Abstract:
A low inductance, hermetically sealed, RF shielded feed-through is provided for exciting low impedance discharges associated with high power CO 2  slab lasers. The feed-through mechanically obtains RF contact, preferably, at the center of the length of the electrodes that are inserted within the long laser housing, thereby making it easier to obtain a uniform electric field distribution along the length of the electrodes.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority from U.S. Provisional Application No. 60/902,967, filed Feb. 23, 2007, titled “High Power Low Inductance RF Hermetic Sealed Feed-Through for Slab CO 2  Lasers.” Provisional Application No. 60/902,967 is hereby incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to carbon dioxide (CO 2 ) slab lasers and, in particular, to techniques for obtaining a low inductance, hermetically sealed, RF shielded feed-through for exciting low impedance discharges associated with high power CO 2  slab lasers. 
       BACKGROUND OF THE INVENTION 
       [0003]    The low impedance (e.g., around several ohms) of high power CO 2  slab laser discharges that are energized by RF frequencies in the neighborhood of 100 MHz places electrical constraints on the hermetically sealed RF feed-through connecting the solid state RF power supply to the parallel electrodes inside the laser tube housing. In order to reduce the circulating currents to a minimum and to have a fast rise time (e.g., below 0.1 microsec. at about 100 MHz) in pulsing the discharge, low series inductance is desired for the RF feed-through. The function of the RF feed-through is to deliver fast rise time, high RF power pulses (e.g., having peak powers of a few thousand watts to over 20,000 watts of RF power to a CO 2  laser having several hundred watts to 1000 W of output IR average output power, respectfully) into a laser discharge within a hermetically sealed metal “tube” chamber without emitting stray RF radiation into the atmosphere. This tube chamber contains the appropriate gas mixture of CO 2 :N 2 :He, etc., at below atmospheric pressure (i.e., typically between 60-150 Torr), the mirrors forming the laser&#39;s optical resonator, and the two long parallel facing metal electrodes between which the laser&#39;s discharge is generated. Depending upon the laser power desired, the gap between the two parallel facing electrodes and the width and the length dimensions of the electrodes varies with the desired output power of the laser, gas pressure, the RF frequency, the gas temperature, etc. For a 500 W CO 2  slab laser, the electrode length is in the neighborhood of 800 mm. and the width is typically slightly larger than one tenth ( 1/10) of the length. The laser tube has a rectangular or cylindrical configuration, typically fabricated from aluminum. For a 500 W laser, the open ends of the laser tube are typically of the order of 150 mm. The distance between the mirrors of the resonator is slightly longer than the length of the electrodes. For an electrode length of approximately 800 mm, the distance between the mirrors is approximately 870 mm. 
         [0004]    From uniform discharge considerations, it is desirable to have the electrical RF contact made by the hermetically sealed feed-through to the electrodes occur at the center of the length of the electrode. The small aspect ratio of the contact opening to the length of the laser tube housing makes this contact mechanically difficult to accomplish. Consequently, as disclosed in commonly-assigned U.S. Pat. No. 5,155,739, RF contact has been made to one end of the electrode pair by placing the feed-through at one of the end flanges of the laser housing tube. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides techniques for obtaining a low inductance, hermetically sealed, RF shielded feed-through for exciting low impedance discharges associated with high power CO 2  slab lasers. The invention further provides techniques for mechanically obtaining RF contact at the center of the length of the long electrodes that are inserted within the long laser housing, thereby making it easier to obtain a uniform electric field distribution along the length of the electrodes. 
         [0006]    The features and advantages of the various aspects of the present will be more fully understood and appreciated upon consideration of the following detailed description of the invention and the accompanying drawings, which set forth illustrative embodiments in which the concepts of the invention are utilized. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is an exploded isometric view illustrating a high power, low inductance, RF, hermetically sealed feed-through in accordance with the present invention. 
           [0008]      FIG. 2  is an assembled view of a high power, low inductance, RF, hermetically sealed feed-through in accordance with the present invention from the inside of the laser tube housing. 
