Study on Laser Propagation Characteristic of Downlink Channel in Non-uniform Seawater

In underwater optical communications, accurate evaluation concerning the optical propagation characteristic in seawater is crucial to the design of the communication system. The authors studied the distribution and loss properties of light traveling in seawater using Monte Carlo simulation method for the complex and changeable ocean communication channel. Assuming that the laser intensity is with Gaussian distribution, according to the Argo (Array for Real-time Geotropic Oceanography) data, the non-uniform seawater within 200 meters is layered vertically from the surface down, down, and the scattering and absorption of each layer is deduced. We studied the relation among the spot profile in receiving plane, emitted photons, transmission distance, and attenuation coefficient by using tracking the photon transmission process, and its correction is verified by using Monte Carlo simulation. Experimental results demonstrate that the scattering times of photons in seawater increase with the scattering distance and attenuation coefficient, and result in the enlargement of the receiving spot. Besides, if the seawater is considered as non-uniform, then the spot energy of the downlink is more concentrated than that in uniform seawater. In other words, the beam energy is more concentrated and suitable for long-distance transmission in a vertical direction. Therefore, considering the seawater as non-uniform medium and layering it vertically will make the evaluation of light transmission characteristic closer to practice.