Abstract:
An optical amplifier module contains at least one optical amplifier. The module includes an internal housing having an outer dimension substantially equal to an outer dimension of an internal fiber splice housing of an undersea optical fiber cable joint. The internal housing includes a pair of opposing end faces each having a retaining element for retaining the internal housing within an outer housing of the undersea optical fiber cable joint. The internal housing also includes a sidewall interconnecting the opposing end faces, which extends between the opposing end faces in a longitudinal direction. The sidewall includes a receptacle portion having a plurality of thru-holes each being sized to receive a passive optical component employed in an optical amplifier.

Description:
STATEMENT OF RELATED APPLICATION 
   This application claims the benefit of priority to U.S. Provisional Patent Application 60/427,769 filed Nov. 19, 2002, and entitled “Optical Amplifier Module Housed In A Universal Cable Joint.” 

   FIELD OF THE INVENTION 
   The present invention relates to the field of optical repeaters, and more particularly to an optical repeater employed in an undersea optical transmission system. 
   BACKGROUND OF THE INVENTION 
   In undersea optical transmission systems optical signals that are transmitted through an optical fiber cable become attenuated over the length of the cable, which may span thousands of miles. To compensate for this signal attenuation, optical repeaters are strategically positioned along the length of the cable. 
   In a typical optical repeater, the optical fiber cable carrying the optical signal enters the repeater and is coupled through at least one amplifier and various components, such as optical couplers and decouplers, before exiting the repeater. These optical components are coupled to one another via optical fibers. Repeaters are housed in a sealed structure that protects the repeaters from environmental damage. During the process of deployment, the optical fiber cable is coiled onto large drums located on a ship. Consequently, the repeaters become wrapped about the drums along with the cable. Due to the nature of the signals, and the ever increasing amount of information being transmitted in the optical fibers, repeaters are getting larger, and their increased length creates problems as they are coiled around a drum. Although the drums may be up to 9-12 feet in diameter, current repeaters may be greater than 5 feet in length, and, therefore, are not able to lie flat, or even substantially flat, along a drum. Tremendous stresses due to forces on the order of up to 100,000 pounds are encountered at the connection point between the repeater and the fiber optic cable to which it is attached, especially during paying out and reeling in of the cable. The non equi-axial loading across the cable may arise as a result of severe local bending that is imposed on the cable at its termination with the repeater. This loading would inevitably lead to failure of cable components at loads well below the tensile strength of the cable itself. 
   To prevent failure of the cable during deployment of the repeater, a bend limiter is often provided, whose purpose is to equalize the forces imposed on the cable. In addition, a gimbal may be provided at each longitudinal end of the repeater to which the bend limiting devices are attached. The gimbal provides free angular movement in two directions. The bend angle allowed by the gimbal between the repeater and bend limiting device further reduces the local bending that is imposed on the optical fiber cables. 
   The large physical size of conventional repeaters increases their complexity and cost while creating difficulties in their deployment. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, an optical amplifier module is provided that contains at least one optical amplifier. The module includes an internal housing having an outer dimension substantially equal to an outer dimension of an internal fiber splice housing of an undersea optical fiber cable joint. The internal housing includes a pair of opposing end faces each having a retaining element for retaining the internal housing within an outer housing of the undersea optical fiber cable joint. The internal housing also includes a sidewall interconnecting the opposing end faces, which extends between the opposing end faces in a longitudinal direction. The sidewall includes a receptacle portion having a plurality of thru-holes each being sized to receive a passive optical component employed in an optical amplifier. 
   In accordance with one aspect of the invention, the plurality of thru-holes laterally extend through the receptacle portion of the sidewall in the longitudinal direction. 
   In accordance with another aspect of the invention, the internal housing has a generally cylindrical shape. The receptacle portion of the sidewall has a curvature that defines a diameter of the cylindrical shape. 
   In accordance with another aspect of the invention, the undersea optical fiber cable joint is a universal joint for jointing optical cables having different configurations. 
