Abstract:
A method and apparatus for terminating an end of an optical cable is disclosed such that coaxial tubes of the cable and fibers therein are all prevented from moving relative to one another. For some embodiments, the coaxial tubes crimp together by a mechanical crimp that compresses the outer tube onto the inner tube without roller crimping. A fiber retention subassembly crimps to one of the coaxial tubes, and an adhesive fills the fiber retention subassembly, thereby fixing the fibers therein and isolating tension from the ends of the fibers that extend from the fiber retention subassembly. The ends of the fibers of the optical cable connect with fibers of another optical cable or device by a fusion splice. A splice cover holds and/or seals the spliced section and prevents relative movement between the optical cables or the optical cable and the device at the spliced section.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the invention generally relate to a splice for an optical cable. More particularly, the invention relates to a method and assembly for securing a spliced section of the optical cable. 
   2. Description of the Related Art 
   Optical sensors used in harsh environments such as within a wellbore of an oil or gas well communicate readings from within the wellbore to optical signal processing equipment located at the surface. Surface equipment transmits optical signals to the downhole optical sensors via optical cables which transmit return optical signals to an optical signal processor at the surface. The optical cables may run down the outer surface of one of the tubular strings in the wellbore such as production tubing and clamp thereto at intervals as is known in the art. Since the optical cable is exposed to the harsh effects of chemicals, high pressures, and high temperatures, the optical cables used in harsh environments typically consist of multiple layers. For example, these optical cables may have two concentric metal or alloy tubes disposed around an optical waveguide or fiber that transmits the optical signals. 
   A fusion splice between the ends of two fibers permits repairing a damaged section of the optical cable, coupling the optical cable to the optical sensor or surface equipment, or adding an additional length of optical cable. However, shifting of the concentric tubes or fibers therein relative to each other due to tensile loads or thermal expansion can damage the fiber that extends from the end of the optical cable. Thus, the components of the optical cable are typically secured to one another at the optical cable ends or termination points in an effort to prevent such relative movement. Further, a fusion splice creates a weak point in the fiber. A virgin fiber can accept approximately a 700,000 psi dynamic tensile load before breaking while the fiber at the splice can accept only approximately a 150,000 psi dynamic tensile load before breaking. This weakening of the fiber can effect long-term fiber reliability even under low static tensile loads. Therefore, tension on the fiber at the fusion splice must be isolated from tension forces applied to the rest of the fiber and optical cable. 
   A conventional method for preparing an optical cable for splicing is a complex and time consuming process. The previous method utilized roller crimping of the optical cable which could take at least an hour to prepare a single optical cable end due to the need for the operator to continually check the depth of the crimp. Even with such constant care, roller crimping subjects the process to operator variability. Further, roller crimping thins the outer coaxial tube wall making it subject to breaking and subsequent bond failure. A member crimped directly to the fiber in the previous method can cause attenuation or power loss in the fiber, especially in small (e.g. one eighth inch diameter) optical cable that has a thin buffer layer surrounding the fiber. Benefits of small diameter cable such as increased length of cable per spool, ease in handling, flexibility without kinking, and lower cost make small diameter cable preferable over larger diameter optical cable for many applications. 
   Therefore, there exists a need for a simplified method and assembly that secures a spliced section of any diameter optical fiber cable and fibers within the cable, preferably while eliminating or minimizing attenuation or power loss. 
   SUMMARY OF THE INVENTION 
   The invention generally relates to a method and assembly for terminating an end of an optical cable such that coaxial tubes of the cable and fibers therein are all prevented from moving relative to one another. For some embodiments, the coaxial tubes crimp together by a mechanical crimp that compresses the outer tube onto the inner tube without roller crimping. A fiber retention subassembly crimps to one of the coaxial tubes, and an adhesive fills the fiber retention subassembly, thereby fixing the fibers therein and isolating tension from the ends of the fibers that extend from the fiber retention subassembly. The ends of the fibers of the optical cable connect with fibers of another optical cable or device by a fusion splice. A splice cover holds and/or seals the spliced section and prevents relative movement between the optical cables or the optical cable and the device at the spliced section. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a view of an end of a coaxial cable having an inner tube and armor cut to accept a fiber retention subassembly. 
       FIG. 2  is a view of the end of the coaxial cable having crimps that secure the inner tube to the armor to provide a prepared cable end. 
