Abstract:
A pulsed, solid laser having a solid-state gain-medium and Q-switch is optically-pumped by a diode-laser array controlled to deliver pump-light pulses to the gain-medium. The Q-switch and the diode-laser array are cooperatively controlled by a controller such that laser output-pulses produced in response to pump-light pulses have the same energy independent of the time-interval between laser output-pulses. Pump-light pulses may be provided by the controller operating a switchable current supply which supplies current-pulses to the diode-laser array for causing pump-light pulses to be delivered to the gain-medium. A controller may also be arranged to drive the diode-laser to provide continuous pump-light output and to operate a light modulator located between the diode-laser array and the gain-medium to cause pump-light pulses to be delivered to the gain medium.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates in general to diode-pumped pulsed solid state (DPSS) lasers. The invention relates in particular to DPSS laser including a Q-switch, and wherein a diode-laser for delivering pump-light pulses is driven by a pulsed or switched power-supply and the Q-switch is synchronously activated at the termination of pump-light pulses. 
     DISCUSSION OF BACKGROUND ART 
     Rapidly pulsed DPSS lasers are increasingly being used for precision, fine machining operations in electronics and related industries. Rapidly pulsed, here, refers to a range of pulse repetition rate between about 1 kilohertz (KHz) and several megahertz (MHz). One example of such a machining operation is trimming of resistors using a pulsed Nd:YAG laser at a wavelength of 1064 nanometers (nm). This operation requires the use of variable cutting speeds. When cutting speed is varied, pulse repetition rate must be varied correspondingly to maintain a constant width of cut. In order for a predetermined relationship between pulse repetition rate and cutting speed to be reliable, the energy-per-pulse must remain constant as the pulse repetition rate is varied. 
     Another example of a use of rapidly pulsed DPSS lasers is engraving images in plastic cards such as identification cards. A pulsed Nd:YAG laser at a wavelength of 1064 nm is also useful in this operation. Typically an image is engraved by laser machining a series of pits having variable spacings therebetween, the real concentration of pits determining the form of the image. Variable spacing is achieved by varying pulse repetition rate at a constant machining speed. Here again, the operation is most effective if the energy-per-pulse remains constant as pulse repetition rate is varied. 
     Prior-art rapidly-pulsed DPSS lasers are typically continuously pumped, and pulses are formed by repeatedly opening and closing a Q-switch, located in the laser&#39;s resonant-cavity. In these prior-art lasers, at pulse-repetition rates greater than about 1/τ m  m (where τ m  is the characteristic lifetime of excited states of the gain medium), energy-per-pulse is inversely dependent on the pulse-repetition rate. For a gain medium with a relatively long characteristic lifetime such as Nd:YLF, this dependence begins at pulse-repetition rates above about 1 KHz. For a gain-medium with a shorter characteristic lifetime, for example, Nd:YAG, the dependence begins above about 4 KHz. 
     There is a need for a pulsed-laser which provides laser output-pulses having a constant energy-per-pulse independent of the time-interval between the pulses. Preferably, laser output-pulses should have constant energy-per-pulse even if in a series of laser output-pulses the time interval between pulses varies. 
     SUMMARY OF THE INVENTION 
     A pulsed laser in accordance with the present invention provides output-pulses of constant energy-per-pulse, independent of the interval between pulses, even when intervals between pulses are randomly varying. 
     In one aspect, a laser in accordance with present invention comprises a laser resonant-cavity or laser resonator having a solid-state gain medium. A source of pump-light is provided for energizing the gain medium. The pump-light source is arranged to provide a series of pump-light pulses for energizing the solid-state gain-medium. Each of the pump-light pulses has the same duration, however, the time-period between pump-light pulses is variable. 
     A Q-switch is located in the resonant-cavity. The Q-switch is arranged to retard operation of the resonant-cavity until a pump-light pulse is terminated. Termination of a pump-light pulse provides a trigger signal for opening the Q-switch, thereby allowing operation of the resonant-cavity for generating a laser output-pulse. The pump-light source has an essentially constant output throughout each pump-light pulse, whereby each laser output-pulse has about the same energy, independent of the time-period between pulses. 
