Calibration system and method for calibrating a coherent imaging measurement system

Abstract

Systems and methods for static and dynamic calibration can be used to provide alignment of a measurement beam from a coherent imaging (CI) measurement system relative to a processing beam from a material processing system. In these systems and methods, calibration measurement outputs can be obtained from the CI measurement system and / or from auxiliary sensors. Future measurements performed by the CI measurement system can be modified based, at least in part, on the calibration measurement outputs.

Description

[0001] Related applications

[0002] This application claims the benefit of U.S. Provisional Application Serial No. 63 / 011,235, filed April 16, 2020, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to coherent imaging measurement systems and methods for monitoring high energy beam processing applications, and more particularly, to static and dynamic calibration techniques for aligning a measurement beam in coherent imaging measurement systems and methods. Background Art

[0004] Using coherent imaging (CI) measurement systems (e.g., the IPG Photonics LDD700 system) to monitor laser processing applications (e.g., laser welding, laser additive manufacturing, laser marking, laser material removal, laser cleaning, etc.) offers significant benefits for process outcomes. The benefits of CI systems are primarily realized in their spatial and temporal relationship to the laser process itself. For example, a more specific relationship involves the temporal and spatial relationship of the CI measurement beam to the processing laser beam spot on the workpiece being processed. A typical single-shot measurement from a CI system, referred to herein as an A-line, is defined based on the context in which it is acquired.

[0005] This meaning is specified based in part on where the measurement is made: in space relative to the workpiece (e.g., subcomponent A and subcomponent B in a joining application to form part AB); in space relative to the processing unit (e.g., relative to the workpiece fixture, relative to the unit coordinate system, relative to the tool coordinate system, etc.); in space relative to the process (e.g., before the process path vector); in time relative to the process (e.g., 100 ms after the process is complete); in space relative to the machining laser focal spot (e.g., 0.1 mm after the process beam); in time relative to machining laser power modulation (e.g., 0.5 ms after each laser pulse in a series of laser pulses); and in time relative to the machining laser beam modulation pattern (e.g., 0.05 ms after the start of a circular laser oscillation pattern).

[0006] In the example of laser welding, during laser processing, CI measurements can be taken at a specific distance in front of the processing laser focal spot so that the measurement is not affected by the process itself, but close enough to the focal spot so that accurate measurements of the part assembly geometry (e.g., the seam line between two components 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 further be 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 process—a key metric for many laser welding processes.

[0007] For many laser processing applications, registration of CI measurements with the laser focal spot and temporal power modulation curve must be performed with sufficient accuracy and precision to be useful in process monitoring and process control environments. The specific requirements may vary depending on the application and the type of CI measurement being performed for that application. For example, in multi-mode laser welding applications, it is typically sufficient to spatially register the CI measurement with the process beam on the surface of the workpiece to within a distance of approximately tens to hundreds of microns. In single-mode laser welding applications, registration distances of approximately tens of microns to a single micron may be desirable.

[0008] Similarly, for keyhole depth measurements in pulsed welding applications, temporal registration with the temporal power profile of the machining laser, on the order of tens to hundreds of microseconds, is sometimes desirable. Such registration may be desirable to ensure that CI measurements are performed while the keyhole steam passage is open.

[0009] The examples above help illustrate the importance of CI systems for the spatiotemporal registration of processing lasers in some laser processing applications. However, the broader importance extends far beyond the limited examples provided above. Such registration can be used to modify measurement system behavior to obtain more useful measurement results, as well as to determine the usefulness of measurement results (e.g., by quantifying their lack of registration) and modify measurement data processing accordingly.

[0010] In some applications, stringent registration requirements require highly trained personnel to commission the CI system on-site at the laser processing station. In some applications, the commissioning procedure may need to be performed iteratively until the specific requirements are achieved. In these cases, the registration requirements imposed by the application can be very time-consuming. The minimal knowledge and time required to perform such a procedure can be a significant barrier to future modifications to the laser processing station, as it may involve the redeployment of highly trained personnel, extensive remote support from trained personnel, or extensive upfront training for the laser processing station operator.

[0011] In many applications, registration procedures and CI system calibrations are specific to the hardware configuration (e.g., specific to a laser processing station) and need to be performed for each deployed CI system. Engineering and manufacturing tolerances may necessitate calibration for each laser processing station, even if the stations are nominally identical. Similarly, in some applications, further calibration procedures may be desirable to perform registration procedures specific not only to the laser processing station but also to the process itself. Consequently, a CI system used to monitor a variety of laser processing applications, or even different types of the same laser processing application, may involve numerous calibrations.

[0012] In cases where the CI system has some resonant components, wobble properties, or other characteristics where the phase of the A-line acquisition timing cycle is difficult to control and / or predict, it may sometimes be necessary to retrospectively determine the temporal and spatial relationship between the A-line and the rest of the process. However, even in these cases, the techniques and apparatus described herein provide equivalent benefits when compared to implementations where the A-line can be triggered "on demand."

[0013] Manufacturing tolerances, combined with very tight registration requirements, can necessitate recalibration of the CI system following common, but relatively infrequent, laser processing station procedures. These procedures can include: changing the laser head's protective cover slide; changing the focal length of the laser head's focusing optics; replacing the laser delivery fiber; replacing the laser head with a spare; and adjusting the CI system's delivery optics. In such applications, the CI system's end user may need to monitor the system's registration throughout its use or may need to perform multiple calibrations throughout its operational lifecycle.

[0014] External environmental influences (e.g., changes in temperature, vibration, humidity) and maintenance (process fiber replacement, system cleaning) can cause changes in the registration between the process and imaging optics. Because these influences cannot always be isolated to a subset of the system's components, it is desirable to directly co-register the various imaging and energy beam systems, taking into account the net impact of all these influences on co-registration. In many cases, the best way to do this is to use (multiple) imaging systems to observe the interaction of the process beam with the material.

[0015] Therefore, there is a need for a new device and method that: improves the accuracy of the registration procedure; improves the precision of the registration procedure; reduces the time requirement of the registration procedure; automates the registration procedure (semi-automated or fully automated); reduces the minimum knowledge and experience requirements to perform the registration procedure; or some combination thereof. Summary of the Invention

[0016] Consistent with one aspect of the present disclosure, a system includes a material processing system, a coherent 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 coherent imaging (CI) measurement system includes a coherent imaging (CI) core unit for generating a measurement beam and a CI scanning module for delivering the measurement beam to the target, wherein 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 use in monitoring and / or controlling the material processing system. The one or more controllers are further configured to receive the CI measurement output from the CI core unit for use in monitoring and / or controlling the material processing system. The controller(s) are further configured to receive a calibration measurement output and control the CI measurement system based at least in part on the calibration measurement output, such that the controller(s) are configured to control the CI measurement system to modify future measurements performed by the CI measurement system for alignment between the CI measurement system and the processing beam.

[0017] Consistent with another aspect of the present disclosure, a method for calibrating a coherent imaging (CI) measurement system is provided. The method includes providing a material processing system configured to generate a processing beam and transmit it to a target; providing a coherent imaging (CI) measurement system configured to generate a measurement beam and transmit it 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 from 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 modify future measurements performed by the CI measurement system for alignment of the CI measurement system and the processing beam. Modifying future measurements performed by the CI measurement system may include performing a system-level calibration, performing a process-level correction, and / or performing an intra-process-level correction.

[0018] In some embodiments of the method, the CI measurement system is automatically controlled to modify future measurements performed by the CI measurement system such that a static calibration is performed on the alignment of the CI measurement system and the processing beam. In other embodiments of the method, the CI measurement system is automatically controlled to modify future measurements performed by the CI measurement system such that a dynamic calibration is performed on the alignment of the CI measurement system and the processing beam.

[0019] Consistent with another aspect of the present disclosure, a system includes a material processing system, at least one material processing system controller, a coherent 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 is configured to generate an output that is substantially a single spatial mode having an M-squared value of less than 2.0. The material processing system controller(s) are configured to cause the material processing system to generate the laser processing beam and direct it to the target for calibration measurement.

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

[0021] The CI measurement system includes a coherent imaging (CI) core unit for generating a measurement beam and a CI scanning module for transmitting the measurement beam to the target, wherein the CI measurement system produces a CI measurement output. The (multiple) CI system controllers are configured to receive the CI measurement output from the CI core unit for monitoring and / or controlling the material processing system. The (multiple) CI system controllers are 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 (multiple) CI system controllers are configured to control the CI measurement system to modify future measurements performed by the CI measurement system for alignment of the CI measurement system and the processing beam. In some embodiments of the system, the CI measurement system may include an inline coherent imaging (ICI) measurement system, and the CI measurement system may be configured to produce the calibration measurement output. In other embodiments, an auxiliary sensor may be configured to produce the calibration measurement output.

[0022] Consistent with yet another aspect of the present disclosure, a method for calibrating a coherent imaging (CI) measurement system is provided. The method includes providing a material processing system comprising a laser configured to generate and deliver a laser processing beam to a target; providing a coherent 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 calibration measurements; obtaining calibration measurement output from the CI measurement system and / or from 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 modify future measurements taken by the CI measurement system for alignment of the CI measurement system and the processing beam. In some embodiments of the method, the laser may be configured to generate an output that is substantially a single spatial mode having an M-squared value of less than 2.0.

[0023] In some embodiments of the method, the laser machining beam produces a physical modification on the target, the physical modification corresponding to the position of the machining beam. In these embodiments, the calibration measurement output can be based on the physical modification, and the CI measurement system can be controlled to align the CI measurement system relative to the physical modification. Obtaining the calibration measurement output can include, for example, measuring the physical modification with the CI measurement system. In other embodiments, the laser machining beam generates localized process radiation on the target, the localized process radiation representing the position of the machining beam. In these embodiments, the calibration measurement output can be based on the localized process radiation, and the CI measurement system can be controlled to align the CI measurement system relative to the localized process radiation. Obtaining the calibration measurement output can include, for example, detecting the localized process radiation with the CI measurement system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] These and other features and advantages will be better understood from the following detailed description read in conjunction with the accompanying drawings, in which:

[0025] Figure 1 is a schematic block diagram of a laser processing system and a coherent imaging (CI) system capable of performing CI system calibration measurements consistent with embodiments of the present disclosure.

[0026] Figure 2 is a schematic block diagram of a laser processing system and a coherent imaging (CI) system with a 2D inline camera for CI system calibration measurements, consistent with other embodiments of the present disclosure.

[0027] Figure 3 is a schematic block diagram of a laser processing system and a coherent imaging (CI) system with a double-clad fiber and a photodetector for CI system calibration measurements consistent with further embodiments of the present disclosure.

[0028] Figure 4 is a schematic block diagram of a laser processing system and a coherent imaging (CI) system with a 2D camera below the laser head for CI system calibration measurements consistent with even further embodiments of the present disclosure.

[0029] Figure 5 is a series of images illustrating an exemplary process beam registration calibration performed using interferometric output measurements acquired during raster scanning of a marked calibration target and processed using a series of image processing algorithms.

[0030] Figure 6 is a block diagram illustrating an example of an interferometric data processing algorithm employed for process beam registration calibration.

[0031] Figure 7 is a schematic illustration of an exemplary CI system scanning module measurement result acquisition scan pattern employed for feature detection in a CI system calibration measurement.

[0032] Figure 8 is a diagram illustrating an example of a time gating strategy used to distinguish between back-reflected process beam light and process radiation exiting the laser interaction region.

[0033] Figure 9 It is a diagram showing the use Figure 1 Graph of an exemplary process radiation line scan performed by the system shown in FIG at two locations within the laser scan head scan field.

[0034] Figure 10 is a schematic illustration of an example of a process beam registration using a device with a 2D inline camera.

[0035] Figure 11 is a schematic illustration of the shared optical path between the CI system scanning module and the inline camera.

[0036] Figure 12 is a diagram that illustrates the Figure 2 Schematic diagram of an exemplary time gating scheme for differentiating between process beam proxy and imaging beam proxy measurements using a 2D camera in a system.

[0037] Figure 13 is a schematic illustration of the calibration used to align the CI system reference frame with the process beam / workpiece reference frame.

[0038] Figure 14It is a diagram of Figure 1 Graphs of process radiometric measurements recorded by the system shown in FIG. 5 (left) and examples of data processing operations (eg, noise suppression and curve fitting) that can be used to find the centroid of the data (right).

[0039] Figure 15 is a schematic illustration highlighting examples of differences in commanded imaging beam position and actual measurement beam position when scanning in opposite directions with correct and incorrect tracking errors.

[0040] Figure 16 is a graph illustrating exemplary signal measurements obtained while scanning a static feature in two opposite directions using correct tracking error (top) and incorrect tracking error (bottom).

[0041] Figure 17 is a schematic illustration of an example of the type of correction that needs to be applied to the scanning module positioning commands of an imaging system to correct for optical distortion in a laser scanning head as the scan field position varies.

[0042] Figure 18 is a schematic illustration of an example of path-specific imaging beam and process beam registration measurement results for an apparatus involving a laser scanning head.

[0043] Figure 19 It is a diagram of Figure 3 Images of calibration measurements performed by a system using a double-clad fiber in combination with an auxiliary photodiode, used to determine the process beam center position (1) and path direction (2) during the laser process.

[0044] Figure 20 It is a diagram of Figure 2 Images of calibration measurements performed by the system, which employs a 2D inline camera, are used to determine the process beam center position and path direction during the laser process.

[0045] Figure 21 are images and diagrams illustrating exemplary calibration measurement techniques performed by the system using point-based measurement acquisition (eg, interferometry output or single-point process radiometry in conjunction with a CI system scanning module) to identify process orientation.

[0046] Figure 22 is a schematic illustration of an example of how changes to the keyhole laser welding process (eg, changes to the process speed) can result in changes to the position of the vapor channel relative to the process beam focus position.

[0047] Figure 23is a schematic illustration of how the use of a measurement line transverse to the nominal laser machining path direction can be used to identify differences between the nominal machining path and the actual machining path.

[0048] Figure 24 is a schematic illustration of the local surface tilt calibration measurement process.

[0049] Figure 25 is an image illustrating an area of a workpiece captured by a CI system scan and including laser spot diameters identified using imaging data collected by the CI system. DETAILED DESCRIPTION

[0050] Consistent with the present disclosure, systems and methods for static and dynamic calibration can be used to provide alignment of a measurement beam from a coherent imaging (CI) measurement system (also referred to as a CI system) relative to a processing beam from a material processing system. The system can be calibrated before a process is performed (i.e., system-level calibration), during a process (i.e., process-level calibration), and / or between processes (i.e., intra-process-level calibration). Although the illustrated embodiments depict a laser processing system, the systems and methods described herein can be used with any high-energy beam processing application (e.g., electron beam), and not just laser processing. Examples of CI measurement systems include, for example, inline coherent imaging (ICI) systems, such as those disclosed in U.S. Patent Nos. 8,822,875, 9,757,817, and 10,124,410, U.S. Patent Application Publication No. 2020 / 0023461, and U.S. Patent Application Serial No. 16 / 721,306 (now U.S. Patent Application Publication No. 2020 / 0198050), all of which are incorporated herein by reference in their entirety.

