Optical Focusing-based Adaptive Modulation for Optoacoustic Communication

Abstract

Wireless communication from air to underwater is a longstanding challenge that can be addressed by the optoacoustic process. We can directly transmit data to underwater submerged nodes from the air with proper modulation technique by varying basic laser parameters, e.g., laser focusing angle from air to water. Laser-induced underwater plasma volume and shape are important because the duration and directivity of the generated acoustic pulses depend on these. Non-spherical shaped plasma generates anisotropic acoustic pressure; thus it is difficult to communicate from air to an unknown positioned underwater node. In this paper, we analyze how to control the shape of the plasma and propose an optical focusing-based adaptive modulation (OFAM) technique that enables transmission to underwater nodes even if the node's position is unknown. Bit error rates (BER) for different underwater node positions are analyzed, and the BER performance is compared with a lower pulse energy laser. Our results indicate that the performance is better if the underwater node position is in the direction of the laser beam, also when the laser focusing angle varies the most and a higher energy laser is used.