Research On Technologies Of Underwater Acoustic Signal Detection Technology Based On PGC Demodulation

The water surface will be stimulated to produce a micro-amplitude wave,when the acoustic field generated by underwater acoustic target propagates to the water-air interface.This micro-amplitude wave of water surface carries a large amount of vibration information of underwater sound source,and it is a feasible solution to detect this information indirectly by detecting the water surface acoustic wave(WSAW).This paper focuses on the Laser interference detection method of WSAW.The waveform of the method is analyzed theoretically and the feasibility of this method for underwater acoustic targets is verified by simulations and experiments.In this paper,the PGC demodulation scheme of optical heterodyne interference is emphatically studied,and the algorithm improvement and innovation are carried out in combination with this research topic.The establishment of WSAW caused by underwater sound target principle model,theoretical analysis of optical heterodyne detection of WSAW signal form,and study of its spectral characteristics.The WSAW has the same frequency and its amplitude is inversely proportional to the frequency of underwater acoustic source and the distance between the source and the water surface.Michelson laser interference light path is used to obtain the detection signal whose spectrum composition and distribution are analyzed by triangular function and expansion of Bessel function.According to the theory and simulation,the frequency spectrum consists of low frequency band and sound source frequency band which has a characteristic of center frequency symmetry.And based on the characteristic,a method of extracting the center frequency is proposed to solve the frequency information.An improved detection scheme of underwater sound source based on PGC demodulation is proposed,and the phenomenon of demodulation distortion caused by orthogonal signal blanking is solved.High frequency water surface fluctuation is introduced as carrier in the reference optical path,and then demodulated by PGC demodulation scheme based on optical heterodyne interference.The classical arctangent algorithm of PGC is improved to increase the estimation of initial phase and modulation depth of the carrier.The demodulation results can completely contain the fluctuation information of water surface,in which we can extract the frequency and intensity change information of the WSAW from the mixed signal.Which means that the scheme can detect the change of underwater sound source and realize tracking detection.A real-time detection system is designed and built,which includes the detection optical path,data acquisition,visualization software interface,demodulation algorithm and other complete modules.A set of interferometric system is built by using discrete optical components in the optical darkroom,and a visual detection interface is designed by using Labview.In the experimental process,it can demodulate and analyze the experimental results.Experiments on multi-frequency-band,large amplitude fluctuation and timevarying intensity signals have verified the anti-interference ability and tracking detection ability of this method.The underwater sound source in the frequency range of 50 Hz-10 kHz has been detected experimentally and good experimental results have been achieved,which confirms the wide detection range of the scheme;the experiment proves that the system has strong anti-interference ability by adding mechanical fluctuations with low frequency and large amplitude to simulate large disturbance of water surface;the experimental result of demodulation of acoustic signals with modulated intensity also proves that the scheme can track and detect time-varying signals.Through theoretical simulation and experimental research,a series of results show that the proposed PGC demodulation technology can effectively and accurately detect the vibration information of underwater acoustic target.This research work has enriched the technical foundation of laser detection of underwater acoustic sources,and also has a reference role for other laser interference vibration measurement fields.