Abstract:
A system and method of forming a hermetic seal between a lens or window and a lid, which minimizes bending moments, and therefore, bifringance on the window or lens is disclosed. This is accomplished by use of a transition member, specific geometries, and selection of the coefficient of thermal expansion of the materials to insure that the solder joint is in compression on cool down and solder strains are maintained within acceptable limits. The transition member may be soldered or welded to lid or integral through machining.

Description:
FIELD OF THE INVENTION 
   This application claims priority to co-pending U.S. Provisional Application entitled, “Hermetically Sealed Lens and Method of Providing the Same,” having Ser. No. 60/501,537, filed Sep. 8, 2003, which is entirely incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   Glass is typically made from fused Silica and may be shaped to form a lens or window. To focus light, glass reflects and refracts light based on its shape, material properties, and an angle of incident light. If the shape of the glass is distorted, for example by external forces, light rays traveling through the glass, or bouncing off the glass surface, may be directed in an undesired direction. This undesired redirection of light can cause problems, especially with optical systems that rely on the stability of optical characteristics. 
   With advancements in technology, lenses have been utilized in many different environments including, but not limited to, outer space, where the lens must be enclosed in an air-tight body. Temperature changes can cause thermal expansion problems in a lens assembly. 
   A typical lens or window assembly contains a lens or a window secured to a lid that may be part of an enclosure for housing electronic components, for example, sensitive detectors. The lens or window may have a metallic coating on one surface of the lens and is secured to the lid by soldering. The lens or window may have a different coefficient of thermal expansion (CTE) than that of the lid. A change in temperature will cause the lid to expand or contract faster or slower than the lens or window, which can impart forces on the lens or window. The lens or window is soldered along one surface and, therefore, the imparted forces are not in the plane of the lens. This can generate a bending moment that can cause the lens to bow enough to adversely affect the optical characteristics of the lens. Bifringence is a by-product of the stresses caused by the bending moment imposed on the glass and may result in unacceptable distortion of images through the lens or window. 
   The lens, frame, and lid may be coupled together using soldering techniques to provide a hermetic seal. Unfortunately, the seal between the lens and the frame may impart detrimental forces to the lens that may change the shape of the lens enough to cause optical problems. The solder joint may also fail due to these external forces, thereby compromising the hermetic seal. Soldering a minimal clearance joint between the lens and the frame results in a very thin layer of solder. While a thin layer of solder exhibits great strength in certain contexts and types of testing, it does not provide for significant radial compliance in the configuration described above. Accordingly, when the lens assembly is subjected to temperature cycling, as is required in the testing of many military components, the solder joint may fail. In addition, the hard solder joint may not fail during testing and may instead fail during use of the lens or window, which may be located miles above the Earth where repairs are expensive to accomplish, if at all possible. 
   Thus, a heretofore unaddressed need exists in the lens industry to address the aforementioned deficiencies and inadequacies. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention provide a system and method for providing a hermetically sealed lens and window assembly. Briefly described, in architecture, one embodiment of the system, among others, can be implemented as follows. The assembly contains a lens having a first surface, a second surface, and a side surface, the lens having a first outside dimension; a frame having a first generally vertical surface, the frame having a first inside dimension greater than the first outside dimension of the lens; and a mechanical joint coupling the lens to the frame, the joint aligned in the plane of the lens to minimize a bending moment being imposed on the lens which may be caused by differing coefficients of thermal expansion of the lens and the frame. 
   The present invention can also be viewed as providing methods for providing a hermetically sealed lens or window assembly. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: inserting a lens or window having a first outside dimension in a frame having a first inside dimension; forming a mechanical joint between an outside surface of the lens and an inside surface of the frame, the mechanical joint being in a plane of the lens to minimize a bending moment being imposed on the lens. 
   Other systems, methods, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1  is a top view of a lens or window assembly coupled to a lid in accordance with a first exemplary embodiment of the invention. 
       FIG. 2  is a side view of the assembly of  FIG. 1 . 
       FIG. 3  is an exploded sectional view of the assembly of  FIG. 1 , taken through line  3 — 3  in  FIG. 1 . 
       FIG. 4  is a sectional view of a frame of the assembly of  FIG. 1 . 
       FIG. 5  is a sectional view of the assembly of  FIG. 1 . 
       FIG. 6  is a top view of a lens or window assembly coupled to a lid, in accordance with a second exemplary embodiment of the invention. 
