Abstract:
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, which is formed from a thermally conductive material, includes a receptacle portion having a plurality of thru-holes each being sized to receive a passive optical component employed in an optical amplifier. The module also includes at least one circuit board on which resides at least one voltage dropping element for conveying voltage from the conductor to electronics also residing on the circuit board and associated with the optical amplifier. An isolated electrical path provides electrical power received from a conductor in at least one optical fiber cable to the at least one circuit board. The voltage dropping element is in thermal communication with the sidewall.

Description:
RELATED APPLICATIONS  
       [0001]     This application is related to U.S. patent application Ser. No. 10/687,547 filed Oct. 16, 2003, entitled “Optical Amplifier Module Housed In A Universal Cable Joint”.  
         [0002]     This application is also related to U.S. patent application Ser. No. 10/715,330 filed Nov. 17, 2003, entitled “Method and Apparatus For Electrically Isolating An Optical Amplifier Module Housed In A Universal Cable Joint.” 
     
    
     FIELD OF THE INVENTION  
       [0003]     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  
       [0004]     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.  
         [0005]     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.  
         [0006]     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.  
         [0007]     The large physical size of conventional repeaters increases their complexity and cost while creating difficulties in their deployment.  
       SUMMARY OF THE INVENTION  
       [0008]     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, which is formed from a thermally conductive material, includes a receptacle portion having a plurality of thru-holes each being sized to receive a passive optical component employed in an optical amplifier. The module also includes at least one circuit board on which resides at least one voltage dropping element for conveying voltage from the conductor to electronics also residing on the circuit board and associated with the optical amplifier. An isolated electrical path provides electrical power received from a conductor in at least one optical fiber cable to the at least one circuit board. The voltage dropping element is in thermal communication with the sidewall.  
         [0009]     In accordance with one aspect of the invention, at least one optical pump source is in thermal contact with one of the end faces.  
         [0010]     In accordance with another aspect of the invention, the end faces each include at least one inwardly extending boss. The optical pump source residing on one of the inwardly extending bosses.  
         [0011]     In accordance with another aspect of the invention, a first side of the circuit board resides on a surface extending through the sidewall. A thermally conductive pad is mounted to the first side of the circuit board and provides a thermally conductive path between the voltage dropping element and the sidewall.  
         [0012]     In accordance with another aspect of the invention, the voltage dropping element is mounted to the thermally conductive pad.  
         [0013]     In accordance with one aspect of the invention, the undersea optical fiber cable joint includes a pair of cable termination units in which end portions of optical fiber cables to be jointed are respectively retained. The retaining elements are each connectable to one of the cable termination units.  
         [0014]     In accordance with another aspect of the invention, the conductor of each of the optical fiber cables to be jointed are in electrical contact with one of the retaining elements.  
         [0015]     In accordance with another aspect of the invention, the isolated electrical path includes a power conductor located within the circuit board that is in electrical contact with one of the retaining elements.  
         [0016]     In accordance with another aspect of the invention at least one voltage dropping element is provided for conveying a portion of voltage from the power conductor to the electronics associated with the optical amplifier.  
         [0017]     In accordance with another aspect of the invention, the voltage dropping element is a zener diode.  
         [0018]     In accordance with another aspect of the invention, the circuit board comprises a pair of circuit boards, and the isolated electrical path further includes at least one electrically conductive pin electrically connecting the power conductors of the pair of circuit boards.  
         [0019]     In accordance with another aspect of the invention, the plurality of thru-holes laterally extend through the receptacle portion of the sidewall in the longitudinal direction.  
         [0020]     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.  
         [0021]     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.  
         [0022]     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.  
         [0023]     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.  
         [0024]     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.  
         [0025]     In accordance with another aspect of the invention, the internal housing is formed from a pair of half units that each include one of the retaining elements.  
         [0026]     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.  
         [0027]     In accordance with another aspect of the invention, the outer dimension of the internal housing is less than about 15 cm×50 cm.  
         [0028]     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  
       [0029]      FIG. 1  shows an example of an undersea optical fiber cable.  
         [0030]      FIG. 2  shows a simplified schematic diagram of a universal cable joint for jointing fiber optic cables for use in undersea optical telecommunication systems.  
         [0031]      FIG. 3  shows a particular example of a universal cable joint that is available from Global Marine Systems Limited and the Universal Joint Consortium.  
         [0032]      FIG. 4  shows a side view of an optical amplifier module constructed in accordance with the present invention.  
         [0033]      FIG. 5  shows a perspective view of one of the half units that form the optical amplifier module depicted in  FIG. 4 .  
         [0034]      FIG. 6  shows a side view of one of the half units that form the optical amplifier module depicted in  FIG. 4 .  
