WO2019028452A1

WO2019028452A1 - Material layer detection and processing

Info

Publication number: WO2019028452A1
Application number: PCT/US2018/045347
Authority: WO
Inventors: David Squires; Andrew Budd; Joseph Dallarosa
Original assignee: Ipg Photonics Corporation
Priority date: 2017-08-04
Filing date: 2018-08-06
Publication date: 2019-02-07

Abstract

Material layer detection and depthwise layer processing is disclosed. A depthwise material profile is generated based on detected optical characteristics of layer material and laser processing is controlled based on a comparison of the depthwise material profile and measured optical properties of a layer. Laser processing may be controlled to avoid damage to underlying material. Laser process parameters may be modified based on measured optical properties of a layer measured to provide optimized laser processing.

Description

MATERIAL LAYER DETECTION AND PROCESSING

CROSS-REFERENCE TO RELATED APPLICATIONS This patent application claims priority under 35 U.S.C. § 1 19(e) to U.S. Provisional Application No. 62541,282 titled "MATERIAL LAYER DETECTION AND PROCESSING," filed on August 4, 2017, which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The field of the invention is generally directed to laser-based cleaning and overlayer material stripping processes. More particularly, the invention relates to detection and removal of single layers and multiple layers of material from a substrate.

Background of the Invention

High power lasers are useful for cleaning materials, for example removing debris or oxidation from a variety of substrates. Likewise, high power lasers are useful for selectively removing one or more layers of material disposed on a substrate, such as an overlayer, a layer, or strata that overlies the substrate. For example, high power lasers are used in paint stripping applications.

In conventional layer removal applications, such as paint removal (e.g., stripping, depainting, de-coating), chemical and mechanical removal processes are used with many undesirable aspects such as relatively slow removal rates (ft²/hr), high water use, potential carcinogen release, attendant requirements for Personal Protective Equipment (PPE), and masking of areas for material removal. With high water use, a large waste volume is generated, which increases the cost and complexity of waste disposal. In comparison with chemical and mechanical techniques, laser based material removal can minimize water use, reduce waste volume, improve waste extraction efficiency and minimize hazardous material exposure.

The increased availability of high power laser sources has made laser based material removal a viable and affordable process. In the past, high power CO2 laser systems with wavelengths at or near 10 microns were used for paint removal. C0₂ lasers are capable of removing organic material, are insensitive to visible color variation, and have low damage potential to metallic substrates. However, for depthwise removal of selected layers, and for removal of layers overlying composite substrates, C0₂ lasers are relatively indiscriminate with regard to material removal. Penetration depth can be limited due to the shallow surface ablation depth, and multiple passes may be needed to achieve the required material removal depth.

In contrast to C0₂ lasers, high power fiber lasers that operate at shorter wavelengths near

1 micron are capable of providing color selectivity and increased depth penetration, particularly with transparent resin-based overlayer materials. Depth penetration of these types of lasers is also greater than that achieved by C0₂ lasers, and therefore removal rates are increased such that in some instances thicker layers can be removed in a single pass.

In certain instances, the substrate has a high laser damage threshold relative to the ablation threshold of material deposited on the substrate that is to be removed. Therefore, substrate damage can be avoided when the laser fluence remains below the laser damage threshold of the substrate. In this case, a laser processing window can be determined, which includes a range of laser processing parameters that can be used that provide not only the minimum fluence needed to remove selected layers of material, but also do not risk damage to the substrate. The laser processing window functions to provide a suitable range of laser processing parameters, where the substrate effectively provides a depthwise stop to material removal, thereby providing effective removal of complete overlying strata. Non-limiting examples of laser processing parameters include pulse energy and pulse duration, pulse width, spot size, the pulse repetition rate, the wavelength of the laser beam, the energy density (fluence) of the laser beam, the number of laser pulses (shots) applied to the material sample, and the beam profile (e.g., Gaussian, "top-hat" or flattened).

