Abstract:
An undersea optical repeater is provided. The repeater includes a pressure vessel for use in an undersea environment. The pressure vessel includes a pressure housing and at least two cable reeving elements disposed on opposing ends of the pressure housing for respectively receiving ends of optical cables that each include an electrical conductor therein. The cable receiving elements are adapted to be in electrical contact with the respective electrical conductors in the optical cables. At least one optical amplifier is located in the pressure vessel. The optical amplifier includes at least one electrical component adapted to receive electrical power from the electrical conductors in the optical cables. The pressure housing includes a dielectric layer having sufficient dielectric properties for electrically isolating the cable receiving elements from one another to provide a voltage thereacross.

Description:
RELATED APPLICATIONS 
   This application is related to U.S. application Ser. No. 10/687,547, filed Oct. 16, 2003, entitled “Optical Amplifier Module Housed In A Universal Cable Joint For An Undersea Optical Transmission System,” and U.S. application Ser. No. 10/967,720, entitled “Ternary Ceramic Dielectric Coating For An Optical Repeater Pressure Vessel,” filed on even data herewith, and is a continuation-in-part and claims the benefit of priority of U.S. application Ser. No. 10/715,330, filed Nov. 17, 2003 now U.S. Pat. No. 6,917,465, entitled “Method and Apparatus for Electrically Isolating an Optical Amplifier Module Housed In A Universal Cable Joint.” Each of these prior applications is incorporated herein by reference in its entirety. 

   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 
   Undersea optical communication systems include land-based terminals containing transmitters and receivers connected by a cabled-fiber-transmission medium that includes periodically spaced repeaters, which contain optical amplifiers whose purpose is to compensate for the optical attenuation in the cabled fiber. In a bidirectional transmission system each repeater will generally contain two or more optical amplifiers, one for each of the oppositely-directed transmission paths. As the repeaters are usually placed undersea and away from power sources, power must be supplied remotely to the repeaters. The cabled fiber therefore usually contains a cooper conductor to carry electrical power to the repeaters from the terminals. These undersea systems serve to carry optical communication signals (i.e., traffic) between the terminals. The traffic on these systems can consist of voice, data, television, Internet traffic, international telephone traffic, etc. Consequently, the revenue lost when the system is down can be significant. Therefore, these systems must have high reliability and availability. 
   Recently, ultra-small form factor optical repeaters for undersea use have been developed which have dimensions that are substantially smaller than that of conventional undersea optical repeaters. One example of such a repeater is disclosed in co-pending U.S. application Ser. No. 10/687,547 and U.S. application Ser. No. 10/715,330, which are hereby incorporated by reference in their entirety. One example of the repeater shown in these references has dimensions of only about 7.5 cm×15 cm. 
     FIG. 1  shows a side view of such an ultra-small form factor repeater. The repeater  100  includes a pressure vessel comprising a cylindrical metallic housing  110  and metallic end caps  120   1  and  120   2  that are secured to opposing ends of the cylindrical housing  110 . The cylindrical housing  110  must withstand high undersea hydrostatic pressures and remain hermetic for at least 25 years. The pressure vessel must also be corrosion resistant or at least capable of being coated with an anticorrosion component. Suitable materials that are often employed include a high-strength grade of copper-beryllium and steel. 
   Optical cables  130   1  and  130   2  enter the repeater  100  through the end caps  120   1  and  120   2 , respectively. Optical cables  130   1  and  130   2  include an electrical conductor for supplying electrical power to the electrical components located in the repeater  100 . The electrical conductors in the optical cables are in electrical communication with the respective end caps  120   1  and  120   2 . In order to drop power to the electrical components a voltage must be established between the end caps  120   1  and  120   2 . To accomplish the necessary voltage drop, electrical continuity must be interrupted between the end caps  120 . Accordingly, some provision for interrupting electrical continuity needs to be provided since the housing  110  is generally formed from a metallic material. 
