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 <i>f<sub>sam</sub></i> and the heterodyne frequency <i>Δf</i>. The sampling frequency and the heterodyne frequency should be satisfied with the Nyquist sampling theorem which <i>f<sub>sam</sub></i> will be two times larger than <i>Δf</i>, 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.
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.