In some instances, the substrate laser damage threshold may be on the same order of magnitude or may be below the laser ablation threshold of the overlying material. The laser processing window may therefore be small or essentially non-existent, and there is an attendant risk of substrate damage when removing any overlying material. Thus, material removal processes requires careful control to ensure that material is removed without unacceptable substrate damage. Preferably, the material is completely removed without any substrate damage. Paint removal from a composite substrate material like fiberglass is one example where the laser power and ablation processes may need to be controlled to avoid substrate damage. Substrate coatings such as paint can be built up layer by layer with different materials forming strata. The resulting strata structure includes the layers of overlying material and the underlying substrate. Material to be removed may include one or more of the layers of different materials overlaying the substrate. When material is selectively removed depthwise, the removal may include a subset of the layers deposited on the substrate. For example, it might be desirable to leave a primer paint layer intact when removing a topcoat layer. It will be appreciated that layers of different materials may have different properties and that layer identification is an important aspect of process control to differentiate a material for removal from an underlying material that is not to be damaged or removed.

By way of example, the cleanLASER system by C 1 ean- Lasersy steme GmbH

(Herzogenrath, Germany), is directed to selective paint removal applications, where thin paint layers are laser stripped layer by layer. As stated at http://www.cleanlas_.er.de, "Top coats can be laser removed to expose base material primers," and applications for the system include

"Aerospace: Paint-removal while maintaining primer." In another example, General

Lasertronics Corporation (GLC) in San Jose, CA (USA), provides closed loop paint stripping systems. The Lasertronics web site, http://www.lasertronics.com, states that "Our pulse-by-pulse closed-loop control technology uses powerful off-the-shelf industrial lasers to strip coatings quickly and safely. Our color-selective workhead allows stripping directly to the substrate or to intermediate layers." US Patent No. 7,633,033, assigned to GLC, discloses illuminating target material in an optical path that is independent from a detection optical path, and the detected illumination is used to sense color.

In yet another example, SurClean of Wixon, MI (USA), demonstrates a robotic, scanned laser depainting system in the video at https :/,· www .youtube. com/watch? v-2rW V1G IXaU. which depicts paint being stripped down to a yellow primer layer. The www.surclean.net website states that "Surclean, Inc.'s laser process controller allows for precise removal of individual coating layers in surface treatment applications. The process controller can differentiate between top coat and primer in paint removal applications and adjust laser power to exclusively remove the top coat while leaving the primer intact, for example." US Patent No. 9,573,226, assigned to SurClean, describes a coating removal process that utilizes first and second streams of electromagnetic radiation. Some systems use camera or lens-based image acquisition to acquire target images and identify target materials. For example, a lens-based imaging system may form an image of a target area and features of the target area such as color may be used to identify the target material. However, image acquisition and processing speeds can be camera limited, and the effective frame rate may be less than a laser processing repetition rate. As a result, many laser processing periods may elapse before adjacent images are acquired. Thus, the sampling rate afforded by camera detection may be insufficient to provide high resolution material

identification for process control for certain applications.

With regard to spatial resolution associated with camera or lens-based imaging systems, a relatively large target area size may be needed to identify a target area of material, and this target area size may result in an imaged pixel size that is large with respect to the laser processing spot size. With large pixels, the imaging resolution of the camera-based system may be inadequate to accurately identify material and verify that material removal is complete. With color cameras using Bayer filters, fine features of discontinuous material layers may not be adequately represented in the resulting interpolated color images, which further limits the effective camera resolution. Other well-known imaging limitations, such as non-planar and inclined surface imaging can further degrade imaging resolution. Thus, camera-based imaging can be limited and fail to resolve materials on the order of a laser processing spot.

Resolution on the order of a laser processing spot size at laser processing pulse rates is a demanding requirement for fast and accurate material identification. Improved high-speed, high- resolution material identification is of interest to provide high-speed fiber laser based selective material removal.

BRIEF SUMMARY OF THE INVENTION

In at least one embodiment of the present invention, a method of laser processing one or more layers of material of a stratified workpiece at target locations is provided. Reference optical properties are measured and a depthwise strata profile is generated. A laser processing beam removes material during sequential processing periods. First and second optical properties of a stratum are measured at a target location, and compared to the first and second optical properties of the strata profile. A control signal is generated based on the comparison, the laser processing beam is attenuated in response to the control signal, and laser damage to underlying material is reduced.

