Abstract:
A woven fiber protection cable assembly for use in an optical fiber hydrophone module. The assembly comprises an elastic woven fiber strap with at least one tube attached to one or more sides of the strap in a sinusoidal pattern. The strap at a first end and longitudinal middle portion is substantially aligned with the central axis of the hydrophone module. Two layers of the strap are fastened together in the longitudinal middle portion, and the first end of the strap comprises a loop. The two layers at the second end of the strap are spatially separated and on opposite sides overlap a fiber transition segment, around which one end of the tube is coiled. The elongation of the strap causes the period of the sinusoidal pattern to increase without imparting damaging stress to the optical fiber.

Description:
FEDERAL RESEARCH STATEMENT 
     This invention was made with Government support under Contract N00024-98-C-6308. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF INVENTION 
     The science of underwater sonar equipment is increasingly relying on the use of fiber-optic technology. This reliance is driven by the requirement to have more acoustic sensors per sonar system, with higher sensitivities and lower cost. 
     Passive sonar arrays towed from submarines or surface ships are excellent candidates for fiber-optic sensor technology. In this area, an all optical fiber hydrophone assembly is a recent development by the United States Naval Research Laboratories. The hydrophone assembly consists of a series of air-backed plastic cylinders, called mandrels, which are helically wrapped with optical sensing fiber. The hydrophone senses sound pressure levels through the strain induced in the optical sensing fiber as the sound pressure wave deforms the mandrel. Strain is imparted in the fiber in direct proportion to the pressure induced strain in the mandrel. The characteristics of the light signal transmitted through the fiber change in relation to the strain in the fiber, allowing measurement of the sound pressure level based on the change in the light signal. The mandrels are interconnected by axial interconnect springs to form a line of mandrels that make up the hydrophone assembly. The hydrophone assembly is integrated into a discrete thin-line acoustic module. Modules typically range from 50 to 250 feet in length. End-to-end connection of modules forms long optical hydrophone sonar arrays. Bulkhead couplings are located at each end of the modules and provide connections between adjacent modules. The active sensing hydrophone fiber must transition into and through the module couplings. This requires the interconnection of optical fibers. 
     While large bandwidth capabilities and small size make optical fibers desirable for use, optical fibers are mechanically fragile. Tow-induced loads may cause the fibers to fracture. Such loads may be induced in deployment and recovery operations of the acoustic array, in towing of the acoustic array by drag loading induced elongation, and in bending of the optical fiber. The optical fiber is bent when the optical hydrophone sonar array is wound on a handling system reel. Radial compressive loads may cause degraded light transmission as the result of a phenomenon known as microbending loss. Stress corrosion is another cause of failure of optical fibers, and is a stress-accelerated chemical reaction between the optical fiber glass and water that can result in microcracks in the glass, adversely effecting fiber performance. 
     To accommodate desired growth in the field of optical fiber hydrophones, it is necessary that new apparatus and methods for use with optical hydrophone sensor technology be developed to protect the fibers from mechanical failure. For example, while the optical fiber is relatively well protected while wound on the hydrophone mandrels and interconnect springs, there is a need for a reliable means of transitioning optical fibers on and off the optical hydrophone assembly in the critical areas at each end of the module where the fiber transitions to the bulkhead coupling. There is also a need for protecting the optical fibers as they make the transition from the optical hydrophone sensors to optical-mechanical terminations that provide interconnectivity through the bulkheads to other towed sonar array modules. 
     Optical fibers serving individual modules are limited in the number of light transmission channels available for communication with the monitoring equipment in the vessel. Multiple optical fibers may therefore be required to service an entire hydrophone array. These bypass fibers are needed in order to serve aft modules in the hydrophone array. Optical fibers that service modules aft of the forward module must bypass the hydrophone assembly of one or more modules by a route outside of the hydrophone assembly, creating the need for protection of the bypass fibers. The bypass fibers are aligned with the module central axis proximate to each end of the module. The bypass fibers transition to be substantially parallel to the module central axis and alongside the hydrophone assembly. Bypass fibers must be protected from strain resulting from tow speed induced-drag loading. Reliable end terminations are also required. 
     Modules require a fill fluid in order to have neutral buoyancy. Means for filling the module that provide a seal for both the module and for the fiber that passes through the module seal are needed. There is also a need for improvement in the physical connections between the optical fibers of adjacent modules. Existing optical towed sonar arrays use various configurations of standard optical connector technologies. Specially designed optical-mechanical connectors are available, but require large physical space envelopes, both in diameter and length. Such connectors include fiber splice trays, which are commercially available, but are too large for retrofitting into thin-line towed sonar arrays. 
     A general splicing technique with proven reliability is also needed. Fiber splicing is a necessary step in integrating prefabricated subcomponents of hydrophone assemblies into the towed array optical module assembly. The optical fiber end terminations should be fabricated off-line, eliminating the need, and the risk of damage, for integrating the active sensing fiber into the end termination components. The splicing apparatus should also be effective in repairing an optical fiber break during the hydrophone winding process during fabrication of the optical hydrophone assembly. 
     SUMMARY OF INVENTION 
     The present invention is for use in an optical fiber hydrophone module. The woven fiber protection cable assembly provides a protected means for terminating the optical fiber at the end of the module, without subjecting the fiber to damaging tow induced drag loading or the loading incurred during handling of the module. A woven fiber protection cable assembly having features according to the present invention comprises an elastic woven fiber strap with a tube attached to one side of the strap in a sinusoidal pattern. In one embodiment the elastic woven fiber strap has two sides, two layers, two ends, and a longitudinal middle portion. The strap at a first end and longitudinal middle portion is substantially aligned with the central axis of the hydrophone module. The two layers are fastened together in the longitudinal middle portion and the first end of the strap comprises a loop. The two layers at the second end of the strap are spatially separated and on opposite sides overlap a fiber transition segment. The fiber transition segment is a conical shaped element that transitions the optical fiber from a wound helical orientation on the last interconnect spring of the hydrophone assembly to a straight configuration at the module central axis. 
     A first tube in which an optical fiber is disposed is attached to the strap in a sinusoidal pattern to at least the longitudinal middle portion of one layer of the strap. A first end of the strap is attached to a relatively fixed element and the two layers at the second end of the strap are attached to a relatively movable element. The elongation of the strap causes the period of the sinusoidal pattern to increase without imparting damaging stress to the optical fiber. 