           [0009]      FIG. 3  is an assembled view of a high power, low inductance, RF, hermetically sealed feed-through in accordance with the present invention from the outside looking in to the laser tube housing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]      FIG. 1  shows an exploded isometric view of a high power, low inductance, RF, hermetically sealed feed-through used to connect RF power from external to the laser tube housing to the discharge electrodes within the laser tube housing. The exploded isometric view is shown looking out toward one of the side walls (not shown in  FIG. 1 ) from inside the laser tube&#39;s hermetically sealed housing onto which a metal flange  9  is bolted. The metal tube housing serves as RF ground. Moving from left to right in  FIG. 1 , two RF contacting springs  1  and  2  are used to make electrical contact to the edge surfaces of the hot electrode. The contact to the electrode is preferably made at the center of the length of the electrode. Springs  1  and  2  are attached to a Nickel plated rectangular Copper plate  3 . The attachment can be made, for example, by soldering springs  1  and  2  to the Copper plate  3  with a 95% Sn: 5% Sb solder. The combined assembly of springs  1  and  2  and Copper plate  3  is attached by a screw  4  to a Nickel plated disk  5  that is in electrical contact with a Kovar cap  6  which is, in turn, connected to a Copper rod (shown as item  15  in  FIG. 3 ) that extends through the center of the protrusion consisting of a ceramic insulator  7  to a Kovar metal (items  6  and  8 ) hermetic seal. The assembly of the metal to ceramic hermetically sealed feed-through comprises the hot RF Nickel plated Copper disk contact  5 , the Kovar ( 6 ,  8 ) sealed to the ceramic  7 , and the metal flange  9 . Metal flange  9  is preferably bolted onto a wall of the metal laser tube housing (not shown) by six bolts (not shown) inserted through the six holes  14  formed in the flange. The metal flange  9  serves as an electrical ground when bolted onto the laser tube metal housing. 
         [0011]    The feed-through assembly in this embodiment of the invention is preferably a modified brazed metal to ceramic hermetically sealed feed-through provided by Ceram Tech of North Carolina, U.S.A. (Part Number Cerama Seal 6700-01-CF). One modification that was made to this standard part was to have a specific length for the protrusion into the laser tube housing to accommodate the distance from outside the laser tube to the edge surface of the hot electrode so that springs  1  and  2  are compressed against the edge surface of the hot electrode to provide a good RF contact. 
         [0012]    As shown in  FIG. 1 , another modification that was made to this standard part was to have a “lip”  13  machined in the stainless steel flange  9  that also protrudes past the inside surface of the laser tube housing. An “O”-ring recess  10  is provided to maintain the hermetically sealed condition of the laser tube housing when the RF feed-through metal to ceramic flange assembly is bolted onto an outside surface of the laser tube housing. 
         [0013]    A Nickel plated Aluminum RF shield  12  is press fitted over the stainless steel lip  13  to ensure a good electric contact between the shield  12  and the lip  13 . The RF shield  12  serves as the outer ground conductor of a short co-axial transmission line the characteristic impedance of which is designed to match the input impedance of the laser discharge, as well as the output impedance of the impedance matching network that feeds RF power from external to the laser tube housing to the laser discharge within the hermetically sealed laser housing. This short co-axial cable structure is responsible for the low series inductance characteristics of this RF feed-through design. The design of such a co-axial transmission line to obtain a given impedance is well known to those experienced in the RF state of the art, as exemplified by Fig. A on page 589 of the 4th edition of “RF Data for Radio Engineers”, published by the IT&amp;T Corp., 1956. 
         [0014]    As shown in  FIG. 1 , a spring  11  is also attached to the RF shield  12  and serves as the RF ground when pressed against the edge surface of the ground electrode. The length of the shield assembly is designed so that when assembled, the surfaces of springs  1 ,  2  and  11  are the same distance from and perpendicular to a common flat surface, namely the edge surfaces of the hot and ground electrodes. 
         [0015]    The diameter of the opening cut into the laser tube housing is selected to be larger than the outside diameter of the shield  12  that is press fitted over the stainless steel lip  13 , but smaller than the inside diameter of the O-ring recess  10  so that an hermetical seal can be maintained, 
         [0016]      FIG. 2  shows the assembled high power, low inductance RF feed-through of  FIG. 1  from the inside of the laser tube housing. 
         [0017]      FIG. 3  shows the assembled hermetically sealed, high power, low inductance, RF feed-through of  FIG. 1  from the outside looking in to the laser tube housing. Item  15  in  FIG. 3  is the hot Copper center conductor the opposite end of which is attached to item  5  of  FIG. 1 . Note that  FIG. 3  shows that the springs  1  and  2  and  11  are pressed against the thin sides of the hot electrode  16  and the ground electrode  17 , respectively, thereby providing easy RF contact through a side of the laser tube housing to the middle of the length of the electrodes. 
         [0018]    It should be understood that the particular embodiments of the invention described above have been provided by way of example and that other modifications may occur to those skilled in the art without departing from the scope and spirit of the invention as expressed in the appended claims and their equivalents.