We experimentally demonstrate a high-speed air-water optical wireless communication system with both downlink and uplink transmission employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) and a single-mode pigtailed green-light laser diode (LD). This work is an important step towards the future study on optical wireless communications between underwater platforms and airborne terminals. Over a 5-m air channel and a 21-m water channel, we achieve a 5.3-Gbps transmission without power loading (PL) and a 5.5-Gbps transmission with PL in the downlink. The corresponding bit error rates (BERs) are 2.64×10⁻³ and 2.47×10⁻³, respectively, which are below the forward error correction (FEC) criterion. A data rate of 5.5 Gbps with PL at a BER of 2.92×10⁻³ is also achieved in the uplink.

We first study the transmission property of red light in water in terms of extinction coefficient and channel bandwidth via Monte Carlo simulation, with an interesting finding that red light outperforms blue-green light in highly turbid water. We further propose and experimentally demonstrate a broadband underwater wireless optical communication system based on a simple and cost-effective TO56 red-light laser diode. We demonstrate a 1.324-Gb/s transmission at a bit error rate (BER) of 2.02 × 10⁻³ over a 6-m underwater channel, by using 128-QAM OFDM signals and a low-cost 150-MHz positive-intrinsic-negative photodetector, with a record spectral efficiency higher than 7.32 bits/Hz. By using an avalanche photodetector and 32-QAM OFDM signals, we have achieved a record bit rate of 4.883 Gb/s at a BER of 3.20 × 10⁻³ over a 6-m underwater channel.

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 × 10⁻³, 2.0 × 10⁻³ and 2.3 × 10⁻³, respectively, which are below the forward error correction (FEC) threshold of 3.8 × 10⁻³.

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In this letter, we propose and experimentally demonstrate an underwater wireless optical communication system based on a simple and cost-effective TO56 transversal multi-mode green-light laser diode (LD) and a low-cost 150-MHz positive-intrinsic-negative detector. We use quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing modulation, with fine frequency granularity and high spectral efficiency, to tackle the challenging issue imposed by the ragged and uneven frequency response of the green LD within a limited bandwidth. We have demonstrated that a bit rate of 1.118 Gb/s (net bit rate: 0.927 Gb/s) can be achieved at a bit-error-rate of 2.98 × 10 ^-3 , with the assistance of bit loading (using both 16-QAM and 256-QAM signals). A spectral efficiency as high as 6.18 bit/s/Hz is also obtained in the proposed underwater wireless optical communication system.