     In further accordance with the present invention, the tube has a coiled end that receives the optical fiber as the fiber transitions from the hydrophone assembly and is wrapped around an adjacent fiber transition segment. 
     In another embodiment, the elongation of the strap is at least 50 percent. The elastic woven fiber strap has a fiber made of thermoplastic multi-filament yarn spun from liquid crystal polymer woven into the strap along the borders of the strap to establish the elongation characteristics. One or more tubes may be made of polytetrafluoroethylene. The present invention further comprises an additional tube attached to the other side of the strap in a sinusoidal pattern, this tube receiving at least one bypass fiber from a bypass cable assembly that is a cable that runs alongside the hydrophone assembly. This additional tube may be made of poly-paraphenylene terephthalamide. 
     In still further accord with the present invention, the woven fiber protection cable may be made by folding a length of cable in half, making a loop at the folded end, and inserting a center layer in the middle section. At the folded end the two layers of base material separate to form a loop that is adapted to be fastened to a module oil seal component within a bulkhead coupling and clevis at which the module ends. At the split end of the strap, the two outside layers separate into two branches that extend alongside the fiber transition segment and are fastened to the positioning tapes of an internal strength member, a woven element that encapsulates the entire length of the module between bulkhead couplings. In yet still further accord with the present invention, the tubes are sewn in between layers of the strap. Also provided according to the present invention is an optical fiber hydrophone module having a woven fiber protection cable assembly. 
     Methods are also provided for protecting an optical fiber in a hydrophone module. One method comprises providing an elastic woven fiber strap and attaching a tube in a sinusoidal pattern such that when the strap elongates there is no damaging stress imparted to the fiber. Another embodiment includes providing an elastic woven fiber strap, attaching a tube for holding an optic fiber in a sinusoidal pattern on the strap, coiling one end of the tube and wrapping it around the fiber transition segment, and receiving the fiber as it transitions from the fiber transition segment to the central axis of the module. 
     Features and advantages of the present invention will become more apparent in light of the following detailed description of some of the embodiments thereof, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a more complete understanding of this invention reference should now be had to the embodiments illustrated in greater detail in the accompanying drawings and described below. 
         FIG. 1  is an elevation view of a submarine towing a hydrophone array. 
         FIG. 2  is a plan view of a hydrophone module with some elements cut away, illustrating some of the elements of the present invention. 
         FIG. 3  is a perspective view of a hydrophone segment. 
         FIG. 4  is a perspective view of an interconnect spring used in the hydrophone segment of FIG.  3 . 
         FIG. 5  is a longitudinal section view of the hydrophone module taken along line  55  of FIG.  2 . 
         FIG. 6  is a cross-section view of the hydrophone module taken along line  66  of FIG.  2 . 
         FIG. 7  is a perspective view of the woven fiber protection cable assembly, fiber transition segment, and hydrophone assembly used in the embodiment of FIG.  2 . 
         FIG. 8  is a perspective view of a woven fiber protection cable assembly of the present invention used in the embodiment of FIG.  2 . 
         FIG. 9  is a plan view of the woven fiber protection cable assembly of FIG.  8 . 
         FIG. 10  is an elevation view of the woven fiber protection cable assembly of FIG.  8 . 
         FIG. 11  is a perspective view of a fiber transition segment of the present invention, used in the embodiment of FIG.  2 . 
         FIG. 12  is a section view taken along the longitudinal axis of the fiber transition segment of FIG.  11 . 
         FIG. 13  is a perspective view of a fiber bypass assembly of the present invention. 
         FIG. 14  is an enlarged perspective view of the embodiment of FIG.  13 . 
         FIG. 15  is an exploded perspective view of one end of the embodiment of FIG.  2 . 
         FIG. 16  is section view used in the description of a bulkhead coupling, fiber splice tray, termination assembly, and clevis of FIG.  15 . 
         FIG. 17  is a perspective view of a termination assembly of the embodiment of FIG.  15 . 
         FIG. 18  is an enlarged exploded perspective view of an optical fiber seal of the embodiment of FIG.  15 . 
         FIG. 19  is a perspective view of a fiber seal retainer of the embodiment of FIGS.  18 . 
         FIG. 20  is a longitudinal section view of the fiber seal retainer of FIG.  19 . 
         FIG. 21  is a perspective view of the fiber seal of the embodiment of FIG.  18 . 
         FIG. 22  is a longitudinal section view of the fiber seal of FIG.  21 . 
         FIG. 23  is a perspective view of a compressive tube stop of the embodiment of  FIGS. 17 and 18 . 
         FIG. 24  is a longitudinal section view of the compressive tube stop of the embodiment of FIG.  23 . 
         FIG. 25  is a longitudinal section view of an end cap of the embodiment of FIG.  18 . 
         FIG. 26  is a perspective view of the fiber-optic splice tray of the embodiment of FIG.  15 . 
         FIG. 27  is another perspective view of the fiber-optic splice tray of the embodiment of FIG.  15 . 
         FIG. 28  is a cross-section view of the fiber-optic splice tray taken along line  2828  of FIG.  27 . 
         FIG. 29  is another cross-section view of the fiber-optic splice tray taken along line  2929  of FIG.  27 . 
         FIG. 30  is a perspective view of another embodiment of the fiber-optic splice tray of FIG.  15 . 
         FIG. 31  is a partially exploded perspective view of an optical fiber splicing apparatus of the present invention and a hydrophone mandrel. 
         FIG. 32  is a perspective view of the optical fiber splicing apparatus of  FIG. 31 , installed on a hydrophone mandrel. 
         FIG. 33  is a plan view of the splice protector of the optical fiber splicing apparatus of FIG.  31 . 
         FIG. 34  is a longitudinal section view of the splice protector taken along line  3434  of FIG.  33 . 
         FIG. 35  is a cross-section view of the splice protector taken along line  3535  of FIG.  33 . 
         FIG. 36  is a plan view of a rotation sleeve of the optical fiber splicing apparatus of FIG.  31 . 
         FIG. 37  is a longitudinal section view of the rotation sleeve taken along line  3737  of FIG.  36 . 
