REAPING THE BENEFITS using an all-Raman terrestrial amplified transmission technology in unrepeatered applications Michel W. Chbat and Herve A. Fevrier Xtera Communications, Inc.,
Bridging very long and/or high-loss optical transmission links without a possibility of a repeater has long presented technical and engineering challenges, making unrepeatered systems a distinct category of optical transmission systems. A large number of unrepeatered systems have now been deployed in all parts of the world, with distances up to a few hundred kilometers and capacities of several Gb/s. Historically, the main applications of unrepeatered systems were in coastal festoons, access to or between islands, or lake crossings, all categorized as submarine applications. To that end, the designs of unrepeatered systems have been made following a “submarine link philosophy”. However, unrepeatered transmission is also part of terrestrial networks that have to deal with uneven span distributions, very remote ILA sites (generator or solar powered), exiting or avoiding expensive leased facilities and costly building additions. Submarine unrepeatered applications considerably differ from their terrestrial counterparts in that the former usually involves standalone links where the cable and fiber types are chosen independently of any other network, whereas the latter usually deals with a uniform fiber type throughout the network. For example, submarine unrepeatered links are implemented with a low-loss pure-silica core fiber (PSCF), while terrestrial links can have a large variety of fiber types, differing by their physical characteristics [standard single-mode fiber (SSMF), non-zero dispersion shifted fiber (NZ-DSF) types, dispersion-shifted fiber (DSF)]. An unrepeatered system should be able to support both submarine and terrestrial categories.
In general, unrepeatered systems have to deal with two main physical layer issues: total loss and chromatic dispersion. On the operational side, the major considerations are: safety, equipment reliability, and density. Above all, cost is undoubtedly the largest differentiator among the systems. Here, we will see how the use of a proved-in terrestrial technology can be applied to submarine and terrestrial unrepeatered systems and provide a low-cost, high-density, and high-reliability system, with superior transmission performance on various fiber types. Raman amplification has been widely used in terrestrial and submarine systems in order to increase the overall transmission reach and/or bandwidth. Lumped Raman amplification (LRA) – also referred to as discrete Raman amplification – has been shown to enable the access to previously unused spectral windows, e.g., S-band, since the amplification bandwidth only depends on the wavelengths of the pump sources. Distributed Raman amplification (DRA) is usually used to improve the system noise performance by effectively reducing the amplified spontaneous emission (ASE) noise contribution of each line amplifier, leading to an increase in the transmission distance. This is achieved by allowing the operation at reduced power levels while maintaining the received optical signal-to-noise ratio (OSNR), thus also reducing the impact of optical nonlinear effects, and further increasing the transmission distance. A tight integration of DRA and LRA has been implemented in a terrestrial amplified ultra-long-haul transmission system, leading to several important
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