Abstract:
In order to maintain a constant laser output pulse power in a RF-energized, sealed-off, diffusion cooled, pulsed, CO 2  gas-discharge laser, each laser output pulse is generated by train or burst of shorter RF pulses. When the time between laser output pulses becomes short enough that the power in one pulse would be reduced by gas-discharge heating effects of a previous pulse, power in the RF pulse trains is varied by varying the duration or duty cycle of pulses in the bursts, thereby keeping output-pulse power in the laser output pulses constant. RF pulses in any burst can have a different duration for tailoring the temporal shape of a corresponding laser-output pulse.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority of U.S. Provisional Application No. 61/624,096, filed Apr. 13, 2012, the complete disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to output power control in radio-frequency (RF) energized, pulsed carbon dioxide (CO 2 ) gas-discharge lasers. The invention relates in particular to providing constant power from one laser output pulse to a next, and also to controlling the temporal shape of output pulses of such lasers. 
       DISCUSSION OF BACKGROUND ART 
       [0003]    In a RF-energized sealed-off diffusion cooled pulsed CO 2  gas-discharge laser, if time between pulses becomes short enough, the diffusion cooling can become overwhelmed resulting in a temperature rise in the CO 2  lasing gas mixture. This can affect the characteristics of output pulses, particularly the rise and fall of the output pulses. This can lead to an inconsistency in power from pulse to pulse and accordingly variations in average power. A typical lasing-gas mixture includes helium (He), nitrogen (N 2 ) and CO 2  in proportions of about 80:10:10. 
         [0004]      FIG. 1A  and  FIG. 1B  provide a timing diagram comparing a relationship between an RF pulse ( FIG. 1A ) energizing a prior-art diffusion cooled CO 2  gas-discharge laser and a resulting laser output pulse ( FIG. 1B ) from the laser. It is assumed that the laser is tuned to operate (lase) at a wavelength of about 10.6 micrometers (μm). The RF pulse is initiated at time t 0  and terminated at time t 3 . As depicted here, the RF pulse has a duration of about 100 microseconds or greater. 
         [0005]    Initially there is no laser output as time is required for N 2  in the lasing gas mixture to be energized by the RF and then transfer that energy to the CO 2  by collision. After a relatively short time, there is an initial gain-spike (power spike) in the lasing gas mixture which drops to almost zero at time t 1 . Because of the very short duration, the energy in this power-spike is not significant from the process perspective. Following the power-spike, the laser output power rises progressively. This period of rising power is designated τ R  in  FIG. 1B . During this time, the temperature of the gas mixture is rising, and at time t 2  the gain (at the 10.6 μm CO 2  wavelength for which the laser is tuned) begins to gradually fall as a result of competition between energy transitions for the 10.6 μm wavelength and for another lasing wavelength at about 9.6 μm (for which the laser-resonator is assumed not to be tuned). A detailed description of the physics of the gain-reduction is not necessary for understanding principles of the present invention and is not presented herein. This gradual falling period of the 10.6 μm-power, due to heating of the lasing gas mixture, is designated in  FIG. 1B  as period τ H . At time t 3 , when the RF pulse is terminated, gain, and accordingly the laser pulse power, falls exponentially toward zero over a fall-time designated in  FIG. 1B  as period τ F . 
         [0006]    If the pulse RF-pulse duration is made sufficiently short, the gas-heating effect in an individual leaser pulse as depicted in  FIG. 1B  can be avoided. However, in a train of laser-pulses with a sufficiently short duration between pulses, gas-heating still occurs but the effect is manifest in a different way, discussed below. 
         [0007]      FIG. 2A  and  FIG. 2B  provide a timing diagram comparing a relationship between a train of three RF pulses A RF , B RF , and C RF  ( FIG. 2A ) and resulting laser output pulses A L , B L , and C L , respectively. It is assumed, here, that the RF pulses are delivered at a pulse-repetition frequency F that provides a time T between pulses on the order of 0.15 milliseconds (ms). The pulse duration (T P ) is assumed to be about 25 microseconds (μs). This pulse duration is sufficiently short that the t H  effect of  FIG. 1B  is avoided. The time between pulses however is sufficiently short that the lasing gas does not return to the temperature at which one pulse is initiated before the next is initiated. A result of this is that the peak power P L2  of pulse B L  is less than the peak power P L1  of pulse A L , and the peak power P L3  of pulse C L  is less than the peak power P L2  of pulse B L . This peak power reduction from one pulse to the next continues until a steady state gas-heating condition is reached. In the case of the exemplary 0.15 ms between pulses, and for a lasing gas pressure of about 100 Torr, this may not occur until about 7 pulses have been delivered. 
