Study On A Novel Combined Fiber Optic Vector Hydrophone

As a new type acoustic sensor, fiber optic vector hydrophone shows great advantages in various applications comparing to traditional scale pressure hydrophone. Omnifarious structures spring up, many countries also launch their research plan on fiber optic vector array, but most of them focus on the bottom-mounted array application. For towed array application, streamer's oscillation and changeable towed force raises special request in bringing down sensitivity to axial acceleration, thus it is of importance to carry out research on a novel fiber optic vector hydrophone for towed array application.By analyzing the characteristic and technical requirement of towed vector hydrophone, after comparing the advantages and disadvantages of fiber optic gradient hydrophone with fiber optic inertial vector hydrophone, a combined fiber optic vector hydrophone, with one dimensional gradient sensor in axial and two dimensional inertial sensor in radial, aimed at reducing the influence of axial acceleration, is proposed. The mainly contents and results achieved in this paper are listed as below.1. Design and fabrication of the pressure sensing elements used in gradient hydrophone is studied, thin shell theory is considered for analysis of sensing elements with air cavity structure, the calculated pressure phase sensitivity is concide with the experimental data, with error less than 3 dB.2. Implement of combined fiber optic vector hydrophone is studied. Optical and demodulation system for gradient sensor based on two interferometers scheme is designed, working frequency range, as well as noise characteristic, is analyzed and tested. The sensing elements'acoustic separation must smaller thanλmin/7, and then the mounting structure for combined vector hydrophone is finally decided.3. Methods for calibration of combined vector hydrophone under plan traveling wave field and standing wave field are put up, influence of the sensitivity difference and sensitivity measurement error of the two sensing elements is analyzed in detail. Tests is carried out in standing wave tube, and result shows that the sensitivity of each axis is almost strictly conform to the ideal with respect to frequency ranging from 20 Hz to 1250 Hz. For gradient sensor, the sensitivity is -147.4 dB @ 1000 Hz, and-153.0 dB, -149.9 dB for inertial sensor, the directivity is >35 dB at the four test frequency points from 80 Hz to 630 Hz for gradient hydrophone. Noise test experiment result shows that the noise level of the gradient hydrophone is 10 dB reduced on average compared to single sensing element, and larger than 20 dB reduced for temporal aliasing of relaxation oscillation4. Measurement method of the response for each axis to axial acceleration is put forward, and tests are done according to the method, it shows that each axis' acceleration crosstalk is about -10 dB re. rad/g expect at only few special frequency, it proves that the combined vector hydrophone be superior to three dimensional inertial vector hydrophone in insensitivity to axial acceleration and have great potential in towed array application.