Laser processing may remove one or more paint layers, the substrate may be a structural composite, the optical reference properties may be any of diffuse reflectance, specular reflectance, bidirectional reflectance, scatter, polarization, photometric, radiometric, and spectrometric properties. The target location may be proximate to the laser processing interaction zone, and precede or follow the laser interaction.

In other embodiments of the present invention, a laser based paint stripping system is provided. A fiber laser generates a laser processing output, a beam delivery system delivers the laser processing output to a laser spot on a work piece, a first test laser having a first test wavelength, and a second test laser having a second test wavelength illuminate a target location. First and second detectors responsive to the first and second test laser wavelengths generate respective first and second detector signals. The system provides means for moving the laser spot relative to the work piece, a data storage medium with at least one stored readable strata profile, a controller that reads the strata profile, compares the first and second detector signals with the strata profile, determines a stratum based on the comparison, compares the stratum with the strata profile, and attenuates the laser processing output when removal is complete.

The system may include an effluent removal system and a vehicle, robot arm, or gantry to transport the system or part of the system relative to a painted structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the disclosure will become more readily apparent with the aid of the following drawings, in which:

FIG. 1 is graph of reflectivity data obtained from a spectrophotometer for two different overlying layers of material disposed on a substrate in accordance with one or more aspects of the invention;

FIG. 2 is a schematic of intact strata deposited on a substrate that is to be treated by one example of a laser processing system in accordance with one or more aspects of the invention;

FIG. 3 is a schematic of a first stratum of the strata of FIG. 2 removed in accordance with one or more aspects of the invention; FIG. 4 is a schematic of the first strata and a second strata of FIG. 2 removed in accordance with one or more aspects of the invention;

FIG. 5 is a schematic of all overlying strata of FIG. 2 removed from the substrate in accordance with one or more aspects of the invention; and

FIG. 6 is a schematic of one example of a raster laser scanning application in accordance with one or more aspects of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to embodiments of the invention. Wherever possible, same or similar reference numerals or letters are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. For purposes of convenience and clarity only, directional (up- down, etc.) or motional (forward/back, etc.) terms may be used with respect to the drawings. The term "couple" and similar terms do not necessarily denote direct and immediate connections, but also include connections through intermediate elements or devices.

High power fiber lasers can deliver very good beam quality, which means that the fiber laser output can be collimated, scanned and focused to a spot with substantial scan field size and working distance between the work surface and the scanning optics. For paint removal, and particularly high volume paint removal, a long working distance configuration allows for efficient extraction of effluent generated during the removal process and protection of the scanning system from contamination. According to some embodiments, the system includes an effluent removal system configured to remove the effluent.

Fiber laser power for paint removal can be in the range of hundreds of watts to tens of kilowatts with convenient optical fiber cable delivery and high wall plug efficiency. The fiber laser may be, for example, Q-switch Ytterbium pulsed fiber laser systems (YLP), such as the YLP-HP and YLPN series available from IPG Photonics in the 100 watt to 1000 watt power range (IPG Photonics Corporation, Oxford, MA (USA)). This power range is merely an example and a working average power range will depend on the materials to be removed, the substrate properties, and the removal rate required. For example, faster material removal rates over large areas may be achieved with 10's of kilowatts and commensurate beam delivery system components. Although the discussion herein refers to paint as the material to be removed, other materials and removal applications are also within the scope of this disclosure. For instance, the fiber laser removal processes described herein may be used for cleaning applications of molds or tools, such as tire molds, composite part molds, baking molds, injection molds, dies, rubber seal molds, etc.

The material removal process is generally thermal in nature and heat of combustion of materials may increase the overall removal rate. Temporal properties of delivered laser energy promote a thermal removal process and may be configured as continuous wave or pulsed, with pulse widths having values that extend down to the nanosecond regime.

