Fully distributed hydroacoustic sensing based on ultra-highly sensitive and lightweight fiber-optic hydrophone cable

In this paper, we demonstrate a fully distributed hydroacoustic sensing based on the ultra-highly sensitive and lightweight fiber-optic hydrophone cable assisted with heterodyne phase-sensitive optical time domain reflectometry. The proposed lightweight hydrophone cable is an all-solid state multilayer composite cable, which mainly consists of cable core, acoustic sensitization layer, sensing fiber, and protective layer. Based on the theoretical and simulation analysis results, the acoustic sensitization layer of the hydrophone cable is extruded from thermoplastic polyurethanes to enhance the acoustic sensitivity, and the sensing fiber is uniformly wound around the acoustic sensitization layer with pre-stress. To improve the signal-to-noise ratio of the sensing signal and guarantee the measurement stability in complex underwater environment, the backscattering enhanced optical fiber inscribed with a series of backscattering enhanced points is employed as the sensing fiber to suppress the coherent fading noise. Besides, the differential-polarization-vector-sum algorithm based on the polarization diversity reception is proposed and utilized to suppress the polarization fading noise of the coherent detection system. In the experiments, the results demonstrate an excellent acoustic sensitivity of −137.2 dB re:1 rad/(μPa·m) with a flat frequency response at the range of 5 Hz ∼ 2 kHz, which is consistent with the theoretical analysis and simulation result, as well as it can keep stable whether at the water pressure of 0.3 MPa or the axial elongation of 2%. Besides, it is noteworthy that the acoustic sensitivity is higher in the frequency range of 1 Hz ∼ 5 Hz, which even can reach up to −125.3 dB re:1 rad/(μPa·m) at 1 Hz. Lastly, the noise equivalent pressure of the proposed system is about 48 dB re:1μPa/Hz1/2 at 1 kHz, which is lower than the level of the Deep Sea State Zero. In field tests, the lightweight fiber-optic hydrophone cable can be rapidly deployed, and various emergency intrusive targets like boat, frogman, etc. are all detected and tracked based on the characteristics of high sensitivity and broadband response, which has exhibited the proposed system has tremendous potential for the future surface and underwater surveillance applications.