Abstract:
Methods and apparatus for hermetically sealing an optical fiber in a feedthrough connection. In brief overview, the optical fiber is mounted in a housing whose physical properties differ from that of the fiber by utilizing a structure in accord with the present invention. The structure, typically in transition bushing form, is formed from at least two materials, such that the physical properties of a first material are selected to match the physical properties of the optical fiber, and the physical properties of a second material are selected to match physical properties of the housing. When the matched physical properties are the coefficients of thermal expansion (CTE) of the fiber and the housing, the result is a fiber optic mounting that remains hermetically sealed despite changes in ambient temperature that would typically induce stresses in the seal, potentially causing its failure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims the benefit of co-pending provisional application No. 60/476,668, filed on Jun. 5, 2003, and co-pending provisional application No. 60/433,320, filed on Dec. 13, 2002, the entire disclosures of which are incorporated by reference as if set forth in their entirety herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to seals and more specifically to the sealing of an optical fiber feedthrough connection.  
         BACKGROUND OF THE INVENTION  
         [0003]    Telecommunications devices utilizing optical fibers typically have hermetically-sealed metal housings to protect the internal components of the device from temperature variations, moisture, and other environmental contaminants. These sealed housings have at least one aperture that permits an optical fiber to enter and exit the device. It is desirable to maintain a hermetic seal between the optical fiber and the aperture to prevent those contaminants excluded by the housing from entering through the aperture.  
           [0004]    These device housings are typically made from a ferrous material or a non-ferrous metallic material such as titanium alloy, magnesium alloy, or aluminum alloy. The choice of a non-ferrous metallic housing may be advantageous due to a number of design factors, such as weight, heat transfer properties, CTE properties, or configurations. However, the use of a non-ferrous metallic housing prohibits the use of soldering to establish a hermetic seal between the optical fiber and the aperture, since there is a poor CTE match between a glass fiber and a non-ferrous metallic material such as titanium alloy, magnesium alloy, or aluminum alloy. Moreover, non-ferrous alloys are not easily plated and, therefore, are not easily receptive to a solder joint.  
           [0005]    A need therefore exists for methods and apparatus that permit the establishment of a hermetic seal between an optical fiber and a non-ferrous metallic housing.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention relates to methods and apparatus for hermetically sealing an optical fiber in a feedthrough connection. In brief overview, the optical fiber is mounted in a housing whose physical properties differ from that of the fiber by utilizing a structure in accord with the present invention. The structure, typically in transition bushing form, is formed from at least two materials, such that the physical properties of a first material are selected to match the physical properties of the optical fiber, and the physical properties of a second material are selected to match physical properties of the housing. When the matched physical properties are the coefficients of thermal expansion (CTE) of the fiber and the housing, the result is a fiber optic mounting that remains hermetically sealed despite changes in ambient temperature that would typically induce stresses in the seal, potentially causing its failure.  
           [0007]    In one aspect, the present invention provides a method for hermetically sealing an optical fiber including providing an optical fiber, providing a transition bushing, and mounting the fiber in the transition bushing. These individual steps may be performed substantially simultaneously. In one embodiment, mounting the fiber involves metallizing the optical fiber and soldering the metallized fiber to the transition bushing. The fiber is metallized using plasma deposition, electroplating, or both. In another embodiment, mounting the fiber utilizes a glass sealing process. In a further embodiment, the transition bushing is mounted in a housing using, e.g., welding or laser welding.  
           [0008]    The transition bushing has a first section and a second section, the first and second sections of the transition bushing having different physical properties. In one embodiment, the first and second sections of the transition bushing have different CTEs. The CTE of a section of the transition bushing is matched to the CTE of a housing for attachment to the transition bushing. The housing and the section with the matching CTE may be formed from welding-compatible materials; the section with the matching CTE may be formed from a non-ferrous material such as titanium alloy, magnesium alloy, or aluminum alloy.  
           [0009]    In another aspect, the present invention provides a method for hermetically sealing an optical fiber including providing an optical fiber mounted in a ferrule, providing a transition bushing, and mounting the ferrule in the transition bushing. These individual steps may be performed substantially simultaneously. In one embodiment, mounting the fiber includes soldering the ferrule to a section of the transition bushing. In a further embodiment, the transition bushing is mounted in a housing using, e.g., welding or laser welding.  
