Patent Description:
Lasers are known to be important tools for processing a wide range of materials. In particular, lasers are very well suited to and see wide application for processing of metals, polymers, ceramics, semiconductors, composites and biological tissue. By focusing a laser beam, it can be possible to achieve improved precision of the laser's action in a direction transverse to the beam axis. However, localizing the laser's action in the axial direction of the beam can be difficult. During processes such as laser welding, a phase change region (PCR) is created where the material localized to the bonding region changes dynamically from solid to a liquid and/or a gas state and back to a solid again at the completion of the weld. In some cases the material may change multiple times between the various states and also interact with other substances present in the weld zone including other solids, liquids and gasses. Controlling this phase change region (PCR) is important to control the quality of the weld and the overall productivity of the welding system. The high spatial coherence of laser light allows good transverse control of the welding energy deposition, but thermal diffusion limits the achievable aspect ratio of welded features when the energy is transmitted through the material with conduction alone. For higher aspect ratio features, the more dynamic and unstable process of keyhole welding is used to allow the conversion of optical to thermal energy to occur deeper in the material. Here, axial control (depth of the PCR) is even more problematic. In keyhole welding, the depth of the PCR and the absorption of the laser may extend deep into the material (for example, depths from <NUM> micrometers to tens of millimeters). Here, the beam intensity is sufficient to melt the surface to open a vapor channel (also known as a capillary or "the keyhole") which allows the optical beam to penetrate deep into the material. Depending on the specific application, the keyhole may be narrow (e.g., less than <NUM>) but several millimeters deep and sustained with the application of optical power (for example in the range from <NUM>-<NUM> W to <NUM>,<NUM> W or more). As a result, the light-matter interaction region inside the PCR can be turbulent, unstable and highly stochastic. Unfortunately, instability of keyhole formation can lead to internal voids and high weld porosity resulting in weld failure, with potential catastrophic consequences. Similarly, keyhole instability can result in spatter that contaminates nearby system components, complicating the application of laser welding in systems such as vehicular transmissions. Weld quality verification is usually required, often using expensive ex-situ and destructive testing. Welding imaging solutions are offered but are limited in their capabilities and usually monitor regions either before or after the PCR, to track the weld joint, or record the top surface of the cooled weld joint. <CIT> (basis for the preamble of claims <NUM> and <NUM>) discloses a system and a method for coherent imaging and feedback control for modification of materials.

The invention relates to characterization of morphology, for example, including one or more of length, width, depth, size, shape, and aspect ratio of the keyhole and surrounding material over time by directing an interferometry measurement beam (including, for example, a low-coherence interferometry measurement beam) into the PCR and surrounding area. The beam may be moved along an x- or y-axis and/or θ/φ (i.e., theta/phi, angle may change from normal).

According to one aspect of the invention, there is provided an apparatus comprising: an imaging optical source that produces imaging light that is applied to a material processing system, wherein the material processing system implements a material modification process and creates a phase change region (PCR) in a material; at least one element that directs the imaging light at a plurality of imaging beam positions proximate the PCR; at least one input-output port that outputs a first component of the imaging light to an optical access port of the material processing system and that receives a reflection component of the imaging light; an optical combiner that combines the reflection component and at least another component of the imaging light to produce an interferometry output, the interferometry output based on a path length taken by the first component and the reflection component compared to a path length taken by the at least another component of the imaging light; and an interferometry output processor that processes the interferometry output to determine at least one characteristic of the PCR.

The apparatus further comprises a material processing beam source that produces a material processing beam that is applied to the material in the material modification process, wherein the material processing beam creates the PCR in the material.

According to another aspect of the invention, there is provided an apparatus for modifying a sample, the apparatus comprising: a material processing beam source that produces a material processing beam that is applied to the sample at a sample location in a material modification process wherein the material processing beam creates a phase change region (PCR) in the sample; an imaging optical source that produces imaging light that is applied at a plurality of imaging beam positions proximate the PCR (i.e., in the vicinity of the PCR and/or within the PCR); an optical interferometer that produces an interferometry output for each imaging beam position using at least a component of the imaging light that is delivered to the sample, the interferometry output based on at least one optical path length to the sample compared to another optical path length; and an interferometry output processor that processes the interferometry outputs to determine at least one characteristic of the PCR.

According to another aspect of the invention, there is provided an apparatus for use with a material processing system that implements a material modification process and creates a phase change region (PCR) in a material, the material processing system having an optical access port, the apparatus comprising: an imaging optical source that produces imaging light that is applied at a plurality of imaging beam positions proximate the PCR; at least one input-output port that outputs a first component of the imaging light to the optical access port of the material processing system and that receives a reflection component of the imaging light; an optical combiner that combines the reflection component and at least another component of the imaging light to produce an interferometry output, the interferometry output based on a path length taken by the first component and the reflection component compared to a path length taken by the at least another component of the imaging light; and an interferometry output processor that processes the interferometry outputs to determine at least one characteristic of the PCR.

According to another aspect of the invention, there is provided a method comprising: applying an imaging light to a material processing system, wherein the material processing system implements a material modification process and creates a phase change region (PCR) in a material; using at least one element to direct the imaging light at a plurality of imaging beam positions proximate the PCR; outputting a first component of the imaging light to an optical access port of the material processing system and receiving a reflection component of the imaging light; combining the reflection component and at least another component of the imaging light to produce an interferometry output, the interferometry output based on a path length taken by the first component and the reflection component compared to a path length taken by the at least another component of the imaging light; and processing the interferometry output to determine at least one characteristic of the PCR.

The method further comprises applying a material processing beam to the material in the material modification process, wherein the material processing beam creates the PCR in the material.

According to another aspect of the invention, there is provided a method for modifying a sample, the apparatus comprising: producing a material processing beam that is applied to a sample at a sample location in a material modification process wherein the material processing beam creates a phase change region (PCR) in the sample; producing imaging light that is applied at a plurality of imaging beam positions proximate the PCR; producing an interferometry output for each imaging beam position using at least a component of the imaging light that is delivered to the sample, the interferometry output based on at least one optical path length to the sample compared to another optical path length; and processing the interferometry outputs to determine at least one characteristic of the PCR.

Embodiments will be described below, by way of example, with reference to the accompanying drawings, wherein:.

In all embodiments described herein, a material modification beam, also referred to as a material processing beam, is used. Examples of a material processing beam include a laser beam, an electron (or other particle) beam, plasma beam, electric arc, or water jet. Auxiliary laser beams and combinations of these (e.g., a laser beam guided by a water jet, hybrid laser arc welding) are also encompassed. Thus, whereas most embodiments are described as using a laser beam, it will be understood that the invention is not limited thereto.

As used herein, the terms "keyhole", "capillary", and "vapour channel" are considered to be equivalent and are intended to refer to the gaseous cavity that exists in a phase change region of a material during a material modification process using a material modification beam.

<FIG> is a cross section diagram of a typical material welding process featuring coherent imaging in accordance with an embodiment of the invention. Two metal samples <NUM> and <NUM> are to be joined together in a continuous welding (CW), keyhole welding laser process. The laser beam <NUM> is moved across the surface in the direction indicated by arrow <NUM>.

The PCR (phase change region) comprises a liquid region <NUM>, a gas or keyhole region <NUM>, and a bonded solid region <NUM>, the solid having been reformed from the other two states. In general, if keyhole welding is occurring successfully, there will be three phases, as depicted in <FIG>. However, in some embodiments, the apparatus and method are used to detect the lack of keyhole formation, in which case there may be only liquid and solid states, or only a solid state.

A plurality of imaging beams <NUM> (herein depicted as 20a through 20i) are introduced at multiple points and/or at multiple incident angles in, and optionally near, the PCR. In the specific example depicted, there are seven beams 20a, 20b, 20c, 20d, 20e, 20f, <NUM> that are substantially normal to samples, and two beams 20i, <NUM> that have incident angles that are not normal to the samples. The imaging beams <NUM> are used to generate measurements using low-coherence interferometry at each of the multiple points and/or multiple incident angles. While <FIG> shows a specific plurality of imaging beams <NUM> introduced at a specific set of points and incident angles, more generally, a plurality of measurements at some set of imaging beam positions are taken. The multiple imaging beam positions may involve one or a combination of:.

In some embodiments, one of the plurality of imaging beam positions is created by the multiple internal reflection of an imaging beam inside of an optical element (which may also be referred to herein simply as an "optic") that the imaging light interacts with inside of the beam delivery system. This multiple reflection introduces additional optical path length (thus shifting the location of the reflection to a depth in the image and allowing it to be distinguished from the another beam measuring something else such as the keyhole depth) and a transverse shift of the focus of the beam. This allows for convenient simultaneous measurement of the top surface reference point(s) (TSRP) and weld depth. Top surface reference points are discussed in further detail below. An image showing such simultaneous measurement capability is shown in <FIG>. The reflection showing the TSRP is located at an indicated depth of approximately <NUM>.

