Photonic-Crystal-Based Fiber Hydrophone With Sub- $100~\mu $ Pa/ $\surd $ Hz Pressure Resolution

A Fabry-Perot fiber hydrophone with improved fluidics over a previous design to increase the pressure sensitivity by two orders of magnitude and the response at low frequencies is reported. The interferometer is made of a photonic-crystal diaphragm placed at the tip of a single-mode fiber coated with a dielectric reflector. To increase the sensitivity, the water that fills the sensor head is connected to the surrounding environment through holes in the protective housing. The response extends below 100 Hz, with a measured average sensitivity of ~0.23 Pa
^-1
and a record average minimum detectable pressure of ~90 μPa/√Hz from 100 Hz to 2.5 kHz.