           [0010]    The ferrule is made of a ferrous alloy. The transition bushing has a first section and a second section, the first and second sections of the transition bushing having different physical properties. In one embodiment, the first and second sections of the transition bushing have different CTEs. The CTE of a section of the transition bushing is matched to the CTE of a housing for attachment to the transition bushing. The housing and the section with the matching CTE may be formed from welding-compatible materials; the section with the matching CTE may be formed from a non-ferrous material such as titanium alloy, magnesium alloy, and aluminum alloy.  
           [0011]    In another aspect, the present invention provides a hermetically sealed optical fiber. The optical fiber is mounted in a transition bushing having a first section and a second section, the first and second sections of the transition bushing having different physical properties. The fiber is mounted in the transition bushing using a solder joint or, in another embodiment, using a glass sealing process. In a further embodiment, the transition bushing is mounted in a housing using, e.g., welding or laser welding.  
           [0012]    In one embodiment, the first and second sections of the transition bushing have different CTEs. The CTE of a section of the transition bushing is matched to the CTE of a housing for attachment to the transition bushing. The housing and the section with the matching CTE may be formed from welding-compatible materials; the section with the matching CTE may be formed from a non-ferrous material such as titanium alloy, magnesium alloy, or aluminum alloy.  
           [0013]    In still another aspect, the present invention provides a hermetically sealed optical fiber. The optical fiber is mounted in a ferrule, the ferrule being mounted in a transition bushing having a first section and a second section having different physical properties. The ferrule is mounted in the transition bushing using a solder joint or, in another embodiment, using a glass sealing process. In a further embodiment, the transition bushing is mounted in a housing using, e.g., welding or laser welding.  
           [0014]    The ferrule is made from a ferrous alloy. In one embodiment, the first and second sections of the transition bushing have different CTEs. The CTE of a section of the transition bushing is matched to the CTE of a housing for attachment to the transition bushing. The housing and the section with the matching CTE may be formed from welding-compatible materials; the section with the matching CTE may be formed from a non-ferrous material such as titanium alloy, magnesium alloy, or aluminum alloy.  
           [0015]    The foregoing and other features and advantages of the present invention will be made more apparent from the description, drawings, and claims that follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The advantages of the invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings in which:  
         [0017]    [0017]FIG. 1 depicts an embodiment of a transition bushing for hermetically sealing an optical fiber in a feedthrough utilizing a ferrule in accord with present invention;  
         [0018]    [0018]FIG. 2 presents a flowchart of a method for hermetically sealing an optical fiber in a feedthrough utilizing a ferrule and a transition bushing in accord with present invention;  
         [0019]    [0019]FIG. 3 depicts an embodiment of a transition bushing for directly sealing an optical fiber in a feedthrough in accord with present invention;  
         [0020]    [0020]FIG. 4 presents a flowchart of a method for directly sealing an optical fiber in a feedthrough utilizing a transition bushing in accord with present invention; and  
         [0021]    [0021]FIG. 5 presents an embodiment of a transition bushing adapted for sealing multiple optical fibers in a feedthrough, utilizing direct sealing or ferrule techniques.  
         [0022]    In the drawings, like reference characters generally refer to corresponding parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed on the principles and concepts of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The present invention permits the hermetic sealing of an optical fiber into a non-ferrous metallic housing to exclude external environmental influences—such as moisture, dust, and air—that typically cause problems with the operation of the electro-optical device contained within the housing. When direct mounting of the fiber to the non-ferrous metallic housing is impractical, the use of a bimetallic structure in accord with the present invention (e.g., in transition bushing form) permits the hermetic sealing of the optical fiber to the housing, as discussed below. This is an exemplary use, as embodiments of the present invention are generally suitable for sealing.  
         [0024]    Referring to FIG. 1, in one embodiment of the present invention a ferrule  100  is used to hermetically seal a fiber  104  into a transition bushing  108 . The transition bushing  108  is, in turn, hermetically sealed to a housing  112 .  
         [0025]    In one embodiment, the optical fiber  104  is mounted in the ferrule  100  using a solder technique. First, the surface of the optical fiber  104  is metallized using a suitable plating technique, such as plasma deposition or electroplating. After metallization, the optical fiber  104  is soldered into place in the ferrule  100 , forming an optical fiber solder joint  116 . Alternately, a fiber  104  is mounted in the ferrule  100  through a glass sealing technique that utilizes an appropriately chosen glass frit material.  
         [0026]    The material forming the ferrule  100  is typically selected for a CTE that matches the CTE of the optical fiber  104 . Matching CTEs helps maintain the hermetic quality of the seal between the optical fiber  104  and ferrule  100  despite temperature excursions in the ambient environment during installation or operation. Suitable materials for the ferrule  100  include ferrous alloys such as KOVAR. Like the optical fiber  104 , the ferrule  100  may be metallized to facilitate its mounting in the transition bushing  108 .  
