Abstract:
Systems and techniques for processing materials using wavelength beam combining for high-power operation in concert with interferometry to detect the depth or height of features as they are created.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/973,353, filed on Apr. 1, 2014, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    In various embodiments, the present invention relates generally to beam-emission systems, and more particularly to systems and techniques for processing materials. 
       BACKGROUND 
       [0003]    High-power lasers are used in many cutting, etching, annealing, welding, drilling, and soldering applications. An advantage to these systems is the precision with which cuts can be made and recesses etched into a wide variety of materials. 
         [0004]    One challenge in implementing high-precision laser-based processing systems is determining the height or depth of a feature as it is formed. Material anisotropy, or even ordinary compositional variations across a nominally uniform material, can affect the amount of material removed by a laser at a given power level. Without real-time knowledge of feature depth or height during processing, and feedback capability to alter the beam in response thereto, it is impossible to guarantee precise dimensions that do not vary across the feature particularly if the feature extends across a considerable (e.g., more than a few microns) length, since the more material that is processed, the greater will be the likelihood of encountering compositional variations that affect material response. 
       SUMMARY 
       [0005]    Embodiments of the invention provide systems and techniques for processing materials using wavelength beam combining for high-power operation in concert with interferometry to detect the depth or height of features as they are created. 
         [0006]    Interferometry makes use of the principle of superposition to combine waves such that their combination is diagnostic of the original state of the waves. This works because when two waves with the same frequency combine, the resulting pattern is determined by the phase difference between the two waves—i.e., waves that are in phase will undergo constructive interference while waves that are out of phase will undergo destructive interference. Thus, depending on the height or depth of a feature in relation to the wavelengths of the incident radiation, some wavelengths, when combined with their reflections, will undergo these changes, which indicate the true depth/height of a feature—e.g., a cut, weld, or mesa. By utilizing a two-dimensional (2D) beam, the structure of a 3D feature can be determined. Feature information can be used in a feedback configuration to ensure that the entire feature remains uniform as it is created. 
         [0007]    Accordingly, in a first aspect, the invention pertains to a system for processing a workpiece. In various embodiments, the system comprises a wavelength beam combining (WBC) emitter for emitting a multi-wavelength output beam comprising optical radiation having a plurality of wavelengths; a movable surface reflective to all of the wavelengths in the multi-wavelength output beam; a photodetector; a beamsplitter for (i) diverting a portion of the multi-wavelength output beam to the movable reflective surface and (ii) diverting a portion of a reflection of the multi-wavelength output beam from a surface of the workpiece to the photodetector, the photodetector being aligned with the movable reflective surface to receive therefrom a reflection of the diverted portion of the multi-wavelength output beam; and a controller, coupled to the WBC emitter device and the photodetector, for computing a height or depth of a feature on the surface of the workpiece based at least in part on a signal from photodetector. 
         [0008]    The height or depth may be determined based on a distance between the beamsplitter and the surface of the workpiece. This, in turn, may be determined from the signal from the photodetector, which may indicate the degree of interference between the reflection of the multi-wavelength output beam from the surface of the workpiece and the reflection from the mirror of the diverted portion of the multi-wavelength output beam. 
         [0009]    In various embodiments, wherein the controller is configured to control a parameter of the multi-wavelength output beam to maintain a target distance between the beamsplitter and the surface of the workpiece. For example, the controlled parameter may be one or more of power or beam parameter product. The WBC emitter may comprise a plurality of beam emitters each emitting a beam; a combining optical element arranged to receive the plurality of beams and cause a chief ray of each of the beams to converge along a beam-combining axis; a dispersive element, positioned along the beam-combining axis, to receive and transmit the converging chief rays; and a partially reflective output coupler arranged to receive the transmitted beams from the dispersive element, to reflect a portion of the transmitted beams toward the dispersive element, and to transmit the multi-wavelength output beam. 
         [0010]    In another aspect, the invention pertains to a method for processing a workpiece. In various embodiments, the method comprises the steps of causing emission of an output beam from a WBC emitter, where the output beam comprises optical radiation having a plurality of wavelengths; diverting a portion of the output beam to a movable surface reflective to all of the wavelengths in the multi-wavelength output beam; diverting a portion of a reflection of the output beam from a surface of the workpiece to a photodetector; receiving, at the photodetector, a reflection from the movable reflective surface of the diverted portion of the output beam, the reflection including all of the wavelengths in the multi-wavelength output beam; and computing a height or depth of a feature on the workpiece based at least in part on a signal from the photodetector. 
         [0011]    The height or depth may be determined based on a distance between the surface of the workpiece and a location where the portion of the reflection is diverted. For example, the signal from the photodetector may indicate a degree of interference between the reflection of the multi-wavelength output beam from the surface of the workpiece and the reflection, from the movable surface, of the diverted portion of the multi-wavelength output beam. 
         [0012]    In some embodiments, the method further comprises the step of controlling a parameter of the multi-wavelength output beam to maintain a target distance between the beamsplitter and the surface of the workpiece. For example, the controlled parameter of the multi-wavelength output beam may be power and/or beam parameter product. 
