Abstract:
A method of delivering a beam of laser-radiation to a workpiece for processing the workpiece comprises transmitting the beam twice through an acousto-optic modulator (AOM) crystal in opposite zero-order directions of the AOM at separate locations on the AOM crystal, before delivering the beam to the workpiece.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the priority of U.S. Provisional Application No. 62/367,581, filed Jul. 27, 2016, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to laser-machining and laser processing methods and apparatus. The invention relates in particular to laser-machining methods in which a laser beam for effecting the machining is modulated by an acousto-optic modulator (AOM). 
       DISCUSSION OF BACKGROUND ART 
       [0003]    One commonly used laser-machining (laser-processing) method involves modulating a continuous wave (CW) or pulsed laser-beam using an AOM. Radiation used for the machining is admitted to a workpiece via the AOM at one angle of incidence thereon for effecting the machining, and directed away from the workpiece via the AOM at another angle of incidence thereon during a pause in the machining. 
         [0004]    Traditionally, the beam is admitted to the workpiece by the AOM by diffracting the beam at a first-order diffraction angle (direction) of the AOM, and directed away from the workpiece by transmitting the laser-beam through the AOM at a zero-order incidence angle (direction) of the AOM. More recently, however, it has been found advantageous to use the zero-order transmission of the AOM to admit the laser-beam to the workpiece, and the first-order diffraction to direct the laser-beam away from the workpiece. 
         [0005]    This latter method is preferred for laser-beams having a relatively broad spectral content, such as beams from carbon monoxide (CO) lasers, as no dispersion of the laser-beam occurs on zero-order transmission. This avoids a need to provide means to correct dispersion before delivering the laser-beam to the workpiece for the machining or processing. A method in which machining is effected by a first order AOM-diffracted beam is described in U.S. Pre-grant Publication No. 20150083698, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference. A method in which machining is effected by a zero-order AOM-transmitted beam is described in U.S. Pre-grant Publication No. 20170050266, assigned to the assignee of the present invention, and the complete disclosure of which is also hereby incorporated herein by reference. 
         [0006]    Laser-beams used in AOM-modulated laser processing methods are typically plane-polarized, and diffraction by an AOM is polarization sensitive. It has been found in cases where diffraction by the AOM is used to direct a laser-beam away from a workpiece that there is some “leakage” of laser-radiation along the zero-order transmission direction. This has been found to be as much as about 2% of the incident laser-radiation. The leakage can be due to deviation from exact plane-polarization of the laser-beam or by slight misalignment of the polarization-plane with the AOM. 
         [0007]    In many applications, such a leakage may be below a threshold value at which a workpiece could be altered or damaged in some way and can accordingly be ignored. In some sensitive applications, however, or in an application where the leakage may strike repeatedly on a workpiece at the same spot, the threshold could be exceeded with negative consequences. There is a need to reduce such leakage, preferably by about an order of magnitude. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect, a method in accordance with the present invention for delivering a beam of laser-radiation to a workpiece for processing the workpiece, comprises delivering the beam of laser radiation to an acousto-optic modulator (AOM). The beam is then transmitted through the AOM first and second times, in respectively first and second zero-order directions of the AOM, at respectively first and second separate locations thereon, before delivering the beam of radiation to the workpiece. 
         [0009]    The method is applicable to modulating a CW beam for proving a train of laser pulses on the workpiece, or to modulating a beam of laser pulses for temporally shaping the laser pulses. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  schematically illustrates one preferred embodiment of laser-machining apparatus in accordance with the present invention including a laser delivering a laser beam and an AOM, with the laser beam being transmitted through AOM in forward and reverse passes in zero-order directions to reach a workpiece, with a single reflector arranged to direct the laser-beam back to the AOM between the forward and reverse passes. 