         [0027]    The combination of the ferrule  100  and the optical fiber  104  is soldered in place in the transition bushing  108 , forming a ferrule solder joint  120 . In accord with the present invention, the transition bushing is fabricated from disparate materials. For example, the bushing  108  in FIG. 1 is formed from a ferrous portion  124  and a transition alloy portion  128 .  
         [0028]    The materials used in the transition bushing are, in general, selected so that the CTE of each portion of the bushing matches the CTE of the material to which the portion of the bushing is hermetically sealed. For example, the bushing  108  in FIG. 1 has a portion  124  that is soldered to ferrule  100  and another portion  128  that is welded to housing  112 , forming a hermetic joint  132 . Accordingly, portion  124  of the bushing  108  is fabricated from a ferrous material (such as KOVAR) whose CTE matches the CTE of the ferrule  100 . Similarly, portion  128  of the bushing  108  is fabricated from a material whose CTE matches the CTE of the housing  112 . Typical materials for portion  128  include titanium alloys, stainless steel alloys, magnesium alloys, aluminum alloys—such as aluminum 4032 or aluminum 4037—and other metal alloys. Matching CTEs helps maintain the hermetic quality of the seals with the bushing  108  despite temperature excursions during installation or operation.  
         [0029]    The materials used in the portions of the transition bushing are also typically selected for compatibility with a desired welding technique, such as laser welding. In one embodiment, where the transition alloy portion  128  of the bushing  108  is selected to be a direct alloy match to the housing  112 , the transition bushing  108  is joined directly to the housing  112  using conventional joining techniques such as TIG or laser welding.  
         [0030]    In one embodiment, the bushing  108  is fabricated from disparate materials using an explosion bonding process, joining the disparate materials and creating a hermetic seal  136  between them. More specifically, this process permits the hermetic joining of, e.g., an aluminum, magnesium, or titanium alloy to a ferrous alloy through the controlled detonation of an explosive charge. In another embodiment, the transition bushing  108  is machined from explosion-bonded bimetallic plate stock, and finished with appropriate plating for subsequent solder operations. The transition bushing  108  is also suited to fabrication using other techniques.  
         [0031]    [0031]FIG. 2 presents a flowchart of an embodiment of a method for hermetically sealing an optical fiber in accord with the present invention. Referring to FIG. 2 (and still to FIG. 1), an optical fiber  104  mounted in a ferrule  100  is provided (STEP  200 ). A transition bushing  108  having multiple sections with different physical properties is also provided (STEP  204 ). The ferrule  100  is mounted in the transition bushing  108  (STEP  208 ), for example, using a solder joint  120  between the ferrule  100  and a ferrous portion  124  of the transition bushing  108 . The transition bushing  108  is mounted in a housing  112  (STEP  212 ).  
         [0032]    In one embodiment, the fiber  104  is sealed in the ferrule  100  using a glass sealing technique (STEP  200 ). In another embodiment, the fiber  104  is soldered into the ferrule  100 , forming a solder joint  116  (STEP  200 ). In further embodiments, the fiber  104  is metallized or plated before soldering; the ferrule  100  may also be plated or metallized.  
         [0033]    As discussed above, the transition bushing  108  is fabricated from disparate materials and selected so that the CTE of each portion of the bushing matches the CTE of the material to which the portion of the bushing is hermetically sealed (STEPS  208  &amp;  212 ). The materials used in the portions of the transition bushing are also typically selected for compatibility with a desired welding technique, such as laser welding.  
         [0034]    In one embodiment, the bushing  108  is fabricated from disparate materials using an explosion bonding process, joining the disparate materials and creating a hermetic seal  136  between them. In another embodiment, the transition bushing  108  is machined from explosion-bonded bimetallic plate stock, and finished with appropriate plating for subsequent solder operations. The transition bushing  108  is also suited to fabrication using other techniques.  
         [0035]    In another embodiment, shown in FIG. 3, the optical fiber  104  is directly mounted in the transition bushing  108  without using a ferrule. In various embodiments, the fiber  104  is mounted using a soldering technique, a glass sealing technique, or another suitable mounting technique. When a soldering technique is used, the fiber  104  may first be metallized or plated to facilitate the creation of a solder joint  116 . The transition bushing  108  is, in turn, hermetically sealed to a housing  112 .  
         [0036]    As discussed above, the transition bushing  108  is fabricated from disparate materials and selected so that the CTE of each portion of the bushing matches the CTE of the material to which the portion of the bushing is hermetically sealed. The materials used in the portions of the transition bushing are also typically selected for compatibility with a desired welding technique, such as laser welding.  
