The Research Of Fiber Optic Vector Hydrophone Based On Fiber Bragg Grating Fabry-perot Cavity

The development of marine resources and the protection of territorial rights have been paid more and more attention by the state.Underwater acoustic technology is the main means of studying and exploring the ocean.As the core component of underwater acoustic detection,hydrophones' performance determines the performance of the entire system.Fiber optic hydrophone has the advantages of reliable performance and easy to multiplex,but there are also some shortcomings such as insufficient sensitivity of bare fiber and fiber itself is fragile,so how to overcome these shortcomings and effectively improve its detection sensitivity and working frequency bandwidth has become a hot research topic.Based on the sensing characteristics of fiber Bragg grating based Fabry-Perot(FBG-FP)cavities,the paper introduces the sensor theory of thin-walled hollow rigid cylindrical structure,and establishes the theoretical model of FBG-FP vector acoustic sensor.The simulation results show that the acceleration sensitivity and resonant frequency of the sensor are affected by the structural parameters.Using the digital PGC demodulation method to build all-fiber vector hydrophone system,to achieve a high sensitivity vector acoustic wave detection.The main work of this paper:1.The spectral characteristics of FBG-FP cavity are studied.The optical transmission characteristics and reflection spectral characteristics of the fiber Bragg grating(FBG)and the fiber Fabry-Perot(F-P)cavity are deduced.The optical path of the coherent detection system based on the unbalanced Michelson interferometer is designed and the schemes of anti random phase fading and polarization fading are presented.The visibility function of the interference fringes of the system is deduced based on the weak reflection FBG-FP sensing principle.The influence of the spectrum and the optical path mismatch on the optical path coherence is analyzed.Which provides a basis for realizing high sensitivity vector acoustic wave detection system.2.The theoretical model of the co-vibrating fiber optic vector hydrophone based on thin-walled cylinder is established.The effects of the core material and structure parameters on the sensitivity and frequency characteristics of the vector acoustic sensitive probe are simulated and analyzed.By optimizing the parameters,a number of acoustic sensitivity probes are designed and fabricated.The experimental results show that the acceleration sensitivity of the probe can be effectively improved by increasing the inertia element quality,reducing the wall thickness of the cylindrical shell and increasing the height of the cylinder.3.The digital phase generated carrier(PGC)demodulation scheme and the signal processing algorithm are studied,and the feasibility of the all-digital signal processing algorithm is verified by simulation.This paper presents an all-fiber vector hydrophone system with low reflectivity FBG-FP,and analyzes the principle of vector acoustic signal detection.The experimental results show that the acceleration sensitivity of the difference between the vertical direction is-25 dB,which means the vector hydrophone has the good pointing performance.4.A full fiber optic vector hydrophone system and vector hydrophone test system is designed.The experiment of directivity,sensitivity and frequency response of hydrophone system are carried out.The vector sonic detection on the frequency band of 0.2-2 kHz is achieved with an acceleration sensitivity of 28 dB re rad/g.The pointing performance of the vector hydrophone system in the two-dimensional plane is tested by experiments.