         [0008]      FIG. 3  is a graph schematically illustrating measured amplitude ratio (indicated by diamonds) between isolated laser-output pulse pairs as a function of time between pulses in a prior-art diffusion-cooled CO 2  laser. The circle indicates that pulse separation time that corresponds to a pulse repetition frequency of 3 kilohertz (kHz). The RF pulse duration (excitation pulses) in this case was about 25 μs, but similar results were obtained with RF pulses having a duration of 50 μs. It can be seen that amplitudes of consecutive pulses become about equal when time between pulses is between about 750 and 1000 microseconds or greater. 
         [0009]    For delicate laser machining operations, such as laser drilling of via holes in printed circuit boards (PCBs), maintaining pulse-to-pulse consistency is very important. An onset of pulse inconsistency defines an upper limit to the pulse-repetition rate that can be used, and accordingly a limit to the throughput of the operation. In most cases, the steady-state condition is not an option, because a particular drilling sequence requires require pulses-on-demand at irregular intervals. Accordingly, there is need for a method and apparatus that can maintain pulse-to-pulse consistency of laser output pulses triggered on demand with relatively short intervals therebetween. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is directed to a method of operating a gas-discharge laser for providing laser output in the form of laser output pulses. The laser includes spaced apart gas-discharge electrodes powered by a radio frequency power supply (RFPS), with a laser-resonator formed between the discharge electrodes. 
         [0011]    In one aspect of the invention the method comprises delivering a first train of RF pulses from the RFPS to the gas-discharge electrodes for energizing the laser-resonator. The RF pulses in the first train thereof are temporally spaced apart by a time sufficiently short that the laser-resonator responds to the first train of RF pulses as though the first train of RF pulses were a single RF pulse, whereby the laser-resonator delivers a first laser-output pulse in response to the energizing by the first train of RF pulses. 
         [0012]    In another aspect, the method may further include, following delivery of the first train of RF pulses, delivering a second train of RF pulses from the RFPS to the gas-discharge electrodes for energizing the laser-resonator. The RF pulses in the second train of RF pulses are also temporally spaced apart by a time sufficiently short that the laser-resonator responds to the second train of RF pulses as though the second train of RF pulses were a single RF pulse, whereby, following delivery of the first single laser output pulse, the laser-resonator delivers a second laser-output pulse in response to the energizing by the second train of RF pulses. One of the duration and number of RF pulses in the second train thereof is selected such that the first and second laser output pulses have about equal power. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1A  and  FIG. 1B  form a timing diagram schematically illustrating a relationship between power as a function of time of a RF pulse and a corresponding laser output pulse in a prior-art diffusion-cooled CO 2  laser. 
           [0014]      FIG. 2A  and  FIG. 2B  form a timing diagram schematically illustrating a relationship between power as a function of time of a train of RF pulses and a corresponding train of laser-output pulses in a prior-art diffusion-cooled CO 2  laser, wherein peak pulse power is reduced from one pulse to the next as a result of lasing-gas heating by the RF pulses. 
           [0015]      FIG. 3  is a graph schematically illustrating measured amplitude ratio between isolated laser-output pulse pairs as a function of time between pulses in a prior-art diffusion-cooled CO 2  laser. 
           [0016]      FIG. 4A  is a graph schematically illustrating first and second consecutive prior-art RF excitation pulses assumed to have a time therebetween sufficiently short to cause in corresponding laser output pulses the peak power reduction of  FIG. 2B . 
           [0017]      FIG. 4B  is a graph schematically illustrating intra-pulse modulation in accordance with the present invention, wherein the prior-art laser pulses of  FIG. 4A  are replaced with consecutive first and second equal-duration trains of shorter pulses, also of equal duration, but with the pulse repetition frequency of the second train of shorter pulses being increased to compensate for the peak power reduction effect of  FIG. 2B  by including more shorter pulses in the second train than in the first train. 
           [0018]      FIG. 4C  is a graph schematically illustrating intra-pulse modulation in accordance with the present invention, wherein the prior-art laser pulses of  FIG. 4A  are replaced with consecutive first and second equal-duration trains of shorter pulses, with the same number of pulses in each train but with the pulses in the second train having a longer duration than the pulses in the first train to compensate for peak power reduction effect of  FIG. 2B . 
