Research On Key Technology Interferometric Hydrophone Of Fiber Bragg Grating

Hydrophone, constructed by optical fiber grating, possess a series of advantages such as anti-electro-magnetic field, small size, easy to build the detector array and so on. For water sound engineering the hydrophone with high sensitivity and the anti-acceleration as well as resist resting water pressure capability is specially required. This dissertation investigates the passive and active fiber grating to realize such character.At first the resent research of fiber grating hydrophone in the world is introduced. In Chapter2the principle of fiber grating sensor is introduced. Through the coupling mode theoretical and elastooptic theory the sensitivity of fiber grating hydrophone is analyzed. In Chapter3the principle and key technique including the sensitivity enhancement, signal accepting and demodulating by unbalanced interferometer, the anti-resting water pressure and anti-acceleration technique is introduced.The passive optical fiber Bragg grating hydrophone is designed and executed, and its sensitivity enhanced through side pressure and end surface pulling is investigated in Chapter4. Two methods of filling epoxy and casing using elastic slice are used to enhance the sensitivity of the optical fiber grating sensor. The research results indicate that the resonance frequency using the filling method is lower (300Hz), and high frequency sensitivity is too low (<-205dB); otherwise the techniques request is complex. The Be-Cu(Beryllium-Copper)cupper slice is used to enhance sensitivity. In a frequency range of100～1000Hz the sensitivity reached-175±2dB. Make the casing of the enhance sensitivity sensor and apply it to water sound detection indicate that its sensitivity and frequency response are satisfied to application request.The design and execution of active optical fiber grating sensor is investigated in Chapter5, which possess not only a high sensitivity to sound in-water, but also better anti-acceleration characterizer and resisting to resting water pressure. This hydrophone is constructed of an active phase-shifted grating namely the Distribution Feedback Fiber laser (DFB FL), which consists of Er+3doped fiber grating with λ/4phase-shift pumped by980nm laser diode, and emits the laser with narrow line width smaller than2KHz. When the grating suffers an acoustic pressure underwater the grating space and position of the phase-shift point will be changed. As a result the laser wavelength emitted from DFB FL will be changed, which corresponding to the acoustic pressure to be measured. Casing this active FBG element with two thin elastic slices in both sides symmetrically, the sensitivity of hydrophone is increased. The disturber of the axial acceleration could be compensated due to the symmetrical encapsulation of slices, and therefore the detection ability is markedly enhanced. In a frequency range of100Hz～1000Hz the sound-phase sensitivity reached to-133.3±0.7dB (OdB=lrad/μPa) and the anti-acceleration sensitivity is meliorated for20dB (0dB=1rad/g). Using the pre-stress on the fiber grating the compression resistance of hydrophone is increased. The resisting to resting water pressure capability reaches2MPa, which corresponding to200m depth under water.At last the detector array using passive and active fiber gratings is investigated, which is suited for the towing and fixing sonar. Furthermore the deduction of system noise and sound responsibility by burying fiber grating is studied. So through the experiment and design the applying possibility of fiber grating is validated.