Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
  • H01S3/104—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation in gas lasers
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03K—PULSE TECHNIQUE
  • H03K7/00—Modulating pulses with a continuously-variable modulating signal
  • H03K7/08—Duration or width modulation ; Duty cycle modulation
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/097—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser
  • H01S3/09702—Details of the driver electronics and electric discharge circuits
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/22—Gases
  • H01S3/223—Gases the active gas being polyatomic, i.e. containing two or more atoms
  • H01S3/2232—Carbon dioxide (CO2) or monoxide [CO]

Definitions

• the present invention relates in general to carbon dioxide (C0 2 ) gas discharge lasers powered by a radio frequency (RF) power supply.
• the invention relates in particular to methods of pulse width modulation (PWM) for selectively varying and controlling the average power output of the RF power supply.
• PWM pulse width modulation
• a C0 2 gas discharge laser is typically powered by a high-voltage RF power supply (RFPS).
• RFPS RF power supply
• the power supply applies RF voltage to electrodes of the gas laser, which excite a discharge in a lasing gas mixture including C0 2 and inert gases.
• the discharge takes place within a laser resonator.
• the discharge energizes the lasing gas such that the energized gas provides optical gain causing laser radiation to circulate in the laser resonator.
• a fixed, predetermined portion of the circulating radiation is coupled out of the laser resonator as output radiation.
• the laser is typically operated in a pulsed manner and delivers pulses at a predetermined peak power, for a given pulse duration, and at a predetermined pulse repetition frequency (PRF).
• PRF pulse repetition frequency
• the PRF is between about 1 kilohertz (kHz) and 200 kHz.
• the average power in a laser pulse is related to the average power delivered by the RF power supply during the duration of the pulse.
• the RF power supply typically operates at a predetermined fixed (RF) frequency between about 10 megahertz (MHz) and 150 MHz with 100 MHz being typical, i.e., much higher than the highest contemplated PRF of the train of pulses.
• the power in the laser output pulses is controlled by modulating the width of the individual RF pulse from the RF power supply.
• This power control method is called pulse width modulation (PWM).
• PWM pulse width modulation
• the RF power supply is periodically turned (fully) on and (fully) off, thereby generating a train of RF pulses which are provided to the laser discharge.
• the RF pulses in the train have the same on time, and the same off time between pulses.
• the pulse train is characterized by a duty cycle which is equal to the pulse duration of one pulse within the pulse train divided by the repetition period of the pulse train.
• RF power delivered to the laser is controlled by varying the duty cycle, which is effected by varying the duration (modulating the temporal width) of the RF pulses during the repetition period. Whatever the duty cycle, the width of all RF pulses in a train thereof is the same.
• the duration of the pulses in a digital pulse width modulator is digitally controlled, so a pulse in a train can only be lengthened or shortened by fixed increments, the length of an increment being determined by the frequency of a system clock delivering clock pulses.
• the number of RF pulses in a train is fixed (again digitally) at some value required to provide that the train average power can be considered as equivalent to a steady state value that the train is attempting to simulate. Accordingly the resolution, i.e., the accuracy to which the average RF power can be controlled, and the corresponding power of a laser pulse, is determined by the clock-pulse period relative to the repetition period of the RF pulse train.
• the laser PRF 1 kHz (corresponding to the frequency of delivery of RF pulse trains)
• the present invention is directed to a method and apparatus for controlling, by pulse width modulation, the output power of a pulsed gas discharge laser powered by a pulsed RF power supply.
• the pulse width modulation method comprises delivering a train of digital pulses to the RF power supply.
• the train has a predetermined number of pulses therein, and each pulse in the train has an incrementally variable duration.
• the power supply is arranged to deliver a train of RF pulses
• each train of RF pulses having an average power dependent on the duration of the RF-pulse train and the aggregate duration of pulses in the RF-pulse train.
• the average power in the RF-pulse train can be varied by incrementally varying the duration of one or more, but less than all, of the digital pulses in the train thereof.
• the duration of the digital pulses in the train thereof is controlled by pulses delivered by a digital clock.
• the incremental variation of the duration of the one or more digital pulses is one or more of the pulse repetition periods of the digital clock.
• the inventive pulse width modulation method is referred to herein as a dual modulus digital pulse width modulation (DMDPWM) method.
