Fiber Optic Acoustics Hydrophone for Underwater Monitoring

This paper presents the development of a fiber optic hydrophone (FOH) specifically designed for underwater monitoring. The FOH uses a Michelson interferometer mechanism with a fiber coil wound on a compliant mandrel to detect sound-induced deformations, and it uses fiber Bragg gratings to measure strain. The FOHs were designed to be highly sensitive at zero sea state. The performance of the developed FOHs was evaluated in an instrumented tank with acoustic tones ranging from 3 to 25 kHz. The hydrophone showed a sensitivity of about 19 nm/Pa between 3 and 10 kHz, which corresponds to a resolution of 0.3 mPa/Hz^1/2. In addition, the FOH was used as the basis for developing a five-element towed array, with the multiplexed hydrophones interrogated simultaneously to determine their sensitivity. The instrumented tank tests showed excellent repeatability of hydrophone sensitivity and confirmed the potential of fiber optic technology for underwater applications, especially due to its lightness and compactness, which make it well suited for use on unmanned underwater vehicles.