Abstract:
A pressure vessel joint for repeaters in submarine optical communication systems is described. The joint employs a breech ring which secures an endcap to the pressure vessel. The breech ring and pressure vessel have corresponding keys. The breech ring slides over the endcap and is rotated such that the breech ring keys engage the mating keys formed in the pressure vessel. The endcap is seated on a pre-loaded C-ring which seals the secured structure. Additional, redundant piston seals can also be provided to further prevent leakage.

Description:
BACKGROUND  
         [0001]    From the advent of the telephone, people and businesses have craved communication technology and its ability to transport information in various formats, e.g., voice, image, etc., over long distances. Typical of innovations in communication technology, recent developments have provided enhanced communications capabilities in terms of the speed at which data can be transferred, as well as the overall amount of data being transferred. As these capabilities improve, new content delivery vehicles, e.g., the Internet, wireless telephony, etc., drive the provision of new services, e.g., purchasing items remotely over the Internet, receiving stock quotes using wireless short messaging service (SMS) capabilities etc., which in turn fuels demand for additional communications capabilities and innovation.  
           [0002]    Recently, optical communications have come to the forefront as a next generation communication technology. Advances in optical fibers over which optical data signals can be transmitted, as well as techniques for efficiently using the bandwidth available on such fibers, such as wavelength division multiplexing (WDM), have resulted in optical technologies being the technology of choice for state-of-the-art long haul communication systems.  
           [0003]    For long haul optical communications, e.g., greater than several hundred kilometers, the optical signal must be periodically amplified to compensate for the tendency of the data signal to attenuate. For example, in the submarine optical communication system  10  shown in FIG. 1, the terrestrial signal is processed in WDM terminal  12  for transmission via optical fiber  14 . Periodically, e.g., every 75 km, a repeater  16  amplifies the transmitted signal so that it arrives at WDM terminal  18  with sufficient signal strength (and quality) to be successfully transformed back into a terrestrial signal.  
           [0004]    Conventionally, erbium-doped fiber amplifiers (EDFAs) have been used for amplification in the repeaters  16  of such systems. As seen in FIG. 2( a ), an EDFA employs a length of erbium-doped fiber  20  inserted between the spans of conventional fiber  22 . A pump laser  24  injects a pumping signal having a wavelength of, for example, approximately 1480 nm into the erbium-doped fiber  20  via a coupler  26 . This pumping signal interacts with the f-shell of the erbium atoms to stimulate energy emissions that amplify the incoming optical data signal, which has a wavelength of, for example, about 1550 nm. One drawback of EDFA amplification techniques is the relatively narrow bandwidth within which this form of resonant amplification occurs, i.e., the so-called erbium spectrum. Future generation systems will likely require wider bandwidths than that available from EDFA amplification in order to increase the number of channels (wavelengths) available on each fiber, thereby increasing system capacity.  
           [0005]    Distributed Raman amplification is one amplification scheme that can provide a broad and relatively flat gain profile over a wider wavelength range than that which has conventionally been used in optical communication systems employing EDFA amplification techniques. Raman amplifiers employ a phenomenon known as “stimulated Raman scattering” to amplify the transmitted optical signal. In stimulated Raman scattering, as shown in FIG. 2( b ), radiation from a pump laser  24  interacts with a gain medium  22  through which the optical transmission signal passes to transfer power to that optical transmission signal. One of the benefits of Raman amplification is that the gain medium can be the optical fiber  22  itself, i.e., doping of the gain material with a rare-earth element is not required as in EDFA techniques. The wavelength of the pump laser  24  is selected such that the vibration energy generated by the pump laser beam&#39;s interaction with the gain medium  22  is transferred to the transmitted optical signal in a particular wavelength range, which range establishes the gain profile of the pump laser.  
           [0006]    Although the ability to amplify an optical signal over a wide bandwidth makes Raman amplification an attractive option for next generation optical communication systems, the use of a relatively large number of high power pump lasers (and other components) for each amplifier in a Raman system has hitherto made EDFA amplification schemes the technology of choice for long haul optical communication systems. However, as the limits of EDFA amplification are now being reached, recent efforts have begun to explore the design issues associated with supplementing, or replacing, EDFA amplification technology with Raman amplification technology.  
