The availability of the underwater wireless optical communication (UWOC) based on red (R), green (G) and blue (B) lights makes the realization of the RGB wavelength division multiplexing (WDM) UWOC system possible. By properly mixing RGB lights to form white light, the WDM UWOC system has prominent potentiality for simultaneous underwater illumination and high-speed communication. In this work, for the first time, we experimentally demonstrate a 9.51-Gb/s WDM UWOC system using a red-emitting laser diode (LD), a single-mode pigtailed green-emitting LD and a multi-mode pigtailed blue-emitting LD. By employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) modulation in the demonstration, the red-light, the green-light and the blue-light LDs successfully transmit signals with the data rates of 4.17 Gb/s, 4.17 Gb/s and 1.17 Gb/s, respectively, over a 10-m underwater channel. The corresponding bit error rates (BERs) are 2.2 × 10, 2.0 × 10 and 2.3 × 10, respectively, which are below the forward error correction (FEC) threshold of 3.8 × 10.

With the increasing demands for underwater monitoring and military applications, underwater wireless optical communication (UWOC) is desired to be an alternative approach to provide higher data rate than acoustic communication. Twisted light carrying orbital angular momentum (OAM) has recently gained increasing interest in diverse areas, especially in free-space and fiber-based optical communications. OAM-based UWOC between underwater and aerial users, a promising technique to enable a variety of applications, which however, has not yet been reported so far. Here we experimentally demonstrate an adaptive water-air-water data information transfer using OAM. According to the feedback information of the received intensity distribution, the reflection element is adjusted for mitigating the misalignment-induced degradation effect due to water level change. The experimental results show favorable performance of the feedback-assisted water-air-water twisted light data information transfer.

Spatial modes have attracted increasing interest in free-space and fiber-based optical communications. Underwater wireless optical communication is becoming a promising technique in marine exploration. Here we investigate the underwater wireless optical communications using different spatial modes, i.e., traditional Gaussian modes, orbital angular momentum modes having helical phase fronts, and diffraction-free and obstruction-tolerant Bessel modes. We evaluate the underwater transmission performance of three spatial modes subjected to dynamic bubbles, which cause similar power fluctuations, regardless of spatial modes. We also demonstrate an underwater transmission link subjected to static obstructions using three spatial modes carrying 1.4 Gbaud orthogonal frequency division multiplexing 16-ary quadrature amplitude modulation (16-QAM) signals. The Bessel mode shows the best performance against obstructions.