Abstract:
A method for securing a glass fiber to a housing includes positioning the glass fiber in proximity to the wall of a housing and applying a quantity of solder glass frit to the surface of the glass fiber. Sufficient solder glass frit is employed so as to occupy the gap between the glass fiber and the housing. The solder glass frit has a melting point lower than that of the glass fiber. The surface of the glass fiber is not metallized. Upon heating the solder glass frit softens and adheres to both the glass fiber and the wall of the housing. The fused solder glass frit secures the glass fiber to the housing and forms an hermetic seal therebetween.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an optical device package, and more particularly to an optical device package including one or more hermetically bonded optical fibers, and a method for making same. 
     2. Background of the Art 
     Optical device packages are known in the art. Typically such packages include one or more optical fibers such as those used in communications, a housing, and means to secure the terminal portion of the optical fiber within the housing. Optical fibers are typically fabricated from fused silica or specialty glasses having very low absorption of light for wavelengths used for communications. The housing can be adapted to engage other optical devices so as to act as a connector. The optical fiber can act as a transmitter, wherein the optical signal carried by the fiber exits the end of the fiber, or a receiver, in which the end of the fiber receives a signal from an external source. Optical device assemblies can include both transmitters and receivers. 
     Because the optical fibers are usually of very small diameter and require a highly precise alignment, it is necessary to stabilize the position and orientation of an optical fiber in an optical device by securing it to the housing. One way of securing the optical fiber includes the use of a metal solder, e.g., tin-lead alloy, which has a melting point lower than that of the fiber. For example, U.S. Pat. No. 4,708,429 to Clark et al., which is herein incorporated by reference, discloses a method of securing the optical fiber by metallizing a portion of the length of the fiber with chromium and gold to enhance the subsequent wetting of the fiber by metal solder. 
     However, this method has disadvantages in that it requires the step of applying a coating of metal to the glass fiber to promote adhesion of a metal solder. This step adds to the time and cost of manufacture. It would be desirable to have a simpler method of securing a glass fiber to a housing. 
     SUMMARY 
     A method for securing a glass fiber to a housing. The method comprises the steps of: (a) providing a housing having a wall; (b) positioning the glass fiber in proximity to the wall, the glass fiber having a non-metallized surface; (c) applying a quantity of solder glass frit to the glass fiber, the solder glass frit being in contact with both the housing wall and the non-metallized surface of the glass fiber and having a melting point below the melting point of the glass fiber; and (d) heating the solder glass frit to a temperature sufficient to at least soften the solder glass frit so as to form a seal between the non-metallized surface of the glass fiber and the housing wall. 
     The method herein advantageously avoids the step of metallizing the surface of the glass fiber, which would be necessary if metal solder were used instead of fiber glass frit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various embodiments are described below with reference to the drawings wherein: 
     FIG. 1 is a cutaway partly sectional perspective view of an optical fiber secured in a housing; 
     FIG. 2 is a cross-sectional view of an optically coupled device package; 
     FIG. 3 is a perspective view of an optical cable prepared for assembly; 
     FIG. 4 is a perspective view of the support sleeve of FIG. 2; 
     FIG. 5 is a perspective view partially cutaway of an optical fiber assembly; and 
     FIG. 6 is a perspective view of the fiber assembly of FIG. 5 in cooperation with the housing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Optical fibers for use in the present invention are fabricated from a glass typically composed of fused silica with or without other components and are characterized by a melting point of at least about 1,000° C., preferably at least about 1,100° C., and more preferably at least about 1,200° C. 
     In general, the method herein broadly relates to the securing of a optical glass fiber in a housing. Referring now to FIG. 1, a fiber optic device  1  includes a tubular housing  2  having an interior wall  3  defining an axial aperture  4  having a first diameter. The housing  2  can be made from a metal such as steel, copper, brass, aluminum, or nickel, and alloys such as Kovar, Invar, and copper-tungsten (Cu—W). Alternatively, housing  2  can be fabricate from glass, ceramics and the like. An optical glass fiber  5  is inserted into axial aperture  4 , the optical glass fiber  5  having a non-metallized circumferential surface  6  with a second diameter. The optical glass fiber second diameter is less than the axial aperture first diameter so as to define a gap between the interior wall  3  of the housing and the outer circumferential surface  6  of the glass fiber. A quantity of solder glass frit  7  is applied to a portion of the outer non-metallized circumferential surface  6  of the optical glass fiber sufficient to completely occupy the gap between the surface  6  of the optical glass fiber  5  and the interior wall  3  of the housing. The solder glass frit is heated to a temperature sufficient to at least soften the solder glass frit so as to form an hermetic seal between the non-metallized surface  6  of the glass fiber and the interior wall  3  of the housing. The fiber optic device  1  can be an optical connector and can be a transmitter or receiver of optically transmitted information carrying signals such as used in communication networks. 
     To provide a more detailed illustrative embodiment, the invention herein is discussed below in conjunction with an optical fiber assembly such as that disclosed in U.S. Pat. No. 4,708,429 to Clark et al. However, such use is exemplary and not contemplated as limiting the scope of the present invention. 
     Referring now to FIG. 2, an optically coupled device package  10  comprises a frame  12  with an aperture  13  extending through one of its walls. Optionally, a neck  14  can extend through the aperture  13 . Mounted within the frame  12  is an optional thermoelectric cooler  16  which controls the temperature of the base plate  18 , and an optical device  20  mounted thereon. The optical device  20  can be a laser, LED, or detector. An optical fiber assembly  25  comprising an optical cable  28  secured within a cable housing  26  is sealed within the aperture  13  or neck  14 . Housing  26  can be fabricated from the materials indicated above with respect to housing  2 . A lid, not shown, is ultimately sealed to the top of the frame  12 . The frame  12 , neck  14  and lid can be of a metal such as copper or brass. For a hermetic package, the seals between the glass optical fiber  24  and housing  26 , the housing  26  and aperture  13  or neck  14 , the neck  14  and the aperture  13 , and the lid and the frame  12 , should be hermetically tight. 
     In FIG. 3 the cable  28  includes an outer jacket  30  around a fibrous support layer  32  (e.g., of Kevlar®, a registered trademark of the DuPont Company). Inside the fibrous layer  32  is a protective coating  34  which overlies the optical fiber  24 . The protective coating  34  is typically a resilient synthetic material, e.g., acrylate, polypropylene, nylon, teflon or the like. This protective coating  34  should be stripped away to expose a length of the optical fiber  24  sufficient for coupling. In contrast to the method disclosed in U.S. Pat. No. 4,708,429, the optical fiber  24  is not metallized. The soldering agent, i.e., solder glass frit, is applied directly to the uncoated surface of the optical fiber  24 . Overlying a portion of the exposed length of the optical fiber  24  is a solder preform  40  which may be a tight wire wrap or a cylindrical preform of the desired solder glass frit material. The preform  40  is heated sufficiently so that the preform  40  wets the optical fiber  24 . The outside diameter of the preform  40  should be about equal to that of the protective coating  34 . 
     Solder glass frit for use in the present invention is a low melting point glass, for example, lead borate glass, lead-zinc-borate glass and the like. A composition range for a solder glass frit suitable for the present invention is as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Oxide 
                 Broad Range 
                 Usual Range 
               