           [0007]    In order to design a wideband, Raman-amplified optical communication system, however, a much larger number of active and passive optical and electrical components need to be housed in each repeater  16  than were previously needed in conventional submarine optical communication systems. Additionally, the amount of optical fiber, and the number of fiber splices, needed to interconnect the optical components will also increase dramatically. For example, Applicants have estimated that implementation of wideband, Raman-amplified optical communication systems may require repeaters which have 150-300 (or more) lasers, 500 to 800 (or more) passive optical components and 600-900 (or more) optical splices.  
           [0008]    Even as the number of components, length of fiber and amount of power needed to operate those components has increased, the physical size of the repeater  16  is restricted by, for example, operational, deployment, transportation and storage considerations. Thus, according to exemplary embodiments of the present invention, it is preferable to design structures and techniques for accommodating the aforedescribed optical components and fiber (as well as other components) within a repeater  16  having substantially the dimensions (in millimeters) illustrated in FIG. 3.  
           [0009]    Each repeater  16  typically also includes one or more removable endcaps  28 . Conventionally, these endcaps can be secured to the body of the repeater  16  using threads, bolted flanges or both (not shown in FIG. 3). However, repeater endcaps having threaded connections require a large amount of torque to install. Moreover, repeater endcaps using bolted flanges increase the outer diameter of the repeater by the width of the flanges, which is undesirable for repeaters with restricted size that have a large number of components to house within their inner diameter.  
           [0010]    Thus, it would be desirable to provide another method and structure for joining the endcaps of repeaters in submarine optical communication systems to their pressure vessels.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    These, and other, drawbacks, limitations and problems associated with conventional optical communication systems are overcome by exemplary embodiments of the present invention, wherein a pressure vessel is machined from a cylindrical section, without any bosses or flanges for endcap attachment. Instead, the endcap is secured using a breech ring. The assembly is sealed using, for example, a face seal and one ore more piston seals, all of which are embedded within the thickness of the pressure vessel. The piston seal(s) provide redundant sealing of the unit. The bell housing can also be secured to the pressure vessel using a keyed arrangement similar to that of the breech ring so that axial loads are passed directly to the pressure vessel from the bell housing.  
           [0012]    According to one exemplary embodiment of the present invention, a repeater includes a pressure vessel having a plurality of engaging tabs formed therein, an endcap; and a breech ring, having a plurality of engagement elements formed thereon, for securing the endcap to said pressure vessel. In this way, a secure, removable connection between the endcap and the pressure vessel is provided without increasing the outer diameter of the pressure vessel by using flanges or bosses, while at the same time permitting the axial load to pass directly from the bell housing to the pressure vessel.  
           [0013]    Repeaters and pressure vessel joints according to the present invention have a number of benefits over conventional structures. First, the joint is relatively simple to manufacture and assemble. Second, structures according to the present invention reduce the cost of material associated with manufacturing the pressure vessel by minimizing machining waste. Third, efficient load paths are created which transfer loads directly through the pressure vessel and avoid unloading the seals.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a schematic diagram of an optical communication system in which the present invention can be implemented;  
         [0015]    [0015]FIG. 2( a ) is a conceptual diagram of a conventional erbium-doped fiber amplifier;  
         [0016]    [0016]FIG. 2( b ) is a conceptual diagram of a conventional Raman amplifier;  
         [0017]    [0017]FIG. 3 is a depiction of a repeater showing dimensions thereof;  
         [0018]    [0018]FIG. 4 is a block diagram of an exemplary terminal unit of an optical communication system including link monitoring equipment according to exemplary embodiments of the present invention;  
         [0019]    [0019]FIG. 5 is a block diagram of an exemplary repeater of an optical communication system in which the present invention can be implemented;  
         [0020]    [0020]FIG. 6 is another block diagram of an exemplary repeater of an optical communication system including an exemplary Raman pumping architecture;  
         [0021]    [0021]FIG. 7 depicts the various shell layers associated with repeaters according to exemplary embodiments of the present invention;  
         [0022]    [0022]FIG. 8 illustrates an exploded view of one end of a repeater having a pressure vessel joint according to an exemplary embodiment of the present invention;  
         [0023]    [0023]FIG. 9 shows a cross-sectional view of a repeater having a pressure vessel joint according to an exemplary embodiment of the present invention;  
         [0024]    [0024]FIG. 10 is an enlarged, cross-section of the interior of a pressure vessel according to an exemplary embodiment of the present invention; and  
         [0025]    [0025]FIG. 11 illustrates a breech ring and breech ring installation tool according to an exemplary embodiment of the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0026]    In the following description, for the purposes of explanation and not limitation, specific details are set forth, such as particular systems, networks, software, components, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of known methods, devices and circuits are abbreviated or omitted so as not to obscure the present invention.  
