Link performance evaluation for air-sea free-space optical communications

Effective communication between underwater platforms and aerial platforms has been a challenging issue in a long-time, due to the fact that either acoustic waves or electromagnetic waves can efficiently transmit only in the sea water or air, rather than both. As laser beams are able to penetrate a decent depth of sea water, free-space optical communications (FSOC) is considered to be a good substitutive approach. As is well known, the attenuation caused by absorption and scattering has proved to be the most significant adverse factor for underwater laser propagation, which, however, can only be compensated by a larger power margin. Nonetheless, even if the launching power is large enough to allow for affordable receiver sensitivity, the intensity fluctuation induced by atmospheric and oceanic turbulence can degrade the link performance to a great extent. This study addresses the turbulence effects on FSOC links between an underwater vehicle and an aerial platform. By use of wave optics simulation (WOS), the propagation of both the Gaussian beams and the annular beams in an air-sea two-section link is examined. The difference in performance between the uplink and the downlink is compared and explained according to numerical results. Generally, uplink suffers more from turbulence because the majority of turbulence lies nearer to its transmitter. Moreover, it is found that an annular beam always delivers a smaller scintillation index and a greater signal-to-noise ratio. This study is supposed to benefit the research and development of future air-sea optical communication systems.