   Unfortunately, changing the material from which the housing  110  is formed from a conductive to a dielectric material is problematic because of the substantial structural and thermal demands placed on it. Not only must be the housing  110  be formed from a material strong enough to withstand the hydrostatic pressures of the undersea environment, but it must also be sufficiently thermally conductive to dissipate the waste heat generated by the electrical components within it. Very few available materials can provide the strength needed in such a small volume with the required thermal conductivity. Moreover, most materials that can provide the required strength and thermal conductivity are also good electrical conductors since thermal and electrical conductivity usually go hand in hand because they both arise from the mobility of electrons within the material). 
   Accordingly, it would be desirable to provide a pressure vessel for an undersea optical repeater that meets the stringent structural, thermal and electrical properties that such a pressure vessel requires. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, an undersea optical repeater is provided. The repeater includes a pressure vessel for use in an undersea environment. The pressure vessel includes a pressure housing and at least two cable receiving elements disposed on opposing ends of the pressure housing for respectively receiving ends of optical cables that each include an electrical conductor therein. The cable receiving elements are adapted to be in electrical contact with the respective electrical conductors in the optical cables. At least one optical amplifier is located in the pressure vessel. The optical amplifier includes at least one electrical component adapted to receive electrical power from the electrical conductors in the optical cables. The pressure housing includes a dielectric layer having sufficient dielectric properties for electrically isolating the cable receiving elements from one another to provide a voltage thereacross. 
   In accordance with one aspect of the invention, the dielectric layer is an oxide layer. 
   In accordance with another aspect of the invention, the pressure housing is formed from a metallic material. 
   In accordance with another aspect of the invention, the pressure housing is formed from a metallic material. 
   In accordance with another aspect of the invention, the metallic material includes stainless steel having chromium therein. 
   In accordance with another aspect of the invention, the said oxide layer is a chromium oxide layer. 
   In accordance with another aspect of the invention, the oxide layer is an oxide layer formed from oxidation of the pressure housing. 
   In accordance with another aspect of the invention, the oxide layer is an oxide layer applied to the pressure housing. 
   In accordance with another aspect of the invention, the applied oxide layer is applied by a technique selected from the group consisting of thermal spraying and chemical vapor deposition. 
   In accordance with another aspect of the invention, the pressure housing is formed from an electrically conductive ceramic material. 
   In accordance with another aspect of the invention, the oxide layer arise from oxidation of the electrically conductive ceramic material. 
   In accordance with another aspect of the invention, the pressure vessel is a pressure vessel adapted for an undersea optical fiber cable joint. 
   In accordance with another aspect of the invention, the pressure vessel is a pressure vessel adapted for a universal cable joint for jointing optical cables having different configurations. 
   In accordance with another aspect of the invention, an optical amplifier module located within the pressure vessel is provided for containing the optical amplifier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a side view of a conventional pressure vessel for an undersea optical repeater. 
       FIG. 2  shows a side view of a pressure vessel having a dielectric layer constructed in accordance with the present invention. 
       FIG. 3  shows a side view of an optical amplifier module that may be employed in a repeater constructed in accordance with the present invention. 
       FIG. 4  shows a perspective view of one of the half units that form the optical amplifier module depicted in  FIG. 3 . 
       FIG. 5  shows a side view of one of the half units that form the optical amplifier module depicted in  FIG. 3 . 
       FIG. 6  shows a cross-sectional side view one of the half units that form the optical amplifier module depicted in  FIG. 3 . 
       FIG. 7  is cross-sectional side view of the optical amplifier module shown in  FIG. 3 . 
       FIG. 8  is an enlarged, cross-sectional side view of the portion of the optical amplifier module that interconnects with the end cap. 
   

   DETAILED DESCRIPTION 
   The present inventors have recognized that a dielectric layer can be applied to the outer surface of a pressure vessel that serves as an optical repeater housing. The dielectric layer, which in some embodiments of the invention is an oxide layer, has a resistivity that is sufficient to establish a voltage between the end caps of the pressure vessel, which voltage can be used to power the electrical components contained within the optical repeater. 
