Abstract:
The present invention provides a hermetically sealed module to be located in an external pressure vessel providing protection from external pressure in an undersea environment. The hermetically sealed module includes at least one optical amplifier and an hermetically sealed housing for containing the optical amplifier. The housing has a retaining element for retaining the housing within the external pressure vessel. The module also includes a plurality of ports for conveying into the housing, in an hermetically sealed manner, at least one optical fiber and a conductor incorporated in an undersea optical fiber cable. The conductor supplies electrical power to the optical amplifier. At least one conductive terminal is located in the housing for establishing electrical contact with the conductor traversing each of the plurality of ports. The conductive terminal supplies electrical power from the conductor to the optical amplifier.

Description:
STATEMENT OF RELATED APPLICATION  
       [0001]    This application claims the benefit of priority to U.S. Provisional Patent Application 60/434,753, filed Dec. 19, 2002, entitled “Hermetically Sealed Optical Amplifier Module To Be Integrated Into A Pressure Vessel For Undersea Applications. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to the field of optical repeaters and more particularly to an optical repeater for use in an undersea optical communication system.  
         BACKGROUND OF THE INVENTION  
         [0003]    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.  
           [0004]    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.  
           [0005]    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.  
           [0006]    The large physical size of conventional repeaters increases their complexity and cost while creating difficulties in their deployment.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a hermetically sealed module to be located in an external pressure vessel providing protection from external pressure in an undersea environment. The hermetically sealed module includes at least one optical amplifier and an hermetically sealed housing for containing the optical amplifier. The housing has a retaining element for retaining the housing within the external pressure vessel. The module also includes a plurality of ports for conveying into the housing, in an hermetically sealed manner, at least one optical fiber and a conductor incorporated in an undersea optical fiber cable. The conductor supplies electrical power to the optical amplifier. At least one conductive terminal is located in the housing for establishing electrical contact with the conductor traversing each of the plurality of ports. The conductive terminal supplies electrical power from the conductor to the optical amplifier.  
           [0008]    In accordance with one aspect of the invention, a pressure seal is located between each of the ports and the conductor.  
           [0009]    In accordance with another aspect of the invention, the pressure seal is a polyethylene seal.  
           [0010]    In accordance with another aspect of the invention, the undersea optical fiber cable further includes an electrically insulating sheath surrounding the optical fiber and the conductor. The pressure seal is located between the port and the electrically insulating sheath.  
           [0011]    In accordance with another aspect of the invention, the conductive terminal includes a through hole traversed by the optical fiber.  
           [0012]    In accordance with another aspect of the invention, a ferrule is located in the through hole. The ferrule is traversed by the optical fiber and provides a hermetic seal therewith.  
           [0013]    In accordance with another aspect of the invention, an end portion of the optical fiber includes a metallized coating for soldering the optical fiber within the housing.  
           [0014]    In accordance with another aspect of the invention, the retaining element includes an adjustable expansion mechanism located on an outer surface of the housing for exerting pressure against an inner wall of the pressure vessel so that the housing is retained therein.  
           [0015]    In accordance with another aspect of the invention, the adjustable expansion mechanism includes a plurality of pivotable members.  
           [0016]    In accordance with another aspect of the invention, the adjustable expansion mechanism includes an alignment member for aligning the housing within the pressure vessel.  
           [0017]    In accordance with another aspect of the invention, the alignment member is selected from the group consisting of a boss, tab, tang and slot.  
           [0018]    In accordance with another aspect of the invention, the adjustable expansion mechanism provides continuous indexing variability.  
           [0019]    In accordance with another aspect of the invention, a gas fill port extends into the housing for supplying gas to an interior of the housing.  
           [0020]    In accordance with another aspect of the invention, a fiber tray is located in the housing for supporting optical fiber employed in the optical amplifier.  
           [0021]    In accordance with another aspect of the invention, a plurality of receptacles are provided which are sized to receive a passive optical component employed in the optical amplifier.  
           [0022]    In accordance with another aspect of the invention, the plurality of receptacles are integrally formed with said fiber tray.  
           [0023]    In accordance with another aspect of the invention, the optical amplifier includes a circuit board located in the housing.  
           [0024]    In accordance with another aspect of the invention, the optical amplifier includes at least one optically active element mounted to the circuit board. The optical amplifier comprises a rare-earth doped optical amplifier.  
