Ultraviolet Band Based Underwater Wireless Optical Communication

Abstract

Underwater wireless optical communication (UWOC) has attracted increasing interest for data transfer in various underwater activities. However, the complexity of the water environment poses considerable challenges to establish aligned and reliable UWOC links. Therefore, solutions that are capable of relieving the requirements on positioning, acquisition and tracking (PAT) are highly demanded. Different from the conventional blue-green light band utilized in UWOC, ultraviolet (UV) light is featured with low solar background noise, non-line-of-sight (NLOS) and good secrecy. The proposed work is directed towards the demonstration and evaluating the feasibility of high- speed NLOS UWOC for easing the strict requirement on alignment, and thus circumvent the issues of scintillation, deep-fading, and complete signal blockage presented in conventional LOS UWOC. This work was first started with the investigation of proper NLOS configurations. Path loss (PL) was chosen as a figure-of-merit for link performance. With the understanding of favorable NLOS UWOC configurations, we established a 377-nm laser-based, the first-of-its-kind NLOS UWOC link. The practicality of such NLOS UWOC links has been further tested in a field trial. Besides the underwater communication links, UV-based NLOS is also appealing to be the link for direct communication across the wavy water-air interface. Investigations for such a direct communication link have been carried out to study data rate, coverage and robustness to the dynamic wave movement, based on the performance of different modulation schemes, including non-return-to-zero (NRZ)-OOK and quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM). Further this study, an in-Red Sea canal field in-situ test has been conducted, showing strong robustness of the system. In addition, an in-diving pool drone-aided real-application deployment has been carried on. The trial results indicate link stability, which alleviates the issues brought about by the misalignment and mobility in harsh environments, paving the way towards real applications. Our studies pave the way foreventual applications of UWOC byrelieving the strict requirements on PAT using UV-based NLOS. Such modality is much sought-after for implementing robust, secure, and high-speed UWOC links in harsh oceanic environments.