Abstract:
A solid-state laser comprises an excitation light source having a semiconductor laser array, a plurality of chopper circuits arranged between a direct-current power supply and an output terminal of the semiconductor laser array and connected in parallel with each other, for applying resultant power of the chopper circuits to the semiconductor laser array, a current controller for controlling the plurality of chopper circuits to modulate the resultant power at time resolution ranging from 1 μs to 100 μs, a solid-laser medium excited by a laser beam emitted from the semiconductor laser array, and a pair of mirrors for resonating a laser beam generated by exciting the solid-state medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-302127, filed Oct. 25, 1999, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an output control technique of controlling a laser pulse of a solid-state laser, which is excited by a semiconductor laser array, in time-function waveform. 
     Recently, the number of cases where a very thin material is processed using a solid-state laser (e.g., welding of an aluminum plate having a thickness of 0.1 mm or less and deposition of plastic components) has been increased as electronic components are miniaturized. Therefore, as shown in FIG. 5A, the pulse width ranges from 100 μs to 500 μs, and the pulse waveform of the solid-state laser has to be controlled by setting the current of one pulse as a function of time. 
     As a method of controlling the pulse output from a solid-state laser in arbitrary waveform, it can be thought to optically form a pulse through a Q-switching operation or under the control of a power supply. 
     In the Q-switching operation, a pulse is formed optically by the operation of a high-speed shutter set in an optical resonator. In other words, when the high-speed shutter is opened, a laser oscillator consumes energy accumulated in an excitation medium and starts to oscillate. The laser oscillator stops oscillating when it completely consumes the energy. The time period during which the laser oscillator oscillates corresponds to the pulse width. Usually, the pulse width is of the order of nanoseconds and the pulse is in a single-peak shape. Since the Q-switching operation is a self-excited oscillation, it is difficult to control one pulse in arbitrary form. 
     In the self-excited oscillation, a very-high-density inverted population is achieved if a loss of the optical resonator is increased to prevent the laser oscillator from oscillating while the laser oscillator is pumping laser materials and, if the loss is suddenly decreased to obtain a large Q value which is advantageous for oscillation, the accumulated energy is released explosively in several nanoseconds to several tens of nanoseconds. The Q-switching operation is performed based on the above principle. It is therefore difficult to control one pulse of a laser beam in arbitrary form at time resolution of 10 μs to 500 μs. 
     According to the power supply control, a pulse is excited by controlling electric energy applied to a flash lamp serving as an excitation source. Since, in this case, the power supply control is electric control, the pulse width and pulse shape can be controlled relatively easily if the pulse width is larger than a certain value. For example, Jpn. Pat. Appln. KOKAI Publication No. 4-42979 discloses a technique of controlling heat input to a process point by controlling one pulse in arbitrary waveform when the pulse width ranges from 1 ms to 20 ms. 
     Since, however, the start-up responsivity of the flash lamp falls within a range of 100 μs to 500 μs, it is extremely difficult to control one pulse in arbitrary form when the pulse width ranges from 10 μs to 500 μs. 
     FIGS. 5A to  5 C are graphs of pulse waveforms controlled by the excitation of a flash lamp. FIG. 5A shows a preset waveform in which the pulse width is set to five steps which differ from each other by 20 μs within 100 μs. FIG. 5B shows a current waveform. However, it does not correspond to the preset waveform at all when the pulse width is 100 μs but simply exhibits a single curve. FIG. 5C shows a laser waveform. Like the current waveform, the laser waveform does not correspond to the preset waveform at all when the pulse width is 100 μs. There is no correlation between the laser waveform and the preset waveform when the pulse width is 100 μs. The laser waveform also simply exhibits a single curve. 
     In other words, the flash lamp excitation requires response time of at least 100 μs, so that the current and laser waveforms cannot follow the preset waveform varying 20 μs by 20 μs and greatly differ in shape from the preset waveform. 
     As the rising characteristics of pulses of the flash lamp, the flash lamp is large in individual difference and easy to vary with time. For example, when a sharp peak is set at the beginning of a pulse waveform which corresponds to the pulse width of 100 μs, the peak is always susceptible to individual differences of the flash lamp and variations with time thereof, which greatly influences a process using the flash lamp. 
     Because of the above-described characteristics of the flash lamp, it is very difficult to control one pulse in time-function waveform when the pulse width is 500 μs or less. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to achieve a high-precision laser process by controlling one pulse in time-function waveform even when the pulse width is 500 μs or less. 