Each of a plurality of imaging beams (e.g., beams 20a-20i) originates from a semi-coherent light source, although as described above multiple beams may originate from a single light source. A very specific example of this type of light source is a superluminescent diode with a spectrum ranging from <NUM>-<NUM> and output power of <NUM> mW coupled into a single mode optical fiber such as Corning H1780. Light sources meeting these criteria are commercially available and manufactured by Superlum Diodes Inc. (Ireland) and other manufacturers. The beam from the light source is carried, directed and manipulated through various media and components that might include fiber optic cables, air (or other gases), mirrors (or semi-reflective mirrors), lenses, or other optics. The fiber optic cables can be of the single-mode, multimode, and/or polarization maintaining types. The light source beam is split into two or more beams, for example using a semi-reflective mirror. One beam known as the imaging beam or sample beam is directed towards the sample; each of the beams depicted in the figure as 20a to 20i is such a sample beam. Another beam known as the reference beam is reflected off a reference surface (e.g., a mirror). The sample beam and the reference beam are then optically recombined, for example by the same semi-reflective mirror, so that they create and interference pattern. While a Michelson-style interferometer was just described, other interferometer configurations such as Mach-Zehnder (including the use of optical circulators), Sagnac, and common-path may also be applied in some embodiments. The interference pattern, I(k), will vary depending on the path length of the reflected imaging beam relative to the path length of the reference beam, Δz, according to the relationship <MAT>. These interferometry patterns are then captured and digitized using a commercially available spectrometer and camera such as the DeepView™ NIR spectrometer (BaySpec, Inc. San Jose, USA). Additional established optical coherence tomography techniques and those from inline coherent imaging are then used to calculate depth relative to a known reference position. Specific examples of interferometry systems will be described below with reference to <FIG> and <FIG>.

The following are examples of methods that may be used calculate keyhole PCR characteristics and/or parameters. A reference position(s) is established using points on the sample surface identified to be TSRPs. In the case where the sample is substantially flat, at least one TSRP can be used to define a top surface reference plane. Additional top surface reference points can be determined based on the top surface reference plane without taking corresponding additional measurements. Alternatively, multiple top surface reference points are used to calculate depth of the process.

The reference position, such as the TSRP, may be set, measured, or calibrated before, during, or after the welding process. This may be achieved by taking a baseline depth measurement or measurements at locations on the sample unaffected by the welding process, such as the location illuminated by beam <NUM> in <FIG>. The TSRP can also be defined in real time by simultaneously imaging the top surface and keyhole bottom either through the use of multiple imaging channels or by enlarging the imaging spot to simultaneously or dynamically (i.e., sometimes the top, sometimes the bottom based on keyhole oscillations) cover both locations. In the simplest case, the TSRP can be determined by taking one or more measurements of the material immediately before the weld begins. If the material is sufficiently flat relative to the weld motion, then this initial measurement can define the TSRP for the rest of the weld. In other cases the TSRP is mechanically fixed at a specific distance or may be measured using other standard electrical, mechanical, or optical means. An example of this would be a beam delivery system that rolls across the workpiece(s). In this case, the virgin material surface would be a known distance away from the welding optics that is directly related to the distance between the unit's wheel(s) and the optics. Another example would be a welding system that utilizes a fixture or clamp to hold the workpiece(s). Again, since the distance between the optics and the fixture is known, the distance between the optics and the virgin surface of the material is known.

The imaging beams, such as beams <NUM> of <FIG>, are used to measure, instantaneously and/or over a period of time, one or a combination of two or more of keyhole length, width, depth, surface shape, sub-surface profile, wall slope, collapse, instability, undercut, and other physical parameters of the PCR. Specific example methods of calculating each of these values will now be described. More generally, what constitutes length, width, depth, surface shape, sub-surface profile, wall slope, collapse, instability, undercut, or the other physical parameters of the PCR can be defined on an implementation-specific basis.

A single depth measurement is generally defined as the distance below the TSRP measured by the imaging beam. Note that depth can be a negative value if the measurement is above the TSRP.

For the following examples, the imaging beams are normal or close to normal to the sample surface.

All of the examples above rely on imaging beams that are normal or substantially normal to the plane of the sample surface. In some cases it may be advantageous to take coherent imaging readings at angles that are not normal to the plane of the sample surface. Readings from imaging beams <NUM> and 20i of <FIG> would be examples of this. For example, in some embodiments, these off-normal imaging beams are used to determine or contribute to the determination of one or more of wall instability, partial keyhole collapse or situations that could lead to voids or porosity in a welding process. Particularly at high welding speeds at deeper depths, the keyhole vapour channel may undercut some of the liquid (towards the trailing edge of the weld) such that there is not a direct optical path to the bottom of the keyhole that is also normal to the material surface. This situation is particularly vulnerable to unstable pathological behaviour and may be detected by comparing signals from imaging beams 20b and <NUM> (or one similarly angled to reach the bottom of a undercut keyhole).

The dynamics of the liquid region of the PCR are examined. This can be done, for example, by taking multiple imaging beam measurements in and around where the liquid phase region is expected to be located. For the example of <FIG>, multiple imaging beam measurements near imaging beam 20e may be used to look at the slope, changes, waves, or other characteristics of the liquid.

In some embodiments, the interface between the liquid/solid region of the PCR is located using the measurements. In a specific example, multiple imaging beam measurements are taken in and around where the interface is expected to be located (for example in and around the location of beams 20e, 20f of <FIG>). The liquid will oscillate and detectably change its position whereas the solid region will be static, thus producing measurable contrast between the two phases.

In some embodiments, waves are excited and generated in the liquid region of the PCR using acoustic and/or optical energy source techniques to assist with generating imaging contrast and understanding PCR geometry, dynamics, and characteristics such as viscosity. For example, an acoustic vibration may be excited in the liquid at a frequency that is smaller than the imaging sample rate. An imaging beam observing such a liquid region would be able to measure the phase and amplitude of the geometric distortion that follows the acoustic vibration, thereby confirming the liquid state of the point being imaged.

In some embodiments of the invention, at least one of the plurality of imaging beams positions is outside the PCR. Beams 20f and <NUM> are examples of this in <FIG>.

In some embodiments of the invention, light is applied to at least two of the plurality of imaging beams positions simultaneously. The multiple imaging beams can be generated in this case using multiple beam sources, or by using a single beam source and one or more splitters.

In some embodiments of the invention, light is applied to at least two of the plurality of imaging beam positions sequentially. The sequentially applied imaging beams can be generated using multiple beam sources that are activated in sequence, or by using a single beam source that is reconfigured to produce each of the beams in sequence.

In some embodiments of the invention, the plurality of imaging beam positions are achieved by changing the position and/or angle of at least one imaging beam relative to the processing beam during the welding process.

In some embodiments of the invention, the number of positions where the plurality of imaging beams is applied to the sample is changed during the welding process.

In some embodiments of the invention, at least one of the plurality of imaging beam positions does not have an incident position that is on a line formed by the material processing beam. For example, in <FIG>, laser beam <NUM> moves in direction <NUM> and traces out a path. One or more of the imaging beams can be applied off this path. This can be used, for example, to determine keyhole width.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beams positions is used to determine the width or diameter of the keyhole when viewed from the same direction as the material processing (e.g., laser) beam is applied.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is focused to a diameter that is smaller than the diameter of the laser beam.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is focused to a diameter that is similar to the diameter of the laser beam.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is focused to a diameter that is larger than the diameter of the laser beam.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is focused to a diameter that encompasses the PCR.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is focused to a diameter that is larger than the PCR.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is used to take successive readings at a frequency of approximately <NUM> or more.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is used to determine the maximum depth achieved by the keyhole over a period of time. In some embodiments, this determination is used to control at least one parameter of the welding process to reduce the number of instances where welding will penetrate beyond a specified depth and/or into a specified material, including reducing the number of instances to zero.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is used to determine the minimum depth achieved by the keyhole over a period of time. In some embodiments, this determination is used to control at least one parameter of the welding process to reduce the number of instances where welding fails to penetrate beyond a specified depth and/or into a specified material, including reducing the number of instances to zero.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is used to determine the shape and size of the keyhole over time.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is used to determine if the keyhole collapses or fails to maintain a specified depth.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is used to calculate an optimal speed for the welding process.

In some embodiments of the invention, imaging light applied to at least one of the plurality of imaging beam positions is used to calculate an output power level for the laser beam.

In some embodiments of the invention, measurements for at least one of the plurality of imaging beam positions are processed, output, and fed back to the laser process control system to provide closed loop operation.

<FIG> is a schematic diagram of an example of an apparatus that implements the coherent imaging in a material welding process. A material processing laser <NUM> produces a laser beam that is carried via a fiber optic cable <NUM> and connects through fiber attachment connector <NUM> to a laser head <NUM> which outputs laser beam <NUM>. Embodiments described herein involve the use of a laser. The laser beam <NUM> is collimated by optic <NUM> and focused by optic <NUM> such that keyhole welding is achieved on the sample <NUM>, <NUM>. PCR <NUM>, <NUM>, <NUM> is depicted. A movable dichroic mirror <NUM> is shown.

