Abstract:
Affixing together two articles includes placing a frit preform with an adhesive material on the surface of either article. The preform and the articles are heated to cause the frit material to liquify and then cooled to affix together the two articles at the junction of the two articles.

Description:
FIELD OF THE INVENTION 
     This invention is related to hard vacuum seals. More particularly, this invention is related to a method of providing a gas laser with a hard vacuum frit seal for laser components attached to a gas discharge cavity body. 
     BACKGROUND OF THE INVENTION 
     In many laser devices, a gas discharge cavity is provided by a tube or block, herein referred to as the laser block, consisting of glass, ceramic, glass ceramic, metal, or the like. Generally, attached to the laser block is a plurality of electrodes typically consisting of a metal such as aluminum, nickel-iron alloys, and the like. Further, attached to the laser block is at least two mirrors for establishing an optical path. For ring lasers, at least three mirrors are attached to the laser block for establishing a closed-loop optical path. 
     In order to provide low cost manufacturing of laser devices, the electrodes and mirrors may be attached to the laser block by a frit seal, sometimes referred to as a solder glass seal. Of course, for lasers, the frit seal must provide a hermetic seal between the components and the laser block, as particularly taught in U.S. Pat. No. 4,817,112, entitled “Low Cost Ring Laser Angular Rate Sensor”, issued to Weber et al. 
     Another patent which shows the use of a frit seal is U.S. Pat. No. 4,233,568, entitled “Laser Tube Mirror Assembly”, issued to Hamerdinger et al. As particularly described therein, a selected glass frit is chosen on the basis of its coefficient of thermal expansion. More particularly, the coefficient of thermal expansion of the frit material is generally selected to closely match the components which are being affixed to each other. 
     In order to apply the frit material to either a junction of two articles or sandwiched between mounting surfaces of the two articles, the frit material is typically mixed with a low viscosity vehicle to form a resultant slurry, with the frit material suspended therein. U.S. Pat. No. Re. 25,791 is exemplary in describing the characteristics of a solder glass or frit sealing process. 
     In manufacturing of lasers using the above glass frit/slurry, the components to be attached to the laser block, for example the electrodes and mirrors, are placed in position, and held in place with a light force, e.g. a spring loaded force. In turn, the frit/slurry, or simply “the slurry”, is applied to the peripheral junction of the component to be attached and the laser block by a variety of techniques, including, among others, dipping, pressure flow, brushing, use of a syringe, or other “toothpaste-like” dispensing techniques. 
     After the slurry has been applied to the peripheral junction of the component and the laser block, the temperature of the components, laser block, and slurry are raised to a fritting temperature sufficient to allow the frit material to change to its liquid or fluid state which results in a liquid glass surrounding the aforementioned junction. This step is followed by a cooling period during which time the frit material returns to its “glass” state thereby forming a hermetic seal at the junction of the component to the laser block. 
     The aforementioned process is described in further detail in the aforementioned U.S. Pat. No. 4,233,568. However, with respect to laser devices, it should be noted that the aforementioned process is preferably one of placing the frit material around the junction of the components and the laser block, as opposed to being sandwiched between the mounting surface of the component and a mounting surface of the laser block. 
     Although the “slurry” process as just described provides an adequate hermetic seal, it has a number of disadvantages. For example, the frit/slurry combination is sensitive to the “mix ratio” of the frit material and the vehicle. This may lead to inconsistencies in the completeness, strength, coefficient of thermal expansion induced stress, and the hermeticity of the seal. Secondly, the slurry process does not lend itself to consistent application of the slurry material from one component to the next, thereby decreasing the reliability of the seal in high volume production. Lastly, the frit/slurry process does not lend itself to “automated” production techniques because of the difficulties encountered in the application of the slurry surrounding the junction of the component and the laser block. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a frit seal without the disadvantages of a frit/slurry seal method. 
     Another object of the invention is to provide a consistent frit seal between two articles intended to be affixed to each other. 
     The present invention pertains to both a method and apparatus of a novel seal particularly useful in the manufacture of lasers, and other kinds of gas discharge devices. More particularly, this invention pertains to the attachment of electrodes and/or mirrors to a body which provides a gas discharge cavity. 
