Research On Laser Remote Sensing To Detect Underwater Low Frequency Acoustic Signals

With the continuous development of economy and national defense construction, accurate detection of underwater target plays an important role in the development and utilization of the ocean. Becauses of its shortcomings such as the poor mobility, easy to be interferenced, the widely used underwater detecting sonar technology is unable to meet the requirements of underwater target detection. In the field of water and acoustic combining research, laser non-contact detection of underwater acoustic signal technology arises at the historic moment. Laser remote sensing technology was usually used to detect the high frequency underwater acoustic signal, however, in this paper, a method of detecting low frequency underwater acoustic signal is proposed which is based on the theory of acousto-optic diffraction.Based on light diffraction effect and the wetting effect, a simple and novel optical detection system is established, which can be used to detect underwater target objects that can send out low frequency acoustic signal. What's more, the method is real-time and nondestructive. Place a aluminum cylinder on the liquid surface, due to the wetting effect the liquid surface was bent so that the low frequency underwater acoustic signal was able to transmit to the water-air interface generating liquid surface wave. An incident laser light whose amplitude and phase were modulated by the wave come into being diffraction. Thus, acousto-optic diffraction of low frequency underwater acoustic signal was achieved. Clear and stable diffraction patterns are obtained. Laser diffraction method for low-frequency liquid surface wave theory has been quite perfect, through the study of low frequency liquid surface wave, related information of the low frequency underwater acoustic signal which generate the wave can be obtained. The related content of this article mainly has the following four points:Firstly, the principle of formation of low-frequency liquid surface waves was analysed based on the theory of fluid mechanics. Then further, the basic law of acousto-optic diffraction of low frequency underwater acoustic signal was studied based on wave optics theory accordingly. The low-frequency liquid surface waves were regarded as reflex type phase grating. When the laser oblique incidence to the grating surface, there will be a diffraction phenomenon.Secondly, this paper analyzed the form principle of low-frequency liquid surface wave disturbed by the low frequency underwater acoustic signal. On the basis of experiment, this paper analyzed and explained special diffraction properties of liquid surface wave formed by the low frequency underwater sound source. By observing diffraction pattern of the experiment, found that the distribution of diffraction spot has obvious asymmetry. For the same order diffraction spot, compared with the positive order diffraction spot, the negative order diffraction spot is more far away from the center of the zero order diffraction spot. Through theoretical analysis, gives a detailed explanation of this special diffraction phenomenon.Thirdly, by experiment inquiry, we found that when the depth of the tablet to enter into the water is changed, the observed diffraction pattern change obviously. Diffraction classes and the relative intensity of diffraction spots have significant change. According to Fourier optical related knowledge and the nature of Bessel function obtained the computing analytic way of amplitude of low frequency water surface wave. The liquid surface wave formed by the low frequency underwater acoustic signal is related to the depth of the tablet to enter into the water. The amplitude of liquid surface wave with the increase of tablet into the water depth increases after the first decreases, and exist an optimal depth for the tablet to enter into the water.Fourthly, change the depth of the sound source, using CCD photograph different diffraction spots when underwater sound source at different depth. The amplitude of liquid surface wave attenuated with the increase of the sound source depth. Based on this, find out an optical, non-contact method based on laser diffraction prospecting the depth of underwater low-frequency sound source. By means of numerical fitting, the analytical equation about surface wave amplitude and the depth of the sound source was obtained and found that the surface wave amplitude decreases with the increase of sound source depth. Thus, according to the sound source surface wave amplitude, the depth of the sound source in the water can be known. Change the vibration frequency of source, when frequency grows, amplitude attenuation along with the depth of the sound source speed up. This shows that with the propagation distance increases, the attenuation of low frequency acoustic signal of different frequency situation is different. The higher the frequency, attenuation to the faster.