RF-excited, hermetically sealed, pulsed CO2 lasers are gas discharge lasers widely used in material processing and laser machining applications such as via hole drilling in printed circuit boards and glass-plate scribing for TV screen manufacture. Such a laser includes a laser gas mixture including CO2 and inert gases. A gas discharge is ignited in the laser gas to energize the CO2 for providing optical gain. In order to be adaptable to a variety of applications, such a laser should be capable of operating in a wide variety of pulse formats including a wide range of constant pulse repetition frequencies (PRF) to random sequences of changing PRF. An RF-excited, hermetically sealed, pulsed CO2 laser typically requires pre-ionization of the laser gas in order to provide near-immediate ignition of the discharge in response to a user command signal with minimal variation in delay time between receipt of the command pulse and the ignition. Response-delay time variations are commonly referred to as “pulse-time jitter” by practitioners of the art.
In an RF-discharge gas laser the RF resonant circuit (which includes the lasing gas between discharge electrodes) has a high Q and a higher resonant frequency when the discharge is un-lit. High Q is associated with high impedance at resonance. Once the discharge is lit, the impedance, and accordingly the Q, drops significantly and the resonant frequency of the RF circuit drops correspondingly. It is easier to achieve ignition of a gas discharge with a high-Q resonant circuit than with a low-Q resonant circuit. This resonant frequency-shift presents a problem in the design of RF excited CO2 lasers, as the frequency of the RF supply to the electrodes must be selected to provide a compromise between optimum ignition effectiveness and efficiency of operation once the discharge is ignited (lit). The problem is complicated by the fact the longer a discharge is not lit the more difficult it is to reignite the discharge. The resonant-frequency-shift problem is described briefly below with reference to FIGS. 1A-C and FIGS. 2A-C.
FIG. 1A schematically illustrates partially in cross-section a typical arrangement 10 of a CO2 laser-head. Laser head 10 includes gas housing and electrode assembly 11, including a hermetically sealed, metal enclosure 12 which contains a lasing-gas mixture including CO2 and inert gases. Within enclosure 12 are elongated electrodes 14 and 16 parallel to each other parallel to each other and spaced apart by dielectric spacers 18. Spacers 18 are usually of a ceramic material such as aluminum oxide or beryllium oxide. RF power is delivered to the laser head from an RF power supply (RFPS), not explicitly shown, via an LC impedance matching network 20. The matching network is usually adjusted for the RFPS to see a matched 50 Ohm (50Ω) load (Z0) looking into an equivalent electronic resonant circuit of the electrode assembly where a discharge is lit between the electrodes. The RF power is connected to electrode 14, usually referred to as the “hot” electrode, via a hermetically sealed insulating feed-through 22. Electrode 16 is grounded via enclosure 12. A plurality of inductors Lt (only one shown in FIG. 1A) are provided along the length of the hot electrode and ground. These are adjusted to maintain an about uniform distribution of RF voltage along the length of the electrodes. A detailed description of such inductors in a laser head is provided in U.S. Pat. No. 4,443,877.
Depending on the applied RF voltage and frequency either free-electrons are generated or a diffuse discharge is lit in space 24 between the electrodes. The laser is completed, as is known in the art by an optical resonator having a longitudinal axis generally perpendicular to the plane of the drawing. It should be noted, here, that while laser head 10 generally represents a so-called slab laser, in which a laser mode is constrained in one transverse axis by the electrodes, principles discussed herein are equally applicable to any other gas laser that has waveguide modes of free-space Gaussian modes.
FIG. 1B schematically illustrates an equivalent electronic resonant circuit of laser head 10 when RF power is applied to electrode 14 but a discharge is not lit between the electrodes. Cft and Lft represent capacitive and inductive reactance, respectively, associated with the hermetically sealed RF feed-through 22. Inductance Lt is discussed above. Resistance Rt is resistance associated with this inductance. Ce is a capacitance associated with the electrodes and ceramic spacing material therebetween.
