Abstract:
A gas discharge laser includes a laser housing including a laser gas and an electrode-assembly for lighting a discharge in the laser gas. The electrode assembly has a first resonant frequency when the discharge is not lit and a second resonant frequency when the discharge is lit. RF power delivering circuitry of the laser includes an arrangement for determining and recording the two resonant frequencies. RF power is applied to the electrodes at the first recorded resonant frequency to facilitate lighting of the discharge, and thereafter at the second resonant frequency to light and sustain the discharge.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority of U.S. Provisional Application No. 61/057,392, filed May 30, 2008, the complete disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to pre-ionization methods and apparatus for facilitating ignition of a gas discharge. The invention relates in particular to igniting a gas discharge in a radio frequency (RF) excited, hermetically sealed carbon dioxide (CO 2 ) laser. 
       DISCUSSION OF BACKGROUND ART 
       [0003]    RF-excited, hermetically sealed, pulsed CO 2  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 CO 2  and inert gases. A gas discharge is ignited in the laser gas to energize the CO 2  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 CO 2  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. 
         [0004]    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 CO 2  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 . 
         [0005]      FIG. 1A  schematically illustrates partially in cross-section a typical arrangement  10  of a CO 2  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 CO 2  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 (Z 0 ) 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 L t  (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. 
         [0006]    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. 
         [0007]      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. C ft  and L ft  represent capacitive and inductive reactance, respectively, associated with the hermetically sealed RF feed-through  22 . Inductance L t  is discussed above. Resistance R t  is resistance associated with this inductance. C e  is a capacitance associated with the electrodes and ceramic spacing material therebetween. 
         [0008]      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 C s  and a resistance R d  in series. C s  is a capacitance created by a sheath of electrons generated just beneath “hot” electrode  14  when the electrodes are energized with RF power. R d  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 R d  is a reason why the equivalent resonant circuit in the lit-discharge condition has a low Q. 
         [0009]      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 Z UL  at a frequency f UL  for the unlit-discharge circuit and a peak impedance Z L  at a frequency f L  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. 
         [0010]      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. 
         [0011]      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. 
         [0012]    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. 
         [0013]    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. 
         [0014]    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. 
         [0015]    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. 
       SUMMARY 
       [0016]    In one aspect of the present invention, a gas discharge laser comprises a laser housing including a laser gas and an electrode-assembly for lighting a discharge in the laser gas. The electrode assembly has a first resonant frequency when the discharge is not lit, and a second resonant frequency when the discharge is lit. The electronic circuitry is arranged to determine and record at least the first resonant frequency, apply RF power to the electrodes at the recorded first resonant frequency for a first predetermined time period insufficient to light a discharge in the laser gas but sufficient to create sufficient free electrons in the gas to facilitate lighting of the discharge, and thereafter light the discharge by applying RF power to the electrodes at the second resonant frequency. 
         [0017]    In a preferred embodiment of the invention the electronic circuitry is also arranged to determine and record the second resonant frequency. By providing the laser with circuitry for determining particularly the first (unlit-discharge) resonant frequency for that particular laser, the above discussed problems of variations of the unlit-discharge resonant frequency between lasers of a given model are avoided. The monitoring and recording of the lit and unlit discharge resonant frequencies can be carried out by a few simple, readily-available electronic components in a few seconds, as will be evident from the detailed description of the invention presented herein below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1A  schematically illustrates partially in cross-section prior-art gas-laser apparatus including a CO 2  laser-head including an electrode assembly contained within a hermetically sealed enclosure containing a laser-gas mixture with RF power being applied to the electrode assembly via an impedance matching network. 
           [0019]      FIG. 1B  schematically illustrates an equivalent electronic circuit of the electrode assembly of FIG. A when the RF power to the electrode assembly does not light a discharge in the laser-gas. 
           [0020]      FIG. 1C  schematically illustrates an equivalent electronic circuit of the electrode assembly of  FIG. 1A  when the RF power to the electrode assembly lights and sustains a discharge in the laser-gas. 