• FIGS. 1A, IB, 1C, and ID are graphs of voltage as a function of time schematically illustrating principles of the (DMDPWM) method of the present invention and depicting digital pulse parameters as a function of clock cycle periods in a simplified example of the method.
• FIG. 2 is a high level circuit block diagram schematically illustrating one preferred embodiment of digital pulse width modulator circuitry for implementing the method of the present invention including a signal processor for translating user requests into three digital words, period and pulse width counting circuitry responsive to two of the digital words and N-modulo counter circuitry responsive to the other digital word.
• FIG. 3 is a circuit diagram schematically illustrating one preferred configuration of the N-modulo counter circuitry of FIG. 2
• FIG. 4 is a logic circuit diagram schematically illustrating a preferred configuration of the period and pulse-width circuitry of FIG. 2, and details of the interaction of that circuitry with the N-modulo counter circuitry of FIG. 3.
• FIGS. 1A-D schematically illustrate digital pulse parameters as a function of clock cycle periods in a simplified example of dual modulus digital pulse width modulation (DMDPWM) method of the present invention.
• F frame
• P integer value
• pulse frames would normally be delivered repetitively in practical use of a laser but may be given different parameters from one repetition to the next, as needed.
• the clock cycle period t is equal to 1/f, where f is the clock frequency.
• all of the pulses have a basic width (duration) W equal to 2t.
• the voltage amplitude of the pulses is represented by V.
• the power delivered in a pulse frame can be given various average values by incrementally increasing (stretching) the duration of one or more pulses in the frame beyond the basic duration.
• the amount of stretching is one clock-cycle period t.
• the amount of stretching for each of the stretched pulses is one clock-cycle period t.
• each of the four pulses in the frame is stretched by one clock-cycle period.
• the pulse trains of FIGS. 1A-D are trains of digital pulses that are delivered to the RFPS by inventive DMDPWM circuitry described in detail further hereinbelow.
• the pulse trains would command an RFPS to deliver corresponding trains of laser output pulses of RF energy, which would produce corresponding pulses of laser energy.
• the envelope of the laser output pulses would be similar to the envelope of the digital pulses except for slower rise and fall times.
• envelope is used here in recognition that the RF pulses would be voltage varying at RF frequency under the envelope.
• the average power of a frame of pulses can be represented by a duty cycle D, which is the sum of the duration of all pulses within the frame divided by the duration of the frame.
• the average value of the pulse train determines the average power of laser output.
• the width of all pulses in a train is incremented to increase the duty cycle. Accordingly, the resolution is limited by the number of clock cycles in a pulse repetition period of the laser.
• the resolution is effectively increased by 1/F where F is the number of pulses in a frame than can be stretched.
• Stretched pulses can be evenly distributed throughout a frame. This results in a smoother output waveform than occurs in the case where all the stretched pulses are bunched together. This smoothing of the output waveform is important for minimizing the peak-to- peak amplitude ripple of the output of the RFPS driving the laser. This smoothing of the RFPS output, translates to a smoother power output from the laser. Stretching the duration of pulses from a basic value by only one clock cycle, which can be a very small time increment, and which can be important in minimizing this ripple.
• DMDPWM is given by an expression:
• the effective duty cycle control resolution has been improved by 1/F. If F consists of 8 bits, giving a frame of 256 pulses, the duty cycle control resolution
• the resolution is improved by stretching some of the pulses by one clock cycle t.
• N 255, every pulse but one is stretched. If N is incremented past 255, N rolls back to zero and generates a "carry" which is used to increment W. The result is that 256 pulses out of 256 can be stretched. Effectively, W and N can be concatenated into a single digital word WN where each move of one bit position to the left represents a 2x increase in duty cycle.
• FIG. 2 is a high level circuit block diagram schematically illustrating a preferred embodiment 10 of a dual modulus digital pulse width modulator (DMDPWM) in accordance with the present invention.
• the DMDPWM includes a signal processor (microprocessor) 12 including a system clock 14.
• the DMDPWM also includes pulse width modulating circuitry 16 including a period and pulse width (Period/PW) counter 18 and an N-modulo counter 20.
• Clock 14 delivers clock pulses to the Period/PW counter.
