Fiber Fabry–Pérot Hydrophone Based on Push–Pull Structure and Differential Detection

The differential interferometric interrogation of fiber Fabry-Pérot (FFP) hydrophones based on air-backed mandrels is investigated. The hydrophone consists of two FFP interferometers wrapped on the outer and inner shells of the push-pull structure respectively, which are constructed by fiber Bragg gratings with equal distance and interrogated by the same compensating interferometer. An FFP hydrophone using a cavity length of about 10 m was calibrated with an acoustic pressure sensitivity of -143.9 dB re rad/μPa and a fluctuation of ±0.8 dB from 80 Hz to 2.5 kHz. Approximate 40 dB noise reduction in the interrogation interferometer and 5.7 dB sensitivity enhancement were achieved by the differential detection approach.