Abstract:
Laser-drilling apparatus includes a gas-discharge for laser emitting laser-radiation pulses, and two acousto-optic modulators (AOMs). The laser radiation pulses are characterized as having two temporal central portions between temporal leading and trailing edge portions. The AOMs are arranged to spatially separate the central temporal portions of the pulses from each other and from the leading and trailing edge portions of the pulses.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority of U.S. Provisional Application No. 62/251,941, filed Nov. 6, 2015, the complete disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to methods of operating a pulsed carbon monoxide (CO), gas-discharge laser. The invention relates in particular to methods of temporally shaping pulses from such a CO laser. 
       DISCUSSION OF BACKGROUND ART 
       [0003]    CO gas-discharge lasers with average output power greater than 250 Watts (W) have recently become commercially available. Such lasers have been accepted as advantageous for certain laser-machining operations, particularly laser-drilling of via-holes in printed circuit boards PCBs. 
         [0004]    Via-hole drilling using a pulsed CO laser is described in detail in U.S Pre-grant Publication No. 2015/0083698, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference. In this method, pulses from a CO laser are temporally shaped by removing leading and trailing edges of the pulse with slow rise and fall times. An acousto-optic modulator (AOM) is used for this operation. The AOM is operated to pass the unwanted leading and trailing edges of pulse directly with the AOM turned “off”, and to diffract a wanted, temporally shaped portion, of the pulse along a path to the workpiece (PCB). Means are provided for compensating for spectral (chromatic) dispersion introduced into the temporally shaped pulse by the diffraction of the AOM. 
         [0005]    In preferred embodiments of the above referenced via-hole drilling operation, the drilling is performed on one workpiece at a time. Operational productivity could be increased by adapting the method for operating on two or more workpieces at a time. 
       SUMMARY OF THE INVENTION 
       [0006]    In one aspect, laser apparatus in accordance with the present invention comprises first and second acousto-optic modulators (AOMs) and a gas-discharge laser for emitting laser-radiation pulses. The laser-radiation pulses have a temporal rising-edge portion, a temporal falling-edge portion, with first and second temporal central portions therebetween. The first and second AOMs are arranged and operated to spatially separate the first and second temporal central portions of the pulses from the leading-edge and trailing-edge portions of the pulses, and spatially separate the first and second temporal central portions of the pulses from each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1A ,  FIG. 1B ,  FIG. 1C , and  FIG. 1D  schematically illustrate a preferred embodiment of pulse-dividing apparatus in accordance with the present invention at four different time intervals during passage of a pulse of laser-radiation through the apparatus. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    Turning now to the drawings,  FIG. 1A  schematically illustrates a preferred embodiment  10  of pulse-dividing apparatus in accordance with the present invention for time-dividing a pulse P into useful portions B and C, and leading and trailing edge portions A and D, respectively. Pulse-portion A occurs between times to (pulse-initiation) and time t 1 . Pulse-portion B occurs between times t 1  and t 2 . Pulse-portion C occurs between times t 2  and t 3 . Pulse-portion D occurs at times greater than or equal to time t 3 . 
         [0009]    Apparatus  10  comprises first and second acousto-optic modulators (AOMs)  12  and  14 , respectively. Pulse P is delivered by a carbon monoxide (CO) laser  16 , or any other gas discharge laser capable of lasing simultaneously at a plurality of different wavelengths between a shortest wavelength λ S  and a longest wavelength λ L . In a CO laser, λ S  may be about 4.5 micrometers (μm) and λ L  may be about 6.0 μm. 
         [0010]    The drawing depicts three possible beam-paths through apparatus  10 . Beam-path  1  is the path of radiation transmitted through AOM without diffraction, i.e., beam-path  1  is a zero-order path of AOM  12 , and is parallel to the incidence direction of pulse P on AOM  12 . When AOM  12  is in an “ON” condition, i.e., with radio-frequency (RF) power applied to the AOM, the longest and shortest wavelengths are diffracted at different angles, as illustrated in the drawing. The diffraction angles are dependent on the applied radio-frequency. 
