Performance assessment of Laguerre–Gaussian OAM modes for high-capacity underwater optical wireless communication

In this paper, a four Laguerre–Gaussian modes ( LG 0 0 LG₀⁰ , LG 18 0 LG₁₈⁰ , LG 45 0 LG₄₅⁰ , and LG 75 0 LG₇₅⁰ ) based underwater optical wireless communication (UOWC) system for transporting 10 Gbps data per mode is presented. The proposed link is modeled and evaluated for 5 distinct water types namely Pure sea water, Clear ocean, Coastal ocean, Harbor I, and Harbor II. The effect of the beam divergence angle of 1.0 mrad, 1.1 mrad, and 1.2 mrad is evaluated at the maximum evaluated ranges using log(BER), Q factor, and eye diagrams. It is found from the results that the performance of the link decreases with increasing turbidity of water, and the Pure sea water gives the largest transmission range, whereas Harbor II gives the least transmission range. In every type of water, the smallest beam divergence of 1.0 mrad consistently produces the best performance and 1.2 mrad produces the highest attenuation and poorest signal quality. The results show that OAM multiplexing is an effective method to increase the capacity of underwater links with acceptable performance in moderately clear channels. The results also demonstrate the need for beam divergence optimization in order to make the system robust and extend its underwater range of operation.