| In recent years, application technologies of fiber optic hydrophones are developing rapidly and will play an important role in large-scale underwater observation systems and novel hydroacoustic equipments, while the conventional package techniques based on discrete components can not meet the practical engineering requirements of the fabrication of a larger-scale and high-reliability array with a mass-product capability. The investigation of the fiber Bragg grating (FBG) hydrophones with high sensitivity based on interferometric detection and the integrated manufacturing techniques of doped fiber fabrication are the front edges of the fiber optic hydrophone developments. A fiber Fabry-Perot (F-P) cavity can be constructed easily by a pair of FBGs fabricated in a single piece of fiber. Taking advantage of this kind of sensing structure is a major prospect of the research on novel fiber optic hydrophones. In this thesis, we investigate the fiber F-P hydrophone techniques to fulfill the application requirements of fiber optic hydrophones by means of corresponding reasearches on fiber Bragg grating based Fabry-Perot (FBG-FP) cavities and the FBG-FP sensors.Firstly, starting with spectral analyses of FBG-FP caivities, the phase characteristics of FBGs and the constructed F-P cavity are investigated, and a scheme of cavity length estimation is designed with a high engineering applicability. The interferometrically interrogated FBG-FP sensor system is descripted systematically, and the freatures of the sensivitivies of the FBG-FP sensing structures is investigated theorectically and experimentally. A strong FBG based asymmetric F-P sensor system is constructed to analyze experimentally the sensing characteristics of the interrogation of multiple reflection beams in the cavity with detectioin resolution and sensitivity enhancements. The noises in an interferometrically interrogated FBG-FP sensor system are modeled, and the phase detection resolution of the system is investigated theoretically and experimentally, accordingly the constructed pico-strain sensor system achiceves an optimized noise level on the verge of the theoretical limit. Finally, a differentially interrogated FBG-FP hydrophone based on air-backed mandrels is investigated. The effects of the sensitivity enhancement and the noise mitigation in the method of differential detection are tested experimentally, and the measured sensivity and the flatness of the frequency response are able to meet the practical application requirements.The main results are provided as follows:1. The phase spectra characteristics of FBGs and the constructed FBG-FP cavity are investigated. Considering the features of structure parameters in a FBG-FP cavity, firstly the linear phase spectrum of a weak grating is obtained, and the three-segment linear approximation is proposed for strong gratings with a concise and clear mathematical expression, which provide valid methods for the experimental analyses of FBG-FP hydrophone and the examination of the cavity production. Then based on the feedback characteristics of FBGs, spectral characteristics of FBG-FP cavities are investigated by the conception of equivalent cavity length. A scheme of measuring the effective cavity of a FBG-FP cavity is designed, which estimates the FBG-FP cavity length with a high degree of accuracy. The scheme can provide accurate parameters for the description of the spectral characteristics and the signal interrogation of FBG-FP hydrophones.2. The interferromatrically interrogated FBG-FP acoustic sensor system is established with signal demodulation of a high degree of stability. The the acoustic pressure sensivity of the FBG-FP cavity constructed by bare fiber is calibrated. The sensivity composition of the sensor system is analyzed theoretically and experimentally, and the influence of the disturbation of FBGs on the system stability is studied experimentally. The corresponding analyses are both instructive for the performance optimization of the interferromatric FBG-FP sensor system and the package with sensitivity enhancement of FBG-FP hydrophone element.3. An asymmetric intensive FBG defined F-P sensor system is constructed. Taking advantage of the amplification effect of the fiber strain on the phase of the light and the band filtering effect in the cavity to promote the spectral profiles of the interference lights, the detection sensivity and resolution both achieves enhancement by decoding precisely different order reflection beams in the cavity. The influnences of the interrogation of different order reflection beams, system paremters, band filtering effect in the cavity and the light pulse intervals to the system performance are all analyzed theoretically and experimentally. The results implys that the interrogation of the fourth order reflection beam can achieve 9.8dB sensitivity enhancement and 3dB resolution promotion compared with the result using the second order reflection beam. The sensing structure provides a new scheme for constructing fiber optic hydrophone arrays with different application demands4. The noises in an interferromatric FBG-FP sensor system are modeled. The effects of the imbalance path length differences of the two tandem interferometers and the spectral mismatch of the two FBGs on the phase detection resolution of the system are investigated. Especially the variation of visibility induced by spectral mismatch is analyzed based on the approximation of Gaussian profiles, which makes a valid supplyment to the corresponding former researches. In order to eliminate the enviromantal disturbances to the sensor system, the antinoise techniques based on differential detection utilizing reference sensor and reference wavelength are investigated experimentally. The results imply that the referentce wavelength method has the noise mitigation effect of 40dB for both single-frequency and broadband environmental disturbances without increasing the complexity of the sensor array and without reducing detection resolution. Finally, a pico-strain sensor system is established based on former analyses. An optimum phase resolution close to the theoretical noise limit is achieved, which permits the 4m-long FBG-FP sensor a detection resolution of 1.84 pε/Hz1/2 above 60 Hz. The analyses can be used to the stability promotion and the performance optimization of the sensor system.5. The differential interferometric interrogation of FBG-FP hydrophones based on air-backed mandrels is investigated. The hydrophones take advantage of the combination of optical differential detection and push-pull structures to promote the effect of noise elimination and sensitivity enhancement at the same time. A FBG-FP hydrophone with an external diameter of 12mm using a cavity length of 10m is calibrated with the acoustic pressure sensitivity of -143.9dB re rad/μPa and a fluctuation of±0.8dB from 80 Hz to 2.5 kHz. The scheme satisfies the fabrication of ultra-thin FBG-FP hydrophone with compact structures and can promote the detenction performance of the system in low frequency band.
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