     Preferably the pump-light source is further arranged to deliver sufficient pump-light to the gain-medium, between termination of each pump-light pulse and initiation of a subsequent pump-light pulse, that gain in the gain-medium is the same at the initiation of each pump-light pulse independent of the time interval between the pump-light pulses. 
     In one preferred embodiment, the pump-light source is a diode-laser array driven by a regulated current-supply and a controller. The controller is arranged such that the current-supply delivers a series of current-pulses to the diode-laser array. The diode-laser array responsively generates a series of pump-light pulses for energizing the solid-state gain-medium. 
     The controller is further arranged such that each of the current-pulses and corresponding pump-light pulses has the same duration, and such that the current-pulses and corresponding pump-light pulses may have a variable time-period therebetween. Termination of a current-pulse provides a trigger-signal for opening the Q-switch, thereby allowing operation of the resonant-cavity for generating a laser output-pulse. The controller is further arranged such that the diode-laser array has an essentially constant output throughout each pump-light pulse, whereby each laser output-pulse has about the same energy, independent of the time-period between laser output-pulses. 
     In another preferred embodiment the pump-light source is a diode-laser array driven by a regulated current-supply and a controller and provides a continuous pump-light output. A light-modulator is located between the diode-laser array and the solid-state gain-medium in the path of the pump-light output of the diode-laser array. The controller is arranged to operate the light-modulator such that the pump-light output from the diode-laser array is delivered to the gain medium as a series of pump-light pulses having the same duration but having a variable time-period therebetween. A Q-switch is located in the resonator, the Q-switch is arranged to retard operation of the resonator until a pump-light pulse is terminated, the termination of the pump-light pulse proves a trigger-signal for opening the Q-switch, thereby allowing delivery by the resonator of a laser output-pulse. The controller is further arranged such that the diode-laser array has an essentially constant output throughout each pump-light pulse, whereby each laser output-pulse has about the same energy, independent of the time-period between laser output-pulses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. 
     FIG. 1 schematically illustrates one preferred embodiment of a Q-switched, DPSS laser including a laser-diode driver and controller in accordance with the present invention. 
     FIG. 2 is a timing-diagram schematically illustrating one preferred temporal relationship of diode-laser current, current-switching signals, Q-switch trigger-signals, laser-gain, and laser output-pulses in the DPSS laser of FIG.  1 . 
     FIG. 3 is a timing-diagram schematically illustrating another preferred temporal relationship of diode-laser current, current-switching signals, Q-switch trigger-signals, laser-gain, and laser output-pulses in the DPSS laser of FIG.  1 . 
     FIG. 4 is a block diagram schematically illustrating functional elements of the driver and controlled of FIG.  1 . 
     FIG. 5 schematically illustrates another preferred embodiment of a Q-switched, DPSS laser in accordance with the present invention including a diode-laser pump-light source, a light-modulator for modulating output of the pump-light source and a laser-diode driver and controller. 
     FIG. 6 is a timing-diagram schematically illustrating one preferred temporal relationship of light-modulator voltage, voltage-switching signals, Q-switch trigger-signals, laser-gain, and laser output-pulses in the DPSS laser of FIG.  5 . 
     FIG. 7 is a timing-diagram schematically illustrating another preferred temporal relationship of light-modulator voltage, voltage-switching signals, Q-switch trigger-signals, laser-gain, and laser output-pulses in the DPSS laser of FIG.  1 . 
     FIG. 8 is a block diagram schematically illustrating functional elements of the driver and controller of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, wherein like features are designated by like reference numerals, FIG. 1 depicts one preferred embodiment a pulsed laser  20  in accordance with the present invention. Laser  20  includes a “folded” resonant-cavity or resonator  22  formed between an output-mirror  24  and a maximum reflecting mirror  26 . Laser  20  is pumped by a diode-laser array  29 . Folding of the resonator is accomplished by mirrors  21  and  23 , of which mirror  21  is transparent to light emitted by laser-diode  29 . A Q-switch  28 , for example, an acoustic optic Q-switch, is located at the end of resonant-cavity  22  closest output-mirror  24 . A solid-state gain-medium or crystal  30  is located at the end of resonant-cavity  22  closest to maximum reflecting mirror  26 . Gain-medium  30  may, for example, the neodymium-doped yttrium aluminum garnet(Nd:YAG), neodymium-doped yttrium orthovanadate (Nd:YVO 4 ) or neodymium-doped yttrium lithium fluoride (Nd:YLF). These examples, however, should not be considered as limiting the present invention. 
