Abstract:
In a CO 2  laser a pre-ionizer is assembled in a flange configured to be attached to a laser-gas enclosure of the laser over an aperture in a wall of the enclosure. An aperture in the base of the flange is aligned over the aperture in the enclosure wall. The aperture in the pre-ionizer flange is covered by a ceramic membrane. A disc electrode is in contact with the ceramic membrane on a side of the membrane outside of the laser-gas enclosure. An RF potential applied to the disc electrode creates a corona discharge on the side of the ceramic membrane inside the enclosure. The corona discharge ionizes laser gas in the enclosure before RF power is applied to electrodes of the slab laser.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to pulsed gas-discharge lasers. The invention relates in particular to pre-ionizing devices for such lasers. 
       DISCUSSION OF BACKGROUND ART 
       [0002]    A pulsed gas-discharge laser usually includes a sealed enclosure filled with a gas mixture (laser gas). A series of electric discharges is struck in the lasing gas in a discharge region between spaced-apart electrodes. This is accomplished by applying a repetitively pulsed electrical potential across the electrodes. A laser resonator is arranged with an axis thereof extending through discharge region. The discharge energizes the gas mixture and the energized gas mixture provides optical gain. Laser output is delivered from the resonator in a series of optical-radiation pulses having a repetition frequency corresponding to the repetition pulsed electrical potential. 
         [0003]    A pulsed gas-discharge laser commonly used in industrial applications is a pulsed carbon-dioxide (CO 2 ) laser commonly referred to as a slab laser. In such a laser the spaced apart electrodes are elongated electrodes (“slab” electrodes), usually having a plane face of one arranged face-to-face and parallel to a corresponding plane face of the other. In such a CO 2  laser, the lasing gas pressure is usually between about 50 Torr and 150 Torr. The pulsed electrical potential is applied as a pulsed radio frequency (RF) potential. The RF potential (power) during each pulse ignites and sustains the gas discharge. It is usual to provide a pre-ionizing device to create ionization in the lasing gas before the pulsed RF-power is applied. 
         [0004]    In the absence of such a pre-ionizing device, the time required to ignite the discharge between the slab electrodes and obtain pulsed laser output can vary randomly. Such a random ignition time would be undesirable for applications requiring precise laser turn-on and turn-off time, such as in drilling, marking, engraving, scribing, and cutting. In addition, in order to ignite the discharge without a pre-ionizer, it would usually be necessary to increase the RF power to a level two or more times greater than the power necessary to sustain the discharge once it has been ignited. This adds complexity and cost to the RF power supply. 
         [0005]    One prior-art approach to providing pre-ionization in a pulsed CO 2  laser is described in U.S. Pat. No. 5,434,881. In this approach, the pre-ionization is provided by repeatedly striking a spark discharge between two auxiliary spaced-apart electrodes located in the vicinity of the discharge region. It has been found, however, that these auxiliary electrodes are rapidly eroded by the repetitive sparking, and that the eroded (sputtered) material of the electrodes can contaminate the lasing gas and shorten the lifetime of the laser. 
         [0006]    One device designed to overcome the sputtering and contamination problems of the approach of the &#39;881 patent is described in U.S. Pat. No. 6,963,596, to Shackleton et al., assigned to the assignee of the present invention and incorporated herein by reference. In this device, a pre-ionizing discharge is formed between two pin-like electrodes (pin-electrodes), each thereof covered by a dielectric jacket. The dielectric jacket for the pin electrodes is provided by a ceramic crucible having hollow extension portions protruding from a base of the crucible, and shaped to accommodate the pin-electrodes. The crucible is clamped into an aperture of the lasing gas enclosure, and a separate assembly including the pin-electrodes is clamped to the crucible. The dielectric-covered pin-electrodes are energized by a low-power RF power source. 
         [0007]    The dielectric covering of the pin-electrodes of Shackleton et al. device essentially eliminates problems of sputtering and related contamination of the laser. However, parts for the device, have been found to be difficult to fabricate, intricate to assemble and relatively fragile. There is a need for a simpler, more robust device that is equally effective at eliminating sputtering and contamination problems of prior art pre-ionization approaches. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a pre-ionizer for a gas discharge laser. In one aspect of the invention, the pre-ionizer comprises a metal body having an passage extending therethrough. A membrane of a dielectric material is sealably retained within the metal body and closes the passage through the metal body. A planar electrode is retained within the metal body, in contact with the dielectric membrane on one side thereof and electrically isolated from the metal body. 