           [0011]      FIG. 2  schematically illustrates another preferred embodiment of laser-machining apparatus in accordance with the present invention, similar to the embodiment of  FIG. 1 , but wherein two reflectors are arranged to direct the laser-beam back to the AOM between the forward and reverse passes. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Turning now to the drawings wherein like features are designated be like reference numerals,  FIG. 1  schematically illustrates a preferred embodiment  10  of laser-machining apparatus in accordance with the present invention. A laser  12  delivers a beam  14  of laser radiation to be used for the laser machining. An AOM  16  is provided for amplitude modulating beam  14 . 
         [0013]    AOM  16  is activated by an acoustic wave delivered to the AOM in a direction indicated by arrow A when it is required to interrupt passage of the laser beam to a workpiece not shown. Activation of AOMs is well known in the art to which the present invention pertains. A detailed description of such activation is not necessary for understanding principles of the present invention, and, accordingly, is not presented herein. 
         [0014]    Beam  14  is first incident on AOM  16  (at a location B thereon) at an angle θ B  which is the first order diffraction angle of the AOM when the AOM is activated. When AOM  16  is not activated, beam  14  is transmitted through the AOM along a zero-order direction, i.e., the beam is not diffracted by the AOM, and leaves the AOM at the incidence angle θ B . Preferably beam  14  is polarized in a direction indicated by double arrow P, parallel to the plane of incidence of beam  14  on the AOM, for minimizing reflection losses on transmission. The first-transmitted beam  14  is directed by a reflector  18  arranged such that beam  14  is again incident on AOM  16  (at a location C thereon) at angle θ B  and is transmitted a second time through the AOM along a zero-order direction. Twice-transmitted beam  14  is then directed by a reflector  26  past the AOM toward the workpiece. Those familiar with the art will recognize that beam  14  may be incident on beam-directing, beam-shaping or focusing optics before actually being incident on the workpiece. 
         [0015]    When it is desired to interrupt passage of beam  14  to the workpiece, AOM  16  is activated by the acoustic wave and the AOM becomes essentially a diffraction grating. At the first incidence of beam  14  on the AOM, a substantial portion of the beam is diffracted along a first diffraction order direction of the AOM as indicated by dashed line  18  is captured by a beam-trap (beam-dump)  20 . As the diffraction process is less than 100% efficient, there will be some “leakage” of laser-radiation along the zero-order transmission direction. This can be as much as about 2% of the incident laser-radiation as discussed above. 
         [0016]    The leaked radiation proceeds along the beam- 14  path and, with AOM still active, the leaked radiation is diffracted again by the AOM in a first diffraction order direction of the AOM as indicated by dashed line  22 . The leaked radiation is captured by another beam-dump  22 . Again, because of a less than 100% diffraction efficiency, the will be some leakage of leaked radiation long the transmission direction, but this will have less than 0.2% of radiation power first incident on the AOM. 
         [0017]      FIG. 2  schematically illustrates another preferred embodiment  30  of laser-machining apparatus in accordance with the present invention, similar to the embodiment of  FIG. 1 , but with exceptions as follows. In apparatus  30  two reflectors  32  and  34  are used to direct first-transmitted beam  14  back to AOM to make a second incidence thereon. A reflector  36  directs a twice transmitted beam  14  back to the workpiece. The effectiveness of apparatus  30  in reducing leaked radiation during interruption of operations on a workpiece is comparable to that of apparatus  10  discussed above. 
         [0018]    Those skilled in the art will recognize from the description presented above that the present invention could be used with any wavelength of laser-radiation for which AOMs are available. Those skilled in art the will also recognize that the invention may be used with lasers delivering either continuous wave (CW) or pulsed radiation. Further, while in the embodiments of the present invention described above, first and second zero-order passes through the AOM take place in opposite (forward and reverse) directions, a suitable arrangement of reflector could be used to cause the first and second zero-order passes to occur in the same direction. 
         [0019]    In summary, the present invention is described in terms of preferred embodiments. The invention, however, is not limited to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto.