Use of Gaussian beam divergence to compensate for misalignment of underwater wireless optical communication links

The vast majority of underwater wireless optical communication systems use collimated blue/green laser beams to deliver high-speed data over a transmission range of a few metres to tens of metres. However, such systems are extremely susceptible to misalignment of the transmitter and the receiver. The mitigation techniques for misalignment reported in the literature are complex and costly at times. In this study, the authors consider the simple approach of increasing the divergence angle of the transmitted Gaussian beam to mitigate misalignment. Both plane and spherical beams are considered as the limitation cases. Using Monte Carlo simulations, the authors show that an optimum divergence angle for the maximum acceptable lateral offset exists with respect to the receiver sensitivity in clear waters while this is not an efficient method in harbour waters. Results demonstrate that there is a design trade-off between acceptable lateral offset, power loss and channel bandwidth. Furthermore, the authors show how the proposed scheme of beam divergence affects the maximum allowed link span as well as the channel bandwidth for a given distance.