   In accordance with another aspect of the invention, the universal joint includes a pair of cable termination units in which end portions of the optical cables to be jointed are respectively retained. The retaining elements are each connectable to one of the cable termination units. 
   In accordance with another aspect of the invention, the retaining elements each include a flange through which at least one optical fiber extending from the end portion of one of the optical cables extends into the internal housing. 
   In accordance with another aspect of the invention an optical fiber storage area is located within the internal housing. 
   In accordance with another aspect of the invention, a support member is provided for supporting at least one circuit board on which reside electronics associated with the optical amplifier. 
   In accordance with another aspect of the invention, the optical fiber storage area includes at least one optical fiber spool around which optical fiber can be wound. 
   In accordance with another aspect of the invention, the internal housing is formed from a pair of half units each having a mating surface extending in a longitudinal plane that includes the longitudinal direction. The half units are interconnected to one another along the mating surfaces. 
   In accordance with another aspect of the invention, the support member supporting the circuit board is arranged so that the circuit board and the mating surface form a substantially continuous plane. 
   In accordance with another aspect of the invention, the support member is defined by a lip extending inwardly from, and recessed below, the mating surface. 
   In accordance with another aspect of the invention, the sidewall includes a pair of ribbed members extending longitudinally from the receptacle portion of the sidewall. The ribbed members each have a tension rod thru-hole extending laterally therethrough in the longitudinal direction for supporting a tension rod employed by the undersea optical fiber cable joint. 
   In accordance with another aspect of the invention, the outer dimension of the internal housing is less than about 15 cm×50 cm. 
   In accordance with another aspect of the invention, the outer dimension of the internal housing is about 7.5 cm×15 cm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an example of an undersea optical fiber cable. 
       FIG. 2  shows a simplified schematic diagram of a universal cable joint for jointing fiber optic cables for use in undersea optical telecommunication systems. 
       FIG. 3  shows a particular example of a universal cable joint that is available from Global Marine Systems Limited and the Universal Joint Consortium. 
       FIG. 4  shows a perspective view of optical amplifier module constructed in accordance with the present invention. 
       FIGS. 5 and 6  show different perspective views of one of the half units that form the optical amplifier module depicted in  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   The present inventors have recognized that a substantially smaller repeater can be achieved by first reducing the length of the repeater so that the stresses placed upon it during its deployment are greatly reduced, thereby eliminating the need for gimbals. The elimination of the gimbals, in turn, allows further reductions in the dimensions of the repeaters. 
   The present inventors have further recognized that a repeater substantially reduced in size can be housed in a unit formed from off-the-shelf components that have been qualified for the undersea environment. In particular, the inventors have recognized that a housing conventionally used for interconnecting different undersea optical fiber cables can also be used as an ultra-small form-factor repeater housing. As discussed below, one such housing, commonly referred to as the Universal Joint, has become the defacto worldwide standard for maintaining submarine cables and has a lengthy history of successful deployment. The present invention thus provides a repeater that, because of its small size, is easily deployed and which is located in an economical, submarine qualified housing that is already well established in the undersea optical communications industry. Moreover, because the Universal Joint can interconnect different optical fiber cables, the repeater can be used to interface with a variety of cables and systems from different manufacturers. 
   To facilitate an understanding of the present invention, an example of an undersea optical fiber cable will be described in connection with  FIG. 1 . While different cable manufactures employ cables having different configurations and dimensions, most cables employ most of the components depicted in  FIG. 1  in one form or the other. Optical cable  330  comprises a single, centrally located gel-filled buffer tube  332  made from a metal such as aluminum or stainless steel. The gel-filled buffer tube  332  contains optical fibers  335 . In some cases the buffer tube  332  is replaced with a centrally disposed kingwire that is surrounded by optical fibers that are embedded in a polymer. Two layers of strandwires, which serve as strength members, are wound around the buffer tube. One layer includes strandwires  338  and the other layer includes strandwires  339 . A copper conductor  340  surrounds the strandwires and serves as both an electrical conductor and a hermetic barrier. An outer jacket  342  formed from polyethylene encapsulates the copper conductor  340  and serves as an insulating layer. 