According some embodiments, the fiber laser output may be collimated with a collimator focal length up to 170 mm and scanned with a galvanometer based scan head with an

approximately 33 mm aperture. The scanned beam can be focused with a 254 mm scan lens to form a spot size of 2 mm or less. For example, the scan head may be IPG Photonics model IPG 2D High Power Scanner and the scan head may be positioned relative to the work piece and configured with a gantry or robotic arm positioning system. These exemplary optical system parameters are considered non-limiting and routine optimization for certain applications may indicate a different collimator focal length, scan aperture and scan lens focal length to achieve a spot size, scan field size, and working distance. Other types of beam delivery include spinner type fast-axis scanning and other raster capable beam delivery systems. The scanning system may translate the laser spot at a high speed, for example up to 45 m/s.

To meet the needs for laser based overlayer material removal, methods and systems that provide high resolution material detection are provided. In at least one aspect of the invention, target material layers are characterized and a characteristic difference or differences between layers or between a layer and the substrate are identified. Preferably, these differences provide robust and reliable differentiation between materials to be removed and materials not to be removed. The material characterization is used to determine one or more material comparisons that are used to identify material to be removed or to identify completion of a material removal process. According to various aspects, differences in spectral, polarization, and scattering properties may be used for purposes of material identification and process control. Non-limiting examples of optical reference properties that may be used to characterize the layers of material include diffuse reflectance, specular reflectance, bidirectional reflectance, scatter, polarization, photometric, radiometric, and spectrometric properties.

A wide range of materials with diverse optical properties are contemplated for removal with the present invention. A variety of illumination parameters may be used to characterize material including spectrum, polarization, and material incidence. Further, it is recognized that material properties may change during processing, and material characterization may include unprocessed material as well as representative processed and partially processed material samples. Material samples may be characterized with conventional tools, such as reflectometers, spectrometers, spectrophotometers, polarimeters and the like.

In at least one embodiment, layer materials and the substrate are characterized using a spectrophotometer by reflecting light off respective layers over a range of wavelengths, such as a wavelength range from 200 nm to 2000 nm. In some instances, materials may be characterized at multiple incident angles. This information may be used to generate reference optical properties for each of the materials and may be stored in a database that can be accessed by a controller, as described in further detail below. In some embodiments the reference optical properties are generated before the laser removal process begins.

In accordance with at least one embodiment, multiple wavelength channels may be used for purposes of material comparison based on the stored reference optical properties. For instance, low and high detection wavelength channels may be selected based on measured layer to layer contrast to enhance layer identification selectivity. For example, a low wavelength channel may be a band centered at 405 nanometers and a high detection channel may be a band centered at 980 nanometers with respective 45 degree angles of incidence. FIG. 1 is a graph illustrating reflectance data obtained from a spectrophotometer for two different layers (denoted Layer 1 and Layer 2 in FIG. 1 ) across a wavelength range from 380 mm to 980 nm. The reflectance data indicates contrast variation between the two layers at 405 nm (i.e., a low detection wavelength channel), where layer 1 has relatively high reflectivity and layer 2 has relatively low reflectivity. In contrast, at 980 nm (i.e., the high detection wavelength channel), both layer 1 and layer 2 exhibit relatively high reflectivity. However, at 830 nm, layer 2 has relatively high reflectivity and layer 1 has relatively low reflectivity. As explained further below, two or more of these wavelength channels may be used for purposes of material identification and/or comparison. For purposes of identifying material for the removal process, two or more wavelength channels may be selected based on the spectral response (e.g., reflectivity) of each material and the ability of the wavelength channel (s) (in combination) to distinguish between the two materials. For instance, the material characterization information may be used to set a unique combination of control limits for detected low and high band radiation such that a layer material is uniquely identified by detected radiation falling within both the low band control limit range and the high band control limit range. For example, a first layer may have relatively high and consistent reflectivity at 405 nanometers and 980 nanometers, whereas a second material may have relatively low reflectivity at 405 nanometers and high reflectivity at 980 nanometers, as described above in reference to Layers 1 and 2 of FIG. 1. Thus, each material is uniquely characterized by the high and low channels and signals falling within or outside respective control limits. As will be appreciated, more than two wavelength channels (e.g., three, or more) may be used for purposes of material characterization. In some instances, a user may review the reference optical properties for different materials used in a particular removal application and select the respective wavelength channels and control limits. In other instances, a controller (e.g., computer processor) may be programmed or otherwise possess the ability to review the reference optical properties and determine appropriate wavelength channels and control limits.