     Pump-light PL from laser-diode array  29  is delivered by an optical-fiber  31  to a lens  32 , and focused by lens  32  through mirror  21  onto gain medium  30  for energizing the gain-medium. It should be noted here that the terminology “diode-laser array”, as used in the context of this embodiment of the inventive laser and the appended claims is meant to encompass even a single diode-laser. Further it should be noted that while a so-called end-pumped arrangement is depicted in FIG. 1 for gain-medium  30 , principles of the present invention are equally applicable to side-pumped (laterally-pumped) arrangements for solid-state gain-media. 
     Laser  20  is driven and controlled by a driver/ controller  40  (hereinafter controller  40 ). A control panel  42  is used to provide user instructions to controller  40  via a lead  43 . Controller  40  operates Q-switch  28  via a lead  44 . A portion  45  of output-power of laser  20  is sampled by a beamsplitter  46  and directed by the beamsplitter to a power monitor  48  such as a photodiode or the like. Output of power monitor  48  is communicated to controller  40  via a lead  50 . Controller  40  provides regulated current to diode-laser  29  via a lead  52 . 
     Referring now to FIG. 2, one preferred mode of operation of laser  20  by controller  40  is depicted in the form of a timing diagram. In this mode of operation, laser  20  delivers a series of pulses of essentially equal energy, but having varying time interval therebetween. Diode-laser array  29  is supplied constantly during operation of laser  20  with a essentially-constant low level L of current I D  which generates only sufficient pump-light to maintain gain in gain-medium  30  at a minimum level G MIN . After delivery of a laser-pulse, there is some residual gain in a gain-medium, which decays exponentially with time due to fluorescence. Maintaining minimum gain ensures that conditions in the gain-medium are the same at the delivery of each new pump-light pulse regardless of the interval between pump-light pulses. 
     On receipt of a trigger-signal TR, controller  40  raises current I D  to the diode-laser to an essentially-constant value H for a time-period A and then allows the current to fall to the minimum value. This raised current time-period may be referred to as a pump-pulse (current-pulse) which generates a corresponding pump-light pulse from diode-laser array  29 . 
     It should be noted here, that the terminology “essentially-constant” and “essentially-equal” mean respectively constant or equal within normal limits of electronic control. Further, it should be noted that operation at pulse-repetition rates less than about 200 KHz is contemplated for laser output-pulses delivered by this embodiment of the inventive laser. Accordingly it is assumed that the rise and fall times of current in a current-pulse and pump-light in a corresponding pump-light pulse are negligibly short compared with the total length of the pulses. Operation at higher pulse-repetition rates, however, is not precluded. 
     From initiation to termination of a current-pulse, i.e., during time-period A, the gain of gain-medium  30  rises from minimum value (G MIN ), sustained by the level L of diode-laser current I d , to a maximum value (G MAX ) which is determined by the level H of the diode-laser current and the length of time-period A. 
     At the end of time-period A, i.e., with the falling-edge (termination) of the current-pulse or pump-pulse, a radio-frequency (RF) signal is switched to zero (switched-off) thereby opening Q-switch  48  and allowing delivery of a laser output-pulse from resonator  22  via mirror  24 . The RF signal is switched off for a brief time-period or timeout T/O which is selected to be shorter than the shortest interval between termination of one pump-pulse and initiation of the next. 
     By operating laser  20  in this manner, the energy in a laser output-pulse is determined entirely by the duration of the pump-pulse. Accordingly, regardless of the time-interval between pump-pulses, in a corresponding series of laser output-pulses, the energy-per-pulse is substantially constant from pulse to pulse. By time-interval between pump-pulses, here, is meant the time-interval between initiation of pump-pulses. In a laser in accordance with the present invention an energy-per-pulse repeatable within about 2 percent or less is achievable. 