         [0013]    As used herein, the term “optical element” may refer to any of lenses, mirrors, prisms and the like which redirect, reflect, bend, or in any other manner optically manipulate electromagnetic radiation. The term “beam” includes any form of directed electromagnetic radiation, and may be single-wavelength or multi-wavelength. Beam emitters, emitters, or laser emitters, or lasers include any electromagnetic beam-generating device such as semiconductor elements, which generate an electromagnetic beam, but may or may not be self-resonating. These also include fiber lasers, disk lasers, non-solid state lasers, vertical cavity surface emitting lasers (VCSELs), etc. Generally, each emitter includes a back reflective surface, at least one optical gain medium, and a front reflective surface. The optical gain medium increases the gain of electromagnetic radiation that is not limited to any particular portion of the electromagnetic spectrum, but that may be visible, infrared, and/or ultraviolet light. An emitter may include or consist essentially of multiple beam emitters such as a diode bar configured to emit multiple beams (i.e., each diode in the bar emits a single beam). 
         [0014]    The term “substantially” or “approximately” means ±10% (e.g., by weight or by volume), and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Nonetheless, such other materials may be present, collectively or individually, in trace amounts. Reference throughout this specification to “one example,” “an example,” “one embodiment,” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present technology. Thus, the occurrences of the phrases “in one example,” “in an example,” “one embodiment,” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, routines, steps, or characteristics may be combined in any suitable manner in one or more examples of the technology. The headings provided herein are for convenience only and are not intended to limit or interpret the scope or meaning of the claimed technology. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The foregoing will be more readily understood from the following detailed description of the invention, in particular, when taken in conjunction with the drawings, in which: 
           [0016]      FIGS. 1A and 1B  schematically illustrate a conventional beam-combining system that may be used to pattern or cut material. 
           [0017]      FIGS. 2A and 2B  schematically illustrate shortened WBC systems with non-confocal combining optics. 
           [0018]      FIG. 2C  illustrates a compact non-confocal dual lens WBC system. 
           [0019]      FIG. 3  illustrates a position-to-angle WBC system devoid of an optical combining element. 
           [0020]      FIG. 4  schematically illustrates a WBC laser system using a curved grating to increase brightness. 
           [0021]      FIG. 5  schematically illustrates a feedback system for controlling laser operation in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Aspects and embodiments relate generally to beam sources that achieve high power and high brightness using wavelength beam combining (WBC). The approaches and embodiments described herein may apply to 1D and 2D beam-combining systems along the slow-axis, fast-axis, or other beam-combining dimension. In addition, the techniques may apply to external- and non-external-cavity WBC systems. 
       Representative WBC Systems 
       [0023]    A conventional external-cavity 1D WBC system that may be utilized with embodiments of the present invention is shown in  FIG. 1 . The illustrated system utilizes a 1D bar  102  of diode lasers having a back-reflective surface  104 , a gain medium  106  with two or more diode emitters, and a front reflective surface  108 . The system also includes a combining optic  110  (e.g., a lens), a dispersive element  112 , and a partially reflecting output coupler  114 . The combining optic or lens  110  is located a focal distance  120   a  away from the front reflective surface  108  of the diode bar  102 , and the dispersive element  112  is located a focal distance  120   b  away from the optic  110 ; typically, the focal distances  120   a ,  120   b  are identical and correspond to the focal planes of the optic  110 . The output coupler  114  reflects a portion  122  of the generated beams to the dispersive element  112  and allows the remaining portion  125  to pass through as the system output. 
         [0024]    For explanatory purposes,  FIG. 1A  shows a single beam. In fact, the diode bar  102  generates a plurality of beams  130  as illustrated in  FIG. 1B . The combining lens overlaps the chief rays from all of the emitting elements on the dispersive element  112 , and collimates each beam in both axes orthogonal to the direction of propatation. 
         [0025]    A more compact WBC system may be achieved as shown in  FIGS. 2A and 2B  by intentionally placing the diode bar  102  or the dispersive optic  112  at a position other than the focal plane of the combining optical element  110 . If the combining optical element  110  is disposed less than a focal length from the diode bar  102 , one or more additional collimating optics  210  may be located in front of or behind the dispersive element  112  and in front of the partially reflective output coupler  114 . This arrangement can reduce the optical path length between the diode bar  102  and output coupler by almost a full focal length of combining element  110 , particularly when the combining element  110  is placed adjacent to the front surface/facet  108  of the diode bar  102 . 
         [0026]    In a variation of this embodiment, also shown in  FIG. 2A , a collimating optic  212  may be disposed in front of each emission point along the front surface/facet  108  of the diode bar  102  and in front of the combining optical element  110 , which still results in a shortened WBC system. In this variation, the collimating optic(s)  212  may comprise or consist of an array of micro-optical fast-axis collimating (FAC) lenses, slow-axis collimating (SAC) lenses, or combination of both. By collimating each beam, proper wavelength stabilization feedback into each of the diode elements is ensured. This enables each emission element to produce a unique wavelength that is stabilized with less susceptibility to shifting, producing a multi-wavelength output beam profile of high brightness and power is achieved. 
         [0027]    As shown in  FIG. 2A , the dispersive element or diffraction grating  112  is placed substantially at the back focal plane of the lens  110 . As drawn (the first approximation), the lens  110  with focal length F 1  only converges the chief rays for each of the diode elements. This can be understood from the Gaussian beam transformation by reference to the lens equation: 
         [0000]    
       
