Theoretical model on underwater sound detection based on Laser heterodyne method

Laser-acoustic joint detection technology is an emerging technology in the field of space-underwater communication, underwater target detection and ocean monitoring in the marine environment. It plays an important role in many new high-end marine equipment manufacturing, deep-sea exploration and security fields. However, accurate detection of multiple sound sources in the case of spectrum aliasing of detection signals has been a technical bottleneck. The purpose of this paper is to extract the underwater sound field information from the sound waves on the water surface, and demodulate the sound frequency of the sound sources close to the underwater frequency. According to the modulation theory of incident laser on the surface of water, this paper introduces the basic principle of laser interferometry to detect sound waves on water surface. This paper proposes a method for detecting the frequency of underwater acoustic signals using optical heterodyne. The expression of the photodetector output current is derived under a plurality of underwater sound source signals. The time domain and frequency domain characteristics of the detected surface wave interference signals are analyzed by simulation. And the feasibility of the method was verified. The results show that the method can detect the frequency and amplitude of the ideal surface wave. In order to obtain a more accurate audible frequency of underwater sound source, this paper proposes a frequency demodulation method based on Hilbert transform. And specific mathematical expressions are derived. This solves the frequency demodulation problem of spectral aliasing of the coherent detection signal. It provides a new method for the detection and processing of underwater acoustic signals.