High-capacity, long-distance underwater wireless optical communication (UWOC) technology is an important component in building fast, flexible underwater sensing networks. Underwater communication with light as a carrier has a large communication capacity, but channel loss induced by light attenuation and scattering largely limits the underwater wireless optical communication distance. To improve the communication distance, a low-power 450 nm blue continuous wave (CW) laser diode (LD)-based UWOC system was proposed and experimentally demonstrated. A communication link was designed and constructed with a BER of 3.6 × 10−3 in a total link loss of 80.72 dB in c = 0.51 m−1 water with a scintillation index (S.I.) equal to 0.02 by combining with 32-pulse-position modulation (32-PPM) at a bandwidth of 12.5 MHz and single photon counting reception techniques. The allowable underwater communication distance in Jerlov II (c = 0.528 m−1) water was estimated to be 35.64 m. The attenuation lengths were 18.82, which were equal at link distances of 855.36 m in Jerlov I (c = 0.022 m−1) water. A receiving sensitivity of 0.34 photons/bit was achieved. To our knowledge, this is the lowest receiving sensitivity ever reported under 0.1 dB of signal-to-noise ratio (SNR) in the field of UWOC.

As a new type of technology, the integrated system of underwater wireless optical communication and radar will play a huge role in realizing flexible and high-speed communication links between underwater vehicles, underwater monitoring points, and marine vessels. It plays an important role in wireless sensor networks, ocean exploration and detection. This paper proposes an integrated system of underwater wireless optical communication and radar, which integrates the functions of communication and radar in the same system. A time-slot synchronous clock recovery method is proposed to recover communication signals and achieve high-reliability communication; a high-precision target imaging algorithm based on the first photon is proposed to achieve high-precision radar imaging. The communication performance is verified by simulation, and the influence of radar imaging quality is verified by experiment. The results show that the system can not only achieve the function of single-photon wireless optical communication, but also achieve the highquality target imaging of single-photon level.