         
           
             
               
                 1 
                 
                   s 
                   + 
                   
                     
                       Z 
                       R 
                       2 
                     
                     
                       s 
                       - 
                       f 
                     
                   
                 
               
               + 
               
                 1 
                 
                   s 
                   ″ 
                 
               
             
             = 
             
               1 
               f 
             
           
         
       
     
         [0000]    where s and s″ are the input and output waist locations, Z R  is the Raleigh range, and f is the focal length. Thus, the chief rays  160  are overlapping at the grating  112  while each individual beam is still diverging (as indicated at  162  by dashed lines). The diverging beams  162  may or may not be later collimated by an optical element, such as the optic  210 . With all the diode element beams overlapped on the dispersive element  112 , the output beam quality is generally that of a single emitter. Again, one advantage of this system is the size may be considerably smaller than, for example, a two-focal-length spacing between diode elements and the dispersive element  112 . In some instances cases, the beam path is reduced by almost half or more. The spacing as described herein may be slightly longer, equal to, or slightly shorter than F 1 . 
         [0028]    Alternatively, an embodiment devoid of collimating optic(s)  210  is illustrated in  FIG. 2B . The combining optical element  110  is placed a focal length from the front facet  108  of the diode bar  102  and, as a result, collimates the light from each diode element. A reduced system size is still achieved by placing the dispersive element  112  less than a focal length away from the combining optical element  110 . The brightness of the multi-wavelength beam is still increased as compared to the initial array of beams produced by the diode bar  102 , but there may be some degradation in the output beam quality. In one variation of this embodiment, the diode elements  102  are a single 10-mm wide bar with  47  emitters. Each emitter may have a FAC lens (not shown) and no SAC lens. Inclusion of a SAC lens does not change the results. The focal length of the FAC lens in this variation may be 910 μm, and the diode bar may operate at a 1 μm wavelength. With each beam being diffraction-limited along the fast axis, the typical full divergence after the FAC lens is about 1 milliradian (mrd). Along the slow axis, the beam diverges about 100 mrd. We assume that the combining optical element  110  or transform lens has a focal length of 150 mm. The output beam quality M 2  is approximately: 
         [0000]    
       
         
           
             
               M 
               2 
             
             = 
             
               
                 πθ 
                 
                   4 
                    
                   λ 
                 
               
                
               
                 
                   
                     
                       ( 
                       
                         zx 
                         / 
                         f 
                       
                       ) 
                     
                     2 
                   
                   + 
                   1 
                 
               
             
           
         
       
     
