Single-pixel, single-photon underwater laser communication with a coiled-fiber receiver.

Underwater wireless optical communication (UWOC) offers high data rate and low latency, making it an important complement to underwater acoustic communication. In particular, single-photon UWOC (SP-UWOC) has attracted increasing attention because of its high sensitivity and potential for long-range transmission. However, the dead time of a single-photon detector makes it difficult to achieve effective pulse recognition and communication with a single-pixel detection scheme. In addition, the highly collimated laser beam, water turbulence, and underwater platform motion impose stringent optical alignment requirements between the transmitter and receiver. Here, a single-pixel SP-UWOC based on a coiled-fiber receiver is proposed and demonstrated. By using a coil of bare optical fiber as the receiver, the proposed system not only relaxes the alignment requirement between the transmitter and receiver but also exploits the position-dependent propagation delays along the coiled fiber to temporally spread simultaneously incident photons. This enables a single-pixel single-photon detector to receive and resolve multiple photons within a single transmitted pulse, thereby achieving effective communication. Experimental results show that, with a bit error rate (BER) below the forward error correction (FEC) threshold of 3.8 × 10-3, the proposed system supports a data rate of 0.8 Mbps over an 85 m link at an attenuation length (AL) of 4.65 m and over a 9.6 m link at an AL of 0.89 m. These results demonstrate the feasibility of the proposed approach and highlight its potential for long-range, robust, and alignment-relaxed UWOC.