Abstract:
A method and apparatus for protecting polished surfaces of optical crystals includes placing a thin film of index-matching liquid between the polished surface and an optical flat, and hermetically sealing the liquid in place.

Description:
This application is a continuation of U.S. application Ser. No. 08/045,760, filed Apr. 14, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates, in general, to optical crystals and more particularly to a method and apparatus for protecting polished surfaces of such crystals. 
     It is known that polished optical surfaces on hygroscopic crystals such as are used in electro-optic light modulators or other laser light applications, for example, can deteriorate very quickly when exposed to a humid atmosphere and, in the past, protection of such surfaces has been very difficult. Typically, a polished crystal has been protected by submerging it in an index-matching oil contained in a sealed liquid cell so that the oil prevents ambient humid air from reaching the crystal surface. Such a liquid cell has windows that allow light to reach the crystal, and the external surfaces of such windows typically are provided with anti-reflection coatings. Because electric fields are usually also applied to such crystals by way of electrodes on the crystals, such prior liquid-filled cells must be constructed to provide for such electrodes and for their corresponding electrical leads which must be brought into and out of the cells through the respective cell walls. Leak-proofing such a cell is difficult. 
     In addition, because light passing to and from the submerged optical crystal passes through the liquid cell, optical absorption in the liquid path produces the well-known &#34;thermal blooming effect.&#34; As a result, maintenance of the optical beam profile at high light intensity levels is always a problem and can limit the use of such liquid cells to relatively low power levels. In spite of the foregoing problems, almost all commercial electro-optic modulators and Q-switches for lasers use one form or another of liquid cells of the same basic design. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for protecting the polished surface of an optical crystal without the need for submerging the crystal in a liquid cell. 
     It is a further object of the invention to provide apparatus for producing a thin protective film on a polished optical crystal surface and for hermetically sealing the film to protect the crystal surface. 
     Briefly, the present invention is directed to hermetically sealed polished optical surfaces of optical crystals, either hygroscopic or nonhygroscopic, and to a method of sealing such surfaces. Examples of commonly-used optical crystals are the so-called ADP-isomorphs, which include NH 4  H 2  PO 4 , (Amonia Di-hydrogen Phosphate, or ADP), KH 2  PO 4  (KDP), NH 4  D 2  PO 4 , (deuterated ADP, OR AD*P), KD 2  PO 4  (KD*P), CsH 2  AsO 4  (CDA), CsD 2  AsO 4  (CD*P), etc. or the borates such as β-BaB 2  O 4  (BBO), LiB 3  O 5  (LBO) etc., as well as others. The polished surfaces of the crystals are sealed by a thin film of a commercially available index-matching liquid, such as oil, which will not damage the crystal surface. Such liquids typically are a mixture of two inert organic liquids, one with a high index of refraction and the other with a low index. By using different proportions of these liquids in the mixture, an index ranging from, for example, 1.4 to 1.75 can be achieved. This liquid is placed in a protective region between a polished optical surface of the crystal and an adjacent corresponding inner surface of an &#34;optical flat&#34; which is a standard optical quality glass of fused or crystalline quartz having optically flat inner and outer surfaces which may or may not be parallel to each other. The optical flat has an inner surface approximately the same dimensions as the polished surface to be protected, has approximately the same index of refraction as the crystal and the liquid film, and also includes an anti-reflection coating on its outside surface. A small quantity of oil is introduced into the region between the polished surface of the crystal and the inner surface of the optical flat, and the optical flat is pressed against the crystal surface to cause the oil to spread and to form a thin, even film. The resulting liquid thin film serves the purposes of not only protecting the polished surface of the crystal but also of ensuring index matching between the surfaces of the crystal and the optical flat. 
     After the liquid thin film is in place, the optical flat is hermetically sealed to the crystal, as by a bead of sealant such as ultraviolet light-cured or thermally cured epoxy around the periphery of the oil-filled region between the flat and the crystal. After curing, the epoxy sealant holds the oil and the flat in place and prevents ambient air and moisture from displacing it, thereby ensuring protection of the polished surface of the crystal. 
