Underwater Free Space Optics

Abstract

Radio waves propagate poorly in water and acoustics have been the dominant method for undersea communications, but its data rates are bandwidth limited. Free Space Optics potentially provides an alternative solution to acoustical communications with wider bandwidth and wireless flexibility that would benefit many undersea applications over short ranges.

Compared to atmospheric propagation, ocean waters are a more complex medium for light propagation presenting a very high attenuation, depending on a variety of different parameters and conditions. In this work, a model is investigated to that estimates the total attenuation of the light propagation in natural waters in the context of optical communications. Using a one parameter model of absorption and scattering, a general framework has been structured in MathCAD, where the absorption and scattering coefficients are iteratively calculated for a variable vertical chlorophyll profile. The beam spread function is implemented to estimate the total beam attenuation. Link budget simulations are also computed to address the feasibility of the underwater free space optical links. Laboratory experiments of light attenuation in sea water with red (633nm) and green (532nm) wavelengths corroborate simulated results obtained with the beam spread function within 25% error margin. Particulate scattering experiments with polystyrene latex spheres of sizes 500nm and 6μm were carried out to confirm its dependence on wavelength and particle size. These phase function of the measured experimental results are in agreement with the previously published data.