           [0019]      FIG. 4D  is a graph schematically illustrating intra-pulse modulation in accordance with the present invention similar to the scheme of  FIG. 4C , but wherein the first pulse in each train thereof has a longer duration that the other pulses in the train for decreasing the rise time of corresponding laser pulses generated by the pulse trains. 
           [0020]      FIG. 5  is a reproduction of an oscilloscope trace schematically illustrating a measured laser pulse generated by a 25 microsecond-long RF pulse-train similar to the pulse-trains of  FIG. 4D  but wherein there are 12 sub-pulses following the longer first sub-pulse. 
           [0021]      FIG. 6  is a high level circuit diagram schematically illustrating experimental apparatus used for the generation of the laser pulse of  FIG. 5 . 
           [0022]      FIG. 7  is a high level circuit diagram schematically illustrating one example of circuitry for implementing pulse-power control by intra-pulse modulation in accordance with the present invention. 
           [0023]      FIG. 7A  schematically illustrates a gas-discharge laser incorporating the circuitry of  FIG. 7 . 
           [0024]      FIGS. 8A ,  8 B,  8 C, and  8 D are graphs schematically illustrating one mode of operation in the circuitry of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIGS. 4B ,  4 C, and  4 D schematically illustrate three different embodiments of inter-pulse modulation of RF exciting pulses in accordance with the present invention.  FIG. 4A  is provided for comparison with  FIGS. 4B-D  and schematically illustrates first and second consecutive un-modulated (prior-art) RF pulses P 1  and P 2  with which modulated pulse trains of  FIGS. 4B-D  can be compared. In  FIG. 4A  each of the prior-art un-modulated pulses has the same duration T A . There is a time T B  between the termination of pulse P 1  and the initiation of pulse P 2 . The pulse repetition period is T A +T B , equivalent to a PRF of 1/(T A +T B ). A preferred value for TA is between about 25 μs and about 50 μs. A preferred value for TB is greater than about 200 μs. 
         [0026]    For purposes of this description, it is assumed that time T B  is in the about 750 μs-or-less range in which the power of a laser output pulse is reduced by the above-discussed gas-heating during the generation of an immediately preceding pulse (see  FIG. 3 ). It is assumed that a look-up table has been formed by tabulating measured power ratio of consecutive pulses as a function of the inter-pulse period TB as depicted in  FIG. 3 . 
         [0027]    In the inter-pulse modulation scheme of the present invention, in all embodiments thereof, the un-modulated (continuous) pulses of the prior art are replaced by bursts or trains of sub-pulses with a time between sub-pulses sufficiently short that the laser responds to the pulse train as though the train were a single continuous pulse. In order for this to occur, the inter-pulse period of the sub-pulses should be about 1 μs. A pulse train can be considered as a modulated pulse having the duration of the train of pulses, with a square modulation envelope, and 100% modulation depth. This provides for varying the power in so-modulated pulse conveniently by pulse-width modulation or PRF variation. 
         [0028]    In the embodiment of  FIG. 4B , continuous pulse P 1  of  FIG. 4A  is replaced by a train PT A  of five sub-pulses of equal amplitude. The pulses all have the same (sub-pulse on) duration T D  and the same inter-pulse (sub-pulse off) period T E  for a percent duty cycle for the modulation in the pulse train of 100*T D /(T D +T E ). From the experimentally determined look-up table and knowledge of T B , it is determined how much additional power must be in a consecutive pulse-train PT B  (having the same duration as train PT A ) such that consecutive laser pulses generated by the consecutive pulse trains have the same power. Here, it is assumed that PT B  must have 1.2 times the power of PT A  for the corresponding laser pulses to have equal power. In order to achieve this, PT B  is a train of six sub-pulses having the same duration and amplitude as the sub-pulses of PT A  and the duration of trains. This provides the additional power in the train by providing an extra sub-pulse, effectively increasing the duty cycle of PT B  by a factor of 1.2 (6/5 in rational-fraction terms). 
         [0029]    In the embodiment of  FIG. 4C , continuous pulse P 1  of  FIG. 4A  is again replaced by a train PT A  of five sub-pulses of equal amplitude and duration as discussed above. A second pulse train PT C  has the same number of sub-pulses at the same PRF as in train PT A  but the duration (temporal “width”) of the of the sub-pulses in PT C  is increased by the 1.2 factor to provide the additional RF power for keeping the laser-pulse power constant. Again, as the duration of PT C  is the same as that of PT A , the duty cycle is PT C  is 1.2 times that of PT A . 
