Laser sources enable highly efficient optical communications links due to their ability to be focused into very directive beam profiles. Recent atmospheric and space optical links have demonstrated robust laser communications links at high rate with techniques that are applicable to the undersea environment. These techniques contrast to the broad-angle beams utilized in most reported demonstrations of undersea optical communications, which have employed LED-based transmitters. While the scattering in natural waters will cause the beam to broaden, a narrowly directive transmitter can still significantly increase the optical power delivered to a remote undersea terminal. Using Monte Carlo analysis of the undersea scattering environment, we show the two main advantages of narrow-beam optical communication: increased power throughput and decreased temporal spread. Based on information theoretic arguments, gigabit-per-second class links can be achieved at 20 extinction lengths by utilizing pulse position modulation, single-photon-sensitive receivers, and modern forward error correction techniques.
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