments, the system 100 shown in FIG. 1 can use native process emissions as a contrast mechanism. In these embodiments, the hardware of the CI measurement system can be configured, operated, or extended in such a way that the optical components and detection devices of the CI system can detect process emissions resulting from laser processing applications. In this embodiment, the system 100 can still incorporate auxiliary sensors, but their measurements are not used as calibration inputs for the CI measurement system.

[0055] In one implementation, using native process emissions involves an imaging light source that can be turned off or reduced to a low enough power that it does not interfere with process radiation detection, and an imaging system detector that can be operated to detect process radiation at a level above background noise. Spectral-domain CI systems are particularly well suited for this task because they can conveniently limit detection of process radiation to the wavelength band used for the subsequent coherent imaging task. This reduces and / or eliminates alignment errors due to chromatic aberrations.

[0056] In more specific implementations of system 100, the CI measurement system may be extended to accept light transmission outside the spectral band of the CI source to facilitate detection of process emissions by the CI system components. In other more specific implementations, the CI measurement system may be extended to include components for controlling the amount and / or color of light entering the detection system. Such components may include one or more of the following: specific wavelength filters, neutral density filtering with adjustable filter levels, fiber bend radius variations, fiber attenuators, and aperture adjustments (through irises). Any of these elements may be flexibly removed and / or introduced into the optical path by actuators known to those skilled in the art.

[0057] In some implementations, the processed light returning through the optical path of the imaging system can be diverted to a secondary detection device (e.g., a photodetector) better suited for such measurements, or the CI detection device can be specifically modified to enable better detection of the calibration signal.

[0058] An example calibration measurement routine, process beam alignment, enabled by an embodiment of system 100 using the native process emission contrast mechanism is described in more detail below. The process beam location in the field of view is achieved by turning off or reducing to a negligible level the CI system's light source. The CI measurement system's light delivery path is used to transmit the process radiation emanating from the laser interaction zone through the laser head to the CI measurement system's detection device. The CI light delivery path may be temporally modified (e.g., by changes in optical filters or apertures), and / or the CI detection device may have its settings temporally changed (e.g., by increasing the integration time in a line-scan camera in an SD-OCT system) for the purpose of increasing the signal-to-noise ratio (SNR) of the process radiation (e.g., blackbody radiation) measurement. In an SS-OCT system employing a balanced photodiode detection scheme, for example, one of the inputs to a channel pair may be disabled (e.g., blocked) to avoid common-mode cancellation of the calibration signal.

[0059] Certain laser processes can be implemented to generate a localized process radiation source (e.g., a blackbody emitter) that is used as a proxy for the process beam center. Certain workpiece / specimen material types can be used to enhance the proxy signal to the detection device. Tungsten can be used to allow for increased process beam energy density and resist surface melting. Other materials (types and shapes) may be used to specifically optimize for particular blackbody radiation spectra. Certain laser process parameters (e.g., low-power pulses, high-power pulses) may be used to enhance the proxy signal or prevent damage to the workpiece. Damage-free laser processes can generate process radiation for proxy detection while protecting the underlying parts, allowing alignment procedures to be performed directly on production parts and within the production environment.

[0060] Additional auxiliary laser processing considerations may be used to further increase the workpiece's resistance to damage and / or to enhance the measured process radiation signal. Shielding gas deployed over the workpiece surface during laser processing helps stabilize the surface temperature, helps mitigate workpiece oxidation, and helps keep the detected signal level consistent. An air knife may be used to prevent process vapors from obscuring the optical path to the detector and adversely affecting the SNR.

[0061] In some cases, such as in scanner-based laser processing, where chromatic aberrations can cause problems with accurate processing beam alignment, the CI light delivery path can be intentionally or unintentionally, permanently or temporarily, modified in production or during calibration procedures to improve rejection of spectral bands outside the band of the CI source. For proxy-specific purposes, transmission of only the spectral components of the processing radiation that overlap with the CI light source can mitigate any alignment errors associated with chromatic aberrations in the laser head.

[0062] In other cases, where systematic errors associated with chromatic aberration are negligible or nonexistent, the CI light delivery path may be modified to transmit a broader spectral band to improve the recovered process radiation signal. In some cases, the CI light delivery path may be modified to target a specific spectral band or bands of process radiation. Targeting a specific spectral band helps improve alignment accuracy because the specific spectral band may be more strongly associated with the proxy, while other spectral bands may be by-products of the environment or the process used to create the proxy.

[0063] For example, a laser pulse is used to establish a laser interaction zone on a steel sheet to act as a blackbody radiation source (proxy), with the peak of the spatial blackbody radiation profile coinciding with the center of the treatment beam. The primary spectral components associated with the proxy are primarily in the infrared portion of the spectrum. Plasma can be generated as an unintentional byproduct of laser processing, which can further increase treatment radiation levels in the visible and ultraviolet spectral regions. Due to the processing environment and processing equipment (e.g., shielding / cover gas flow), the spatial location associated with the peak plasma radiation signal may differ from the spatial location associated with the center of the blackbody radiation source, potentially causing errors when determining the center location of the proxy. By eliminating spectral components outside the infrared region, the signal associated with the plasma can be eliminated, and the center of the blackbody radiation source can be detected with improved accuracy. Similarly, a notch filter can be employed in the optical path of a CI system to block back-reflected treatment beam light to better target the blackbody radiation.

[0064] In some embodiments, filtering the processing beam radiation with optical components may not be feasible. In this case, temporal gating can be used to separate the processing radiation in the laser interaction zone from the back-reflected processing beam light. Examples of such temporal gating include pulse-based laser processing waveforms in which processing radiation measurements are only performed (or ensured) during the off portion of the pulse's duty cycle. As shown in the example of temporal signal gating in FIG. 8, a temporal gating strategy can be used to distinguish back-reflected processing beam light from processing radiation emanating from the laser interaction zone. Knowing the laser power profile (solid line) allows for detector signal measurements (dashed line) to be taken only when the laser is off so as to be isolated. If the laser power profile is unknown, specific detector characteristics, such as the tail region (target signal), can be used to isolate the detector signal associated with the desired processing event. This type of gating ensures that only radiation generated at the workpiece surface is collected by the measurement system. Similar temporal gating techniques may be used to effectively filter out undesired spectral components that may have shorter or longer decay times.

[0065] In some implementations, precise temporal gating can be achieved through access to the laser gating drive signal (or its instructions). However, in some cases, implementing temporal gating via this method may not be feasible or accurate. As a result, characteristics of the acquired signal itself may be used to implement temporal gating. As an example, identification of a large spike followed by a tail region may be used to isolate the desired spectral component in the form of the signal tail (see FIG. 8). In other implementations, an additional photodiode may be used to optically identify the laser in time by measuring back-reflected processing beam light.

[0066] In other examples, if the wavelengths of the processing beam and the imaging beam are close enough that chromatic aberration produces negligible errors, or if the optical delivery system is constructed in such a way that chromatic aberration is not an issue, the back-reflected processing beam light itself may be used as a proxy for detection. In such examples, the laser processing may be selected so that the laser intensity is too low to produce significant heating of any workpiece or absorption of the processing beam. Instead, the processing beam light is back-reflected from the workpiece, and a portion of it is transmitted through the CI light delivery path to the detection device. In this example, the peak spatial intensity of the back-reflected light is used as a proxy for the processing beam center.

[0067] Similar scanning module scanning strategies and signal processing strategies (see, e.g., FIG. 7 ) as previously described can be applied to acquire and analyze the detected process radiation signal. The measured process radiation for points along a linear scan through the laser interaction zone is shown in FIG. 9 . FIG. 9 shows process radiation measurements performed at two positions within the laser scanning head scanning range using a sweep along the X axis. The intensity profile (e.g., its peak or centroid) is used as a proxy for the position of the process beam within the scanning module frame of the CI system. Similar measurements can be performed to determine alignment along the Y axis. As shown in FIG. 9 , at the center of the laser head scanning range, the CI reference frame alignment to the process beam is good. As a result, the intensity profile of the sweep is maximized at a distance of 0 μm along the sweep performed in the CI reference frame. Due to chromatic aberration, at an off-center point in the laser head scanning range (top right), the same measurement sweep performed in the CI reference frame exhibits a peak intensity profile signal at −200 μm along the sweep. Future CI measurement beam positioning operations can then take this error into account to improve positioning accuracy.

[0068] Similar alignment measurements can be performed along other CI reference frame axes at other locations within the process beam scan range to improve overall calibration. Repeated scanning techniques similar to "beam movement" may be applied to improve measurement accuracy. Averaging of detector measurements at the same CI scan module position can be performed to increase SNR and eliminate variations due to process non-idealities.

[0069] System 100' with 2D in-line camera 2 further comprises a 2D in-line camera 230 coupled, for example, at a 2D camera port included in the energy beam delivery system. In an embodiment, the CI controller 156 may comprise at least one processing unit for extracting and analyzing the 2D camera measurements, and data acquired by the 2D camera measurements may be communicated to a feedback controller responsible for performing calibration of the CI system.

[0070] In most implementations, the CI system hardware can turn off the imaging light source or reduce it to a sufficiently low power so that it does not interfere with the process radiation detection. Alternatively or additionally, the imaging light source can be modified in time so that it does not interfere with the process radiation detection for at least one instance in time.

[0071] In many implementations, the 2D camera measurements are extracted by a camera controller and communicated to another controller that is responsible for performing data processing operations. However, in some cases, the camera controller can take over this responsibility. Data processing can be performed by other dedicated pieces of hardware, or can be performed in existing hardware such as a feedback controller.

[0072] A 2D camera port included in the energy beam delivery system (e.g., laser head 112) may be used in an in-line camera configuration (e.g., looking through the beam delivery system). In other implementations, the 2D camera may be set off-axis to avoid sharing the processing beam path with the imaging beam path. In an off-axis configuration, multiple 2D cameras may be used to achieve multiple viewing angles during calibration measurements. In either configuration, the 2D camera's optical path may include additional optics (e.g., focusing lenses, filters, apertures, mirrors) to improve the quality of the calibration measurements. The optical path may be designed to better transmit certain spectral bands (e.g., NIR) and specifically reject other spectral bands (e.g., VIS).

[0073] The particular optical paths of the 2D camera 230 relative to the processing beam 111 and the measurement beam 121 can be variable. In some embodiments, the camera port can share at least some components of the optical path with the measurement beam 121. In other embodiments, the optical paths do not overlap. The 2D camera hardware may have adjustable characteristics (e.g., exposure time, frame rate, field of view) that are configurable depending on the type of calibration measurement being performed.

[0074] An example calibration measurement routine, processing beam alignment, enabled by an embodiment of system 100′ involving 2D in-line camera 230 is described in more detail below. Processing beam alignment is achieved by aligning the imaging beam to an in-line camera frame of reference (e.g., camera frame pixels) and aligning the processing beam to the same in-line camera frame of reference. When both beams, or more specifically, proxies for their respective beams, are aligned in the camera frame of reference, the separation distance within the camera frame can be used to quantify the degree of alignment of the imaging and processing beams (or conversely, to quantify the degree of misalignment).

[0075] FIG. 10 shows an example of aligning the processing beam and imaging beam proxies using a 2D camera. The 2D in-line camera detects back-reflected imaging beam light (the imaging beam proxy) and processing radiation (the processing beam proxy). Image processing is used to determine the degree of misalignment and correct the imaging beam position accordingly. The image in FIG. 10 shows misalignment of the imaging beam proxy and the processing beam proxy. To align the imaging beam proxy to the processing beam proxy, the processing beam proxy position on the 2D camera can be recorded. The scanning module of the CI system is used to actively adjust the imaging beam position until its proxy position overlaps (within an acceptable degree) with the processing beam proxy. In the event that a specific calibration of the imaging beam position to the 2D camera frame has been performed a priori such that the location of the imaging beam proxy on the camera is mapped to the CI scanning module position, alignment of the imaging beam to the processing beam can be performed by back-calculating the correction from the displacement between the imaging beam proxy and the processing beam proxy on the camera.

