To date, underwater optical communications using blue or blue-green lasers or LEDs are considered optimal to achieve high bandwidth and high data transmission rate. State-of-the-art wireless systems using single laser diodes can reach a transmission rate of 2.7 Gbit/s at a distance up to 34.5 m. Recently, researchers at Fudan University (Shanghai, China) and the University of Toronto (Toronto, ON, Canada) have demonstrated red/green/blue (RGB) laser-diode systems with transmission rates up to 3.4 Gbit/s at distances up to 2.3 m for a single laser diode and aggregate 9.7 Gbit/s using an on-off keying (OOK) modulation scheme for RGB laser-diode-based wavelength-division multiplexing (WDM). Furthermore, the multiplexed configuration also offers an RGB white-light underwater illumination capability that is not possible with single-laser-diode systems. 

Using WDM technology, both underwater communications and underwater solid-state lighting are possible to cover a range of subsea explorations. The experiment used commercial gallium-nitride (GaN)-based 450 nm (blue) and 520 nm (green) laser diodes, as well as an aluminum-gallium-indium-phosphide (AlGaInP) 660 nm (red) laser diode in standard off-the-shelf packages, stabilized to around 25°C using a Peltier thermoelectric cooler. In addition to demonstrating high-bit-rate communication rates, the neutral-density filters and diffusers were used to balance the color outputs of the laser diodes to produce high-quality white light simultaneously with the communications channel, with respectable CIE color values and up to 7084 lux of illumination. Reference: X. Liu et al., Opt. Express, 26, 15, 19259–19274 (2018).