Abstract:
A centered annular ring, or moat, is disposed on each of the six planar sides of a RLG block. The surface of each moat is slightly below the surrounding surface of each side of the block. Because each moat is recessed relative to the respective side surface, the moat surface is less likely to become scratched, and therefore allows for a better seal between the block and the components attached thereto.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     None. 
     BACKGROUND OF THE INVENTION 
     The present invention is a moat feature for achieving high-integrity, airtight seals between an optical apparatus block and the components attached to the block. 
     One embodiment of this invention is applied to a ring laser gyroscope (RLG). A RLG is commonly used to measure the angular rotation of a vehicle, such as an aircraft. Such a gyroscope has two counter-rotating laser light beams which move within a closed loop path or “ring” with the aid of successive reflections from multiple mirrors. The closed path is defined by an optical cavity which is interior to a structural gyroscope frame or “block”. In one type of RLG, the block includes planar top and bottom surfaces that are bordered by six planar sides that form a hexagon-shaped perimeter. Surfaces on each of the sides define mounting areas for components such as mirrors and electrodes. For example, three planar non-adjacent sides of the block form the mirror mounting surfaces for three mirrors at the corners of the optical path, which is triangular in shape. 
     Operationally, upon rotation of the RLG about its input axis (which is perpendicular to and at the center of the planar top and bottom surfaces of the block), the effective path length of each counter-rotating laser light beam changes, and a frequency differential is produced between the beams that is nominally proportional to angular rate. This differential is then measured by signal processing electronics to determine the angular rotation of the vehicle. 
     Typically in a RLG block having a triangular shaped optical path incorporating three mirrors, one of the mirrors has a concave reflective surface while the other two mirrors have planar reflective surfaces. The curved mirror serves two main purposes. First, the curvature of the reflective surface controls the diameter and the primary mode of the counter-rotating laser light beams. Second, the curvature of the reflective surface is used to align the counter-rotating laser light beams within the optical cavity so that the light beams are at substantially maximum intensity to minimize RLG bias errors. In particular, this latter purpose is accomplished due to the inherent attributes of the concave reflective surface. Additionally, a typical RLG block has three electrodes, which are disposed one on each of the three planar side surfaces not occupied by mirrors. 
     It is important that the mirrors and electrodes are securely attached to their respective surfaces, and that the seal between the block and those components is airtight so that a vacuum inside the block is preserved. Typically, the components are soldered onto their respective planar surfaces. However, these surfaces often contain scratches, chips, and fractures, formed during manufacture or handling, which prevent the formation of an airtight seal. There is a need for an improved device and method for achieving a high integrity seal between a RLG block and the components attached thereto. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a device and method for achieving an airtight seal between a RLG block and the components attached thereto. The invention comprises disposing a centered annular ring, or moat, on each of the six planar sides of a RLG block. The surface of each moat is slightly below the surrounding surface of each side of the block. Because each moat is depressed relative to the respective side surface, the moat surface is less likely to become scratched, and therefore allows for a better seal between the block and the mirror or electrode attached thereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a RLG block incorporating the moats of the present invention. 
     FIG. 2 is plan sectional view of a RLG block incorporating the moats of the present invention, taken along line A-A of FIG.  1 . 
     FIG. 3 is a plan section view of a RLG block incorporating the moats of the present invent with attached mirror and electrode components. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a ring laser gyroscope (RLG) block  10  with moats  12  in accordance with the present invention. Block  10  is preferably formed of a glass, glass ceramic, or like material on turning center  13 . Suitable block materials include a glass ceramic material marketed under the trademarks “Cervit” and “Zerodur”. An example of a suitable glass material is a borosilicate glass marketed under the trademark “BK-7”. 
     Block  10  is generally triangular shaped with a hexagonal outer periphery. The hexagonal outer periphery includes three planar non-adjacent sides that form first, second and third mirror mounting surfaces  14 ,  16  and  18 , respectively, and three further planar non-adjacent sides  20 ,  22  and  24 , respectively. The mounting surfaces  14 ,  16 , and  18  and sides  20 ,  22 , and  24  form a border for planar top and bottom surfaces  26  and  28 , respectively, of block  10 . 
     Recessed moats  12  may be of various shapes and configurations, but in a preferred embodiment, circular moats  12  are machined into block  10  about wells  32  and  34  so that rings  30  are formed substantially concentric to moats  12 . The interior edge of each ring  30  is defined by a well  32  or  34  into the interior of block  10 . The exterior edge of each ring  30  is defined by the interior edge of corresponding moat  12 . The face surfaces of rings  30  are co-planar with the surfaces of planar sides  14 ,  16 ,  18 ,  20 ,  22 , and  24 . In comparison, the surfaces of moats  12  are below the surfaces of rings  30  and sides  14 ,  16 ,  18 ,  20 ,  22 , and  24 . Preferably, moats  12  may all be the same depth for ease of manufacture. 