[0076] In this type of setup, the optical path configuration and 2D camera configuration allow for detection of both the imaging beam proxy and the process beam proxy. The process beam proxy can be established similarly to that described above for system 100 using native process emission contrast mechanisms. The imaging beam proxy can be established using at least some component of the imaging beam light that is back-reflected from the workpiece surface. Similar to what was described for workpiece material and shape optimization for system 100 using native process emission contrast mechanisms, the workpiece material and shape can be selected to deflect a particular amount of the back-reflected imaging beam to the 2D camera.

[0077] If the camera optical path and the CI system optical path are shared, diverting some of the back-reflected beam from the CI system to the camera can be achieved by a beam splitter, as shown in FIG. 11 . Depending on the specifications and requirements of the CI system's detection device and camera, the splitter ratio (e.g., 50:50 vs. 90:10 vs. 99:1) can be device-specific. Splitting the light may be achieved using a polarized CI light source and a polarizing beam splitter. Other possible means of directing the back-reflected imaging beam light to the camera include a mechanically adjustable reflector (e.g., a mirror) to direct light to the CI measurement system when desired for coherent measurements and to direct light to the camera when desired for processing beam alignment.

[0078] When the camera's optical path is separate from the CI system's optical path (e.g., an off-axis camera), the workpiece shape and / or material type and / or fixture position may be intentionally selected to induce diffuse scattering or angled specular reflection to increase the amount of light directed along the camera's optical path.

[0079] The fact that both the imaging beam proxy and the processing beam proxy are aligned in the 2D camera poses some additional challenges to individual detection. As the alignment of the imaging beam to the processing beam improves, the two proxies begin to overlap in the camera and may become indistinguishable. If chromatic aberration is not significant and the processing beam proxy consists of different spectral components than the imaging beam proxy, adjustable spectral filtering may be applied to resolve each beam proxy in the detection device. However, if chromatic aberration is not negligible or a simpler implementation is preferred, the two spots may be resolved temporally. Figure 12 shows an example of a 2D camera temporal gating scheme used to distinguish between processing beam proxy and imaging beam proxy measurements using a 2D camera. As shown, the temporal modulation of the laser power (solid line in the top graph), the temporal modulation of the imaging beam power (dashed line in the top graph), and synchronization to the 2D camera frame allow the frame to be classified as a processing beam proxy frame (lower left graph) or an imaging beam proxy frame (lower right graph).

[0080] For example, a process beam proxy can first be detected by the camera by turning off or blocking the CI light source. A laser process similar to that described for system 100 using a native process ejection contact mechanism can be used to generate the process beam proxy in the detection device. Once the process beam proxy is identified in the camera frame, its center pixel location is saved and the laser process is stopped. The CI light source is then turned on. The imaging beam light reflected back from the workpiece provides the imaging beam proxy to the camera. The center pixel of the imaging beam proxy can then be recorded and compared to the process beam proxy center to determine the degree of alignment. This information can then be fed back to the CI system (as described above) to further improve calibration.

[0081] Depending on the frame rate of the 2D camera, the rise / fall time of the processing beam, the rise / fall time of the CI light source, and the workpiece, the two proxies may be resolved in time by performing a pulsed laser processing sequence. While a multi-pulse sequence is not strictly required to perform co-alignment of the two beam proxies, frame averaging can be performed to increase the SNR associated with centroiding each proxy and enable more accurate measurements. Frame averaging can be used to combat noise from various sources, including camera shot noise, laser processing non-idealities, workpiece surface imperfections, low signal levels, insufficient camera contrast, etc.

[0082] In addition to temporally gating the processing beam proxy signal and the imaging beam proxy signal, the system settings may also need to be specifically modified for processing beam proxy acquisition, imaging beam proxy acquisition, or both. Due to differences in the properties of each radiation source, it is likely that the radiation intensity levels may be different when measuring each proxy. As a result, techniques such as dynamic optical filtering, adjusting camera exposure time, processing power adjustment, or imaging beam power adjustment may need to be applied to ensure that the optical intensity levels of each proxy are within the measurable range of the detection device.

[0083] System 100'' with double-clad fiber The system 100'' shown in FIG. 3 further comprises a double-clad fiber 330 for transmitting coherent imaging system light in its core and for transmitting process radiation in its cladding. In this system 100'', at least one photodetector 332 is used to measure the process radiation from the cladding of the double-clad fiber, and data processing hardware (e.g., CI controller 156) is used to extract the photodetector measurements. Details of this configuration are described in U.S. Patent No. 6,273,999, which is incorporated herein by reference in its entirety. In most implementations of the system 100'', the light source of the CI system is adjustable to facilitate or not interfere with the system's calibration measurements.

[0084] An example calibration measurement routine for process beam alignment enabled by system 100'' is similar to that described above for system 100 using a native process emission contrast mechanism. In this example, a proxy for the process beam is measured by collecting process radiation, transmitting it through the inner cladding of a double-clad fiber, and aligning it with a photodetector. Unlike system 100 using a native process emission contrast mechanism, process radiation collected by the CI system's detection device is not used for the calibration measurement. The photodetector measurement is synchronized to the position of the useful CI system's scanning module to map the measurement to the useful CI system's reference frame.

[0085] Chromatic aberrations between the coherent measurement beam spectral band and the spectral components collected by the inner cladding can introduce chromatic aberrations when attempting to co-align the reference frame of the imaging system to the process radiation. To account for these errors, the effects of chromatic aberrations can be corrected by data processing (e.g., modeled pre-calibration) or can be neutralized by optically filtering process radiation spectral components that are sufficiently far from the measurement beam spectral band.

[0086] The 2D camera is then positioned in the system 100'' The system 100''' shown in Figure 4 further comprises a 2D camera 430 positioned sufficiently close to the processing plane (i.e., the plane containing the workpiece surface). The system 100''' also comprises optical filtering elements (not shown) for controlling the amount and spectral content of light reaching the 2D camera and at least one processing unit (e.g., CI controller 156) for extracting and analyzing the 2D camera measurements. Data acquired by the 2D camera measurements can be communicated to a feedback controller responsible for performing calibration of the CI system.

[0087] An example calibration measurement routine for processing beam alignment enabled by system 100''' is similar to that described above for system 100' with a 2D in-line camera. In this case, however, 2D camera 430 is positioned at the processing plane and used directly to detect the processing and imaging beams. 2D camera 430 also typically has additional filtering to ensure that the measurement and processing beams are sufficiently attenuated to a level that can be measured by the detection device.

[0088] The system 100''' may include additional automated hardware and controls to enable automatic positioning of the 2D camera 430 to the correct calibration measurement position while the calibration is being performed, and to position the 2D camera 430 out of the correct calibration measurement position when the laser processing is being performed. In other systems, the positioning of the 2D camera 430 may be performed manually by the system operator.

[0089] Techniques similar to those described for system 100' can be used to synchronize the measurements at camera 430 with the beam positions so that each beam type can be distinguished. In many cases, further synchronization of the power levels of the processing source and the imaging light source is desirable to achieve readings at the range of the detector (i.e., readings above the detector noise and below the detector saturation value).

[0090] In some cases, it may not be possible to reduce the power of the processing source to a low enough level to be measured by the detection device. To accommodate this, some systems may utilize source pump leakage light instead of the operating beam. In other systems, additional optical beam sampling components may be used to reduce the intensity of the processing beam before it reaches the detection device. Some systems may also include a calibration light source with similar spectral quality to the processing source but with a substantially lower output power, which may be directed substantially along the processing beam optical path. This calibration light source may be used as a proxy for the processing beam source for 2D camera measurements. While the power level of the imaging light source can generally be reduced to a low enough level to be detected, the same technique may also be applied to the imaging beam.

[0091] In further embodiments, combinations of any of the above systems 100, 100', 100'', 100''' may be useful in achieving different detection contrast mechanisms, as different techniques may be better suited to particular types of alignment procedures and laser processing.

[0092] Calibration Type Various types of calibrations can be performed using the systems described above to benefit different types of laser processing applications. The examples described herein are non-limiting examples commonly encountered in laser welding applications. While these examples are described in the context of laser welding, such examples and alignment processes can be extended to other forms of energy beam welding and other laser processing applications.

[0093] Generally, calibration measurements can be performed by a system in the absence of a priori input to the calibration process. However, in some cases, a priori information can be useful to reduce calibration time and increase calibration accuracy. The a priori information can be in the form of one or more of the following: a default calibration compiled from a series of previous calibration measurements of the same type on similar subsystems; previous calibration measurements on the same subsystem; and output from an optical model or optical simulation, such as that employed by computational optical simulation software such as Zemax Optical Design.

[0094] For some applications, a priori information can be used to achieve sufficient calibration, but in most applications, a priori information is used to supplement the measurement system's calibration routines.

[0095] Static Calibration The foregoing system can be used to perform static (i.e., non-process-specific) calibration measurements. Such measurements are typically performed when the CI measurement system is initially integrated into a laser processing station or when modifications are made to the CI delivery optics and / or process beam delivery optics. For applications with high precision calibration requirements (e.g., single-mode laser processing), such calibrations may be performed more frequently to account for equipment tolerance limits, temperature drift, environmental variations, mechanical vibrations, dynamic forces during processing, etc. By communicating with a process master and / or external equipment, one or more of these static calibrations can be performed in the context of using the system in serial production.

[0096] One or more static calibration procedures may be performed on workpieces being processed by the system periodically after a certain threshold of failing quality measurements, after a temperature change, after maintenance, or after one or more components of the processing system are replaced. Because the available space on the workpiece may have geometric complexity near the calibration indicia, certain embodiments include the ability to suppress portions of the scanned image data so that these areas do not erroneously engage and confuse the contrast detection algorithm. When multiple calibration procedures are performed, the results can be averaged and the distribution analyzed to determine whether the alignment of the processing and measurement beams is likely to be correct. For example, if the width of the distribution of measurements is equal to or greater than the expected size of the processing beam at the workpiece, the alignment may not be very good. However, if the distribution of measurements is much smaller than this value, good alignment is likely.

[0097] The following examples are described first in the context of calibration. Additional alternative examples are used to illustrate how the previously described systems / contrast mechanisms can be deployed to perform such calibrations. While a few select alternative examples are described herein, the previously described systems and contrast mechanisms may be used in other alternative examples not explicitly described herein. Certain calibrations can be achieved using embodiments of the previously described systems and contrast mechanisms.

[0098] Process beam / workpiece reference frame alignment In integrating a CI system with a laser head, the alignment / positioning of the process beam provides a meaningful origin for the CI system's scanning module reference frame. The imaging system's scanning module's hardware origin (electrical / mechanical reference frame origin) is inherently misaligned with the process beam center without calibration or alignment. Therefore, alignment measurements of the process beam center are advantageous for aligning the imaging system's scanning module origin with the process beam center. As illustrated by the alignment of the imaging system's reference frame shown in FIG. 13, calibration can be used to align the CI system's reference frame (imaging system frame) to the process beam / workpiece reference frame (head reference frame).

[0099] Measurement of the process beam center in the XY frame of the imaging system's scanning module provides XY correction to the imaging system to center the imaging system's reference frame about the process beam origin. Similar calibrations can be used to align the XY orientation of the imaging system's scanning module with the XY orientation of the workpiece / head, and to provide scaling calibrations to match the XY scale of the imaging system's scanning module to the XY scale of the workpiece / head.

[0100] Detection of the process beam center can be achieved through one or more of the proxy and contrast mechanisms previously described (e.g., sensing based on process radiation, workpiece surface differentiation and subsequent optical path length, workpiece surface differentiation and subsequent back-reflected beam intensity, etc.). Subsequent detection of the process beam center can be performed with one or more of the sensors previously described. The following examples illustrate possible implementations of the process beam center alignment procedure, although many other variations are possible.

[0101] In one implementation, the processing beam is used to mark small features on a workpiece positioned beneath the head. The features are small enough to locate their centroids within an acceptable tolerance (e.g., ±10-20 μm), but large enough to produce distortions to the part surface (e.g., variations in height, changes in surface reflectivity, changes in polarization) that are detectable by a coherent measurement system. Distortions whose maximum or minimum distortions coincide with the processing beam center are generally desirable, such as laser ablation processes where the resulting hole has a maximum depth that coincides with the processing beam center. Other laser marks and / or spot welds may also be used. Materials for this calibration include anodized aluminum, aluminum, steel, stainless steel, copper, nickel-plated copper, and other commonly available metals. Polymers and graphite may also be used. Graphite, in particular, is advantageous because it has no liquid phase at normal atmospheric pressure. This means that the boundaries of the laser mark are easily defined and segmented with image processing algorithms. In some embodiments, the material being marked is the product being manufactured in the cell, which allows for in-situ recalibration and / or verification of the co-alignment of the beams in production.

