Underwater turbulence on scintillating-fiber based omnidirectional underwater wireless optical communication system

Abstract

The underwater wireless optical communication (UWOC) technology is vastly developing due to its advantages of high bandwidth, large capacity, and low latency. However, the complex underwater channel characteristics and strict requirements on pointing, acquisition, and tracking (PAT) systems hinder the performance and augmentation of UWOC. A large-area scintillating-fiber-based UWOC system is proposed to solve the PAT issue while offering high-speed, omnidirectional data detection over turbulent underwater channels. In this work, we utilized 120-cm² coverage area scintillating fibers as a photoreceiver. The large area scintillating fibers realize omnidirectional signal detection by absorbing an incident optical radiation, re-emitting it at a longer wavelength, and then guided to the end of the fibers connected with an avalanche photodetector. The UWOC system offers a 3-dB bandwidth of 66.62 MHz, and a 250 Mbit/s data rate is achieved using non-return-to-zero on-off keying (NRZ-OOK) modulation. The system was tested over a 1.5-m underwater channel under turbulences of air bubbles, temperature, salinity, and turbidity. We generated bubbles by blowing 0.20, 0.63, and 1.98 mL/s speeds of Nitrogen gas flow. A temperature gradient of 1.33 and 2.67 Celsius/m was introduced by circulating warm and cold water at the two tank ends, respectively. Salinity concentrations at 35 and 40 ppt were introduced to emulate the salinity in the Red Sea. Lastly, different volumes of Maalox^TM were added into pure water to emulate pure sea, coastal ocean, and turbid harbor water. The fiber-based UWOC system operates under those turbulence conditions with error-free communication and 0% outage probability.