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.
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.
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.
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.
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 xe2x80x9cstimulated Raman scatteringxe2x80x9d 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""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.
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.
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.
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.
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.
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.
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.
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.
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.