     As will be shown in FIG. 3, on sides  14 ,  16 , and  18 , mirrors rest on the top surface of each of the rings  30 . On sides  20 ,  22 , and  24 , electrodes rest on the lower surface of each of the moats  12 , and so rings  30  are not necessary on those sides. However, the raised surface of rings  30  relative to the recessed surface of moats  12  help to protect the surface of moats  12  from scratches and other damage. Additionally, the combination of rings  30  and moats  12  offers an assembler a “bulls-eye” target for the centering of each component directly above its respective well. 
     FIG. 2 is a plan sectional view of a RLG block incorporating moats  12  of the present invention, taken along line A—A of FIG.  1 . Parts in FIG. 2 are numbered the same as the corresponding parts in FIG.  1 . As shown best in FIG. 2, in a first preferred embodiment, wells  32  on mounting surfaces  14 ,  16 , and  18  are generally partially conical, while wells  34  on sides  20 ,  22 , and  24  are generally cylindrical. In alternative embodiments, wells  32  and  34  may have a variety of shapes, so long as they do not interfere with the path of the light beam. 
     Moats  12  may be formed by any means known in the art. Preferably, they are machined into block  10  by the same machine that drills the internal structures of block  10 . In a preferred embodiment, a CNC (Computer Numerical Control) machine is used. 
     In a preferred embodiment, an internal optical cavity  36  of the block  10  comprises three substantially straight laser bores  38 ,  40  and  42 , that are interconnected at mounting surfaces  14 ,  16  and  18  by wells  32 . Each well  34  is also in communication with internal optical cavity  36 . Bores  38 ,  40  and  42  and wells  32  and  34  are machined within block  10  to form a triangular shaped closed loop optical path, with mounting surfaces  14 ,  16  and  18  located at corners of the optical path. 
     After machining, block  10  is treated to reduce the size of any surface scratches, fractures, or other deformations formed during machining. In one embodiment, block  10  is treated in an acid etch bath. This is especially effective for eliminating minute imperfections that cannot be seen, yet which could contribute to air leakage if not removed. A relatively smooth surface of moat  12  is important to creating an air-tight seal. 
     FIG. 3 is a plan section view of a RLG block incorporating the moats of the present invention, with attached mirror and electrode components. The components may be attached in any known manner. In a preferred embodiment, a drop of adhesive such as amyl-acetate is placed in the bottom of each moat  12 . Curved mirror  44  is centered on ring  30  of surface  14 , directly above well  32 , so that the curved surface of the mirror contacts the outside edge of ring  30 . Flat mirrors  46  and  48  are centered on rings  30  of surfaces  16  and  18 , respectively, directly above wells  32 . Electrodes  50 ,  52 , and  54  are placed into moats  12  of sides  20 ,  22 , and  24 , respectively, directly above wells  34 . 
     Then, a pre-formed, doughnut-shaped frit seal  56  is placed at the base of each mirror  44 ,  46 , or  48  or electrode  50 ,  52 , or  54 , so that it presses onto the lower surface of corresponding moat  12  and surrounds the respective mirror or electrode. Preferably, frits  56  are made of a solder-glass material with a lower melting point than that of block  10  and a similar coefficient of thermal expansion as that of block  10 . Then, the RLG is heated, in an oven for example, so that frits  56  melt to permanently and tightly seal each component to block  10 . Each preformed frit  56  is initially annular in shape. However, as it melts, the material of frit  56  adheres to the contact surfaces of, and conforms to the shape of, the respective moat  12  and mirror  44 ,  46 , or  48  or electrode  50 ,  52 , or  54  it contacts. 
     Moats  12  serve several purposes. First, they facilitate achieving an airtight seal by reducing the incidence and size of chips, scratches, or fractures of block  10  in the region of each seal. Because the surface of each moat  12  is slightly recessed relative to side surfaces  14 ,  16 ,  18 ,  20 ,  22 , and  24 , the surface of each moat  12  is less susceptible to scratching, chipping, and other damage. Second, the present invention increases the yield of usable blocks  10  because moats  12  can be disposed even on scratched blocks to repair the surface in the vicinity of each seal, leading to significant savings. Moreover, economic savings are realized because the invention allows blocks to be produced with less stringent surface finish requirements without affecting the quality of the resulting RLG. Production of blocks with reduced requirements leads to significant cost savings. Additionally, moats  12  aid in the assembly of RLG&#39;s by providing a visual marker by which to center the mirrors and electrodes over the wells on each side of block  10 . 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.