     According to the present invention, there is provided a power supply unit for a solid-state laser using a semiconductor laser array as an excitation light source, comprising a plurality of chopper circuits arranged between a direct-current power supply and an output terminal of the semiconductor laser array and connected in parallel with each other, for applying resultant power of the chopper circuits to the semiconductor laser array, and a current controller for controlling the plurality of chopper circuits to modulate the resultant power at time resolution ranging from 1 μs to 100 μs. 
     The present invention allows one pulse to be modulated and controlled in time-function waveform at time resolution ranging from 1 μs to 100 μs even when the pulse width is 500 μs or less, with the result that a high-precision laser process can be executed. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1A is a schematic diagram showing the structure of a solid-state laser according to an embodiment of the present invention; 
     FIG. 1B is a schematic diagram showing the structure of a laser-beam generator according to the embodiment of the present invention; 
     FIG. 2 is a circuit diagram of a power supply having a waveform control function which is incorporated into the solid-state laser; 
     FIG. 3A is a graph explaining waveforms of dither signals; 
     FIG. 3B is a graph explaining waveforms of output currents; 
     FIGS. 4A to  4 C are graphs showing waveform control of the present invention by excitation of an LD; and 
     FIGS. 5A to  5 C are graphs showing prior art waveform control by excitation of a lamp. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A is a schematic diagram showing the structure of a solid-state laser  10  according to an embodiment of the present invention, and FIG. 1B is a schematic diagram showing the structure of a laser-beam generator  10 A according to the embodiment of the present invention. 
     The solid-state laser  10  includes a laser medium  11  using a YAG rod as a solid-state medium and a reflector  12  and an output mirror  13  arranged at both ends of the laser medium  11  along an optical axis thereof to constitute an optical resonator. The laser  10  also includes a semiconductor laser array  14  for excitation and a heat sink  15  for cooling. The semiconductor laser array  14  is arranged in parallel with the optical axis of the laser medium  11  and integrally with the heat sink  15  as one component. The output of the semiconductor laser array  14  is controlled by a power supply  20  having a waveform control function using a dither circuit as is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 8-317655. 
     The laser-beam generator  10 A converges laser beams, which are emitted from the semiconductor laser array  14 , on a lens  16 . The converged laser beam is output from the lens  16 . 
     The power supply  20  is so constituted as shown in the circuit diagram of FIG.  2 . In FIG. 2, reference numeral  21  denotes a DC power supply,  22   a  to  22   d  indicate switching elements (IGBT) for current control,  23   a  to  23   d  show reactors for smoothing a DC current,  24   a  to  24   d  are diodes,  25   a  to  25   d  represent current monitors,  26  indicates an output terminal, and  30  shows a control circuit. The switching elements  22   a  to  22   d , reactors  23   a  to  23   d , and diodes  24   a  to  24   d  constitute four chopper circuits of a current control type which are connected to each other. 
     The control circuit  30  includes a current reference signal generator  31 , an oscillator circuit  33 , dither circuits  35   a  to  35   d  (which will be described later) connected to the oscillation circuit  33 , comparators  32   a  to  32   d  connected to the current reference signal generator  31  through the dither circuits  35   a  to  35   d , and driver circuits  36   a  to  36   d  connected to the comparators  32   a  to  32   d.    
     Upon receiving a start-up signal, the current reference signal generator  31  outputs a current reference signal SO having a preset voltage pattern (time-function waveform). The comparators  32   a  to  32   d  compare dither superimposition signals Sa to Sd (which will be described later) and output signals S 1  to S 4  corresponding to output currents I 1  to I 4  of the chopper circuits detected by the current monitors  25   a  to  25   d , and output on/off signals. The oscillator circuit  33  generates clock pulses Ra to Rd whose phases differ from each other by 900° at a fixed frequency. The dither circuits  35   a  to  35   d  generate saw-tooth dither signals Da to Dd which are synchronized with the clock pulses Ra to Rd and gradually decrease as shown in FIG. 3A, and superimpose the dither signals on the input current reference signal SO, thereby outputting dither superimposed signals Sa to Sd. The driver circuits  36   a  to  36   d  turn on/off the switching elements  22   a  to  22   d  in response to the signals from the comparators  32   a  to  32   d.    
     Paying attention to one chopper circuit, the control of switching elements will now be described. Upon receiving a start-up signal, the current reference signal generator  31  outputs a current reference signal SO having a time-function waveform. The oscillator circuit  33  outputs a clock pulse Ra from which a dither signal Da is generated. The dither signal Da is superimposed on the current reference signal SO and the dither superimposition signal Sa is input to the comparator  32   a.    