     FIG. 2  shows a side view of one example of a repeater in which the present invention may be employed. The repeater  200  includes a pressure vessel comprising a cylindrical metallic housing  210  and metallic end caps  220   1  and  220   2  that are secured to opposing ends of the cylindrical housing  210 . Optical cables  230   1  and  230   2  enter the repeater  200  through the end caps  220   1  and  220   1 , respectively. End caps  220   1  and  220   2  are coupled via intermediate coupling means (not shown), for example, using a threaded connection, so that mechanical loads may be transferred from cable  230   1  to cable  230   2 , and vice versa, such that mechanical continuity is provided to the larger communication cable formed by the joining of the cable segments. End caps  230   1  and  230   2 , in this illustrative example, are shaped as a frustum. However, it is emphasized that the selection of this particular shape for end caps  230   1  and  230   2  is merely illustrative, as the invention is intended to encompass other shapes as well. The large end of the cone abuts the end of cylindrical housing  210  and the smaller end of the cone includes an opening to permit passage of the cables  230   1  and  230   2  into the interior space of repeater  200 . In some designs, the end caps  220   1  and  220   2  may be fastened to the housing  210 , using, for example, conventional fastening means, such that the housing  210  also is a load-bearing member of the repeater. 
   Pressure vessel housing  210  is utilized to create an interior space in repeater  210  which contains the various electrical and optical components of the repeater such as erbium doped fiber, pump sources, couplers and the like. It is noted that the interior space of housing  210  and the contents therein, are not particularly pertinent to the invention at hand, and therefore, except in one example presented below for illustrative purposes, no further details regarding such space and contents are provided herein. 
   As previously mentioned, end caps  220   1  and  220   2  are electrically active because they are in contact with the power conductor located in cables  230   1  and  230   2 , respectively. In order to drop a predetermined voltage to the electrical components within the repeater, housing  210  must be configured so that is does not provide an electrically conductive path between the end caps  220   1  and  220   2 . 
   In accordance with one embodiment of the present invention, an oxide layer  250  is applied to the outer surface of the pressure vessel housing  210 . The oxide layer  250  should have a sufficient thickness to provide the necessary dielectric properties and still remain structurally strong. For example, if the housing is formed from stainless steel having a sufficient chromium content, the housing can be oxidized to form a chromium oxide dielectric surface layer. Alternatively, such an oxide dielectric layer can be applied to the stainless steel housing by a variety of techniques such as thermal spraying and chemical vapor deposition, for example. 
   In other embodiments of the invention the housing  210  may be formed from various electrically conductive ceramics instead of stainless steel. Suitable ceramics include those on which a stable oxide layer can be formed. For example, ceramics that include aluminum, silicon, and titanium can be oxidized to provide the outer dielectric layer. 
   In some embodiments of the invention the internal electrical and optical components of the repeaters are located in an optical amplifier module  400  of the type depicted in  FIGS. 3-6  and which is disclosed in copending U.S. application Ser. No. 10/687,547 and U.S. application Ser. No. 10/800,424. Optical amplifier module  400  is designed to fit within a pressure vessel that typically serves as a universal cable joint for jointing fiber optical cables for use n undersea optical telecommunications systems. The optical amplifier module  400  depicted in the figure 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. 
   A side view of optical amplifier module  400  is shown in  FIG. 3  with the end caps  13  (corresponding to end caps  220  in  FIG. 2 ) in place but without the cylindrical housing  210 . The module  400  is defined by a generally cylindrical structure having flanges  402  (seen in  FIG. 4 ) 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. 4 , 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. 4  shows a perspective view of one of the units  404 . In the embodiment of the invention depicted in  FIGS. 3-8 , each half unit  404  houses two erbium-doped fiber amplifiers. 
   Flanges  402  mate with cable termination units (not shown) of the aforementioned universal joint. As seen in the cross-sectional views of  FIGS. 6 and 7 , 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 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 . 
   As shown in  FIGS. 3-5 , 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 ceramic boss  440  is located on the end of the spinal member  406  remote from the end flange  403 . As shown in  FIGS. 4 and 6 , 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. 
   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. 
   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. 
   End faces  403  each include a pair of pump support bosses  403   a  (see  FIGS. 5 and 6 ) 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 .