[0051] refer to Figures 1 to 4 Consistent with embodiments of the present disclosure, systems 100, 100', 100", 100'" generally include a material processing system, a coherent 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., a laser 110) that delivers a processing beam 111 to a workpiece 102 via an energy beam delivery system (e.g., a 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 interferometric measurement output from at least one component of the measurement beam 121 that is directed to and reflected from the workpiece 102. One or more of the controllers / processors (e.g., a laser controller 154 and a CI controller 156) can be used as a feedback controller to monitor at least one processing parameter of the material modification process and to adjust the behavior of the coherent imaging system based on at least one calibration measurement.

[0052] The coherent imaging system (i.e., CI core unit 120) includes an imaging optical source (not shown) that generates imaging light (i.e., measurement beam 121) and an optical interferometer (not shown) that generates an interferometric measurement output using at least one component of the imaging light transmitted to the workpiece 102. The coherent imaging system also includes a scanning module 122 that directs at least one component of the imaging light relative to the 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 interferometric measurement output, as well as a controller that coordinates measurement acquisition, directs the scanning module position, and communicates with external components. The CI controller 156 may act as a feedback controller that uses at least one of the interferometric measurement outputs to calibrate the behavior of the imaging system via measurement acquisition and scanning module positioning. In some embodiments, measurements used to calibrate the behavior of the imaging system are additionally or alternatively obtained by one or more auxiliary sensors or detectors.

[0053] The workpiece 102 undergoing material processing may be a component or subcomponent having a specific functionality, or it may be a calibration target whose purpose is to achieve calibration measurements of a coherent imaging system.

[0054] The material processing beam source may be a laser 110 or an electron beam source. The laser beam 111 may be a single-mode beam or a multi-mode beam. A single-mode beam includes a beam having an M-squared value of 2 or less, and more specifically, 1.5 or less. The laser beam source 110 may include, but is not limited to, a fiber laser, a disk laser, a solid-state laser, a diode laser, or a CO2 laser. The laser beam 111 may have a spectral component in the ultraviolet region, the visible region, or the infrared region.

[0055] 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 optics head, a swing head, a front objective scanning head, or a rear objective scanning head. The laser head 112 may include an instrument port for interfacing with a coherent imaging system and introducing a measurement beam 121 substantially coaxially with the laser beam 111. In the case of an auxiliary sensor included in the system, the laser head 110 may include additional instrument ports, such as an inline camera port (see, e.g., Figure 2 ). Additional auxiliary sensors can include inline cameras (e.g. Figure 2 2D inline camera 230 in), photodiodes (e.g., Figure 3 The auxiliary sensors may also have their own sensor control modules to analyze and process the measurements. The output from the auxiliary sensors is fed back to the CI controller 156 and used for coherent imaging system calibration.

[0056] The material modification processes performed using the systems described herein may include, but are not limited to, one or more of the following: welding; drilling; cutting; routing; perforating; brazing; sintering; surface treatment; additive manufacturing; and subtractive manufacturing. An example of using ICI to monitor and control a swing process is disclosed in more detail in U.S. Patent Application Publication No. 2020 / 0023461, which is incorporated herein by reference in its entirety.

[0057] The apparatus may also include other processing equipment, such as a motion control console (eg, motion table 104 ), a robotic arm, a processing atmosphere system, an air knife, a shielding gas, and a process controller.

[0058] The CI measurement system can be one of or a variation of the more common CI measurement system variants, 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; low coherence interferometry (LCI); and inline coherence imaging (ICI).

[0059] In SD-OCT and its variants, a low-coherence light source generates light in a limited spectral band (e.g., 830 to 850 nm). This light is injected into an interferometer and ultimately measured using a spectrometer, which can include a grating and a line scan camera. Examples of light sources used in SD-OCT systems include superluminescent diodes (SLDs) and frequency combs.

[0060] In SS-OCT and its variants, a tunable narrowband light source produces a narrowband of light that is injected into an interferometer. The central wavelength of the injected light is tuned across a spectral band to produce a single interferometric output. The light is typically measured by a photodiode or balanced photodetector synchronized with the source's spectral sweep. Examples of light sources used in SS-OCT include Fourier domain mode-locked (FDML) sources and vertical cavity surface-emitting lasers (VCSELs).

[0061] In general, CI measurement system variants share the following features: a beamsplitter (e.g., a fiber 50:50 evanescent mode coupler) that splits light from a light source into a sample arm, which delivers the imaging beam to the workpiece being measured, and a reference arm, which contains the interferometer reference optical path and can be static or adjustable. Light backreflected from the sample and reference arms is recombined at the beamsplitter and transmitted to the imaging system detector. CI measurement systems also include a signal processor to interpret the measurements from the detector.

[0062] In the system 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. A CI controller 156 can apply system calibrations to influence how the imaging or measurement beam 121 is positioned relative to the processing beam 111, when measurements are taken, and / or how the measurement results are processed and analyzed. The CI scanning module can include any device and / or optics capable of moving or deflecting one or more measurement beams, including but not limited to a galvanometer scanner, one or more moving lenses, a piezoelectric scanner, a MEMS scanner, a KTN scanner, an electro-optical deflector (EOD), and an acousto-optic deflector (AOD).

[0063] The system further includes a feedback controller or processor (e.g., CI controller 156) to utilize the coherent measurements to make determinations regarding process quality. The determinations can be Boolean OK / NOK determinations or can involve more complex process analysis. The feedback controller or processor can also utilize the measurement data to adjust process parameters to improve future aspects of the process or future processes.

[0064] The system may also 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 output power distribution, timing, monitor laser health, and communicate with other submodules or a process master. Motion control devices and process auxiliary subsystems may have their own controllers 158 to control operation and communicate with other devices. The laser head 112 may also have its own controller 152 to monitor laser head health, drive optomechanical components, and communicate with other devices. 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.

[0065] Examples of systems and contrast mechanisms

[0066] According to various embodiments, different systems and contrast mechanisms can be used to provide input to the feedback controller to calibrate the measurement results. As used herein, "contrast mechanism" refers to a physical property and / or measurement principle that allows one or more aspects of a workpiece and / or phase change region to be distinguished from one another. Examples of these systems and contrast mechanisms are described below.

[0067] System 100 using a coherent imaging contrast mechanism

[0068] According to some embodiments, Figure 1The system 100 shown in FIG4 can use interferometry output measurements to provide calibration measurements to a feedback controller (e.g., CI controller 156). In this embodiment, the system 100 can still incorporate auxiliary sensors, but their measurements are not used as CI measurement system calibration inputs.

[0069] In this system 100, the measurement system hardware responsible for generating the interferometric output measurements of the laser process is the same measurement system hardware used to generate the calibration measurements of the coherent imaging system. In most cases, this hardware maintains a similar configuration when performing calibration measurements as when performing process measurements. However, in some calibrations, it may be desirable to modify the hardware configuration, behavior, and / or parameters to improve the quality of the calibration measurement signal.

[0070] For example, in SD-OCT-based approaches, the detector integration time can be increased to improve the SNR when measuring specific calibration features. In general interferometry setups, the reference optical path can be modified to better match the optical path length associated with the calibration artifact. Similarly, the imaging optical source power level can be similarly modified to produce a back-reflected imaging beam level that is more conducive to calibration target measurement.

[0071] The following describes an exemplary calibration measurement routine—process beam registration—implemented by an embodiment of the system 100 using a coherent imaging contrast mechanism. CI system registration relative to the process beam 111 is achieved by analyzing CI system measurements taken when the CI system scanning module 122 is directed at a specific position within the scanning module reference frame. Such reference frames may include: mechanical mirror deflection angles in the case of a galvanometer-based scanning module; analog drive voltage signal levels in the case of an analog galvanometer; digital drive voltage signal levels in the case of a digital galvanometer; piezoelectric drive voltage; and a scanning module standard reference frame, which may or may not be head-independent.

[0072] The precise position of process beam 111 within the reference frame is unknown and is the target of calibration routines, but mechanical alignment of optomechanical components generally ensures that process beam 111 falls within the scanning field of the scanning module. In such systems, process beam registration is achieved by analyzing coherent measurements of workpiece 102, which has undergone localized physical modifications due to process beam 111. The physical modifications on workpiece 102 are used as a proxy for the process beam spot. Because all beam delivery elements in the system are used to create this mark and deliver / collect imaging beam 121, nearly all anomalous effects that would affect the CI measurement system's ability to maintain accurate and precise co-registration with process beam 111 can be immediately measured and compensated for.

[0073] A typical example of this contrast mechanism involves marking a small spot on the workpiece 102 with the process beam 111, such that the spot differs in height and / or reflectivity compared to the unprocessed material. The coherently measured optical path length change (i.e., height) aspect can be used to provide path length-based contrast to identify the spot on the material. The coherently measured backreflection intensity aspect can be used to provide reflectivity-based (and, to some extent, geometry-based) contrast to identify the spot on the material. A combination of height- and intensity-based contrast mechanisms can also be used to identify the process beam position.

[0074] By performing coherent measurements of the process-sensing spot at various points within the scanning module reference frame, signal processing and statistical algorithms can be used to identify a region within the scanning module reference frame as the process beam (representative) center. For example, a processed spot may form a small cavity in the workpiece 102. Height-based measurements of the cavity may exhibit a behavior in which the surface height decreases from the unmodified material surface toward the cavity center (representative of the process beam center location). In simple embodiments, this location can be identified by taking the position (within the scanning module frame) that exhibits the lowest coherent measurement height. In more complex embodiments, image processing algorithms or fitting operations can be employed to identify the centroid region.

[0075] In addition to identifying centroids, the systems described herein may also be used to determine bounding regions, centers of mass, centers of power, or other such definitions for positioning a process beam.

[0076] A similar procedure may alternatively be performed using backscattered intensity levels instead of measured altitude. In other procedures, a combination of the two measurements (eg, intensity-weighted altitude measurement, altitude-weighted intensity measurement, etc.) may be used as the signal for processing.

[0077] like Figure 5 As shown in the overview of the speckle processing algorithm illustrated in , process beam registration calibration can be performed using output measurements acquired during a raster scan of a marked calibration target and processed using a series of image processing algorithms. Figure 6 A more detailed example of an interferometric data processing algorithm that may be used for process beam registration calibration is illustrated.

[0078] In addition to changes in the signal type that can be used to identify the process beam representative, different scanning module search strategies can be deployed to speed up the acquisition process, improve detection accuracy, simplify data processing, or some combination thereof. Scanning strategies (within the scanning module reference frame) can include a 2D raster pattern over the representative, orthogonal scan vectors with iteratively updated process beam center positions (e.g., to achieve a process similar to "stepping the beam" to determine the centroid of a Gaussian distribution by adjusting two orthogonal axes), spiral scan patterns, discrete point measurements, feedback-based random step scans, etc. Figure 7 As illustrated in the scan pattern examples in , different scan module measurement acquisition scan patterns can be used for feature detection in CI system calibration measurements, including square grid formation, cross pattern, and spiral pattern.

[0079] Averaging of detector measurements at the same CI scanning module position can be performed to enhance the SNR and exclude variations due to process non-idealities. Differential measurements can similarly be used to enhance the SNR and reduce the effects of characteristic speckle patterns commonly found in CI images. In this context, differential measurements can include 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 process beam. The two images are then compared using one or more differential calculations known to those skilled in the art.

[0080] System 100 using intrinsic process emission contrast mechanism

[0081] According to other embodiments, Figure 1 The system 100 shown in FIG. 1 can use intrinsic process emissions as a contrast mechanism. In these embodiments, the CI measurement system hardware can be configured, operated, or expanded so that the CI system optics and detectors can detect process radiation caused by the laser processing application. In this embodiment, the system 100 can still include auxiliary sensors, but their measurements are not used as CI measurement system calibration inputs.

[0082] In some embodiments, the use of intrinsic process emissions involves: an imaging light source that can be turned off or reduced to a sufficiently low power so that it does not interfere with process radiation detection; and an imaging system detector that can be operated so that it can 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 their detection of process radiation to the wavelength band used for subsequent coherent imaging tasks. This reduces and / or eliminates registration errors arising from chromatic aberrations.

[0083] In more specific embodiments of system 100, the CI measurement system can be expanded to accommodate the transmission of light outside the spectral band of the CI source to facilitate detection of process radiation by CI system components. In other more specific embodiments, the CI measurement system can also be expanded 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 filtering levels; varying fiber bend radius; fiber attenuators; and aperture adjustment (via an iris). Any of these elements can be flexibly removed and / or introduced into the optical path via actuators known to those skilled in the art.

[0084] In some embodiments, process light returning through the imaging system optical path can be diverted to a second detector (eg, a photodetector) that is more suitable for such measurements, or the CI detector can be specifically modified to allow better detection of the calibration signal.

[0085] An exemplary calibration measurement routine, process beam alignment, implemented by an embodiment of the system 100 using an intrinsic process emission contrast mechanism is described in more detail below. The process beam position in the field of view is achieved by turning off the CI system light source or reducing the CI system light source to a negligible level. The optical transmission path of the CI measurement system is used to transmit process radiation transmitted from the laser interaction region through the laser head and to the detector of the CI measurement system. For the purpose of increasing the signal-to-noise ratio (SNR) of the process radiation (e.g., blackbody radiation) measurement, the CI optical transmission path can be temporarily modified (e.g., a change in the optical filter or aperture) and / or the CI detector can have its settings temporarily changed (e.g., an increase in the integration time on a line scan camera in an SD-OCT type system). In an SS-OCT system employing a balanced photodiode detection scheme, for example, one of the inputs of a channel pair can be disabled (e.g., blocked) to avoid common-mode cancellation of the calibration signal.

[0086] Specific laser processes can be performed in order to generate a local process radiation source (e.g., a blackbody emitter) to be used as a proxy for the center of the process beam. Specific workpiece / test piece material types can be used to enhance the proxy signal on the detector. Tungsten can be used to allow increased processing beam energy density and resistance to surface melting. Other materials (types and geometries) can be used for specific optimization for specific blackbody radiation spectra. Specific laser processing parameters (e.g., low power pulses versus high power pulses) can also be used to enhance the proxy signal or prevent damage to the workpiece. Non-destructive laser processes allow the generation of process radiation for proxy detection while preserving the underlying part - making it possible to perform registration procedures directly on the production part and directly within the production environment.

[0087] Additional auxiliary laser process considerations can be used to further enhance the workpiece's resistance to damage and / or enhance the measured process radiation signal. During laser processing, a shielding gas deployed above the workpiece surface can help stabilize the surface temperature, mitigate workpiece oxidation, and maintain consistent detected signal levels. An air knife can be used to prevent process vapors from obscuring the optical path to the detector, which would adversely affect the SNR.

[0088] In some cases where chromatic aberrations can cause problems for accurate process beam registration, such as in scanner-based laser processing, the CI optical transmission path can be modified, intentionally or unintentionally, permanently or temporarily, at production time or during calibration procedures to improve the suppression of spectral bands outside the CI source band. For the purpose of representative identification, transmitting only the spectral components of the process radiation that overlap with the CI light source can mitigate any registration errors associated with chromatic aberrations of the laser head.

