Abstract

The transmission distance, alignment requirement and stability of underwater wireless optical communication (UWOC) systems are usually difficult to trade off. In this article, to ensure that the optical spot at the receiver end has sufficient optical power and uniform power distribution, the transmitter of a UWOC system is designed based on a single-color light-emitting diode (LED) array with a beam shaping system. The lens group-based beam shaping scheme not only extends the transmission distance, but still maintains the easy-alignment nature and stability of the LED-based UWOC link. At the receiver, a diversity reception scheme with an array of photomultiplier tubes (PMTs), and a high-gain scheme combining a PMT with an optical antenna, are adopted and compared. To alleviate the inter-symbol interference (ISI) and compensate for the nonlinear distortion of the signal, hardware pre-equalization and nonlinear post-equalization based on Volterra are adopted at the transmitter and receiver, respectively. A UWOC transmission with a distance of 50 m and a data rate of 300 Mbps is realized, which is the highest rate-distance product realized by an ordinary monochromatic LED-based transmitter ever reported. The stability and practicability of the proposed UWOC system with relaxed alignment requirement are verified in laboratory and swimming pool, as well as in a real harsh sea environment.