         [0030]    In the embodiment of  FIG. 4D , pulse P 1  is replaced by a pulse-train PT D  including an initial sub-pulse SP 1  having a duration T G  that is relatively long compared with the duration of the train, for example, about one-half of the duration of the train. Remaining sub-pulses have a duration and duty cycle comparable to the duration and duty cycle in the of sub-pulse pulses in the embodiments of  FIGS. 4B and 4C . Here, the sub-pulses are designated as having duration T D . 
         [0031]    A reason for initiating train PT D  with a relatively long sub-pulse is to increase the power in the leading edge of a corresponding laser pulse to provide a temporal shape to the laser pulse that is “square” or “rectangular” compared to the “peaked” pulses of  FIG. 2B . Such “square” or “rectangular” pulses are desirable in certain laser processing operations. This intra-pulse modulation scheme is useful even in cases where laser output pulse temporal separation is long-enough that the same modulation can be applied to a subsequent pulse. If correction is required, however, this can be provided by proportionally increasing the duration of all sub-pulses in a next exciting pulse train. This is illustrated in  FIG. 4D , wherein the duration of all sub-pulses in a train PT E  is increased by a factor of 1.2. 
         [0032]    It should be noted here that the pulses and sub-pulses schematically depicted in  FIGS. 4A-D  are actually envelopes of the RF frequency oscillations (not shown) of the power supply. This RF frequency is typically on the order of 100 megahertz (MHz), i.e., an oscillation period is on the order of 0.01 μs. 
         [0033]      FIG. 5  is a reproduction of an oscilloscope trace schematically illustrating a measured laser pulse generated by a 25 microsecond-long RF pulse-train consisting of an initial sub-pulse having a duration of 13 μs followed by twelve sub-pulses delivered with a duty cycle of 65%. The above-discussed “gain-spike” effect at the beginning of the pulse is clearly evident and is real. The apparent modulation of the “flat” portion of the pulse is electronic noise or “ringing” in the detector used for the pulse measurement, and is not actually present in the laser pulse. It is not the laser response to the modulated RF pulse train. 
         [0034]    Experimental apparatus  100  used to generate the pulse of  FIG. 5  is schematically illustrated in  FIG. 6 . A pulse-delay generator  102  was used to generate two synchronized pulses  104  and  106  out of terminals A and B, respectively. The pulse-durations and separation of the pulses are adjustable. The delay-generator was Model DG  535  generator, made by Stanford Research Systems Inc. of Sunnyvale, California. Pulse  104  is emitted earlier than pulse  106 . The duration of the pulse from terminal A was selected to be the desired duration of the initial long sub-pulse in the pulse-train (duration T G  in terms of train PT D  in  FIG. 4D ). 
         [0035]    Pulse  106  is used to trigger a function generator  100 . The function generator used was a Model No. 33220A function generator available from Agilent Inc of Sunnyvale, Calif. The function generator was adjusted to emit a train of pulses (here, 3 pulses) having a duration than T G  (duration T D  in terms of train PT D  in FIG. 
         [0036]      4 D). Duration T D  and the chosen duty-cycle and duration of the pulse train determines the intra-pulse duration (duration T E  in terms of train PT D  in  FIG. 4D ). The pulse train is triggered by pulse  106  at a time TG+TE after t 0  and appears as pulse train  112  in  FIG. 5 . Pulse  104  and pulse train  112  are connected to diodes  108  and  114 , connected in opposition. The pulse trains are summed at node  116  between the diodes to provide a pulse train  118  (equivalent of pulse train PT D  in  FIG. 4D ) which is used to command RF pulses from a RFPS  120  connected to discharge-electrodes of the laser. 
         [0037]    It should be noted here that the experimental circuit of  FIG. 6  is capable merely of forming a modulated pulse (pulse train) of the type depicted in  FIG. 4D  for “squaring-up” laser output pulses. The circuit does not have any provision for implementing pulse-to-pulse output-power control by the inventive intra-pulse modulation in any of the modes of  FIGS. 4B-D . 
         [0038]      FIG. 7  is a high level circuit diagram  130  schematically illustrating one example of circuitry for implementing pulse-to-pulse pulse-power control by intra-pulse modulation in accordance with the present invention. A description of the circuitry is set forth below with reference in addition to  FIGS. 8A ,  8 B,  8 C, and  8 D, and to  FIG. 4A  and  FIG. 4D . In this description, it is assumed that a sub-pulse train of the “squaring-up” type depicted in  FIG. 4D , for example, sub-pulse train PT D , has already been generated. 