• a user inputs to the signal processor a desired pulse repetition period Pi, a desired basic (minimum) pulse width Wi, and a desired resolution in the form of a number of pulses Nl to be stretched.
• the signal processor translates these inputs into digital words P (22), W (24) and N (26).
• the digital P and W words are provided to counter 18 and the digital N word is provided to counter 20.
• the circuitry functions as follows.
• counter 18 Every time counter 18 counts clock pulses up to a period P, the counter resets to zero and the signal out of the DPWM i.e., out of counter 18, to the RF power supply goes to a high value. Every time the counter 18 counts up to W, i.e., the basic pulse width, the signal out of the DPWM, to the RFPS goes to a low value. Part of the DPWM out signal is directed to the N-modulo 256 counter 20 to serve as a clock for the N-modulo 256 counter. Every time the PWM out signal goes high, counter 20 advances by one count. Counter 20 produces a high output signal N times out of 256 pulses of the DMDPWM. Whenever the output signal of counter 20 is high, the pulse width is W + 1 instead of W. Counter 20 provides this carry out information to counter 18 as indicated in FIG. 2.
• FIG. 3 schematically illustrates one example of an arrangement of N-modulo 256 counter 20 which accomplishes this task.
• counter 20 includes an 8-bit adder 28 and an 8-bit D flip-flop 30. It should be noted here that adder 28 and flip-flop 30 should handle the same number of bits, whatever that number of bits may be.
• 8 bits are used to improve the basic PWM resolution by 256, i.e., 2 8 .
• flip-flop 30 Every time flip-flop 30 is clocked by the output of the counter 18 of FIG. 2, the contents of flip-flop 30 are incremented by N.
• the output of flip-flop 30 can be thought of as the "present state” of circuit 20, and the input of the flip-flop can be thought of as the "next state” of circuit 20.
• the present invention is described above in a context of extending the resolution of a basic (prior-art) DPWM by 8 bits.
• the choice of 8-bits, here is arbitrary, but practical.
• the resolution increase can be chosen to increase by a greater or lesser amount.
• an N-modulo 1024 counter could be used, and the "stretched" pulses would be distributed over frames of 1024 output pulses.
• the resolution in theory at least could easily be extended to an even higher number of bits. At some level, however, there will be a diminishing of returns because periodic ripple components at some fraction of the laser output frequency will be generated.
• Circuitry 16 functionally described above with reference to FIG. 2 and FIG. 3, can be implemented in a single commercially available complex programmable logic device
• Circuitry 16 of FIG. 4 operates as follows.
• a counter 32 in circuitry 18 counts up by one count with every transition of the clock signal 14 from the signal processor (see FIG. 2).
• the output of counter 32 is fed to two digital comparators 34 and 36. Whenever the data at the A and B inputs of any one of the comparators are equal, the output of that comparator goes to logic 1. If A and B are not equal in any of the comparators, the output of that comparator will go to logic zero.
• a PWM output pulse cycle begins when output signal of counter 32 equals the value P input from the signal processor.
• the output of comparator 36 goes to logic 1, causing counter 32 to reset to zero count, and setting a Set-Reset (SR) flip-flop 38 to logic 1. This marks the beginning of a PWM output pulse out of the SR flip-flop 38.
• SR Set-Reset
• Circuitry 20, comprising adder 28 and D flip-flop 30 (cooperative with an inverter 46 in circuitry 18) "decides” if AND gate 40 should be enabled or not, i.e., if the PWM output pulse should be "normal” or “stretched". The operation of circuitry 20 for making the "decision" is described above with reference to FIG. 3.

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• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
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• 2010
  • 2010-03-30 US US12/749,781 patent/US8351480B2/en active Active
  • 2010-10-07 JP JP2012534239A patent/JP2013507790A/ja active Pending
  • 2010-10-07 EP EP10768132.2A patent/EP2489124B1/en active Active
  • 2010-10-07 KR KR1020127012305A patent/KR20120098688A/ko not_active Ceased
  • 2010-10-07 WO PCT/US2010/051819 patent/WO2011046804A2/en not_active Ceased
  • 2010-10-07 ES ES10768132T patent/ES2728249T3/es active Active
  • 2010-10-07 CN CN201080046197.2A patent/CN102668379B/zh active Active

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