[0089] In other cases where systematic errors associated with chromatic aberration are negligible or nonexistent, the CI optical transmission path can be modified to transmit a wider spectral band to improve the collected process radiation signal. In some cases, the CI optical transmission path can be modified to target one or more specific spectral bands of the process radiation. Targeting specific spectral bands helps improve registration accuracy because certain spectral bands may be more strongly associated with the representative, while other spectral bands may be byproducts of the environment or the process used to create the representative.

[0090] For example, laser pulses can be used to create a laser interaction zone on a steel plate that acts as a blackbody emitter (a proxy), where the peak of the spatial blackbody emission distribution coincides with the center of the process beam. The primary spectral components associated with the proxy fall primarily in the infrared portion of the spectrum. Plasma may be generated as an unintended byproduct of the laser process, which in turn can increase process radiation levels in the visible and ultraviolet spectral regions. Due to the process environment and process equipment (e.g., shielding / blanket gas flows), the spatial location associated with the peak plasma radiation signal may differ from that associated with the center of the blackbody emitter and may introduce errors in determining the center position of the proxy. By suppressing spectral components outside the infrared region, the signal associated with the plasma can be suppressed, and the center of the blackbody emitter can be detected with improved accuracy. Similarly, notch filters can be employed in the CI system optical path to block back-reflected process beam light to better target the blackbody radiation.

[0091] In some embodiments, it may not be feasible to filter out process beam radiation with optical components. In such cases, time gating can be used to separate process radiation in the laser interaction zone from back-reflected process beam light. Examples of such time gating include pulse-based laser machining waveforms where process radiation measurements are performed (or saved) only during the off portion of the pulse duty cycle. Figure 8 As shown in the temporal signal gating example in Figure 1, a temporal gating strategy can be used to distinguish between backreflected process beam light and process radiation emanating from the laser interaction zone. Knowledge of the laser power profile (solid line) allows the isolation of detector signal measurements (dashed line) taken only when the laser is off. If the laser power profile is unknown, specific features of the detector signal, such as the tail region (target signal), can be used to isolate the detector signal associated with the desired process event. This type of gating ensures that only radiation generated at the workpiece surface is collected by the measurement system. Similar temporal gating techniques can be used to effectively filter out unwanted spectral components that may have shorter or longer decay times.

[0092] In some embodiments, precise time gating can be achieved by accessing (or commanding) the laser gate drive signal. However, in some cases, performing time gating via this method may not be feasible or accurate. Therefore, characteristics of the acquired signal itself can be used to perform time gating. As an example, identifying a large peak followed by a tail region can be used to isolate a desired spectral component in the form of a signal tail (see Figure 8 ). In other embodiments, additional photodiodes may be used to optically identify laser on periods by measurement of back-reflected process beam light.

[0093] In another example, where the processing and imaging beam wavelengths are close enough that chromatic aberration produces negligible errors, or where the optical transmission system is configured so that chromatic aberration is not an issue, the backreflected process beam itself can be used as a proxy for detection. In such an example, the laser process can be selected so that the laser intensity is too low to produce any significant heating of the workpiece or absorption of the processing beam. Instead, the processing beam light is backreflected from the workpiece, and a portion of it is transmitted through the CI optical transmission path and onto the detector. In this example, the peak spatial intensity of the backreflected light is used as a proxy for the center of the process beam.

[0094] The same as described above (see, for example, Figure 7 ) Similar scanning module scanning strategy and signal processing strategy to acquire and analyze the detected process radiation signal. Figure 9 The measured process radiation at points along a linear scan through the laser interaction region is shown in . Figure 9 Process radiation measurements are shown at two locations within the laser scan head scan field using a sweep along the X-axis. The intensity distribution (e.g., its peak or centroid) is used as a proxy for the process beam position within the CI system scan module. Similar measurements can be performed to determine alignment along the Y-axis. Figure 9 As shown in Figure 3, the CI reference frame is well aligned with the process beam at the center of the laser head scan field. Therefore, the intensity distribution of the sweep is maximized at a distance of 0 µm along the sweep performed in the CI reference frame. Due to chromatic aberration, the same measurement sweep performed in the CI reference frame at an off-center point in the laser head scan field (top right) exhibits a peak intensity distribution signal at -200 µm along the sweep. Future CI measurement beam positioning operations can then account for this error to improve positioning accuracy.

[0095] Similar registration measurements can be performed elsewhere within the process beam scan field and along other CI reference frame axes to improve overall calibration. Iterative scanning techniques similar to "stepping the beam" can be applied to improve measurement accuracy. Averaging of detector measurements at the same CI scan module position can be performed to enhance SNR and exclude variations due to process non-idealities.

[0096] System 100' with 2D inline camera

[0097] Figure 2 The system 100' shown in FIG further includes, for example, a 2D inline camera 230 coupled to a 2D camera port included in the energy beam delivery system. In some embodiments, the CI controller 156 may include at least one processing unit for extracting and analyzing 2D camera measurements, and data obtained from the 2D camera measurements may be passed to a feedback controller responsible for implementing CI system calibration.

[0098] In most embodiments, the CI system hardware allows the imaging light source to be turned off or reduced to a sufficiently low power so that it does not interfere with process radiation detection. Alternatively or in addition, the imaging light source can be modified in time so that for at least one time instance, it does not interfere with process radiation detection.

[0099] In many implementations, the 2D camera measurements are extracted by the camera controller and passed to another controller responsible for performing data processing operations. However, in some cases, the camera controller can assume this responsibility. Data processing can be performed by another dedicated piece of hardware, or it can be performed on existing hardware (e.g., a feedback controller).

[0100] A 2D camera port included in the energy beam delivery system (e.g., laser head 122) can be used in an inline camera configuration (e.g., looking through the beam delivery system). In other embodiments, the 2D camera can be positioned off-axis to avoid sharing the processing and imaging beam optical paths. In an off-axis configuration, multiple 2D cameras can be used to achieve multiple viewing angles during calibration measurements. In either configuration, the 2D camera optical path can also include additional optics (e.g., focusing lenses, filters, apertures, mirrors) to improve calibration measurement quality. The optical path can also be designed to better transmit certain spectral bands (e.g., NIR) and specifically suppress other spectral bands (e.g., VIS).

[0101] The specific optical path of 2D camera 230 relative to processing beam 111 and measurement beam 121 can be variable. In some embodiments, the camera port can share at least some components of the optical path with measurement beam 121. In other embodiments, there is no overlap in the optical paths. The 2D camera hardware can also have adjustable properties (e.g., exposure time, frame rate, field of view) that are configurable depending on the type of calibration measurement being performed.

[0102] An exemplary calibration measurement routine—process beam registration—implemented by an embodiment of system 100′ having a 2D inline camera 230 is described in greater detail below. Process beam registration is achieved by performing registration of the imaging beam to an inline camera reference frame (e.g., camera frame pixels) and registration of the processing beam to the same inline camera reference frame. When the two beams, or more specifically, their individual beam representatives, are registered in the camera reference frame, the separation distance within the camera frame can be used to quantify the degree of registration of the imaging and processing beams (or conversely, the degree of misalignment).

[0103] Figure 10 An example of registering a process beam and an imaging beam representative using a 2D camera is shown. The 2D inline camera detects back-reflected imaging beam light (imaging beam representative) and process radiation (process beam representative). Image processing is used to identify the degree of misalignment and correct the imaging beam position accordingly. Figure 10The image in Figure 3 shows a misalignment between the imaging beam representative and the processing beam representative. To align the imaging beam representative to the processing beam representative, the processing beam representative position on the 2D camera can be recorded. The CI system scanning module is used to actively adjust the imaging beam position until its representative position overlaps with the processing beam representative (within an acceptable range). If a specific calibration of the imaging beam position relative to the 2D camera frame has been performed a priori so that the imaging beam representative position on the camera is mapped to the CI scanning module position, alignment of the imaging beam with the processing beam can be performed by back-calculating the correction from the displacement between the imaging beam representative and the processing beam representative on the camera.

[0104] In this type of setup, the optical path and 2D camera arrangement permit detection of an imaging beam representative and a processing beam representative. The processing beam representative can be established using an intrinsic process emission contrast mechanism, similar to the processing beam representative described above for system 100. The imaging beam representative can be established by using at least some component of the imaging beam light back-reflected from the surface of the workpiece. Similar to what has been described for workpiece material and geometry optimization of system 100 using an intrinsic process emission contrast mechanism, the workpiece material and geometry can be selected to divert a specific amount of the back-reflected imaging beam toward the 2D camera.

[0105] In the case where the camera optical path and the CI system optical path are shared, such as Figure 11 As shown in , diverting some of the back-reflected beam from the CI system to the camera can be achieved by a beam splitter. Depending on the specifications and requirements of the CI system detector and camera, the ratio of the beam splitter (e.g., 50:50 vs. 90:10 vs. 99:1) can be device-specific. Splitting the light can also be achieved using a polarized CI light source and a polarizing beam splitter. Other possible means of directing the back-reflected imaging beam light onto the camera include a mechanically adjustable reflector (e.g., a mirror) to direct the light to the CI measurement system when coherence measurement is desired, and to direct the light to the camera when process beam alignment is desired.

[0106] In cases where the camera optical path is separated from the CI system optical path (e.g., an off-axis camera), the workpiece geometry and / or material type and / or fixture position can be intentionally selected to cause diffuse scattering or angled specular reflections with the goal of increasing the amount of light directed along the camera optical path.

[0107] The fact that both the imaging beam representative and the processing beam representative are registered on the 2D camera presents some additional challenges for individual detection. As the registration of the imaging beam with the processing beam improves, the two representatives begin to overlap on the camera and may not be individually distinguishable. If chromatic aberration is insignificant and the processing beam representative consists of different spectral components than the imaging beam representative, adjustable spectral filtering can be applied to resolve each beam representative on the detector. However, if chromatic aberration is not negligible, or if a simpler implementation is advantageous, the two spots can be resolved temporally. Figure 12 An example of a 2D camera time gating scheme used to distinguish between process beam representative and imaging beam representative measurements using a 2D camera is shown. As shown, temporal modulation of the laser power (solid line in the top graph) and the imaging beam power (dashed line in the top graph) and synchronization with the 2D camera frame allows for classification of frames as process beam representative frames (bottom left graph) or imaging beam representative frames (bottom right graph).

[0108] For example, the processing beam representative can be first detected on the camera by turning off or blocking the CI light source. The processing beam representative can be generated on the detector using a laser process similar to the laser process described for system 100 using an intrinsic process emission contrast mechanism. Once the processing beam representative is identified in the camera frame, its center pixel position is stored and the laser process is stopped. The CI light source is then turned on. The imaging beam light back-reflected from the workpiece provides the imaging beam representative on the camera. The center pixel of the imaging beam representative can then be recorded and compared with the center of the process beam representative to determine the degree of alignment. This information can then be fed back to the CI system (as described above) to further improve the calibration.

[0109] Depending on the frame rate of the 2D camera, the rise / fall times of the machining beam, the rise / fall times of the CI light source, and the workpiece, the two representatives can also be temporally resolved by performing a pulsed laser machining sequence. While a multi-pulse sequence is not strictly necessary to perform co-registration of the two beam representatives, it does allow frame averaging to be performed to enhance the SNR associated with the center identification of each representative and allow for more accurate measurements. Frame averaging can be used to combat noise caused by various sources, including camera shutter noise, laser process non-idealities, workpiece surface defects, low signal levels, poor camera contrast, etc.

[0110] In addition to temporally gating the processing beam representative signal and the imaging beam representative signal, system settings may need to be modified specifically for processing beam representative acquisition, imaging beam representative acquisition, or both. Due to the different properties of each radiation source, it is likely that the radiation intensity level will be different when measuring each representative. Therefore, it may be necessary to apply some techniques (e.g., dynamic optical filtering, adjusting camera exposure time, processing power adjustment, or imaging beam power adjustment) to ensure that the optical intensity level of each representative is within the measurable range of the detector.

[0111] System 100" with double-clad fiber

[0112] Figure 3 The system 100" shown in further includes a double-clad fiber 330, which is used to transmit coherent imaging system light within its core and process radiation within its cladding. In the system 100", at least one photodetector 332 is used to measure the process radiation from the double-clad fiber cladding, and data processing hardware (e.g., CI controller 156) is used to extract the photodetector measurements. The details of this configuration are described in U.S. Patent No. 10,124,410, which is incorporated herein by reference in its entirety. In most embodiments of the system 100", the CI system light source is controllable so that it can be adjusted to facilitate or not interfere with system calibration measurements.

[0113] The exemplary calibration measurement routine implemented by system 100″—process beam registration—is similar to the exemplary calibration measurement routine described above for system 100 using an intrinsic process emission contrast mechanism. In this example, a process beam representative is measured by collecting process radiation, transmitting it through the inner cladding of a double-clad fiber, and registering it on a photodetector. Unlike system 100 using an intrinsic process emission contrast mechanism, the process radiation collected by the CI system detector is not used for calibration measurements. The photodetector measurements are synchronized with the CI system scanning module position to map the measurement results to a useful CI system reference frame.

[0114] When attempting to co-register the imaging system reference frame with the process radiation, differences in the color image between the coherent measurement beam spectral band and the spectral components collected by the inner cladding can lead to chromatic aberrations. To account for these errors, chromatic aberration effects can be corrected through data processing (e.g., pre-calibration, modeling), or eliminated by optically filtering out process radiation spectral components that are sufficiently distant from the measurement beam spectral band.

[0115] System 100''' with a 2D camera positioned below

[0116] Figure 4The system 100''' shown in FIG further includes a 2D camera 430 positioned sufficiently close to the working plane of the process (i.e., the plane containing the workpiece surface). The system 100''' also includes an optical filtering element (not shown) for controlling the amount and spectral composition of light reaching the 2D camera, and at least one processing unit (e.g., CI controller 156) for extracting and analyzing 2D camera measurements. The data obtained from the 2D camera measurements can be passed to a feedback controller responsible for implementing CI system calibration.

[0117] The exemplary calibration measurement routine enabled by system 100'''—process beam registration—is similar to the exemplary calibration measurement routine described above for system 100' including a 2D inline camera. However, in this case, 2D camera 430 is positioned at the work plane and is 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 detector.

[0118] The system 100''' may also include additional automation hardware and controls to automatically position the 2D camera 430 at the correct calibration measurement location while calibration is being performed, and to position the 2D camera 430 so that it is out of the way when laser processing is being performed. In other systems, positioning of the 2D camera 430 may be performed manually by a system operator.

[0119] The positions and measurements of the beams on camera 430 can be synchronized using techniques similar to those described for system 100' to allow differentiation of each beam type. In many cases, further synchronization of the processing and imaging light source power levels may be desirable to achieve readings within the detector's range (i.e., above the detector's noise and below its saturation value).