           [0021]      FIG. 2A  is a graph schematically illustrating the form of relative impedance of the electrode assembly of  FIG. 1A  for the unlit-discharge and lit-discharge conditions. 
           [0022]      FIG. 2B  is a graph schematically illustrating the form of relative impedance of the electrode assembly of  FIG. 1A  for the unlit-discharge and lit-discharge conditions of the electrode assembly. 
           [0023]      FIG. 2C  is a graph schematically illustrating the form of reflected RF power from the impedance matching network of the laser of  FIG. 1A  for the unlit-discharge and lit-discharge condition of the electrode assembly. 
           [0024]      FIG. 3  schematically illustrates a preferred embodiment of a gas laser in accordance with the present invention electronic circuitry arranged for determining and recording unlit-discharge and lit-discharge resonant frequencies of an electrode assembly and applying RF power at the recorded frequencies in sequence to the electrode assembly. 
           [0025]      FIGS. 4A-E  is a timing diagram schematically illustrating a preferred mode of operating the laser of  FIG. 3  using the lit-discharge and unlit-discharge resonant frequencies determined and recorded by the electronic circuitry. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 3  and  FIGS. 4A-E  schematically illustrate a preferred embodiment  40  of a gas-discharge (CO 2 ) laser in accordance with the present invention.  FIG. 3  schematically illustrates the laser including inventive electronic circuitry in block-diagram form.  FIG. 4  is a timing diagram schematically illustrating a preferred functioning of the electronic circuitry. 
         [0027]    Referring first to  FIG. 3 , apparatus  40  includes a direct digital frequency synthesizer (DDS)  42 , in communication with an RF power supply (RFPS)  44 . In this arrangement, the DDS provides a digitally derived RF frequency signal having any value selected within a predetermined range. The RFPS is an RF amplifier having sufficient bandwidth to amplify signals within the predetermined frequency range. 
         [0028]    The DDS is in communication with a microprocessor (MP)  46  which preferably includes an electrically erasable read-only memory (EEPROM) for electronic storage. Output of the RFPS is connected to an LC matching (impedance-matching) network  48  via a directional coupler sensor  50 . The LC matching network is connected to the laser discharge housing  12 , i.e., to a “hot” discharge-electrode within the housing, similar to electrode  14  of  FIG. 1A . Directional coupler sensor  50  provides for monitoring the connection between RFPS  44  and LC matching network. Sensor  50  monitors both the forward RF-wave (power) delivered to the matching network and the reflected RF-wave (power) from the matching network. Monitored values are communicated to microprocessor  46 . 
         [0029]    ON-commands are delivered simultaneously to “one shot” mono-stable multi-vibrators (MSMVs)  52  and  54  (also designated MSMV # 1  And MSMV # 2 , respectively, in  FIG. 3 ), and to a logic OR-gate  56 . Such MSMVs include amplifiers cross coupled by resistors and capacitors, and are triggered from a stable state to an unstable state by either the rising (positive) edge or falling (negative) edge of a command pulse. The MSMVs return to the stable state, without further triggering, after a time determined by the value and arrangement of the capacitors and resistors. 
         [0030]    MSMV  52  communicates with DDS  42  for selecting the frequency at which the DDS operates and the time for which the frequency is the unlit-discharge resonant frequency f UL . OR-gate  56  communicates the laser-ON command or the output of MSMV  54  to the DDS for providing an RF ON/OFF signal. MSMV  52  is a positive-edge (of a command signal or pulse) triggered device. MSMV  54  is negative-edge (of a command signal or pulse) triggered device. The manner in which the MSMVs operate cooperatively as timers is described in detail further hereinbelow. 
         [0031]    In one preferred method of operating the laser, the RF frequency corresponding to the minimum of the unlit-discharge curve of  FIG. 1C , i.e., the minimum reflected RF power, and the RF frequency corresponding minimum of the lit-discharge curve of  FIG. 1C  are determined. These determinations are preferably performed by a manufacturer of the laser before the laser is delivered to a user. A signal to perform the search for these frequencies is illustrated in  FIG. 3  as being delivered via a fixed connection  58  to microprocessor  46 . Alternatively, the signal can be provided by a temporary connection to the microprocessor. The search is preferably carried out after tuning-inductors of the laser, corresponding to inductor L t  of  FIG. 2 , have been adjusted by the manufacturer for a desired voltage-flatness along the length of the electrodes. 
