Research On Key Technology For Ultrathin Fiber-optic Hydrophone Based On Draw Tower Fiber Bragg Grating Array

Fiber-optic hydrophone has been widely studied in the world due to its advantages of anti-electromagnetic interference,easy reuse,high sensitivity and long-distance transmission.In recent years,with the continuous development of underwater target vibration reduction noise reduction technology,sonar for 20 to 30 k Hz hydroacoustic wave detection distance is constantly compressed,so the high coherence,transmission distance of low frequency sound wave detection become the latest research direction.In summary,the development of high sensitivity,cable with ultrathin diameter,largescale reuse of fiber-optic hydrophone for low frequency hydroacoustic detection is the future development trend.The conventional hydrophone is based on several independent fiber-optic units,by means of fusion connection and parallel connection,the separate sensor units are assembled and reused.The large number of components and complex structure greatly increase the workload and reuse difficulty.So that,it is very important to find a simple and reliable solution with a large number of multiplexes but without breakpoint,and above problems must be solved in the application of fiber-optic hydrophone array with large scale,ultrathin cable,high stability and high sensitivity.Therefore,it is of great significance and broad application prospect to study the hydrophone technology with the above characteristics.In order to meet the needs of large scale,miniaturization,high stability and high sensitivity of fiber optic hydrophone array,in this dissertation,this article provides an in-depth analysis of the fiber-optic hydrophone system with ultrathin diameter based on draw tower fiber Bragg grating array,and deeply study key technologies.The main research contents of this dissertation are as follows:(1)Analysis of the principle of draw tower fiber Bragg grating hydrophone array.In order to meet the requirement of high sensitivity detection of fiber-optic hydrophones,we adopt the draw tower fiber Bragg grating array as the light reflection unit,and realize the interferometry by using the prepared weak reflection gratings,then deduce the sensing mechanism of fiber in hydroacoustic system.The demodulation principle for interferometric hydrophone is derived by combining the existing multiphase demodulation method and phase generation carrier demodulation method.We conduct theoretical analysis and simulation verification for axial length sensitization and radial coating sensitization simultaneously.(2)Design and optimization of signal demodulation algorithm based on reference sensor.Based on the problems existing in the phase generator carrier method,we set up a reference sensor by using the good consistency of the draw tower fiber Bragg grating array,design a simple and effective signal demodulation method and further use the reference sensor to achieve adaptive noise suppression and signal optimization.Based on it,we demonstrate the demodulation effect through theoretical calculation,simulation analysis and experimental test,and verify the effectiveness of noise suppression and signal optimization methods quantitatively.The experiment is carried out by using piezoelectric ceramics to generate vibration.The algorithm is preliminarily verified by combining the time domain results,demodulation linearity and SNR analysis,and compare with the existing phase-generated carrier algorithms in terms of carrier frequency drift and modulation depth.The adaptive noise optimization of optical path is also realized by constructing reference input based on reference sensor,and finally the algorithm error caused by the non-ideal device in the optical path is fitted with parameters to realize the optimization of the tuning signal.(3)Implementation of the cable forming method for integrated sensitization of draw tower fiber Bragg grating hydrophone array.We carry out crosstalk theory and performance analysis for the initial preparation of the draw tower fiber Bragg grating array,and then based on the axial and radial theory,we design the initial single-element sensitization test,grope the appropriate process parameters,determine the best parameters for material to be applied into cable,and finally make before and after comparison of the samples of the hydrophone array.At the same time,we verify the increasing sensitivity of the acoustic pressure sensitivity for fiber-optic hydrophone with the change of cavity length and radial coating.In addition,the wavelength,reflectivity,formation and other factors before and after parameters are compared and analyzed to prove whether the integrated coating and cable process meets the demand of the ultrathin line of the draw tower fiber Bragg grating hydrophone array with good preparation performance.(4)Underwater experimental study for draw tower fiber Bragg grating hydrophone array.We analyze the hydroacoustic calibration method for the draw tower fiber Bragg grating interferometer hydrophone,analyze the factors affecting the sensitivity calibration in the scheme,then determine the optimal sound pressure field measurement scheme.We design the underwater test,and compare with the signal generation carrier algorithm for total harmonic distortion and signal-to-noise ratio.Then we use the prepared fiber-optic hydrophone cable to conduct the time domain signal test,based on it,verify the sensitivity effect of the prepared fiber-optic hydrophone cable.At last,we teste the demodulation results and equivalent noise pressure of the 256-element hydrophone array,which prove the practical application ability of the low frequency detection of the draw tower fiber-optic hydrophone.