[0102] The scanning module of the imaging system moves the imaging spot on the workpiece in a defined scanning pattern, and a desired measurement quality (e.g., depth, intensity, etc.) is recorded at each location. The processing beam center is identified as the imaging spot location within the scanning module's reference frame with a target signal value (e.g., local maximum or minimum). Additional data processing techniques, such as fitting, averaging, and filtering, may be employed to improve detection accuracy.

[0103] A specific implementation of this process involves two measurement acquisitions of the calibration target: one before laser spot treatment and one after laser treatment. The pre-fire acquisition is obtained from an untreated calibration target or an untreated area of ​​the calibration target. The post-fire acquisition is obtained after performing laser treatment to create features in the target. This treatment can be a marking-type treatment or an ablation-type treatment. Example laser treatment parameters include a 3 ms, 600 W pulse generated by a continuous wave single-mode laser and a 2 ms, 2000 W pulse from a continuous wave multimode laser. The material type of the calibration target is described above. Each acquisition is a 4D data set representing the intensity of the backreflection as a function of 3D position in space, obtained by performing coherent measurements while scanning the imaging beam over a rectangular raster pattern. The absolute difference between the acquisitions is calculated. A series of image processing operations (eg, intensity thresholding, contour calculation, and convex hull calculation) are then performed to increase image contrast and to detect feature points.

[0104] A hierarchical filtering scheme, with each subsequent stage having fewer data points than the previous stage, is used to remove the data. This scheme is divided into a pre-processing stage, a feature point extraction stage, and a shape fitting stage. The pre-processing stage utilizes nearest-neighbor denoising and image intensity thresholding techniques to extract the largest, most visible intensity blobs in the acquired data. The feature point extraction stage relies on blob approximation and convex hull calculations to calculate feature points in the extracted intensity blobs from the previous stage. The shape fitting stage fits circles to the extracted feature points, and bounding boxes and minimum bounding circles are calculated for the feature points extracted in the previous stage. The output of the shape fitting is used to determine a proxy position for the processing beam and evaluate the accuracy of the algorithm. The fitted centroid is assigned as the proxy position for the processing beam. The fitted radius is used to determine the accuracy of the fit by comparison with the expected range of values ​​governed by the processing beam spot size and processing parameters.

[0105] Different scan pattern strategies and search optimization strategies may be implemented to increase detection speed and accuracy, as shown in Figure 7. Simple scan patterns include rectangular grid, circular grid, spiral grid, rectangular raster scan, spiral scan, and crosshair scan patterns. More complex searches may involve an iterative approach with progressively finer search locations, more complex search strategies including random walks, the use of a priori measurements, and the use of models.

[0106] Inputs to the calibration routine may include, for example, optical modeling of the nominal processing and imaging beam paths through the laser head, collection of previous calibration point values, and / or collection of similar calibration point values. These inputs may be used to provide a starting point around which to center the calibration scan pattern outlined above. A more accurate starting center point may involve fewer measurement iterations to achieve the same level of calibration accuracy. A more specific example involves the use of a computational optical model to generate a correction vector for the imaging beam to align the imaging beam with the processing beam near the workpiece surface for the nominal optical configuration. A series of correction vectors may be generated for different optical configurations, such as the angle of the scanning mirror in the laser scanning head or different defocus positions. The correction vectors are used by the measurement system during its calibration routine to center a rectangular raster scan pattern about the nominal corrected position (i.e., generated from the computational optical model). This increases the likelihood that the calibration proxy will be located within the scan field of view of the calibration measurement, reducing the need for search routines and repeat measurements for calibration of the same system.

[0107] In other implementations, the process beam is used to create a blackbody radiation source at the surface of the workpiece via heating. The radiation source is most commonly a blackbody emitter, but in some implementations, it can be a plasma generated by the process. Through careful selection of process parameters, it is possible to create a radiation source at the surface of a material without causing significant damage to the material itself. Such processes can be beneficial for target reuse and detection accuracy, and can even allow production parts to be used for processing. The use of production parts as calibration targets can enable reduced downtime in high-volume production lines and allow more frequent recalibration. Because a blackbody source is primarily mediated by energy absorption and conduction into the mass of the material, its apparent location should be very stable and symmetrical around the center of the process beam. This can provide very accurate and repeatable alignment of the beam.

[0108] The radiation produced by the source can be captured directly using the optics of a coherent measurement system (e.g., system 100 shown in FIG. 1 ), using the cladding of a double-clad fiber in combination with a photodiode (e.g., system 100″ shown in FIG. 3 ), by a 2D in-line camera (e.g., system 100′ shown in FIG. 2 ), or by some combination thereof, as previously described. In the case of direct coherent measurement system detection or double-clad fiber detection, the measurement spot is moved in a scanning pattern as previously described, and the detected intensity is recorded at each position. The scanning position associated with maximum intensity (or a similar prominent feature, such as the center of the signal cluster) is identified as the processing beam origin. Spectral filtering may be applied to the processing radiation before detection to improve accuracy.

[0109] An example of a process radiation measurement recorded directly by a coherent measurement system during a line scan along one of the axes of the scanning module of an imaging system is shown in FIG. 14. The process radiation line scan fit shown in FIG. 14 shows the process radiation measurement recorded by system 100 (left plot) and the data processing operation (right plot). The plot on the left shows the radiation intensity measurement as a function of the measurement position of the imaging system along the search axis. The plot on the right shows the result of additional data processing and fitting algorithms, such as noise removal and curve fitting, to find the centroid. In this case, the centroid of the fit represents the process beam center position along the scan axis of the CI system. Here, the distance scale on the scan axis has been adjusted to correspond to the center of the fit to indicate the position of the zero-position corrected CI beam.

[0110] In the case of a 2D in-line camera, the processing radiation is recorded on the camera. The back-reflected imaging beam can be recorded on the camera simultaneously or later (see FIG. 10). Possible sequences of such detection are described in more detail above. If the in-line camera has already been calibrated to the imaging system, the correction of the imaging beam position can be used to directly calculate the correction of the imaging beam to the scanning module. However, if such calibration has not been performed, additional acquisitions of the back-reflected imaging beam can be performed at different imaging beam positions (as instructed by the scanning module) until the error in the positioning of the imaging beam (relative to the processing beam center at the camera) is sufficiently minimized. Additional data processing and spectral filtering can be applied to improve performance.

[0111] In many implementations, the treatment beam center position is detected automatically or with minimal user input and saved as a general system calibration.

[0112] Similar techniques can be used to identify and calibrate the scaling and rotation of a CI system with respect to a more convenient reference frame, such as the laser head frame, the workpiece frame, or the world frame. While scaling and rotation are the primary correction transformations used by a CI system, additional transformations, such as reflection and distortion corrections, may be desirable to improve the positioning accuracy of the measurement system. These calibration measurement aspects allow the same calibration process to be used to identify multiple transformation corrections for the CI system. However, calibration measurements for these transformation types may be performed independently.

[0113] Transform calibration generally involves more than process beam origin type calibration due to the nature of the geometry in question. Transform calibration involves creating two or more features on the workpiece (or underneath the laser head), such as a mark or radiation emitter on the calibration target / workpiece. To determine the scaling factor, for example, two features of known separation distance are created in a reference frame to which the CI system is aligned. The features are identified in the CI system's reference frame, and the separation distance between the features in the CI frame is determined. The ratio of the distance in the CI reference frame to the distance in the target reference frame is used to determine the corrective scaling factor for the CI system.

[0114] The same identified features may be used to determine the desired rotational correction for the CI system: if the features are marked with reference to a known axis or set of axes in the target's reference frame, the location of the feature relative to a particular axis in the CI system's reference frame can be used to calculate the rotation to align the two reference frames.

[0115] For these types of calibration processes, synchronization with the laser source and / or cell (eg, through standard communication protocols or indirectly via a cell operator) may be advantageous.

[0116] The above procedure may be extended to include more complex feature generation to account for reference frame distortions and reflections, for example, by creating features with specific asymmetries about multiple axes in the target's reference frame.

[0117] In the optical path configuration of a CI system, multiple transformation correction factors (e.g., multiple scaling factors) of the same type may be used to account for multiple axes (not necessarily orthogonal) in the CI system's reference frame, multiple axes (not necessarily orthogonal) in the target's reference frame, optical distortions in the CI system's beam path (e.g., optical distortions associated with the CI scanning module), optical distortions in the laser head's beam path (e.g., lens distortions in the scan head), different shapes in the target's reference frame (e.g., cylindrical, spherical, and planar), and tracking errors of the CI scanning module.

[0118] The scanning module of a CI system is used to position and scan the imaging beam relative to the target's reference frame. Scanning is enabled by optical devices that are subject to inertial and finite acceleration and / or slew-rate limitations. As a result, the true position of the imaging beam lags behind the commanded position by a tracking error. In its simplest form, the tracking error appears as the position of the imaging beam a fixed time lag (e.g., 0.1 ms) after the commanded position. Often, the tracking error follows a more complex system response and is often dependent on the input drive command itself.

[0119] While the examples presented herein illustrate the simplest forms of tracking error, the techniques outlined can be extended to identify and correct for more complex models of tracking error. By creating one or more features in the target reference frame (e.g., height markings or process radiation emitters) and instructing the scanning module of the CI system to scan the features in different directions and along different axes, the tracking error can be determined.

[0120] FIG. 15 illustrates the difference in positioning in the target's reference frame when tracking error is accurately and inaccurately accounted for. Specifically, the tracking error schematic shown in FIG. 15 highlights the difference between the commanded imaging beam position and the actual measurement beam position with accurate and inaccurate tracking errors when scanning in opposite directions. The inaccurate tracking error case highlights how the distance between measured static features is measured, while a scan in one direction and a scan in the other direction can be used to calculate the tracking error distance. When accurately calibrated, scanning across locations in the target's reference frame produces the same measurement signal independent of the scan direction. When incorrectly calibrated, the actual position sampled in the target's reference frame systematically deviates from the commanded position in the same reference frame. While the example in FIG. 15 shows the true position lagging behind the commanded position, it is possible that the true position can yield the commanded position for certain types of inaccurate tracking errors.

[0121] Given the correct type of target and imaging system scan pattern, imprecise tracking errors can be identified in measurements of features in the target's reference frame of the imaging system (or auxiliary system), as shown in FIG. 16. Specifically, FIG. 16 shows example signal measurements obtained while scanning across a static feature in two opposite directions using a correct tracking error (top) and an imprecise tracking error (bottom). As shown, scanning in one direction produces a peak signal, such as that associated with a peak radiation intensity from a processing radiation emitter at a certain position / time in the CI system's scan path, while scanning with the same pattern across the same feature in the opposite direction produces a peak at a different position / time in the CI system's scan path. The difference in time / position in the scan path is used to determine the tracking error. In some cases, this difference is used in combination with additional knowledge or modeling of the calibration of the scanning module or other CI system to determine the tracking error.

[0122] As previously identified, the tracking error may depend on aspects of the drive command. Examples of such dependencies may include the commanded velocity, commanded displacement, and axes of the scanning module. To improve the accuracy of the CI system, multiple tracking error calibrations may be performed under different input conditions.

[0123] Similarly, to properly obtain a calibration measurement of the tracking error, the CI system and, if applicable, its auxiliary sensor may be modified (relative to the measurement operating conditions) to recover a calibration measurement signal. For example, the integration time of the CI system's detector may be extended, or additional measurement averaging may be performed to achieve a better SNR from the processing radiation emitter. Special laser processing may also be performed to generate such a signal.

[0124] It may be sufficient to only perform a tracking error calibration of the CI scanning module during production of the CI system, however, tracking error calibration may also be performed in the end-use application for improved measurement accuracy.

[0125] Long-term drift of the CI scanning module Engineering tolerances can result in long-term drift of the scanning module of a CI system, in which case the accuracy of the calibration (e.g., target reference frame alignment, tracking error, etc.) deteriorates over time. Changing environmental conditions, such as temperature, can accelerate the process.