         FIG. 38  is a cross-section view of the rotation sleeve taken along line  3838  of FIG.  36 . 
         FIG. 39  is a partial longitudinal section view of the fastening of open cell foam to a positioning tape of the embodiment of the present invention in FIG.  2 . 
         FIG. 40  is a partial longitudinal section of the fastening of a bypass cable assembly to a positioning tape of the embodiment of the present invention in FIG.  2 . 
         FIG. 41  is an elevation view of a hydrophone assembly of the present invention. 
         FIG. 42  is an exploded perspective view of a hose pulling assembly used to assemble the embodiment of FIG.  2 . 
         FIG. 43  is a perspective view of a hose pulling assembly used to assemble the embodiment of FIG.  2 . 
     
    
    
     DETAILED DESCRIPTION 
     Certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention. For example, words such as forward, aft, upper, lower, left, right, horizontal, vertical, upward, and downward merely describe the configuration shown in the Figures. The components may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise. Also, the scope of the invention is not intended to be limited by the materials or dimensions listed herein, but may be carried out using any materials and dimensions that allow the construction and operation of the hydrophone module. 
     Referring now to the drawings, wherein like reference numerals illustrate corresponding or similar elements throughout the several views, there is shown in  FIG. 1  a submarine  90  using a tow cable  92  to tow an optical hydrophone sonar array  100 . The hydrophone array  100  is a linear array of modules  102  connected end-to-end. Intermodule mechanical connectors  104  fasten the modules  102  together and to the tow cable  92 . A ship could also tow the array  100 . The modules  102  may range in length from 50 feet to over 250 feet, and arrays  100  are commonly several hundred to several thousand feet in length. 
     A module  102 , shown in  FIG. 2 , comprises at a forward end  106  a fiber splice tray  108 , a female bulkhead coupling  110  that is part of the intermodule mechanical connector  104  shown in  FIG. 1 , a clevis  112 , woven fiber protection cable assembly  114 , and a fiber transition segment  116 , then a hydrophone assembly  118 , and at the aft end  120  a fiber transition segment  116 , a woven fiber protection cable assembly  114 , a clevis  112 , and a male bulkhead coupling  121  that with the adjacent female bulkhead coupling  110  comprises the intermodule mechanical connector  104 . The male bulkhead coupling  121  may be designed by one of ordinary skill in the art to mate with a female coupling. Optical sensing fiber  124 , shown in  FIG. 2  within tubing, spans the length of the module. The module  102  is surrounded by a hollow cylindrical open pore polyurethane foam  126 , split longitudinally to allow it to be placed around the hydrophone assembly  118 . Gabardine weave polyester cloth may be used with adhesive tape to connect adjacent portions of foam.  FIG. 3  shows a hydrophone assembly  118  with two hydrophone segments  128 . The two hydrophone segments  128  each comprise an air-backed plastic mandrel  136  and semi-circular steel cages  130 ,  132  that encapsulate and protect the mandrels  136  (one is removed to expose the mandrel  136 ). Interconnect springs  134  connect hydrophone segments  128 . 
     The air-backed plastic mandrels  136  are acoustically sensitive hollow cylinders that may be fabricated from a polycarbonate resin such as LEXANÂ®104 (LEXAN is a registered trademark of General Electric Plastics). In one embodiment, the mandrels  136  have a ⅜-inch outside diameter and are approximately 4.5 inches long. The mandrels  136  are plugged at each end (plugs not visible) with plugs made of the same material as the mandrel  136 , solvent bonded to the mandrel  136 . The optical fiber may be a low bend loss single mode fiber. 
     The perforated steel cage halves  130 ,  132  surround and protect the mandrels  136 . The steel cage halves  130 ,  132  may be fabricated from  40  percent open,  18  or  20  gauge, staggered perforated low carbon steel sheet. Some of the steel cage halves  130  are slotted  131 , while other steel cage halves  132  are not slotted. Slotted cage halves are placed at the forward, mid and aftmost channels of the hydrophone assembly  118  to allow use of tape at those locations or at any desired interval as deemed appropriate by one skilled in the art. 
     The plastic interconnect spring  134  ( FIG. 4 ) connecting the mandrels may also be the same material type as the mandrels  136 . Solvent bonding is used to connect the mandrels  136  and springs  134 . The interconnect springs  134  mechanically separate the individual mandrels  136 , providing points of flexure for the assembly  118 , and facilitate the handling of long continuous hydrophone assembly lengths. Optical fiber is received in a groove  135  in the interconnect springs  134  to transition the optic fiber from one mandrel  136  to the next. The pitch and dimension of the helical void  137  may be as selected by one of ordinary skill in the art. 
     Several additional components of the module appear in  FIGS. 5 and 6 . The air-backed mandrel  136  inside surface  138  defines a cylindrical void terminated at each end of the mandrel with a plug  140 . An internal strength member assembly is designed to carry the load applied to the module  102  and comprises an internal strength member  141 . The internal strength member  141  is a woven member of aramid fibers that surrounds the hydrophone assembly  118  and is attached to each clevis  112  and includes two positioning tapes  142 ,  143 . Certain components of the hydrophone assembly  118  are fastened to the two positioning tapes  142 ,  143  within the internal strength member  141  in order to maintain their relative positions, including the foam  126  with polyester thread  145 . In  FIG. 5  the optical sensing fiber  124  is only shown in tubing; no bare sensing fiber is shown. 
     A bypass cable assembly comprises additional jacketed optical fibers, which are bypass fibers  144 , attached to a woven cable  146 . The woven bypass cable  146  carries the bypass fibers  144  to aft modules by transitioning the fibers from the forward woven fiber protection cable assembly  114 , around the hydrophone assembly  118  outside of the foam  126 , to the aft woven fiber protection cable assembly  114 . A hose  148  is pulled over the entire assembly and fastened to the female bulkhead coupling  110  and male bulkhead coupling  121  ( FIG. 2 ) at each end. A termination assembly  150  resides within the bulkhead couplings  110 ,  121 . 
     The fiber transition segment  116  and the woven fiber protection cable assembly  114  are integrated as shown in  FIG. 7  for use in the present invention. Together these elements protect the optical sensing fiber as it transitions between the central axis of the module  102  and the hydrophone assembly  118  at both ends of the module. 
