Experimental demonstration of half-duplex underwater wireless optical communication based on a single-photon avalanche diode

Underwater wireless optical communication (UWOC) has attracted considerable attention recently because of its high communication speed. In applications, it is important to improve the receiving sensitivity of UWOC to increase communication distance, as there is significant attenuation underwater. To address this issue, the photon-counting method is considered useful because it can detect weak light at the photon level. However, the output signals from such a detector usually are discrete voltage pulses because light is extremely weak and quantum property is quite evident. This brings difficulty to data recovery. In this work, a low-pass filter (LPF) was used to convert these discrete pulses into continuous signals to identify information with the traditional method. Its performance was demonstrated in a half-duplex UWOC system, where two single-photon avalanche diodes (SPADs) were used as detectors. In the system, the modulation rate of communication light is 2 megabits per second (Mbps) with the OOK format. The photon-counting rate of each SPAD is 40 mega-counts per second (Mcps), and the bandwidth of each LPF is 2 MHz. The results show that, by this system, files can be sent at a speed of 278.78 Kbps from one computer to another via the file transfer protocol (FTP) when the average numbers of pulses per bit from the two SPADs are 13.50 and 7.30, respectively. The experiment verified the feasibility of the method presented.