[0120] In some cases, it may not be possible to reduce the processing source power to a low enough level for it to be measured by the detector. To accommodate this, some systems may utilize source pump leakage light rather than the operational beam. In other systems, additional optical beam sampling components may be used to reduce the intensity of the processing beam before it reaches the detector. Some systems may further include a calibration light source having similar spectral qualities to the processing source but at a substantially lower output power, which can be directed substantially along the processing beam optical path. This calibration light source can be used as a proxy for the process beam source for 2D camera measurements. While the imaging light source power level can typically be reduced low enough to be detected, the same techniques can also be applied to the imaging beam.

[0121] In further embodiments, combinations of any of the above systems 100 , 100 ′, 100 ″, 100 ′″ may be used to implement different detection contrast mechanisms, as different technologies may be better suited for certain types of registration procedures and laser processes.

[0122] Calibration type

[0123] Various types of calibration can be performed using the system 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 procedures can be extended to other forms of energy beam welding and other laser processing applications.

[0124] Typically, calibration measurements can be performed by the system without 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. A priori information can take the form of one or more of the following: a default calibration compiled from a series of past calibration measurements of the same type on similar subsystems; past calibration measurements for the same subsystem; and output from an optical model or simulation, such as that employed by computational optics simulation software (e.g., Zemax Optical Design).

[0125] For some applications, a priori information can be used to achieve adequate calibration, but in most applications, a priori information is used to supplement the measurement system calibration routine.

[0126] Static calibration

[0127] The above 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 first 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 compensate for equipment tolerances, temperature drift, environmental fluctuations, mechanical vibrations, dynamic forces during processing, and the like. By communicating with a process master and / or external devices, one or more of these static calibrations can be performed in the context of mass production using the system.

[0128] One or more static calibration procedures can also be performed on workpieces periodically processed by the system, after a certain threshold of failed quality measurements, after a temperature change, after maintenance, or after one or more parts of the processing system are replaced. Because the available space on the workpiece may have geometric complexity near calibration marks, some embodiments include the ability to suppress portions of the scanned image data so that these areas do not erroneously attract and confuse the contrast detection algorithm. In cases where multiple calibration procedures are performed, the results can be averaged and their distribution analyzed to determine whether the registration of the process and measurement beams is likely correct. For example, if the width of the distribution of measurement results is equal to or greater than the expected size of the process beam at the workpiece, the registration is likely not very good. However, if the distribution of measurement results is much smaller than this value, the chances of good registration are high.

[0129] The following examples are first described in the context of calibration. Additional sub-examples are used to illustrate how the system / comparison mechanism described above can be deployed to perform such calibration. Although only a few selected sub-examples are described herein, the system and comparison mechanism described above can be used in other sub-examples not explicitly described herein. Specific calibrations can be implemented using embodiments of the system and comparison mechanism described above.

[0130] Process beam / workpiece reference system registration

[0131] In CI system integration with laser heads, process beam alignment / registration provides a meaningful origin for the CI system scanner module reference frame. The hardware origin (electrical / mechanical reference frame origin) of the imaging system scanner module does not naturally coincide with the process beam center without calibration or alignment. Therefore, process beam center registration measurement is beneficial for aligning the imaging system scanner module origin with the process beam center. Figure 13 As illustrated in the imaging system reference frame alignment shown in , calibration can be used to align the CI system reference frame (imaging system frame) with the process beam / workpiece reference frame (laser head reference frame).

[0132] Measuring the process beam center in the imaging system scan module XY frame provides XY calibration for the imaging system so that its reference frame is centered about the process beam origin. Similar calibration can be used to align the imaging system scan module XY orientation with the workpiece / laser head XY orientation, and to provide scaling calibration to match the imaging system scan module XY scale with the workpiece / laser head XY scale.

[0133] Process beam center detection can be achieved using one or more of the above-described representatives and comparison mechanisms (e.g., process radiation, workpiece surface demarcation and subsequent optical path length-based sensing, workpiece surface demarcation and subsequent back-reflected beam intensity, etc.). Subsequent detection of the process beam center can be performed using one or more of the above-described sensors. The following examples illustrate possible implementations of the process beam centering procedure, but many other variations are possible.

[0134] In one embodiment, a process beam is used to mark a small feature on a workpiece positioned below a laser head. The feature is marked small enough to locate its centroid within an acceptable tolerance (e.g., ±10-20µm), but large enough to produce distortions to the part surface (e.g., height deformation, changes in surface reflectivity, polarization changes, etc.) that can be detected by a coherent measurement system. Distortions where the maximum or minimum distortion coincides with the center of the process beam are typically desirable—for example, in laser ablation processes, where the resulting hole has a maximum depth that coincides with the center of the process beam. Other laser marking and / or spot welding methods can also be used. Materials suitable for this calibration include anodized aluminum, aluminum, steel, stainless steel, copper, nickel-plated copper, and other common metals. Polymers and graphite can also be used. Graphite is particularly advantageous because it has no liquid phase at normal atmospheric pressure. This means that the boundaries of the laser mark can be clearly defined and easily segmented using image processing algorithms. In one embodiment, the marked material is a product manufactured on a cell, allowing for real-time recalibration and / or verification of beam co-registration during production.

[0135] The imaging system's scanning module moves an imaging spot over the workpiece in a defined scanning pattern and records the desired measurement quantity (e.g., depth, intensity, etc.) at each position. The process beam center is identified as the imaging spot position within the scanning module's reference frame using a target signal value (e.g., a local maximum or minimum). Additional data processing techniques (e.g., fitting, averaging, and filtering) may also be employed to enhance detection accuracy.

[0136] A specific embodiment of the process involves two measurement acquisitions of a calibration target - one before laser spot processing and one after laser processing. The pre-excitation acquisition is obtained from an unprocessed calibration target or an unprocessed area of the calibration target. The post-excitation acquisition is obtained after laser processing is performed to create features on the target. The process can be a marking process or an ablation process. Exemplary laser processing parameters include: 3 ms, 600 W pulses generated with a continuous wave single mode laser; and 2 ms, 2000 W pulses generated with a continuous wave multimode laser. The material type of the calibration target is as described above. Each acquisition is a 4D data set representing the intensity of back reflections as a function of 3D position in space and is obtained by performing coherence measurements while scanning an imaging beam over a rectangular grating pattern. The absolute difference between the acquisitions is calculated. A series of image processing operations (e.g., intensity thresholding, contour calculation, and convex hull calculation) are then performed to increase image contrast and detect feature points.

[0137] The data is cleaned using a hierarchical filtering scheme in which each subsequent stage has fewer data points than the previous stage. The scheme is divided into preprocessing, feature point extraction, and geometric fitting stages. The preprocessing stage utilizes nearest neighbor denoising and image intensity thresholding techniques to extract the patches of maximum and most visible intensity in the acquired data. The feature point extraction stage relies on patch approximation and convex hull calculation to calculate feature points in the extracted intensity patches from the previous stage. The geometric fitting stage fits circles to the extracted feature points - computing bounding boxes and minimum enclosing circles for the feature points extracted in the previous stage. The output of the geometric fitting is used to determine the process beam representative position and evaluate the accuracy of the algorithm. The fitted centroid is designated as the process beam representative position. The fitted radius is used to determine the accuracy of the fit by comparing it to the expected range of values dictated by the machining beam spot size and process parameters.

[0138] Different scanning patterns and search optimization strategies can be implemented to enhance detection speed and accuracy, e.g. Figure 7 . Simple scan patterns include rectangular grids, circular grids, spiral grids, rectangular raster scans, spiral scans, and crosshair scan patterns. More complex searches may involve iterative methods with progressively finer search positions, more complex search strategies including random walks, the use of prior measurements, and the use of models.

[0139] Inputs to the calibration routine can include, for example, optical modeling of the nominal process beam path and imaging beam path through the laser head, a collection of previous calibration point values, and / or a collection of similar calibration point values. These inputs can be used to provide a starting point around which the calibration scan pattern outlined above is centered. A more accurate starting center point can require fewer measurement iterations to achieve the same level of calibration accuracy. A more specific example involves using a computational optical model to generate imaging beam correction vectors for a nominal optical configuration to align the imaging beam with the processing beam near the workpiece surface. A series of correction vectors can be generated for different optical configurations (e.g., scanning mirror angles or different defocus positions in the laser scan head). The correction vectors are used by the measurement system during its calibration routine to center the rectangular raster scan pattern around the nominally corrected position (i.e., the position generated from the computational optical model). This increases the likelihood that the calibration representative is within the calibration measurement scan field of view, thereby alleviating the need for search routines and iterative measurements for the same system calibration.

[0140] In another embodiment, a blackbody radiation source is generated on the surface of the workpiece via heating using a process beam. The radiation source is most commonly a blackbody emitter, but in some embodiments can be a plasma generated by the process. By carefully selecting the process parameters, it is possible to generate a radiation source on the surface of the material without causing significant damage to the material itself. Such a process can be beneficial for target reuse, detection accuracy, and may even allow production parts to be used in the process. Using production parts as calibration targets can allow for reduced downtime on high-throughput production lines and enable more frequent recalibration. Because the blackbody source is primarily mediated by energy absorption and conduction into the bulk of the material, its apparent position should be extremely stable and symmetrical about the center of the process beam. This can provide very accurate and repeatable beam alignment.

[0141] As described above, it is possible to directly use the coherent measurement system optics (e.g. Figure 1 ), using a cladding-bonded photodiode in a double-clad fiber (e.g., Figure 3 ”), 2D inline cameras (e.g., Figure 2 The radiation generated by the source is captured using the system 100′ shown in FIG. 1 , or some combination thereof. In the case of direct coherence measurement system detection or double-clad fiber detection, the measurement spot is moved in a scanning pattern, as described above, and the detected intensity is recorded at each position. The scanning position associated with the maximum intensity (or a similar prominent feature, such as the center of signal mass) is identified as the process beam origin. Spectral filtering can be applied to the process radiation before detection to improve accuracy.

[0142] Figure 14An example of process radiation measurements directly recorded by a coherent measurement system during a line scan along one of the imaging system scan module axes is shown in FIG. Figure 14 The process radiation linescan fit shown in Figure 1 illustrates process radiation measurements recorded by system 100 (left graph) and data processing operations (right graph). The left graph shows the radiation intensity measurements as a function of the imaging system's measurement position along its search axis. The right graph shows the results of additional data processing and fitting algorithms (e.g., noise suppression and curve fitting) to find the centroid. In this case, the fitted centroid represents the process beam center position along the CI system's scan axis. Here, the scan axis distance scale has been adjusted so that the zero position corresponds to the center of the fit to illustrate the corrected CI beam position.

[0143] In the case of a 2D inline camera, the process radiation is recorded on the camera. The back-reflected imaging beam can be recorded on the camera simultaneously or subsequently (see Figure 10 ). Possible sequencing of such detections is described in more detail above. If the inline camera has been calibrated relative to the imaging system, the imaging beam position corrections can be used to directly calculate corrections to the imaging beam 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 directed by the scanning module) until the imaging beam positioning error (relative to the process beam center on the camera) has been sufficiently minimized. Additional data processing and spectral filtering can also be applied to improve performance.

[0144] In many embodiments, the process beam center position is detected automatically, or with minimal user input, and saved as a general system calibration.

[0145] Similar techniques can be used to identify and calibrate the scale and rotation of a CI system relative to a more convenient reference frame (e.g., the laser head frame, the workpiece frame, or the world frame). While scale and rotation are the primary corrective transformations used by CI systems, additional transformations (e.g., reflection and distortion corrections) may be desirable to improve the positioning accuracy of the measurement system. Aspects of these calibration measurements can allow the same calibration process to be used to identify multiple transformation corrections for the CI system. However, these transformation-type calibration measurements can also be performed independently.

[0146] Due to the geometric nature of the problem, transformation calibrations are typically more involved than process beam origin calibrations. They typically involve creating one or more features on the workpiece (or beneath the laser head), such as a mark or radiation emitter on the calibration target / workpiece. To determine the scaling factor, for example, two features with known separation distances are created in the reference frame to which the CI system is to be aligned. The features are identified in the CI system reference frame, and the separation distance between the features in the CI frame is determined. The ratio of the CI reference frame distance to the target reference frame distance is used to determine the corrective scaling factor for the CI system.

[0147] The same identification features can be used to determine the desired rotation correction for the CI system. If the features are marked with reference to a known axis or set of axes in the target reference frame, the feature position relative to a specific axis in the CI system reference frame can be used to calculate the rotation to align the two reference frames.

[0148] For these types of calibration processes, synchronization with the laser source and / or the unit (eg, through a standard communication protocol or indirectly via a unit operator) may be advantageous.

[0149] The above procedure can be extended to include more complex feature generation to account for reference frame distortions and reflections, for example by creating features with significant asymmetry about multiple axes in the target reference frame.

[0150] In some CI system optical path configurations, similar types of multiple transformation correction factors (e.g., multiple scaling factors) can be used to account for: multiple axes (not necessarily orthogonal) in the CI system reference frame; multiple axes (not necessarily orthogonal) in the target reference frame; optical distortions in the CI system beam path (e.g., optical distortions associated with the CI scanning module); optical distortions in the laser head beam path (e.g., lens distortion in the scanning head); different geometries in the target reference frame (e.g., cylindrical surface versus spherical surface versus planar surface); and CI scanning module tracking errors.

[0151] The CI system scanning module is used to position and scan the imaging beam relative to the target reference frame. Scanning is achieved using optical devices that are limited by inertia and finite acceleration and / or slew rate. Consequently, the actual position of the imaging beam lags behind the commanded position by a tracking error. In a simple form, the tracking error manifests as a constant time lag (e.g., 0.1 ms) in the imaging beam position after the commanded position. Typically, the tracking error follows a more complex system response and often depends on the input drive command itself.

[0152] The examples presented herein illustrate the simplest form of tracking error, but the techniques outlined can be extended to identify and correct more complex models of tracking error. Tracking error can be determined by creating one or more features (e.g., height markers or process radiation emitters) in the target reference frame and commanding the CI system scanning module to scan over the features in different directions and along different axes.

[0153] Figure 15 The figure shows the difference in positioning in the target reference frame when the tracking error is correctly taken into account and when it is incorrectly taken into account. Specifically, Figure 15 The tracking error schematics shown in highlight the differences in the commanded imaging beam position and the actual measurement beam position when scanning in opposite directions with correct and incorrect tracking error cases. The incorrect tracking error case highlights how the tracking error distance can be calculated using the distance between a static feature measured when scanning in one direction and the static feature measured when scanning in the other direction. When calibrated correctly, scanning over a position in the target reference frame produces the same measurement signal regardless of the scanning direction. When calibrated incorrectly, the actual position sampled in the target reference frame deviates systematically from the commanded position in the same reference frame. Although Figure 15 The example in shows that the true position lags the commanded position, but for certain types of incorrect tracking errors, the true position may lead the commanded position.

