Low-frequency Underwater Sound Source The Laser Detection Research

By using laser detection underwater acoustic signal can not only realize the laser remote sensing technology, but also it can further underwater target detection. Because it has the wide prospects of application, so the extensive attention was caused. A technology has been developed to detect the low-frequency underwater acoustic signal by laser and a detection apparatus is developed in this paper. The sound wave produced by sound source under water propagates to the water surface through water medium and then forms the surface acoustic wave (SAW). When the laser beam illuminates obliquely on the surface acoustic waves at a certain angle, these waves result in the modulation effect on a laser beam reflected by the water surface. Then the steady and visible diffraction pattern is formed on the observing screen. The relationship between the diffraction fringe spacing and the wave frequency was obtained in experiment. A method that can calculate the wavelength produced by the low frequency underwater acoustic is proposed. According to the dispersion relations, a method for detecting underwater sound frequency is presented. The attenuation relationship between the SAW amplitude and the propagation distance is analyzed.This paper is composed with the following four parts:The first part in this paper, the significance and the background was mainly introduced for the underwater detection. And a method using laser is putted forward to detect the underwater target. The progress of the laser sonar detection technology and the remote sensing was detailed. The optical ranging method was summarized to divided into two kinds:coherent measurement and incoherent measurement. Laser doppler measurement technology, laser triangulation and laser direct intensity modulation was especially focused on.In this second part, the generation and mechanism of the surface acoustic wave was introduced. Based on the fluid mechanics, the liquid surface acoustic wave and boundary condition was provided. Therefore one-dimensional surface sound waves that can be seen as the ideal sine type reflex type the phase grating have a modulation effect for the incident beam. The principle of the acoustic-optic effect and its application significance was given. The formula from the surface acoustic wave and light interaction was got. The reflected light was composed of a set of grades added by the waves in n diffraction light, and spread with a certain diffraction angle.The third part in this paper, a technology has been developed to detect the low-frequency underwater acoustic signal by laser. And a detection apparatus is developed. For dozens of Hertz low frequency surface acoustic wave, the clear stability diffraction pattern was observed. When the surface wave frequency was changing, the stripe spacing of the diffraction pattern was also varied, and became wide with the increase of frequency. Based on the principle of wave optics, the analytical formula of the surface with fringe spacing was analyzed. Then the wavelength and the wave vector were determined. From the angle of the fluctuation, the dispersion relationship with SAW was given. According to the computer program, the log-log plot of the angular frequency versus the wave number k on the liquid surface was fitting. And the regression equation of the value was got. the experimental result coincided with theory dispersion relationship. Then the feasibility of the experiment equipment was validated. Through the experiment system, we can detect the frequency information with the SAW.The fourth part in this paper, based on the detection devices, the diffraction patterns collected when the propagation distance was changed and the surface wave frequency was keeping constant. From diffraction intensity theory, the strength of the light diffraction formula was got. According to the formula, the surface wave amplitude values were provided under the different propagation distance through the computer program. When the propagation distance was increased, the amplitude values were decreased. Therefore the attenuation effect existed in the liquid surface wave amplitude. According to the least-square method, the relative transmission distance curve about the amplitude logarithm with the propagation distance was obtained. The corresponding attenuation coefficient was calculated. And we find that when the frequency was increased, the attenuation coefficient of the surface wave was increased. The surface wave attenuation was enhanced as frequency increased.Based on the experiment in this paper, the low-frequency acoustic signal was produced through stimulate the underwater source. The surface wave signal was formed. The liquid surface acoustic wave developed the diffraction effects, and the diffraction pattern was formed. The fringe spacing and the wavelength values were obtained. And the regression equation of the dispersion relations was fitting out. And the surface wave amplitude attenuation characteristics was analysed. The liquid surface wave amplitude was decreased when the propagation distance was increased, and the attenuation coefficient was increased with the frequency.