[0126] To address long-term drift and ensure sufficient scanning module positioning accuracy, scanning module calibrations of the type described above may be performed periodically. A coherent measurement system that can automatically detect and implement updated corrections would be highly beneficial to the end user. Automation and synchronization of the imaging system with the laser processing cell could further simplify the recalibration process, allowing measurement generation and detection to be performed automatically (or semi-automatically).

[0127] Scanner Range Correction In a fixed optical laser head, alignment of a single process beam / workpiece reference origin may be sufficient because the process beam position relative to the laser head is fixed. However, further extensions to fixed optical implementations may involve multiple joint alignments for different defocus positions (e.g., different planes relative to the focal plane of the head).

[0128] Complexity to the co-alignment calibration process arises when the laser head includes beam scanning optics, which can move the focal point of the processing laser beam relative to the laser head. Examples of laser scanning heads include, but are not limited to, 2D high-power scanners and 3D high-power scanners available from IPG Photonics Corporation.

[0129] In such a scanner-based implementation, the reference frame of the imaging system may be aligned to the processing beam position (and reference frame) at multiple locations throughout the laser head scan field. In some applications, alignment requirements can be relaxed by restricting the effective working area (or workspace) of the scan field. Similarly, in applications with relaxed alignment accuracy requirements, calibration at multiple locations within the scan field may not be necessary.

[0130] However, there are applications and scan head hardware where such additional alignment may be desirable or advantageous. Example applications include single-mode laser processing applications, such as keyhole welding of electrically moving components, where the phase change region created on the workpiece surface by the narrow diameter single-mode beam is small relative to the error in the joint alignment of the measurement system at different locations in the scan field. In such applications, the lack of additional calibration positions could cause measurements to be off-target in one or more regions within the scan field. The relevant alignment scale for some applications may be on the order of a few microns to tens of microns.

[0131] An example hardware deployment in which additional co-alignment may be desirable or advantageous is a system in which the measurement spectral band is different from the spectral band of the processing beam. In such a system, chromatic aberrations induced by various optical components in the head cause the measurement spot to deviate from the focal position of the processing beam by different amounts for different positions within the scan range. Such deviations may be large enough to cause measurements to be taken at inaccurate positions relative to the phase-change region or to particular features in the workpiece.

[0132] An example of the types of corrections that can be applied to the scanning module positioning commands of the imaging system as a function of the position of the scan field to correct for optical distortions in the laser scanning head is shown in Figure 17. This example shows the types of 2D corrections that can be used for a 2D scanning head. Similar 3D corrections can be used to correct a 3D scanning head.

[0133] In addition to scan field co-alignment errors resulting from color effects, co-alignment errors at different locations in the scan field can result from other optical and geometric effects. For example, misalignment of the CI measurement system beam path and the processing laser beam path through the laser head can result in similar scan field co-alignment errors. Thermal effects on the head, such as thermal lenses, can also produce co-alignment errors.

[0134] Depending on the system and application, a coarse calibration throughout the scan range may be sufficient to co-align the measurement and processing beams. However, in systems or applications involving a very high degree of co-alignment, it may be desirable to perform a more specific co-alignment calibration around the processing laser path, as shown in FIG. 18 . FIG. 18 shows an example of path-specific imaging and processing beam alignment measurements for an apparatus with a laser scanning head. Specific calibration can reduce overall calibration time (e.g., by limiting calibration locations to those relevant to the process) and improve accuracy (e.g., by introducing calibration locations among a coarser, coarse calibration grid). As shown in the illustrated example, such calibration can be performed at specific locations in the processing path, or may be performed at locations around the path (e.g., at the bounding perimeter).

[0135] Similar calibration measurements may be used to compensate for optical path length changes and changes to the measurement beam focus associated with different positions within the scan field. Synchronization of the imaging system with the laser processing system facilitates such calibration. In this case, synchronization with the system motion control (e.g., scanner position) can be used to automatically direct (or read) the focus position of the processing laser. Synchronization with the processing laser source, or its controller, can be used to indicate when the calibration process should be performed and when the imaging system is ready to perform the next calibration measurement.

[0136] Tracking and position errors of the processing beam of the main scanner Similar to the corrections for the scanning module of a CI system, the laser scanning head may use corrections to account for its own tracking errors, optical distortions, mechanical positioning, and thermal effects. A laser processing system with an integrated coherent measurement system can utilize the CI measurement system (or its auxiliary detection device) to determine the laser head scanner corrections, which are the primary scanner corrections.

[0137] Similar strategies described above for calibrating a CI system can be used to perform primary scanner calibration. For example, the primary scanner can be used to generate a series of calibration marks on a target below the laser head. Imaging system measurements (coherent, processed radiation, or some combination thereof) can then be used to determine tracking or positioning errors associated with the primary scanner.

[0138] A more specific, non-limiting example is to use the primary scanner to mark a series of points on a workpiece below the head while traveling in one direction and another series of points while traveling in the opposite direction. The scanning module of the CI measurement system can then be used to perform coherent measurements of the marked surface while the primary scanner is held in a stationary position. The height-based measurements of the coherent measurement system can be used to determine the location of the marked features on the workpiece surface. The positions of the marked features in one direction and the other can then be used to determine the tracking error of the primary scanner.

[0139] The primary scanner may be used instead of (or in addition to) the scanning module of a CI system to position the measurement beam at specific locations on the workpiece for the purpose of calibration measurements. In this case, synchronization is provided between the position of the primary scanner and the measurements of the imaging system. While it may be useful to perform such measurements with a fully calibrated imaging system, the primary scanner calibration measurements may be performed in a way that does not require full imaging system calibration.

[0140] Calibration of the primary scanner may only be performed at system start-up or production, however in some cases periodic calibration may be performed to account for optomechanical degradation or changes in environmental conditions (e.g., thermal lensing).

[0141] Dynamic Calibration The systems described above may be used to perform dynamic (i.e., process-specific) calibration measurements, as described in the following examples. Such measurements are typically performed for each type of laser processing application and for major changes to the application's process parameters. Calibrations / adjustments may be performed continuously during a process application to achieve high calibration accuracy and to reduce laser station downtime (i.e., time not spent performing laser processing). Minimal system downtime is important in many high-volume laser processing production environments.

[0142] Similar to static calibration, dynamic calibration may need to be performed periodically in high precision processes to compensate for equipment tolerance limits, temperature drift, environmental variations, mechanical vibrations, dynamic forces during production, etc.

[0143] Dynamic calibration is also used to align the imaging system measurements to aspects of the laser process that are specific to the laser process and / or workpiece. Aspects of the laser process can be detected as part of the dynamic calibration of the CI system and fed back (or feedforward) to influence future CI measurements of specific aspects of the process. These aspects of the laser processing include, without limitation, process path shape, process path speed, process path direction, process path tangent angle, process time, laser power time profile, optical path length changes along the process path and its local vicinity, workpiece displacement from the laser head along the process path and its local vicinity, joint alignment of the imaging system and process beam along the process path and its local vicinity, changes in measurement beam focal position along the process path and its local vicinity, phase change region shape, phase change region extent, phase change region position, phase change region radiation intensity (i.e., temperature), melt pool position, melt pool shape, keyhole position, keyhole shape, laser beam spot size, laser beam energy distribution, laser beam temporal characteristics (e.g., continuous wave or pulsed, pulse width, pulse duration, pulse frequency, pulse shape), measurement beam reflectivity profile (i.e., surface reflectivity and / or subsurface reflectivity), workpiece fiducial mark / reference / datum, and measurement beam dispersion.

[0144] Dynamic calibration is performed by taking measurements of a similar process prior to the process in a process measurement (e.g., quality assurance determination). These calibration measurements can be stored so that they can be performed for the target process. Changes to the target process generally prompt a recalibration step.

[0145] In other dynamic calibration implementations, calibration measurements are performed during the process when process measurements are performed. These calibration measurements can be performed immediately at the start of the process so that the remainder of the process can be monitored with a calibrated system. Calibration measurements can be interleaved with or performed simultaneously with the process measurements for improved accuracy.

[0146] Phase change region measurement In addition to providing process monitoring quality determination, measurements of the phase change region may be used to provide dynamic measurement system calibration measurements. For example, measurements of the process radiation of the phase change region allow calibration measurements of the process beam co-alignment to be performed during laser processing instead of taking the system offline. This may be used in applications with high-volume processing requirements. Figure 19 shows how a coherent imaging system employing a double-clad fiber in combination with an assist photodiode (e.g., system 100'' shown in Figure 3) can be used to determine the process beam center position (1) during laser processing. In this example, the double-clad fiber may comprise a Double-Clad Fiber Coupler manufactured by Castor Optics. Similarly, Figure 20 shows how 2D in-line camera measurements of the phase change region (e.g., using system 100'' shown in Figure 2) and the coherent measurement beam can be used to determine process beam alignment during laser processing applications. Similar measurements may be performed using other contrast mechanisms, such as CI system optical path measurements, to determine process beam alignment during processing.

[0147] Phase change region measurements may be used directly or indirectly to perform other process-based calibrations. For example, FIG. 19 shows how the tail region of a phase change region (PCR) can be detected by a double-clad fiber setup and used to determine the process direction / tangent angle (2). Such measurements typically rely on the existence of a calibration of the imaging system's reference frame to the target's reference frame. Similarly, FIG. 20 shows how a 2D in-line camera image of the process can be used to identify the process direction from the PCR tail region. Such processes generally include an additional calibration step to align the in-line camera's reference frame to at least one of the imaging system's reference frame, the scan head's reference frame, and the workpiece's reference frame.

[0148] FIG. 21 shows how point-based measurements (e.g., coherent height measurements, process emissions measurements through a coherent system, process emissions measurements through a double-clad fiber) can be scanned for a PCR. By synchronizing single-point measurements to positions around the scan pattern of a circular (or similar) scanning module, the resulting signals and features in the measurement signal profile can be used to identify the direction of the PCR tail centerline and, ultimately, the process direction. The centerline can be identified via a local maximum or peak in the signal (as shown in FIG. 21) as a function of position in the scan path. The position in the scan path can then be mapped to the CI system's reference frame (e.g., to the right in the upper example and to the left in the lower example) to determine the quasi-instantaneous path direction in the CI system's reference frame.

[0149] Similarly, PCR tail length measurements can be used to determine processing rates or changes in processing rates. PCR tail length generally increases with processing rate. However, the exact nature may depend on other processing factors, such as material type, material shape, laser spot size, and assist gas. A priori knowledge of such relationships, or process modeling (e.g., based on cooling rates), can be used along with tail length measurements to determine processing rates. The processing rate can then be used to direct the measurement system to measure specific locations or times before or after processing.

[0150] Measurement of other aspects of the phase change region can be important for measurement positioning during certain laser processing applications. For example, in keyhole laser welding, alignment of the measurement beam to the vapor passage formed during welding is established to perform keyhole depth measurements using a coherent imaging system. Changes to the keyhole laser welding process, such as changes in process speed, can change the location of the vapor passage relative to the process beam focal position, as shown in FIG. 22. As shown in FIG. 22, an increase in welding speed can result in an increase in the lag distance of the vapor passage position after the process beam. Similarly, a change in welding direction can result in a change in the lag direction of the vapor passage after the process beam. Additional process effects, such as changes in material shape or composition or the periodic wobble of the process beam, can also result in changes in the vapor passage position. Calibration measurements of the vapor passage position at one or more locations along the weld path are useful for achieving measurement beam positioning for keyhole depth measurements and can be used to set the imaging beam position for keyhole depth measurements during the same process or for subsequent processes.

[0151] Measurements of the CI measurement system may be used to identify the location of the vapor path based on the occurrence of a particular depth characteristic (e.g., deepest measured depth, clustering of points at a particular depth, measurements with a particular type of depth change, measurements with a particular signal intensity level or change, etc.). Measurements of process radiation may also be used to identify this vapor path location. Such measurements may include peak radiation levels, particular changes in radiation levels, radiation in one or more particular spectral bands, particular characteristics in the local radiation measurements of the PCR, etc.

