Research On Active Arc-tangent Demodulation Method Of Ultra-weak Fiber Bragg Grating Based On Fizeau Interferometer Hydrophone Array

With the deepening of marine research,the realization of weak underwater acoustic signal detection in very low frequency bands(?10Hz)is of great significance to China's military and national defense.The ultra-weak fiber Bragg grating hydrophone system based on Fizeau interference researched in this paper has the advantages of large multiplexing scale,long detectable distance,thin array cable diameter,high sensitivity and low cost.At present,it is the main means and key development direction of underwater exploration.By combining the advantages of classical algorithms,an active arc-tangent demodulation algorithm for analyzing weak and very low frequency hydroacoustic signals is proposed,which realizes simultaneous detection of underwater acoustic signals at different positions in the 2-3000 Hz wide frequency band.The main research contents of this project are as follows:(1)Based on Fizeau interference structure principle and fiber Bragg grating theory,we introduced the optical path of ultra-weak fiber Bragg grating hydrophone array system based on Fizeau interference,the theoretical analysis and formula derivation of the system sensing principle,device selection and parameter setting in the optical path were carried out.(2)We introduced the theory of the 5 existing classical phase demodulation algorithms: the NPS and NRL algorithms of 3×3 coupler demodulation,arc-tangent algorithm,the PGC-DCM and PGC-Arctan algorithms of PGC demodulation,and analyzes their advantages and disadvantages through simulation experiments.Based on this way,we proposed an active arc-tangent demodulation algorithm,the simulation and experimental results show that the proposed algorithm can elimate the variation of light intensity and the influence of random disturbance of the initial phase of system on the demodulation result,can be used to detect weak underwater acoustic signals at very low frequency.(3)We conducted experimental research on the ultra-weak fiber grating Fizeau interference hydrophone array system by building a hardware optical path.Firstly,the measurement of weak underwater acoustic signal in 500-3000 Hz frequency band is realized by using acoustic transducer,and the test results have good linearity.Secondly,in order to detect the VLF underwater acoustic signal,four adjacent Fizeau interference ultra-weak fiber Bragg grating hydrophones in the ultra-weak fiber Bragg grating array with a length of 1km were randomly selected.The experiment was carried out by means of vibrating liquid column method and the test results were compared with the demodulation results of the standard hydrophone.The experimental results show that the system can simultaneously measure the amplitude,frequency and phase of underwater acoustic signals at different positions in the 2-2000 Hz broadband band.When the signal is 10 Hz,the signal-to-noise ratio is 43.785 dB,and the sensitivity of the system is 2755.49(?pa/?Hz).The phase acoustic-pressure sensitivity of the system increases with the increase of the sensor's cavity length and has very good stability,the response is excellent in very low frequency band,when the length of sensor cavity is 200 m,the phase pressure sensitivity of underwater acoustic signal with frequency of 2 Hz can reach-135.81 dB(re rad/?pa).In this paper,the Fizeau interferometric ultra-weak fiber Bragg grating hydrophone array system based on the active arc-tangent algorithm we proposed is demonstrated,the result shows that the system can realize high sensitivity detection of weak underwater acoustic signals in 2-3000 Hz broadband,and the theoretical sensing distance is up to 5km.Therefore,the system has a broad application prospect in underwater precise positioning,VLF acoustic detection,deep-sea monitoring and other aspects.