According to various embodiments, the choice of wavelength channel may also depend at least in part on the availability or capability of providing an illumination/light source (discussed further below) having the particular desired wavelength. In addition, the light source(s) used for measurement and/or for processing cannot interfere with the operational aspects of the measurement and or removal process. For instance, if the processing laser has a different wavelength than a measurement (test) laser, and the processing laser illuminate the measurement area at the same time as the measurement laser, then the measurement data will not be accurate. As discussed further below, in certain instances laser processing control parameters may be put into place (i.e., used) to avoid this type of situation.

In accordance with certain aspects, sample areas may be measured in a laser processing system. These measurements may be used in combination with the reference optical properties for identifying materials in the removal process. Material properties can be measured as part of a processing job set-up procedure (e.g., to accumulate the reference optical properties), intermittently between processing operations, and or during a processing operation, or after a processing operation. During a laser processing application, light sources used for purposes of measurement may be supplied from a number of different sources, including auxiliary illumination (i.e., laser light source(s) other than the processing laser light source) that can be used to illuminate the workpiece at wavelengths corresponding to the low and high detection channels, illumination provided directly by the processing laser source, as well as collateral illumination supplied, for example, by plasma generated during material removal.

In at least one aspect of the invention, and referring now to FIG. 2, fiber laser system 20 generates laser output 21 and laser processing head 22 delivers laser processing output as laser processing beam 23 to strata structure 10, which includes layers of overlying material and an underlying substrate. A first test beam source 24A (e.g., an auxiliary illumination source as described above) illuminates target material 25 with first test beam 26A at a first test wavelength (e.g., 405 nm), and a second test beam source 24B {e.g., another example of an auxiliary illumination source as described above) illuminates target material 25 with second test beam 26B at a second test wavelength (e.g., 980 nm). The target material reflects a portion of the first test beam 26A to generate a first test illumination 27A (e.g., spectral response such as reflectivity) that is detected by detector 29A, which is configured to generate detector signal 28A based on the detected first test illumination 27A. In addition, the target material reflects a portion of the second test beam 26B to generate a second test illumination 27B that is detected by detector 29B configured to generate detector signal 28B based on the detected second test illumination 27B. A plurality of test beam sources may therefore be used to illuminate target material, where the target material reflects a portion of each of the test beams, and respective detectors detect this reflected light as test illumination and generate a corresponding plurality of detector signals. For example, a third test beam could be used. In addition, the processing laser beam may also be used for illuminating the target material for purposes of obtaining reflection or other spectral response data. For instance, the processing laser beam may be operated in pulsed mode, where the test beam configured at a first desired wavelength may be applied for purposes of obtaining measurements at the first desired wavelength in between the pulses of the processing laser (or in other regimes where the processing laser is turned off) and reflection data from light applied by the processing laser, configured at a second desired wavelength, may be used to obtain measurements at the second desired wavelength (e.g., either during the pulses or at other regimes where the processing laser is applied and the test beam is turned off). It will also be appreciated that a single detector, responsive to illumination from more than one test beam may be used in place of multiple detectors. Likewise, a broadband illumination source may illuminate target material at multiple wavelengths for detection and generation of multiple detection signals.

The material removal process is controlled by a controller based at least in part on the detected test illumination signal(s). As described above, selection of the test illumination source(s)may be determined based on a stored reference material characterization. A

comparison of the respective detector signals and a stored strata profile can provide the basis for a processing control parameter. The processing control parameter may also direct the test beam laser source to vary the spot size of the test beam, and/or have the test beam scan with the ablation beam.

The processing control parameter can be also be used to prevent undesirable

(underlyling) material damage in an overlayer removal process.

