Abstract:
An fiber optic splice having provide substantially enhanced reliability and broadened operating temperature range uses a light-cured index matching fluid to splice the facing ends of the optical fibers in between two metallic tubes forming a leak-tight, thermally insulating, and mechanically robust outer package.

Description:
GOVERNMENT SUPPORT  
       [0001]     This invention was made with Government support under contract No. N68335-05-C-0045 awarded by Naval Air Warfare Center AD. The Government may have certain rights in the invention. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to the field of optical fiber communication and more particularly to the reconstruction of an optical fiber cable.  
       BACKGROUND ART  
       [0003]     In the past decade, applications involving optical fiber based communication systems are becoming more practical and are gradually replacing copper based systems. A common task required by these applications is to repair damaged fiber optic cables. There are two prior art technologies that are used to repair fiber-optic cables and the most relevant patents to this invention appear to be the one by Thomas Scanzillo, Aug. 10, 2004, U.S. Pat. No. 6,773,167 and by Toshiyuki Tanaka, Oct. 5, 1999, U.S. Pat. No. 5,963,699. These patents are thereby included herein by way of reference.  
         [0004]     A typical prior art mechanical fusion spliced fiber optic cable is illustrated in  FIG. 1 . The splice consists of an input optical fiber  110  with a protective coating  120 , and an output optical fiber  115  with a protective coating  125 . The optical fibers are joined at their interface  130  using an automated apparatus following precision alignment and discharge induced fusion splicing process. In order to protect the splicing region, a rigid rod  150  is used and typically the splice and the rigid rod are both enclosed in a heat shrinking enclosure  140 .  
         [0005]     A typical prior art mechanical fiber-optic splice is illustrated in  FIG. 2 . The splice consists of an input optical fiber  210  with a protective coating  220 , an output optical fiber  215  with a protective coating  225 , a capillary glass tube  250  with a precision through channel, placed in side of a protective outer tube  240  and with protective end caps  260  and  265 . Typically the input and output fibers are placed inside of the glass capillary, an index matching fluid  230  is used to form an air free contact. For certain splices, there is an added small perpendicular channel in the capillary tube  255 . To aid the fiber insertion into the glass capillary tube, two ends of the capillary tube are normally tapered to form interfacing cones. The inner diameter of the capillary tube is made substantially close to the outer diameter of the optical fiber with typical tolerances within one micrometer for a single-mode fiber splice, and a few micrometers for a multimode fiber splice. The index matching fluid is transparent and has a refractive index very close to that of the core of the optical fiber. Frequently, the optical fiber cable-splice interfaces are further protected by flexible boots  270  and  275 . The prior art fiber splice is often protected with a plastic outer package (not shown) for mechanical stability.  
         [0006]     A related prior art fiber optic cable is illustrated in  FIG. 3 . The cable consists of coating protected optical fiber  310 , loose buffer tube  320 , cable strengthening fibers  380  and outer jacket  390 . These cables are designed for reliable operation in challenging environments.  