         [0009]    In another aspect of the invention, the metal body of the pre-ionizer is in the form of a flange and is sealed to a wall of the discharge laser with the side of the dielectric membrane in the flange opposite the electrode aligned over an aperture in a wall of a laser-gas enclosure of the gas discharge laser. When an RF potential is applied to the planar electrode an electric discharge is formed adjacent the dielectric membrane within the enclosure. The discharge ionizes laser-gas contained in the enclosure. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
           [0011]      FIG. 1  is a cutaway side-elevation view schematically illustrating an RF energized slab laser having a laser enclosure at low pressure containing slab electrodes and a laser gas, the laser enclosure being surmounted by a power enclosure at atmospheric pressure, the power enclosure for housing an integrated RF power supply for the laser and also including components of a pre-ionizer in accordance with the present invention. 
           [0012]      FIG. 2  is a perspective view schematically illustrating detail of a preferred arrangement of integrated laser and power enclosures in one example the laser of  FIG. 1 . 
           [0013]      FIG. 3  is a plan view from above schematically illustrating one preferred embodiment of a pre-ionizer in accordance with the present invention. 
           [0014]      FIG. 4  is a fragmentary cross-section view seen generally in the direction  4 - 4  of  FIG. 3 , schematically illustrating details of the pre-ionizer of  FIG. 3 , the pre-ionizer including a mounting flange for mounting the pre-ionizer over an aperture in a wall of the laser enclosure of  FIG. 1 , the mounting flange having an aperture therein aligned with the enclosure-wall aperture, the aperture of the mounting flange being covered by a dielectric membrane provided by the base of a dielectric crucible, the crucible being clamped and sealed within the flange, and the pre-ionizer having a disc electrode adjacent the dielectric membrane on a side thereof opposite the flange aperture. 
           [0015]      FIG. 5  is an exploded three-dimensional view schematically illustrating further details of components and assembly thereof in the pre-ionizer of  FIGS. 3 and 4 . 
           [0016]      FIG. 6  is a graph schematically illustrating measured light intensity of ionizing discharges produced by examples of the pre-ionizer of  FIGS. 3-5  as a function of power supply input voltage for various dielectric materials of the dielectric crucible. 
           [0017]      FIG. 7  is a graph schematically illustrating measured light intensity of ionizing discharges produced by an example of the pre-ionizer of  FIGS. 3-5  having an alumina crucible as a function of power supply input voltage at various pressures of lasing gas in the laser enclosure. 
           [0018]      FIG. 8  and  FIG. 9 , are respectively plan and cross-section views, with  FIG. 9  seen generally in the direction  9 - 9  of  FIG. 8 , schematically illustrating another preferred embodiment of a pre-ionizer in accordance with the present invention, similar to the pre-ionizer of  FIGS. 3 and 4 , but wherein the ceramic membrane is a simple disc brazed into the flange and covering the aperture therein. 
           [0019]      FIG. 10  is an exploded three-dimensional view schematically illustrating further details of components and assembly thereof in the pre-ionizer of  FIGS. 8 and 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  and  FIG. 2  schematically illustrate a slab laser  18  including a pre-ionizer  20  in accordance with the present invention. Laser  18  includes an upper enclosure  22  and a lower enclosure  24 . Upper enclosure  22  is at atmospheric pressure and contains RF power supply circuitry (not shown) for powering the laser and the pre-ionizer. Enclosure  22  is referred to hereinafter as the power enclosure. Lower enclosure  24  is at a low (less than atmospheric) pressure, for example, between about 50.0 Torr and 150.0 Torr. Enclosure  24  contains lasing gas and components of the slab laser including slab electrodes  26  and  28 . Slab electrode  26  receives radio frequency (RF) power from a supply thereof (not shown) in power enclosure  22  via an electrode  30 . Slab electrode  28  is connected to ground by an electrode  32 . Mirrors (not explicitly shown) for forming a laser resonator are held at opposite ends of the laser enclosure in mirror mounts  34  and  36 . The laser resonator extends through a gap  38  between slab electrodes  26  and  28 . A discharge is formed in the gap when sufficient RF power is applied to electrodes  26  and  28 . An output beam of the laser exits enclosure  24  via a window  40  laterally offset in mirror mount  36 . 
         [0021]    It should be noted here that only sufficient description of laser  18  is provided herein to indicate how preferred embodiments of the inventive pre-ionizer may be integrated into this and other slab lasers. A detailed description of the construction and operation of RF-energized slab lasers in general is provided in U.S. Pat. No. 5,123,028, the complete disclosure of which is hereby incorporated by reference. 
         [0022]      FIG. 3 ,  FIG. 4 , and  FIG. 5  schematically illustrate a preferred embodiment  20 A of pre-ionizer  20  mounted on a metal wall  25  of enclosure  24  of  FIG. 1 . Pre-ionizer  20 A is assembled in a metal body  50  that serves as a mounting-flange for attaching the pre-ionizer to enclosure  24  and is referred to as a flange hereinafter. Flange  50  has an upper circular recess  52  and a lower circular recess  54 . An aperture  56  extends from the lower circular recess through a base or mounting face  58  of the flange. Together, the recesses and the aperture provide a passage through the flange. It should be noted here that the terms “upper”, “lower” and base as applied to the recesses and the flange are used here merely for convenience of description. The inventive pre-ionizer is not limited to being used in the orientation depicted in  FIGS. 3-5 . Flange  50  is preferably formed from aluminum, but this should not be considered as limiting the present invention. 