A low-coherence interferometry imaging light source <NUM> produces imaging light that is carried via fiber optic cable <NUM> and connected through fiber attachment connector <NUM> to the laser head <NUM> where the imaging light is used to form an imaging beam <NUM>. A collimating lens <NUM> directs the imaging beam <NUM> towards the mirror <NUM>. The mirror <NUM> is actuated by a motorized system (not shown) such that the imaging beam <NUM> can be directed to multiple locations within the PCR <NUM> on the sample <NUM>, <NUM>.

In the illustrated example, the laser and the imaging beam (more generally light applied to at least one of the imaging beam positions) are focussed by a common focusing lens <NUM>. In some embodiments of the invention the imaging beam <NUM> and laser beam <NUM> may each have their own separate focusing lenses which focus the beams before they are combined by mirror <NUM> and then delivered to the sample <NUM>, <NUM>. This may be particularly useful in some applications as it separates the optical requirements for imaging (e.g., multiple, possibly aspheric optical elements for field distortion reduction) and high power beam delivery optical requirements (minimal focal shift and power absorption). It also makes it easier to position a scanning device near the back focal position of a scanning lens, which is often desired for reducing optical path distortion when using telecentric scanning optics.

The scanning device is used to change the angle and/or position of the imaging beam and/or the processing beam. Examples of a scanning device are a galvanometer-mirror scanner (e.g., available from SCANLAB AG in Germany), a piezoelectrically actuated mirror, and a motor actuated mirror. A scanning lens is an optical device that is used in conjunction with or as a part of the scanning device to direct an optical beam. An example of a scanning lens is an LSM05 from Thorlabs Inc. (Newton, USA).

Embodiments described herein provide adjustment (automatic or otherwise) of the imaging beam's position relative to the processing beam, to compensate for motion/misalignment. For example, such adjustment may be carried out automatically, to adjust the imaging beam for changes in the velocity of the processing beam, and/or changes in the direction of the processing beam, and/or changes in the relative velocity between the beam delivery system and the workpiece, and/or changes in the process beam velocity and/or the velocity of the process beam's focus relative to the workpiece. In some embodiments the imaging beam may be adjusted to compensate for flexing and dynamic misalignment in the processing beam head and/or motion system. In some embodiments, the scanning device is used to adjust the imaging beam's position to compensate for flexing and dynamic misalignment of the head, the optics contained therein, and any motion system, such as, e.g., a robot or gantry motor that is utilized to effect the material modification process.

In some embodiments of the invention, an auxiliary measurement system that uses electronic, mechanical, optical, and/or capacitive techniques is provided. It is used, for example, to determine the distance from the laser head to the sample, and this information used as part of the interferometry and/or feedback processing.

For example, in some embodiments, at least one imaging beam is used to determine depth of the processing beam penetration in a material, i.e., depth of the keyhole. In such an embodiment the at least one imaging beam position is inside the keyhole. In various embodiments the at least one imaging beam position inside the keyhole is used together with at least one other measure to determine the depth of the keyhole. The at least one other measure may be obtained, for example, using a mechanical sensor in contact with a surface of the material being modified, an electronic sensor, a capacitive sensor, or optically. For example, in some embodiments, at least two imaging beam positions include a position outside the keyhole and a position inside the keyhole.

Determination of keyhole depth according to embodiments described herein may be carried out simultaneously during the material modification process. Such determination provides for dynamic control of the material modification process, thereby providing quality control, and improving quality.

The imaging beams are used to measure, either on a one time basis, once per workpiece basis, over a period of time, at regular or pre-set intervals, continuously, or substantially continuously, one or a combination of two or more keyhole features or parameters. The keyhole features, which are dynamic as they may change throughout the material modification process, include, but are not limited to, keyhole depth, location of maximum keyhole depth, average keyhole depth, keyhole location, keyhole width, keyhole length, surface shape, subsurface shape, subsurface keyhole length, subsurface keyhole width, wall slope, sidewall angle, keyhole collapse, keyhole stability, dynamics of liquid region of the PCR, location of interface between liquid and solid region, other physical parameters of the PCR.

Embodiments described herein allow dynamic and simultaneous, or substantially simultaneous, tracking of the material modification process, to achieve greater control of the process and higher quality results. In some embodiments, such tracking is carried out in respect of the processing beam, and accordingly at least one imaging beam is substantially aligned with the processing beam. In other embodiments, such tracking is carried out with respect to the PCR, and in particular, in respect of one or more dynamic keyhole features such as those described above. Accordingly, in such embodiments, the imaging beam is aligned so as to determine the one or more dynamic keyhole feature(s). Such alignment of the imaging beam may not be with processing beam; rather, in such embodiments the imaging beam may track the PCR (i.e., a PCR feature) optionally by a selected distance or with an offset determined so as to optimize the tracking.

The embodiments discussed in the description of <FIG> may be implemented using the arrangement of <FIG>.

<FIG> is a schematic diagram of an example of another apparatus that implements the coherent imaging in a material welding process. A material processing laser <NUM> produces a laser output that is carried via a fiber optic cable <NUM> and connects at fiber optic connector <NUM> to a laser head <NUM> which outputs laser beam <NUM>. A laser beam <NUM> is substantially collimated by optic <NUM>, reflected by dichroic mirror <NUM> and then focused by optic <NUM> such that keyhole welding is achieved on the sample <NUM>, <NUM>. In some embodiments, mirror <NUM> is a movable mirror.

An imaging system <NUM> produces imaging light that is carried via fiber optic cable <NUM> and connected to fiber optic connector <NUM> to the laser head <NUM> where the imaging light is used to form an imaging beam <NUM>. A collimating lens <NUM> substantially collimates the imaging beam and directs the imaging beam <NUM> towards the mirror <NUM>. Adjustment of the relative positions and angles of <NUM>, <NUM>, <NUM>, and <NUM> may be used to control the position and focal plane of the imaging beam. A movable mirror is shown at <NUM>. The mirror <NUM> is actuated by a motorized system (not shown) such that the imaging beam <NUM> can be directed to multiple locations within the PCR <NUM>, <NUM>, <NUM> on the sample <NUM>, <NUM>. Instead of or in addition to a motorized mirror, an acousto-optic deflector, electro-optic deflector, or other device known to those of ordinary skill in the art to statically or dynamically change the angle of a beam could be used. In some cases, additional mirrors may be used to fold the beam path to allow it to fit into smaller linear spaces.

The movable mirror <NUM> and the dichroic mirror <NUM> are used to allow the laser beam and the imaging beam to be directed towards the sample and closely aligned. Depending on the angles of the mirrors <NUM>, <NUM>, at a given instant in time, the laser beam and imaging beam may be parallel, or at some angle relative to each other.

The imaging beam <NUM> is used to measure, over a period of time, one or a combination of two or more of the length, width, depth, surface shape, sub-surface shape, sidewall slope, collapse, instability, and/or other physical parameters of the PCR <NUM>, <NUM>, <NUM>.

Features denoted by reference characters <NUM>, <NUM>, <NUM> appearing in <FIG> will be detailed below in the discussion of working examples.

In some embodiments of the invention, a sacrificial covering glass is provided to protect the rest of the optics from emissions from the material modification process. The use of cover glass in laser materials processing is widely known and used by those of ordinary skill in the art.

In some embodiments of the invention, a cross jet of gas from one or a plurality of gas-fed pressurized orifices (known by some as an air knife) is employed to protect the optics from emissions from the material modification process by blowing them away. The use of a cross jet is widely known and used by those of ordinary skill in the art.

In some embodiments of the invention, a cover gas applied by one or more nozzles located above and/or below the workpiece and is employed to produce/prevent specific chemical effects (e.g., reduce oxidation) on the material being modified. The use of a cover gas is widely known and used by those of ordinary skill in the art.

The use of cover and cross jet gases may also provide the benefit of suppressing the amount of plasma and/or debris inside the beam path, which changes how the energy from the high power processing beam is absorbed in the sample.

Coherent imaging is particularly well suited to observing and controlling high energy material modification processes for several reasons. It is resistant to sensing process light, it is virtually immune to blackbody radiation, it has high sensitivity, high dynamic range, high speed, and is easy to integrate into existing optical systems.

In some embodiments, the beam delivery system may be modular to allow multiple configurations of optics, gas, material processing beams, and instrumentation to be used. Such instrumentation includes auxiliary measurement tools that can be used to influence the feedback and/or imaging output. Several examples follow:.

In some embodiments, optical path length compensation for gas pressure in the coherent imaging beam path is performed. The process controller and/or image processor may receive inputs from one or more pressure sensors that read the ambient pressure inside locations in the beam delivery system. Gas pressure changes the optical path length registered by the coherent imaging system. In some embodiments, these measurements are used to digitally (i.e., in processing the signal electrically or in software) or physically (e.g., by modifying the reference arm length) compensate for optical path length changes due to high pressure gas.