     FIG. 2  shows a simplified schematic diagram of a universal cable joint for jointing fiber optic cables for use in undersea optical telecommunication systems. Such a joint is referred to as a universal cable joint because it can interconnect many different types of undersea optical telecommunication cables, regardless of manufacturer. The cable joint includes a common component assembly  10  in which an optical fiber splice is located. The fiber splice is formed from two fibers that respectively originate in two cables that each terminate in cable termination units  12 . A protective assembly  15  surrounds common component assembly  10  and cable termination units  12  to provide protection from the external environment. 
     FIG. 3  shows a particular example of a universal cable joint that is available from Global Marine Systems Limited and the Universal Joint Consortium, which, as previously mentioned, is often simply referred to as the Universal Joint. In  FIGS. 2 and 3 , as well as the figures that follow, like reference numerals indicate like elements. In  FIG. 3 , the protective assembly  15  depicted in  FIG. 2  comprises a stainless steel sleeve  14  that surrounds the common component assembly  10  and a polyethylene sleeve  16  that is molded over the common component assembly  10 . The stainless steel sleeve  14  provides resistance to tensile, torsional and compressive loads and further provides an electrically conductive path through which electrical power can be transmitted from the copper conductor of one cable to the copper conductor of the other. 
   The jointing process begins by stripping back the various layers of the cable to reveal predetermined lengths of the outer jacket, copper conductor, strandwires, and the fiber package (e.g., the buffer tube containing the optical fibers or the kingwire surrounded by the optical fibers). The strandwires are clamped in a ferrule assembly located in the cable termination units  12 . The fiber package extends into the common component assembly  10 , where it is held in place by a series of clamps. In the common component assembly  10  the individual fibers are separated and spliced to their corresponding fibers from the other cable. The splices, along with excess fiber, are looped and wound in channels that are formed within the common component assembly  10 . The common component assembly  10  is inserted in the stainless steel sleeve  14  and end caps  13  are screwed to each end of the assembly  10 . Two tension rods  17  and  19  extend through the end caps  13  and the common component assembly  10 . The tension rods  17  and  19  are designed to carry the tension loads that are placed on the universal joint during the deployment process as the joint is transferred from a ship to its undersea environment. Finally, the joint is laid in a mold that is injected with molten polyethylene to provide an insulate (i.e., polyethylene sleeve  16 ) that is continuous with the outer jacket of the cables. 
   The present inventors have recognized that a cable joint such as the universal cable joints depicted in  FIGS. 2-3  can be modified to serve as a repeater housing in which 1 or more optical amplifiers are located.  FIGS. 4-6  show one embodiment of an optical amplifier module  400  that replaces the common component assembly  10  seen in  FIGS. 1-4 . The optical amplifier module  400  must have substantially the same dimensions as the common component assembly, which is only about 7.5 cm×15 cm. As previously mentioned, this is far less in size than conventional repeater housings, which are often several feet in length. The optical amplifier module  400  depicted in the figures can support 4 erbium-doped fiber amplifiers (EDFAs), physically grouped as a dual amplifier unit for each of two fiber pairs. Of course, the present invention encompasses optical amplifier modules that can support any number EDFAs. 
   Each optical amplifier includes an erbium doped fiber, an optical pump source, an isolator and a gain flattening filter (GFF). The amplifiers are single-stage, forward pumped with cross-coupled pump lasers. A 3 dB coupler allows both coils of erbium doped fiber in the dual amplifier to be pumped if one of the two pump lasers fails. At the output, an isolator protects against backward-scattered light entering the amplifier. The gain flattening filter is designed to flatten the amplifier gain at the designed input power. An additional optical path may be provided to allow a filtered portion of the backscattered light in either fiber to be coupled back into the opposite direction, allowing for COTDR-type line-monitoring. Of course, optical amplifier module  400  may support EDFAs having different configurations such as multistage amplifiers, forward and counter-pumped amplifiers, as well as fiber amplifiers that employ rare-earth elements other than erbium. 
