Abstract:
A method and apparatus of selecting a predetermined number of pulses from a stream of laser pulses is described. The number of pulses is determined by the operator from the laser pulse repetition rate and the desired number of pulses. The invention comprises synchronizing the laser pulses with the start of the pulse selection, allowing the predetermined count of laser pulses to pass a blocking mechanism; and finally blocking the pulses after the pulses have passed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a utility application of our provisional application, serial No. 60/171,412, filed Dec. 21, 1999 entitled “Method and Apparatus to Select a Predetermined Number of Pulses from a Laser,” currently pending attorney docket number RD-26978). 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to the field of pulsed laser control. More specifically, the present invention describes an external apparatus and method adaptable to most lasers for selecting a predetermined number of laser pulses. 
     During material processing with lasers it is often necessary to apply a single pulse or a low number of pulses to a spot of the device or material being treated. In principle, this could be done by firing the laser from an external trigger when a pulse is required (single pulse operation). However, most lasers, especially high power lasers, are designed such that the thermal focal power of the gain medium (i.e., the laser rod or slab) is part of the laser resonator. When the laser is turned on it takes several shots for this focal power to develop and for the resonator to function properly. During these first few pulses the output energy and spatial profile of the laser beam are unpredictable. Obviously, the same problem arises when the laser is run in single pulse operation. Hence, single pulse operation is not an option when, as in many critical applications, well characterized laser pulses are required. In addition, to generate the current pulses for the lamp(s), conventional laser power supplies typically use a “pulse-forming network” (PFN) that is charged from a reservoir capacitor bank with a technique called “resonance charging.” This technique also requires a few pulses before the PFN produces reproducible current pulses. To overcome these problems, it is customary to have the laser running continuously in thermal and electrical steady-state, and to have the laser beam physically blocked. In the prior art, blocking of the laser beam is typically achieved inside the resonator for those lasers capable of limiting the number of pulses. It is advantageous to block the beam external to the resonator. With external blocking the energy is extracted from the gain medium to avoid uncontrolled parasitic oscillation. Further, lasers without the ability to limit the number of pulses require an external method of blocking the beam if they are to be made capable of limiting the number of pulses emitted. Then, when one or a few pulses are required, the operator manually removes the block for an appropriate amount of time and then blocks the beam again after the desired number of pulses has passed. This approach becomes less and less feasible as the pulse repetition rate of the laser increases. The repetition rate of the laser pulses at some point becomes too rapid for the operator to physically move a block and, at all repetition rates; it is possible that a partial pulse could be allowed to pass. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention discloses an apparatus and method that provides for the application of a predetermined number of lasers pulses in situations where manual operation is prohibited by physical limitations imposed by high repetition rate lasers. The approach, however, is not limited to high repetition rate lasers nor high power lasers. The approach can be applied to low repetition rate lasers and low power lasers as well. The disclosed invention can be fabricated as an external assembly to the laser and, therefore, adapted to lasers not having the ability to limit the pulses to a predetermined number. 
     The present invention is divided into three major blocks. In a first operation the initiation of the selection of pulses is synchronized with the operation of the laser. That is, initiation of the sequence for application of the laser pulse(s) is delayed until the advent of the next laser pulse. In this manner the timing of the second operation is aligned with the repetition rate of the laser. 
     The second operation begins with the occurrence of the first laser pulse after the initiation. In this operation, a delay generator allows laser pulses to pass for a duration of time predetermined by the number of laser pulses desired. 
     The third operation is the placement of a physical shutter that blocks or allows the laser pulses depending upon the status of the delay generator output of operation two. 
    
    
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings, which set forth an illustrative embodiment in which the principles of the invention are utilized. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The details of the invention will be described in connection with the accompanying drawings, in which: 
     FIG.  1 . Block diagram of the disclosed invention; and 
     FIG.  2 . Diagram of a method and apparatus to select a predetermined number of pulses from a laser. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The relationship of the parts of the invention can be seen by referring to FIG.  1 . wherein laser controller  110  generates a trigger signal  112  that causes a laser  114  to emit a laser pulse from a stream of laser pulses  116 . A pulse detector  122  detects the occurrence of the laser pulses  116  and thereupon generates a first signal to a synchronizer  120 . Initiation of the invention occurs at a box  118  (operation initiation may be through a human operator or a non-human operator, such as a machine tool after it determines that a workpiece is in proper position to receive laser pulse treatment), which generates a second signal to the synchronizer circuit  120  to await the occurrence of a next laser pulse from the laser pulse stream  116 . At the occurrence of the first laser pulse detected by the pulse detector  122  after initiation  118 , the synchronizer  120  issues a third signal to a delay generator  124 . The delay generator  124  provides a timing pulse signal for a shutter controller  126  to move shutter  128  out of the path of the laser pulse stream  116  thereby allowing the laser pulse stream to reach a target (not shown). The delay generator  124  continues to provide the timing pulse signal for the shutter controller  126  until a predetermined number of pulses, n, preset by the operator at box  130  have passed by the shutter. The preset number of pulses, n, and the frequency of the laser pulses, f, determine the length of the delay generator timing pulse signal. The length of the delay generator timing pulse signal is n/f although any suitable counting method could be used to achieve the same purpose. After the delay generator has timed the predetermined number of laser pulses, the delay generator removes the timing pulse signal from the shutter controller and the shutter is caused to return to a position wherein the laser pulse stream  116  is again blocked. 
