FEATURE
FIBER TECHNOLOGY FOR SUBMARINE SDM CONTEXT
BY SERGEI MAKOVEJS
launch power into the fiber to achieve highest signal-to-noise Repeatered submarine optical fiber systems are unique ratio (SNR) for any given transmission reach and data rate in the sense that they have inherent limitations on how requirements. Figure 1 shows an example of what such an opmuch electrical power can be remotely delivered from the timization process may look like for a transpacific 10,000 km shore to submarine cable repeaters. This is different to link with 60 km repeater spacing for the two fiber types. Both terrestrial fiber systems, in which there are no fundamental fibers have the same typical 1550 nm attenuation of 0.150 limitations on how much power can be brought locally to dB/km, but different effective areas (Aeff ) – 115 µm2 and power transmission equipment, although the cost of power 150 µm2 (also typical at 1550 nm). Due to its larger Aeff, 150 delivery can vary. Electrical power constraint is also the µm2 fiber allows for a 1 dB increase in optimum launch power reason why submarine systems historically operated with relative to 115 µm2 fiber resulting in 1 dB SNR increase. a relatively low number of fibers, typically 12 fibers or less, The quest for higher capacity per fiber is exactly why ultraconsistent with the limit on the number of amplifiers that low-loss 150 µm2 fiber has been the primary choice for many can be powered from the constrained source at the shore. repeatered submarine links deployed over the last few years, Due to such a limited number of fibers in the cable, the or those that are currently in the final years of construction. fiber has always been considered as Perhaps one of the most prominent a precious resource, and maximizaexamples where 150 µm2 fiber was tion of capacity per fiber has hisused was MAREA – a transattorically been a priority. As a result, lantic submarine route co-owned submarine links have always relied by Facebook and Microsoft. At on the latest innovations in systems the time MAREA was designed and fibers such as, the lowest atit represented one of the most tenuation silica-core fibers with the technologically advanced submalargest possible effective area, and rine projects. However, the conacceptable microbend and macrostantly evolving macroenvironment bend performance. To maximize requires to continue to increase capacity per fiber (referred to as the transoceanic bandwidth at an “traditional” approach throughout unprecedented rate. As a result, the this article), the goal of submarine current belief within the industry Fig. 1 Signal-to-noise ratio (SNR) as a function of fiber launch power per wavelength channel for a 10,000 km link with 60 km span length network engineer is to optimize is that traditional ways to design
28
SUBMARINE TELECOMS MAGAZINE