A CO2 gas discharge laser is typically powered by a high-voltage RF power supply (RFPS). The power supply applies RF voltage to electrodes of the gas laser, which excite a discharge in a lasing gas mixture including CO2 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). Typically 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). 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 (DPWM) 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. Similarly 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.
By way of example if a DPWM has a clock frequency f=10 MHz, each clock cycle period is 1/f=0.1 microsecond. If the laser PRF=1 kHz (corresponding to the frequency of delivery of RF pulse trains) a complete pulse width modulation period would contain 10 MHz/1 kHz=10,000 clock cycles and the resolution would be 10,000, i.e., 0.01%. If the laser PRF is increased to 100 kHz with the same clock frequency the resolution falls to 10 MHz/100 kHz=100, i.e., 1.0%. In order to obtain the resolution possible in the 1 kHz-PRF case at 100 KHz, the clock frequency would have to be increased to 1 gigahertz (GHz). This higher frequency is not practical in a commercial laser as it requires the uses of correspondingly faster circuit components and wider counters, all of which increases the cost of a laser.
In laser processing application for which CO2 lasers are used, for example in semiconductor device processing applications, there is an increasing trend towards using higher pulse repetition frequencies, for example, up to 200 kHz or greater. Power control accuracy significantly better than 1% is generally desired. There is a need for a PWM method that would allow this control accuracy with reasonable clock frequencies, for example between about 1 MHz and 10 MHz.