[0152] Similar features to the vapor passages in laser keyhole welding can be identified and calibrated in other laser processing applications such as load manufacturing, marking, cleaning, and cutting.

[0153] Pre- and post-treatment calibration measurements Measurements of one or more features created in a workpiece as a result of laser processing, and measurements of one or more features of a workpiece intended to undergo laser processing, may be used for dynamic system calibration. Such post-processing features may be measured by the system (with or without auxiliary sensors) by measuring sufficiently far after processing, in space, time, or both. Because the measurements are performed after laser processing, the measurements typically rely on height-based contrast mechanisms or contrast mechanisms based on the intensity of external radiation.

[0154] In many laser processing applications, the process creates changes (e.g., changes in height, reflectivity, polarization, material density, etc.) that are detectable by a coherent measurement system or auxiliary detection device. Examples include weld beads in laser welding applications, hardened beads in additive manufacturing, changes to glass reflectivity in scribing applications, and areas of removed material in laser cleaning. By performing measurements at specific locations relative to the origin of the process beam's reference frame and with specific measurement scan patterns, additional characteristics of the laser process can be determined and used to direct future measurement system positioning. These dynamic calibration measurements can be used to construct laser processing paths and other laser processing calibrations with limited or no a priori knowledge of the path.

[0155] In other cases, calibration measurements may be used to determine corrections to apply to a set of nominal process parameters to correct for non-idealities in the process. For example, a coherent measurement system may have access to a nominal process path input to a laser scanning head. However, physical limitations in the scanning head (e.g., finite acceleration) may cause the true path to deviate slightly from the commanded one. Dynamic calibration may be used to correct for these types of deviations.

[0156] A specific example of these types of calibration measurements involves a series of linear sweeps of a coherent measurement beam, perpendicular to and centered on a nominal weld path, across the hardened weld bead (or similar markings along the weld path) while a motion control system (e.g., a scan head, robot, linear stage, rotary stage, etc.) performs a motion path such as that shown in FIG. 23. FIG. 23 illustrates how the use of measurement lines transverse to the nominal laser processing path can be used to identify differences between the nominal and actual processing paths. The transverse sweeps are often offset (before or after) from the processing beam along the weld path. In this example, measurements based on the height of features created by the laser processing (e.g., marked lines) are used as calibration targets.

[0157] When the true or actual motion path is the same as the nominal motion path (bottom schematic), the line appears centered in each of the measurement profiles of the traverse, and the feature appears as a straight line in the center of the image reconstructed from the linear sweep. As the discrepancy between the actual and nominal paths increases (top schematic), the position of the line in the measurement profile of each traverse changes depending on the deviation in path position and tangent angle. Deviations from the nominal motion path cause features to appear shifted and / or distorted (e.g., wider) in the images. These deviations can evolve as a function of position along the path because the misalignment between the nominal and actual paths can vary as a function of position along the path.

[0158] The example shows path error detection using a height-based contrast mechanism. Similar strategies may be employed using a back-reflected intensity-based contrast mechanism, a process radiation contrast mechanism, or a combination of all three. Additional sets of vertical scans at different lead and lag distances from the process beam during the motion path may be used to more precisely identify corrections for such motion paths.

[0159] The above examples are simple illustrations of the types of scan patterns and strategies used to identify laser processing motion paths. More complex scan patterns may be used to implement dynamic corrections. Other examples may involve coherent imaging speckle correction, coherent imaging height measurement correction, 2D in-line camera image correction, tracking motion similar to that in an optical computer mouse, process ejection detection, or the use of a back-reflected light beam.

[0160] Examples of dynamic calibrations (identification and / or correction) that may be applied to an imaging system for monitoring the application of laser processing include, without limitation, process path error, process path tangent angle, process path velocity, optical path length change induced by beam delivery optics, laser interaction area extent, laser interaction area position, phase change area extent, phase change area position, workpiece tilt, and workpiece surface curvature.

[0161] For example, local workpiece surface tilt can be calibrated by performing a repeated circular scan pattern of the measurement beam (in the reference frame of the scanning module) while the laser station motion control system (or laser scanning head) drives the processing beam across its processing path. In most implementations, the processing beam is disabled during this process so as not to damage the workpiece. The circular scan pattern is typically performed at a high frequency to achieve a large number of complete circular periods measured during the laser processing path. The frequency is often dictated by the hardware limitations of the scanning module of the coherent imaging system.

[0162] For each circular scan, the CI system height measurements taken throughout the circle are mapped to their corresponding XY positions around the circular scan pattern. A planar fit to the XYZ data produces the tilt of the plane in the CI system's reference frame. By performing a series of consecutive scans, the evolution of the tilt angle of the plane as a function of position along the path (mapped to the number of circular scans) can be generated. The change in tilt angle as a function of the weld path can then be used to modify the CI system measurement scan strategy and data processing algorithms while monitoring the laser process.

[0163] Such a local surface tilt calibration measurement process is shown in FIG. 24. As shown in FIG. 24, while the laser head moves along its motion path trajectory across a part with surface curvature, the scanning module of the CI system scans the measurement beam in a periodic circular pattern taking CI measurements synchronized to the measurement beam position. In this example, the circular pattern is generated relative to the scanning module's reference frame (as opposed to the workpiece surface) to measure the workpiece's tilt relative to the CI system's scanning module's reference frame. CI height measurements are mapped to corresponding circular beam scan pattern repetitions and further mapped to corresponding XY positions within the pattern. Once the CI height measurements within a cycle are mapped to XY positions, a planar fit is applied to the data (X: X position, Y: Y position, Z: CI height measurement). The coefficients of the planar fit produce a surface normal vector representing the instantaneous local surface tilt. The surface normal vector is then mapped to an angular coordinate frame convenient for the system (e.g., tilt angle along the weld path direction). This process is repeated for each scan cycle. A timestamp is assigned to each cycle (e.g., the time the cycle midpoint occurs relative to the start time of the path), allowing the surface tilt angle to be plotted as a function of time along the laser processing path. The tilt as a function of processing path position may be used by the CI system to provide positioning corrections for the CI system scanning module and may also be used to correct the CI system height measurement (not shown).

[0164] While a circular scan pattern was used in this example, other scan patterns may be used for this calibration. Such scan patterns may include, but are not limited to, sampling a finite number of points distributed about the XY scan range, scanning with a crosshair pattern, and scanning with a rectangular raster pattern. Any scan pattern that provides enough points (at least three) to reliably fit the surface will suffice for this calibration.

[0165] Similarly, although a planar fit is used in this example, other data processing algorithms and surface fitting techniques may be used for this calibration. For example, a more complex surface curvature fit may be used to identify first-order tilt corrections and additional surface distortions. The data processing and fitting techniques may take an iterative approach or may perform direct calculations. Any processing technique that generates one or more surface tilt or curvature metrics is sufficient for this calibration.

[0166] Identifying and aligning the wobble pattern of the processing beam For certain types of laser processing applications (e.g., welding highly reflective materials such as copper or aluminum), it is common to wobble the processing beam in a short-term manner to improve processing results. Measurement techniques similar to those described above can be used to clearly identify aspects of the wobble process and improve the alignment of the measurement beam with respect to the wobbling processing beam. These measurements may be used to influence how the measurement data is subsequently processed and analyzed. Examples of relevant aspects of the wobble process include, without limitation, wobble phase, wobble shape, wobble period, geometric deviation from the nominal wobble shape, and changes to the phase change region induced by the wobble process.

[0167] Example workflow benefiting from calibration measurements The following sections provide examples of the types of workflows that benefit from or are enabled by the calibration measurements described above. The workflows described herein may be implemented by substituting the device embodiments, contrast mechanisms, and calibration types described above. Similarly, the workflows may be implemented through the use of different communication methods, such as those described below.

[0168] Automated calibration and cell synchronization The automation, or even semi-automation, of much of the calibration process identified herein provides many benefits to the end user. Automation minimizes user interaction with the laser processing and imaging systems, thereby transferring fewer resources (e.g., fewer trained personnel, less training, less time, etc.) to the end user to operate the equipment. Minimal user interaction also reduces the risk of user error or the consequences of user error. User error during the system calibration process can have consequences for measurement accuracy and process reliability because it is propagated to all subsequent measurements performed by the system. Similarly, automation reduces the overall time to perform calibration, allowing calibration to be performed at a moment that is more convenient for the laser processing or production environment rather than at a time that is more convenient for the operator.

[0169]

[0003] Different calibration processes involve different levels of synchronization between the laser processing system and the imaging system. Synchronization can be achieved through direct communication or signal transmission between the laser processing system and the imaging system, or indirectly through communication with one or more process controllers. Synchronization between the processing system and the imaging system can include, but is not limited to, one or more of the following: imaging system control of the processing laser position, imaging system control of the processing laser power profile, imaging system control of a predetermined processing laser job, imaging system synchronization to the processing laser position, imaging system synchronization to the processing laser power profile, imaging system synchronization to a predetermined processing laser job, and a predetermined processing laser job that is matched to a predetermined imaging system calibration job and synchronized via a common start signal. Forms of synchronization can include, without limitation, a common external start signal, a common external synchronization signal, user configuration of corresponding imaging system jobs and processing system jobs, positioning signals, power profile signals, digital signals, analog signals, optical signals, optical signals generated by the laser processing itself, and common industrial communication protocols (e.g., TCP / IP, Ethernet-IP, Profinet, etc.).

[0170] Synchronization may be used to ensure that the imaging system receives sufficient signal for the calibration measurement. Synchronization may be used to ensure that the laser processing performed for the calibration does not create excessive damage to the calibration workpiece.

[0171] In addition to calibration of the process and imaging systems, synchronization between the measurement system and the process system / cell may be used to offset degradation in process performance over time. General wear and tear on laser processing station components (e.g., optics, fixtures, gas delivery systems) can slowly degrade laser processing performance with continued system use. Measurement of laser processing station performance by the imaging system may be used to identify such degradation (e.g., through a reduction in process radiation levels) and correct system performance (e.g., increasing commanded laser power to compensate, automatically replacing system components, providing user instructions, etc.). Similar detection and correction may be performed for process-induced damage to the station, such as contamination of the laser head's protective optics by process ejecta.

[0172] Calibration at the time of manufacture During manufacturing, it is common for coherent imaging systems to undergo calibration procedures to improve the accuracy of the measurement system, for example, variations in sub-components introduced as a result of design tolerances and their own manufacturing variability are measured and calibrated.

[0173] When possible, calibration of the CI system is performed with a measurement system integrated into the laser head used in the laser processing application. However, due to real-world limitations, integration with the laser head before the CI system arrives at its final use location is not always possible. In this case, the CI calibration may be performed with the same model laser head, a similar type laser head, or a substitute calibration head. The deviation between the laser head used for calibration and the final use laser head may be negligible, and no further calibration is performed at system start-up. In some applications, such as those with high accuracy requirements, further calibration operations may be performed at system start-up, as described below.

[0174] The following example illustrates aspects of the calibration process associated with integrating a CI system into a laser scanning head. The order of the calibration process described herein is not limiting, and in some implementations, certain calibration operations may be performed in parallel.

[0175] Before the calibration process is performed, the CI system is mechanically integrated with the laser scan head. Some laser heads, such as those available from IPG Photonics Corporation, include a port dedicated to CI system integration. Other laser heads may require mating hardware to attach the CI system to an existing in-line camera (or other sensor) port. Other laser heads may include mechanical adjustment points, optical beam paths, and physical modifications to the delivery optics to accept the coherent imaging system.

[0176] Following mechanical integration, electrical and communication interfaces with the laser head and CI system are established. In a fully automated calibration process, the laser head, CI system, laser source, and, if applicable, cell motion control equipment are all in communication (directly or indirectly) with each other. The component terms described herein refer to specific hardware items and hardware controllers.

[0177] In a semi-automated calibration process, some of the above communication links may be established, but the user is also involved in the process, providing the missing communication and synchronization elements. For example, when the laser head and CI system are primed and ready to perform their calibration operation routines, the user presses a button to activate and fire the laser. The laser also provides a synchronization signal to initiate operation of the laser head and CI system.