         [0011]    AOM  14  can be operated at the same frequency as AOM  12  for compensating dispersion introduced in a diffracted beam by AOM  12 , as is known in the art. Beam-path  2  is the diffracted first-order beam from AOM  12 , dispersion-compensated by AOM  14 . When the AOMs are operated at a common frequency, beam-path  2  is parallel to beam-path  1 . Beam-path  2  is spread in the plane of the drawing dependent on the longest and shortest wavelengths diffracted, and on the separation distance of AOMs  12  and  14 . Beam-path  3  is a zero-order beam-path of AOM  14 . Radiation on this path is not dispersion-compensated. 
         [0012]    It should be noted here that only sufficient description of dispersion compensation is provided herein for understanding time-division principles of the present invention. A more detailed description of various AOM dispersion-compensation arrangements is provided in the above-referenced 2015/0083698 publication. 
         [0013]    Continuing with reference to  FIG. 1A , in a time interval between time to and time t 1  corresponding to leading edge portion A of a pulse P, AOM  12  is in an “ON” condition (RF applied and diffracting) and AOM  14  is in an “OFF” condition, i.e., with zero or low RF applied. Radiation from pulse P is diffracted by AOM  12  along a first-order beam-path toward AOM  12  and is transmitted along the zero-order beam-path of AOM  12  into beam-path  3  to a radiation dump (not shown). 
         [0014]    Referring now to  FIG. 1B , at time t 1 , AOM  12  is switched OFF, allowing radiation from pulse B to be transmitted along beam-path  1  (the zero order beam-path of AOM  12 ) to a first target (not shown). Both AOMs remain “OFF” until time t 2  such that portion B of pulse P is transmitted along beam-path  1  to the target. Here it should be noted that having AOM  14  “OFF” during the period between is preferred for preventing any “leakage” of radiation along the first-order beam-path of AOM  12  from entering beam-path  2 . Weak RF power could be selectively applied to AOM  12  to adjust the amplitude of pulse-portion B. 
         [0015]    Referring to  FIG. 1C , at time t 2  and until time t 3 , AOM  12  and AOM  14  are both switched “ON” causing radiation from pulse P to be diffracted by AOM  12  along the first-order beam-path thereof to AOM  14 , then diffracted by AOM  14 , thereby directing portion C of pulse P along beam-path  2  to a second target (not shown). It is assumed here that during the time interval between time t 2  and time t 3 , both AOMs are operating at the same frequency. RF power to AOM  14  could be adjusted to control the power in pulse-portion C. 
         [0016]    Referring to  FIG. 1D , at time t 3 , AOM  14  is switched “OFF”, allowing the remainder of radiation in pulse P (trailing-edge portion D thereof) to be transmitted along beam-path  3 . The above-described switching sequence can be repeated with the arrival of each pulse arriving at AOM  12  from the CO laser. 
         [0017]    Those skilled in the art will recognize that certain modifications of apparatus  10  are possible without departing from the sprit and scope of the present invention. By way of example, the spread due to dispersion in the first order beam-path from AOM  12  to AOM  14  could be reduced by inserting a unit-magnification optical relay between AOM  12  and AOM  14 . Various beam-steering or beam-shaping arrangements could be included between AOMs  12  or  14  and the first or second targets. Typically, beam-path  1  and beam-path two would each include achromatic focusing elements for focusing the pulse portion on the respective targets, as described in the above-referenced 2015/0083698 publication. 
         [0018]    From the description of the present invention provided above, those skilled in the art will recognize that further time-division of a CO laser pulse is possible, in theory at least, without departing from the spirit and scope of the present invention. By way of example, if in the time-interval between t 2  and t 3  of  FIG. 1C , the radio-frequency applied to AOMs  12  and  14  were switched to a different, common frequency, then dispersion-compensated first-order, beam-path  2  from AOM  14  would be shifted, up or down in the plane of the drawing, to provide a third beam-path, parallel to the second beam-path, for a portion of the transmitted pulse-portion. This would allow a third target to be irradiated by radiation from pulse P. 
         [0019]    The present invention is described above with reference to a preferred embodiment. The invention is not limited, however, to the embodiment described and depicted herein. Rather the invention is limited only by the claims appended hereto.