Research On Technology Of Wide Band Fiber Optic Acoustic Sensor

Fiber optic acoustic sensor is a new-type acoustic measurement instrument. Compared with the traditional e lectronic acoustic sensor, fiber optic acoustic sensor has the advantages of higher sensitivity and larger bandwidth. Especially, all fiber optic acoustic sensor still can work under the high temperature and high pressure and other special conditions thanks to its excellent high temperature resistant characteristics. The sound sensitive unit built by Optical fiber grating Fabry-Perot cavity is a novel all fiber optic acoustic sensor which not only has the advantages of higher sensitivity and all fiber and single end, but also can be used in a large multiplexing array. In this thesis, extensive studies on the sound sensitive structure which is a combination of fiber grating Fabry-Perot cavity and mandrel wound by fiber and the method of demodulation of wide-band acoustic phase signal are carried out and an all fiber optic acoustic sensor which can be applied to high temperature, high pressure and other special circumstances is obtained. The main contents of this thesis are provided as follows:1. The phase characteristics of FBG-FP cavity are investigated and the mathematical expression of the phase is obtained. According to the fringe visibility function, the spectral mismatch of grating, the mismatch of reflectivity of grating and the arm mismatch of optical path as well as their influences on the fringe visibility are simulated and analyzed. A further study on the method of measuring the length of low reflectivity FBG-FP cavity which makes a solid basis for improving the performance of sensor is carried out.2. In this thesis, the theoretical model of broadband fiber optic acoustic sensitive structure based on air-backed mandrel is investigated and the effects of material properties and structural parameters on the the resonant frequency and the acoustic pressure sensitivity are analyzed. The methods of improving the resonance frequency and pressure sensitivity of sound sensitive structure are proposed. Four different types of fiber optic acoustic sensitive structure are designed and constructed, whose freque ncy characters and pressure sensitivity are tested, the results of which demonstrate that using mandrel made of high Young’s modulus materials and reducing the weight of the structure can enhance the resonance frequency of the structure and that using elastic tube made of low Young’s modulus materials and decreasing thickness of elastic tube and increasing the length of fiber can enhance the acoustic pressure sensitivity of the structure.3. A scheme of demodulation technology based on phase generated carrier(PGC) is proposed, in which a high-speed phase modulator is applied. The signal between 0.2 kHz and 20 kHz could be demodulated using this scheme we have proposed.4. The fully-enclosed FBG-FP cavity sound sensitive structure which is applied to the environment of high temperature and high pressure is designed and constructed and the testing system of FBG-FP cavity acoustic sensor is built. The experimental results show that the acoustic sensor with an external diameter of 20 mm using a cavity length of 12.357 m is calibrated with the pressure sensitivity of-151.9 dB re rad/μPa from 0.2 kHz to 20 kHz.