The amount of correction required is obtainable by fixing the location of the process head relative to a solid object and gradually pressurizing the head while tracking the apparent location of the surface. Thus a mapping of path length error as a function of head pressure can be obtained. Similarly, if the gas composition has a significant effect on the path length, this too can be simulated and measured in the same way. After programming this mapping into the process controller/image processor, it can then apply the necessary correction based on the measured pressure inputs.

In some embodiments, compensation for optical path length changes resulting from scanning the imaging/processing beams is performed. This can be done digitally and/or physically by adjusting an optical path length in the interferometer. This can be done, for example, by mirrors or by adding glass.

In some embodiments of the invention, the optical path length imaging beam on the sample is physically modified as a consequence of directing it to a different location on the sample, even if the sample height itself is not changing. This results in a distortion of the morphological data that the imaging system returns. While this can be corrected digitally in some cases, it may also be beneficial to correct for it physically, especially if the distortion is large. Alternatively, it may also be advantageous to intentionally distort the imaging field in order for it to conform to a material geometry that is not flat. The decision whether or not to take any of these approaches is application specific.

The first step to a correction is to measure the distortion. This is accomplished by moving the imaging beam to several locations on an object that is known to be flat, measuring the apparent height of the object in the coherent imaging system and subtracting the desired profile from these results. This data yields the error between the uncorrected and desired profiles.

The next step is to apply the correction. Digitally, this can be accomplished by addition of the known error at given positions to the future measurements at or around those positions. Error maps can be interpolated beforehand or in real time. Physically, distortion correction can be accomplished by manual or automated addition of optical materials to the reference path. This includes modifying the reference arm delay line length. In production, this can be co-ordinated with the scanning device. For example some scanning devices (such as a Cambridge Technology FlexScan-3D™, Cambridge Technology, Inc. , Bedford, USA) utilize a Z-axis actuator to control the focal distance of the laser beam. The control signal for this actuator can be utilized as a command signal (e.g., via digital/analog scaling and subtraction) for the reference arm of the coherent imaging system.

In some embodiments, acoustic excitation is employed to detect liquid vs. solid phases. This might be used, for example, to provide an indication of the viscosity of the melt. Coherent imaging can sense the frequency and amplitude of a vibrating melt pool by monitoring its axial/transverse position and/or its reflectivity (which indicates, amongst other things, the radius of curvature of its surface). If vibration of a melt pool can be excited by optical (e.g., laser) or mechanical (e.g., an acoustic transducer) means, then coherent imaging is able to sense the presence of these vibrations, their frequency and/or their amplitude. This can be used to measure the viscosity of the melt pool in a certain region and to distinguish between liquid and solid metal.

For example, immediately after a spot weld occurs, a molten pool of metal is left oscillating. Coherent imaging data can determine the period and/or phase of the oscillation of the melt pool. This period is characteristic of and can therefore be used to determine at least one of the melt viscosity, material type, material state, and geometry. By sensing the period, (in this case) one can infer that the melt pool width is decreasing over time.

<FIG> shows an example of a coherent image of a laser spot weld. The keyhole is initially opened and increases in depth until the laser pulse (<NUM> duration, <NUM> kW intensity) is terminated at about a quarter of the way into the image. The keyhole remains open for a few tens of microseconds until it is filled in by liquid metal. The liquid metal oscillates which can be seen by the repetitive enhancement and reduction of the signal. Notably, the period of the oscillation is steadily reduced as the size of the oscillating weld pool shrinks due to the periphery fusing.

In some embodiments, spiking in weld depth is detected, and used as an indicator of the presence of porosity in the weld. Porosity indicates strength/longevity and sometimes even corrosion resistance of the weld. It is known to those of ordinary skill in the art that a welding phenomenon called "spiking" exists. Spiking is characterized by rapid, momentary enhancements in the weld penetration depth. In post-weld destructive testing, spiking appears as an inconsistent weld depth with one or many narrow depth enhancements. The underlying cause of spiking is dynamic instability and chaotic behaviour of the materials in and around the keyhole. Coherent imaging techniques are fast enough and have high enough spatial resolution to detect the rapid and momentary enhancements in depth that are the spiking phenomenon. Consequently, coherent imaging techniques can sense keyhole instability.

Also associated with keyhole instability are weld defects such as porosity and bad mixing of dissimilar materials. There is a correlation between the appearance of spiking dynamic behaviour as sensed by coherent imaging techniques and the presence of porosity in a weld. Therefore, by observing the keyhole dynamics of welding, coherent imaging can detect the presence of porosity in the weld.

Furthermore, by sensing the phase of keyhole oscillations with coherent imaging, the process controller can be made to synchronously or asynchronously drive the keyhole oscillations by modulating the power of the high power laser, or through the application of another energy source such as from an ultrasonic transducer.

Zinc is widely used to inhibit corrosion on a variety of metal products. The low vaporization temperature of zinc presents significant challenges to welding of materials that contain zinc (e.g., galvanized steel). The explosive vaporization of zinc can eject material from the phase change region and is difficult to predict. This can result in underfill and unwanted weld depth enhancements. Coherent imaging can sense the occurrence of a gas phase explosion or similar transient and generate an annunciation that can be used to warn the user or a process controller that a gas phase explosion or similar transient may have compromised the quality of a weld. In coherent imaging, this condition looks like a sudden enhancement of the weld depth over the course of a time period, for example ~<NUM>. Coherent imaging can also sense keyhole instability from small transients before a large transient and generate a warning of risk of a future large transient or large gas phase explosion. This may be thought of in the way that small seismic events may herald the coming of a large one in the future. In this case, corrective action can be taken to prevent a large zinc explosion by, for example, reducing the power of the processing beam and/or increasing the gap between two parts to be welded by e.g., tens of micrometers. While zinc is referred to in this paragraph, other materials may have the same effect.

Referring now to <FIG>, in the top view, a coherent image of a lap welding process involving galvanized steel and a <NUM> inch thick stainless steel shim is shown. The material is fed at <NUM>/s and imaged at <NUM>. The image processor is configured to track the bottom of the keyhole and has overlaid a lighter area to enhance the contrast of the image for easier viewing and interpretation of welding depth. The tracking algorithm locked on to signals <NUM> standard deviations above the noise floor and used a transverse correlation length of <NUM>. Horizontal lines are overlaid showing the thickness of the top sheet. This same underlying tracking information can also be used for numerical interpretation of the keyhole depth and instability. Severe spiking and instability are clearly shown by the large depth swings (amplitudes > <NUM> are evident over time scales of <NUM>-<NUM> and lengths of ~ <NUM>). This would likely be a defective weld.

In the bottom view, substantially less instability in a similar continuous wave (CW) laser weld is evident. However, there are a few "spikes" visible at <NUM> and just before <NUM>, <NUM>, <NUM>, and <NUM>. It is more likely that this weld is a good weld than in the top view.

In some embodiments, one or more of the coherent imaging systems or methods described herein is used in conjunction with a mechanically actuated objective lens to keep the focus of the processing beam a certain distance from the material surface. This approach may be particularly useful in laser cutting embodiments as opposed to welding embodiments. In one example, the focus of the processing beam is mechanically actuated over distances between <NUM> and <NUM> along the axis of the beam. This capability is demonstrated without coherent imaging in products such as the Laser Mechanisms FiberCut RA™ ( Laser Mechanisms, Inc. , Michigan, USA). This may be desirable to correct for non-ideal material geometry (thickness, distortion, etc.) and/or non-ideal motion of the beam delivery system (e.g., a robot that is unable to smoothly move over a plate of metal). If a coherent imaging system were to be added to such a scheme, it could be used instead of, or in addition to, a capacitive height sensor. In this case, the distance of actuation is likely to roughly correspond to a change in the working path length between the coherent imaging system and the material. It may therefore be desirable to change the reference path length of the coherent imaging system in a way that is correlated with the actuation of the processing beam's focus, in order to reduce the relative path length between the sample and reference arms. In most coherent imaging systems, this serves the function of keeping the material surface visible inside of the field of view.

One example of how to produce this correlated reference path length actuation is to digitally synchronize a reference mirror actuator (which may be located some distance away from the beam delivery system itself) to the processing beam focus actuator. An example actuator for the reference arm is an Aerotech PRO115™ linear translation stage (Aerotech, Inc. , Pittsburgh, USA). Less expensive and mechanically precise linear actuators may also be used. If the reference path attached to the actuator has multiple passes (i.e., the unidirectional beam path crosses the expanse that is modified by the actuator), then the actuator may be programmed to move a corresponding fraction of the distance moved by the processing beam focal adjuster.