[0154] In case of the correct type of target and imaging system scanning pattern, incorrect tracking errors can be identified in the measurement results of the target reference frame characteristics of the imaging system (or auxiliary system), e.g. Figure 16 Specifically, Figure 16 Example signal measurements obtained while scanning a static feature in two opposite directions using correct tracking error (top) and incorrect tracking error (bottom) are shown. As shown, scanning in one direction produces a peak signal at one position / time in the CI system scan path, such as a peak signal associated with peak emission intensity from a process radiation emitter, while scanning the same pattern over the same feature in the opposite direction produces a peak at a different position / time in the CI system 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 conjunction with additional knowledge or modeling of the scanning module or other CI system calibration to determine the tracking error.

[0155] As previously identified, tracking error is likely to depend on aspects of the drive command. Examples of such dependencies may include command velocity, command displacement, and scanning module axis. To improve CI system accuracy, multiple tracking error calibrations may be performed under different input conditions.

[0156] Similarly, to properly acquire tracking error calibration measurements, the CI system, and in some cases its auxiliary sensors, can be modified (relative to their measurement operating conditions) to collect calibration measurement signals. For example, the CI system detector integration time can be increased, or additional measurement averaging can be performed to achieve a better signal-to-noise ratio (SNR) from the process radiation emitter. Specialized laser processes can also be implemented to generate such signals.

[0157] During production of a CI system, it may be sufficient to perform only a CI scanning module tracking error calibration. However, a tracking error calibration may be performed in an end-use application for improved measurement accuracy.

[0158] CI scanning module long-term drift

[0159] Engineering tolerances may result in long-term drift of the CI system scanning module, where calibration accuracy (eg, target reference frame registration, tracking errors, etc.) deteriorates over time. Changing environmental conditions (eg, temperature) may accelerate the process.

[0160] To combat long-term drift and ensure adequate scanning module positioning accuracy, scanning module calibration of the type described above can be performed periodically. A coherent measurement system that can automatically detect and implement updated corrections greatly benefits end users. Automation and synchronization of the imaging system with the laser processing unit further simplifies recalibration sequences, allowing measurement generation and detection to be performed automatically (or semi-automatically).

[0161] Scanner field correction

[0162] In a fixed optics laser head, a single process beam / workpiece reference origin registration may be sufficient, as the position of the process beam relative to the laser head is fixed. However, further extensions in fixed optics implementations may involve multiple co-registrations for different defocus positions (e.g., different planes relative to the laser head's focal plane).

[0163] The co-registration calibration process becomes complex when the laser head is involved with beam scanning optics. Scanning optics allow the focus of the machining laser beam to be moved relative to the laser head. Examples of laser scanning heads include, but are not limited to, the 2D High Power Scanner and the 3D High Power Scanner available from IPG Photonics Corporation.

[0164] In implementations based on such scanners, the imaging system reference frame can be registered with the process beam position (and reference frame) at multiple locations throughout the laser head's scan field. In some applications, this registration requirement can be relaxed by limiting the effective working area (or volume) of the scan field. Similarly, in applications where registration accuracy is less critical, calibration at multiple locations within the scan field may not be necessary.

[0165] However, there are applications and scan head hardware where such additional registration may be desirable or advantageous. Exemplary applications include single-mode laser machining applications, such as keyhole welding of electromigration components, where the phase change regions produced on the workpiece surface by a narrow-diameter single-mode beam have relatively small co-registration errors relative to the measurement system at different locations within the scan field. In such applications, the lack of additional calibration points may cause measurements to be off-target at one or more regions within the scan field. The relevant positioning scale for some applications may be on the order of a few microns to tens of microns.

[0166] Exemplary hardware deployments where additional co-registration may be desirable or advantageous include systems in which the measurement spectral band differs from the processing beam spectral band. In such systems, chromatic aberrations introduced by various optical components within the head cause the measurement spot to deviate from the focus position of the processing beam by different amounts for different locations within the scan field. Such deviations can be significant enough to cause measurements to be taken at incorrect locations relative to the phase change region or specific features on the workpiece.

[0167] Figure 17 An example of the type of correction that can be applied to the imaging system scan module positioning commands to correct for optical distortion in the laser scan head as the scan field position changes is shown in FIG. This example illustrates the type of 2D correction that can be used for a 2D scan head. Similar 3D correction can be used to correct a 3D scan head.

[0168] In addition to scan field co-registration errors caused by color effects, co-registration errors at different locations in the scan field can be caused by other optical and geometric effects. For example, misalignment of the machining laser beam path through the laser head and the CI measurement system beam path can cause similar scan field co-registration errors. Thermal effects on the head (such as thermal lensing) can also cause co-registration errors.

[0169] Depending on the system and application, a general calibration of the entire scan field may be sufficient to co-register the measurement and processing beams. However, in systems or applications where a very high degree of co-registration is required, it may be desirable to perform a more specific co-registration calibration around the processing laser path, such as Figure 18 As shown in . Figure 18An example of path-specific imaging beam and process beam registration measurements for a device involving a laser scan head is shown. 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 between a coarser, general calibration grid). As shown in the illustrated example, such calibration can be performed at specific locations along the machining path and at locations around the path (e.g., around the perimeter of the boundary).

[0170] Similar calibration measurements can be used to compensate for changes in optical path length and 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 guide (or read) the processing laser focus position. 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.

[0171] Main scanner process beam tracking and position error

[0172] Similar to the calibration of the CI system scanning module, laser scan heads can also use calibration to take into account their own tracking errors, optical distortion, mechanical alignment, and thermal effects. Laser processing systems with integrated coherent measurement systems can utilize the CI measurement system (or its auxiliary detector) to determine the laser head scanner calibration - the main scanner calibration.

[0173] Similar strategies described above for calibrating CI systems can be used to perform main scanner calibration. For example, the main scanner can be used to generate a series of calibration marks on the target below the laser head. Imaging system measurements (coherence, process radiation, or some combination thereof) can then be used to determine tracking or positioning errors associated with the main scanner.

[0174] A more specific, non-limiting example includes using a main scanner to mark a series of points on a workpiece beneath the head while traveling in one direction, and another series of points while traveling in the opposite direction. Then, while the main scanner remains in a static position, a CI measurement system scanning module can be used to perform coherent measurements of the marked surface. The height-based measurements of the coherent measurement system can be used to identify the locations of features marked on the workpiece surface. The locations of features marked in one direction relative to features marked in the other direction can then be used to identify main scanner tracking errors.

[0175] For calibration measurements, the main scanner can be used instead of (or in addition to) the CI system scanning module to position the measurement beam at a specific location on the workpiece. In this case, synchronization is provided between the main scanner position and the imaging system measurement. While it can be useful to perform such measurements with a fully calibrated imaging system, the main scanner calibration measurements can be performed in a manner that does not preclude full imaging system calibration.

[0176] Main scanner calibration may be performed only at system commissioning or production. However, in some cases, periodic calibration may be performed to counteract optomechanical degradation or changes in environmental conditions (e.g., thermal lensing).

[0177] Dynamic calibration

[0178] The system described above can also be used to perform dynamic (i.e., process-specific) calibration measurements, as described in the examples below. Such measurements are typically performed for each type of laser processing application and for major changes in the application's processing parameters. Calibration / adjustments can also be performed continuously during a processing application to achieve high calibration accuracy and reduce laser station downtime (i.e., time not spent performing laser processing). In many high-throughput laser processing production environments, minimal system downtime is crucial.

[0179] As with static calibration, dynamic calibration may also need to be performed periodically in high-precision processes to compensate for equipment tolerance limits, temperature drift, environmental fluctuations, mechanical vibrations, dynamic forces during production, etc.

[0180] Dynamic calibration is also used to register imaging system measurements with aspects of the laser process that are specific to the laser process and / or the workpiece. Aspects of the laser process can be detected as part of the CI system's dynamic calibration and fed back (or forwarded) to influence future CI measurements of specific aspects of the process. These aspects of the laser process include, but are not limited to, the following: process path geometry; process path speed; process path direction; process path tangent angle; process time; laser power temporal distribution; optical path length change with respect to the process path and its local proximity; workpiece offset from the laser head with respect to the process path and its local proximity; imaging system and process beam co-registration along the process path and local proximity; change in measurement beam focus position with respect to the process path and local proximity; phase change region geometry; phase change region extent; phase change region location; phase change region radiation intensity (i.e., temperature); melt pool location; melt pool geometry; keyhole location; keyhole geometry; laser beam spot size; laser beam energy distribution; laser beam temporal characteristics (e.g., continuous wave versus pulsed wave, pulse width, pulse period, pulse frequency, pulse shape); measurement beam reflectivity distribution (i.e., surface reflectivity and / or subsurface reflectivity); workpiece fiducial marks / reference / base surfaces; and measurement beam divergence.

[0181] Dynamic calibration can be performed by taking measurements on a similar process before the process being measured (e.g., for QA determination). These calibration measurements can be stored so that they can be applied to the target process. Changes to the target process typically prompt a recalibration step.

[0182] In other dynamic calibration implementations, calibration measurements are performed during the process of performing process measurements. These calibration measurements can be performed immediately at the beginning of the process so that the remainder of the process can be monitored with a calibrated system. Calibration measurements can also be interspersed or performed simultaneously with process measurements for improved accuracy.

[0183] Phase change region measurement

[0184] In addition to providing process monitoring quality determinations, measurements of the phase change region can also be used to provide dynamic measurement system calibration measurements. For example, process radiation measurements of the phase change region allow in-process beam co-registration calibration measurements to be performed during laser processing rather than taking the system offline. This can be used in applications with high throughput requirements. Figure 19 shows the use of double-clad fibers in combination with auxiliary photodiodes (e.g. Figure 3 ”) can be used to determine the process beam center position during a laser process (1). In this example, the double-clad fiber may include a double-clad fiber coupler manufactured by Castor Optics. Similarly, Figure 20Illustration of 2D inline camera measurement of the phase change region and the coherent measurement beam (e.g. using Figure 2 The system 100' shown in FIG can also be used to determine process beam registration during laser processing applications. Similar measurements can be made using other contrast mechanisms (for example, CI system optical path measurements) to determine process beam alignment during processing.

[0185] The measurements of the phase change region can be used directly or indirectly to perform other process-based calibrations. For example, Figure 19 shows how the tail region of the 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 generally rely on the presence of a calibration of the imaging system reference frame relative to the target reference frame. Similarly, Figure 20 Shown is how a 2D inline camera image of the process can be used to identify the process direction from the PCR tail region.Such a process typically involves an additional calibration step to register the inline camera reference frame with at least one of the imaging system, scan head, and workpiece reference frames.

[0186] Figure 21 Indicates how point-based measurements (e.g., coherence height measurements, process emission measurements through a coherent system, process emission measurements through a double-clad fiber) can be scanned around a PCR. By synchronizing single-point measurements with positions around a circular (or similar) scanning module scanning pattern, the resulting signals and features in the measured signal distribution can be used to identify the direction of the PCR tail centerline, and therefore the process direction. The centerline can be determined via local maxima or peaks in the signal that vary with position in the scan path (e.g., Figure 21 The position in the scan path can then be mapped to the CI system reference frame to determine the quasi-instantaneous path direction in the CI system reference frame (e.g., rightward in the top example and leftward in the bottom example).

[0187] Similarly, measurements of PCR tail length can be used to identify process speed or changes in process speed. The length of the PCR tail generally increases with process speed. However, the exact nature may depend on other process factors, such as material type, material geometry, laser spot size, and assist gas. A priori knowledge of such a relationship or process modeling (e.g., based on cooling rate) can be used with tail length measurements to determine process speed. The process speed can then be used to guide the measurement system to measure at a specific location or time before or after the process.

[0188] Measurement of other aspects of the phase change region can be crucial for measurement positioning during certain laser processing applications. For example, in keyhole laser welding, alignment of the measurement beam with the vapor channel formed during welding is established to perform keyhole depth measurements using a coherent imaging system. Changes to the keyhole laser welding process (e.g., changes in process speed) can cause the position of the vapor channel to change relative to the process beam focus position, such as Figure 22 As shown in Figure 22 As shown in , an increase in welding speed can cause the steam channel position to lag behind the process beam by an increased distance. Similarly, a change in weld direction can cause the steam channel's direction to change, lagging behind the process beam. Additional process effects (e.g., changes in material geometry or composition, or periodic oscillations of the process beam) can also contribute to changes in the steam channel position. Calibration measurements of the steam channel position at one or more locations along the weld path are beneficial for achieving measurement beam positioning for keyhole depth measurement and can be used to set the imaging beam position for keyhole depth measurement during the same process or for subsequent processes.

[0189] CI measurement system measurements can be used to identify the location of steam tunnels based on the presence of specific depth signatures (e.g., the deepest measured depth, clustering of points at a specific depth, measurements with a specific type of depth variation, measurements with a specific signal strength level or variation, etc.). Process radiation measurements can also be used to identify steam tunnel locations. Such measurements can include peak radiation levels, specific variations in radiation levels, radiation in one or more specific spectral bands, specific signatures in PCR topographic radiation measurements, etc.

[0190] Features similar to vapor channels in laser keyhole welding can be identified and calibrated in other laser processing applications such as additive manufacturing, marking, cleaning, and cutting.

[0191] Pre- and post-process calibration measurements

[0192] Measurements of one or more features produced on a workpiece as a result of the laser process, as well as measurements of one or more features of a workpiece intended to undergo laser processing, can also be used for dynamic system calibration. Such post-processing features can be measured by the system (with or without auxiliary sensors) by measuring sufficiently far after the process, either in space or time, or both. Because the measurements are performed after the laser process, they typically rely on height-based or external radiation intensity-based contrast mechanisms.

[0193] In many laser machining applications, the process produces changes (e.g., in height, reflectivity, polarization, material density, etc.) that can be detected by a coherent measurement system or auxiliary detector. Examples include weld beads in laser welding applications, solidified weld beads in additive manufacturing, changes in 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 reference frame and using specific measurement scan patterns, additional characteristics of the laser process can be determined and used to guide future measurement system positioning. These dynamic calibration measurements can be used to construct laser machining paths and other laser machining calibrations when there is limited or no a priori knowledge of the path.

[0194] In other cases, calibration measurements can be used to determine corrections to apply to a set of nominal process parameters to correct for process non-idealities. For example, a coherent measurement system may have access to a nominal machining path input to a laser scanning head. However, physical limitations in the scanning head (e.g., finite acceleration) can cause the actual path to deviate slightly from the commanded version. Dynamic calibration can be used to correct for these types of deviations.

[0195] 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 (or similar marking along the weld path) over the solidified weld bead while a motion control system (e.g., scan head, robot, linear stage, rotary stage, etc.) executes the motion path, e.g. Figure 23 As shown in . Figure 23 This example shows how the use of a measurement line transverse to the nominal laser process path can be used to identify discrepancies between the nominal and actual process paths. The transverse sweep is typically offset (forward or backward) from the process beam along the weld path. In this example, a height-based measurement of a feature created by the laser process (e.g., a marking line) is used as a calibration target.

[0196] If the true or actual motion path is identical to the nominal motion path (bottom schematic), the line appears centered in each of the transverse measurement profiles, and the feature appears as a straight line in the center of the image reconstructed from the linear sweep. As the difference between the actual and nominal paths increases (top schematic), the position of the line within each transverse measurement profile changes depending on the deviation in path position and tangent angle. Deviations from the nominal motion path cause the feature to appear offset and / or distorted (e.g., wider) in the image. These deviations can develop as the position along the path changes, because the misalignment between the nominal and actual paths can change as the position along the path changes.