         [0027]    Repeaters which enable high power Raman-amplified optical signal transmission systems can be employed in systems such as those depicted in FIG. 1, i.e., submarine optical communication systems, or in terrestrial systems. For the purpose of illustration, rather than limitation, an exemplary Raman-amplified system is described below for context. Those skilled in the art will appreciate that many different system configurations could also utilize repeater designs, structures and techniques according to the present invention.  
         [0028]    An exemplary architecture for terminal  12  and  18  is provided in the block diagram of FIG. 4. Therein, the long reach transmitters/receivers (LRTRs)  30  convert terrestrial signals into an optical format for long haul transmission, convert the undersea optical signal back into its original terrestrial format and provide forward error correction. The WDM and optical conditioning unit  32  multiplexes and amplifies the optical signals in preparation for their transmission over cable  34  and, in the opposite direction, demultiplexes optical signals received from cable  34 . The link monitor equipment  36  monitors the undersea optical signals and undersea equipment for proper operation. The line current equipment  38  provides power to the undersea repeaters  36 . The network management system (NMS)  40  controls the operation of the other components in the WDM terminal, as well as sending commands to the repeaters  36  via the link monitor equipment  36 , and is connected to the other components in the WDM terminal via backplane  42 .  
         [0029]    Functional blocks associated with an exemplary repeater  16  are depicted in FIG. 5. Therein, each fiber has a splitter  50  connected thereto to sample part of the traveling WDM data signal. The splitters  50  can, for example, be implemented as 2% couplers. A photodetector  52  receives the sampled optical signal from its respective splitter  50  and transforms the optical signal into a corresponding electrical signal. The photodetector  52  outputs the electrical signal to a corresponding sub-carrier receiver unit  54 , which detects and decodes the commands present in the sub-carrier modulated monitoring signal that has been modulated on the envelope of the WDM data signal. After decoding the command, the particular sub-carrier receiver  54  determines whether the decoded command is intended for it. If so, the action in the command is executed, e.g., measuring the power of the WDM signal, measuring the pump power output from one or more lasers in the pump assembly, or changing the supply current to the lasers of the pump assembly. To this end, the sub-carrier receivers  54  are connected to respective current control and power monitoring units (I settings)  56 , which each include pump power monitors and pump current controls for each laser in the associated pump laser assembly  58 .  
         [0030]    The pump modules  58  provide pump light into the optical fibers to amplify the data signals traveling therein using a Raman amplification scheme, as generally described above. The gain profile for a single pump wavelength has a typical bandwidth of about 20-30 nm. For high capacity WDM communication applications, such a bandwidth is too narrow and, accordingly, multiple pump wavelengths can be employed to broaden the gain profile. FIG. 6 depicts an exemplary pump architecture for providing multiple pump wavelengths in a Raman amplification scheme.  