Another example of how to produce the correlated reference path actuation is to use a surface attached to the processing beam focal actuator itself as the reference surface. In this example, some component of the reference path exists inside the beam delivery system and is reflected by a retroreflector or a mirror that is mounted on the processing beam focal actuator. In this way, as the focal position is changed by the actuator, the reference path length is automatically compensated. It may be desirable to mount a lens above the reference surface in the beam delivery head that converges the beam on said reference surface in order to make the coherent imaging signal less sensitive to mechanical imperfections in the focal actuator. The configuration described in this example also provides the benefit of being used as an autofocus system. The focal actuator is programmed via the image processor and feedback controller to keep the material surface at user-defined distance from the zero-delay point in the coherent imaging system.

In order to reduce dispersion mismatch between the reference and sample paths, it is desirable to use approximately equal amounts of each optical material in both paths (this includes air/vacuum). It may be challenging and/or impractical to produce a sufficiently long air path for the reference entirely within the beam delivery system. It may also be very problematic if the process lens, i.e., lens A in <FIG> or lens <NUM> in <FIG>, was exchanged for one with a different focal length. Therefore, in the example mentioned above, the air component of the reference arm is broken into two sections. The second section is the one that reflects off the processing beam focal actuator as described above. The first section of the reference path is produced at another location inside the coherent imaging system by coupling out of the optical fiber and into the air and then back into fiber before transmitting the reference light to the second section in the beam delivery system. This first part of the path can be adjusted to configure the amount of reference signal power desired, the dispersion characteristics of the reference path, and provide fine adjustment of the zero-delay point relative to the material and/or the focus of the high power processing beam.

In the case that the coherent imaging system does not share the same focal objective as the processing beam, then the focal position of the coherent imaging system may also be actuated axially to keep the focus of the coherent imaging system near the surface of the material being modified.

An example of such a system is depicted in the embodiment of <FIG> shows a reference arm that has an adjustable delay line, and also that has a delay tied to the position of the process lens carrier, an adjustable focal position, and that has capacitive height sensing. More generally, one or more of these features may be implemented. In some embodiments, one or more of these features is implemented in combination with any of the methods/systems described herein.

The coherent imaging system may be of the time-domain, spectral domain (i.e., spatially multiplexed spectral measurements) or swept-source (i.e., temporally multiplexed spectral measurements) types. In the first two cases, a superluminescent diode and/or broadband laser (e.g., mode locked Ti:Sapphire, mode locked fiber laser) is an example of an acceptable light sources for coherent imaging. A microelectromechanical system tunable vertical cavity surface emitting laser, and a MEMS (micro-electromechanical system)-tunable VCSEL (vertical cavity surface emitting laser) (see for example Benjamin Potsaid, et al. "MEMS tunable VCSEL light source for ultrahigh speed <NUM> - <NUM> axial scan rate and long range centimeter class OCT imaging", Proc. SPIE <NUM>, Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XVI, (February <NUM>, <NUM>); see also Thorlabs MEMS VCSEL Swept Source OCT System, Thorlabs Inc. , Newton, USA). Light sources with very long instantaneous coherence lengths allow for longer imaging ranges that may be particularly beneficial for observing and controlling the material modification processes described herein and those described through reference.

Swept-source and time-domain imaging approaches typically do not use detectors that can substantially discriminate between different frequencies of light. This means that they are more vulnerable to being overloaded by the incoherent emissions of the process and/or by the high power modification energy. The addition of blocking filters, either inside the fiber line (e.g., fiber Bragg gratings, etc.) or at the detector, to isolate the imaging light from the unwanted signals may be employed for the material processing applications described herein. Balanced detection is also substantially beneficial for rejecting these unwanted signals.

While the majority of the examples presented here concern fiber-delivered lasers operating in the <NUM> to <NUM> wavelength range, this approach is agnostic to the wavelength of the high powered material modification beam. For example, the wavelength of the modification beam may be between <NUM> and <NUM>.

In some embodiments of the invention, the reference arm mirrors may be moved in order to adjust the optical path length of the reference arm. Due to precision limitations in the motion control and the sensitivity of energy coupling into single-mode optical fiber, the DC intensity of the reference signal may change with the position of the reference arm delay line. In order to reduce the appearance of fixed pattern noise in the image when the DC power level changes, the background subtraction array may be scaled to match the current DC power level. The amount of scaling can be determined a priori/offline by mapping the DC power level for some set of reference arm positions, or in real time by minimizing the DC power signal (after the signal has been converted from an interferogram) through changing the scaling.

In some embodiments, the processing beam and the imaging beam(s) are combined after their respective focal objectives. Such a system has a beam combining device located distally (i.e., towards the sample) after the focusing devices for the processing and imaging beams. The combiner may be used as a cover glass to segregate other optics from the process gas.

In the case of a high power laser processing beam, the combiner may be, for example, a multilayer dielectrically coated optic that transmits the high power processing beam and reflects the imaging beam. Reflecting the high power processing beam and transmitting the imaging beam is also possible.

The combiner may also be used to sample the intensity of the high power processing beam by directing it to an optical power meter subsystem of the beam delivery system.

Independent lenses allow for scanning both the high power processing beam and the imaging beam and maintaining the alignment of the imaging system relative to the process. Otherwise, chromatic aberrations inherent in the focal lenses might cause the imaging and processing beams to walk off of each other, particularly at larger scan angles.

An example of such a system is depicted in <FIG>. The elements of <FIG> include the following:.

Not shown is a power control input to the laser, cover gas nozzle, or wire feed/arc tip, any of which may be included in some embodiments.

Mirror <NUM> in <FIG> could optionally be a pair of mirrors and could be motorized to allow for "remote" welding. Element <NUM> could be a power detector or simply a power absorber. Furthermore, it is noted that either scanning mirror pair, or the adjustable mirror of any of the embodiments described could be replaced with an acousto-optic or electro-optic deflector.

Note that the approach of <FIG> has the benefit of not requiring an F-theta lens to be used with the processing beam.

All of the techniques described herein may be applied to observe hybrid laser arc welding or any other material modification process that has a vapor channel/capillary, such as, for example, material liquefaction.

In some embodiments of the invention, multiple imaging beams are focused on different areas of the sample. It is not required to have a separate light source and detection scheme for each of these points. Instead, they can be multiplexed onto the same detector (as demonstrated in <FIG>). To accomplish this, one may split the sample arm into one or more paths/channels (e.g., by way of an evanescent mode coupler, or multiple internal reflections within an optic) with different optical path lengths. In some cases, the difference in these lengths would be greater than the depth field of view of the coherent imaging system, which may be preferable depending on the application and the number of channels, but not required. The reference arm can be configured to generate one or more reference reflections to correspond to each of these different paths. The actual splitting of the reference arm itself can be accomplished in many ways that would be known to those of ordinary skill in the art (e.g., with semi reflective mirrors, or by splitting the reference into multiple paths each with their own configurable delay), but one must carefully plan the different path lengths to match the expected sample paths during use. Each reference arm path would be configured to add enough additional delay to each channel such that their signals would not overlap. In some embodiments, channels with lower SNRs (e.g., signals from within the keyhole) would be configured to be closer to the zero delay point than those with higher expected SNRs. The result is that each sample image channel would appear at a different location in the processed image and can therefore be simultaneously observed without the need for an entirely separate coherent imaging system.

Another way to effectively multiplex the reference arm is to utilize the secondary coherence subpeaks that are exhibited by some low-coherence light sources (due to residual Fabry-Perot modulation) such as those sold commercially by Superlum (Carrigtwohill, Ireland). These subpeaks create an interference signal typically several millimeters away from the main lobe of the coherence function. This separation can be effectively used as if there was a separate reference surface several millimeters away from the primary one.

In another embodiment of the invention, a birefringent optic (such as a calcite prism) is used to multiplex the imaging beam and spatially separate it onto different parts of the sample. The polarization of the imaging beam into the birefringent optic can then be adjusted to balance the intensity between the two sample paths. Similarly, other birefringent optics can be used in the reference arm to create polarization-specific optical delays for interference comparison and/or to address/access the two polarization channels.

Surface composition and/or quality (including, e.g., an indication of oxidation condition, and roughness) is an important factor in the acceptance of a particular material modification process. As the imaging light from a coherent imaging system interacts with the material surface, it can provide a measurement of surface roughness and/or oxidation. This information can be extracted from relative intensity measurements (i.e., comparing signal intensities from different locations and/or different materials) and spectral measurements (i.e. monitoring signal attenuation in a particular region of the imaging spectrum). All of these signals can be processed through the imaging processor and be used to generate various annunciations and process feedback as desired.

In some embodiments, when processing coherent images, the location of reflecting (and/or scattering) interfaces is determined by locating the depth element with the peak signal intensity (peak pixel identification). In some embodiments, the location of reflecting interfaces is achieved by fitting a quasi-Gaussian curve and extracting its centroid. In some embodiments, the location of reflecting interfaces is achieved by measuring the phase and location of the fringes in the interferogram. In some embodiments, combinations of two or all three of these methods are used to achieve the location of reflecting surfaces. In some embodiments, certain signals are selected and others are disregarded (e.g., in the case of background noise or secondary reflections, to choose a region of interest, or the like).