         [0035]      FIG. 7  shows a cross-sectional side view one of the half units that form the optical amplifier module depicted in  FIG. 4 .  
         [0036]      FIG. 8  is cross-sectional side view of the optical amplifier module shown in  FIG. 4 .  
         [0037]      FIG. 9  is an enlarged, cross-sectional side view of the portion of the optical amplifier module that interconnects with the end cap.  
         [0038]      FIG. 10  shows a plan view of the bottom of one of the circuit boards illustrating the manner in which the zener diodes are mounted to facilitate heat transfer. 
     
    
     DETAILED DESCRIPTION  
       [0039]     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.  
         [0040]     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.  
         [0041]     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.  
         [0042]      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.  
         [0043]      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.  
         [0044]     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.  
         [0045]     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. 4-9  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.  
         [0046]     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.  
         [0047]     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 .  
         [0048]     A side view of optical amplifier module  400  is shown in  FIG. 4  with end caps  13  in place. The module  400  is defined by a generally cylindrical structure having flanges  402  (seen in  FIG. 5 ) located on opposing end faces  403 . A longitudinal plane  405  extends through the optical amplifier module  400  to thereby bisect the module  400  into two half units  404  and  404 ′ that are symmetric about a rotational axis perpendicular to the longitudinal plane  405 . That is, as best seen in  FIG. 5 , 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. 4-9 , each half unit  404  houses two erbium-doped fiber amplifiers.  
         [0049]     Flanges  402  mate with the cable termination units  12  of the Universal Joint shown in  FIG. 3 . As seen in the cross-sectional views of  FIGS. 7 and 8 , 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 .  
         [0050]     As shown in  FIGS. 4-6 , 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 ceramic boss  440  is located on the end of the spinal member  406  remote from the end flange  403 . As shown in  FIGS. 5 and 7 , the thru hole  407  extends through the ceramic boss  440 . As discussed below, the ceramic boss  440  prevents the flow of current from one half unit  404  to the other.  
         [0051]     A circuit board support surface  416  extends along the periphery of the unit  404  in the longitudinal plane  405 . Circuit board  426  is mounted on support surface  416 . When the half units  404  and  404 ′ are assembled, circuit boards  426  and  426 ′ are interconnected by a pair of interlocking conductive power pins  423  that provide electrical connectivity between the two circuit boards  426  and  426 ′. The inner cavity of the unit  404  located between the circuit board support surface  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.  
         [0052]     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.  
         [0053]     End faces  403  each include a pair of pump support bosses  403   a  (see  FIGS. 6 and 7 ) that extend inward and parallel to the circuit board  426 . The circuit board  426  has cut-outs so that the pump support bosses  403   a  are exposed. A pump source  427  that provides the pump energy for each optical amplifier is mounted on each pump boss  403   a.    
         [0000]     Electrical Connectivity  
         [0054]     As previously mentioned, electrical connectivity must be maintained between the cables in the two cable termination units  12 . However, the various components in the optical amplifier module  400  must be electrically isolated to enable a small voltage (e.g., 5-20 v) that must be supplied to the electrical components located on the circuit boards  426 .  
         [0055]     Referring again to  FIG. 3 , the optical amplifier module  400  and sleeve  14  are surrounded by polyethylene sleeve  16 , which serves as a dielectric. Electrical power is taken from the conductor in the cable located in the termination units  12  and transferred through a conductor located in the circuit board  426 . The circuit board is electrically isolated from the optical amplifier module  400 , with the epoxy resin of the circuit board acting as a local dielectric. After the voltage is dropped to the electrical components on one of the circuit boards the voltage is passed from circuit board  426  to circuit board  426 ′ via a pair of complaint conductive pins  423  that each comprise a pin and socket assembly. The pins  423  allow for any axial movement that may occur as a result of tension or hydrostatic pressure.  
         [0056]     More specifically, with reference now to  FIGS. 7 and 8 , power is supplied to the electrical components as follows. Since the cable termination units  12  are electrically powered or active, end caps  13  are also electrically active. A power conductor extends within each of the circuit boards  426  and  426 ′. The power conductors receive electrical power directly from the pump support bosses  403   a . One or more voltage dropping elements such as zener diodes are located on the circuit board  426 . The zener diodes, which electrically couple the power conductors to the other electrical components on the circuit board, drop a voltage that is sufficient to power the electrical components. Electric connectivity extends along the power conductors and is maintained across the circuit boards to the other via the conductive pins  423 . In this way electric conductivity extends from one end cap  13 , through the end flange  403  and pump support boss  403   a  in contact with the end cap  13 , through the power conductor located on the circuit board  426  resting on the pump support boss  403   a , through one of the power pins  423  and through the power conductor located in the other circuit board  426 . Finally, electrical conductivity extends to the other end cap  13  via the other pump support boss  403   a  and end flange  403 .  
