Fiber-Optic Acoustic-Velocity Hydrophone Based on Castor-Oil-Loaded Fiber-Laser Cantilever

We demonstrate a three-axis fiber-optic acoustic-velocity hydrophone, which is based on three orthogonal planar fiber-laser cantilevers packaged in a pressure-sealed spherical shell full of castor oil. The viscous drag force of the castor oil changes the dominant damping mechanism of the cantilever. The proposed hydrophone responds to the acoustic velocity rather than the acoustic acceleration. We theoretically analyze the correspondence of the fiber laser cantilever to the acoustic motion when it is immerged in the air, water, and castor oil, respectively. We fabricated a three-axis fiber-optic hydrophone and the calibration results show that the proposed hydrophone presents operation bandwidth from 5 to 100 Hz. Its pressure sensitivity of the x-, y-, and z-axis is
$2.95\times 10^{{4}}$
,
$2.4\times 10^{{4}}$
, and
$2.31\times 10^{{4}}$
Hz/Pa at 40 Hz, respectively. The directivity index of the three axes is 27.89, 32.87, and 36.04 dB, respectively. It resists 12.5 MPa hydrostatic pressures due to the design of the spherical housing. This fiber-optic acoustic-velocity hydrophone can find many applications in deep-sea low-frequency acoustic field measurement, such as the buoy of deep water for low-frequency signal detection.