Interferometery is used throughout the embodiments described herein to generate the measurements discussed. In some embodiments, the interferometer comprises: a combiner; a reference arm, a first component of the imaging light being applied to an input of the reference arm resulting in an output signal of the reference arm, the reference arm having another optical path length; and a sample arm, a second component of the imaging light being applied to the sample arm resulting in an output signal of the sample arm, at least a component of the output signal of the sample arm including reflections of the component of the imaging light from a sample location, the sample arm having at least one optical path length; wherein the combiner combines the output signal of the reference arm and the output signal of the sample arm to produce a combined signal as an interferometry output; the apparatus further comprising a signal detector configured to produce a first interferogram from the interferometry output.

In some embodiments, the apparatus comprises at least one of: multiple sample arms, a respective interferogram being generated for each sample arm, reference arm combination; multiple reference arms, a respective interferogram being generated for each sample arm, reference arm combination; and multiple reference arms and multiple sample arms, a respective interferogram being generated for each sample arm, reference arm combination.

In some embodiments, the interferometer comprises: at least one splitter and/or optical circulator; and at least one sample arm after the splitter and/or optical circulator, the imaging signal being applied to the sample arm resulting in an output signal of the sample arm, at least a component of the output signal of the sample arm including reflections of the component of the imaging signal from at least two locations in the sample arm and/or the material being processed, the sample arm having at least one optical path length and another optical path length; wherein the splitter and/or optical circulator receives the output signal from the sample arm and directs it towards a detector; the apparatus further comprising a signal detector configured to produce an interferogram from the interferometry output.

<FIG> is a block diagram of a first detailed implementation. In this embodiment, modification beam (ML) <NUM> and imaging beam (SLD) <NUM> sources are shown. In this embodiment, the two light paths are combined by a dichroic or other combining optic (DM) <NUM> after independent focal objectives <NUM>, <NUM>. In this embodiment, the interferometer can be built in single or, in other embodiments, in multi-mode optical fibre. Detection is accomplished by means of a high speed spectral detector (HSS) <NUM>. While the embodiment shown uses a <NUM>:<NUM> power splitting ratio <NUM> between sample arm <NUM> and reference arm <NUM>, in other embodiments other splitting ratios in the interferometer are possible and may depend on the availability of optical power and/or the need for detection sensitivity. In some embodiments, other interferometer configurations, e.g., Mach-Zehnder, Sagnac, common path, etc. may be used. In this embodiment, DM <NUM> is shown to reflect the imaging light and transmit the modification light, however, the reverse can alternatively be implemented. In some embodiments, combination of the beams via polarization-sensitive or neutral reflection optics may be implemented. A person of ordinary skill in the art will understand that detection, processing, and feedback electronics are omitted from the embodiment shown in this figure and such processing steps may be performed within the feedback controller. Feedback controller <NUM> receives the output of the HSS <NUM> and controls the modification laser <NUM> and/or one or more other aspects of the material modification process.

<FIG> is a block diagram of a second detailed implementation. In this embodiment, a high power broadband source is created by coupling short, dispersion-optimized pulses output by broadband source <NUM> into a length of single mode optical fiber <NUM>. This results in an expansion of spectral bandwidth, in some embodiments, on the order of a factor of <NUM>, though in other embodiments, more or less broadening is possible. The embodiment shown features a Ti:AlO3 laser source <NUM> that operates in the region of <NUM> to <NUM>. In other embodiments, spectral ranges from <NUM> to <NUM> from other optical imaging sources may be used. In this embodiment, a Glan-Taylor polarizer (GTP) <NUM>, Faraday optical isolator (ISO) <NUM>, half-lambda waveplate polarization control <NUM>, and Fork prism dispersion compensation <NUM> are shown. In other embodiments, other broadband sources (such as superluminescent diodes, other lasers, and/or other broadening methods) may be substituted for the broadened Ti:AlO3 laser source.

In this embodiment, the modification laser (ML) <NUM> passes through collimator <NUM> and the imaging beam passes through sample arm collimator <NUM> after which the modification laser beam and the imaging beam are combined by an optic component (DM) <NUM> before they are focused by a common focal objective <NUM>.

In such embodiments, the lens may be achromatic, aspheric, and/or conical (e.g., axicon). This beam combination may be focused through an optional nozzle <NUM> that can be used to apply assisting fluids (e.g., compressed gas, water spray) to the modification process. The nozzle spray may also be independent from the optical beam; i.e., the two are delivered to the sample from different points. The Michelson interferometer includes a <NUM>:<NUM> splitter <NUM> (though in other embodiments, other splitting ratios may be used), reference arm collimator <NUM>, and reference mirror <NUM>. Also shown are polarization controllers <NUM>, <NUM>, <NUM>. The spectral detection in this embodiment involves a fiber-coupled reflective grating spectrometer <NUM>. In some embodiments, an additional mirror in front of the lens (ASL) <NUM> can allow the beam to approach and leave the reflective grating <NUM> as close to the Littrow configuration as possible, improving diffraction efficiency. In some embodiments, a transmission grating and/or multi-grating, and/or Fabry-Perot spectrometer may be used. A silicon line camera <NUM> produces an interferogram that is passed to image processing electronics <NUM>, the output of which is passed to feedback controller <NUM>. Feedback controller <NUM> produces a feedback <NUM> to control the modification laser <NUM> or some other aspect of the modification process.

It will be understood that the methods and apparatus described herein may also be used in applications in "remote" welding heads (such as, for example, the Scanlab AG IntelliWELD™ head, Scanlab AG, Germany) where the imaging system is introduced into the camera port of the head. In this case, any splitting or scanning of the imaging beam is still done, but since the processing and imaging beams share the scanning mirrors inside the remote head, the imaging beam is always positioned relative to the processing beam. In other words, the IntelliWELD head's mirrors provide a "common mode" deflection of the imaging and welding beams and an additional scanner/splitter (such as the one depicted in <FIG>, element <NUM>) produces offsets between the imaging and processing beams for the purposes of imaging various locations in the vicinity of the PCR.

In another embodiment, the remote head utilizes scanning mirrors after the focusing objective (i.e., "post-objective scanning"), thereby minimizing chromatic aberrations and/or chromatic walk-off of the processing and imaging beams that might be present when using an F-theta scan lens.

In some embodiments, an active deflection element (such as the one shown in <FIG>, element <NUM>) is programmed to deflect the imaging beam by an angle that is related to the current velocity of the processing beam relative to the workpiece. This is because many of the features of interest of the PCR lag behind the processing beam by an amount that is related to the processing velocity. Such dynamic lag control may be particularly acute when the process is changing velocity, magnitude, and/or direction, such as in a curvilinear weld. The amount of lag may be preprogrammed with the process, automatically looked up from a stored table, or dynamically calculated, e.g., from a fitting function that is preprogrammed at the time of manufacturing or adjusted in the field.

In some embodiments, the apparatus may be programmed to automatically search via scanning for a specific PCR feature, and recognize when proper alignment is achieved. For example, proper alignment with the bottom of the vapor channel is achieved when a sufficient/optimal frequency and strength of a reflected signal is detected from a subsurface depth. Through a process of, for example, initial offset estimation, dithering, and successive iterative adjustment, the optimal PCR lag for a linear or curvilinear weld path may be found. Such automatic alignment lends itself to considerable ease of utilizing the embodiments described herein.

Active deflection adjustment and dithering is also useful for maintaining alignment with the PCR if changes to the optics (e.g., arising from vibration, wear, temperature shifts, thermal lensing, or rebuilding of the optics).

Other examples can be found in <CIT>, and International Patent Application No. <CIT>.

A first experiment will be described in the context of the example apparatus of <FIG>. For the experiment, the laser <NUM> was a <NUM>, <NUM> kW IPG fiber laser, and the laser head <NUM> was a Laser Mechanisms AccuFiber™ head (Laser Mechanisms, Inc. , Michigan, USA). The imaging system <NUM> was connected to the laser head <NUM>, and produced imaging light having a <NUM> central wavelength (and a bandwidth of approximately <NUM> measured at full-width, half-maximum) which was reflected off a movable mirror <NUM>. A dichroic mirror <NUM> was used to allow the laser beam and the imaging beam to be directed towards the sample while being closely aligned. A sample of mild steel plate was moved at rates of approximately <NUM>/s in a linear direction <NUM> while continuous wave keyhole bead on plate welding was performed. Weld depths of approximately <NUM> were achieved. Trials were performed with the imaging beam <NUM> directed at various positions (e.g., 20a-20e of <FIG>) relative to the laser beam <NUM> such that multiple measurements within the keyhole and PCR <NUM>, <NUM>, <NUM> were obtained.

