A phase modulation based interference visibility optimization method for optical fiber hydrophone transmission system

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In long-distance fiber-optic sensing and communication systems, phase modulation methods are usually employed to suppress stimulated Brillouin scattering (SBS), thereby increasing the stimulated Brillouin threshold. However, when phase modulation is added to the light source, the linewidth is broadened by the periodic modulation and the coherence length is reduced. In a large-scale multiplexed fiber optic hydrophone system, the optical pulse interference visibility returned by the fiber optic hydrophone is affected by the optical path difference, phase modulation amplitude and frequency, etc., resulting in reduced visibility of interference fringes. In practical applications, it is difficult to fully compensate the optical path difference between the hydrophone and the time-delay interferometer. In the case of large optical path difference, the phase modulation amplitude and frequency need to be optimized to improve the visibility of the interference pulse. In order to solve this problem, this paper first theoretically analyzes the principle of SBS suppression by light source phase modulation. The laboratory builds a fiber optic hydrophone remote transmission test system, and uses a spectrum analyzer to measure the backscattering spectrum of the transmission fiber to obtain the suppression of SBS. The best parameters for the suppression of the effect, the interference visibility of the hydrophone is tested on this basis, the phase noise of the hydrophone with poor visibility is tested, and the phase noise consistency of the tested hydrophone at 1kHz. By optimizing the modulation parameters of the phase modulator and changing the modulation frequency by using the method of equal step size, the interference visibility of the interference pulse is improved. Improve the visibility of the hydrophone with poor visibility from 0.42 to 0.89, the phase noise of the hydrophone is reduced, effectively balancing the Brillouin threshold and interference visibility, and greatly improving the noise of the hydrophone The consistency is of great significance to the engineering application of the optical fiber hydrophone remote transmission system.