         [0000]    where λ=1 μm, z is the distance after the lens to the grating and center at the back focal plane, x=10 mm is the dimension of the array, and θ is the individual beam divergence after the grating. 
         [0029]      FIG. 2C  illustrates a WBC arrangement that enables a shortened beam pathway, and substantially separates the functions of combining chief rays and collimating diverging rays into two separate optical elements (or systems) positioned before the dispersive element  112 . The combining element  210  is positioned at a distance substantially less than its respective focal length F 1  away from the front aperture  108  on one side and approximately a focal length F 1  away from the dispersive element  112  on the other side. This allows combining element  110  to direct the chief rays from each diode emitter of the diode bar  102  to overlap (or substantially overlap) on the dispersive element  112 . At the same time, the collimating optical element  210  is placed approximately a focal length F 2  away from the front aperture of each diode emitter on one side and a distance less than focal length F 2  from the dispersive element  112  on the other side. The primary function of the collimating optical element  210  is to collimate the diverging rays. One skilled in the art will readily appreciate that both elements  110 ,  210  have optical power along the same dimension and as a result will have some effect on the physical placement of each optical element with respect to the front aperture and dispersive element. This interdependency may managed in part by placing the optical element  110  close to the emission aperture and the optical element  210  close to the dispersive element  112 . This ensures that the combining optical element  110  primarily dominates the combining of the chief rays on the dispersive element  112 , but is influenced by the prescription of the collimating optical element  210  and vice versa. 
         [0030]    Other designs may reduce system size and even the need for optical combining elements through alternative position-to-angle methods. For example,  FIG. 3  illustrates a WBC system devoid of an optical combining element. Each diode element  102  (which may have as few as a single diode emitter) may be mechanically positioned in a manner that the chief rays (solid lines  160 ) exiting the diode elements  102  overlap at a common region on the dispersive element  112  as shown. (In other variations of this embodiment, and similar to that shown in  FIG. 2B , the beams do not completely overlap at the dispersive element, but the spatial distance between each along a combining dimension is reduced.) The diverging rays, illustrated by dashed lines  162 , are collimated by collimating optic(s)  210  positioned between the dispersive element  112  and the partially reflective output coupler  114 . (Some variations of this embodiment include replacing collimating optic  124  with individual FAC and/or SAC lenses positioned at the front surface or facet of each diode bar.) This embodiment thus increases brightness while reducing the number of optical elements required as well as reducing overall system size. 
         [0031]    In another embodiment, shown in  FIG. 4 , a curved diffraction grating  412  is placed a focal length F 1  away from the diode bar  402 . The curved diffraction grating  412  combines the emitted beams into a multi-wavelength beam that is transmitted to the partially reflective output coupler  114 , which reflects a portion of the beams back towards the curved diffraction grating  412 . The wavelengths of the reflected beams are then filtered by the diffraction grating  412  and transmitted back into each emitter of diode bar  102 , whereby each emitter is stabilized to a particular wavelength. The maximum brightness produced by this type of system generally hinges on the amount of power the curved diffraction grating  412  can handle. This optical architecture reduces the number of optical elements and shortens the beam path while increasing the brightness of a multi-wavelength output beam. Degradation of the beam quality results as a function of the beam width  475  over the entire distance of the beam profile  485 . 
         [0000]    Combination with Interferometry 
         [0032]    Any of the foregoing optical architectures can be used in high-power materials-processing applications such as cutting, drilling, and patterning. In accordance with embodiments of the present invention, the output of the WBC source is passed through one or more elements creating an interferometric output that is analyzed to determine, in real time, the depth or height of the surface that the beam strikes. A representative architecture is shown in  FIG. 5 . The multi-wavelength beam output of any of the WBC systems shown in  FIGS. 1-4  (and indicated at  505 ) is used to process a machining surface  510 . The WBC system  505  provides an input for machining a surface  510  to create desired features or cuts. The beam passes through a beamsplitter cube  515  disposed between the multi-wavelength source and the machining surface  510 . A portion  518  of the beam is diverted to a movable mirror  520  (e.g., a surface having a reflectivity ≧99%), which is configured for translation along the optical axis of the beam  518 ; thus, the mirror  520  can shift position to the location indicated in phantom. A portion  522  of the reflected beam is transmitted back through the beamsplitter  515  onto an interferometric detector system  525 , where this beam portion  522  is used as a first reference for comparison. The portion  530  of the beam that passes directly through the beamsplitter  515  is transmitted onto the machining surface  510 , where a portion  532  of the beam is reflected back to the beamsplitter  515  and then diverted onto the interferometric detector as a second reference to be compared with the first reference. In order to accurately determine the change of depth position of the cuts and/or welds on the machined surface, the lateral position of the reflective mirror  520  may be adjusted to match up with the coherence wavelength of one or more wavelengths. This occurs when the a wavelength undergoes constructive or destructive interference at the detector  525 . 