     The woven fiber protection cable assembly  114  is shown in  FIGS. 8 ,  9 , and  10 . The woven fiber protection cable assembly  114  includes a woven fiber protection cable  170  and etched polytetraflouroethylene (PTFE, such as TEFLONÂ®; TEFLON is a registered trademark of the DuPont Corporation) tubing  172 ,  174 . One tube  172  contains the optical sensing fiber and the other tube  174  contains bypass fibers (described below). The woven cable  170  is an elastic member comprising a plurality of parallel elastic strands knitted together. The elastic member is fabricated from parallel strands of elastane (spandex) elastic fibers (such as LYCRAÂ®; LYCRA is a registered trademark of the DuPont Corporation). A single ply polyester yarn is utilized to knit the elastic strands together. The ultimate elongation of the woven elastic is specified to be a minimum of 50 percent. The cable  170  may be approximately 0.50-inches wide. 
     The tubing  172 ,  174  may be, for example, either  21  or  22  gauge. The tubing  172 ,  174  is installed in the cable  170  using a sinusoidal integration scheme. At one end the woven cable  170  is configured to have a loop  176 , and at the other end the two ends  178  of the cable  170  are initially free and extend to either side of the fiber transition segment (not shown). The middle portion  180  of the cable  170  has three layers, a center layer  182  and two outside layers  184 ,  186 . The tubing  172 ,  174  is inserted between the center layer and outer layers  184 ,  186  in a sinusoidal pattern with a period of 0.50 to 0.55 inches. The tubing  172 ,  174  extends over the edges of the cable by approximately 0.05 inches. The end of the tubing  172  proximate to the fiber transition segment  116  is formed into a retractable coil. This design provides protection to the fiber  124  along with the ability to elongate and retract during variations in speed of the tow vessel. A length of optical fiber  124  is installed within the coiled tubing  172  for containment, protection and subsequent splicing with the hydrophone assembly  118 . 
     The woven fiber protection cable assembly  114  may be made by folding a length of cable in half, making a loop  176  at the folded end, inserting the center layer  182  in the middle section  180 , and separating the free ends  178  that go around the fiber transition segment  116 . The outside layers  184 ,  186  of the cable  170  are sewn to the center layer  182  along each side, using two-ply textured polyester yarn. 
     The woven fiber protection cable assembly  114  provides a means for transitioning optical fibers from the optical hydrophone assembly  118  and bypass fiber cable (described below) assemblies to the bulkheads (for example, see forward bulkhead  110  in FIG.  5 ), which allows intermodule connectivity. The fiber is protected from the point at which it transitions from the hydrophone assembly  118  where the fiber starts into or exits its helical pitch, a point where the fiber would otherwise be susceptible to breakage, to the central axis of the module assembly  102  and into and through the module bulkhead coupling  110 , and clevis  112 . The woven fiber protection cable assembly  114  also provides the necessary capability to elongate and retract during variations of tow speed induced drag loading without imparting strain on the fiber. 
     The fiber transition segment  116 , shown in  FIGS. 7 ,  11  and  12 , transitions the optical sensing fiber  124  between the central axis of the module  102  and the hydrophone assembly  118 , and is mounted to interconnect springs  134  at both the forward  106  and aft  120  ends of the hydrophone assembly, as shown in FIG.  2 . The transition segment  116  has an internal groove  190  that is aligned with and has approximately the same pitch as the interconnect spring groove  135  (FIG.  4 ). The fiber transition segment conical portion  192  is molded around an insert  194  that is the same material as the interconnect spring  134 . The insert  194  is solvent bonded to the interconnect spring  134  thereby connecting the fiber transition segments  116  to the end of the hydrophone assembly  118  for transitioning the optical sensing fiber  124  from the hydrophone assembly  118 . 
     The transition segment  116  functions as bend strain relief to provide-a smooth transition for the fiber  124  across the edge face of the cylindrical mandrel  136  and interconnect spring  134 . Mathematically modeling the transition segment  116  as a bend strain relief as known by one of ordinary skill in the art may be performed to define the material properties (i.e., elastic modulus) and shape of the transition segment  116  given the root diameter at its base  196 , minimum bend radius, and the fiber diameter. Given a root diameter of 0.5 inches, for one embodiment a material with an elastic modulus of 8,400 psi was required. The 0.5-inch root diameter was required in order to provide a smooth transition from the interconnect spring  134  to the transition segment  116 . The smaller end  198  of one embodiment of the transition segment  116  has a 0.312-inch diameter, with an end rounded at a 0.16-inch radius. The conical portion  192  of this transition segment  116  is 3.5 inches long. A 90-A durometer polyurethane may be used for fabrication due to its molding characteristics and its compatibility with the module fill fluid, and may be molded around a LEXANÂ®  104  insert  194  to allow solvent bonding to the adjacent interconnect spring  134 . The portion of the insert  194  that extends from the conical portion  192  of the transition segment  116  has an outside diameter that matches the inside diameter of the interconnect spring  134 . 
     The transition segment  116  interfaces with the interconnect spring  134  affixed to the last hydrophone mandrel at each end of the hydrophone assembly  118 . The groove  135  in the spring  134  on the last mandrel at each end of the hydrophone assembly  118  is aligned with the groove  190  in the transition segment  116  and solvent bonded. The hydrophone fiber (not shown) transitions from the last hydrophone mandrel by continuing the spiral rotation of the fiber off of the interconnect spring  134  and onto the transition segment  116 . The bare fiber is laid along the helical groove  190  in the transition segment  116  for two to three revolutions. The coiled portion of the optical sensing fiber tube  172  is wound into the grooves  190  of the transition segment  116  for two to three revolutions. The fiber then transitions into the protective tube  172 . The tube  172  continues along the helical groove  190  for two to three additional revolutions. Both the bare fiber and the etched PTFE tube  172  are bonded in the groove  190 . The tubing  172  exits out of the molded groove  190  into several (three to five) free retractable coils. The tube  172  with the optical sensing fiber inside is then integrated with the woven fiber protection cable assembly  114 . A service length of optical fiber is maintained in order to allow for splicing to the optical hydrophone assembly  118 . 
     The transition segment  116  provides a controlled means of gradually transitioning the fiber to or from a wound helix to an otherwise straight configuration. The fibers are protected within the tube  172 , as they exit the transition segment  116  and pass through the woven fiber protection cable assembly  114  into the module bulkhead coupling  110  and clevis  112 . 