         [0037]    In one embodiment, the bushing  108  is fabricated from disparate materials using an explosion bonding process, joining the disparate materials and creating a hermetic seal  136  between them. In another embodiment, the transition bushing  108  is machined from explosion-bonded bimetallic plate stock, and finished with appropriate plating for subsequent solder operations. The transition bushing  108  is also suited to fabrication using other techniques.  
         [0038]    [0038]FIG. 4 presents a flowchart of an embodiment of another method for hermetically sealing an optical fiber. Referring to FIG. 4 (and still to FIG. 3)—and in contrast to FIGS. 1 and 2—in this embodiment, the fiber  104  is directly mounted in the transition bushing  108  without utilizing a ferrule. A suitable optical fiber  104  is provided (STEP  400 ), as is a suitable transition bushing  108  (STEP  404 ), and the fiber  104  is mounted directly in the transition bushing  108  (STEP  408 ). The transition bushing  108  is mounted in a housing  112  (STEP  412 ).  
         [0039]    In one embodiment, the fiber  104  is sealed in the bushing  108  using a glass sealing technique (STEP  408 ). In another embodiment, the fiber  104  is soldered in place in the bushing  108  (STEP  408 ). In further embodiments, the fiber  104  is metallized or plated before soldering; the bushing  108  may also be metallized.  
         [0040]    As discussed above, the transition bushing  108  is fabricated from disparate materials and selected so that the CTE of each portion of the bushing matches the CTE of the material to which the portion of the bushing is hermetically sealed (STEP  408  &amp;  412 ). The materials used in the portions of the transition bushing are also typically selected for compatibility with a desired welding technique, such as laser welding.  
         [0041]    In one embodiment, the bushing  108  is fabricated from disparate materials using an explosion bonding process, joining the disparate materials and creating a hermetic seal between them. In another embodiment, the transition bushing  108  is machined from explosion-bonded bimetallic plate stock, and finished with appropriate plating for subsequent solder operations. The transition bushing  108  is also suited to fabrication using other techniques.  
         [0042]    As illustrated in FIG. 5, further embodiments of the present invention provide methods and apparatus for hermetically sealing multiple optical fibers in a single transition bushing package  500 . This bushing package  500  allows each fiber to be mounted directly in the package  500  using, e.g., soldering, or indirectly using a ferrule, as discussed above. In one embodiment, some of the fibers are mounted directly in the multi-fiber package  500  while other fibers in the same multi-fiber package  500  are mounted indirectly using ferrules.  
         [0043]    The multi-fiber transition bushing package is fabricated from materials having different physical properties. For example, the multi-fiber bushing  500  in FIG. 5 is formed from a ferrous (KOVAR) portion  504  and a transition alloy (e.g., Aluminum alloy 4047) portion  508 . The materials used in the multi-fiber transition bushing  500  are, in general, selected so that the CTE of each portion of the bushing matches the CTE of the material to which the portion of the bushing is hermetically sealed. For example, the ferrous portion  504  of the multi-fiber bushing  500  is intended to be hermetically soldered to a ferrule (not shown) or a metallized optical fiber, and the transition alloy portion  508  is intended for welding to a housing (not shown). The material for the ferrous portion  504  is selected for a CTE that matches the CTE of the ferrule or the fiber. The material for the transition alloy portion  508  is selected for a CTE that matches the CTE of the housing. Matching CTEs helps maintain the hermetic quality of the seals with the bushing  500  despite temperature excursions during installation or operation. The materials used for the transition bushing package  500  are also typically selected for compatibility with a desired welding technique, such as laser welding.  
         [0044]    In one embodiment, the bushing  500  is fabricated from disparate materials using an explosion bonding process, joining the disparate materials and creating a hermetic seal between them. More specifically, this process permits the hermetic joining of, e.g., an aluminum, magnesium, or titanium alloy to a ferrous alloy through the controlled detonation of an explosive charge. In another embodiment, the transition bushing  500  is machined from explosion-bonded bimetallic plate stock, and finished with appropriate plating for subsequent solder operations. The transition bushing  500  is also suited to fabrication using other techniques.  
         [0045]    Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be expressly understood that the illustrated embodiment has been shown only for the purposes of example and should not be taken as limiting the invention, which is defined by the following claims. These claims are thus to be read as not only including literally what is set forth by the claims but also to include those equivalents which are insubstantially different, even though not identical in other respects to what is shown and described in the above illustrations.