       FIG. 7  is a sectional view of the assembly of  FIG. 6 . 
       FIG. 8  is a sectional view of the assembly of  FIG. 6 . 
   

   DETAILED DESCRIPTION 
   For exemplary purposes, the following describes a hermetically sealed lens assembly and method of providing the same. It should be noted, however, that alternative hermetically sealed lens assemblies may be provided in accordance with the present invention. The present invention is intended to include different lens assemblies where a solder joint between a lens (described below) and a frame (described below) provides a hermetic seal. The present invention may also include hermetically sealing and attaching non-lens assemblies. Reference herein will be made to lenses or window assemblies and is intended to cover, for example, pieces of glass with or without light focusing capabilities. 
     FIG. 1  is a top view of a lens or window assembly coupled to a lid in accordance with a first exemplary embodiment of the invention and  FIG. 2  is a side view of the assembly of  FIG. 1 . In accordance with the first exemplary embodiment of the invention, a lens or window assembly  100  contains a lens or window  120  and a frame  140 , each of which is described in detail below with reference to FIG.  1 – FIG. 5 . The frame  140  may be secured to a lid  180 , which in turn may be secured to an enclosure that houses electronic components associated with the lens or window assembly  100 . The frame  140  provides a transition member between the lens or window  120  and the lid  180 . The lens or window  120  and frame  140  may stick out, stay within, or straddle an enclosure (not shown). The lens or window  120  and frame  140  may straddle an enclosure by providing indentations in the lid  180 . A first generally vertical surface  142  ( FIG. 4 ), which will be described in greater detail herein, fits within the indentation and allows the frame  180  and lens or window  120  to be seated between the surfaces of the lid  180 . The optimum shape of the lens or window  120 , from a stress standpoint, is circular, but other shapes may be used without departing from the present invention. Likewise, the shape of the lid  180  is shown as being rectangular, but other shapes may be used without departing from the present invention. 
     FIG. 3  is an exploded sectional view of the lens or window assembly  100  of  FIG. 1  taken through line  3 — 3  in  FIG. 1 , and  FIG. 4  is a sectional view of the frame  140  of the lens or window assembly  100  of  FIG. 1 . In this embodiment, when assembled, a portion of the lid  180  is located within the frame  140 . The lens or window  120  may have a first surface  122 , a second surface  124 , and a side surface  126 . The first surface  122  and second surface  124  are shown parallel but are not required to be parallel. A first chamfer  128 A may be located between the first surface  122  and the side surface  126 , and a second chamfer  128 B may be located between the side surface  126  and the second surface  124 . The side surface  126  can have a coating or plating, for example, layers of Titanium, Nickel, and Gold, or some combination of material that may be deposited by a sputtering process, a physical vapor deposition process, an evaporative process, or any other process, which provides good adhesion and a solderable surface. 
   Referring to  FIG. 4 , the frame  140  may have a stepped interior surface. The frame  140  may have a first generally vertical surface  142 , a second generally vertical surface  144 , a third generally vertical surface  146 , a fourth generally vertical surface  148 , a fifth generally vertical surface  150 , a first generally horizontal surface  152 , a second generally horizontal surface  154 , a third generally horizontal surface  156 , and a fourth generally horizontal surface  158 . 
     FIG. 5  is a sectional view of the lens or window assembly  100  of  FIG. 1 . A first gap  190  may exist between the fourth generally vertical surface  148  and the side surface  126  of the lens or window  120 ; a second gap  192  may exist between the third generally vertical surface  146  and the side surface  126  of the lens or window  120 ; a third gap  194  may exist between the second generally vertical surface  144  and the lid  180 ; a fourth gap  196  may exist between the first generally vertical surface  142  and the lid  180 ; and a fifth gap  198  may exist between the frame  140  and the lid  180 . The gaps  190 ,  192 ,  194 ,  196 , and  198  allow for manufacturing tolerances and provide space for a solder joint. The purpose of the first gap  190  is to radially locate the lens or window  120  in the frame  140 , and may be as small as possible. The second chamfer  128 B may come into point contact with a corner  162  ( FIG. 4 ) formed between the fourth generally horizontal surface  158  and the fifth generally vertical surface  150 . The gaps  190 ,  192 ,  194 ,  196 , and  198  may have different thicknesses. The second gap  192  and the fourth gap  196  are sized to achieve thick conformal solder joints. A conformal solder joint has a thickness sufficient to absorb strain without failure. The desired maximum width of the second gap  192  and the fourth gap  196  is the maximum width that will still allow capillary action of the solder. The first gap  190  and the third gap  194  are sized, for example, 0.005–″0.006″, to achieve a functional solder joint. A functional solder joint is not too thick or too thin and therefore provides adequate capillary action to form a complete solder joint. A solder joint in the fifth gap  198  is relatively non-functional in terms of leak integrity. 
