Research of very low-frequency(VLF) hydroacoustic detection based on Fizeau interferometry

We demonstrate a novel type of underwater acoustic sensor for very low-frequency hydroacoustic detection by using Fizeau interferometry. The sensor probe consists of a singlemode bare fiber with a distance of 50 meters between two adjacent ultra-weak fiber Bragg gratings(uwFBGs). The hydroacoustic signals are demodulated by using the algorithm of digital arc tangent based on 3 x 3 coupler. Experiments showed that the detection system has a good amplitude-frequency response to the very low-frequency hydroacoustic signal. Using vibration liquid column method and comparing with standard piezoelectric underwater acoustic detector, we obtained an acoustic pressure phase sensitivity about -174dB(re 1rad/^Pa) of sensors in the frequency range of 5-10 Hz. We believe this system is very promising for applications of long distance target detection and seabed seismic wave detection. Due to the ultra weak reflection characteristics of fiber Bragg gratings(FBGs), this method provides the possibility for large-scale multiplexing of the system.