Upgrading WDM Submarine Systems to 40-Gbit/s Channel Bitrate

The introduction of wavelength division multiplexing (WDM) has triggered a tremendous capacity growth in submarine systems, both by the increase of the number of WDM channels and by the increase of the channel bitrate. Starting from 2.5 Gbit/s in the mid-1990s, the bitrate was upgraded to 10 Gbit/s by the end of the century in commercial products. The next generation of submarine systems will likely be based on a 40-Gbit/s bitrate. However, transmissions at a 40-Gbit/s rate are more challenging than transmissions at 10 Gbit/s. The goal of this paper is to provide an overview of the technologies which could be required or used in next-generation submarine systems. In the first part of this paper, an overview of the history of submarine links is provided. Then the technologies used in current N times 10 Gbit/s systems are described. Eventually, the challenges to overcome are discussed, whether they concern the type of fiber, the type of optical amplifier, or the nature of the modulation format