As shown in FIG. 3, top coat 31 is removed by applying the laser processing beam 23 to the top coat 31 to remove and top coat 31 and reveal an intermediate layer 32. In FIG. 4 the intermediate layer is removed via the laser processing beam 23 to reveal a primer layer 41, and in FIG. 5 all overlying layers are removed by the laser processing beam 23, revealing the underlying substrate 51.

By way of example only, FIG. 6 illustrates laser scanning in a raster pattern 61 for material removal.

In accordance with some embodiments, auxiliary illumination may coincide with a point or tip of the laser removal tool for purposes of measuring or otherwise monitoring an active processing area. In some instances, auxiliary illumination may lead a laser processing area to measure material properties immediately prior to laser processing, or may follow a laser processing area to measure post processing material properties immediately following laser processing. Feed forward and feedback process control techniques may be employed to increase processing speed and/or quality.

According to various embodiments, the auxiliary illuminators are high brightness directional sources used to generate illumination for the low and or high detection bands.

Sources may be, for example, 405 nm and 980 nm laser diodes, but are not so limited.

Preferably, the sources illuminate layer material at a bright spot for efficient imaging from a measurement location at a layer surface, which is sufficient to provide response data (e.g., reflected radiation) to one or more control detectors that detect and are responsive to the response data from the low and/or high band detection channels.

In accordance with at least one embodiment, detection is configured to be fast enough to determine a material difference at or less than a laser processing period. For instance, the control detectors may be photodiodes with sufficient selectivity, sensitivity, and temporal response to provide a low-noise high-speed control signal for real time detection of material properties with regard to the control limit of respective detection channels. Sampling of areas for material removal may be sufficient for uniform strata; but detection that is in real time and fast enough for pulse to pulse process control at rates approaching 100 kilohertz is also within the scope of this disclosure. For example, pulse rates may be from 2 - 50 kilohertz.

In at least one embodiment, auxiliary illumination for one or more test channels is incident on a target area at an angle of incidence, and a corresponding detector is aligned to receive specular reflections from the target area. As described above, the target area for measurement may be material currently being processed, may be material about to be processed, or may be material already processed. Multiple illuminators and detectors may be disposed to illuminate and detect the same target area simultaneously.

Illumination and detection may be controlled using a controller, for example, a controller configured with Field Programmable Gate Array (FPGA) with embedded control logic. The control logic may be programmed for example in Labview FPGA, and Labview Real-Time. The controller may be in communication with a user interface, for example at an operator interface of a material removal system, or a remote user interface.

Detector selectivity may be enhanced with optical filters to reject some or substantially all of unwanted radiation, for instance reflected processing laser radiation, collateral radiation generated by plasma, the laser plume, and/or radiation in other test channels. For example, bandpass laser line filters corresponding to the bandwidth of each illuminator source may be located in the optical path between the illumination spot and the detector so as to transmit only the bandwidth or a desired bandwidth of each illuminator source to the detector.

As described above, a set of low channel and high channel control limits for each stratum may also be stored as a strata profile or as a portion of a strata profile (e.g., reference optical characteristics) for reference in a system controller. A processing job performed by the controller may load or otherwise receive the strata profile, select a stop layer within the profile, set laser processing parameters based on overlying layers and the stop layer, and control or otherwise control a processing laser beam to process target material to remove the overlying layers. The processing job may also include scanning and motion control parameters to position the processing beam relative to one or more areas of the substrate. A user may also enter one or more process parameters to the controller, such as the strata structure (materials, thickness, etc.), the desired stop layer, control limits, and or laser processing parameters (e.g., power, repetition rate, pulse duration, fluence, etc.).

in accordance with at least one embodiment, data related to the strata structure that is to be processed is received or otherwise determined by the controller. The strata structure data may include information related to the layers of material that are deposited on the substrate material (e.g., material, thickness), the reference optical properties for each of these materials, the desired stop layer (e.g., top coat layer or substrate) and associated wavelength channel control limits. In addition, processing parameters for the processing and test lasers may also be received or otherwise determined by the controller.