         [0007]     The prior art approaches have several areas for improvements. For example, the plastic protective outer package has a very limited range of operating temperature. Furthermore, in avionics applications, a fast temperature-cycled environment requires additional packaging considerations to ensure stable and reliable operations. Furthermore, in order to splice fiber optic cable such as the one illustrated in  FIG. 3 , one must have structure improvements such that the mechanical and chemical resistance properties of the cable restored. There is a need, therefore, to make improvements to these prior arts, so that highly reliable fiber-optic splices and reconstructed fiber-optic cables can be realized.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention discloses an apparatus and a method for obtaining fiber-optic splice and fiber-optic cable reconstruction that provides substantially enhanced reliability in a temperature cycled environment. This apparatus and method can be applied to applications in many areas such as avionics and automobile and defense related instrumentation. Key steps of the method include a) using light-cured index matching fluid to splice the optical fiber, b) restoring mechanical strength of the fiber optic cable by crimping cable strengthening fibers in between two metallic tubes; and c) forming a leak-tight, thermally insulating, and mechanically robust outer package. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which:  
         [0010]      FIG. 1  shows the structure of a prior art fusion spliced fiber-optic splice;  
         [0011]      FIG. 2  displays the structure of a prior art mechanical fiber-optic splice;  
         [0012]      FIG. 3  illustrates the cross sectional view of a high quality prior art fiber-optic cable;  
         [0013]      FIG. 4  depicts the cross sectional view of an improved fiber-optic splice incorporating a structure for reconstructed cable;  
         [0014]      FIG. 5  shows the cross sectional view of an improved fiber-optic splice incorporating a structure for reconstructed cable and further incorporating thermal and mechanical stress reduction elements;  
         [0015]      FIG. 6  displays the cross sectional view of an improved fiber-optic splice incorporating a structure for reconstructed cable and further incorporating thermal and mechanical and environmental stress reduction elements;  
         [0016]      FIG. 7  illustrates an improved splice fixture consisting of base plate, V-grooved fiber cable guide, guide for glass capillary tube, and UV LED light source for curing the index-matching fluid. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     The present invention discloses a new method and an apparatus to obtain a highly reliable mechanically reconstructed fiber-optic cable. The new method departs from the prior art practice of directly splicing fiber-optic cables. The basic concept is to introduce a leak-tight, thermally shielded, and mechanically robust outer package. In addition, light-cured index matching fluid is used to permanently fix the optical fibers to the glass capillary. The new approach provides a highly reliable reconstructed fiber-optic cable for hash environment and rough handling.  
         [0018]     The first preferred embodiment of the present invention  400  is illustrated in  FIG. 4 . The core of a reconstructed fiber-optic cable consists of an input optical fiber  410  with an outer protective tube  420 , an output optical fiber  415  with an outer protective tube  425 , and a glass capillary tube  450  with a precision capillary channel, and two cable-splice bridging flanges  463  and  468 . Typically, the ends of the optical fibers are stripped and cleaved according to splicing specifications. The ends are then inserted into the capillary tube. To aid the splicing process, the ends of the capillary tube are tapered to allow for the ease of the insertion of the optical fibers and to accommodate the loose tubes outside of the optical fiber. Light-cured index matching fluid is introduced inside of the capillary tube between the optical fiber ends to be spliced, and cured once a desired insertion loss target is achieved. Typically the inner diameter of the capillary tube is very close to the outer diameter of the optical fiber. For single mode optical fibers, the capillary inner diameter is within one micrometer of the fiber diameter, whereas for multimode fibers it is within a few micrometers. In order to restore mechanical strength of the fiber-optic cable, the input cable strengthening fibers  480  are crimped between the cable-splice bridging flange  463  and an outer tube  460 . Similarly the output fiber-optic cable strengthening fibers  485  are crimped in between a bridging flange  468  and its outer tube  465 . The cable mechanical property is restored by crimping an outer tube  445  with both input tube  460  and output tube  465 , at respective locations.  
         [0019]     The second preferred embodiment of the present invention  500  is illustrated in  FIG. 5 . The core of a reconstructed fiber-optic cable consists of an input optical fiber  510  with an outer protective tube  520 , an output optical fiber  515  with an outer protective tube  525 , and a glass capillary tube  550  with a precision capillary channel, and two cable-splice bridging flanges  563  and  568 . Typically, the ends of the optical fibers are stripped and cleaved according to splicing specifications. The ends are then inserted into the capillary tube. To aid the splicing process, the ends of the capillary tube are tapered to allow for the ease of the insertion of the optical fibers and to accommodate the loose tubes outside of the optical fiber. Light-cured index matching fluid is introduced inside of the capillary tube between the optical fiber ends to be spliced, and cured once a desired insertion loss target is achieved. Typically the inner diameter of the capillary tube is very close to the outer diameter of the optical fiber. For single mode optical fibers, the capillary inner diameter is within one micrometer of the fiber diameter, whereas for multimode fibers it is within a few micrometers. In order to restore mechanical strength of the fiber-optic cable, the input cable strengthening fibers  580  are crimped between the cable-splice bridging flange  563  and an outer tube  560 . Similarly the output fiber-optic cable strengthening fibers  585  are crimped in between a bridging flange  568  and its outer tube  565 . The cable mechanical property is restored by crimping an outer tube  545  with both input tube  560  and output tube  565 , at respective locations. In order to improve thermal and mechanical properties of the splice, a thermal insulating tube  555  is placed outside of the splice core whereas two flexible boots  570  and  575  are used to protect the cable-splice interface regions.  