       FIG. 3  is a view of the fiber retention subassembly aligned with the prepared cable end. 
       FIG. 4  is a view of the fiber retention subassembly positioned on the prepared cable end and crimped in place. 
       FIG. 5  is a view of the fiber retention subassembly having an adhesive being injected into it to provide a retained fiber cable end. 
       FIG. 6  is a partial section view of a splice cover slid over a first retained fiber cable end and adjacent a second retained fiber cable end. 
       FIG. 7  is a view of a fusion splice and recoat of fibers extending from the first and second retained fiber cable ends. 
       FIG. 8  is a partial section view of the splice cover secured to a portion of both the first and second retained fiber cable ends to provide a completed cable splice. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The invention generally relates to a method and assembly for securing a spliced section of an optical cable.  FIG. 1  illustrates an end of a coaxial cable  100  having fibers  101 , an inner tube  102  and armor  104  cut to accept a fiber retention subassembly (shown in  FIG. 3 ). While the cable  100  is shown with the inner tube  102 , the cable may not require an inner tube (e.g. the fibers  101  may be coated directly with a protective material). Therefore, the invention in some embodiments does not include the inner tube  102  of the cable  100 . Additionally, the coaxial cable  100  may include a jacket  106  that covers and protects the armor  104 . The jacket  106  is a larger diameter covering that may be made of nylon and clamped to downhole tubing. In order to withstand the environment downhole, inner tube  102  and armor  104  may be made of a metal or alloy. 
   To prepare the end for splicing, an end portion of the jacket  106  is cut and removed from the end of the coaxial cable  100 . An operator can use a knife to cut and remove a long enough portion (e.g. six feet) of the jacket  106  such that the jacket  106  does not interfere during the splicing process. Next, a portion of the armor  104  is removed from the end of the coaxial cable  100 . The operator can use a standard tube cutter to score the outside of the armor  104 , which the operator can then flex to cleave the portion of the armor  104  being stripped. A sufficient length (e.g. twenty inches) of armor  104  is stripped from the coaxial cable  100  in order to leave enough fiber to form the splice. Next, the inner tube  102  is stripped from the coaxial cable  100 . The inner tube  102  is stripped such that a length (e.g. fifteen hundredths inches) of the inner tube  102  extends past the armor  104 . In operation, the operator can score the inner tube  102  with a knife file and flex the inner tube  102  to cleave the portion of the inner tube  102  being stripped. 
   As shown in  FIG. 2 , mechanical crimps  200  secure the inner tube  102  to the armor  104  to provide a prepared cable end  201 . As with all crimps described herein, the operator can produce the crimps  200  using any available mechanical crimping tool that compresses the armor  104 . While roller crimping may be used to perform the crimping, this time consuming and more sensitive procedure may not be required. Thus, a crimping tool having die inserts can be used to provide the crimps  200 . Depending on the shape of the die inserts of the crimping tool, the crimps  200  may be a hex crimp, a circular crimp, or any other shape. While three crimps  200  are shown positioned along the outside of the armor  104 , any number of crimps  200  may be used in succession to create a stronger holding force between the inner tube  102  and the armor  104 . Prior to performing the splice or attaching a fiber retention subassembly (shown in  FIG. 3 ), the operator may clean the fibers  101  using an alcohol or other solvent to remove any protective greases. 
     FIG. 3  illustrates a fiber retention subassembly  300  axially aligned with the prepared cable end  201 . The fiber retention subassembly  300  includes a protective tube  302 , a fill tube  304 , and a section of mating armor  306  all concentrically arranged and crimped together by a crimp  308  in the mating armor  306 . Preferably, the protective tube  302  is made of polyimide, and the fill tube  304  is made of a translucent or transparent polymer. However, the tubes  302 ,  304  may be made of any material such as a hard polymer or metal that is capable of withstanding the environment and protecting and securing the fibers  101 . As shown, the crimp  308  secures the mating armor  306  at an end of the fill tube  304  such that a portion of the mating armor  306  extends past the end of the fill tube  304 . A portion of the protective tube  302  is secured within the fill tube  304  by the crimp  308  while the remainder of the protective tube  302  extends beyond the end of the fill tube  304  that has the mating armor  306  thereon. The fiber retention subassembly  300  may be formed either on location where the coaxial cable  100  is located or manufactured and preassembled offsite. 