     As shown in  FIGS. 13 and 14 , the bypass cable assembly  200  protects optical bypass fibers, and includes a jacketed bundle of bypass fibers  144  attached to a woven cable  146 . The bypass cable assembly  200  provides the capability to transition any number of fibers  144  within the module  102  around the hydrophone assembly  118  to service aft modules  102 . The fibers  144  that service aft modules  102  must pass along side the preceding optical hydrophone assembly  118  without being damaged. The bypass cable assembly  200  provides a protected and reliable means of transitioning a bundle of optical fibers  144  from the forward most bulkhead coupling  110  to the aft most clevis  112  within a module  102 . This design component also provides the necessary capability to elongate and retract during variations of tow speed induced-drag loading without imparting strain on the fiber. 
     Bypass fibers run parallel to the hydrophone assembly to serve as the active-sensing fiber for subsequent and discrete blocks of additional hydrophone channels in aft modules. A number of individual bypass fibers are packaged into the single jacketed bundle  144 , with the jacket in one embodiment being made of a thermoplastic polyether elastomer (such as HYTRELÂ®; HYTREL is a registered trademark of the DuPont Corporation), with an integrated strength member made of para-aramid fiber produced from poly-paraphenylene terephthalamide (such as KEVLARÂ®; KEVLAR is a registered trademark of the DuPont Corporation). This package of bundled fibers  144  is attached in a sinusoidal attachment integration scheme to the woven cable  146  that spans the entire length of the module  102 . In one embodiment, the bundled bypass fibers  144  are also attached to the woven fiber protection cable  114 , and because the bypass fibers  144  are already jacketed, the PTFE tubing used on the optical sensing fiber  124  along the woven fiber protection cable  114  is not needed on the bypass fibers  144 . The bypass cable  146  is woven from 15 parallel strands of elastane (LYCRAÂ® elastic) having a diameter of 0.012 inches. A single ply polyester yarn is utilized to knit the elastic strands together. Two strands of single ply liquid crystal polymer thermoplastic multifilament fiber (such as VECTRANÂ®; VECTRAN is a registered trademark of Hoechst Celanese Corporation) are woven into the cable  146  along the borders to establish the ultimate elongation of the woven cable  146 , which is a minimum of 10 percent. These features provide the necessary elongation characteristics so that the bypass fibers  144  are not strained or broken as a result of towing at high speed. 
     The two positioning tapes  142 ,  143  of the internal strength member assembly run the length of the module and are attached to devises  112  at each end. In one embodiment the tapes are 0.5-inch wide and are made of a synthetic thermoplastic, such as nylon. The woven cable  146  with its integrated fiber bundle  144  is stitched with yarn that is two-ply, textured polyester yarn along one of the elastic component positioning tapes  143  every 12 inches, sandwiching the fiber bundle  144  between the woven cable  146  and the positioning tape  143 . This method of attachment protects the fiber bundle  144 . The cable  146  is left unattached near both ends to allow it to transition into the woven fiber protection cable assemblies  114 . 
       FIG. 15  shows a fiber-optic splice tray  108  and termination assembly  150  in their positions relative to the intermodule female bulkhead coupling  110 , clevis  112 , woven fiber protection cable assembly  114 , and fiber transition segment  116  at the aft end  120  of a module  102 . A similar configuration exists at the forward end  106  of a module  102 . 
     The termination assembly  150  comprises a module oil seal assembly  202  and a fiber seal assembly  203 . The module oil seal assembly  202  is shown in  FIGS. 16 through 18 . The module oil seal assembly  202  is the primary hydrostatic seal for the module fill fluid, and comprises a cylinder  204  with one end open that has a circumferential ring  206  and the other end substantially closed. The module oil seal assembly  202  has a centrally located orifice  208  in the substantially closed end of the cylinder that accepts a threaded check valve  212  and seal screw  210 . The cavity  214  defined by the cylinder  204  also accommodates a self-retracting coiled tube  216 . The module oil seal assembly  202  also provides a threaded termination point  217  for the adjacent end of the woven fiber protection cable assembly  114 , using a fastener such as a machine screw  219 . O-rings  220 ,  222 ,  224  provide seals between mating parts. O-rings  226  on the outside of the bulkhead coupling  110  provide a seal between the bulkhead coupling  110  and the hose (not shown). 
     The female bulkhead coupling  110  has alignment pins  229  (two of three are shown) to facilitate mating with an adjacent male coupling  121  (FIG.  2 ). A port  231  for an additional threaded check valve assembly is also provided. 
     A static seal is formed at the interface of the intermodule bulkhead coupling  110  and clevis  112  by threading the coupling  110  onto the clevis  112 , which compresses the O-ring  220  between the module oil seal ring  206  and the internal face of the coupling  227 , and also mechanically restrains the module oil seal assembly in position. Sealant may be used as an alternative to an O-ring as known by one of ordinary skill in the art. The module oil seal assembly  202  also provides two identical counter-bored cavities  228  with O-ring sealing surfaces, which accept optical fiber seals  230  equipped with radial O-rings  232  for static sealing. The walls of the counter-bored holes  228  are machined to be radial sealing surfaces for the O-rings  232 .The threaded check valve  212  permits the injection or removal of module fill fluid on an individual module basis. Both sensing and bypass optical fibers reside in the self-retracting coiled tube  216 , which when mated to the fiber splice tray  108  provides means for managing the excess optical fiber service length required for accessing the fibers within the fiber splice tray  108 . Extension and retraction of the coiled tube  216  can be accomplished without imparting strain on the optical fibers. The formed, self-retracting coiled tube  216  maintains a constant radius for the fibers residing within, thus preventing damage resulting from violations of the fibers” minimum bend radius. 
