Underwater Low Frequency Acoustic Signal Acousto-optic Diffraction Method For Visual Detection

Obtain physical information of underwater target from liquid surface is an important detection technology, which has been importantly applied on marine biology and military. A novel and simple technique based on light diffraction effect and wetting effect for visualization of low frequency underwater acoustic wave (LFUAW) in real time has been developed in this thesis. The low frequency underwater longitudinal wave can be generated into water surface transversal capillary wave around the cylinder by our technique. Modulating the phase of laser beam which is reflected from water surface by surface acoustic waves (SAWs) can realize acousto-optic effect. Then a steady and visible diffraction pattern is experimentally observed. A physical model of the SAW is established to verify the feasibility of our technique. The main contents are summarized as follows:(1) According to the fluid mechanics knowledge, the surface disturbance and underwater sound source vibration generated by the liquid surface wave grating are discussed. Using the knowledge of wave optics, acousto-optic effect of liquid surface is analyzed, which provides theory basis of the laser diffraction testing technology for the study of surface wave and the feasibility of low frequency underwater acoustic signal.(2) Based on the low frequency underwater acoustic wave diffraction of the wetting effect, the liquid surface wave amplitude is increased with the increasing diameter of the cylinder, when we remain the angle of incidence, the frequency and amplitude of LFUAW constant. We find that the results of our experiment have linear relation. The liquid surface wave amplitude is increased with the increasing amplitude of LFUAW, if the frequency and diameter of the cylinder keep constant. And the results of diffraction stripe light intensity distribution have relation with the liquid surface wave amplitude.(3) Based on the low frequency underwater acoustic wave diffraction of the wetting effect, the angle width of the diffraction stripe is increased with the increasing the excitation frequency, when we remain the diameter of the cylinder, the angle of incidence and the frequency of LFUAW constant. According to the analysis of the diffraction stripe, the analytic formula of the water surface wave is calculated by the grating equation. The experimental results are matched with the theoretical value, which verify the accuracy of the dispersion relation.(4) Based on the wetting effect and acousto-optic diffraction effect, the technique of visual low-frequency underwater acoustic signals is established, which has advantages with accuracy and real-time non-destructive property.