         [0023]    A crucible  60  is located in lower recess  54  of flange  50 . The crucible has a diameter about equal to the diameter of the lower recess. Crucible  60  has a rim portion  63  surrounding a base  62 . Base  62  preferably has a thickness less than about 0.10 inches and most preferably less than about 0.05 inches. The crucible base forms a dielectric diaphragm or membrane covering aperture  56  in flange  50  and closing the passage through the flange. A retaining sleeve  64  retains the crucible in the flange by means of screws  68  (only one shown in  FIG. 5 ) extending through a flanged portion  66  of the sleeve into the flange in upper recess  52  thereof. A sealing ring  69  (see  FIG. 5 ), preferably of a soft metal such as indium, and located in circular groove  70  in the lower recess of the flange, forms a gas tight seal between the base of the crucible and the flange. Sealing ring  69  is not depicted in  FIG. 4  as the scale of the drawing does not permit this to be done with adequate clarity. 
         [0024]    Crucible  60  is preferably formed from an alumina (Al 2 O 3 ) ceramic. A particularly preferred thickness for the base of a crucible  60  of this material is about 0.020 inches. Other dielectric crucible materials and different base thicknesses may be selected without departing from the spirit and scope of the present invention. A comparison of different dielectric materials and different base (membrane) thicknesses is presented further hereinbelow. 
         [0025]    A metal disc (planar) electrode  74  has an integral stem portion  76  thereof extending through bushing  78  of a dielectric material. Aluminum is a suitable metal for electrode  74 , the use of other metals, however, is not precluded. One preferred material for bushing  78  is G-10, which is an electronics-industry-standard, fiberglass-epoxy composite material used in PC-board manufacture, and commercially available from several electronic material suppliers. Bushing  78  is supported on a rim portion  63  of crucible  60  via a cushion washer  88  of a material such as stainless steel. A retaining plate  82  of a dielectric material is clamped to the top of flange  50  by screws  84 . A preferred dielectric material for plate  82  is also G-10. Stem portion  76  of disc electrode  74  extends through an aperture  85  in plate  82 . A spring  86 , compressed between plate  82  and bushing  78 , provides a force that retains planar electrode  74  in contact with base  62  of crucible  60 . The electrode diameter is preferably about equal to or slightly greater than the diameter of aperture  56  in flange  50 . 
         [0026]    Pre-ionizer  20 A is designed to be cooperative with an aperture  27  in a metal wall  25  of laser-gas enclosure  24  of laser  18  of  FIGS. 1 and 2 . The pre-ionizer is clamped on to wall  25  via screws  90  (only one thereof shown in  FIGS. 4 and 5 ). Aperture  56  of the pre-ionizer flange is aligned in aperture  27  in the enclosure wall. A gas-tight seal between flange  50  and enclosure  24  is provided by a sealing ring  92  compressed in a groove  94  in base  58  of the flange. Flange  50  is in electrical contact with wall  25  of enclosure  24 , which is at ground potential. Planar electrode  74  is electrically isolated from the flange. 
         [0027]    Pre-ionizer  20 A is activated by applying RF power to stem  76  of planar electrode  74 . In this configuration, the electrode  74  is the hot or positive electrode and the metal base  54  is connected to ground. Power can be supplied by an RF power supply having a relatively low RF frequency for example between about 300 KHz and 400 KHz. The ability to operate at a low RF frequency enables the utilization of low cost RF power transistors cooperative with a low loss ferrite-core, step-up transformer to provide the high RF voltage to the pre-ionizer electrode. In experiments performed to evaluate materials and performance of the inventive pre-ionizer discussed further herein below, a converter-type RF power supply of a semiconductor H-bridge, ferrite-transformer design, using four IRF0210 power transistors was employed. RF output-power of the power supply was about 5 W. Those skilled in the art may choose to use other RF power supply types or RF output-power without departing from the spirit and scope of the present invention. 
         [0028]    Continuing with reference in particular to  FIG. 4 , when RF power is applied to the inventive pre-ionizer a corona discharge  96  is formed adjacent ceramic membrane  62  on the side thereof facing into enclosure  24 . Corona discharge  96  provides the desired pre-ionization of laser-gas in enclosure  24 . The discharge is a source of ions and also of UV radiation. The UV radiation can provide further pre-ionization. In this arrangement, the rim portion of flange  50  surrounding aperture  56  therein functions as the ground electrode. While this “ground electrode” is not ceramic protected, it is not anticipated that significant erosion will occur. This is because discharge  96  is a relatively low intensity discharge, and the rim of aperture  56  of the flange surrounds only the outer periphery of the discharge. It is believed that erosion can be minimized by plating the rim portion with nickel. 