     The preferred apparatus and method for selecting a predetermined number of pulses from a stream of laser pulses is generally shown as  200  in FIG.  2 . There are three parts to the invention: the first is a synchronizer  210 , the second a delay generator  220 , and the third a shutter controller  230 . 
     The synchronizer  210  coordinates the initiation of the invention and the laser. A pulse detector  202  identifies the occurrence of a laser pulse and outputs a timing signal (first signal) on a line  204 . The timing signal  204  can easily be generated if the laser power supply outputs an electrical signal of a fixed temporal relationship with the current pulse. The current pulse is generated by the power supply to instigate a laser pulse. Any of various other approaches can be used to detect the occurrence of a laser pulse if the electrical signal from the power supply is not available. For example, a current probe can be used to sense the flash lamp current. Alternatively, an optical detector, such as a light sensitive photodetector, can sense either the light from the flash lamps or the fluorescence from the gain medium. In a like manner, an optical detector can sense the laser beam  248  by detecting the scattered light from the beam shutter  250 . In any case, a signal conditioner  206  is arranged to receive the pulse detector  202  output timing signal on line  204  and produce a pulse for a clock signal on a line  208 . Thus the clock signal on line  208  represents the occurrence of a laser pulse and clocks a latch  212 . An initiation device  214  is configured to generate a second signal, when activated, to enable setting of the latch  212  at the occurrence of the next clock pulse  208 . Many initiation devices can be so arranged and a pushbutton is shown for illustration and not limitation. While any latch  212  circuitry can be used to accomplish this purpose, a simple D flip-flop is shown. The latch  212  output (third signal) on a line  216  is normally low. Latch output  216  remains low after each clock pulse  208  while latch D input on a line  218  is low. As long as the initiation device  214  is in the open position, the latch D input  218  is held low through the connection to a resistor  222  connected to a ground  224 . Activation of the initiation device  214  imposes a logic high on latch D input  218 . The next occurrence of a clock pulse  208  sets the latch output  216  to a logic high. The latch output  216  will remain high after each clock pulse  208  until the initiation device  214  is placed in the open position thereby causing a ground voltage level to appear on latch D input  218 . The next clock pulse  208  will reset the latch output  216  to a logic low. Hence, the latch D input  218  reflects the state of the initiation device  214 . The change in state of initiation device  214  reflected in the latch output  216  only at the occurrence of a clock pulse  208  that is present at a fixed time relative to the occurrence of a laser pulse. In this manner the operator activation of the initiation device  214  is synchronized with the laser pulses. 
     The second step consists of using a delay generator  220  to generate a timing pulse signal in response to the setting of the latch output  216  which is indicative of the operator activation of the initiation device  214  delayed so that the setting of the latch output  216  is synchronized with the occurrence of a laser pulse as described above. Latch output  216  is connected to trigger the delay generator  220  to start the timing pulse signal on a line  226 . The timing pulse signal  226  is of a duration appropriate to let pass the desired number of laser pulses. That is, the timing pulse signal ends after the last of the predetermined number of laser pulses. Hence, the number of laser pulses desired and the pulse repetition rate of those pulses determine the duration of the timing pulse signal  226 . Specifically, if n is the number of pulses selected and f [Hz] the repetition rate of the laser, the timing pulse signal duration is n/f seconds. The delay generator  220  can be designed in numerous ways including digital circuitry to count the predetermined pulses. In an exemplary embodiment a programmable delay generator, such as the DG 535 available from Stanford Research Systems, is used. In this embodiment the operator determines the delay time required and programs the delay generator prior to activation of the pushbutton switch. 
     The third step is a shutter control  230  for blocking the laser pulses except during the duration of the timing pulse signal  226  being at a logic high. One method of blocking the laser pulses is illustrated in FIG.  2  and consists of a beam shutter  250  that is physically interposed in the path of the laser pulse stream to block the pulses. A solenoid  242  is activated to cause unblocking of the laser pulse stream and a solenoid  254  is activated to cause blocking of the laser pulse stream by relay drivers  228  and  232 , respectively. Various methods to avoid simultaneous activation of both solenoids in response to the timing pulse signal will be evident to those of ordinary skill in the art. In a preferred embodiment, the relay drivers are two buffers  228  and  232  both connected to the delay generator output timing pulse signal  226 . Buffer  232  has an inverting input and buffer  228  does not. Therefore, the output of buffer  228  (fourth signal) and buffer  232  are always at opposite logic levels. Buffer  228  is connected to a solid-state relay  236  by a line  234 . When activated during the duration of the timing pulse signal  226 , solid-state relay  236  connects 110 Vac on lines  238  and  240  across a solenoid  242  causing the solenoid slug  244  to move in the direction shown and the beam shutter  250  disposed thereon to unblock the laser pulse output beam  248 . Similarly, buffer  232  is connected to a solid-state relay  246  by a line  252 . In the absence (logic low) of the timing pulse signal  226 , solid-state relay  246  connects 110 Vac on lines  238  and  240  across a solenoid  254  causing the solenoid slug  244  to return to the original position and the beam shutter  250  disposed thereon to block the laser pulse output beam  248 . In this manner, the laser pulse stream is unblocked by the shutter during the duration of the timing pulse signal and blocked at other times. The preferred embodiment uses two solenoids, one that pulls the shutter into, and one that pulls the shutter out of, the laser pulse stream. This affords high-speed operation in both directions. The use of two buffers always at opposite logic levels assures that the solenoids are never activated at the same time and the timing synchronization described above guarantees that, at the time a laser pulse occurs, the shutter is always either open or closed and no fractional pulses are passed through. 
     While a preferred embodiment has been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.