Lack of suitable towed array has existed as a main problem for further extended use of small underwater mobile platforms such as UUV. In this paper, an ultra-thin fiber-optic hydrophone towed array is developed based on DFB fiber laser, which diameter is only 12mm. And the performance of the towed array of the fiber-optic hydrophone is tested by towboat. The experimental results show that the towed noise of the array is 69dB re. uPa/√Hz@1kHz. In addition, the fiber towed hydrophone array can precisely determine the spatial characteristics of the underwater acoustic field at 6 knots, which verifies the feasibility of the ultra-thin fiber laser hydrophone towed array in UUV and other underwater platforms.
In order to increase the multiplexing capacity and optimize the noise level and power of distribute feedback fiber laser hydrophone array, the factor of the mode instability has been studied. Mode instability characteristics of DFB fiber laser hydrophone due to multiple external reflections is modeled, and the dependent condition and suppression method of DFB fiber laser array is carefully analyzed. It can be concluded that the mainly reason for the mode instability of the DFB fiber laser hydrophone array is related to the mode splitting caused by external cavity feedback light reflected by the fiber connector, and the reduction of fiber flange connector reflectivity can effectively suppress the mode instability.
Orthogonal demodulation, as an important part of the demodulation process of heterodyne interferometric optic fiber hydrophone signal, directly determines whether the demodulation system can demodulate the underwater acoustic signal without distortion. The phase shift operation of reference signal is the key step of orthogonal demodulation algorithm. In this paper, the influence of phase shift error on the distortion of underwater acoustic signal is introduced and analyzed by combining the interferometric optic fiber hydrophone demodulation system, the phase shift range of undistorted signal is obtained by simulation analysis. Hilbert algorithm is proposed and used for phase shift of reference signal, to improve the phase shift accuracy, and then enhance the performance of orthogonal demodulation. Using the actual system to verify, it is proved that the heterodyne demodulation of optic fiber hydrophone based on Hilbert algorithm can demodulate the characteristics of underwater acoustic signals without distortion.
Interferometric fiber optic hydrophone based on heterodyne detection is used to measure the missile dropping point in the sea. The signal caused by the missile dropping in the water will be too large to be detected, so it is necessary to boost the upper limit of dynamic range (ULODR) of fiber optic hydrophone. In this article we analysis the factors which influence the ULODR of fiber optic hydrophone based on heterodyne detection, the ULODR is decided by the sampling frequency fsam and the heterodyne frequency Δf. The sampling frequency and the heterodyne frequency should be satisfied with the Nyquist sampling theorem which fsam will be two times larger than Δf, in this condition the ULODR is depended on the heterodyne frequency. In order to enlarge the ULODR, the Nyquist sampling theorem was broken, and we proposed a fiber optic hydrophone which the heterodyne frequency is larger than the sampling frequency. Both the simulation and experiment were done in this paper, the consequences are similar: When the sampling frequency is 100kHz, the ULODR of large heterodyne frequency fiber optic hydrophone is 2.6 times larger than that of the small heterodyne frequency fiber optic hydrophone. As the heterodyne frequency is larger than the sampling frequency, the ULODR is depended on the sampling frequency. If the sampling frequency was set at 2MHz, the ULODR of fiber optic hydrophone based on heterodyne detection will be boosted to 1000rad at 1kHz, and this large heterodyne fiber optic hydrophone can be applied to locate the drop position of the missile in the sea.
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