         [0029]    Experiments were performed to evaluate the performance of the inventive pre-ionizer with different ceramic membrane (crucible  60 ) materials and thicknesses. In these experiments the intensity of light produced by the discharge was observed through an aperture  97  in retaining plate  82 , aligned with a hole  98  extending through bushing  78 , and a hole  99  extending through electrode  74 . As membrane  62  in the experiments was never thicker than 0.040 inches, a significant portion of the light generated in discharge  96  was transmitted by the membrane 
         [0030]    Results of the experiments are depicted graphically in  FIG. 6  and  FIG. 7 . In these graphs, the X-axis scale is a DC supply voltage, often termed the link-voltage, applied to the H-bridge RF power supply. The RF power output of the power supply (connected to the pre-ionizer) scales essentially directly with this applied DC voltage. In each case, the RF power to the pre-ionizer was at 350 KHz and the laser-gas mixture was 4:1:1 helium (He): nitrogen (N 2 ); CO 2  mixture. The unsupported membrane diameter, i.e., the diameter of aperture  56  in flange  50 , was 6 millimeters (mm) and the diameter of planar electrode  74  was 7 mm. 
         [0031]      FIG. 6  depicts measured light power in microwatts (μW) as a function of applied link voltage for various membrane materials and different thickness of the membranes. The highest light intensity detected at any applied RF power was obtained with an 85% Al 2 O 3  ceramic membrane having a thickness of about 0.020 inches. In this experiment the laser gas pressure was 20 Torr.  FIG. 7  depicts measured light power in microwatts as a function of applied link voltage for the 85% Al 2 O 3 , 0.020 inches-thick membrane at various laser gas pressures. It can be seen that the discharge intensity is a sensitive function of laser gas pressure at any applied RF power. 
         [0032]      FIG. 7.0  illustrates the amount of pre-ionizing optical radiation emitted from a CO 2  laser gas mixture having, for example, 4 parts He: 1 part CO 2 : 1 part N 2  with increasing DC (link) voltage applied to the H-bridge low frequency RF supply for an 85% grade alumina at gas pressures varying from 80 Torr to 280 Torr. As the DC voltage into the H-bridge increases, the output RF power driving the pre-ionizing discharge also increases. As expected, the emitted pre-ionizing optical radiation decreases with increasing gas pressures. Increasing optical radiation indicates increasing pre-ionization. It becomes more difficult to create a discharge with increasing gas pressure. 
         [0033]    Optimum placement of the inventive pre-ionizer is between about 0.5 inches and 1.5 inches from the main slab electrodes of the laser (depending on laser gas pressure and other factors) and in clear view of the discharge gap between these electrodes. It was also found that the pre-ionizer was effective when located above a slab electrode, with a hole being provided in the electrode to allow ions and UV radiation from the pre-ionizer discharge to enter the discharge gap between the slab electrodes. Clearly more than one of the inventive pre-ionizers may be provided in a laser. 
         [0034]      FIG. 8 ,  FIG. 9 , and  FIG. 10  schematically illustrate another embodiment  20 B of a pre-ionizer in accordance with the present invention. Pre-ionizer  20 B is similar to pre-ionizer  20 A of  FIGS. 3-5  but is of simpler construction. Only principal differences between the two embodiments are discussed below. 
         [0035]    By way of example, flange  50  of pre-ionizer  20 A is replaced in pre-ionizer  20 B by a flange  51  that has only one recess  53  therein, with aperture  56  at the base of this recess completing the passage through the flange. Crucible  60  of pre-ionizer  20 A is replaced in pre-ionizer  20 B by a disc-shaped membrane  61 . This disc-shaped membrane is soldered or brazed to flange  51  covering aperture  56  and closing the passage through the flange. This eliminates the need for the crucible-retaining sleeve  64  of pre-ionizer  20 A components associated therewith. Electrode  74  is retained in contact with membrane  61  via a spring  87  compressed between a retaining plate  82  and a dielectric bushing  79  that bears on the electrode. It can be seen that pre-ionizer  20 B requires only fourteen parts compared with thirty-two parts for pre-ionizer  20 A, the part counts, here, including screws. Potentially, then, pre-ionizer  20 B can be made at lower cost than pre-ionizer  20 A. It should be noted, however, that cost saved in reduced parts, and reduced complexity of parts may be at least partially offset by the cost of a somewhat delicate brazing or soldering operation required to bond ceramic disc  61  to flange  51 . 
         [0036]    In summary, the present invention is described above in terms of two preferred embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.