Demodulation of Amplitude-Modulated Beam Propagating through Seawaters

The modulation transfer function of an amplitude modulated signal propagating through seawater is explored. This function describes the dependence of the signal modulation depth on modulation frequency in the propagation process and is the most important factor determining the optical communication quality. The experimental data received as the laboratory simulation of the radiation transfer through seawaters are compared to results of the theoretical studies. The deployed experimental set-up and media simulating the optical characteristics of seawaters are briefly introduced. Theoretical studies are performed in the framework of the multi-component approach along with the small-angle diffusion approximation of the radiative transfer theory using the software CLIW (Communication with Light In Water) implementing this theory. It is shown that the theoretical approach deployed in the CLIW software ensures the satisfactorily description of the propagating amplitude-modulated signal characteristics and estimation of the parameters of a communication channel in seawaters at sufficiently large distances from the optical source. Beside it is noted there is a need to know the media phase function in a fairly wide range of angles for such comparisons. In this work the effect theoretically predicted in 1971 was experimentally confirmed for the first time. This effect consists in the fact that the asymptotic deep mode for the alternative component of the signal can take place only for a certain range of frequency modulation values. The maximum angular intensity distribution for the alternative component of the signal takes place not in the «forward» direction but at a certain observation angle q > 0°. The position of this maximum depends on the frequency modulation and optical characteristics of medium.

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