In at least one embodiment, a change in specular reflection is detected and based on this detected change, a control parameter is varied in the laser ablation process. As a simple non- limiting example, reflectance data may be obtained or otherwise measured during the removal process (e.g., the test laser(s) may illuminate the target area in between pulses of the processing laser) and the detected signals obtained during these time periods may be compared against the control limits for each wavelength channel. The processing laser may continue to apply the laser beam to the target area until the reflectance measurements fall within the wavelength channel control limits. Once this predetermined threshold has been reached, the controller may attenuate the laser processing beam by adjusting one or more laser processing parameters, such as lowering or eliminating the processing laser fluence applied to the strata structure. In other instances, measurement data may be obtained in between processing periods (e.g., in time regions other than in between pulses of the processing laser). The removal process may be performed such that layers are removed one at a time (i.e., the process is performed with multiple stop layers), or all at once such that the stop layer is associated with the substrate. Removal processes that fall in between these two type of regimes is also possible. In addition, different laser processing parameters may be applied during a removal process. For instance, a first part of a process may use laser processing parameters configured to rapidly remove several upper overlying layers of the strata structure such that an intermediate stop layer is associated with a layer that is positioned in between the top of the strata structure and the substrate. Once the measurement data indicates that this intermediate stop layer has been reached, a second part of the process may use laser processing parameters configured to remove layers more slowly until the final stop layer is reached (i.e., via the measurement data).

In accordance with certain aspects, control comparisons may use characteristics of partially processed material, processed material, or modified material adjacent to a laser processing target area including laterally adjacent areas and underlying areas. In addition to optical characteristics, bulk material properties such as substrate temperature may be used for purposes of process control. For instance, substrate temperature may characterize a maximum permissible substrate temperature resulting from cumulative laser irradiation. The detectors described above may therefore be configured to measure temperature, or additional temperature sensors may be positioned at appropriate locations for obtaining temperature information.

In accordance with at least one aspect, a material difference is detected and one or more laser processing parameters are adjusted based on the material difference. For example, test beam detection (measurement data) obtained during the ablation process may be used to optimize the process. One such example described above includes removing one or more layers of overlying material using a first set of laser processing parameters (e.g., more laser energy applied to achieve a faster removal rate) until the measurement data indicates that an

intermediate stop layer has been reached, at which point a second set of laser processing

parameters may be used (e.g., less laser energy applied to achieve a slower removal rate) until the measurement data indicates that the final stop layer has been reached.

In accordance with at least another aspect, multiple detectors may be associated with one or more wavelengths and other combinations of light detection. For example, light may be detected at multiple polarizations (and therefore measured polarization data may be used for material characterization and process control purposes), or multiple angles relative to the workpiece surface normal. Multiple detectors may be associated with different properties, such as wavelength and intensity, wavelength and polarization, polarization and angle, scattering properties and so on. According to another example, diffuse reflectivity may be used instead of specular reflectivity for purposes of material characterization and process control. In at least one embodiment, a change in diffuse reflection is detected and based on this detected change, a control parameter is varied in the laser ablation process.

Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation many equivalents to the specific embodiments of the invention described herein. The disclosed schematics can be used with any laser welding system, but the impetus for the presently disclosed structure lies in laser-based material removal from strata. It is therefore to be understood that the foregoing embodiments are presented by way of example only and that within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described. The present disclosure is directed to each individual feature, system, material and or method described herein. In addition, any combination of two or more such features, systems, materials and/or methods, if such features, systems, materials and or methods are not mutually inconsistent, is included within the scope of the present invention.

Claims

1. A method of laser processing one or more layers of overlying material disposed on an underlying substrate that form a strata structure of a workpiece with a laser processing beam during sequential processing periods, each material of the strata structure having measurable optical properties, each layer of overlying material characterized by a laser processing fluence sufficient for layer ablation by the laser processing beam, the substrate material having a laser damage threshold below the laser processing fluence of material of the one or more overlying layers, the method comprising:

measuring optical properties to generate reference optical properties of at least two materials from the group consisting of the substrate material and the material of each overlying layer;

generating a depth wise strata profile of the overlying layers and the substrate based on the reference optical properties;

processing material of at least one overlying layer with the laser processing beam;

measuring a first optical property of material of the strata structure at a target location; measuring a second optical property of the material of the strata structure at the target location;

comparing the first and the second optical properties with the strata profile;

generating a control signal based on the comparison; and

attenuating the laser processing beam by adjusting at least one laser processing parameter in response to the control signal, whereby attenuating the laser processing beam reduces laser damage to underlying material.