         [0033]    The wavelength or wavelengths that undergo interference depends on the difference between (i) the distance between the center point  540  of the beampsplitter  515  and the mirror  520  and (ii) the distance between the center point  540  and the surface  510 . Accordingly, adjusting the position of the mirror  520  until one of the output wavelengths undergoes interference allows calculation of the distance to the surface  510  and, hence, the depth of a groove or the height of a feature relative to a baseline—i.e., a neutral level whose distance from the center point  515  was previously established. By utilizing a 2D array of beam sources and causing relative movement between the surface  510  and the beam  530 , a 3D representation of the surface  510  can be built up. 
         [0034]    The detector  525  may report the instantaneous depth/height information to a controller  550 , which controls the operation of the WBC source  505  (i.e., it actives the source  505  and controls beam parameters as appropriate during processing). The controller  550  also operates a conventional positioning system to cause relative movement between the beam output of the WBC source  505  and the surface  510 . The positioning system may be any controllable optical, mechanical or opto-mechanical system for directing the beam through a processing path along a 2D or 3D workpiece. During processing, the controller may operate the positioning system and the WBC source  505  so that the output beam traverses a processing path along the surface  510 . The processing path may be provided by a user and stored in an onboard or remote memory, which may also store parameters relating to the type of processing (cutting, welding, etc.) and the beam parameters necessary to carry out that processing. In this regard, a local or remote database may maintain a library of materials that the system will process, and upon user selection of a material, the controller  550  queries the database to obtain, for example, a relationship between output power and cutting depth. 
         [0035]    As is well understood in the plotting and scanning art, the requisite relative motion between the beam and the workpiece may be produced by optical deflection of the beam using a movable mirror, physical movement of the laser using a gantry, lead-screw or other arrangement, and/or a mechanical arrangement for moving the workpiece rather than (or in addition to) the beam. As the controller  550  receives real-time feedback regarding the depth or height of a feature, it alters the output power or other parameter of the WBC output beam (e.g., M 2 , beam parameter product, etc.) so that the programmed height or depth is maintained notwithstanding variation in material properties. That is, the point on the surface  510  at which the distance to the center point  540  is computed may be just behind the beam (so that, e.g., the depth of the cut just made is measured). The controller  550  may also store, for example, power levels and corresponding cutting depths for calibration or to correct stored values. 
         [0036]    The controller  550  also controls an actuator  555  for translating the mirror  520  along the axis of the beam  518 . For example, the controller  550  may vary the lateral position of the mirror  520  until interference is detected by the detector  525 , or until a particular wavelength undergoes interference. 
         [0037]    The controller  550  may be provided as either software, hardware, or some combination thereof. For example, the system may be implemented on one or more conventional server-class computers, such as a PC having a CPU board containing one or more processors such as the Pentium or Celeron family of processors manufactured by Intel Corporation of Santa Clara, Calif., the 680×0 and POWER PC family of processors manufactured by Motorola Corporation of Schaumburg, Ill., and/or the ATHLON line of processors manufactured by Advanced Micro Devices, Inc., of Sunnyvale, Calif. The processor may also include a main memory unit for storing programs and/or data relating to the methods described above. The memory may include random access memory (RAM), read only memory (ROM), and/or FLASH memory residing on commonly available hardware such as one or more application specific integrated circuits (ASIC), field programmable gate arrays (FPGA), electrically erasable programmable read-only memories (EEPROM), programmable read-only memories (PROM), programmable logic devices (PLD), or read-only memory devices (ROM). In some embodiments, the programs may be provided using external RAM and/or ROM such as optical disks, magnetic disks, as well as other commonly used storage devices. For embodiments in which the functions are provided as one or more software programs, the programs may be written in any of a number of high level languages such as FORTRAN, PASCAL, JAVA, C, C++, C#, BASIC, various scripting languages, and/or HTML. Additionally, the software may be implemented in an assembly language directed to the microprocessor resident on a target computer; for example, the software may be implemented in Intel 80×86 assembly language if it is configured to run on an IBM PC or PC clone. The software may be embodied on an article of manufacture including, but not limited to, a floppy disk, a jump drive, a hard disk, an optical disk, a magnetic tape, a PROM, an EPROM, EEPROM, field-programmable gate array, or CD-ROM. 
         [0038]    The above description is merely illustrative. Having thus described several aspects of at least one embodiment of this invention including the preferred embodiments, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.