     Referring now to FIG. 3 another mode of operation of laser  20  is described, also in the form of a timing-diagram. In this mode of operation laser  20  provides a series of laser output-pulses representing a digital signal or communication. A train of clock-pulses at regular intervals sets the interval at which a laser-pulse will be delivered (representing a 1), or not delivered (representing a 0). In this mode of operation, diode-laser current I D  is switched only between 0 and the level H required to provide, corresponding to time-period A, the desired energy-per-pulse. 
     A current-pulse or pump-pulse (P 1 ) of duration A is triggered (clock-pulse C 1  and trigger-pulse T 1 ) and a corresponding laser-pulse LP delivered as described above. At the next clock-pulse (C 2 ), another pump-pulse P 2  is triggered by trigger-pulse T 2 . The time-period D between termination of pump-pulse P 1  and initiation of pump-pulse P 2  is insufficient for gain G to fall completely to zero, and gain G falls to a finite minimum value G MIN . At the next clock-pulse C 3  a laser-pulse is not required, and no trigger-pulse is generated. Instead, clock-pulse C 3  is used to trigger an alternative current-pulse (P 3 ) of a duration B which is shorter than duration A. Current-pulse P 3  also causes delivery of a corresponding pump-light pulse of duration B by diode-laser array  29 . Current-pulse P 3  does not switch off the RF signal to Q-switch  28 . Accordingly, no laser output-pulse is generated in response to current-pulse P 3  and its corresponding pump-light pulse. This corresponding pump-light pulse serves only to raise the gain in gain-medium  30  to some predetermined value G INT  between G MIN  and G MAX . Current-pulse or pump-pulse P 3  may be referred to as a gain-maintenance pulse. As no laser-pulse is delivered to deplete the gain in gain-medium  30  provided by current-pulse P 3 , a longer interval is required for the gain to fall to minimum value GMIN. 
     Time-period B of gain-pulses is selected such that G MIN  is reached at the initiation of any other pump-pulse. If the next pump-pulse is a pulse of duration A (P 4 ), then gain in gain-medium  30  reaches the value G MAX  on termination of the pulse. If at two consecutive clock-pulses (C 5  and C 6 ) laser-pulses are not required, consecutive gain-pulses P 5  and P 6  of duration B are initiated by the clock-pulses, again, with Q-switch  28  closed. 
     FIG. 4 illustrates a preferred arrangement of circuit elements for controller  40 . Controller  40  has, as a central control element, a microprocessor  60 . Controller  40  also includes a regulated current-supply  62  for diode-laser  29 . Current-supply  62  includes an analog current-regulator  64  and a switching unit  66 . Microprocessor  60  receives output from power monitor  48  and compares actual output-pulse energy with the desired output-pulse energy. From the comparison, microprocessor  60  sets (via lead  67 ) a diode-laser current-control setpoint for analog-regulator  64  (I D  of FIG. 2) to a value which will provide the desired output-pulse energy. A current-monitor  68  provides feedback to analog regulator  64 , via lead  69 , of actual current through diode-laser array  29 . Microprocessor  60  also sets (via lead  70 ) low value L of diode-laser current required for minimum-gain maintenance in the operation mode of FIG.  2 . 
     A logic unit  72  is arranged to perform above-described triggering and switching operations. Logic unit  72  includes delay-generators  74  and  76  for controlling above-described pump-pulse and gain-pulse durations A and B respectively. Durations A and B of delay-generators  74  and  76  are set by microprocessor  60  through trigger signals delivered to the delay-generators via leads  78  and  80  respectively. Delay-generators  74  and  76  each communicate with an OR-gate  82  via leads  84  and  86  respectively. Either communication will operate switching unit  66 , via lead  87 , for providing above-described pump-pulses or gain-pulses. 