<FIG> are coherent images of keyhole laser welding. This series of images demonstrates the effect of leading, aligning, or trailing the processing beam with the imaging beam. When the imaging beam leads the processing beam, a poor view of the keyhole depth is achieved in this example, in part because the bottom of the keyhole is poorly illuminated by the imaging beam and in part because much of the imaging light is reflected away and lost by the front wall of the keyhole, never returning to the imaging system. When the imaging beam is aligned with the processing beam, image quality and signal-to-noise ratio (SNR) improves, but there is significant vertical scatter as this location still includes a great deal of the front keyhole wall, which can be quite unstable in the axial direction. Significant improvements to SNR and vertical stability are seen by trailing the processing beam by a fraction of its width. When the processing beam is trailed by an even larger amount, both the top surface and the bottom of the keyhole can be measured. This important capability allows for dynamic determination of the top surface reference points (TSRP) and is useful in cases where there is distortion of the material and/or of the motion system. These images are described in more detail below.

In <FIG>, the x-axis represents time, readings were taken at a rate of <NUM> (time bins on the x-axis are separated by <NUM>), and because the sample was in motion at <NUM>/s, this axis also corresponds to distance along the sample during the weld. The y-axis represents height above the zero delay point of the interferometer. The zero delay point is placed below the surface of the material such that depth is increasing as the signal approaches the zero delay point at the top of the image (labelled <NUM>), i.e., increasing depth is in the upward direction. The black pixels in the image correspond to the depth of the reflection of the imaging beam off the sample surface, darker pixels correspond to a brighter reflection. The black pixels appear to be scattered in part because the keyhole is constantly changing. The diameter of the imaging beam is approximately <NUM> and the diameter of the processing beam is approximately <NUM>.

<FIG> depicts image data where the imaging beam leads the processing beam by <NUM> and roughly corresponds to a position between <FIG>, 20a and 20c. The sample begins to move and reaches a steady speed before the <NUM> time mark (left edge of the image). Imaging begins some <NUM> before the laser turns on and is used to establish the TSRP of <NUM>. After the laser turns on, a keyhole is rapidly created (in the first <NUM> time bins, corresponding to <NUM>) as shown by the black pixels. The keyhole at this position then remains open with measurements of between <NUM> and <NUM>. This gives a depth reading of approximately <NUM> and <NUM>, getting deeper as the weld progresses. The laser turns off at <NUM> time bins and the weld terminates and the motion stops after approx. <NUM> time bins covering a distance of <NUM>. After this time, the imaging system continues to run virgin surface only. In <FIG>, the front wall of the keyhole very close to the leading edge is being examined. It can be seen that there is some scatter in the data representing the dynamic variations of the keyhole front wall position and slope.

<FIG> depicts image data where the imaging beam aligned with the processing beam and roughly corresponds to position 20b of <FIG>. This image has a much higher signal to noise ratio (i.e., the signal is brighter), making it easier for a viewer or image processor algorithm to locate the keyhole bottom. As a result, one can more clearly see the initial formation the keyhole as the black pixels move deeper in the first <NUM> time bins of the image. The keyhole depth remained fairly constant between <NUM> and <NUM> below the TSRP (located at <NUM> in this image), getting slightly shallower towards the end of the weld. This image also demonstrates a feature of some embodiments of the invention. Here, in addition to the main imaging beam measuring the bottom of the keyhole, a secondary beam path is created by a multiple reflection inside the dichroic (beam combining) mirror. This secondary beam is delayed relative to the first (i.e., appears at a different depth in the image) and impinges on the sample at a separate transverse location substantially outside of the PCR on the virgin metal. This secondary path also creates an interference signal that shows up in the image at the indicated depth <NUM>. Because this transverse location is substantially outside of the PCR, the capability to track the virgin surface simultaneously with imaging the bottom of the keyhole is maintained. This allows for dynamic measurement/calculation of the TSRP and/or height.

<FIG> depicts image data where the imaging beam trails the processing beam by <NUM> and roughly corresponds to position 20c of <FIG>. The TSRP was measured to be at <NUM> in this image. Here the keyhole appears even brighter than in <FIG> with a depth of between <NUM> and <NUM>. There is less vertical scatter in the image showing that the keyhole has more stability at this 20c location.

<FIG> depicts image data where the imaging beam trails the processing beam by, e. g, <NUM>, and roughly corresponds to position 20d of <FIG>. After initiation the keyhole depth is approximately <NUM>. The TSRP was measured to be at <NUM> at the beginning of the image, and <NUM> at the end of the weld. As a result, the weld depth appears in this image to change by some <NUM> across the weld, despite not actually changing. This image illustrates the challenge of applying coherent imaging techniques accurately without a well-defined TSRP. One way of obtaining a TSRP for all points during the image is to assume that the change in TSRP is linear with time and so can be interpolated from the beginning and end TSRPs. In many cases, this is sufficient. Note that this imaging beam is trailing the processing beam sufficiently to illuminate the keyhole bottom and the top surface of the rear of the weld simultaneously. If one draws an imaginary line between time <NUM>, depth <NUM> and time <NUM> and depth <NUM>, several (approximately <NUM>) points of enhanced signal can be seen. These are reflections from just behind the keyhole in the region that will form the solidified surface of the weld. Since the top of the weld is at about the same height as the virgin surface of the material, these signals can be used for additional TSRP measurements. This feature of the embodiments is clarified further in the next figure.

<FIG> depicts image data where the imaging beam trails the processing beam by <NUM> and roughly corresponds to a position between beams 20d and 20e of <FIG>, such that both the surface of the weld and the bottom of the keyhole are illuminated at some time. The TSRP was measured to be at <NUM> at the beginning of the image. Here the black pixels are gathered in two bands at different depths. The deeper (closer to <NUM> in the image) line corresponds to the bottom of the keyhole at the 20c/20d locations. The shallower upper line (in this example the TSRP) corresponds to reflections off the liquid phase of the PCR as would be seen at location 20e. The signal intensity that returns from either region depends on how much of the imaging beam illuminates and reflects from either region. The two bands in the image are the result of the dynamic balance between the signal returning from the two regions. When the keyhole is more narrow, the shallower band of black pixels corresponding to the liquid phase of the PCR at the top of the keyhole is seen more prominently. When the keyhole is wider, more of the bottom is illuminated, enhancing the deeper band black pixels corresponding to the bottom of the keyhole is seen.

From this one can determine that the keyhole front wall is approximately <NUM> ahead of the central axis of the processing beam and the back wall varies but is approximately <NUM> behind the processing beam, for a total keyhole length of approximately <NUM>. One can also calculate the front wall slope by forming an imaginary line between the depth reading in <FIG> taken at the same (or very nearly the same) instant in time. Other keyhole characteristics can be computed in a similar manner.

Results accuracy can be improved by taking readings at more locations either sequentially through multiple welds with different beam alignments, or dynamically by changing the beam alignment in real time. Other keyhole characteristics can be computed in a similar manner. A leading or trailing imaging spot could also allow measurement of the top surface and bottom surface of the PCR at the same time. Notably, the same approach can be applied to transverse keyhole dynamics just as well as the longitudinal dynamics just discussed.

In some embodiments of the invention, one or more measurement beams will be directed to multiple locations in and near the PCR in rapid succession (perhaps up to <NUM> or more) by using a movable mirror (depicted as element <NUM> in <FIG>) under imaging system control.

In some embodiments, imaging may be done by multiple measurement beams directed at multiple locations simultaneously. In some embodiments, this may be accomplished by multiplexing the imaging beam with a geometric optic such as a multifaceted mirror and/or a prism.

In another experiment conducted using an apparatus based on <FIG>, the laser <NUM> was a <NUM>, <NUM> kW IPG fiber laser which was connected to a Laser Mechanisms AccuFiber laser head <NUM>. An imaging light source <NUM> with a <NUM> wavelength was also connected to the laser head, reflected off a movable mirror <NUM> and through dichroic mirror <NUM> used to allow the laser beam and the imaging beam to be directed towards the sample and closely aligned. A sample of mild steel was moved at rates of approximately <NUM>-<NUM>/min in a linear direction <NUM> while continuous wave keyhole bead on plate welding was performed. Weld depths of up to <NUM> were achieved. Trials were performed with the imaging beam <NUM> directed at various positions relative to the laser beam <NUM> such that measurements within the phase change region (PCR) <NUM> were obtained.

The results demonstrated that by taking measurements at multiple points within the PCR, information about the maximum weld depth, depth variability, keyhole stability, and weld quality could be obtained. In a specific example, the processing beam incident point is depicted at <NUM>, and this moves with the sample, and the imaging beam is aligned to be incident at a point <NUM> which trails the processing beam incident point <NUM> by about <NUM>-<NUM>. This particular approach reduced the vertical scatter of the maximum weld depth measurements substantially when compared to imaging without such an offset. For larger keyholes or those that are deeper with different shapes, the optimum offset will be different and can be determined experimentally. As before, a leading or trailing imaging spot could also allow measurement of the top surface and bottom surface of the PCR at the same time. In some embodiments, imaging would be done at multiple locations in and near the PCR (depicted in <FIG> as imaging beams 20a through 20i) in rapid succession (perhaps up to <NUM> or more). In some embodiments, this is achieved by using an electronically movable mirror (depicted in <FIG> as <NUM>) under imaging system control.