         [0031]    Therein, a number N of pump radiation sources  110  are optically coupled to a respective one of N pump radiation combiners  112 . Each of the pump radiation sources  110  generate various pump wavelengths at various pump powers using individual radiation emitters  114 . The individual radiation emitters  114  can, for example, be lasers, light emitting diodes, fiber lasers, fiber coupled microchip lasers, or semiconductor lasers. The combiners  112  combine the various outputs of their respective pump radiation sources, e.g., by wave division multiplexing, and outputs the combined optical pumping signal to coupler  118 . Coupler  118  can be an N×M coupler which takes contributions from all N inputs to provide a representative output at each of M output ports. Energy from the coupler  118  is pumped into the optical fiber(s) via pump signal combiners  122 . In general, Raman pump architectures couple the light generated by pump lasers at various wavelengths and various powers to the optical fibers to pump the optical data signals. Those skilled in the art will appreciate that many other types of pumping architectures can be employed to provide Raman amplification to optical data signals in accordance with the repeater structures described below. For example, pumping architectures described in commonly assigned, U.S. patent application Ser. Nos. 09/832,175, 09/838,218, 09/838,594, and 09/865,440 filed on Apr. 11, 2001, Apr. 20, 2001, Apr. 20, 2001, and May 29, 2001, respectively, may also be used, the disclosures of which are incorporated here by reference.  
         [0032]    [0032]FIG. 7 illustrates the general shell structure of a repeater  16  according to the present invention from a layered perspective. Specifically, the cylindrical shell  200  is part of the pressure vessel that protects the interior structure and optical/electrical components from a multitude of stresses, both before and after deployment underwater. Desirable physical characteristics for the pressure vessel include high strength, good resistance to corrosion and good thermal conductivity, i.e., to aid in minimizing temperature rises within the repeater due to heat dissipation. Additionally, the pressure vessel should have an extremely low leak rate and provide a wide range of motion for the optical/power cable which enters therein. The pressure vessel can, for example, be fabricated from beryillium-copper alloys (e.g., 1.9% Be), although any materials having the aforementioned characteristics can be used. Pressure vessels according to the present invention can, for example, be approximately 1580 mm long, have an outer diameter of about 380 mm and a wall thickness of about 30 mm.  
         [0033]    Inside of the pressure vessel shell  200  is a dielectric layer  202  that electrically insulates the pressure vessel from the optical/electrical components housed therein. As mentioned above, Applicants anticipate that repeaters  16  operating in next generation, high power optical communication systems, e.g., Raman systems, may require more than 40 kV to be supplied thereto. Accordingly the dielectric layer  202  should have a relatively high breakdown voltage while at the same time having a high thermal conductivity. Selection of an appropriate dielectric material given the need for high breakdown protection and thermal conductivity in repeaters according to the present invention is discussed in U.S. Provisional Patent Application Serial No. entitled “Repeater Shell Structure for High Power Optical Communications”, filed on an even date herewith, the disclosure of which is incorporated here by reference. The thickness of the dielectric layer  202  can, for example, be about 6 mm.  
         [0034]    Inside of the dielectric layer  202 , resides an inner frame structure  203 . In the example of FIG. 7, this inner frame structure  203  is depicted in four sections  204 ,  206 ,  208  and  210 . However, as described below, the number of elements which make up the frame structure is not particularly important and it can be fabricated from more or fewer than four elements. If multiple elements are used for the inner frame structure  203 , then those elements can be separated by springs or wedge-locks (not shown in FIG. 7). The inner frame structure  203  provides an area within which the optical/electrical components are mounted, the optical fiber is run and the various power connections are made, to provide the optical architectures, e.g., those described above, for amplifying the optical signals passing through the repeaters  16 . Having provided an overview as to the various layers and components found in repeaters  16 , various features of repeater pressure vessels according to the present invention will be discussed individually below.  
         [0035]    Referring now to FIG. 8, an exemplary pressure vessel assembly according to an exemplary embodiment of the present invention is depicted in an exploded view. Therein, various components that are used to removably seal one end of the pressure vessel  200  can be seen. The elements shown therein are designed to seal the interior of the pressure vessel  200  against water, while at the same time provide an entry/exit for the power cable and optical fibers (not shown in this figure). The pressure vessel assembly includes a C-ring  210 , a pressure vessel endcap  212  with at least one gasket  214 , a breech ring  216  having engagement elements  230 , a bell housing  218  having engagement elements  236  and a gimbal  220 .  