     The module oil seal design accommodates the optical fiber seal assembly  203 , shown together in FIG.  18 . Bare optical fibers  124 ,  233  pass through a fiber seal retainer  234  ( FIGS. 19 and 20 ) and the fiber seal  230  (FIGS.  21  and  22 ). The inner cavity of the fiber seal  230  is then back filled with an epoxy potting compound, which is compatible with the module fill fluid. The potting compound forms a reliable hydrostatic seal between the fibers  124 ,  233  and the metallic casing of the seal  230 . A radial O-ring  232  is installed onto the fiber seal  230  and the potted seal is inserted into the counter-bored cavity  228  of the module oil seal cylinder  204 . The fiber seal retainer  234  is threaded into the module seal cylinder  204  in order to secure the fiber seal  230  in place. The etched PTFE tubes  172 ,  174  (only  172  is shown) extending from the woven fiber protection cable assembly  114  are installed over tubes  238  in the fiber seal retainer  234  and secured with the compressive tube stops  240  (only one shown in  FIG. 18 ; FIGS.  23  and  24 ). Retainer caps  242  ( FIG. 25 ) are threaded over the tube stops  240  onto the fiber seal retainer  234  to ensure that the PTFE tubes  172 ,  174  are securely held in place. After this procedure has been completed, the coupling  110  is threaded onto the clevis  112  forming the module seal as described above. This establishes a reliable hydrostatic seal, which in one fabricated embodiment was demonstrated to withstand pressures in excess of 3,000 psi. 
     The termination assembly  202  is a protected means for providing hydrostatic module and optical fiber seals at the forward and aft bulkhead couplings  110  (aft bulkhead coupling is not shown) within a module  102 . They also provide the capability to inject fill fluid to or remove fill fluid from each discrete module  102  prior to integration into a full towed acoustic array module string. These attributes are a prerequisite for making each module  102  a stand-alone entity that can be fabricated, optically tested and oil filled for neutral buoyancy. The incorporation of these components into the overall system design permits interchangeability between and within towed sonar acoustic arrays. 
     The integrity of the optical fiber is maintained (i.e., no induced strain or violation of minimum bend radius) within the seal assembly due to the fact that the fiber is fully protected over its entire transition length through the seal. The self-retracting coiled tube  216  located within the module seal cylinder  204  provides a controlled method of transitioning the optical fibers from the fiber seal assembly  203  to the fiber splice tray  108 . The coiled tube  216  also provides flexibility (i.e. service length) to permit the removal and re-insertion of the fiber tray  108  to support the requirements of module interchangeability and array re-configuration. 
     The miniature fiber-optic splice tray assembly  108  for use in a hydrophone module  102  according to the present invention is shown in  FIGS. 26  to  30 . The fiber splice tray  108  houses spliced fibers at the connection between modules  102 . The fiber tray  108  has both entry and exit points  250  for the fiber at either end of the tray  108 . Two pairs of entry and exit points  250  are provided in the event that one pair is inadequate to accommodate the fibers in use. The bottom section  252  of the tray  108  mates with the top section  254 , and an internal groove  256  in the bottom section is of sufficient depth to accommodate several meters of fiber in order to provide adequate service length for performing fusion splices during initial assembly or subsequent repair operations. 
     Each splice is surrounded by a rigid fusion splice sleeve that acts as a splint to protect the fiber at and adjacent to the splice. The rigid sleeve bends very little, and because of the miniature size of the tray, cannot accommodate the tight radii of the bends in the internal groove  256 . Therefore, the sleeve must be located within a straight section of the internal groove  256 . A means of manipulating the rigid fusion splice sleeve to a position within the straight sections is required. The internal groove  256  is designed with multiple alternative fiber paths. In one embodiment, shown in  FIGS. 26 through 29 , the fiber tray  108  has two alternative paths  258 ,  260  at each end of the tray  108  and has an additional two paths  262 ,  264  that cross in the middle of the tray as alternatives to the two parallel straight sections  266 ,  268 . In another embodiment shown in  FIG. 30 , the tray  108   a  bottom section  252   a  has only two alternative paths at each end  258 ,  260 . The fiber may be wound within the groove  256  (not shown), selecting the paths as required to place the sleeve in a straight section  266 ,  268  of the groove  256 . The mating top  254  for the tray  108 ,  108   a  ensures that the fiber is totally encapsulated or captured for further protection during any assembly or repair operation. The tray  108 ,  108   a  may be fabricated from, for example, ASTM A276 stainless steel rod, and has a diameter of between 0.608 and 0.612 inches. The radii of the arcs at each end may be on the order of 0.3 inches or less. 
     An advantage of the fiber-optic splice tray  108 ,  108   a  is that it can house fusion-spliced fibers, protective splice sleeves, and excess fiber service length in a small physical space envelope. The tray  108 ,  108   a  provides access to the optical fibers as they transition between modules  102  and serves as a protective housing for those components. The miniature splice tray  108 ,  108   a  accomplishes this within the bore of the thin-line towed sonar array intermodule coupling, thus providing an effective means of enabling and managing fusion-spliced fibers within a tightly confined volume. The small size of the splice tray  108 ,  108   a  is compatible with the physical geometry of existing towed array mechanical connectors and thus maintains commonality with existing handling system requirements, notably overall rigid length. A threaded boss  270  is provided at each end of the tray  108 ,  108   a  and is used as a temporary attachment point for a housing/booting fixture to maintain the splice tray in a fixed relative position during hosing of the module  102 . The threaded boss  270  can also accept a plunger that opens the check valve  212  when the tray  108 ,  108   a  is inserted into the bulkhead coupling during mating of hydrophone modules  102 . 
     An apparatus to allow splicing of a fiber across a mandrel is depicted in  FIGS. 31 through 38 . Short optical end terminations, referred to as fiber pigtails  278 , remain after transitioning active sensing hydrophone fiber  124  into and through the intermodule mechanical connector  104 . These fiber ends  278  must be spliced to the active sensing fiber  124  of the hydrophone assembly  118 . The fiber splicing assembly comprises a mandrel body  136 , a splice protector  280 , a splinted fusion splice sleeve  281 , and rotation sleeves  282 . The splice sleeve is utilized to protect the fiber splice from the optical hydrophone assembly  118  and the woven fiber protection cable assembly  114 . The splice sleeve may be polyvinylidene flouride (PVDF) heat shrinkable tubing with an interior coating of thermoplastic adhesive that is reinforced with a brass rod that minimizes bending. In one embodiment, the splice sleeve is approximately 0.9-long and is slid over the fiber just prior to making the splice. The splice protector  280  provides a recessed cavity  284  for supporting and protecting the splinted fusion splice sleeve. The rotation sleeves  282  facilitate the winding of excess fiber required for the fusion splicing operation down onto the mandrel body  136 . 