[0197] This example illustrates path error detection using an altitude-based contrast mechanism. Similar strategies can be employed using a backreflection 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 can also be used to more accurately identify such motion path corrections.

[0198] The above examples are simple illustrations of the types of scanning patterns and strategies that can be used to identify laser machining motion paths. More complex scanning patterns can be used to construct dynamic corrections. Other examples may involve the use of coherent imaging speckle correlation, coherent imaging height measurement correlation, 2D inline camera image correlation, tracking operations similar to those in an optical computer mouse, and detection of process-emitted or back-reflected beams.

[0199] Examples of dynamic calibration (identification and / or correction) that can be applied to imaging systems for monitoring laser machining applications include, but are not limited to: machining path error; machining path tangent angle; machining path speed; optical path length change caused by beam delivery optics; laser interaction region extent; laser interaction region location; phase change region extent; phase change region location; workpiece tilt; and workpiece surface curvature.

[0200] For example, local workpiece surface tilt can be calibrated by performing a repeated circular scan pattern of the measurement beam (within the scan module reference frame) while the laser station motion control system (or laser scan head) drives the process beam along its machining path. In most implementations, the process beam is disabled during this process to avoid damaging the workpiece. This circular scan pattern is typically performed at a high frequency to achieve a large number of fully circular cycles measured during the laser machining path. The frequency is typically dictated by the hardware limitations of the coherent imaging system's scan module.

[0201] For each circular scan, the CI system's height measurements taken across the entire circle are mapped to their corresponding XY positions around the circular scan pattern. Planar fitting to the XYZ data yields the tilt of the plane in the CI system's reference frame. By performing a series of consecutive scans, the evolution of the plane's tilt angle as a function of position along the path (which is mapped to the circular scan number) can be generated. This change in tilt angle over the weld path can then be used to modify the CI system's measurement scanning strategy and data processing algorithms while monitoring the laser process.

[0202] exist Figure 24 The local surface tilt calibration measurement process is shown in FIG. When the laser head moves along its motion path over a part with a surface curvature, as shown in FIG. Figure 24As shown in , the CI system scanning module scans the measurement beam in a periodic circular pattern, thereby performing CI measurements synchronized with the measurement beam position. In this example, a circular pattern is generated relative to the CI system scanning module reference frame (relative to the workpiece surface) to measure workpiece tilt relative to the scanning module reference frame. CI height measurements are mapped to their corresponding circular beam scanning pattern iterations and further mapped to their corresponding XY positions within the pattern. Once the CI height measurements within a cycle are mapped to their XY positions, a plane fit is applied to the data (X: X position, Y: Y position, Z: CI height measurement). Based on the plane fit coefficients, a surface normal vector representing the instantaneous local surface tilt is generated. The surface normal vector is then mapped to a system-favored angular coordinate system (e.g., the tilt angle along the weld path direction). This process is repeated for each scanning cycle. Each cycle is assigned a timestamp (e.g., the time at which the midpoint of the cycle occurs relative to the path start time), allowing the surface tilt angle to be plotted as a function of time along the laser machining path. The tilt that varies with machining path position can be used by the CI system to provide CI system scanning module positioning corrections and can be used to correct CI system height measurements (not shown).

[0203] While a circular scan pattern is used in this example, other scan patterns can also be used for this calibration. Such scan patterns may include, but are not limited to, sampling a limited number of points distributed around the XY scan field, a scanning crosshair pattern, a scanning spiral pattern, and a scanning rectangular raster pattern. Any scan pattern that provides enough points (at least three) to reliably fit the surface is sufficient for this calibration.

[0204] Similarly, although plane fitting is used in this example, other data processing algorithms and surface fitting techniques can be used for the calibration. For example, a more involved surface curvature fitting can be used to identify first-order tilt corrections and future surface distortions. The data processing and fitting techniques can take iterative approaches or can perform direct calculations. Any processing technique that produces one or more surface tilt or curvature metrics is sufficient for the calibration.

[0205] Processing beam wobble pattern recognition and registration

[0206] For certain types of laser processing applications, such as welding highly reflective metals (e.g., copper or aluminum), the process beam is often oscillated in a small, periodic manner to improve process results. Measurement techniques similar to those described above can be used to specifically identify aspects of the oscillation process to improve the registration of the measurement beam relative to the oscillating process beam. These measurements can also be used to influence how the measurement data is subsequently processed and analyzed. Examples of relevant aspects of the oscillation process include, but are not limited to: oscillation phase; oscillation shape; oscillation period; geometric deviations from the nominal oscillation shape; and changes in the phase change region caused by the oscillation process.

[0207] Example workflow that benefits from calibration measurements

[0208] 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 here can be implemented by interchanging the device embodiments, comparison mechanisms, and calibration types described above. Similarly, the workflows can also be implemented by using different communication methods (e.g., those described below).

[0209] Automatic calibration process and unit synchronization

[0210] Automation and even semi-automation of many of the calibration processes identified herein provide numerous benefits to the end user. Automation minimizes user interaction with the laser processing system and imaging system. This means that the end user uses fewer resources to operate the equipment (e.g., fewer trained personnel, less training, less time, etc.). Minimal user interaction also results in a reduced risk or reduced consequences of user error. User error during the system calibration process can have an impact on measurement accuracy and process reliability because the error propagates to all subsequent measurements performed by the system. Similarly, automation reduces the overall time to perform calibration and allows calibration to be performed at a time that is more convenient for the laser process or production environment, rather than at a time that is more convenient for the operator.

[0211] 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 signaling between the laser processing system and the imaging system, or it can be achieved indirectly via 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 predefined processing laser job; synchronization of the imaging system with the processing laser position; synchronization of the imaging system with the processing laser power profile; synchronization of the imaging system with a predefined processing laser job; and a predefined processing laser job that is matched to a predefined imaging system calibration job and synchronized via a common start signal. Forms of synchronization can include, but are not limited to: a common external start signal; a common external synchronization signal; user configuration of corresponding imaging and processing system jobs; a positioning signal; a power profile signal; a digital signal; an analog signal; an optical signal; an optical signal generated by the laser process itself; and common industrial communication protocols (e.g., TCP / IP, Ethernet-IP, Profinet, etc.).

[0212] Synchronization can also be used to ensure that the imaging system receives sufficient signal for calibration measurements. It can also be used to ensure that the laser processing performed for calibration does not cause too much damage to the calibration workpiece.

[0213] In addition to calibration of the processing and imaging systems, synchronization between the measurement system and the processing system / cell can also be used to compensate for degradation of processing performance over time. As the system continues to be used, general wear and tear of the laser processing station components (e.g., optics, fixtures, gas delivery systems) can slowly degrade laser processing performance. Measurements of laser processing station performance by the imaging system can be used to identify such degradation (e.g., by reducing process radiation levels) and correct system performance (e.g., increasing commanded laser power to compensate, automatically replacing system components, providing instructions to the user, etc.). Similar detection and correction can be performed for process-induced damage to the station, such as contamination of the laser head protective optics by process exhaust.

[0214] Calibration during manufacturing

[0215] During manufacture, coherent imaging systems typically undergo a calibration operation to improve the accuracy of the measurement system. For example, sub-component variability introduced, for example, due to design tolerances and their own manufacturing variability, is measured and calibrated.

[0216] Whenever possible, CI system calibration is performed using a measurement system integrated into the laser head that will be used in the laser processing application. However, due to real-world constraints, it is not always possible to integrate the CI system with the laser head before it reaches its final location. In such cases, CI calibration can be performed using the same model laser head, a similar type laser head, or an alternative calibration head. The deviation between the laser head used for calibration and the final laser head may be negligible, and no further calibration is performed during system commissioning. In some applications (e.g., those with high accuracy requirements), further calibration operations can be performed during system commissioning, as described below.

[0217] The following examples illustrate aspects of the calibration process related to integration of a CI system into a laser scanning head.The order of the calibration process described herein is not a limitation, and in some embodiments, certain calibration operations may be performed in parallel.

[0218] Before performing the calibration process, the CI system is mechanically integrated with the laser scanning head. Some laser heads (e.g., those available from IPG Optoelectronics) include ports specifically designed for CI system integration. Other laser heads may require interfacing hardware to attach the CI system to an existing inline camera (or other sensor) port. Still other laser heads may include physical modifications to the mechanical adjustment points, optical beam path, and transfer optics to accommodate a coherent imaging system.

[0219] After mechanical integration, the electrical and communication interfaces with the laser head and CI system are established. During the fully automated calibration process, the laser head, CI system, laser source, and, where applicable, the unit motion control device all communicate with each other (directly or indirectly). The component terms described in this article refer to specific pieces of hardware and the hardware controller.

[0220] During the semi-automated calibration process, some of the aforementioned communication links can be established, but the user also participates in the process, providing the missing communication and synchronization elements. For example, when the laser head and CI system are ready and ready to perform their calibration routines, the user can press a button to trigger the laser firing. The laser, in turn, provides a synchronization signal to begin laser head and CI system operation.

[0221] In a fully automated environment, communication between the laser head, CI system, laser source, and motion control subsystems is typically controlled by a device acting as a process master. The process master possesses knowledge of all aspects of the laser process (or calibration process) and understands how the subsystems should be synchronized for successful operation. In some applications, the process master can be implemented through additional hardware (e.g., a programmable logic controller (PLC), a robotic interface, an external computer / server, a smartphone, a tablet, or a microcontroller). In other applications, one of the subsystems (e.g., the laser head controller, the CI system controller, or the laser controller) can assume the role of the process master. Communication is typically achieved directly via two-way communication between the process master and each subsystem. However, in some applications, some subsystems may synchronize with other subsystems and communicate indirectly with the master. Communication can take the form of digital signaling, analog signaling, a networked communication protocol (e.g., TCP / IP), or some combination thereof.

[0222] Once the mechanical, electrical, and communications integration has been 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 has been 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 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 can include 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 own subcomponents based on the measurements. In most calibration routines, the electromechanical adjustments and subsequent measurements are performed iteratively until the resulting measurement results are sufficiently optimized or the target value is reached. Once the calibration is complete, the CI system signals the PM.

[0223] During this calibration routine, the laser head scanning functionality and laser output are not required.The PM powers down or places these subsystems in an idle state so that their operation does not affect the calibration process.

[0224] The CI system scanning module tracking error calibration can be similarly calibrated automatically. In some tracking error calibrations, only a calibration target and the CI system are performed. In these calibrations, a calibration target with specific features (e.g., sharp edges) is set below the laser head. Once the target has been set, the PM commands the system to perform its tracking error calibration. The tracking error calibration measurements can be performed as previously described and illustrated. In an automated environment, the calibration measurement results 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 sufficiently known that a non-iterative approach is sufficient. However, in other cases, the calibration measurements and scanning module behavior adjustments are performed iteratively until the tracking error is sufficiently calibrated. Once the calibration is complete, the CI system signals the PM.

[0225] In some tracking error calibration routines, the calibration target may not have specific features designed for calibration. In these cases, a laser head, a laser, a motion control device, 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 creation device to ensure that the features are created in specified locations or at specified times to allow calibration to be performed. In some cases, this may involve marking similar features on the calibration target with a laser, and communicating the locations of these features (in space and / or time) to the CI system so that the CI system knows when and where to perform its calibration routine. In cases where process radiation is used (as opposed to features that are more permanently marked on the material surface) to create the measurement signal for the calibration measurement, precise time synchronization between the laser excitation event and the CI system may be desirable to ensure that the CI system measurements are performed when the process radiation is emitted.

[0226] The PM can deliver specific process parameters to the laser, laser head, and motion control equipment as needed to enhance the signal detected by the CI system. These parameters can be known a priori, or they can be 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 about 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, the CI system uses it to more accurately synchronize the CI measurements with the positions commanded by the CI scanning module and the final measured positions on the workpiece. Instead of direct low-level control of process parameters by the PM, each subsystem can have a predefined set of process parameters (i.e., tasks) that are called by the PM in response to feedback from the CI system.

[0227] The process beam and workpiece reference frame registration can be similarly automated through synchronization between the process master and the appropriate subsystems. An automated version of these calibration routines can be implemented as follows. The PM signals the cell to automatically load the calibration target under the head. This can be achieved through an automated robotic arm or by notifying the cell operator. The cell then responds to the PM (e.g., through a response from the robot or input from the cell operator) to indicate that the target has been loaded. Once the target has been loaded, the PM signals the laser scanning head to position the laser beam to its own reference frame origin. In some cases, accurate process beam co-registration can involve an initial CI system "pre-scan" to obtain background signal levels or reference geometry. In these cases, the PM signals the CI system to perform a pre-scan measurement. Once the measurement is complete, the CI system signals the PM. Then, for example, as described above, the PM signals the laser to operate to generate a process beam representative signal for the CI system. In some embodiments, the laser is operated to ablate the calibration target, and the CI system measures the resulting ablation feature. In other embodiments, the CI system performs process radiation measurements of the laser interaction zone while the laser is operating (or immediately after the laser has been operated). The PM is responsible for synchronizing the CI system measurements (e.g., by acquiring signals such as digital rising edges) with the laser operation. Once the CI system has completed a calibration measurement, it signals the PM accordingly. The CI system uses this calibration to re-center its reference frame accordingly and performs subsequent measurements in the process beam center frame.

[0228] As described above, in addition to completing signaling calibration, the CI system can also provide feedback to the PM based on the measured signal level / quality. The PM, in turn, adjusts the laser's processing parameters to improve the measured signal level. This feedback can be provided and implemented during the calibration procedure itself or after the calibration measurements are completed, allowing the calibration process to be restarted under improved operating conditions. For some calibrations, the routine can be performed multiple times to improve the calibration results using statistical data processing (e.g., averaging) or fitting. For each iteration, the same calibration target area can be used, a new location on the target can be used, or a new target can be used.

[0229] Workpiece reference frame orientation and scale calibration is performed similarly. The PM signals the unit to load the calibration target (or shift the target to a new location). When the target is ready, the unit responds to the PM. The PM then directs the laser scan head to mark a pattern (e.g., a plus sign or similar feature) to indicate the scan head coordinate axis on the workpiece. Once completed, the scan head signals the PM, which in turn directs the CI system to perform its calibration measurements. An example of this might be a CI system performing a series of coherent measurements while its scanning module drives the measurement beam along a rectangular raster scan pattern over the marked features. The CI system data processing unit then expands the coherent measurements to map them back to a rectangular grid on the part's surface and uses image processing algorithms to identify features in the pattern marked on the workpiece surface. The CI system then calculates the orientation and scale of these features relative to its own reference frame. The CI system then uses this information to update its own reference frame calibration. Once calibration is complete, the CI system signals the PM. The CI system automatically applies these reference frame calibrations to its subsequent measurement operations and scanning module commands to achieve positioning of the measurement beam in the target reference frame.