     In the present invention, two articles are provided with mounting surfaces which are to be joined together. The first article is held against second article forming a peripheral junction. A ring shaped preform is placed on the mounting surface of the second article in proximity of the first and second articles. The preform material consists substantially of a frit material for forming a seal between the two articles which are intended to be attached to each other. The combination of the two articles and preform is heated to a sufficient temperature to cause the frit material to change to a liquid state to thereby form a hermetic seal after cooling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a laser. 
     FIG. 2 a  is planar view of a frit seal in accordance with the present invention. 
     FIG. 2 b  and FIG. 2 c  are cross-sectional views of the frit seal before and after the fritting process in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, thereshown is a cross-sectional view of a laser  10  similar to that illustrated in U.S. Pat. No. 4,612,647, issued to Norvell, entitled “High Performance Laser And Method Of Making Same”. The last referred-to patent describes a method for securing an electrode to a laser body or block using field-assisted bonding. Laser  10  includes a laser block  12  which may be provided by a variety of glass, glass ceramics, metals, or the like so as to provide a lasing cavity  14  within laser block  12 . Highly polished mirrors  16  and  18  are fixed to the laser block  12  at opposite ends of the lasing cavity  14  so as to provide an optical path to establish, lasing operation in a well known manner. Two electrodes, anode  20  and cathode  22 , communicate with the lasing cavity  14  through upright bores  24  and  26 , respectively. 
     Cathode  22  is generally hemispherical, with a mounting surface  32 . Cathode  22  may be constructed from a wide variety of materials including, among others, aluminum, nickel alloys, beryllium, and even glass with an electrically conductive coating such as taught in U.S. Pat. No. 4,612,647. The details of cathode  22 , as well as anode  20 , are beyond the scope of the present invention. However, common to both electrodes is that they generally have a ring-shaped mounting surface intended to be attached to a flat mounting surface of laser block  12  or other body forming the gas discharge cavity. 
     For example, in FIG. 1, anode  20  is illustrated as a button-like electrode sealed to laser block  12  with an indium seal  36 . Cathode  22  is illustrated fixed to laser block  12  by an indium seal  34 . In the present invention, indium seals  34  and  36  are replaced by a glass frit seal, the description of which follows. 
     Mirrors  16  and  18  are commonly joined to laser block  12  by an optical contact, or frit seal. The stability of the seal is particularly critical for ring lasers since the laser beams therein must traverse a polygonal ring path. Therefore, alignment of the mirror surfaces, at least three, relative to each other, is critical so that an optical closed loop path may be established as defined by the mirror surfaces as is well known. Of course, if a frit seal is chosen as the method of attachment of the mirror component to the laser block, the coefficient of thermal expansion of the frit material must be as chosen to be as close as possible to both the mirror component as well as the laser block so that alignment of the mirrors is minimally altered by temperature effects. 
     In the present invention, the term “frit” is intended to mean any of a wide variety of materials which form a glass or glass-like seal, such materials being either vitreous or non-vitreous. Such frit materials may include other elements, for example a lead-glass or the like. Examples of frit materials are particularly identified in the aforesaid U.S. Pat. No. 4,233,568 which lists frit materials, and their corresponding coefficient of thermal expansion properties and their fritting temperatures, from Corning Glass Works and Schott Optical Glass Company. Examples of frit materials suitable in the present invention for use with a laser block and mirror substrate built from a borosilicate glass such as BK-7 glass, from Coming Glass Works, having a coefficient of thermal expansion of 8.3×10 −6 /° C. are Coming 7570 vitreous frit material having a coefficient of thermal expansion of 8.4×10 −6 /° C., Corning 7575 vitreous frit material having a coefficient of thermal expansion of 8.9×10 −6 /° C. , and Schott G017-340 having a coefficient of thermal expansion of 8.3×10 −6 /° C. 
     Illustrated in FIGS. 2 a ,  2   b , and  2   c  is one embodiment of the invention for attaching mirror component  16  to laser block  12 , like those components illustrated in FIG. 1, by use of a “frit preform”  200 . FIGS. 2 a  and  2   b  illustrate the assembly of the mirror component  16  to laser block  12  prior to the “fritting process”, and FIG. 2 c  illustrates the attachment of the mirror component  16  to laser block  12  after the fritting process. More specifically, FIGS. 2 a  and  2   b  illustrate a ring-shaped frit preform  200  having an aperture therethrough. As is well known in the laser art, mirror  16  includes a mirror coating (not shown) to be in communication with lasing cavity  14 . Mirror  16  is illustrated as being cylindrically shaped, and being disposed within the aperture of preform  200 . 
