| Underwater target detection is of concern in marine defense and marine engineering.The realization of underwater sound field detection is the basis of breaking the underwater target recognition,positioning,tracking and other problems.At present,there are many methods for underwater target detection,among which "laser-sound" joint detection belongs to a cutting-edge basic research method.The lab I'm in has been working on "laser-acoustic" joint exploration of underwater acoustic targets since 2008 and has achieved a series of phased results.However,accurate detection of multi-source sources in the case of aliasing of the detection signal has been a technical bottleneck.Because when there are multiple underwater sound sources,and the sound source frequency is close,the laser coherence detection signal spectrum will be mixed together,can't intuitively determine the number of sound sources,and also can't use the traditional method of direct access to water.In order to achieve the underwater target detection,this paper is based on the principle of laser Doppler,detecting the surface acoustic wave through the laser coherent detection system,and demodulating the surface acoustic wave related information from the detection of the interference signal.Around this theme,the main research work is carried out as follows:First of all,the theoretical model of underwater multi-source frequency demodulation is proposed and simulated.The characteristics of radiated noise of underwater targets,the attenuation characteristics of noise propagation in aqueous media,and the principle that noise is transmitted to the water surface to form water surface acoustic waves are analyzed.For underwater multi-sources,this paper proposed a method of frequency demodulation based on Hilbert transform,the specific mathematical expression is deduced.Simulation verified the correctness and feasibility of the demodulation method.Secondly,the frequency demodulation extraction method of the underwater complex acoustic signal is studied.According to the derivation of the theoretical model and the analysis of the actual detection experiment,the MATLAB algorithm is used to realize the demodulation algorithm of the measured signal.The pretreatment of the detection signal is carried out by combining the Smoothing Method with 5.3 Smoothing,which minimizes the influence of the frequency band caused by natural surface fluctuation on the signal demodulation.The band of useful signals is stripped from the interference signal by means of wavelet packet decomposition and reconstruction.Through the signal Hilbert transformation and modulus calculation and smoothing filtering and other processes so that the original aliasing together to separate multiple frequency bands.Finally,the sound frequency of multiple sound sources in underwater is obtained by frequency center search.Thirdly,a laser coherence detection system was designed and built in the optical darkroom.According to the characteristics of the signal detection and the requirements of the actual detection technology,a laser coherence detection system is designed,and some key devices are selected and analyzed according to the actual detection requirements.After that,the experimental system was built in the optical darkroom and debugged so that it could meet the detection requirements.Finally,In order to verify the feasibility and accuracy of the demodulation method proposed in this paper,the underwater target detection of multiple frequency combinations with sound frequency between 1kHz and 10 kHz is carried out by using the built-in laser coherent detection system,and the detection efficiency was evaluated.The experimental results showed that the signal method proposed in this paper can realize the detection of the frequency information of the underwater complex sound signal,which provides a new idea for the underwater acoustic signal processing. |
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