           [0025]    In accordance with another aspect of the invention, the rare-earth doped optical amplifier includes a rare-earth doped fiber for imparting gain to an optical signal propagating therethrough, a pump source for supplying pump power to the rare-earth doped fiber, and a coupler for coupling the pump power to the rare-earth doped fiber. The rare-earth doped fiber and the coupler each reside in one of the plurality of receptacles. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 shows an example of a pressure vessel that can be inserted in a fiber optic cable for use in undersea optical telecommunication systems.  
         [0027]    [0027]FIG. 2 shows one embodiment of the optical amplifier module (OAM) constructed in accordance with the present invention after it has been assembled and sealed.  
         [0028]    [0028]FIG. 3 shows the OAM depicted in FIG. 2 as it is situated within the pressure housing.  
         [0029]    [0029]FIG. 4 shows the OAM of FIG. 2 with its outer cover removed.  
         [0030]    [0030]FIG. 5 shows the feed-through arrangement for providing the conductor tube and the optical fibers into the OAM in an hermetically sealed manner.  
         [0031]    [0031]FIG. 6 shows an end view of the OAM depicted in FIG. 2 as it is situated within the pressure housing.  
         [0032]    [0032]FIG. 7 shows a cross-sectional view through both the OAM and the pressure housing. 
     
    
     DETAILED DESCRIPTION  
       [0033]    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.  
         [0034]    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. 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.  
         [0035]    The present invention provides an optical amplifier module (OAM) for use in undersea optical communication systems. The OAM is designed to be located in a pressure vessel that is used to interconnect two fiber optic cables. The pressure vessel provides protection to the OAM from external sources of pressure and tension while the OAM provides a hermetic seal for the various components that are contained therein. One important advantage of the invention is that the OAM is a sealed device in which its operational details are not discernable, except through defined optical, electrical and mechanical interfaces. Thus, the party responsible for integrating the OAM within the pressure vessel only needs to connect it along these interfaces and the OAM will function to its design parameters. No other action needs to be taken by the integrator. In this way the OAMs, which generally contain complex electronic and optical components, can be built up as separate sub-assemblies from the mechanics of the pressure vessel, thereby providing more flexibility in manufacturing. Moreover, the integration between the pressure vessel and the OAM can take place in a different location from where the OAMs are manufactured, but since the OAM is a sealed functional unit, it can be transported and stocked without concern that its internal electronic and optical components will be damaged.  
         [0036]    [0036]FIG. 1 shows an example of a pressure vessel  100  that can be inserted in a fiber optic cable for use in undersea optical telecommunication systems. The pressure vessel includes a pressure housing  110  and cable termination units  114 . The cable termination units  114  provide mechanical, electrical and optical continuity to the outboard ends of the cable in which the pressure vessel is inserted. The cable termination units  114  each include a splice bottle  112  in which the fiber optic splice is located. The cable termination units  114  are bend limited to prevent cable damage. The pressure housing  110  primarily serves to protect the internal components from external pressure and is not necessarily hermetically sealed. One example of a pressure vessel  100  is available from NSW. The NSW pressure vessel is sometimes conventionally used to house a remote optically pumped amplifier (ROPA), in which the active components (e.g., the pump sources and associated electronics) are located on shore and only the passive optical components (e.g., the erbium doped fibers, coupler, and isolators) of the amplifier are located in the pressure vessel. That is, optical pump energy is provided to the pressure vessel from the shore so that the pressure vessel need not contain any components that require the provision of electrical energy. By contrast, in the present invention the entire optical amplifier, active and passive components included, are all located in the pressure vessel, thus requiring that electrical power be supplied to the pressure vessel.  
         [0037]    [0037]FIG. 2 shows the OAM  200  after its been assembled and sealed while FIG. 3 shows the OAM  200  as it is situated within the pressure housing  110 . FIG. 7 shows a cross-sectional view through both the OAM  200  and the pressure housing  110  when the OAM  200  is properly situated within the pressure housing  110 .  
         [0038]    The exemplary embodiment of the OAM  200  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. 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.  