FIG. 1C schematically illustrates an equivalent electronic resonant circuit of laser head 10 when RF power is applied to electrode 14 and a discharge is lit between the electrodes. The equivalent resonant circuit is similar to that in the unlit condition with an additional capacitance Cs and a resistance Rd in series. Cs is a capacitance created by a sheath of electrons generated just beneath “hot” electrode 14 when the electrodes are energized with RF power. Rd is a resistive loading provided by ionized gas between the electrodes. This sheath capacitance and the lit-discharge resistance are causes of the shifting of the resonant frequency between the unlit and lit discharge conditions. The existence of the resistance Rd is a reason why the equivalent resonant circuit in the lit-discharge condition has a low Q.
FIG. 2A graphically schematically illustrates relative impedance as a function of frequency for the unlit-discharge (solid curve) and lit-discharge (dashed curve) resonant circuits of FIGS. 1B and 1C, respectively. It can be seen that the difference Z′ between a peak impedance ZUL at a frequency fUL for the unlit-discharge circuit and a peak impedance ZL at a frequency fL for the lit-discharge circuit is about an order of magnitude. An operating RF frequency of the laser of 100 megahertz (MHz) is assumed arbitrarily.
FIG. 2B graphically schematically illustrates relative reactance (imaginary part of the impedance) as a function of frequency for the unlit-discharge (solid curve) and lit-discharge (dashed curve) resonant circuits of FIGS. 1B and 1C. It can be seen that at the two resonant frequencies of FIG. 2 the reactance passes through zero for each of the curves. At this zero crossing the RF power delivered to the electrode assembly of the laser is deposited entirely in the discharge resistance and any other resistance that is included in the electrode assembly.
FIG. 2C graphically schematically illustrates reflected RF power (from the impedance-matching network) as a function of frequency for the unlit-discharge (solid curve) and lit-discharge (dashed curve) resonant circuits of FIGS. 1B and 1C. It can be seen that at the two resonant frequencies of FIG. 2C the reflected power in each case is at a minimum. In the unlit discharge condition, however, the minimum is extremely sharp and narrow.
The resonant frequency shift between the lit-discharge and unlit-discharge conditions of a gas-laser electrode-assembly has been recognized in the prior-art and schemes for dealing with the shift have been proposed. By way of example, in U.S. Pat. No. 5,150,372, a scheme is proposed wherein frequency of the RF power from an RFPS is frequency-swept downward from a frequency higher than the resonant frequency of the unlit-discharge condition, through the resonant frequency of the unlit-discharge condition, to the resonant frequency of the lit-discharge condition. The discharge is lit near the end of the sweep and the RF frequency is maintained at the end-frequency (lit-discharge frequency) while laser radiation is being delivered.
It will be evident from FIG. 2C that a problem with this approach is that, during the period of the sweep, the frequency is at some value other than the actual resonant frequency which will mean that there is a significant reflected RF power during the sweep. This will be the case every time a laser radiation pulse is required. This reflected power places additional stress on the RFPS. The reflected RF power results in few free electrons generated in the lasing gas which could result in erratic discharge ignition. It is also possible that continually subjecting the RFPS to the reflected power could cause early deterioration of components of the RFPS.
Another dual-frequency scheme is described in U.S. Pat. No. 6,181,719. Here, two separate sources of RF pulses are provided with the pulses amplified by a common RF amplifier. A solid-state switching arrangement connects either one or the other source to the amplifier. The first source is connected to the amplifier for providing pulses at about the unlit-discharge resonant frequency for providing pre-ignition. When laser output is required the second source is connected to the amplifier.
It has been determined by the inventors of the present invention that the frequency at which the very sharp minimum of reflected power occurs for the unlit-discharge condition can vary significantly between lasers of the same model. That is to say, slight variations in components, assemblies, gas composition, or gas pressure, which are otherwise within manufacturing tolerances, can produce significant variations in the unlit-discharge resonant frequency. Accordingly, providing a separate RF frequency source at some nominal value of this frequency for a particular model of a laser will mean that for most lasers of that model produced the actual unlit discharge resonant frequency will be different from the nominal frequency. This will mean that for those lasers the above discussed potential adverse effects of reflected RF power may be encountered to some degree. There is a need for a dual-frequency discharge ignition approach that can accommodate variations within a group of lasers of the unlit-discharge resonant frequency.