2. The method as in claim 1 , wherein laser processing comprises removing at least one layer of overlying material.

3. The method as in claim 1 , wherein the substrate comprises a structural composite material and wherein attenuating includes attenuating the laser processing fluence such that structural damage to the composite is reduced.

4. The method as in claim 1 , wherein laser processing comprises depainting the substrate.

5. The method as in claim 1 , wherein the laser processing beam is resumed in a subsequent processing period based on the control signal.

6. The method as in claim 1, wherein the substrate comprises a composite material.

7. The method as in claim 1 , wherein the optical reference properties are selected from the group consisting of diffuse reflectance, specular reflectance, bidirectional reflectance, scatter, polarization, photometric, radiometric, and spectrometric properties.

8. The method as in claim 1 , wherein the target location is proximate to a laser processing interaction zone where the laser processing beam processes material of the at least one layer.

9. The method as in claim 1 , wherein the target location is disjoint from a laser processing interaction zone where the laser processing beam processes material of the at least one layer, and wherein measurement of the target location precedes laser processing of the target location.

10. The method as in claim 1 wherein the target location is disjoint from a laser processing interaction zone where the laser processing beam processes material of the at least one layer, and wherein measurement of the target location follows laser processing of the target location.

1 1. A laser based material removal system comprising:

a processing laser configured to generate a laser processing output;

a beam delivery system configured to deliver the laser processing output to a laser spot on a workpiece;

a first test laser having a first wavelength and configured to illuminate a target location on the workpiece;

a second test laser having a second test wavelength and configured to illuminate the target location;

a first detector configured to be responsive to the first test laser wavelength and configured to generate a first detector signal; a second detector configured to be responsive to the second test laser wavelength and configured to generate a second detector signal;

means for moving the laser spot relative to the workpiece;

a data storage medium with at least one stored readable strata profile, the strata profile including reference optical characteristics of materials of a strata structure, the strata structure disposed on the workpiece and having one or more layers of overlying material disposed on an underlying substrate; and

a controller configured to:

(a) read the strata profile;

(b) receive a target stratum of the strata profile;

(c) activate the beam delivery system to process at least one layer of the one or more overlying layers with the laser processing output;

(d) receive the first and the second detector signals from the first and the second detectors;

(e) compare the first and the second detector signals with the strata profile to determine a comparison result; and

(f) repeat steps (c)-(e) until the comparison result reaches a predetermined threshold indicating that the at least one layer of the one or more overlying layers has been removed, and then attenuating the laser processing output.

12. The system of claim 1 1, further comprising an effluent removal system.

13. The system of claim 1 1 , further comprising means for transporting the processing laser, the beam delivery system, the first and the second test lasers, the first and the second detectors, and the laser spot moving means relative to the workpiece.

14. The system of claim 13, wherein the means for transporting is selected from the group comprising a robot arm, a vehicle, and a gantry.

15. The system of claim 1 1 , further comprising means for translating the beam delivery system, the first and the second test lasers, the first and the second detectors, and the laser spot moving means relative to the workpiece.

16. The system of claim 15, wherein the means for translating is selected from the group comprising a worker, a robot arm, a vehicle, and a gantry.

17. The system of claim 1 1 , wherein the processing laser comprises a fiber laser.

18. The system of claim 17, wherein the fiber laser is configured as a pulsed fiber laser, and the controller is configured to receive the first and the second detector signals in between pulses of the pulsed fiber laser.

19. The system of claim 1 1 , wherein the processing laser is configured as the first test laser to illuminate the target location on the workpiece.

20. The system of claim 1 1 , wherein the first and the second detectors are configured to generate the first and the second respective detector signals based on one of diffuse reflectance, specular reflectance, bidirectional reflectance, scatter, polarization, photometric, radiometric, and spectrometric properties measured at the target location.