               
                   
                   
               
             
             
               
                   
                 lead oxide 
                 70-85 
                 75-85 
               
               
                   
                 (PbO) 
               
               
                   
                 zinc oxide 
                  0-20 
                 2-16 
               
               
                   
                 (ZnO) 
               
               
                   
                 boron oxide 
                  5-15 
                 8-15 
               
               
                   
                 (B 2 O 3 ) 
               
               
                   
                 silica 
                  0-10 
                 0-5 
               
               
                   
                 (SiO 2 ) 
               
               
                   
                 barium oxide 
                 0-3 
                 0-2 
               
               
                   
                 (BaO) 
               
               
                   
                 tin oxide 
                 0-5 
                 0-2 
               
               
                   
                 (SnO 2 ) 
               
               
                   
                   
               
             
          
         
       
     
     A suitable solder glass for use in the present invention is disclosed in U.S. Pat. No. 5,560,760 to Toeppen and is commercially available under the designation FK-3 from Schott Glass Technologies of Duryea, Pa. Also suitable are high lead content glass frit slurries available from Corning which can be applied like solder to a glass fiber. Generally, this solder glass frit has a melting point no higher than about 500° C., preferably no higher than about 450° C., and most preferably no higher than about 400° C. The coefficient of thermal expansion of the solder glass frit preferably matches that of the glass fiber and the housing material. 
     FIG. 4 shows the housing  26  of FIG. 2, which comprises a body with an axial opening therethrough. Anchor holes  42 , extending to the opening, are provided through the walls of the housing  26  near a first end. A test port  44  is provided through the side of the housing  26  and is located between the anchor holes  42  and a solder window  46 . The outside diameter of the housing  26  is about equal to the inside diameter of the neck  14  of FIG.  2 . The inside diameter of the housing  26  is about equal to the diameter of the protective coating  34  of the optical cable  28  of FIG.  3 . The housing  26  can be of a metal, such as copper or brass. 
     FIG. 5 illustrates an optical fiber assembly  25  comprising an optical cable  28  which has been inserted in one end of, and axially through, said housing  26 . The cable  28  is positioned such that the protective coating  34  is adjacent the anchor holes  42 . Nodules  48 , integral with the protective coating  34 , are formed in the anchor holes  42 . This can be done by causing the protective coating  34  to flow into the anchor holes  42  forming the nodules  48 , e.g. by heating. Alternatively, small portions of the material that can be fused with the protective coating  34  can be inserted into the holes  42  to form the nodules  48 . The nodules  48  make up a physical anchor  50  in the area of the anchor holes  42  giving the optical cable  28  stability within the housing. Typically, the solder glass preform  40  is located adjacent window  46  of FIG.  4 . At this point, the housing is heated while a solder glass frit is introduced through the window  46  which wets to the preform  40  and the inside of the housing  26  to form the seal  52 , which can be hermetic. 
     Since the optical fiber  24  is now physically anchored and sealed with the housing  26  of the optical fiber assembly  25 , breakage of the fiber  24  near the area of the protective coating  34  is eliminated. The optical fiber  24  may be of any desired length within, or extending from, housing  26 . 
     The fiber assembly  25  is inserted into the neck  14  as shown in FIG.  6 . Specifically, a second end of the support sleeve  26  is positioned within the neck  14  such that the optical fiber  24  can be aligned with the optical device  20  of FIG.  2 . The optical fiber  24  can be held in alignment by a bonding material of solder glass frit  22 , as shown in FIG.  2 . The housing  26  may be fastened in this position within the neck  14  by any convenient means. For example, an aperture  54  may be provided in the side of the neck  14  through which a molten metal solder or solder glass frit can be added. 
     If the package  10  is to be hermetically sealed prior to sealing the lid (not shown) onto the frame  12 , the reliability of seal  52  can be verified by known techniques such as applying a vacuum to the top of the frame  12  and introducing a source of helium gas near to the joints to be tested. The optional test port  44  in FIG. 6 is provided for introduction of helium. The test port  44  of FIG. 6 is located between the physical anchor  50  and the seal  52  to insure that it is the seal  52  that is tested during a leak check. Therefore, as shown in FIG. 5, the protective coating  34  should be stripped back beyond the port  44 . 
     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.