     The presence of the index-matching oil film in the region between the opposed surfaces of the crystal and the optical flat eliminates air between them, thereby eliminating the large reflections at the interfaces produced when air is present. Such reflections are caused by the very different indices of refraction of air, the crystal and the optical flat. Furthermore, since air contains moisture which can damage the polished surface of the crystal, the elimination of air between these opposed surfaces also removes this source of damage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and additional objects, features and advantages of the present invention will become apparent to those of skill in the art from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a rod-shaped crystal having a thin film of protective oil hermetically sealed to a polished surface thereof; and 
     FIG. 2 is a cross-sectional view thereof, taken along line 2--2 of FIG. 1. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in FIG. 1 an optical crystal 10 having polished optical surfaces 12 and 14. The crystal 10 may be any hygroscopic crystal material suitable for use in an optical application, as noted above, for example KD*P, and is illustrated in the shape of a cylindrical rod having polished opposite end surfaces 12 and 14 which are perpendicular to the axis of the crystal. These optical end surfaces are subject to deterioration in a humid atmosphere and thus must be protected to preserve the optical quality of the crystal so that light beams can pass longitudinally through the crystal and through the polished surfaces as indicated by arrow 16. 
     The polished surfaces 12 and 14 are protected by means of sealing assemblies 18 and 20, respectively. Assembly 20 includes an optical flat 22 having inner and outer surfaces 24 and 26. The optical glass or like optical material of flat 22 is not subject to deterioration in the presence of humidity and is selected to have an index of refraction substantially the same as that of the crystal 10. The surfaces 24 and 26 of flat 22 need not be parallel to each other; in fact, there is usually a small wedge between them to avoid optical resonance in the flat. Preferably, a conventional, commercially available anti-reflection coating 28 is provided on the outer surface 26. 
     A thin layer 30 of index-matching liquid is located in the region between the inner surface 24 of flat 22 and the opposed end surface 14 of crystal 20, the liquid being selected to have an index of refraction substantially the same as that of the crystal. Any index-matching liquid with an index of approximately 1.5 can be used with the above-mentioned crystal materials. Such a liquid is commercially available as index-matching oil. 
     In fabricating the sealing assembly 20, the oil is placed on either of the opposed surfaces 14 or 24 and the flat 22 is pressed against the crystal to form a thin film of oil therebetween. The flat 22 is then sealed to the crystal 10, as by a bead 32 of UV-cured epoxy surrounding and closing the oil-filled region between the surfaces 14 and 24. The pressure on the oil film exerted by pressing the flat 22 against the polished optical surface 14 removes air and other contaminants from the oil-filled region, removes damaging humidity from the surface 14. The sealant 32 hermetically seals the region between surfaces 14 and 24 to prevent re-entry of air, water or other contaminants, thereby protecting the crystal surface from damage. 
     The thickness of the oil film 30 is microscopic, the exact thickness depending upon the flatness and smoothness of the two opposed surfaces. Since the optical flat and the crystal are pressed together, they are very close and the liquid layer is very thin. As a result, there is very little heat generated in it by optical absorption, so that thermal blooming is eliminated. Further, the structure allows the application of electrodes directly to the surfaces of the crystal for the application of electric fields to the crystal, in conventional manner, thereby avoiding the problem associated with the introduction of electrical leads and electrodes into and out of prior liquid cells. 
     It will be understood that the sealing assembly 18 is substantially identical to assembly 20. Further, the provision of anti-reflection coatings on the other surfaces of the optical flats effectively provides anti-reflection optical end surfaces for the crystal 10. 
     Although the invention has been described in terms of a preferred embodiment, it will be understood that variations and modifications can be made without departing from the true spirit and scope thereof, as defined in the following claims.