     When the level of an output signal S 1  of the current monitor  25   a  is lower than that of the dither superimposition signal Sa, the comparator  32   a  outputs an on-signal to turn on the switching element  22   a  through the driver circuit  36   a . If the switching element  22   a  turns on, the DC power supply  21  supplies power and the current I 1  gradually increases through the reactor  23   a.    
     When the increase of current I 1  makes the level of the output signal S 1  of the current monitor  25   a  higher than that of the dither superimposition signal Sa, the comparator  32   a  outputs an off-signal to turn off the switching element  22   a  through the driver circuit  36   a . If the switching element  22   a  turns off, the power accumulated in the reactor  23   a  returns through the output terminal  26  and the diodes  24   a  to  24   d  and attenuates gradually. 
     The above operation is performed at high speed for each period of clock pulse Ra, and the output terminal  26  is supplied with a direct current I 1  having almost the same waveform as that of the dither superimposition signal Sa. 
     Similarly, the output terminal  26  is supplied with direct currents I 1  to I 4 . Since the periods of clock pulses output from the oscillator circuit  33  are shifted from each other by 90°, the direct currents have waveforms as shown in FIG.  3 B. The output terminal  26  is supplied with a combined current of I 1  to I 4  and finally supplied with a smooth direct current with few ripples. 
     The dither circuits  35   a  to  35   d  are provided for the following reason. The comparators  32   a  to  32   d  compare the output signals of the current monitors  25   a  to  25   d  with reference to a fixed voltage. The switching elements  22   a  to  22   d  turn on when the output signals are lower than the reference voltage, and they turn on when the output signals are higher than the reference voltage. In other words, the switching elements  22   a  to  22   d  turn on/off in accordance with a slight variation in voltage. It is thus likely that the switching elements will be operated frequently and destroyed accordingly. 
     Even though the dither signals turn off the switching elements  22   a  to  22   d  and lower the voltage from the current monitors, the reference voltage is also lowered by the dither signal and thus the voltage does not lower than the reference voltage. Consequently, an interval between the turn-on and turn-off of the switching elements  22   a  to  22   d  can be set to more than a fixed one by the dither signals. 
     As described above, the output terminal  26  supplies the semiconductor laser array  14  with power having the same time-function waveform as that of the current reference signal SO. 
     It is therefore the feature of the present invention that one pulse output from a solid-state laser excited by a semiconductor laser array of QCW (quasi-CW) specifications can be controlled in time-function waveform using a dither circuit. 
     In a conventional solid-state laser excited by a semiconductor laser array of QCW specifications, a rectangular-wave operation was recommended and thus the power supply to the semiconductor laser array was limited to a rectangular-wave operation to be performed using a stabilized power supply. As compared with the conventional solid-state laser which outputs a rectangular-wave pulse or outputs a single-peak pulse using a Q switch, the control of the present invention is improved in precision more greatly and one pulse can be controlled in time-function waveform more exactly. 
     In the foregoing embodiment, four circuits are arranged in parallel according to the dither control method in which the dither circuits are used for the power supply of the semiconductor laser array; consequently, response time of 10 μs can be achieved at 100 A at a switching frequency of 75 kHz×4 (circuits). 
     FIGS. 4A to  4 C are diagrams explaining the waveform control of the solid-state laser  10  excited by the semiconductor laser array. FIG. 4A shows a preset waveform in which the pulse width is set to five steps differing from each other by 20 μs within 100 μs. 
     FIG. 4B shows a current waveform in which the pulse width is set to deformed five steps corresponding to those of the preset waveform shown in FIG.  4 A. Similarly, FIG. 4C shows a laser waveform in which the pulse width is set to deformed five steps corresponding to those of the preset waveform shown in FIG.  4 A. 
     Unlike the prior art flash lamp excitation shown in FIGS. 5A to  5 C, the laser array excitation allows a shape closely analogous to a preset time-function waveform to be obtained since the response time is 10 μs which is shorter than 20 μs. According to the present invention, one pulse can be controlled in time-function waveform even when the pulse width is 500 μs or smaller. 
     The present invention is not limited to the above-described embodiment. In the embodiment, the voltage applied to the semiconductor laser array is controlled. However, the current supplied to the semiconductor laser array or the power applied thereto can be controlled. The number of chopper circuits is not limited to four. In the embodiment, a power supply circuit of a switching system using a dither circuit is employed; however, it can be replaced with a dropper system. Needless to say, various changes and modifications can be made without departing from the scope of the subject matter of the present invention. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.