     If delay generator  74  is triggered (via lead  100 ), at the end of the delay-period, i.e., on termination of the corresponding current-pulse, a signal is transmitted via lead  88  to a Q-switch timeout generator  90 . Timeout generator  90  is connected via lead  92  to an RF-amplifier  94 . On receipt of the delay-termination (falling-edge) signal from delay generator  74 , timeout generator  90  turns RF-amplifier off thereby opening Q-switch  28  for delivery a laser-pulse. Timeout interval T/O, for which Q-switch  28  is open, is set by microprocessor  60  via a lead  91 . The RF-amplitude of RF-amplifier  94  is set by microprocessor  60  via lead  96 . If delay-generator  76  is triggered (via lead  98 ), no signal is transmitted to timeout generator  90 . Accordingly Q-switch  28  remains closed during delivery of above-described gain-pulses. 
     In the foregoing description of controller  40 , clock and trigger signals may be externally supplied to logic unit  72  via leads  98  and  100  respectively. Such externally-supplied signals may be supplied, for example, from a cooperative apparatus, or from a personal computer or the like. A trigger signal for delay generator  74  may even be delivered from something as simple as a manually-operated switch or button. 
     In one example of a pulsed solid-state laser in accordance with the above described first embodiment of the present invention, arranged to operate in accordance with the timing scheme of FIG. 2, a YVO 4  gain-medium is end-pumped by 808 nm radiation from an 8.0 watt fiber array package. The laser provides better than 2% RMS repeatability of energy-per-pulse over a range of pulse energies between about 10.0 and 20.0 microjoules per pulse (μJ/pulse) at a range of pulse repetition-rates between about 55 kHz and 85 kHz. Those skilled in the art will recognize from the description of the present invention presented herein that principles of the present invention are applicable to lasers including different gain-media with different pulse energies and repetition rates. Accordingly, the above exemplified pulse energies and repetition rates should not be construed as limiting the present invention. 
     Referring now to FIG. 5, another embodiment  110  of a pulsed-laser in accordance with the present invention is illustrated. Components of laser  110  is similar in most respects with those of laser  20  with the exception that a light-modulator  112 , such as an acousto-optic modulator (AOM) or an electro-optic modulator (EOM) is added between diode-laser array  29  and mirror  21 , i.e., between diode-laser array  29  and gain-medium  30 . This embodiment is preferred for operating at pulse-repetition rates greater than 200 KHz and up to several MHz, operation, however, is not limited to these high rates. 
     Switching a regulated power supply at sufficiently high frequency for providing these high pulse-repetition rates becomes difficult, and the rise and fall time of current pulses as percentage of a pulse duration can no longer be assumed to be insignificant. This is overcome in laser  110  by operating diode-laser array  29  to provide continuous pump-light output and modulating the pump-light output of the diode-laser array with light-modulator  112 . Modulation of the light-output of diode-laser array  29  provides that pump-light is delivered to gain-medium  30  as pump-light pulses, and operation of Q-switch  24  is performed synchronously with the falling edge of a pump-light pulse as in laser  20 . Driver and controller  41  (described in detail further hereinbelow) is similar to driver and controller  40  of laser  20  but is modified for driving light-modulator  112  to produce pump-light pulses. 
     FIGS. 6 and 7 are timing diagrams corresponding to above-discussed timing diagrams of FIGS. 2 and 3 respectively. It can be seen that the only difference is that diode-laser current I D  of FIGS. 2 and 3 is replaced with a light-modulator voltage V M . Temporal relationship of signals is otherwise identical. 
     FIG. 8 illustrates a preferred arrangement of circuit elements for controller  41 . This is similar in most respects and functions to controller  40  of FIG. 4 with the following exceptions. The switched current-supply  62  of controller  40  is, in controller  41 , simply a regulated current supply  113 . The maximum level of a pump-light pulse is determined by a maximum diode-laser current communicated to current supply  113  via lead  67 . Switching unit  66  of controller  40  is replaced, in controller  41 , by an AOM/EOM driver  115 , which is a regulated voltage source. AOM/EOM driver  115  receives switching signals from logic unit  72  via lead  87 . 
     The minimum gain of FIG. 6 is provided in that light-modulator  112  (see FIG. 1) is not completely opaque at its minimum transmission. This minimum transmission, and, accordingly minimum gain, is established by a corresponding voltage delivered from AOM/EOM driver  115  via lead  117 . This minimum voltage is set by microprocessor  60  via lead  114 . 
     The present invention is described above in terms of a preferred and other embodiments. The present invention is not limited, however, by the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.