Vibration measurement technique for repeated fiber-optic hydrophone transmission cable system

Abstract

Fiber-optic hydrophone can be used to monitor and detect the weak acoustic signal in the marine environment. It has the characteristics of high sensitivity, good frequency response and wide dynamic range. In order to detect the acoustic signal in the open sea, the repeated fiber-optic hydrophone transmission system is necessary to realize the long-distance transmission. Due to the unidirectional optical isolation of the repeater in the optical transmission path, the Rayleigh back-scattered light of the optical cable after the repeater cannot be returned to be detected, so there is no means to monitor the status of the optical cable after the repeater. In order to realize the monitoring of the vibration state of the submarine cable after the repeated fiber-optic hydrophone transmission systems, a vibration measurement technique based on the combination of distributed acoustic sensing(DAS) technology and the optical cross coupling technology between each amplifier pair of the repeater is proposed. In order to reduce the influence of optical surge caused by the returned optical pulse amplification, the multi-wavelength pulse of optical fiber hydrophone is used as the filling light pulse of distributed vibration detection. The Rayleigh back-scattered light with vibration information is amplified by the repeater through the cross-coupling path, then detected and demodulated by Heterodyne detection technology, and the vibration information is acquired. A distributed fiber-optic vibration sensing system for repeated fiber-optic hydrophone transmission system is established in the laboratory. The system can not only use the multi-wavelength light of fiberoptic hydrophone as the filling light of distributed vibration detection, but also filter the Rayleigh back-scattered light induced by the multi-wavelength light , so as to improve the signal-to-noise ratio of detection. The system can accurately locate the vibration of optical cable within 5km after the repeater in real time, and the maximum measured vibration frequency is 2kHz. The power spectrum density fluctuation at 1kHz is less than 2dB. The experimental results show that the coupling optical path does not affect the demodulation of the fiber-optic hydrophone array signal by the fiber- optic hydrophone demodulation system.