Underwater Acoustic Signal Detection Based On Laser Doppler Vibrometry Technology

Detection of underwater acoustic signal has important values in the marine resources development and protection, marine environment monitoring, marine surveying and exploration. In the military field, acoustic signal is often used in underwater communication as carrier signal because it has smaller wave attenuation and long transmission distance than light wave and electromagnetic. The method can detect submarine accurately and communication directly between air and underwater equipments. It increases the concealment of the communication and improve the transmission rate in real-time communication.When a laser beam irradiate onto the disturbance surface of the underwater sound source, the scattering light can produce Doppler frequency shift with underwater sound source information. We can acquire and demodulate the vibration from the interference signal between the surface scattering light and reference light. This method is referred to as "laser-sound" joint detection technology. It uses water as the carrier medium to complete communication directly. The kind of detection can overcome the small coverage of sonar technology and little depth of the blue and green laser. In addition, the laser Doppler measurement of acoustic vibration has high precision, non-contact measurement, high spatial resolution and the advantages of real-time performance. The water environment is relatively low by using frequency modulation.This paper mainly studies the technology of laser Doppler vibration of underwater acoustic signal detection. The main research content is as follows:1. This paper introduces the background and development of the underwater acoustic signal research. We summarizes the photoelectric methods using laser detection of the underwater acoustic signal and expand the laser Doppler vibration measurement using in multiple applications. According to the propagation of underwater acoustic signal to analysis the surface water wave theory and theoretical analysis of laser coherent detection.2. In this paper, the main frequency of underwater acoustic vibration is from 10Hz to 20 kHz. According to the signal frequency characteristics and detection requirements to design the optical interferometer and experimental apparatus, select the appropriate laser, detector and amplifying filter circuit. Because the signal magnitude is in the voice range, we use Labview programming to control computer sound card in signal getting and processing. The signal is amplified by the circuit and acquired by PC. Using professional MATLAB and Audition to demodulate signal frequency analysis. Signal generated by the signal generator.3. This paper analyzes the broadband frequency response of underwater acoustic signal. The experimental data shows that underwater acoustic signals frequency can be detected accurately at the 10Hz to 20 kHz band through the water surface wave. Error rate is less than 2.3% at 10Hz to 100Hz, error rate is less than 0.02% at 100Hz to 10 kHz. It greatly improved the measurement accuracy. When the frequency is more than 3 kHz, the signal has the higher accuracy and the biger difference. Error rate is between 0.1-0.2% at 10 k to 20 kHz. It has best proformance at 1k-2.5 kHz. System noise is mainly concentrated in the 1k-3 kHz, which is related to the speaker’s own frequency response.