     FIG. 4  shows the fiber retention subassembly  300  positioned on the prepared cable end  201  and crimped in place by a crimp  400 . In order to position the fiber retention subassembly  300  on the prepared cable end  201 , the fibers  101  are run through the inner bores of the protective tube  302  (not visible) and the fill tube  304 . The portion of the protective tube  302  that extends beyond the fill tube  304  inserts into the inner diameter of the inner tube  102  (not visible) of the prepared cable end  201 . In this manner, the protective tube  302  protects the fibers  101  from any burrs on the end of the inner tube  102  along where the inner tube  102  was cut. The mating armor  306  of the fiber retention subassembly  300  butts against the cut end of the armor  104  of the prepared cable end  201  and surrounds the exposed portion of the inner tube  102  of the prepared cable end  201 . Thus, the crimp  400  connects the fiber retention subassembly  300  to the prepared cable end  201  by crimping the mating armor  306  against the outside of the inner tube  102  of the prepared cable end  201 . 
     FIG. 5  illustrates an adhesive, shown diagrammatically as arrow  502 , injecting into a fill port  500  of the fill tube  304  to provide a retained fiber cable end  501  once the adhesive cures. If the fill tube  304  does not include a fill port  500 , the adhesive  502  may inject directly into the exposed end of the fill tube  304 . Applying heat or light to the adhesive cures the adhesive and secures the fibers  101  relative to the fill tube  304  that is secured to the prepared cable end  201 . Thus, the portions of the fibers  101  that extend from the retained fiber cable end  501  are isolated from forces applied to the fibers opposite the fill tube  304 . When the adhesive  502  is photocurable, the fill tube  304  is made of a translucent or transparent material that passes light from an ultra violet (UV) lamp. 
     FIG. 6  shows a partial section view of a splice cover  600  that protects the spliced fibers and further prevents tension force on the fibers at the splice. In operation, the splice cover  600  slides over a first retained fiber cable end  601  such as the retained fiber cable end  501  described above with reference to  FIG. 5 . The first retained fiber cable end  601  aligns adjacent a second retained fiber cable end  602 . The second retained fiber cable end  602  may be a lead from a sensor, instrument, gauge, or connector and may actually be a cable end that has not been prepared as a retained fiber cable end as described herein. As shown in  FIG. 7 , a fusion splice connects the ends of fibers  701  extending from each cable end  601 ,  602 . Known techniques and commercially available equipment may be utilized to form the fusion splice that connects the ends of the fibers  701 . Making the fusion splice generally involves stripping the coating from the fibers, cleaning the fibers, cleaving the fibers to get a flat polished end, precisely aligning and heating the ends to join the fibers, and recoating the fibers along the spliced portion. 
   After the fusion splice and recoating of the fibers  701 , the splice cover  600  slides across cable ends  601 ,  602  as illustrated in  FIG. 8 . The splice cover  600  includes a union fitting cut into a first half  801  and a second half  802  with a section of tubing  803  welded between the two halves  801 ,  802  of the union fitting. Thus, each half  801 ,  802  of the union provides a compression fitting. The length of the tubing  803  covers the excess fiber necessary for the fusion splicing and permits fiber stowage if necessary. Alternatively, the splice cover  600  may only include the first half  801  of the union welded to the tubing  803  that has an opposite end exposed for subsequent coupling via a weld, thread, or otherwise to a connector or other device. As shown, the splice cover  600  secures to a portion of both the first and second retained fiber cable ends  601 ,  602  with the compression fittings  801 ,  802  that tighten against an outside surface of armor  804  of the cable ends  601 ,  602  to provide a completed cable splice. In this manner, the splice cover  600  holds the armor  804  to prevent relative movement between the cable ends  601 ,  602  across the fused fibers. The splice cover  600  may additionally seal the stripped portions of the cable ends  601 ,  602  from the outside environment. 
   The lengths of the portions of jacket  106 , armor  104 , and inner tube  102  that are stripped from the end of the coaxial cable  100  are not fixed and depend on the lengths of the splice cover  600 , the fiber retention subassembly  300 , the portion of the mating armor  306  of the fiber retention subassembly  300  that extends beyond the fill tube  304 , and the length of fiber  101  needed to complete the fusion splice. Thus, the lengths given are only examples for one embodiment. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.