Research On The Characteristics Of Extended Fiber Optic Hydrophone

Flow noise elimination for all optic towed arrays is one of the key technologies for fiber optic hydrophone. Some scientists point out that the flow noise can be eliminated with a hydrophone called extended fiber optic hydrophone that has a series sensing fiber rings unit connected together. The array gain can be improve by this hydrophone and that such an array has a good performance with less expenses. The characteristics of the extended fiber optic hydrophone are researched in this paper. Flow noise elimination of this hydrophone is not revolved.The sensing sound model of the extended hydrophone is developed based on the principle of the ordinary fiber optic hydrophone. The directivity, sensitivity and gain of the extended hydrophone are researched based on the model. The hydrophone is the bigger enough compared to the wavelength of the sound, the directivity is the more distinctness. The hydrophone sensitivity is much higher than the ordinary hydrophone. The directivity of the array consisting of extended fiber optic hydrophone is computed based on Bridge multiple theory. The gain of the extended fiber optic hydrophone and the hydrophone array is related to the directivity of the hydrophone and the noise model used to calculate.An extended hydrophone based on the fiber optic Michelson interferometer consist of four same sensing units is constructed, of which every unit is reeled on a mandrel with polymer to enhance the sensitivity. Every unit of the hydrophone is designed to has the same sensitivity -163dB (0dB=1rad/uPa) with an ordinary hydrophone. The length between each unit is 0.16m, and the whole hydrophone is 0.52m long. In a normal cistern, the directivity and sensitivity of the hydrophone is tested with an impulse method. When the signal frequency is 4688Hz, the width of the main lobe of the directivity is 28 degrees and the theoretical value is 26 degrees. The average sensitivity from 1500Hz to 5000Hz is -151.50dB,the max value is-147.43dB,the min value is 153.40dB,with a±3dB fluctuation,while the theoretical value is-151dB.The results of the experiment prove that both the model of the extended hydrophone sensing sound signal and the theoretical analyze based on the model are right. This is an important step for the research on the flow noise elimination of the extended fiber optic hydrophone towed array.