Simultaneous underwater beam steering and PAM4 transmission enabled by the acousto-optic effect

In this paper, an underwater wireless optical communication (UWOC) system that can simultaneously achieve beam steering and pulse amplitude modulation (PAM) enabled by the acousto-optic effect is proposed and experimentally demonstrated. An approach to manipulate the driving signal of an acousto-optic modulator (AOM) is utilized to simultaneously achieve precise laser emission angle control and signal modulation, which enables high-speed link-switching for multi-user access. The system is tested in a 7-m water tank with a water attenuation coefficient of 0.87 dB/m, and a bit error rate (BER) of 2.99 × 10^-3 is realized at a maximum data rate of 1.6 Mbps. The theoretical deflection angle of a 450-nm laser is 4.6 mrad, while the actual steering range could reach up to 5.2 mrad with a horizontal displacement of 36.4 mm at the receiver plane under experimental conditions. We further measured the relationship between the diffraction efficiency of the 1st-order diffracted light and the amplitude as well as the central frequency of the driving signal, which is essential for the generation of the driving signal. The system provides an effective solution for link-switching in the Internet of Underwater Things (IoUT) and high-throughput underwater wireless optical networks.