[0178] In a fully automated environment, communication between the laser head, CI system, laser source, and motion control subsystems is generally controlled by one device acting as a process master. The process master has knowledge of various aspects of the laser process (or calibration process) and knows how the subsystems should be synchronized to achieve successful operation. In some applications, the process master may be implemented by additional hardware such as a programmable logic controller (PLC), robotic interface, external computer / server, smartphone, tablet, or microcontroller. In other applications, one of the subsystems (e.g., the laser head controller, the CI system controller, or the laser controller) may assume the role of process master. Communication is often achieved directly through bidirectional communication between the process master and each subsystem. However, in some applications, one subsystem may be synchronized with another subsystem or may communicate indirectly with the master. Communication can take the form of digital signals, analog signals, network communication protocols (e.g., TCP / IP), or some combination thereof.

[0179] Once mechanical, electrical, and communication integration is established, a calibration procedure can be performed. A calibration target can be used at the working distance of the laser head. Once the calibration target is set by the user or automatically by other equipment in the cell, the user or cell indicates to the Process Master (PM) that the target is in place. The PM then commands the CI system to automatically perform its depth-of-field calibration, which can include adjusting the CI system delivery optics to better focus the measurement beam on the workpiece and adjusting the reference optical path within the CI system to better match the laser head beam delivery path. During depth-of-field calibration, the CI system performs measurements of the calibration target and electromechanically adjusts its subcomponents based on the measurements. In most calibration routines, the electromechanical adjustments and subsequent measurements are performed iteratively until the resulting measurements are fully optimized or reach a target value. When calibration is complete, the CI system signals the PM.

[0180] During this calibration routine, the scanning functionality and laser power of the laser head are not required, and the PM keeps these subsystems stopped or in a standby state so that their operation does not affect the calibration process.

[0181] Calibration of the tracking error of the scanning module of a CI system can be performed automatically as well. In some tracking error calibrations, only the calibration target and the CI system are performed. In these calibrations, a calibration target with a specific feature (e.g., a sharp edge) is placed below the laser head. Once the target is placed, the PM commands the system to perform its tracking error calibration. The tracking error calibration measurements can be performed as described and illustrated above. In an automated environment, the calibration measurements are processed by the CI system and fed back to the CI system to adjust its scanning module behavior accordingly. In some cases, the mapping between the calibration measurements and the scanning module behavior changes may be known well enough that a non-iterative approach is sufficient. However, in other cases, the calibration measurements and adjustments to the scanning module behavior are performed iteratively until the tracking error is fully calibrated. When the calibration is complete, the CI system signals the PM.

[0182] In some tracking error calibration routines, the calibration target may not have specific features designed for calibration. In these cases, a laser head, laser, motion control equipment, or some combination thereof may be used to create the specific features used for tracking error calibration. In these cases, the PM communicates with both the CI system and the feature creating equipment to ensure that the features are created at specific locations or times to perform the calibration. In some cases, this may involve marking similar features on the calibration target with a laser and communicating the locations (in space and / or time) of these features to the CI system so that it knows when and where to perform the CI system calibration routine. When process radiation (as opposed to features permanently marked by a material surface) is used to create measurement signals for the calibration measurements, precise time synchronization between the laser firing events and the CI system is desirable to ensure that the CI system measurements are performed when the process radiation is emitted.

[0183] The PM can communicate specific process parameters as desired to the laser, laser head, and motion control equipment to enhance the signal detected by the CI system. These parameters can be known a priori or optimized via feedback from the CI system. For example, the PM can fire the laser with a known set of parameters, command the CI system to perform its measurements, receive feedback from the CI system regarding the measured signal level (e.g., no signal, low signal, good signal, high signal), and then adjust the laser parameters accordingly. Once the tracking error is calibrated, it is used by the CI system to more accurately synchronize the CI measurement to the position commanded by the CI scanning module and ultimately to the measured position on the workpiece. Instead of direct low-level control of process parameters by the PM, each subsystem can have a predetermined set of process parameters (i.e., a job) that the PM invokes in response to feedback from the CI system.

[0184] Alignment of the process beam and workpiece reference frames may similarly be automated through synchronization between the process master and appropriate subsystems. Automated versions of these calibration routines may be implemented as follows: The PM signals the cell to automatically load a calibration target under the head. This may be accomplished by an automated robotic arm or by notification to the cell operator. The cell then replies to the PM (e.g., through a response from the robot or through input by the cell operator) to indicate that the target is loaded. Once the target is loaded, the PM signals the laser scanning head to position the laser beam at the origin of its own reference frame. In some cases, accurate process beam co-alignment may first involve a CI system "pre-scan" to obtain background signal levels or a reference shape. In these cases, the PM signals the CI system to perform pre-scan measurements. Once the measurements are complete, the CI system signals the PM. The PM then signals the laser to operate, for example, to generate a process beam proxy signal for the CI system, as described above. In one implementation, the laser is operated to ablate a calibration target, and the CI system measures the resulting ablated features. In another implementation, process radiation measurements of the laser interaction zone are performed by the CI system while the laser is operating (or immediately after the laser is operated). The PM is responsible for synchronizing the CI system measurements with the laser operation (e.g., through an acquisition signal such as a digital rising edge). When the CI system completes the calibration measurements, it signals the PM accordingly. The CI system re-centers its reference frame accordingly and uses this calibration to perform subsequent measurements in the central frame of the process beam.

[0185] In addition to signaling the calibration completely as described above, the CI system may provide feedback to the PM based on the level / quality of the measured signal. The PM may then adjust the laser's processing parameters to improve the measured signal level. This feedback may be provided and implemented during the calibration procedure itself or after the calibration measurement is completed so that the calibration process can be resumed with improved operating conditions. For some configurations, the routine may be performed multiple times to improve the calibration results using statistical data processing (e.g., averaging) or fitting. For each repetition, the same calibration target area may be used, a new location on the target may be used, or a new target may be used.

[0186] Calibration of the orientation and scaling of the workpiece's reference frame is performed similarly. The PM signals the cell to load a calibration target (or move the target to a new location). When the target is ready, the cell replies to the PM. The PM then instructs the laser scanning head to mark a pattern (e.g., a plus sign or similar feature) to point the scan head coordinate axes at the workpiece. Once completed, the head signals the PM, which in turn instructs the CI system to perform further measurements of the calibration. An example of such a system might be a CI system that performs a series of coherent measurements while the scanning module drives the measurement beam along a rectangular raster scan pattern over the marked features. The CI system's data processing unit then unpacks the coherent measurements to map them back to a rectangular grid on the part's surface and uses image processing algorithms to identify the features of the pattern marked on the workpiece surface. The CI system then calculates the orientation and scaling of these features relative to its own reference frame. The CI system then uses this information to update the calibration of its own reference frame. Once the calibration is complete, the CI system signals the PM, which automatically applies these reference frame calibrations to subsequent measurement operations, commanding the scanning module to achieve positioning of the measurement beam in the target's reference frame.

[0187] Because multiple calibration sequences are typically used by a CI system, the CI system may maintain its own calibration status history to indicate, for example, which calibrations have been performed, when a particular calibration was performed or updated, which calibrations are still being performed, and which calibrations are scheduled for future updates. Once a calibration is complete, it is applied by the CI system to perform calibrated measurements of the laser processing application. The PM or cell may maintain a calibration status history of the CI system. The manufacturer of the CI system can use this information to identify when the system is fully calibrated.

[0188] For applications involving CI system integration into a laser scanning head, it is often advantageous to perform co-alignment of the CI system's measurement beam to the laser beam at multiple locations throughout the scanning field of the laser scanning head to allow for optical distortions such as chromatic aberration. Synchronization between the proxy generation of the processing beam for CI detection and the CI system measurement is achieved similarly to that described for the processing beam alignment process previously described. However, calibration of the scanning field involves additional automation considerations to synchronize the co-alignment process to various locations about the scanning field.

[0189] The PM can have knowledge of the calibration locations in the scan field. The PM can have this information preprogrammed or can request the information from the laser scan head and / or CI system. Similarly, the laser scan head and CI system can have preloaded configuration settings to perform this type of calibration, or the PM can be used to synchronize processing and measurements at each location in the scan field without knowing the exact scan field coordinates.

[0190] The PM directs the laser scanning head and CI system to a first position within the scan range. The PM then performs a similar sequence of operations to the co-alignment of the process beams described above to achieve synchronization between the scan range position, laser firing, and CI measurement. Once calibration is completed for the first position, the CI system signals the PM. The PM directs the scanning head and CI system to the next position where the process is repeated. This procedure is performed until the position within the scan range is calibrated. The CI system uses scan range calibration to perform CI system positioning corrections at various positions within the scan range. In most applications, the CI system involves signaling the scan range position (either directly from the scan head controller or indirectly via the PM) so that the appropriate scan range correction can be applied. The CI system can achieve this correction using a calibration measurement or a look-up table generated by a model (e.g., fitting) of the calibration measurement.

[0191] Scan head applications may involve calibrating measurements of the optical path length of the CI system as a function of position in the scan field. In most laser scan heads, the optical path length of the CI system measurement beam changes as a function of position in the scan field due to, for example, increased geometric path length as the scan head deflects the beam outward from its origin, or due to changes in the amount of thickness of the focusing optics through which the beam passes at a given scanner position.

[0192] The optical path calibration may be automated, as may the calibration of the co-alignment of the process beam in the scan field. Synchronization to the laser is not required for this calibration. The PM commands the laser to turn off or wait. The PM signals the cell to load a flat calibration target or a calibration target with known surface curvature and tilt. Once the part is loaded, the cell signals the PM. The PM then directs the scan head to a specific location in the scan field and waits for the appropriate position signal from the scan head. Once in the appropriate position, the PM instructs the CI system to perform its calibration measurement. This measurement is often in the form of an optical path length measurement. The measurement is then processed by the CI system to determine the optical path deviation. For surfaces with known curvature or tilt, the CI system's processing unit applies a flattening correction to the optical path deviation measurement. The optical path deviation is typically measured relative to the optical path length to the workpiece at the origin of the scan field. However, other references (e.g., a plane 2 mm out of focus from the scan head processing plane) may also be used.

[0193] Once the CI system has performed its measurement at a particular location, it signals the PM, which in turn signals the scan head to move to the next location, and the process is repeated. Once the optical path calibration is performed at target points throughout the scan field, the CI system processing module creates a lookup table or model (e.g., fit) to perform optical path length corrections as a function of scan field position when performing process measurements. When co-alignment of the process beam with the scan field is performed, the CI system uses knowledge of the scan field path and synchronization to the scan field path during the laser process application to perform the corrections. This is typically achieved through direct or indirect communication with either the PM or the scan head. This communication can occur at various times, including before the process begins to give the CI system an opportunity to perform calculations to prepare for the process measurements, in real time during the laser process application, and after a test run of the application.

[0194] Similar calibration measurements can be performed on the scan field to calibrate the following: optical dispersion of the measurement beam as a function of scan field position, changes in the focal length of the measurement beam as a function of scan field position, changes in scaling of the workpiece reference frame as a function of scan field position, and changes in rotation of the workpiece reference frame as a function of scan field position.

[0195] Although the automated calibration procedures are described herein in the context of a completed cell assembly, it is possible to perform certain calibrations with only the associated subsystems and sub-modules, for example, calibration of the optical path may be performed on a cell without an operational laser.

[0196] System startup To minimize the workload during system startup, as much calibration processing as possible may be performed at the time of manufacturing. However, due to the constraints described above, it is not always possible to integrate the CI system with the cell or laser head prior to startup. Similarly, while calibration may be performed during system manufacturing, recalibration of certain aspects of the system may be desired as a result of misalignment due to shipping and installation.

[0197] Typically, the calibration procedures performed during system startup closely resemble those performed at manufacturing, or a subset of the procedures. In this case, the process master may similarly be the CI system or controller at the point of end use. Instead of performing some of the calibration routines in their entirety, automated startup checks may be performed to determine which calibrations are still accurate and which require recalibration. These automated startup checks generally involve the same kind of synchronization and communication between the various subsystems and the process master as is performed for a full calibration; however, the operation is generally less time-consuming. For example, verifying the optical path calibration may involve performing measurements of the optical path deviation at a small subset of target points within the scan field. If these measurements match measurements taken during system manufacturing, the calibration is considered accurate. If the check fails, the optical path calibration is performed again.