[0230] Because multiple calibration sequences are often used by CI systems, the CI system can keep track of its own calibration status to indicate, for example, which calibrations have been performed, when certain calibrations have been performed or updated, which calibrations are still to be performed, and which calibrations will be updated in the future. Once a calibration is complete, it is applied by the CI system to perform calibration measurements for laser processing applications. The PM or unit can also keep track of the CI system's calibration status. The CI system manufacturer can use this information to identify when the system is fully calibrated.

[0231] For applications involving the integration of a CI system into a laser scan head, it is often advantageous to perform co-registration of the CI system's measurement beam with the laser beam at multiple locations throughout the laser scan head's scan field—to accommodate optical distortions (e.g., chromatic aberration). Synchronization between process beam proxy generation and CI system measurements for CI detection is achieved similarly to the synchronization described above for the process beam registration process. However, scan field calibration involves additional automation considerations to synchronize the co-registration process with various locations about the scan field.

[0232] The PM may have knowledge of the scan field calibration positions. It may have this information pre-programmed, or it may request this information from the laser scan head and / or CI system. Similarly, the laser scan head and CI system may have pre-loaded configuration settings for performing this type of calibration, and the PM can be used to synchronize machining and measurement at every position within the scan field without knowing the precise scan field coordinates.

[0233] The PM guides the laser scan head and CI system to a first position within the scan field. It then performs a sequence of operations similar to the process beam co-registration steps described above to achieve synchronization between the scan field position, laser excitation and CI measurement. Once calibration has been completed for the first position, the CI system signals the PM. The PM guides the scan head and CI system to the next position where the process repeats. This procedure is performed until the position within the scan field has been calibrated. The CI system uses the scan field calibration to implement CI system positioning corrections at various positions within the scan field. In most applications, the CI system involves signaling of the scan field position (directly from the scan head controller or indirectly via the PM) so that it can apply the appropriate scan field correction. The CI system can implement this correction using a lookup table generated by the calibration measurement or a model (e.g., fit) of the calibration measurement.

[0234] Scan head applications can also involve calibration measurements of the CI system's optical path length as a function of scan field position. In most laser scan heads, the CI system measures how the optical path length of the beam changes as the scan field position changes (for example, due to the added geometric path length as the scan head deflects the beam outward from its origin, or due to changes in the amount of focusing optics thickness the beam passes through at a given scanner position).

[0235] Similar to the scan field process beam co-registration calibration, the optical path calibration can be performed automatically. This calibration does not require synchronization with the laser. The PM commands the laser to be in an off or idle state. The PM signals the unit to load a flat calibration target or a calibration target with known surface curvature and tilt. Once the part is loaded, the unit signals the PM. The PM then guides the scan head to the specified position within the scan field and waits for the position signal from the scan head. Once in position, the PM guides the CI system to perform its calibration measurement. This measurement is usually in the form of an optical path length measurement. The measurement results are then processed by the CI system to determine the optical path deviation. In the case of known curved or tilted surfaces, the processing unit of the CI system performs a flattening correction on the optical path deviation measurement. The optical path deviation is usually measured relative to the optical path length to the workpiece at the origin of the scan field. However, other references can be used (for example, a plane defocused up to 2 mm from the scan head working plane).

[0236] 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 repeats. Once optical path calibration has been performed at target points throughout the scan field, the CI system processing module creates a lookup table or model (e.g., a fit) to implement optical path length corrections that vary with scan field position when performing process measurements. The CI system uses knowledge and synchronization of the scan field path during the laser processing application, as performed for process beam co-registration with respect to the scan field, to implement corrections. This is typically accomplished via direct or indirect communication with the PM or scan head. This communication can be performed at various times, including: before the process begins to give the CI system an opportunity to perform calculations to prepare for process measurements; in real time during the laser processing application; and after a test run of the application.

[0237] Similar calibration measurements can be performed on the scan field to calibrate for the following: optical dispersion of the measurement beam as the scan field position changes; changes in the focus of the measurement beam as the scan field position changes; changes in the scale of the workpiece reference frame as the scan field position changes; and changes in the rotation of the workpiece reference frame as the scan field position changes.

[0238] While the automatic calibration procedures are described herein in the context of a completed cell assembly, it is also possible to perform certain calibrations only in the context of relevant subsystems and submodules. For example, optical path calibration may be performed on a cell without an operational laser.

[0239] System debugging

[0240] To limit the workload during system commissioning, as much calibration as possible can be performed during manufacturing. However, due to the constraints described above, it is not always possible to integrate the CI system with the unit or laser head prior to commissioning. Similarly, while calibration may have been performed during system manufacture, it may be desirable to recalibrate certain aspects of the system due to misalignments caused by shipping and installation.

[0241] Generally speaking, the implementation of the calibration procedure during system commissioning reflects the situation or a subset of the procedure at the time of manufacturing. In this case, the process master can similarly be the controller of the CI system or end-use location. Instead of performing some of the calibration routines as a whole, automatic commissioning checks can be performed to determine which calibrations are still accurate and which need to be recalibrated. These automatic commissioning checks typically involve the same type of synchronization and communication between the various subsystems and the process master as performed for the full calibration. However, the operation is generally less time-consuming. For example, a check of the optical path calibration can involve performing optical path deviation measurements at a small subset of target points within the scan field. If these measurements are consistent with the measurements made during system manufacturing, the calibration is considered accurate. If the check fails, the optical path calibration is re-performed.

[0242] As described above, the CI system or process master can keep track of its calibration status for display to the individual commissioning the system. This can be used, for example, to inform the individual of remaining procedures, to block certain unit operations until commissioning is complete, or to provide a notification to the user that commissioning is complete. While PM can be used to automate the commissioning process, limitations in the unit's signaling and communication schemes may require some manual interaction (e.g., loading a calibration target and pressing a button while loading).

[0243] Measurement work calibration monitoring and updating

[0244] In laser processing applications, CI system measurements of phase change regions provide valuable information about the process. In many of these applications, CI measurements are performed targeting specific sub-regions of the PCR to obtain targeted process information. For example, in keyhole laser welding, it may be desirable to align the measurement beam with the vapor path created in the welded part during the welding process to measure the keyhole depth.

[0245] In many laser welding processes, the position of the steam channel relative to the process beam depends on a number of factors, including but not limited to: material type; material geometry; process speed; process power; process path; oscillation pattern; shielding gas deployment; plume suppression techniques; environmental conditions; and defocus. Typically, for a given process or sub-area of a process, a calibration is performed by the CI system to determine the position of the steam channel relative to the process beam. Once this calibration has been established, it is generally accurate for small perturbations in the process parameters and conditions described above. However, if there are large-scale changes, it may be desirable to recalibrate the steam channel position. Similar calibrations for aspects of the process parameter space can be performed in other laser processing applications.

[0246] In laser processing cells with an integrated CI system, automated workflows can be set up to perform process-specific calibrations. For example, in the case of keyhole alignment during laser welding, specific alignment jobs can be defined within the CI system. These alignment jobs can be run before each new type of process to determine the calibration. They can also be run periodically, interspersed with measurement jobs of the laser process, to verify or update the calibration as needed.

[0247] A pinhole calibration task can be a sequence of coherence measurements about the center of the process beam that targets specific coherence measurement signal characteristics (e.g., maximum depth, specific depth variation, specific depth range, specific intensity signal level, etc.). Similarly, as described above, process radiometry can also be used to perform pinhole calibration measurements. Once the measurements are performed, the CI system processing unit executes an algorithm or image recognition routine to identify the specific calibration.

[0248] In addition to updating how the CI system positions its measurement beam to measure steam channels, the pinhole calibration measurement can also be used to assign a goodness metric to the laser process's measurement work (or aspects of the measurement work). This goodness metric can be used as a confidence indicator for the output, or can be used to provide a notice to the user or process master that operation should be stopped until a new calibration is applied.

[0249] In some embodiments of the apparatus, the CI system subcomponent can allow for the execution of pinhole calibration measurements in parallel with the pinhole measurements of the laser process itself. Here, the calibration measurements do not need to be interspersed or interspersed with the target measurements of the process. Instead, the pinhole calibration measurements can be performed during the process itself and applied to the CI system in real time or near real time.

[0250] It is also possible to perform such calibration measurements alongside process measurement work using resource-sharing techniques. For example, a pinhole calibration measurement can be performed alongside the pinhole depth measurement by adjusting the target position of the measurement beam relative to the expected pinhole position at intervals for a portion of the pinhole depth measurement cycle. This signal can be compared to the nominal pinhole position signal to determine if there is improved calibration. This comparison can be based on signal characteristics such as signal strength, signal density, signal variance, etc.

[0251] Although the process may be nominally constant, periodic orifice recalibration may be desirable due to uncontrolled changes in the process environment. These uncontrolled changes include, but are not limited to: thermal changes in process equipment, mechanical wear of process equipment, contamination of laser head optics, changes in process raw materials and component specifications, process fixture changes, thermal changes in the CI system, and mechanical changes in the CI system.

[0252] In some embodiments, calibration measurements are performed by the CI system, processed by the CI system, and applied automatically by the CI system without user intervention. However, in other embodiments, physical or regulatory constraints may require some level of user interaction (e.g., to confirm an updated calibration) before a new calibration is implemented.

[0253] Automatic process maintenance, logging and user / external communications

[0254] Automated calibration measurements taken during a series of similar laser processes provide data for generating long-term statistics, analyzing process trends, and monitoring the health of the cell. For example, by collecting the intensity of the backscattered light from the workpiece surface over time, it is possible to visualize a decrease in intensity over time. As more material is processed, decreasing intensity can be associated with contamination of the laser head cover glass. Similarly, it can be associated with an aging light source or more material contamination. By setting up additional calibration target points, such as those within the laser head and below the laser head, it is possible to isolate the source of the intensity decrease. This isolation helps identify the contaminated cover glass. Once identified, the CI system can provide a notification to the user, either directly or indirectly via the process master, indicating that a new cover glass should be used. In other systems, this signal can be automatic, prompting more frequent replacement of the cover glass. This notification to the user can take the form of a visual signal on the unit (e.g., LED, display), an audio signal from the unit (e.g., chime, buzzer), or an electronic notification (e.g., smartphone app, tablet app, wearable technology, PC program, email).

[0255] Similarly, records of other calibration measurements can be used to identify subtle process changes. For example, small changes in pinhole position calibration over time can be correlated to changes in the process environment and part fixtures. These long-term trends can be used to identify the source of the changes and make corrections to the process and its equipment. Records of process beam co-registration calibration can be used to identify problems with optical components associated with beam delivery in the head. Trends can be compared over time and with other data sources to identify problems such as thermal lensing, damage to optical components, and loose optical components.

[0256] In many laser processing applications, knowledge of the process beam focus allows for adequate process results. 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 the process. Typically, the material undergoing laser processing is positioned so that its surface is at the focal plane of the laser beam to maximize energy coupling at the material surface. However, it is also common to offset the material surface from the focal plane (commonly referred to as defocusing) to distribute the laser beam energy over a larger surface area. Those skilled in the art can intentionally defocus the beam into or away from the material. Similarly, the ability to register the spatial position of the laser spot above the workpiece surface can ensure that the targeted material area undergoes processing.

[0257] On laser processing stations with an integrated CI system, it is possible to use the CI system as a guidance system to position the focal plane of the process beam relative to the surface of the part, as well as to locate the lateral (XY) position of the process beam spot above the material surface. The information provided by the CI system regarding the 3D focus of the process beam can be used by the machine operator during setup on a new part, or it can be automatically communicated to the machine controller (e.g., a PLC, robotic controller, scan head controller, etc.) to actuate optical or mechanical adjustments to position the process beam focus in a desired position relative to the workpiece. Such adjustments may be desirable for a variety of applications, including, but not limited to, high-tolerance processes (e.g., laser processing with a single-mode beam) and assembly line processes where material tolerance variations or process fixture degradation may require active compensation from one part to the next.

[0258] For a CI system used as a guidance system, the CI system can be calibrated to the process beam focal plane and lateral position. Other examples illustrated in this document outline possible techniques for calibrating to the process beam lateral position on the surface of the material. These techniques can also be applied in the following examples to calibrate the CI system to the process beam 3D focal position. For simplicity, the examples here include scanning the CI system measurement beam in a rectangular pattern around the surface of the material and using the CI system height measurement results to resolve spots on the surface of the material marked by the process beam. However, other CI system measurement modes (e.g., blackbody emission measurement) and other scanning strategies can also be employed. Similarly, the CI system can communicate directly with the process controller to automatically perform the calibration process, or user interaction may be required in some cases. An exemplary automatic calibration process is described in more detail below.

[0259] The process controller communicates with the motion control device to position the laser head relative to the calibration target or workpiece. This may involve loading the target material under the laser processing head or positioning the laser processing head in a new position to point at the target material.

[0260] 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 parameters of the process may be predefined by the controller, preset within the laser itself, or obtained through communication with the CI system. Once the spot has been formed, the process controller commands the CI system to perform its calibration operations. The CI system may perform a rectangular (or other pattern as disclosed herein) scan over the surface of the part to capture a demarcated area on the surface of the workpiece, such as Figure 25 As shown in the image.

[0261] Using the contrast detection algorithm described in this paper, the CI system uses the imaging data it collects to identify the laser spot diameter (representative), e.g. Figure 25 Although speckle appears in the image, the detection algorithm is resistant to the effects of speckle, and the speckle size can be reduced by increasing the numerical aperture of the CI system's beam delivery system.

[0262] Once the CI system calibration measurements are acquired, the CI system processes the information to automatically calculate quantities about the demarcated speckle, such as its center location and diameter. Note that by fitting or centroiding the peaks on the CI system's axial measurements, the effects of speckle and quantization noise in the image can be reduced. In some embodiments, a safety software feature allows manufacturers to limit the axial resolution available to end users to better comply with government regulations regarding device performance.

[0263] Further extensions of this procedure include iterating the CI system scan to tile different search areas and / or modify the scan area to better locate the spots. Similarly, if necessary, the CI system can iteratively communicate with the process controller (or directly with the laser) to excite additional spots to enhance contrast or provide more precise time synchronization to the laser processing event.

[0264] The spot XY center position is used as a proxy for the lateral position of the process beam on the surface of the workpiece. The spot diameter can be used as a proxy for the process beam diameter at the surface of the material, although more indirectly, because process heat effects typically produce marks on the material surface that are much larger than the process beam spot size.

[0265] A further extension of this calibration procedure consists in performing a series of this type of calibration at varying laser head offset distances (ie, the distance between the laser head and the workpiece surface).

[0266] This series of measurements generates a spot (representative) diameter that varies with offset distance. During such measurements, the offset distance can be communicated to the CI system via the process controller, or can be measured directly by the CI system itself (via its height measurement capability). For example, the CI system height measurements near the perimeter of the rectangular scan area (i.e., the area that does not contain the laser marking spot) can be averaged to produce a CI system measurement of the offset distance to the workpiece surface.

[0267] The spot (representative) diameter is then analyzed as a function of offset distance to determine the focal plane of the laser beam. In some cases, this analysis can include finding the offset distance associated with the minimum spot diameter. In some cases, fitting this data to a function (e.g., a Gaussian beam width equation) can be used to increase calibration robustness.

