Fiber optic hydrophones for underwater monitoring

We report on the development and field test of fiber optic hydrophones (FOHs). In details, we focused on the development of acoustic hydrophones for towed arrays applications and seismic hydrophones for seismological monitoring applications. In both cases, the sensing configuration is based on a Michelson interferometer where a fiber coil is wrapped around a compliant mandrel acting as a sensitive element. For the first application, acoustic hydrophones were characterized in an instrumented tank at Leonardo Finmeccanica premises. The hydrophone exhibited a responsivity of about 19 nm/Pa in the frequency range 3÷10 kHz with a resolution down to 300μPa /√Hz. The same FOHs were used as a basic building block to develop a towed array with five elements. By using the same enabling technology, but tailoring the physical and geometrical properties of hydrophones, we developed seismic hydrophones. The sensing system was integrated in the seismologic monitoring system and installed at Campi-Flegrei caldera. During the field trials, we detected several earthquakes occurred in the area and compared the results with a reference piezoelectric hydrophone. The seismic sequence was used to retrieve the sensor responsivity in the frequency range 1-80Hz. The sensing system exhibited a responsivity of about 300nm/Pa and an average noise floor level down to 100μPa/√Hz. The reported field trials demonstrated the capability of FOHs to operate in relevant environments and realistic scenarios.