         [0032]    When microprocessor  46  is commanded (by whatever means) to initiate the frequency search, i.e., to go into a search mode, the microprocessor steps the frequency of pulses delivered by DDS  42  over a predefined range, while monitoring the reflected power in first an unlit-discharge condition of the laser housing, and then in a lit-discharge condition. The reflected power values are communicated to the microprocessor from directional coupler sensor  50 , as discussed above. The microprocessor calculates the reflected power as a fraction of the incident (forward) power. Frequencies at which the minimum reflected power is determined in the two conditions (f UL  for the lit condition, and f L  for the unlit condition, see  FIG. 1C ) are stored in electronic memory of microprocessor  46 . 
         [0033]    Preferably, the pulse width of the search mode RF pulses is maintained relatively short, especially for the unlit-discharge condition, to minimize thermal stress on the final amplifier of the RFPS caused by the reflected power from the unmatched load which is encountered during the search at non-minimum values. Keeping the RF pulses short prevents laser action (discharge lighting) while the unlit resonant frequency is being searched. While the lit-discharge resonant frequency is being determined, the pulses must be long enough to initiate and sustain laser action. By way of example, for determining the unlit-discharge frequency, pulses may be about 1 to 2 microseconds (μs) in duration, and for determining the lit-discharge frequency, pulses may be about 6 to 7 μs in duration. 
         [0034]    As discussed above, even for lasers which have nominally the same electrode arrangement the unlit-discharge and lit-discharge resonant frequencies may be somewhat different. These frequencies can be affected by variations (within manufacturing tolerances) in the dielectric constant of the ceramic material spacing the electrodes, in the separation distance between the electrodes, in the closeness of the coils of the electrode tuning inductors (L t ), in the dimensions of the RF feed through, and in the variations in the values of the impedance matching network components. 
         [0035]    Because of the Q of the electrode structure in an unlit-discharge condition is relatively high, errors in the determination of f UL  can greatly influence the number of free electrons generated per-unit-time in the lasing gas between the electrodes and, accordingly, affect the ignition of the laser. Because of this, it is important to control the frequency of DDS  42  precisely in the search mode, in order to accurately locate the sharp minimum of reflected power. Determination of the resonant frequency in the lit-discharge condition does not need to be as precise because the Q of the electrode structure is lower in this condition. 
         [0036]    A frequency sweep time for DDS  42  on the order of 5 to 10 seconds is usually sufficient to locate the unlit-discharge resonance frequency with adequate precision. Because of the lower Q in the lit-discharge condition, a shorter search time can be taken for locating the lit-discharge resonant frequency resonance. 
         [0037]    Continuing with reference to  FIG. 3  and with reference in addition to  FIG. 4 , in this example of operation of laser  40 , a user delivers a command signal simultaneously to MSMVs  52  and  54  and OR gate  56  as noted above. The signal is positive-going (rising) at a time T 0  and negative-going (falling) at a time T 2  (see  FIG. 4A ) where T 2  minus T 0  equals T L , and T L  is the desired duration of an output pulse from the laser. 
         [0038]    The rising edge of the command pulse is transmitted to DDS  42  via OR-gate  56  and turns the DDS, and accordingly RF power to the laser electrodes, on. The rising edge of the command pulse also triggers MSMV  52  into an unstable state and the MSMV  52  sends a signal to the DDS to select the stored frequency f UL  from microprocessor and operate at that frequency. At a time T 1 , MSMV  52  returns to a stable state (see  FIG. 4B ) and the DDS is switched to operate at the stored frequency f L  for the lit-discharge condition (see  FIG. 4D ). T 1  minus T 0  is equal to T UL  which is selected as discussed above to create sufficient free electrons to provide pre-ionization, without causing laser action. 
