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 and 2.47×10, 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 is also achieved in the uplink.

The transmission distance of underwater wireless optical communication (UWOC) is severely limited by the rapid decay of light intensity in water. Power-efficient pulse position modulation (PPM) and ultra-sensitive multi-pixel photon counter (MPPC) open the door toward designing long-reach UWOC systems. In this paper, a 46-m UWOC system based on PPM and MPPC was proposed and experimentally demonstrated with ultra-low transmitting power into the underwater channel. Clear eye diagrams without any slot error for ten different PPM signals were obtained in the 46-m experiment with data rates of Mbps level. The received optical power was as low as -39.2 dBm for the 10-MHz 4-PPM signal, when the laser worked under the stimulated state. Meanwhile, the received optical power can be reduced to -62.8 dBm, for the 5-MHz 64-PPM signal when the laser worked under the spontaneous state.

In this work, for the first time, we uncover that the level of security we have traditionally taken for granted on underwater wireless optical communication (UWOC) may not always be there. We first numerically investigate the security weaknesses of UWOC via Monte Carlo simulation. With the link distance increasing or the water becoming more turbid, the simulation results indicate that the possibility of information leakage increases, which may pose a great threat to the security of UWOC. By using a high-sensitivity multi-pixel photon counter (MPPC) placed aside the water tank, a 5-MHz square wave signal is successfully tapped at 1-m, 3-m, and 5-m underwater transmission distances, which preliminarily verifies the probability of information leakage. We further experimentally demonstrate an UWOC system with potential eavesdropping employing a 2.5-Gb/s orthogonal frequency division multiplexing (OFDM) signal. After transmitting through a 15-m underwater channel, the OFDM signal is eavesdropped by a mirror at 7.8 m. Both the normal receiver at 15 m and the eavesdropping receiver at 7.8 m can achieve a bit error rate (BER) below the forward error correction (FEC) limit of 3.8 × 10, which validates that UWOC indeed suffers potential safety hazard.