         [0020]     The third preferred embodiment of the present invention  600  is illustrated in  FIG. 6 . The core of a reconstructed fiber-optic cable consists of an input optical fiber  610  with an outer protective tube  620 , an output optical fiber  615  with an outer protective tube  625 , and a glass capillary tube  650  with a precision capillary channel, and two cable-splice bridging flanges  663  and  668 . Typically, the ends of the optical fibers are stripped and cleaved according to splicing specifications. The ends are then inserted into the capillary tube. To aid the splicing process, the ends of the capillary tube are tapered to allow for the ease of the insertion of the optical fibers and to accommodate the loose tubes outside of the optical fiber. Light-cured index matching fluid is introduced inside of the capillary tube between the optical fiber ends to be spliced, and cured once a desired insertion loss target is achieved. Typically the inner diameter of the capillary tube is very close to the outer diameter of the optical fiber. For single mode optical fibers, the capillary inner diameter is within one micrometer of the fiber diameter, whereas for multimode fibers it is within a few micrometers. In order to restore mechanical strength of the fiber-optic cable, the input cable strengthening fibers  680  are crimped between the cable-splice bridging flange  663  and an outer tube  660 . Similarly the output fiber-optic cable strengthening fibers  685  are crimped in between a bridging flange  668  and its outer tube  665 . The cable mechanical property is restored by crimping an outer tube  645  with both input tube  660  and output tube  665 , at respective locations. In order to improve thermal and mechanical properties of the splice, a thermal insulating tube  655  is placed outside of the splice core whereas two flexible boots  670  and  675  are used to protect the cable-splice interface regions. The splice is further protected by a heat shrinking outer tube  678 .  
         [0021]     In the disclosed preferred embodiments outlined above, typically, the Outer package tubes related to crimping ( 445 ,  460 ,  463 ,  465 ,  468 ,  545 ,  560 ,  563 ,  565 ,  568 ,  645 ,  660 ,  663 ,  665 , and  668 ) are metallic and can be made with low thermal expansion alloys such as Invar which is a commercially available alloy formed primarily of iron and nickel, and Kovar which is a commercially available alloy formed primarily of nickel, cobalt and iron. The flexible boots ( 570 ,  575 ,  670 ,  675 ) are made of rubber materials that can withstand extreme temperature conditions (from −60 to 150° C.). The insulating layer ( 555  and  655 ) can be made with materials such as insulation fiberglass or Teflon fibers.  
         [0022]     The forth preferred embodiment of the present invention is illustrated in  FIG. 7 . The alignment fixture of the fiber optic splice consists of a base plate  720 , V-grooved guides for optical fiber cables  727 , for glass capillary  740 , and UV-LED sources. In a typical operation, optical fiber ends are stripped and cleaved in accordance to operating specification. The optical fibers are then attached to the V-grooved guide and pushed towards the glass capillary (which is also fixed to a v-grooved guide). The light-curing index matching fluid is then applied between the mating ends of the optical fibers. Once the optical fibers are brought together and certain insertion loss target is achieved, UV-LED will be powered and the splice is cured. Typical index matching liquids are optical adhesives such as NOA6I from Norland, OG142-13 from Epotek, and UV15 from Master Bond.  
         [0023]     It will be apparent to those with ordinary skill of the art that many variations and modifications can be made to the method and apparatus for splicing fiber-optic cables disclosed herein without departing form the spirit and scope of the present invention. It is therefore intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.