Underwater Acoustic Measurement Based On Equivalent Phase Shift Fiber Bragg Grating

An optical fiber hydrophone based on equivalent phase shift fiber Bragg grating (EPS-FBG) with temperature compensation provides an improvement of sensitivity in underwater acoustic measurement at wide frequency range, from 2.5 kHz to 12 kHz. The acoustic pressure is transduced into elastic vibration of a circle metal disk, resulting in an intensity modulation of light wave when reflected back from the Fiber Bragg grating (FBG). For the purpose of testing the performance of EPS-FBG hydrophone, we proposed a demodulation system based on narrow-line-width tunable laser. It can adjust operating point dynamically in order to cancel outside interference. The experiment within the underwater acoustic frequency range from 2.5 kHz to 12 kHz, shows the packaged optical fiber hydrophone with EPS-FBG has achieved an average sensitivity of 3.64 nm/MPa and a minimum detectable acoustic pressure of-500μPa/(?) @5 kHz, also proves about 18 dB improvement of acoustic pressure sensitivity compared with an optical fiber hydrophone with a regular apodized FBG. According to the one-to-one correspondence between the Bragg wavelength of FBG hydrophone and depth of water, we designed a water-depth measurement system based on tunable Fabry-Perot filter. With a high speed TI TMS320F2812 DSP chip embedded in the control circuit, the system has achieved a high scan frequency of 50 Hz, and also promoted its signal processing capability and extensibility. Experiment shows that the system has a resolution of 0.2 m and the measurement error is about±0.5 m.