Underwater Low-frequency Liquid Surface Wave Diffraction And Diffraction Of Sound And Light Stripes Asymmetry

The methods for using optical to measure underwater acoustic signal has a lot of kinds, such as laser interference method, laser doppler method, and flux variation method. The measurement accuracy is very high. But it is very strict for the target reflective surface. There have a lot of inconvenience in the actual application. Therefore the method for measuring surface wave is put forward by using of laser diffraction. It's measure frequency range is big. And it has many advantages of real-time, no contact, nondestructive and more extensive application. Based on the acousto-optic effect, the underwater acoustic signals has detected and treated. In this paper it mainly includes the following four parts:1,The liquid surface wave was introduced. The optical detection method for liquid surface wave was inducted. Six kinds of optical methods has been introduced to detect the surface wave. And the of detection principle, suitable scope, advantages and disadvantages are summarized. The future of the detection technology for the liquid surface wave is discussed by laser.2,The acousto-optic effect was emphasis on. the principle of effect and surface wave grating were analysised. And it was laying the ground work for the behind of the analysis.3,The relationship between the order of liquid surface wave diffraction and the incidence angle was discussed. To Fourier transformation for the incident light field, the analytical expression of the light field was obtained. We analysis further that the interval of the diffraction stripe is not only related to the grating constant, but also to the incident angle. When was much more than the incident light wavelength, there is a critical value to observe the clear diffraction stripe; when the incident Angle was more than the critical value, we can observed the stripe, conversely can't. The critical value was given theoretically in this paper.4,According to the principle of the acousto-optic effect for the liquid surface wave, a experimental set up to detect the low frequency underwater sound signals. Underwater acoustic signal spread to the surface, and the sound wave is formed in the liquid surface. When the laser beam irradiated the liquid surface, the diffraction pattern is formed after the reflected light modulate by the surface wave. It is collected with photoelectric detector to transfer to the computer. Based on the acousto-optic effect the sound wave in the medium is similar to a diffraction grating. For dozens of Hertz of low frequency liquid surface wave, it's grating constant is in mm magnitude. And this is much more than the incident light wavelength. Under the condition of incidence obliquely for the laser beam, the asymmetry of the diffraction stripe was observed obviously. According to the general formula of the grating equation, the analytical expression of the diffraction stripe is received. The position of the diffraction stripe and the flare strength asymmetry and its mechanism can be explained by this expression.