         [0057]     The electrical path is isolated from the optical amplifier module  400  as follows. An electrically insulating pad is located between the circuit board support surface  416  and the circuit board  426 . In this way the pump support boss  403   a  is electrically isolated from the circuit board  426 , except through the aforementioned power conductor. Ceramic isolators  442  surround the bolts that secure the circuit board  426  to the sidewalls  412  of each half unit  404 . The ceramic isolators  442  prevent electrical discharges from the bolts to the components located on the circuit board  426 . The ceramic boss  440  located on each half unit  404  electrically isolates the spinal member  406  to which it is connected from both the end cap  13  and the end flange  403  with which it is in contact.  
         [0058]      FIG. 9  shows the manner in which the tension rods  409  extending through thru-holes  407  are electrically isolated from the end caps  13 . As shown in  FIG. 9  for the left-most end cap  13 , a ceramic washer  444  surrounds the head of each tension rod  409 . The ceramic washer  444  electrically isolates the end cap  13  from the tension rod  409 . Because the seal established by the ceramic washer  444  is not hermetic, copper washers  446  and  448  are also provided to ensure that such a hermetic seal is achieved between the tension rod and the end cap  13 . The threaded end of the tension rods  409  terminate in the opposing end cap  13  and the threaded ends are not electrically isolated from the end cap  13 .  
         [0059]     Since the sleeve  14  contacts the end caps  13 , sleeve  14  should preferably be formed from a non-conductive material. For example, sleeve  14  may be formed from a thermally conductive ceramic, which is advantageous because of its strength. However, because such ceramics are often nominally electrically conductive they need to be provided with an oxide surface in order serve as a dielectric. The surface finish of the oxide is preferably polished to facilitate formation of a hermetic seal.  
       Thermal Management  
       [0060]     The pump sources  427  and zener diodes generate a significant amount of heat that must dissipated to ensure that the temperature of the various components do not exceed their operational limits. This is a particularly challenging problem because the pump sources  427  and zener diodes may generate several watts of power over a small area. Moreover, the thermal energy must be dissipated while simultaneously achieving electrical isolation of these same components, two goals which are clearly somewhat at odds with one another. As detailed below, a number of features of the optical amplifier module  400  enhance thermal management so that the heat is adequately dissipated.  
         [0061]     As previously mentioned, pump sources  427  are mounted on the pump support bosses  403   a  of the end flange  403 . The heat from the pump sources  427  is thereby conducted through the pump support bosses  403   a  to the end flange  403 , which has a relatively large mass so that it serves as an effective heat sink. The end flange  403  in turn conducts the heat to the end caps  13  seen in  FIG. 3 .  
         [0062]     The sidewalls  412  of the optical amplifier module  400  are made from a thermally conductive material such as a metal, preferably aluminum. Since the sidewalls  412  have a relatively large surface area, they serve as a spreader that distributes the heat over its surface in a uniform manner so that its local and overall temperature rises are kept to a minimum. The zener diodes are preferably situated as close to the sidewalls  412  as possible to so that the heat generated by the diodes can be readily conducted to the sidewalls  412 .  
         [0063]     For example, as best seen in  FIG. 10 , in one embodiment of the invention the zener diodes  484  are located on the bottom of the circuit board  426  (i.e., the side of the circuit board opposite from that on which the pump sources  427  reside). Copper pads  480  are located on this bottom surface, below each of the ceramic isolators  442  that isolates bolts  482  that secure the circuit board  426  to the support surface  416 . The zener diodes  484  are mounted on the copper pads  480 , adjacent to the bolts  482 . The copper pads  480  serve as one of the electrical contacts for each of the zener diodes  484 , the other of which is denoted by reference numeral  486 . A portion of each copper pad  480  resides on the circuit board support surface  416 . The copper pads  480  contact the electrically insulating pad on which the circuit board  426  rests. The electrical insulating pad is a relatively good thermal conductor and thereby conducts the heat generated by the zener diodes  484  from the copper pads  480  to the circuit board support surface  416  of the optical amplifier module  400 . In this way heat flows from the zener diodes  484 , through the copper pads  480  and the electrical insulating pad, and into the optical amplifier module  400 . Once the heat has been distributed over the sidewalls  412  of the module  400  the heat is directly conducted to the stainless steel sleeve  14  that surrounds module  400 .  
         [0064]     The wide distribution of heat over the relatively large surface area of the end caps  13  and the tension sleeve  14  allows the heat to be effectively conducted through the surrounding polyethylene sleeve  16 , which is not a particularly good thermal conductor, to sea water.