     FIG. 2 c  diagrammatically illustrates the resulting frit seal  200   a  after the combination of the mirror  16 , laser block  12 , and frit preform  200  have been heated to the fritting temperature, and subsequently cooled to form the glass frit seal. At the fritting temperature, the frit material changes to a liquid state. The components as illustrated in FIGS. 2 a  and  2   b  are held in place by an adhesive. 
     With the adhesive, the process to hold the frit preform  200  in place can be performed on a non-horizontal surface while the frit seal  200   a  forms. This process is performed by tacking the frit preform  200  in place with adhesive so that no fixturing is required. The frit preform  200  is tacked by a material that has a capability to bind in volatile matrix solvents such as a lacquer. The tacking material is placed on the surface to form a film. This holds the frit preform  200  lightly against the block. A benefit to the non-horizontal process is that manufacturing could be performed in a much less complex manner by forming multiple frits at one time rather than forming one frit at a time. A benefit to the use of the tacking material is that it burns off completely after the heating process. Therefore, no residue or debris is left that would contaminate or add stress to the frit seal  200   a.    
     After the fritting process, the combination as aforesaid is allowed to cool, resulting in a hermetic frit seal  200   a  surrounding the peripheral junction  220  of mirror  16  and laser block  12 . The use of the ring shaped preform  200  in accordance with the present invention results in the flit preform  200  “shrinking” around the junction  220  of the mirror  16  and laser block  12  during the fritting and wetting process thereby enhancing the seal over that of using a frit/slurry. 
     The dimensional aspects of mirror  16 : and preform  200  may have wide variations, all of which are considered to be within the scope of the present invention. One example of a practical embodiment of the present invention is one in which preform  200  has an outside diameter of 0.398 inches, inside diameter of 0.320 inches, and having a thickness of 0.035 inches; and mirror component  16  is composed of BK-7 glass having an outside diameter of 0.300 inches. 
     It should be noted that frit preform  200  consists generally of a frit material held together by any of a variety techniques. For example, Corning Glass Works provides a product under the registered trademark of “Multiform and Clearform” products. These products are intricate non-porous, vacuum tight bodies of pressed glass made by the “powder processing” of glass. Granulated glass particles are dry-pressed into shape and fired at high temperature to fuse them into a tight shaped structure. Other types of preforms are within the scope of the present invention including sintered glass preforms, as well as those preforms held together by a “wax-like” binder for maintaining the preform shape. In the present invention, use of the preforms as aforesaid permits the fritting process requiring only one heating step, the temperature being only sufficient to cause the frit material to change to a liquid state. 
     The description of the invention as thus described with reference to FIGS. 2 a ,  2   b , and  2   c  are applicable to any component other than mirror component  16  being attached to laser block  12 . That is, anode  20  or cathode  22  may be fixed to laser block  12  in the same manner. With reference to FIG. 2 b , mirror  16  may be replaced by anode  20 , cathode  22 , or any other article in practice of the present invention. 
     All those skilled in the art will recognize that only preferred embodiments of the present invention have been disclosed herein, and that the embodiments may be altered and modified without departing from the true spirit and scope of the invention as defined in the accompanying claims. More specifically, the Figures generally depict laser components as articles which are mounted to another article shown as laser block  12 . The Figures, furthermore, generally depict an article which has an annular or ring-shaped mounting surface which when joined to the laser block form an annular junction between the component and the laser block. It is intended that components other than having an annular mounting surface are within the scope of the present invention. 
     The frit preforms illustrated in the accompanying drawings have also been shown to be ring-shaped construction. When such ring-shaped preforms are applied around components which are also annular, the frit process lends itself to the frit preform shrinking around the peripheral junction of the component to the laser block as a result of the fritting process. Although a ring shape preform is preferred, other shapes, for example rectangular-shaped preforms, are also within the scope of the present invention since they too will provide wetting and shrinking around the junction of the component and the article which is intended to be joined thereto.