         [0036]    [0036]FIG. 9 depicts a cross-sectional view of the elements of FIG. 8 in their assembled state. Therein, it can be seen that the breech ring  216  removably secures the pressure vessel end cap  212  within pressure vessel  200  by engaging tabs fabricated within the pressure vessel  200  and a lip formed on the outer surface of pressure vessel endcap  212 . The lip of the pressure vessel endcap  212  is, in turn, pressed against the C-ring  210  that is seated in the pressure vessel  200 , as well as a shelf  213  formed in the pressure vessel. FIG. 9 also illustrates the penetration of the power cable/optical fibers  224  into repeater  16  through the gimbal  220  at pigtail  222  and, subsequently, through the pressure vessel endcap  212  via waterblock  226 . The gasket  214  is, in this example, provided as two piston seals, e.g., to provide redundant sealing against water entry into the pressure vessel  200 . Those skilled in the art will appreciate that more or fewer than two such seals may be used. The distribution of the engagement features around the circumference of the pressure vessel in which the C-ring  210  and gasket  214  (wedge pack piston seals) are located is designed to avoid concentration of stresses caused by depth pressure when the repeater  16  is deployed. This, in turn, maintains the C-ring  210  and gasket  214  in their loaded states.  
         [0037]    The engagement features of the pressure vessel  200  that co-operate with the breech ring  216  to lock the pressure vessel endcap  212  in place can be seen in FIG. 10. Therein, a series of indexed tabs  228  are formed inside pressure vessel  200 . The tabs  228  and corresponding engagement elements  230  on the breech ring  216  can be tapered, e.g., at about 2 degrees, to promote a compression fit between the tabs  228  and engagement elements  230  when assembled. Tabs  228  and breech ring  216  maintain a large force (e.g., 1000 lb/inch) on C-ring  210  to create a low leak rate pressure vessel. A similar set of tabs  234  can be provided axially outwardly of the tabs  228  inside of pressure vessel  200  to lock bell housing  218  into place using its corresponding engagement elements  236 . Once this is accomplished, the bell housing can be further secured by pins (not shown) that are inserted through holes  260  in the bell housing  218  and corresponding holes  262  in the pressure vessel  200  (FIG. 8). Those skilled in the art will appreciate that other locking mechanisms can be used as alternatives. These pins provide resistance against any torsional loads that may be encountered during deployment of the repeater  16 . Also seen in FIG. 10 is the seat  232  for the C-ring  210 .  
         [0038]    To secure the pressure vessel endcap  212  in place, a breech ring installation tool  240  (FIG. 11) can be used. The breech ring tool  240  has a series of indents  242  and tabs  244  which mate with corresponding tabs  246  and indents  248  on an upper side of the breech ring  216 . With the pressure vessel endcap  212  seated against C-ring  210  and shelf  213 , the breech ring installation tool is inserted into mating relationship with breech ring  216  and a suitable amount of torque is applied to rotate the breech ring  216  such that engagement elements  230  are positioned under the indexed tabs  228 . In accordance with one exemplary embodiment of the present invention, the C-ring  210  is pre-loaded with a press (e.g., a 20+ ton arbor press, not shown) during the breech ring locking operation. In this way, the amount of torque needed to secure the breech ring is relatively low. Accordingly, the breech ring tool  240  can be manually operated using handles  250  and  252 . Those skilled in the art will appreciate that although a manual operation is discussed and illustrated here, that breech ring installation tool  240  could also be implemented using an automated power source.  
         [0039]    Manufacturing of the aforedescribed elements of pressure vessel assemblies according to the present invention can be accomplished in a number of different ways. For example, the pressure vessel  200  and endcap  212  can be forged and post machined. Since no flanges are provided for endcap attachment, extrusions and rolled rings can be used for the cylindrical section from which the pressure vessel is machined, which reduces machining waste associated with flanged forged pressure vessels. The bell housing  218  can be centrifugally cast or sand cast and post machined. The breech ring  216  can be ring forged or machined from plate metal stock.  
         [0040]    The preferred embodiments have been set forth herein for the purpose of illustration. However, this description should not be deemed to be a limitation on the scope of the invention. For example, although the foregoing exemplary embodiments depict the endcap as having a generally hemispherical shape, those skilled in the art will appreciate that the endcap could be elliptical or flat. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the scope of the claimed inventive concept.