In a third experiment, an embodiment similar the one shown in <FIG> was used to capture an interferometry signal from the PCR of a laser weld at a plurality of positions ranging from in front of the processing beam to behind the processing beam. The result is shown in <FIG>. Before the processing beam, the information may be used to determine the material position, surface quality, and geometrical compliance of the incoming material. Additionally, scanning using, e.g., <FIG>, element <NUM>, on another axis may locate a surface feature (e.g., a groove) which can be used to provide seam tracking capabilities before the processing beam.

As the imaging beam is located closer to the processing beam (labelled "capillary" in <FIG>), it begins to sense the front wall and the bottom of the vapor component (i.e., keyhole) of the PCR. This permits the sensing of many keyhole dynamics such as depth, width, collapse, melt ejection, pore creation, and instability that may be important quality indicators for the process.

As the imaging beam is located behind the processing beam, it begins to sense the melt pool surface. The rapidly changing height of this surface is an indicator of its liquid state since a solid material would not change height so rapidly. Therefore, the length of the surface disruption is indicative of the melt pool length as shown in the figure. The length of this melt pool may be an important indicator of many quality aspects for the process including proper mechanical contact/connection between parts that are to be joined. It may be possible to assume that multiple welds that have the same melt pool length would share similar quality aspects and therefore this information is useful for industrial quality control applications.

As the imaging beam is located further behind the processing beam, the solid surface of the material is sensed. The roughness, height, and depth of this surface may be an important quality indicator. For instance, many surface ripples indicate an unstable weld process that may have created high porosity and therefore compromised hermetic sealing and/or strength.

Furthermore, quantifying the amount of underfill along the surface of the seam is important information for determining the strength of the weld, its resistance to corrosion, and its compatibility with subsequent coating processes such as priming and painting.

As the imaging beam is located even further behind the processing beam, the signal returning from the solid material surface becomes weaker. This is because oxidation of the surface has occurred, making it is less reflective. This information is indicative of cooling rate and shielding gas quality/coverage. Such information may be used, for example, to correct deficiencies in the shielding gas and/or reject parts.

In a fourth experiment, an embodiment similar to the one shown in <FIG> was used to capture an interferometry signal from the PCR of a laser weld at a plurality of positions ranging from the left to the right of the processing beam. As shown in <FIG>, the resulting data allows estimation of a number of characteristics of the PCR including the melt pool width, the keyhole width, and keyhole depth.

The width of the melt pool may be an important indicator of many quality aspects for the process, including proper mechanical contact/connection between parts that are to be joined. It may be possible to assume that multiple welds that have the same melt pool length would share similar quality aspects, and therefore this information is useful for industrial quality control applications.

The width of the keyhole and asymmetries thereof may indicate the degree of alignment of the process laser beam with joints of the material being processed (e.g., in a butt or fillet weld) and may therefore be used to guide a seam-tracking process to better process the material.

In a fifth experiment, an embodiment similar to the one shown in <FIG> was used. For this experiment, the laser <NUM> was a <NUM>, <NUM> kW IPG fibre laser, and the laser head <NUM> was a Laser Mechanisms AccuFiber™ head. An orthogonal pair of movable mirrors, actuated by galvanometers, was inserted between the collimating lens <NUM> and the dichroic mirror <NUM> to enable rapid adjustment of the imaging beam alignment. A sample of stainless steel plate was moved at rates of <NUM>/min and <NUM>/min while continuous wave keyhole bead on plate welding was performed. Trials were performed with the imaging beam aligned coaxially with the processing beam, as in previous experiments, and with the galvanometer-driven movable mirrors adjusted to place the imaging beam slightly ahead of or behind the processing beam.

<FIG> are coherent images of keyhole laser welding with the imaging beam aligned ahead of or behind the processing beam. Each of these images contains data from two similar welds, performed in opposite directions. These images are oriented with the top surface of the material towards the top of the image, and the zero delay point (at the bottom of the image) deep inside the material. In some cases, the samples have warped with heat; this is visible as a tilt or curvature along the length of each of the two opposite-direction welds in each image.

In <FIG>, the welds were performed at <NUM> W laser power with a feed rate of <NUM>/min. The imaging beam was steered, using the galvanometers, to a focal spot <NUM> behind the processing beam in the first of the two welds. The alignment of the imaging beam was held constant when the direction of travel was reversed for the second weld in this image, so the imaging beam was then <NUM> ahead of the processing beam. At high feed rates, it is expected that the keyhole will lag slightly behind the processing beam. Aligning the imaging beam to a position behind the processing beam led to a stronger signal from the bottom of the keyhole; when the direction was reversed and the imaging beam was ahead of the processing beam, the bottom of the keyhole was not visible and strong signals from near the top of the front wall of the keyhole were obtained.

<FIG> shows the result of a similar experiment, identical to that of <FIG> except for the feed rate (<NUM>/min instead of <NUM>/min) and the imaging beam alignment set by the galvanometers (<NUM> behind the processing beam instead of <NUM>). The first of the two welds in this image shows a strong reflection from the bottom of the keyhole, with weaker signals from near the top of the back wall of the keyhole. When the direction was reversed for the second weld, and the imaging beam was leading the processing beam, the signal was dominated by the reflection from the surface of the melt pool ahead of the keyhole, with occasional glimpses of the front wall of the keyhole.

<FIG> illustrate the difference between fixing the alignment of the imaging beam at a set position relative to the processing beam, and using the galvanometer-actuated mirrors to align the imaging beam at a set distance behind the processing beam given the current direction of travel. The former case (<FIG>), the first weld of the pair, performed with the imaging beam slightly lagging the processing beam, yielded a good view of the bottom of the keyhole, but the second weld - performed in the opposite direction - yielded a poor image. In <FIG>, the alignment was adapted according to the direction of travel, and welds in both directions yielded images of similar high quality.

While the effect of lagging/leading with the imaging beam is shown here with linear welds, it is understood that a <NUM>-axis galvanometer mirror system (for example, a Scanlab AG Scancube™ <NUM>) allows this technique to be generalized to <NUM>-D curvilinear welds. It is also understood that this approach may be generalized to "remote" welding heads such as the HighYAG RLSK™ (HIGHYAG Lasertechnologie GmBH, Germany), TRUMPF PFO™ series (TRUMPF Laser- und Systemtechnik GmbH, Germany), and Scanlab AG IntelliWELD™. In one embodiment, illustrated in <FIG> for a pre-objective scanner and in <FIG> for a post-objective or "F-Theta" scanner, this can be accomplished by using a collimating lens <NUM>, a <NUM>-axis galvanometer scanner <NUM> and a refocusing lens <NUM>, selected such that the output numerical aperture of this system substantially approximates the supported numerical aperture of the remote head's camera port, to image the fibre core onto the focal plane <NUM> where a camera sensor would normally be. By synchronizing the motion of this <NUM>-axis galvanometer scanner (and optionally the imaging system's reference arm) with the remote head's motion then the optimal lag between imaging and process beams can be achieved. Programming of this optimal lead/lag can be done manually or automatically through SNR evaluation in iterative and/or dithering processes. Some embodiments of the invention may use a lookup table and/or memory of these optimal lead/lag positions, as functions of the velocity and/or position of the focal spot for various combinations of laser settings, that can be accessed and updated offline or in real time.

Claim 1:
An apparatus and a material processing system, the material processing system having an optical access port, the apparatus comprising:
an imaging optical source (<NUM>) that is configured to produce imaging light (<NUM>) that is applied to the material processing system, wherein the material processing system is configured to implement a material modification process and to create a phase change region (PCR) in a material;
a material processing beam source that is configured to produce a material processing beam that is applied to the material in the material modification process, wherein the material processing beam creates the PCR in the material;
at least one element (<NUM>) that is configured to direct the imaging light (<NUM>) at a plurality of imaging beam positions (20a-20f, 20i, <NUM>) proximate the PCR;
at least one input-output port (<NUM>) that is configured to output a first component of the imaging light to the optical access port of the material processing system and that receives a reflection component of the imaging light;
an optical combiner (<NUM>, <NUM>) that is configured to combine the reflection component and at least another component of the imaging light to produce an interferometry output, the interferometry output based on a path length taken by the first component and the reflection component compared to a path length taken by the at least another component of the imaging light; and
an interferometry output processor (<NUM>) that is configured to process the interferometry output to determine at least one characteristic of the PCR; and
characterized by further comprising:
a mechanically actuated objective lens (A, <NUM>, <NUM>) that is controlled to keep focus (<NUM>) of the material processing beam a certain distance from the sample location, wherein the another path length is a reference path length which is changed in correlation with actuation of the mechanically actuated objective lens.