         [0039]    When DDS  42  is switched to the lit-discharge frequency f L , laser action is initiated and the laser begins to delivers output radiation (see  FIG. 4E ). At time T 2 , the falling edge of command pulse switches DDS  42  and accordingly RF output from RFPS  44  is turned off. However, essentially simultaneously, the falling edge of the command pulse also triggers MSMV  54  into an unstable state, and the MSMV delivers an ON-pulse to the DDS to sustain the RF output at frequency f L  (see  FIG. 4C ). The terminology “essentially simultaneously”, as used here, means that, as the switching times on and off for the DDS are finite, and greater than the switching delay of MSMV  54  and the lifetime of electrons in the lit discharge plasma, the RF power can be considered as continuously applied to the electrodes and the discharge stays lit. 
         [0040]    At a time T 3 , MSMV  54  returns to a stable state and the ON-pulse is terminated, which turns the DDS and RF output (at f L ) of RFPS  44  off (see  FIG. 4D ), and accordingly terminates laser action (see  FIG. 4E ). The difference between T 3  and T 2  is T LS . T LS  is made equal to T UL , which provides that laser output is delivered for time T L , i.e., the duration of the laser-ON command signal. Note that RF power is delivered to the laser electrodes for a total time equal to T L  plus T LS  (see  FIG. 4D ), but the RF power is only at the lit-discharge frequency f L  for a period equal to T L . A T L  of about 7 μs is adequate for most CO 2 -laser material-processing applications. 
         [0041]    The switching time for the DDS is about 75 nanoseconds (ns) and, in this example, the switching time for the MSMVs is about 15 ns. The total switching time for the f L  frequency generated by the DDS, including an OR-gate delay, is accordingly about 0.1 μs. This switching time is fast enough to be acceptable for generating sufficiently fast laser pulse rise and fall times for most CO 2 -laser material processing applications. 
         [0042]    The MSMV (chip) parts suitable for MSMVs  52  and  53  of  FIG. 3  can be generic parts available from a number of suppliers as part number #74123. OR-gate (chip)  56  is also generic and commonly available from a number of suppliers as part number #7432. A DDS suitable for DDS  42  is commercially available from Analog Devices of Norwood, Mass. as model number AD9954. This DDS has a 32 bit phase accumulator and a 400 MHz clock and can achieve a 0.09 Hertz frequency reset-ability up to 200 MHz. This reset-ability is more than adequate for finding the frequencies f uL  and f L  to the accuracy required for ease of free electron generation and igniting the discharge. A microprocessor suitable for microprocessor  46  is available from Microchip Technology Inc. of Chandler, Ariz. as model number 18F4680. Directional couplers suitable for directional coupler sensor  50  are available from a number of commercial suppliers. 
         [0043]    It has been assumed in this description that the bandwidth of the RFPS is sufficient to amplify both frequencies f uL  and f L . Various RFPS can be designed by those skilled in the electronics art that would have the bandwidth to accommodate both frequencies. One such power supply is described in detail in U.S. Pre-Grant Publication No. 20080204134, assigned to the assignee of the present invention and the complete disclosure of which is hereby incorporated herein by reference. 
         [0044]    Those skilled in the electronic art will recognize from the description of the present invention provided above, that functions of the electronic circuitry depicted in  FIG. 3  may be carried out by components and arrangements thereof other that those depicted and described. Any such components and arrangements for providing the inventive functions may be deployed without departing from the spirit and scope of the present invention. 
         [0045]    Those skilled in the art will also recognize that the present invention may possibly be implemented without determination of the lit-discharge resonant frequency by the microprocessor, as the low Q in the lit-discharge condition provides for a broad minimum of reflected power which may encompass most variations thereof from laser to laser. In this case, a median lit-discharge resonant frequency value may be selected and programmed into the microprocessor. However, given that the electronic means are available for determining the actual resonant frequency, and that the determination takes a very short time, there is no compelling reason not to make the determination and enjoy the full benefit of the present invention. 
         [0046]    In summary, the present invention is described above in terms of a preferred embodiment. The invention is not limited, however, by the embodiment described and depicted. Rather the invention is limited only by the claims appended hereto.