Gas laser discharge tube with ceramic wafers and cooling wafers in a stack
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a discharge tube for use in a gas laser. 
     2. Description of the Related Art 
     A gas laser is disclosed in U.S. Pat. No. 3,753,144 in which, in one embodiment, a discharge tube is formed of sections of metal, such as copper, secured to cooling plates and spaced from neighboring tube sections. In the disclosed laser, the laser beam, or emission, causes sputtering of the copper from the tube sections which leads to the formation of a metal precipitate on a ceramic outer tube of the laser. The metal precipitation on the outer tube deteriorates the insulation value thereof and is a possible source of electric arcs. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a gas laser discharge tube formed of a stacked structure, the gas laser being capable of producing output powers in the milliwatt range without sputtering away metal from cooling plates. The gas laser of the invention is relatively inexpensive, and also has a low structural height. 
     This and other objects are achieved in a discharge tube for a gas laser having alternate ceramic wafers and cooling wafers. The cooling wafers are generally of copper, although other heat conductive materials may be used, and are kept out of the actual laser discharge space of the discharge chamber so that the copper is largely prevented from being sputtered as a result of the laser irradiation. A gap is formed between neighboring ceramic wafers by the cooling wafers that are disposed therebetween. The cooling wafers are recessed, or set back, from the edges of the ceramic wafers defining the laser discharge channel. The gap traps any possible sputtered copper and causes it to condense therein at least to a slight degree, thereby providing a shielding effect to prevent sputtered metal from reaching other portions of the laser. 
     The ceramic wafers of the present discharge tube include at least one gas return channel formed by bores extending through the wafers generally parallel to the axis of the discharge tube. When more than one gas return channel is provided, the bores are generally equally spaced from the discharge channel. The gas return bores of neighboring ceramic wafers are in alignment, or registration, with one another to form a channel through the stack of ceramic wafers through which a gas return flow occurs. To obtain an optimal resistance to thermal stresses in the direction of the axis of symmetry, two or more bores are distributed circumferentially spaced about each one of the ceramic wafers. The cooling wafers are set back from the bores forming the gas return channels, as well. The cooling wafers, in one embodiment, each have one recess, or opening, into which both the discharge channels and the gas return channels defined by the ceramic wafers discharge. Thus, in this embodiment, an especially deep setback of the cooling wafers from the discharge channel is provided so that there is only an extremely slight risk of sputtering of the copper cooling wafers. The interruptions in the discharge channel caused by the setback cooling wafers do not have a significant disturbing effect on the laser beam emission; furthermore, the interruptions in the gas return channels advantageously aid in local pressure equalization. 
     In another embodiment, the cooling wafers extend relatively close to the discharge channel and the cooling wafers include additional bores interrelated with the bores that form the gas return channels in the ceramic wafers. The additional bores in the cooling wafers at least overlap the bores in the neighboring ceramic wafers. 
     The ceramic wafers are preferably of a relatively inexpensive and nontoxic Al 2  O 3  ceramic. When the cooling wafers are set back from the discharge channel preferably by at least twice their thickness, an optimum heat elimination and a reduced tendency for sputtering is provided. 
     A relatively short, or narrow, structure is provided by the present invention without the risk of re-ignition of the laser discharge occurring in the gas return channels, since in one embodiment the ceramic wafers have first bores aligned with one another to form the discharge channel, yet the further bores which form the gas return channels are offset relative to one another in neighboring ceramic wafers. The further bores overlap the bores of neighboring ceramic wafers sufficiently to provide a gas flow channel, but do not provide a line-of-sight passage through the gas return channel. The bores can be offset relative to one another radially and/or circumferentially. Thus, no laser discharge is possible in the gas return channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a gas laser including a discharge tube according to the principles of the present invention; 
     FIG. 2 is an enlarged cross section of a portion of the discharge tube shown in FIG. 1 taken at the region delineated by the broken circle; and 
     FIG. 3 is an enlarged cross section of a second embodiment of the invention and corresponding to the view of FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a gas laser, a laser discharge tube 1 carries laser reflecting mirrors 2 on additional tube sections 4. The laser discharge tube 1 is composed of alternating ceramic wafers 3 and heat conducting cooling wafers or plates 5. The ceramic wafers of the preferred embodiment are composed of Al 2  O 3  ceramic, which is relatively inexpensive and nontoxic in comparison to BeO ceramic which is frequently utilized in known discharge tubes. 
     With reference to FIG. 2, the ceramic wafers 3 each include mutually aligned bores 6 having a rotational axis coinciding with an axis of symmetry of the ceramic wafers 3. The cooling wafers 5 include bores 8 disposed coaxially relative to the bores 6 in the ceramic wafers 3. The cooling wafers 5 are of a metal having good thermal conductivity, and in one embodiment are of copper. The bores 6 form a discharge channel for the laser beam, while additional bores 7 in the ceramic wafers 3 form gas return channels. 
     The cooling wafers 5 have a thickness that is relatively small in comparison to the thickness of the ceramic wafers 3 so that the laser beam discharge in the discharge channel is not disturbed by large interruptions in the discharge channel caused by the setback cooling wafers 5. For example, the cooling wafers of one embodiment are about 0.5 mm thick and the ceramic wafers are about 3 mm thick. Cooling wafers having a thickness of up to 0.7 mm may be provided, given a ratio of the thickness of the cooling wafers 5 to the thickness of the ceramic wafers 3 of 1:4. This ratio yields stable discharge tubes 1 having high heat dissipation characteristics. 
     In a second embodiment shown in FIG. 3, cooling wafers or plates 5&#39; include bores 9 and 10 which align with the bores 6 and 7, respectively, in the ceramic wafers 3. The bores 9 and 10 in the cooling wafers 5&#39; have diameters that are only slightly larger than that of the corresponding bores 6 and 7 in the ceramic wafers 3. The second embodiment provides an especially efficient heat dissipation. It is preferred that a spacing or set back of at least twice the thickness of the cooling wafers 5&#39; be present between the edges of the bores 6 and the edges of the neighboring bores 9. Such spacing is adequate to prevent any significant sputtering of the copper cooling plate 5&#39;, and further enables any copper which may nonetheless be sputtered off to condense largely in the region of the bores 9 between neighboring ceramic wafers 3 without deposition of a metal precipitate within the bores 6 forming the discharge channel. 
     A compact structure can be provided by the present invention by providing the gas return channels 7 closer to the discharge channel 6 than was previously possible. The proximity of the return channels 7 to the discharge channel ordinarily results in a possible ignition of a laser discharge in the gas return channels. Here, however, the bores 7 defining the gas return channels are offset along the length of the discharge tube 1 so that there is no longer a line-of-sight gas return channel present, thereby eliminating the chance of a laser ignition in the return channels. In the embodiment of FIG. 2, this can be accomplished simply by rotating one or more of the ceramic wafers 3 relative to one another. In FIG. 3, any such rotation must be more carefully performed to maintain a gas flow channel. It is also possible to offset the bores 7 radially from one another to prevent a discharge in the return channels. 
     The bores 6 and 7 are preferably symmetrically arranged on each ceramic wafer 3 for good thermal stability and strength. This also prevents misalignment of the resonator mirrors 2 during operation. 
     As used herein, the term &#34;bores&#34; can refer to any opening extending through the cooling wafers 5 and/or ceramic wafers 3. 
     Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.