     The fiber splice components  280 ,  282  are typically installed after the woven fiber protection cable assemblies  114  are secured in the module  102 . The fiber pigtails  278  from the woven fiber protection cable assemblies  114  are fusion spliced with fiber pigtails  278  from the hydrophone assembly  118 . The splice protector  280  is then bonded to the last hydrophone mandrel at each end of the hydrophone assembly  118 . The splinted protection sleeve is installed and shrunk with the application of heat over the splice and then secured within the recessed cavity  284  on the splice protector  280 . The two rotation sleeves  282  are utilized to wind the excess service length of the fiber down onto the hydrophone mandrel  136  on both sides of the splice protector  280 , and the fiber is placed in the groove  285  in the rotation sleeves. The rotation sleeves  282  are then bonded in place. Although the rotation sleeves are shown aligned with the splice protector  280  and with each other in  FIG. 32 , this may not necessarily be the case. The orientation of each rotation sleeve  282  on the mandrel  136  is determined by the length of the sensing fiber  124  on the respective side of the splice protector  280 . 
     This splicing apparatus and methodology facilitate fusion splice techniques within an optical hydrophone assembly. The new fiber splice apparatus and method provide the capability to cost effectively fabricate sub-components of hydrophone assemblies off-line for later integration into a towed array optical module subassembly. The present invention also provides repair capability in the event of a fiber break during fabrication of the optical hydrophone assembly. The splice components allow control of the placement, as well as protection of, the fusion splice sleeve on the optical hydrophone mandrel. The fiber splice of the present invention provides a controlled geometry that allows safe handling and permanent protection of the optical fiber. 
     There is a reduction in risk of fiber breakage resulting from having to transition the active sensing fiber from the hydrophone assembly into and through the module bulkhead couplings. The splicing technique of the present invention provides the capability to transition an autonomous fiber, which has been integrated into the optical end termination assembly off-line, into and through the module bulkhead couplings. This embodiment of the invention eliminates the potential of sacrificing an entire hydrophone assembly due to one fiber break during the fabrication of the module subassembly. Another major attribute is the ability to reside within the existing physical envelope of optical hydrophone assemblies with minimal impact to the overall sensitivity of the system, enabling intermodule connectivity using low loss optical fiber fusion splicing techniques. 
     The fabrication process of one embodiment of a module begins with the assembly of the hydrophone  118 . The mandrels  136 , plugs  140 , and interconnect springs  134  are assembled, and the optical sensing fiber  124  is wound on these components (FIGS.  2 - 6 ). The steel cage halves  130 ,  132  are added. A 0.5-inch wide woven polyester tape  137  is wrapped around and adhesively bonded to the steel cage  130 ,  132  and through periodically spaced pairs of slots  131  (FIG.  13 ). Utilizing 1.5-inch wide strips of polyester cloth, to which a thermoplastic adhesive is applied, individual foam sections four feet in length are joined together to form a continuous length of hollow open pore foam  126 . Before the foam assembly  126  is installed, the internal strength member  141  (ISM) along with positioning tapes  142 ,  143  are placed under an initial tension to insure that its length is equivalent to the nominal hose length. Then the foam assembly  126  is installed. Next, at the center of the ISM  141 , the foam assembly  126  is secured to both of the internal positioning tapes  142 ,  143  every 18 inches using polyester thread  145  as depicted in  FIG. 39  (only one side of the foam and one positioning tape shown). The entire length of the stitching is between 1.2 and 1.5 inches. 
     The next step in the fabrication process is the installation of the bypass cable assembly  200  that comprises the jacketed bypass fibers  144  and woven fiber bypass cable  146 . The bypass cable  146  is stitched every  12  inches along the positioning tape  143 , as depicted in  FIG. 40 , so that the jacketed bypass fibers  144  are sandwiched between the woven cable  146  and the positioning tape  143 . The stitching  288  is a loop stitch of polyester thread with two or three loops. This method of attachment provides increased protection for the fiber bundle. The cable is left unattached near both ends so that it may be transitioned into the fiber protection components. 
     The ISM  141  and its positioning tapes  142 ,  143  are then placed under additional tension in order to elongate it in preparation for the installation of the hydrophone assembly  118  and other components up to the devises  112 . 
     The length of the ISM  141  for the installation of the hydrophone assembly is based upon several factors: nominal hose length, hose elongation characteristics, number and design of interconnect springs. 
     Elongating the ISM prior to installation of the hydrophone assembly was found in testing a prototype to help optimize the interconnect spring gap spacings under operational tow speeds and during reeling. Adherence to this installation methodology allows full extension of the hydrophone assembly  118  during maximum elongation of the module  112 , which occurs at peak tow speeds. 
     The hydrophone assembly  118  is attached to both positioning tapes  142 ,  143  by passing 0.5-inch wide woven polyester tape  137  ( FIG. 41 ) through and around the slotted cage halves  130  and sewing the free ends to the tape  142 ,  143  through the foam  126 . As previously noted, slotted cage halves may be placed at the forward, mid, and aftmost channels of the hydrophone assembly, or at any desired interval. This technique is used to provide a loosely coupled attachment system. In order to provide enhanced positional stability, each hydrophone  118  element is bonded to the open pore foam  126  using a thermoplastic adhesive. The adhesive bond is formed between the foam  126  and the 0.5-inch wide strip of woven polyester tape  137  that has been wrapped around and adhesive bonded to each set of cage halves  130 . 
     The next step in the fabrication process is the integration of the fiber transition segment  116  with the woven fiber protection cable assembly  114  and the interconnect spring  134  in order to construct the fiber protection assembly for both the forward and aft ends of the module (FIGS.  7 - 12 ). This step may be performed at any time during fabrication since the intent is to fabricate this assembly off-line. The transition segment  116 , which has an internal groove with the same pitch as an interconnect spring  134 , is aligned with a spring  134  and solvent bonded. This attachment scheme is identical to the attachment method utilized for attaching the hydrophone mandrels  136  to the interconnect springs  134 . The retractable coiled tube  172  extending from the woven fiber protection cable assembly  114  is wound into the grooves  190  of the transition segment  116  for two to three revolutions. The bare fiber exiting from within the tube continues along the helical groove  190  for two to three additional revolutions. Both the bare fiber and etched PTFE tube  172  are secured within the groove  190  with ultraviolet curable optical adhesive such as Norland NOA UV curable adhesive available from Norland Products, Inc. of New Brunswick, N.J. The helical winding of the optical fiber is continued as it transitions off of the transition segment  116  and onto the interconnect spring  134 . A service length of optical fiber is maintained (approximately one meter) in order to allow future fusion splicing of the optical fiber to the optical fiber on the hydrophone assembly  118 . 
