| The research on underwater objects detection has always been a highly regarded issue in the military and civilian fields.Excellent implementation of underwater communication,water body detection and underwater target location are based on realization of underwater acoustic signal measurement.It plays an important role in the development and protection of China's Marine resources,far-reaching sea exploration and security,new equipment manufacturing and other fields.In order to improve the anti-interference ability of laser interference underwater acoustic measurement method,a method of amplitude modulation laser interference underwater acoustic measurement is proposed in this paper.The laser signal is amplitude modulated by using the high frequency tuning characteristic of tunable semiconductor laser,and the influence of external noise at the measuring end on the interference measurement results is suppressed without adding additional hardware equipment,so as to improve the signal-to-noise ratio(SNR)of laser interference underwater acoustic measurement results.Centering on this theme,this paper mainly carries out research work in the following aspects:(1)The relevant theoretical knowledge of the vibration of surface water waves caused by underwater sound sources is analyzed,as well as the method of solving surface wave amplitude and frequency based on laser interference underwater acoustic measurement technology.According to the tuning characteristics of tunable semiconductor lasers,high-frequency current is introduced to drive laser interference underwater acoustic signal modulation and phaselocked amplification signal demodulation means,forming an amplitude modulation laser interference underwater acoustic measurement method.Experiments obtain the wavelength variation characteristics of the tunable semiconductor laser under different current driving frequencies and driving amplitude conditions,and master the laser wavelength variation range during the amplitude modulation process.(2)The influence factors in the process of amplitude modulation laser interference underwater acoustic measurement are simulated,and the characteristics of the laser interference underwater acoustic measurement signal are analyzed and compared with the luminous flux method signal.The simulation results show that increasing the amplitude of the modulation signal can increase the signal in the modulation frequency band proportionally,which is beneficial to improve the measurement accuracy.When the laser wavelength change is much smaller than the water surface wave amplitude,the laser wavelength change caused by the amplitude modulation process has no effect on the underwater acoustic signal measurement.Compared with traditional laser interferometric underwater acoustic measurement results,the SNR of amplitude modulation interferometric measurement results can be improved by more than 10 d B under strong noise interference conditions.(3)An amplitude modulation laser interference underwater acoustic measurement system based on a 1405 nm tunable semiconductor laser is built,and the composition of the system and the functions and index parameters of each part are introduced in detail.A large number of experimental tests have been conducted on the surface wave signals caused by underwater sound sources at different frequencies and different vibration intensities.The results show that the amplitude modulation laser interference underwater acoustic measurement method can accurately restore the laser interference underwater acoustic information,and the signal characteristics are consistent with the traditional laser interference underwater acoustic measurement results.In the laboratory,the standard deviation of the frequency measurement result of surface water wave is 0.87 Hz,and the standard deviation of the amplitude measurement result is 0.19 nm,which has good stability and consistency.On this basis,the effects of modulation amplitude and coherent beam intensity changes on the measurement signal are experimentally analyzed.By adding stray light interference at the measurement end,the experiment verifies the anti-interference ability of the amplitude modulation laser interference underwater acoustic measurement method.In this paper,theoretical analysis,simulation and experimental research have verified the ability of amplitude modulation laser interference underwater acoustic measurement method to capture the frequency characteristics of underwater sound sources and anti-interference ability,and realizes accurate measurement of the working characteristics of underwater sound sources.The research work has further developed the related theories and methods of laser-acoustic joint detection technology,which is of great significance for promoting the application of laser interference underwater acoustic measurement technology in underwater target detection. |
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