         [0039]    [0039]FIG. 4 shows the OAM  200  with its outer cover removed to expose the internal components located within a housing  218 . As shown, a fiber tray  212  is located above a circuit board  210  that controls the EDFAs. The fiber tray  212  supports the various passive optical components of the EDFA and the excess fiber that interconnects them. The passive optical components (e.g., erbium doped fibers, couplers, isolators, and gain flattening filters) are located in slots  214  within the fiber tray  212 . The active optical components  216  (e.g., the pump lasers) are mounted directly on the circuit board  210 . The OAM housing  218  has a surface  220  that mates with the cover (shown in FIG. 2) to form a hermetic seal.  
         [0040]    Optical cables for use in undersea optical telecommunication systems generally include a conductor such as a copper tube to provide electrical power to the amplifiers. Means must therefore be provided to convey the electrical power into OAM  200 . In one embodiment of the invention the conductor tube itself penetrates directly into the OAM housing  218 . Accordingly, access into the OAM  200  must be provided for both the conductor and the optical fibers. Such access is provided through ports  222  located on opposing ends of the OAM housing  218 . The optical fibers (not shown in FIG. 4) extend within the conductor tube  230 , which in turn is encased in a polyethylene sheath  234  to electrically insulate the conductor tube  230 . The conductor tubes  230  extend to the splice bottles  112  seen in FIG. 1. The polyethylene sheath  234 , conductor tube  230  and optical fibers extend directly into the OAM  200  through the ports  222 . The conductor tube  230  terminates at a High Voltage (HV) terminal  232  located on the circuit board  210 . The conductor tube  230  is physically connected to the HV terminal  232  to provide good electrical communication between them. The HV terminal  232  is configured as a terminal block with a large surface area that provides good mechanical retention of the conductor tube  230  and a low resistance electrical connection.  
         [0041]    A polyethylene seal  236  is located in the ports  222  to provide a pressure seal between the polyethylene sheath  234  surrounding the conductor tube  230  and the OAM housing  218 . Since polyethylene outgases and does not provide a good seal against hydrogen, additional sealing means must be provided to ensure that the OAM  200  is hermetically sealed. As best seen in FIG. 5, a ferrule  238  resides within the conductor tube  230  and provides a hermetic seal. The four optical fibers that enter the OAM  200  extend through the ferrule  238  and can be sealed to the ferrule with epoxy. As an additional measure to ensure a hermetic seal, the ends of the optical fibers that extend into the OAM  200  through the ferrule  238  may be provided with a metal coating or metallized jacket so that they can be soldered in place.  
         [0042]    Returning to FIGS. 2 and 3, an expansion mechanism  240  resides on the outside of the OAM housing  218 . The expansion mechanism  240  allows the OAM  200  to be inserted into pressure housings (e.g., pressure housing  110 ) of various dimensions. The expansion mechanism  240  can be expanded or retracted to the appropriate size to frictionally engage with the inner wall of the housing  218 . The expansion mechanism  240  may be integrally formed with the OAM housing  218 . As best seen in FIG. 6, the two portions  246  of the expansion mechanism  240  that contact the inner wall of the housing are supported by pivots  242 . An expansion nut  244  drives threaded clevis pins outward into the two pivotable portions  246  of the expansion mechanism  240 , thereby applying pressure to the inner wall of the pressure housing  110 . One advantage of this expansion mechanism is that it is not required to satisfy the same tolerances that would otherwise be required if the OAM  200  were to engage the pressure housing  110  on its opposing ends. The particular expansion mechanism depicted in the figures provides infinite variability in indexing the OAM  200  with the pressure housing  110 . That is, the OAM  200  can be rotated within the pressure housing  110  and locked into any desired position by the expansion mechanism  240 . In other embodiments of the invention a positive alignment mechanism such as a boss, tab, tang or slot may be employed to provide a positive indexing means.  
         [0043]    At the completion of the OAM  200  assembly process, but before the OAM  200  is integrated into the pressure vessel, the various hermetic seals are put in place and the interior of the OAM is filled with nitrogen gas via a fill port  250  that is visible in FIG. 6.  
         [0044]    While the inventive module has been described in terms of an optical amplifier module, the invention more generally may be used to provide a hermetically sealed, functional unit that can be used not only for optical amplification, but for a wide variety of other undersea applications as well. For example, splices, filters, and surveillance sensors, or other electrically active components to which an optical signal is communicated may be located within the inventive module, which can subsequently be integrated into a pressure vessel that can withstand undersea environmental conditions.