     The woven fiber protection cable assembly  114  is secured within the module  102  by stitching one of the two reinforced branches  178  of the woven cable  170  to the internal positioning tape  142  and the other branch  178  to the other positioning tape  143 . The bypass fiber  144  is transitioned from the woven cable  146  to the woven fiber protection cable assembly  114  by maintaining its sinusoidal pattern along one of the branches  178  of the woven fiber protection cable  170 . The jacketed fiber bundle  144  is then transitioned off of the woven fiber bypass cable branch  178  and may enter a separate protective carrier  174  (i.e., PTFE tubing) that is integrated within the woven fiber protection cable  170 , or in the preferred embodiment the jacketed fiber bundle  144  is directly integrated into the woven fiber protection cable  170 . The fiber protection cable  170  is then inserted through the clevis  112  and bulkhead  110 . This design approach ensures the survivability of both the active hydrophone sensing fiber and bypass fibers in this critical area where they are transitioned to the central axis of the module  102 . It also provides the desired characteristics for elongation and retraction. 
     After the forward and aft woven fiber protection cable branches  178  are secured within the module  102 , the fiber pigtails from the transition segments  116  are fusion spliced with the fiber pigtails of the hydrophone assembly  118 . The splice protector  280  is bonded to the last hydrophone mandrel  136  at each end of the hydrophone assembly  118 . A custom designed splinted protection sleeve is installed over the splice and then secured within the recessed cavity  284  on the splice protector  280 . Two rotation sleeves  282  are utilized to wind the excess service length of fiber down onto the hydrophone mandrel  136  on both sides of the splice protector  280 . The two rotation sleeves  282  are bonded in place resulting in the final configuration that is depicted in FIG.  32 . This mandrel  136  is intentionally breached to allow it to free flood in order to make it acoustically insensitive. The cage halves  130  and  132  are then placed around the mandrel body  136  and bonded in place in the same manner as all other hydrophone mandrels. 
     The woven fiber protection cables assemblies  114  are attached to the module oil seal  202  as shown in  FIGS. 15 and 16 . The fiber protection is attached to the module oil seal  202  with a machine screw  219  and the tray  108  is coupled to the seal via the coiled tubing  216  through which the fibers transition. At this point the fibers (hydrophone and bypass) are transitioned out of the woven cable assembly  170  and into the tubes  238  of the fiber seal retainer  234 , through the coiled tube  216  and into the fiber tray  108 . The PTFE tubes  172 ,  174  for the fiber are terminated at the fiber seal  203  (FIG.  18 ). The woven fiber protection cable assembly  170  is attached to the module oil seal  202  with a fastener, preferably a machine screw  219  ( FIG. 16 ) that ensures that all loads are carried by the woven cable with none being transferred to the protective carrier or fiber. A sufficient length of bare fiber is wound and stored into the internal groove  256  within the fiber tray  108  in order to provide the service length required for performing fusion splices for intermodule connectivity. 
     In order to accommodate the hosing process, where the hose is slid over the module  102 , the forward end of the module  102  is terminated with a termination assembly  150 , a fiber splice tray  108  and a forward bulkhead coupling  110 . The aft end is terminated with a termination assembly  150 , a temporary fiber splice tray  108 , and temporary tooling. The aft end that is not fully terminated has temporary tooling installed on it until the hose is slid over the module. For hosing, the aft end of the ISM  141  must be pulled into and through the hose  148 . Temporary tooling  300 , shown in  FIGS. 42 and 43 , is designed to secure the aft fiber splice tray  108  and coiled tubing  216  within a protective enclosure  302  that is sized to fit within the hose. The temporary tooling  300  comprises the protective enclosure  302 , a pulling adapter  310 , and an eyebolt  312 . The clevis  112  is part of the ISM  141  and has threads that mate with the protective enclosure  302 . The protective enclosure houses the aft fiber splice tray  108 ,  108   a  and secures it to inhibit rotational or extensional movement during the hosing/booting process. The pulling adapter serves as an interface between the enclosure  302  and the eyebolt  312 . It is tapered to accommodate a lead in for smooth entry into the hose. The eyebolt  312  provides for easy attachment to the wire rope that is used to pull  314  the module  102  into the hose  148 . 
     A wire rope is passed through the hose and attached to a swivel and then the eyebolt  312  on temporary tooling  300  at the aft end of the ISM  141 . The hose  148  is tensioned and the ISM  141 , with the hydrophone assembly  118  installed, is pulled into the hose. The temporary tooling  300  is then removed and the remaining module  102  end component is installed, comprising the aft bulkhead coupling  110  as depicted in FIG.  15 . The module  102  is then oil filled to complete the assembly process. The embodiments of the present invention protect the active sensing optical fiber and the bypass fiber from damage that could otherwise result in the normal course of towing and handling optical hydrophone sonar arrays. The effect of elongation and bending requirements imposed on the module are reduced on the optical fiber by the present invention embodiments, which result in a durable and reliable optical hydrophone sonar array. The embodiments of the present invention also facilitate the assembly of the arrays, in that modules may be individually constructed. Subassemblies within the module, such as the hydrophone assembly and the parts of the module from the fiber transition segment to the adjacent end of the module, may be fabricated independently and then combined. 
     Although the present invention has been shown and described in considerable detail with respect to only one exemplary embodiment for each component, it should be understood by those skilled in the art that we do not intend to limit the invention to the one embodiment since various modifications, omissions and additions may be made to the disclosed embodiment without materially departing from the novel teachings and advantages of the invention, particularly in light of the foregoing teachings. For example, the components may be of modified shapes and sizes. Accordingly, we intend to cover all such modifications, omission, additions and equivalents as may be included within the spirit and scope of the invention as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.