         [0039]    At terminal  132  a user provided (from a PC, microprocessor, or the like) pulse-command  134  is provided. Command  134  can be equated to a prior-art (un-modulated) pulse P 2  of  FIG. 4A , with a similar command, corresponding to pulse P 1  of  FIG. 4A  having previously been delivered. Pulse  134  is split at node  136  with one-portion being connected to one input of an AND-gate  138 , another part being connected to an off-time measuring circuit  140 , and another part being connected to a slope-generator  142  including a look-up table (LUT) including data relating time between pulse-commands to pulse-train duty-cycles as described above. Yet another part of the command pulse is connected to both a high-frequency pulse-generator  144  and a start-up timer  146 . The duration of command-pulse  134  determines the duration of the train of sub-pulses corresponding to the command, for example, sub-pulse train PT E  of  FIG. 4D . 
         [0040]    Off-time measuring circuit  140  measures the time t off  that has elapsed since the end of the previous pulse-command. This is transmitted to circuit  142  that determines a duty-cycle value from the stored LUT. The duty-cycle vale is that required to equalize power in the sub-pulse train to be delivered (PT E  of  FIG. 4D ) with that power delivered by the previous sub-pulse train (PT D  of  FIG. 4D ). The required duty-cycle value is communicated to high-frequency pulse-generator  144 . The duty-cycle value is interpreted as a required increase in PRF of sub-pulses in the train (as depicted in  FIG. 4B ) or as an increase in the duration of sub-pulses in the trains ( FIGS. 4C and 4D ). Whatever method is implemented, pulse-generator  144 , in response to receiving a portion of command-pulse  134 , emits a train  148  of negative-going pulses (corresponding to required sub-pulses) having the selected PRF of, or a duration which corresponds to the inter-pulse period of pulses in the desired sub-pulse train. 
         [0041]    In response to receiving a portion of command-pulse  134 , start-up timer  146  generates a positive-going pulse  150  having a duration corresponding to an extended initial pulse in the sub-pulse train. The duration of pulse  150  can be constant from sub-pulse train to sub-pulse train or adjusted from pulse-train to pulse-train using duty-cycle or pulse-duration data from LUT  110 , relayed from high-frequency pulse-generator  144 . Negative going pulse-train  148  and pulse  150  are connected to separate inputs of an OR-gate  152 . When a signal from either pulse is present at one of the two inputs of the OR-gate, an output signal to the other input of AND-gate is provided. The output of the AND-gate is a sub-pulse train  154  similar to sub-pulse PTE of  FIG. 4D . This sub-pulse train is terminated by the falling edge of command-pulse  134 , which also resets pulse-generator  144  and start-up timer  146  in anticipation of another pulse-command. 
         [0042]      FIG. 7A  schematically illustrates the command-pulse modulating circuit of  FIG. 7A  incorporated in a gas discharge laser  160  in accordance with the present invention. Pulse-train (modulated command pulse)  154  from circuit  130  is delivered to a RFPS  162 . In response to receiving the modulated command pulse, the RFPS delivers a corresponding train of RF sub-pulses, i.e., a modulated RF pulse, to an electrode assembly  166  including a live or “hot” electrode  168  and a grounded electrode  170 . Pulses in this train may be characterized as excitation pulses. RFPS  162  includes an impedance matching network to match the output impedance of the RFPS to the impedance of the electrodes, as is known in the art. A laser-resonator, here an unstable laser-resonator) is formed between mirrors  172  and  174 . The electrodes and resonator are in a housing (not shown) including a lasing gas mixture, as is also known in the art. As noted above, in the method of the present invention, the time between sub-pulses is made sufficiently short that the lasing gas mixture energized by sub pulse-train responds as though the sub-pulse train were a continuous pulse. As a result, only a single laser output pulse  176  is delivered by the laser-resonator in response to energizing by sub-pulse train  164 . 
         [0043]    It should be noted here, that the circuit example of  FIG. 7  is but one example of a circuit for implementing the intra-pulse modulation method of the present invention. From this circuit description and the description of the inventive intra-pulse modulation method presented herein, those skilled in the electronic art may devise other circuits for implementing the inventive method without departing from the spirit and scope of the present invention. 
         [0044]    In summary, the present invention is described herein in terms of a preferred and other 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.