Study Of Water Surface Acoustic Detection Technique Based On Laser Interference

The feature extraction and recognition of underwater objects is one of the major techniques to be studied in future ocean engineering. The purpose of the present study is to extract underwater acoustic information from the acoustic waves on the water surface and lay a technological foundation for the further exploration into the aeronautical telemetering of underwater objects,such as submarines.This paper proposes a water surface acoustic detection technique based on laser inference. The frequency and amplitude information of acoustic waves on the water surface is demodulated from the interference signals of the scattered light and reference light on the water surface in order to detect underwater objects. The following work has been done concerning this topic:A theoretical model for the spatial distribution of the intensity of water-surface scattered field is proposed on the basis of Kirchoff's theory and the simulation and experimental verification have been conducted. Green's theorem is used to derive the integral expression of the intensity of the water-surface scattered field and quantitative relations between the intensity of water-surface scattered light and many factors such as incident angle, scattering angle, microscopic fluctuation of the water surface, intensity of incident light, wavelength of incident light, observation distance, and refractive index of medium. Focused discussion is performed about the effects of the geometric conditions of the incident light and scatter light and the amplitude and wavelength of acoustic waves on the water surface on the intensity of scattered light. The further research is done on the probability distribution of the intensity and phase of scattered field. The efficiency of underwater acoustic information detection is evaluated.The basic mechanism by which the laser interference process is used to detect acoustic waves on the water surface is studied, and the time-domain and frequency-domain spectral characteristics of the signals of inference between scattered light and reference light on the water surface are analyzed. Calculation methods for the frequency and amplitude of sound waves on the water surface are explained respectively in the absence of underwater acoustic sources, in the presence of a single underwater acoustic source, and in the presence of multiple underwater acoustic sources. A set of experimental equipment is set up in a photographic laboratory to detect underwater acoustic signals with such methods and the real-time detection is realized for underwater objects with acoustic frequencies ranging 100Hz-18kHz. The underwater signal frequency extraction technique is studied in order to obtain the precise acoustic frequencies of undertaker acoustic sources. For the short-time-scale HF phase changes in the interference fringes, a local data processing technique is used to demodulate the frequency information of underwater acoustic signals. Experimental results show that: the maximum relative error is 0.7% and the standard deviation is smaller than 8Hz. As wavelet analysis is capable of characterizing local information of signals in both the time domain and frequency domain, this paper discusses the application of wavelets and wavelet packets in the detection of underwater sound signals. The components of HF carrier waves of the original inference signals are separated and reconstructed in order to obtain more accurate underground acoustic information. The maximum relative error of the frequencies of demodulated MF and HF underwater sound signals is 0.3% and the standard deviation is smaller than 5Hz.