[0198] As previously described, the CI system or process master may maintain a history of the calibration status for display for each individual who boots up the system. This can be used, for example, to guide the individual through the remaining steps, to block certain cell operations until boot-up is complete, or to provide an announcement to the user that boot-up is complete. While the PM can be used to automate the boot-up process, limitations in the cell's signaling and communication schemes may involve some manual interaction (e.g., loading the calibration target and pressing a button when it is loaded).

[0199] Monitor and update calibration of measurement jobs In laser processing applications, measurements of the CI system in the phase change region provide important information about the process. In many of these applications, CI measurements are performed on specific subregions of the PCR to obtain targeted process information. For example, in keyhole laser welding, aligning the measurement beam with the vapor passage created in the weld during the welding process is desirable to measure the keyhole depth.

[0200] In many laser welding processes, the location of the vapor path relative to the process beam depends on many factors, including, without limitation, material type, material geometry, process speed, process power, process path, wobble pattern, shield gas deployment, plume suppression techniques, environmental conditions, and defocus. Often, a calibration is performed by the CI system to determine the location of the vapor path relative to the process beam for a given process or process subregion. Once established, this calibration is often accurate for small perturbations to the process parameters and conditions described above. However, recalibration of the vapor path location may be desirable when there are large-scale changes. Similar calibrations to aspects of the process parameter space can be performed in other laser processing applications.

[0201] In laser processing cells with integrated CI systems, automated workflows may be set up to perform process-specific calibrations. For keyhole alignment during laser welding, specific alignment jobs may be defined within the CI system. These alignment jobs may be run before each new type of process to determine the calibration. Alignment jobs may be run periodically and interspersed with laser process measurement jobs to verify or update the calibration as desired.

[0202] A keyhole calibration job may be a series of coherent measurements about the process beam center targeting specific coherent measurement signal characteristics (e.g., deepest depth, specific depth change, specific depth range, specific intensity signal level, etc.). Similarly, measurements of the process radiation as described above may be used to perform the keyhole calibration measurements. Once the measurements are performed, the processing unit of the CI system executes an algorithm or image recognition routine to identify the specific calibration.

[0203] In addition to updating how the CI system positions its measurement beam to measure the vapor path, the keyhole calibration measurement can be used to assign a goodness metric to the laser process measurement job (or aspects of the measurement job) that can be used as a confidence indicator for the output or to provide notification to a user or process master that operation should be stopped until a new calibration is applied.

[0204] In certain embodiments of the apparatus, the CI system subcomponents can perform keyhole calibration measurements in parallel with the keyhole measurements of the laser process itself, without the need for interleaved or interleaved calibration measurements with the target measurements of the process. Instead, the keyhole calibration measurements may be performed during the process itself and applied to the CI system in real time or near real time.

[0205] It is also possible to perform such calibration measurements in conjunction with a processing measurement job through resource sharing techniques. For example, a keyhole calibration measurement can be performed in parallel with a keyhole depth measurement by adjusting the target position of the measurement beam relative to the predicted keyhole position at specific intervals over a specific portion of the keyhole depth measurement period. This signal can be compared to the nominal keyhole position to determine whether an improved calibration exists. Such a comparison can be based on signal characteristics such as signal strength, signal density, signal variance, etc.

[0206] Although a process may be nominally constant, periodic keyhole recalibration may be desired due to uncontrolled changes to the process environment, including, without limitation, thermal changes to the process equipment, mechanical wear to the process equipment, contamination to the laser head optics, changes in process raw material and component specifications, changes in process fixtures, thermal changes to the CI system, and mechanical changes to the CI system.

[0207] In some implementations, calibration measurements are performed, processed, and applied by the CI system automatically without user intervention, however, in other implementations, physical or regulatory constraints may require some level of user interaction (e.g., approving an updated calibration) before a new calibration is performed.

[0208] Maintenance of automated processes, records and communication with users / external parties Automatic calibration measurements taken during a series of similar laser processes provide data to enable long-term statistics, analysis of process trends, and monitoring of the cell's operational health. For example, by collecting the intensity of the imaging beam's backscatter from the workpiece surface over time, it is possible to see a decrease in intensity as a function of time. Decreased intensity can be associated with contamination of the laser head's cover glass as more material is processed. Similarly, decreased intensity can be associated with a degraded light source or contamination of more material. Establishing additional calibration targets, such as those within and below the laser head, can isolate the source of decreased intensity. Such isolation is useful in identifying contaminated cover glasses. Once identified, the CI system can provide a notification to the user, either directly or indirectly via the process master, to indicate that a new cover glass should be used. In other systems, this signal may be automated to ensure that cover glasses are replaced more frequently. The notification to the user may be in the form of a visual signal on the cell (e.g., LED, display), an audible signal from the cell (e.g., chime, buzzer), or an electronic alert (e.g., smartphone app, tablet app, wearable technology, PC program, email).

[0209] Similarly, records of other calibration measurements may be used to identify slight process variations. For example, small changes to keyhole position calibration over time may be associated with changes in the process environment and part fixtures. These long-term trends may be used to identify the source of the variation and correct the process and its equipment. Records of process beam co-alignment calibrations may be useful when identifying problems with optical components associated with beam delivery at the head. Trends may be compared with other sources of data over time to identify issues such as thermal lensing, damage to optical components, and loose optical components.

[0210] In many laser processing applications, knowledge of the focus of the processing beam enables processing results of sufficient quality. Positioning the workpiece surface relative to the focal plane of the processing laser beam is important for coupling the appropriate energy into the material during processing. Often, materials undergoing laser processing are positioned with the material surface in the focal plane of the laser beam to maximize energy coupling at the material surface. However, it is also common to move the material surface out of the focal plane (often referred to as defocusing) to distribute the laser beam energy over a large surface area. Those skilled in the art can intentionally defocus by moving the beam closer to or further away from the material. Similarly, the ability to spatially align the laser spot over the surface of the workpiece can ensure that the targeted material region undergoes processing.

[0211] In laser processing stations with integrated CI systems, the CI system can be used as a guidance system to localize the focal plane of the processing beam relative to the part surface and to localize the lateral (XY) position of the processing beam spot on the surface of the material. Information provided by the CI system about the 3D focus of the processing beam can be used by the machine operator during setup for a new part, or can be automatically communicated to a machine controller (e.g., a PLC, robot controller, scan head controller, etc.), to actuate optical or mechanical adjustments to position the processing beam focal point at a desired location relative to the workpiece. Such adjustments can be desirable for a wide variety of applications, including, but not limited to, large-tolerance processing (e.g., laser processing with a single-mode beam) and assembly line processing where varying material tolerances or degradation of processing fixtures may require active compensation from one part to the next.

[0212] For CI systems used as guidance systems, the CI system can be calibrated to the focal plane and lateral position of the treatment beam. Other examples presented herein outline possible techniques for calibrating to the lateral position of the treatment beam at the surface of the material. These techniques may be applied in the following examples to calibrate the CI system to the 3D focal position of the treatment beam. For simplicity, the examples described herein involve scanning the measurement beam of the CI system in a rectangular pattern around the surface of the material and using the height measurement of the CI system to resolve the spot on the surface of the material marked by the treatment beam. However, other modes of height measurement (e.g., blackbody radiometry) and scanning strategies of the CI system may be employed. Similarly, the CI system may communicate directly with a process controller to automatically perform the calibration process or, in some cases, may require user interaction. An example automated calibration process is described in more detail below.

[0213] The process controller communicates with the motion control equipment to position the laser head relative to the calibration target or workpiece, which may involve placing a target material under the laser processing head so that it points at the target material, or may position the laser processing head at a new location.

[0214] The process controller commands the laser (e.g., via digital signaling or a standard communication protocol such as TCP / IP) to mark a spot on the target surface. The process parameters may be predetermined by the controller, preset within the laser itself, or obtained through communication with the CI system. Once the spot is created, the process controller commands the CI system to perform a calibration operation. The CI system can perform a rectangular scan (or other pattern as disclosed herein) across the surface of the part to capture a delimited area on the workpiece surface, as shown in FIG. 25.

[0215] Using the contrast detection algorithm described herein, the CI system determines the laser spot diameter (proxy) using the imaging data it collects, as shown in Figure 25. Speckle appears in the image, but the detection algorithm counters the effect of the speckle, and the magnitude of the speckle can be reduced by increasing the numerical aperture of the beam delivery system for the CI system.

[0216] Once the CI system calibration measurements are acquired, the CI system processes the information to automatically calculate quantities for the delimited spot, such as its center location and diameter. Note that peak-finding or centroid fitting in the CI system's axial measurements can reduce the effects of speckle and quantization noise in the image. In one embodiment, a safety software feature allows manufacturers to limit the axial resolution made available to end users to better comply with government regulations pertaining to device performance.

[0217] Further extensions to this procedure include repeated CI system scans to tile different search areas to better locate the spots and / or to modify the scan area. Similarly, the CI system can repeatedly communicate with the process controller (or directly with the laser) to fire additional spots to enhance contrast, or to provide more precise temporal synchronization of the laser process events, if necessary.

[0218] The XY center position of the spot is used as a proxy for the lateral position of the treatment beam at the surface of the workpiece. The spot diameter can be used as a proxy, albeit more indirectly, for the diameter of the treatment beam at the surface of the material, since the thermal effects of the treatment generally produce a mark on the surface of the material that is much larger than the size of the treatment beam spot.

[0219] A further extension to this calibration procedure involves performing a series of this type of calibration at various laser head offset distances (ie, distances between the laser head and the workpiece surface).

[0220] This series of measurements produces a spot (proxy) diameter as a function of misalignment distance. During such measurements, the misalignment distance can be communicated to the CI system via a process controller or measured directly by the CI system (via the CI system's height measurement capabilities). For example, CI system height measurements near the periphery of the rectangular scan area (i.e., areas that do not include the laser-marked spot) can be averaged to produce a CI system measurement of the misalignment distance to the workpiece surface.

[0221] The spot (proxy) diameter as a function of shear distance is then analyzed to determine the focal plane of the laser beam. In some cases, this analysis may involve finding the shear distance associated with the smallest spot diameter. In some cases, fitting this data to a function (e.g., a Gaussian beam width equation) may be used to increase the robustness of the calibration.

[0222] In other calibration routines using different laser processes, other metrics as a function of offset distance may be used to identify the focal plane, including, for example, maximizing blackbody radiation intensity, maximizing spot depth, maximizing spot size, maximizing spot height variation, minimizing spot height variation, maximizing blackbody radiation, maximizing blackbody emitter diameter, locally maximizing blackbody emitter diameter, minimizing blackbody emitter diameter, maximizing spot symmetry (minimizing astigmatism), minimizing laser scribe feature width, and maximizing laser scribe feature width.

[0223] The diameter of the same spot (proxy) as a function of the offset distance data may be used to generate a proxy for the process beam focal plane. This information may be further stored or analyzed by the CI system to generate an indication of beam quality. The information may be communicated to the laser cell or user for external analysis. Beam quality analysis is useful during laser and cell start-up operations to ensure proper functionality. Similarly, periodic beam quality measurements and analyses, or measurements and analyses following predicted damaging behavior (e.g., sputtering on the laser head cover glass, mechanical system failure), may be used to verify the quality of continued beam delivery and / or identify problems. Similar measurements and analyses may be used to identify suboptimal beam delivery performance (e.g., thermal lensing that creates dislocations at the focal point).

[0224] It should be noted that for a particular energy beam parameter and material combination, a minimum in the apparent spot (proxy) diameter may exist on one or more sides of the true process beam focal minimum. This is because at the highest intensity (tightest focus), a process will initiate the keyhole (vapor capillary) condition sooner, thus absorbing more total energy and resulting in a larger thermal effect and therefore a larger apparent spot (proxy) diameter.

[0225] Regular measurements of the process beam calibration spot may be used over time to track the cell's health and identify process issues. By recording calibration metrics (e.g., spot XY position, spot diameter, blackbody intensity, etc.), the CI system can analyze trends in the data to indicate potential problems with the beam delivery components. For example, the cover glass optics may become contaminated by process ejecta and suffer beam delivery issues over time. This will manifest as a smaller marked spot diameter or, in the case of a blackbody, a weaker blackbody emitter. Similarly, trends in the recording metrics may be observed due to loose, damaged, or suboptimal beam delivery optics. This information may be presented directly by the CI system (e.g., via annunciation) or communicated to the CI system user or laser cell controller for external analysis.