[0268] In other calibration routines using different laser processes, other metrics that vary with offset distance can be used to identify the focal plane. Examples may include: maximize blackbody radiation intensity, maximize spot depth, maximize spot size, maximize spot height variation, minimize spot height variation, maximize blackbody radiation, maximize blackbody emitter diameter, locally maximize blackbody emitter diameter, minimize blackbody emitter diameter, maximize spot symmetry (minimize astigmatism), minimize laser-scribed feature width, and maximize laser-scribed feature width.

[0269] The same spot (representative) diameter as a function of the offset distance data can also be used to generate a representative for the process beam caustics. This information can be further stored or analyzed by the CI system to generate an indication of beam quality. The information can also be transmitted to the laser unit or user for external analysis. Beam quality analysis is useful during laser and unit commissioning operations to ensure proper functionality. Similarly, periodic beam quality measurements and analysis or measurements and analysis after expected damaging behavior (e.g., spatter generation on the laser head cover glass, collision with mechanical systems) can be used to verify ongoing beam delivery quality and / or identify problems. Similar measurements and analysis can be used to identify suboptimal beam delivery performance (e.g., thermal lensing effects that cause focus position drift).

[0270] It should be noted that for certain energy beam parameter and material combinations, the minimum in the apparent spot (representative) diameter can exist on one or more sides of the actual process beam focus minimum. This is because, at highest intensity (tightest focus), some processes will initiate pinhole (vapor capillary) mode sooner and therefore absorb more total energy, resulting in greater heating and, therefore, a larger apparent spot (representative) diameter.

[0271] Regular measurement of the process beam calibration spot can also be used over time to track cell 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 with process effluent and produce beam delivery issues over time. This may manifest as a smaller marker spot diameter or, in the case of a blackbody, a weaker blackbody emitter. Similarly, trends in the recorded metrics may be observed due to loose, damaged, or suboptimal beam delivery optics. This information can be presented directly by the CI system (e.g., via an alert) or can be communicated to the CI system user or laser cell controller for external analysis.

[0272] All of these methods apply variations on the principle that the same energy applied to the same material with the same temporal power distribution should produce very similar results. If any of these measurements of surface markings vary substantially over time, those variations are likely a proxy for performance variations in the laser source and beam delivery system.

[0273] The process beam calibration procedure can be further combined with other CI system calibration measurements to reduce the number of steps or the overall time required for multiple calibrations. For example, an unprocessed area at the perimeter of the rectangular scan area height map can be used to calibrate for workpiece tilt and optical tilt introduced by different beam delivery paths (particularly associated with beam scanning laser heads). Similar recording of these additional CI system calibration metrics can be used to identify additional aspects of cell health and process issues (e.g., wear on cell fixtures).

[0274] During some CI system calibration procedures, it may be sufficient to use readily available materials as calibration targets (e.g., sheet metal). For some CI system calibrations, the calibration target may be a workpiece undergoing laser material processing (i.e., a pre-production part) or a workpiece that has already undergone material processing (i.e., a post-production part). For other CI system calibrations, a manufactured calibration target specific to the calibration may be required.

[0275] In some embodiments, CI system and / or camera data is used to identify the direction of travel of the motion system (e.g., a robot, linear axis, or linear conveyor system) relative to the head. Similar CI system measurements can be used to identify workpiece placement, workpiece orientation, fixture placement, and fixture orientation relative to the head. This allows for accurate calculation of so-called "flying welding," or coordinated motion operations.

[0276] Finally, in some embodiments, the calibration target is rigidly attached to a non-workpiece object, which will allow the position and orientation of the non-workpiece object relative to the beam delivery and motion control (eg, robotic) system.

[0277] CI system health monitoring

[0278] In addition to being used to identify process and laser unit problems, automated calibration measurements can also be used to monitor the status and health of the CI system itself. Calibration measurements can be performed periodically for the express purpose of determining the operational status of the CI system. Calibration measurements can be performed for the express purpose of calibrating another aspect of the CI system, but can also be used or reused to confirm CI system health or changes in CI system conditions.

[0279] In general, repeated process beam alignment or other calibration groupings that are too wide may be an indicator of inadequate laser system or CI system performance, or a deficiency in the automated alignment process. In any of these events, embodiments of the present disclosure can utilize thresholds on the distribution of automatically calculated alignments as triggers for notifications or external signals to request assistance or halt further processing until the fault condition can be remedied.

[0280] Backreflection measurements from a calibration target within the head can be used to identify aging light sources. The declining light source output power trend can then be fed back to the CI system to increase the light source drive current, or similarly command a higher nominal output power, to achieve similar output power levels as performance degrades over time.

[0281] Similar calibrations can be used to detect and implement corrective actions for: conditions of the coherent system reference optical path (e.g., changes in length, refractive index, transmission, dispersion, etc.); conditions of the coherent system sample optical path (e.g., changes in length, refractive index, transmission, dispersion, etc.); spectral misalignment or calibration problems in the CI system detector; and detection of spectral instabilities in the CI system light source.

[0282] In the examples above, one or more corrective actions can be used to correct the problem and allow the CI system to continue operating normally. However, in other cases, a corrective action may have already been applied and can no longer be applied (e.g., maximizing the light source drive current), or no corrective action can be applied. In such cases, the corrective action can be providing a notification to the process master and / or user indicating that the system is no longer operating properly and requires repair or replacement. In some instances, this signal can be used to switch to a backup CI system or backup system subcomponent.

[0283] Monitoring trends in system calibration data can be collected and analyzed to establish statistics on calibration stability and to generate probabilistic failure mode and effects analysis (FMEA) data. For example, in laser keyhole welding, keyhole alignment calibration data can be used to determine the alignment stability for a specific laser welding process. The effect of small changes in alignment on the measurement quality of the CI system can be characterized and used to determine the desired recalibration frequency. Welding processes that are very sensitive to keyhole misalignment may require more frequent recalibration. Similarly, welding processes that are very sensitive to keyhole misalignment are generally more unstable processes. In this case, the keyhole alignment calibration data can be used as a proxy for the stability of the welding process and can be used to determine the process quality and the need for further optimization. When the CI system determines that a specific recalibration is desired or that the calibration is no longer accurate, it can automatically perform a calibration routine, or it can provide an indication to the unit or user that such a procedure should be performed for continued operation.

[0284] Machine-readable code recognition

[0285] In many industries, it is useful to include part identification markings (such as barcodes or QR codes) on the surface of products, components, and subassemblies. Such identification markings can be used for inventory tracking, product end-use tracking, provenance, and security marking.

[0286] In some applications, CI systems can be utilized for such identification mark scanning. CI system calibration can be used to implement the correct measurement beam scanning strategy and measurement processing algorithm (or configuration) to reliably identify identification marks. For example, calibration can be used to accommodate different material backgrounds, identification mark colors, identification mark heights / depths, identification mark positions, and identification mark sizes. In cases where these marks are part of a quality control system, the CI system will be able to capture the mark data and correlate it with other CI system measurements taken before, during, and after measuring the mark.

[0287] CI systems can also be used as part of the process for generating such identification marks. For example, a CI system can be used to measure the mark and determine whether it has sufficient depth / height or sufficient color change. The output from the CI system can be fed back during the identification mark processing phase to modify the marking process. A common implementation of this process is in the context of laser-generated identification marks (e.g., laser marking or laser scribing processes). The laser that marks the workpiece can be the same laser that performs the laser processing of the workpiece. Marking can occur within the context of the laser processing environment or can be performed before or after the process itself.

[0288] In some embodiments, a CI system can be used as an identification mark reader. It may be beneficial for some identification marks to be invisible to traditional readers (e.g., barcode readers, cameras, LED scanners). For example, these marks can be used to conceal product identifiers for security or aesthetic purposes. The CI system can be used to guide the laser process when generating such marks and provide feedback on the quality of the generation process. Such marks can be generated by marking features (e.g., sub-micron features) on the surface of the material that are primarily suitable for CI measurement detection. For optically transparent or translucent materials (at the color of the CI measurement beam or process beam), such features can be marked below the surface of the material. Subsurface features may be useful for marking plastics, glass, or semi-precious and gemstones. When reading these features, the CI system will optionally correct for the part's apparent tilt and / or optical path length distortion from the optical medium (e.g., lens, air, water, oil, etc.) through which the CI system's measurement beam passes.

[0289] In an embodiment, a pulsed laser or stamping is used to very accurately construct a 3D QR code or barcode in relief on the surface of a workpiece, but the height variation between the negative and positive portions of the pattern is so small (10 μm or less, or 50 μm or less), and the transitions are so gradual, that they cannot be read by conventional readers, or perhaps even discernible by the naked eye. If sufficient control of the engraving / stamping system is possible, it may be possible to encode multiple value levels in each lateral position of the 3D QR or barcode, thus significantly increasing the data density that can be produced per unit surface area on the workpiece.

[0290] Optionally, the code can be covered to protect it from scratching 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 producing these features will make them useful in anti-counterfeiting applications.

[0291] Although the principles of the present invention have been described herein, it will be understood by those skilled in the art that this description is by way of example only and is not intended to limit the scope of the invention. In addition to the exemplary embodiments shown and described herein, other embodiments are contemplated as being within the scope of the invention. Modifications and substitutions by those of ordinary skill in the art are considered to be within the scope of the invention, and the scope of the invention is not limited except by the appended claims.

Claims

1. A calibration system comprising: 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 coherent imaging measurement system comprising a coherent imaging core unit for generating a measurement beam and for producing an interferometric measurement output, and a coherent imaging scanning module for delivering the measurement beam to the target, wherein the coherent imaging measurement system uses the interferometric measurement output to produce a coherent imaging measurement output and further uses a contrast mechanism provided by detection in the coherent imaging measurement system to produce a calibration measurement output; and at least one controller configured to receive the coherent imaging measurement output from the coherent imaging core unit for use in monitoring and / or controlling the material processing system, wherein the controller is further configured to receive the calibration measurement output from the coherent imaging measurement system and is configured to control the coherent imaging measurement system based at least in part on the calibration measurement output received from the coherent imaging measurement system, wherein the controller is configured to control the coherent imaging measurement system based on the calibration measurement output received from the coherent imaging measurement system to modify future measurements performed by the coherent imaging measurement system for alignment of the coherent imaging measurement system and the processing beam.

2. The calibration system according to claim 1, wherein: The coherent imaging measurement system detects the interferometric measurement output to provide the contrast mechanism used to generate the calibration measurement output from the coherent imaging measurement system.

3. The calibration system according to claim 1, wherein: The coherent imaging metrology system detects process radiation produced by the machining beam to provide the contrast mechanism used to produce the calibration measurement output from the coherent imaging metrology system.

4. The calibration system according to claim 1, wherein: The material processing system includes a laser processing system.

5. The calibration system according to claim 4, wherein: The laser processing system includes a laser having a substantially single spatial mode with an M less than 2.

0. 2 The output of the value.

6. The calibration system according to claim 1, wherein: The coherent imaging measurement system includes an inline coherent imaging system.

7. A method for calibrating a coherent imaging measurement system, comprising: providing a material processing system configured to generate a processing beam and deliver the processing beam to a target; providing a coherent imaging metrology system configured to generate a measurement beam and transmit the measurement beam to the target, and configured to provide a coherent imaging metrology system output for controlling and / or monitoring the material processing system; obtaining a calibration measurement output from the coherent imaging measurement system, wherein the calibration measurement output is produced by the coherent imaging measurement system using a contrast mechanism provided by detection in the coherent imaging measurement system; and and automatically controlling the coherent imaging metrology system based at least in part on the calibration measurement output obtained from the coherent imaging metrology system, wherein the coherent imaging metrology system is controlled to modify future measurements taken by the coherent imaging metrology system for alignment of the coherent imaging metrology system and the processing beam based on the calibration measurement output obtained from the coherent imaging metrology system.

8. The method according to claim 7, wherein: Obtaining the calibration measurement output from the coherent imaging measurement system includes detecting an interferometric measurement output of the coherent imaging measurement system to provide the contrast mechanism used to generate the calibration measurement output from the coherent imaging measurement system.

9. The method according to claim 8, wherein Obtaining the calibrated measurement output includes: at least temporarily producing a physical modification in the target using the processing beam; obtaining a measurement result of the physical modification portion using the coherent imaging measurement system; and The calibration measurement output is obtained from the coherent imaging measurement system such that the physically modified portion corresponds to a position of the machining beam.

10. The method according to claim 9, wherein: Obtaining the measurement includes scanning the measurement beam across the target adjacent the physical modification in a scanning pattern.

11. The method according to claim 7, wherein: Obtaining the calibration measurement output from the coherent imaging metrology system includes detecting process emissions produced by the processing beam in the coherent imaging metrology system to provide the contrast mechanism used to produce the calibration measurement output from the coherent imaging metrology system.

12. The method according to claim 11, wherein Obtaining the calibrated measurement output includes: Reduce the light source of the coherent imaging system to at least a negligible level; directing the processing beam toward the target; receiving process radiation from the target into the coherent imaging measurement system when the processing beam is directed at the target; and The calibration measurement output is obtained from the coherent imaging measurement system that detects the process radiation.

13. The method according to claim 12, wherein: The coherent imaging measurement system detects the process radiation while the coherent imaging measurement system scans the target.

14. The method according to claim 12, wherein: Directing the processing beam includes moving the processing beam over the target and using the detected intensity of process radiation to determine a position of the processing beam.

15. The method according to claim 7, wherein: Obtaining the calibration measurement output includes directing the machining beam toward the target while at least pulsing the machining beam.

16. The method according to claim 7, wherein Modifying future measurements performed by the coherent imaging measurement system includes at least one of modifying spatial positioning during measurement acquisition, modifying time synchronization during measurement acquisition, and modifying a measurement algorithm.

17. The method according to claim 7, wherein: Modifying future measurements performed by the coherent imaging measurement system includes modifications to correct for optical distortions introduced by the material processing system and / or a beam delivery system in the coherent imaging measurement system.

18. The method according to claim 17, wherein Modifying future measurements performed by the coherent imaging measurement system includes modifications performed where the optical distortion includes at least one of: optical path length variation, chromatic aberration, defocus, field curvature, image distortion, spherical aberration, coma, and astigmatism.

19. The method according to claim 7, wherein: Modifying future measurements performed by the coherent imaging measurement system includes modifications to correct for mechanical distortions of the material processing system and / or the coherent imaging measurement system.

20. The method according to claim 19, wherein Mechanical distortion includes at least one of: vibration, assembly, torsion, extension, compression, translation, and rotation.

21. The method according to claim 7, wherein: Modifying future measurements performed by the coherent imaging measurement system includes modifications to correct for a process synchronization problem selected from the group consisting of process synchronization problems, the group consisting of at least one of: motion control path synchronization, machining laser energy distribution synchronization, workpiece distortion, process input, shielding gas, cover gas, process environment, process feed material, process sacrificial material, and process fixture.

22. The method according to claim 7, wherein Modifying future measurements performed by the coherent imaging measurement system includes the modification to correct for changes in the phase change region.

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