Characterization of the Beam-Spread Function for Underwater Wireless Optical Communications Links

Optical links are currently being considered for high-bandwidth underwater communications at short ranges (<100 m). To predict the performance of these links, a firm understanding of how the inherent optical properties of water affect the encoded optical signal is needed. Of particular interest is the impact of scattering due to particulates. Typically, the link loss is computed using the beam attenuation coefficient, which describes the attenuation of nonscattered light due to absorption and scattering. This approach is insufficient, as it neglects the contribution of scattered light to the total received signal. Given the dynamic nature of underwater platforms, as well as the dynamic nature of the environment itself, knowledge of the angular dependence of forward-scattered light is imperative for determining pointing and tracking requirements as well as overall signal to noise. In this work, the theory necessary to describe spatial spreading of an optical beam in the presence of scattering agents underwater is reviewed. This theory is then applied to a performance prediction model that is validated via laboratory experiments. Finally, the model is used to study the impact of spatial spreading on an underwater optical link.