   The optical amplifier module  400  is designed to be compatible with the remainder of the cable joint so that it connects to the cable termination units  12  and fits within the stainless steel sleeve  14  in the same manner as the common component assembly  10 . 
   A perspective view of optical amplifier module  400  is shown in  FIG. 4 . The module  400  is defined by a generally cylindrical structure having flanges  402  located on opposing end faces  403 . Flanges  402  mate with the cable termination units  12  of the Universal Joint shown in  FIG. 3 . Through-holes  407  extend inward from the end faces  403  through which the tension rod of the universal joint are inserted. The end faces  403  also include clearance holes  430  for securing the end caps  13  of the Universal Joint to the optical amplifier module  400 . The clearance holes  430  are situated along a line perpendicular to the line connecting the tension rods thru-holes  407 . 
   A longitudinal plane  405  extends through the optical amplifier module  400  to thereby bisect the module  400  into two half units  404   1  and  404   2  that are symmetric about a rotational axis perpendicular to the longitudinal plane  405 . That is, rather than dividing the end faces  403  into two portions located on different half units  404 , each half unit  404  includes the portion of one of the end faces  403  on which a respective flange  402  is located.  FIG. 5  shows a perspective view of one of the units  404 . In the embodiment of the invention depicted in  FIGS. 4-6 , each half unit  404  houses two erbium-doped fiber amplifiers 
   As shown in  FIG. 5 , each unit  404  includes curved sidewalls  412  forming a half cylinder that defines a portion of the cylindrical structure. A spinal member  406  is integral with and tangent to the curved sidewalls  412  and extends longitudinally therefrom. The thru hole  407  containing the tension rod of the universal joint extends through the spinal member  406 . A mating surface  408  extends along the periphery of the unit  404  in the longitudinal plane  405 . When the optical amplifier module  400  is assembled, the mating surface  408  of unit  404   1  contacts the mating surface  408  of its corresponding unit  404   2 . Interlocking teeth  410  are located on the mating surface  408  for properly aligning the two units with one another. A circuit board support lip  416  is located radially inward from the mating surface  408 . The support lip  416  is recessed with respect to the mating surface  408  so that when the circuit board is placed on the support lip  416  (see  FIG. 6 ) the circuit board forms a continuous plane with the mating surface  408 . The inner cavity of the unit  404  located between the circuit board support lip  416  and the spinal member  406  serves as an optical fiber storage area. Optical fiber spools  420  are located on the inner surface of the spinal member  406  in the optical fiber storage area. The erbium doped fibers, as well as any excess fiber, are spooled around the optical fiber spools  420 . The optical fiber spools  420  have outer diameters that are at least great enough to prevent the fibers from bending beyond their minimum specified bending radius. 
   The curved sidewalls  412  are sufficiently thick to support a plurality of thru-holes  418  that extend therethrough in the longitudinal direction. The thru-holes  418  serve as receptacles for the passive components of the optical amplifiers. That is, each receptacle  418  can contain a component such as an isolator, gain flattening filter, coupler and the like. Tapped holes  422  are provided in the inner surface of the sidewalls  412  through which screws are inserted to frictionally secure the passive components in their respective receptacles  418 . 
   A pair of pump support members  424  are situated on the side of the support lip  416  adjacent to the end face  403 . The support members  424  are located on opposing sides of the flange  402 . As best seen in  FIG. 6 , the circuit board  426  has cut-outs so that the board  426  fits over the support members  422  when it is placed on the support lip  416 . After the circuit board  426  is in place, a pump source  427  that provide the pump energy for each optical amplifier is mounted on the support member  422 . A heat sink  428  secured to the end face  403  contacts each pump source to conduct away heat generated by the pump source  427 .