   The lens or window  120  may be secured to the frame  140  with a mechanical joint, for example, through a soldering process, to provide a complete hermetic assembly. The solder may be a soft solder, for example, but not limited to, Sn 62, Sn 63, Sn 96, or Sb 5. Soft solder is more desirable because it absorbs strain better than hard solder, for example, 80/20 solder. The solder may fill the first gap  190  and the second gap  192  between the lens or window  120  and the frame  140 ; and the solder may fill the third gap  194 , the fourth gap  196 , and the fifth gap  198  between the frame  140  and the lid  180 . The solder fills the gaps and forms a complete joint around the lens or window, thereby providing a hermetic seal. 
   It is desirable that the solder joint formed between the lens or window  120  and the frame  140  is in compression during cool down of the solder. This may be accomplished by choosing a material for the frame  140  that has a greater CTE than the CTE of the lens or window  120 . For example, if the lens or window  120  is made of fused silica, which has a CTE of less than 1, the frame  140  can be made of Kovar®, which has a CTE of 5. 
   The location of the gap  192  ensures that any stress caused, for example, by differences in the CTE of the lens or window  120  and the frame  140 , is in the plane of the lens, i.e., the forces do not create an appreciable bending moment on the lens or window  120  that would cause the shape of the lens or window  120  to change, thereby affecting the optical properties of the lens or window  120 . 
   If the material for the frame  140  and the lid  180  is the same, the frame  140  and the lid  180  may be joined using a mechanical joint, for example, through a soldering or welding process, to provide a complete hermetic assembly. If the frame  140  and the lid  180  are welded, the frame  140  and lid  180  do not have to be in radial compression. If the material for the frame  140  and the lid  180  are made of materials having differing CTEs, it is desirable to have the solder joint formed between the frame  140  and the lid  180  in radial compression during cool down of the solder. In the configuration shown in  FIGS. 1–5 , the CTE of the frame  140  should be greater than or equal to the CTE of the lid  180 . For example, if the lid  180  is made of kovar, which has a CTE of approximately 5, the frame  140  could be made of alloy 48 or 49, which has a CTE of approximately 7–8. 
     FIG. 6  is a top view of a lens assembly  200 , in accordance with a second exemplary embodiment of the invention, coupled to a lid  280 .  FIG. 7 , which is a sectional view of the lens assembly  200  of  FIG. 6 , and  FIG. 8 , which is a sectional view of the lens assembly  200  of  FIG. 6 , show how a frame  240  for holding a lens or window  220  is hermetically sealed to the lid  280 . In this embodiment, a portion of the frame  240  is located within the lid  280 . In this embodiment of the invention, the lens or window  220  is coupled to the frame  240  with a mechanical joint  290  and  292 , and the frame  240  is coupled to the lid  280  with a mechanical joint  298 . In this embodiment, radially applied stress on the lens or window  220 , resulting from a temperature change, is in the plane of the lens or window  220 . The inside surface of the lid  280  may also include steps as described above. 
   In another embodiment not shown, but similar to the step at mechanical joint  290 , a gap formed by one step may be used to locate the frame  240  relative to the lid  280  prior to mechanically joining the frame  240  and the lid  280 . In yet another embodiment, a gap may be formed by a second step, as shown in  FIG. 8 , located at the formation of mechanical joint  298 . The second step may be used to form the mechanical joint  298 . 
   If the material for the frame  240  and the lid  280  is the same, the frame  240  and the lid  280  may be joined using a mechanical joint, for example, through a soldering or welding process, to provide a complete hermetic assembly. If the material for the frame  240  and the lid  280  are made of materials having differing CTEs, it is also desirable to have the solder joint formed between the frame  240  and the lid  280  in radial compression during cool down of the solder. In the configuration shown in  FIGS. 6–8 , the CTE of the frame  240  should be less than or equal to the CTE of the lid  280 . For example, if the frame  240  is made of Kovar®, which has a CTE of approximately 5, the lid  280  could be made of alloy 48 or 49, which has a CTE of approximately 7–8. 
   It should be emphasized that the above-described embodiments of the present invention are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.