[0226] All of these techniques apply variations of these principles, and the same energy applied to the same material with the same temporal power profile should produce very similar results. If any of these measurements of surface markings change substantially with time, those changes are a likely proxy for changes in the performance of the laser source and beam delivery system.

[0227] The process beam calibration process may be further combined with other CI system calibration measurements to reduce the number of steps or overall time required for multiple calibrations. For example, unprocessed areas around the periphery of a rectangular scan area height map may be used to calibrate workpiece tilt and optics tilt introduced by different beam delivery paths (particularly those associated with beam scanning laser heads). Similar recording of these additional CI system calibration metrics may be used to identify additional aspects of cell health and process issues (e.g., wear to cell fixtures).

[0228] During the calibration procedure of a particular CI system, it may be sufficient to use a readily available material (e.g., a metal plate) as the calibration target. For calibration of some CI systems, the calibration target may be the workpiece undergoing laser material processing (i.e., pre-production part) or the workpiece after undergoing material processing (i.e., post-production part). For calibration of other CI systems, a manufactured and calibrated target that is specific to the calibration may be required.

[0229] In some embodiments, CI system and / or camera data is used to determine the direction of travel for a motion system (e.g., a robot, linear axis, or linear conveyor system) relative to the head. Similar CI system measurements may be used to determine workpiece placement, workpiece orientation, fixture placement, and fixture orientation relative to the head. This allows for so-called "welding on the fly," or accurate calculation of coordinated motion movements.

[0230] Finally, in some embodiments, the calibration target is rigidly attached to the non-workpiece object, which allows for positioning and orientation of the non-workpiece object relative to the beam delivery system and motion control (e.g., robotic) system.

[0231] Monitoring the health of your CI system In addition to being useful in identifying process problems and laser cell problems, automatic calibration measurements are useful in monitoring the status and health of the CI system itself. Calibration measurements can be performed periodically for the express purpose of determining the operating status of the CI system. Calibration measurements may be performed for other aspects of CI system calibration, but may also be used or repurposed to elucidate the health of the CI system or changes in the state of the CI system.

[0232] Generally, grouping of iterative process beam alignments or other calibrations can be an indicator of improper laser or CI system performance, or an indicator of improper processing of automatic alignments. In either of these events, embodiments of the present disclosure can utilize thresholds in the distribution of automatically calculated alignments as triggers for annunciation or external signals to request assistance or to halt further processing until the fault condition can be corrected.

[0233] Measurement of back reflections from calibration targets within the head may be used to identify degraded light sources. The trend of degraded light source output power can then be fed back to the CI system to increase the light source drive current, or similarly command more nominal output power, to achieve similar output power levels as performance deteriorates over time.

[0234] Similar calibrations may be used to detect and take corrective action for the following: conditions of the reference optical path of a coherent system (e.g., changes in length, refractive index, transmission, dispersion, etc.), conditions of the sample optical path of a coherent system (e.g., changes in length, refractive index, transmission, dispersion, etc.), spectral misalignment or calibration issues in the detector of a CI system, and to detect spectral instability in the light source of a CI system.

[0235] In the above example, one or more corrective actions may be taken to correct the problem and allow the CI system to continue functioning properly. However, in other cases, a corrective action may have already been applied and cannot be applied again (e.g., maximizing the drive current of the light source), or no corrective action may be applied. In this case, the corrective action may be to provide a notification to the process master and / or user to indicate that the system is no longer functioning properly and requires maintenance or replacement. In some examples, this signal may be used to switch to a backup CI system or a backup system subcomponent.

[0236] Monitoring trends in system configuration data may be collected and analyzed to build statistics about calibration stability and to obtain probabilistic failure mode and effects analysis (FMEA) data. For example, in laser keyhole welding, keyhole alignment calibration data may be used to determine alignment stability for a particular laser welding process. The impact of small changes in alignment on the CI system's measurement quality may be characterized and used to determine the desired frequency of recalibration. Welding processes that are highly sensitive to keyhole alignment misalignment may require more frequent recalibration. Similarly, welding processes that are highly sensitive to keyhole alignment misalignment are often more unstable processes. In this case, keyhole alignment calibration data can be used as a proxy to roughly indicate the stability of the welding process and can be used to determine process quality and request further optimization. When the CI system determines that a particular recalibration is desired or that the calibration is no longer accurate, the CI system may automatically perform a calibration routine or may provide an indication to the cell or user that such a procedure should be performed for subsequent operation.

[0237] Machine-readable code recognition In many industries, it is useful to incorporate part identification indicia, such as bar codes or QR codes, onto the surfaces of products, components, and subcomponents. Such identification indicia can be useful for inventory tracking, product end-use tracking, source provenance, and security indicia.

[0238] In some applications, a CI system may be utilized to scan such indicia. Calibration of the CI system may be used to achieve the appropriate measurement beam scanning strategy and measurement processing algorithm (or configuration) to reliably recognize the indicia. For example, calibration may be used to accommodate different material backgrounds, indicia colors, indicia heights / depths, indicia locations, and indicia sizes. If these indicia are part of a quality control system, the CI system can capture indicia data and correlate that data with other CI system measurements taken before, during, or after the indicia are measured.

[0239] A CI system may be used as part of the production process for such identification indicia. For example, the CI system may be used to measure the indicia and determine whether the indicia are of sufficient depth / height or sufficient color change. Output from the CI system may be fed back during the processing stage of the identification indicia to modify the marking process. A common realization of this process is in the context of laser-generated identification indicia (e.g., laser marking or laser scribing processes). The laser that marks the workpiece may be the same laser that performs the laser processing of the workpiece. Marking may occur in the context of the laser processing environment, or may occur before or after the processing itself.

[0240] In some embodiments, a CI system may be used as a reader of identifying indicia. It may be beneficial for certain identifying indicia to be invisible to conventional readers (e.g., barcode readers, cameras, LED scanners). These indicia may be used to conceal product identifiers, for example, for security or aesthetic purposes. CI systems may be used to guide laser processing when generating such indicia and to provide feedback on the quality of the generation process. Such indicia may be generated by marking features (e.g., submicron features) on the surface of the material that are primarily useful for CI measurement detection. For optically transparent or translucent materials (in the color of the CI measurement beam or the color of the processing beam), such features may be marked below the surface of the material. Subsurface features may be useful for marking plastic, glass, or semi-precious and precious stones. When reading these features, the CI system optionally calibrates for tilt of the part's appearance and / or distortions in the optical path length from optical media (e.g., lenses, air, water, oil, etc.) traversed by the CI system's measurement beam.

[0241] In embodiments, a pulsed laser or stamp is used to very precisely construct a 3D QR code or barcode in relief on the surface of the workpiece, but the change in height between the negative and positive portions of the pattern is so small (10 um or less, or even 50 um or less) and the transition is so gradual that it cannot be read by conventional reading devices, or perhaps even identified with the naked eye. If sufficient control of the engraving / stamping system is possible, it may be possible to encode multiple value levels at each lateral location of the 3D QR code or barcode, thereby significantly increasing the data density that can be produced per unit of workpiece surface area.

[0242] Optionally, the code may be coated to protect it from scratches or other damage. The coating may be opaque to visible light (but transparent to CI systems) to make visual identification even more difficult. The sheer difficulty of creating these features makes them useful for anti-counterfeiting applications.

[0243] While the principles of the present invention have been described herein, it will be understood by those skilled in the art that this description is made by way of example only, and not as a limitation on the scope of the invention. In addition to the exemplary embodiments shown and described herein, other embodiments are contemplated within the scope of the present invention. Modifications and substitutions made by those skilled in the art are deemed to be within the scope of the present invention, which is not limited except as by the following claims. [Explanation of symbols]

[0244] 100, 100', 100'', 100'''' system 102 Workpiece 110 Material processing beam source, laser, laser beam source 111 Processing beam, laser beam 112 Energy beam delivery system, laser head 120 Coherent Imaging System, CI Core Unit 121 Imaging beam, measurement beam 122 Beam Scanning Module 150 Processing master control device 152 Laser head control device 154 Control device / processing device, laser control device 156 Control / Processing Device, CI Control Device 158 Motion stage control device 230 2D inline camera 330 double clad fiber 332 Photodetector 430 2D camera

Claims

1. a material processing system comprising a processing beam source for generating a processing beam and a processing beam head for delivering the processing beam to a target; a coherence imaging (CI) measurement system comprising: a coherence imaging (CI) core unit for generating a measurement beam and for generating an interference output; and a CI scanning module for delivering the measurement beam to the target, wherein the CI measurement system generates a CI measurement output using the interference output and a calibration measurement output using a contrast mechanism provided by detection in the CI measurement system; at least one controller configured to receive the CI measurement output from the CI core unit for monitoring and / or controlling the material processing system, the controller also configured to receive the calibration measurement output from the CI measurement system and to control the CI measurement system based at least in part on the calibration measurement output, and to control the CI measurement system based on the calibration measurement output to correct future measurements made by the CI measurement system for aligning the CI measurement system with the processing beam; A system comprising:

2. The system of claim 1 , wherein the CI measurement system detects the interference output and provides the contrast mechanism used to generate the calibration measurement output.

3. The system of claim 1 , wherein the material processing system comprises a laser processing system.

4. The laser processing system has an M ２ 4. The system of claim 3, comprising a laser with a single spatial mode output with a value.

5. The system of claim 1 , wherein the CI measurement system comprises an inline coherent imaging (ICI) system.

6. 1. A method for calibrating a coherence imaging (CI) measurement system, comprising: providing a material processing system configured to generate and deliver a processing beam to a target; providing a coherence imaging (CI) measurement system configured to generate and deliver a measurement beam to the target and to provide a CI measurement system output for controlling and / or monitoring the material processing system; obtaining a calibration measurement output from the CI measurement system by detecting an interference output of the CI measurement system, thereby providing a contrast mechanism used to generate the calibration measurement output from the CI measurement system; automatically controlling the CI measurement system based at least in part on the calibration measurement output, wherein the CI measurement system is controlled to correct future measurements made by the CI measurement system for alignment between the CI measurement system and the processing beam; A method comprising:

7. The step of obtaining a calibration measurement output comprises: generating a physical modification in the target at least in time using the processing beam; obtaining a measurement of the physical modification using the CI measurement system; obtaining the calibration measurement output from the CI measurement system such that the physical correction corresponds to the position of the processing beam; The method of claim 6, comprising:

8. The method of claim 7 , wherein obtaining the CI measurement comprises scanning the measurement beam to the target proximate the physical modification in a scan pattern.

9. The method of claim 6 , wherein obtaining the calibration measurement output comprises directing the processing beam toward the target at least while pulsing the processing beam.

10. 7. The method of claim 6, wherein modifying future measurements made by the CI measurement system includes at least one of modifying spatial positioning during measurement acquisition, modifying temporal synchronization during measurement acquisition, and modifying a measurement algorithm.

11. The method of claim 6 , wherein correcting future measurements made by the CI measurement system includes correction to compensate for optical distortions introduced by the material processing system and / or a beam delivery system in the CI measurement system.

12. 12. The method of claim 11, wherein correcting future measurements made by the CI measurement system includes correcting the optical distortions including at least one of optical path length change, chromatic aberration, defocus, field curvature, image distortion, spherical aberration, coma, and astigmatism.

13. The method of claim 6 , wherein modifying future measurements made by the CI measurement system includes modifying to compensate for mechanical distortions of the material processing system and / or the CI measurement system.

14. The method of claim 13 , wherein the mechanical strain comprises at least one of vibration, tension, torsion, extension, compression, translation, and rotation.

15. 7. The method of claim 6, wherein modifying future measurements made by the CI measurement system includes modifications to compensate for process synchronization issues selected from the group consisting of process synchronization issues including at least one of motion control path synchronization, process laser energy profile synchronization, workpiece distortion, process input, shield gas, cover gas, process environment, process feed material, process sacrificial material, and process fixtures.

16. The method of claim 6 , wherein modifying future measurements made by the CI measurement system includes modifying to compensate for changes to a phase change region created by the processing beam on the target.

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