Miniature photonic-crystal hydrophone optimized for ocean acoustics

TL;DR

This paper introduces a miniature, pressure-insensitive optical hydrophone based on photonic-crystal technology, capable of detecting extremely low acoustic pressures across a broad frequency range with high dynamic range and low distortion.

Contribution

It presents a novel photonic-crystal Fabry-Perot hydrophone design with multiple sensors for high dynamic range and a method to suppress sensor cross-coupling, tested successfully in ocean-like conditions.

Findings

Detects pressures as low as 12 μPa

Operates from 1 Hz to 100 kHz with low distortion

Achieves over 160 dB dynamic range

Abstract

This work reports on an optical hydrophone that is insensitive to hydrostatic pressure, yet capable of measuring acoustic pressures as low as the background noise in the ocean in a frequency range of 1 Hz to 100 kHz. The miniature hydrophone consists of a Fabry-Perot interferometer made of a photonic-crystal reflector interrogated with a single-mode fiber, and is compatible with existing fiber-optic technologies. Three sensors with different acoustic power ranges placed within a sub-wavelength sized hydrophone head allow a high dynamic range in the excess of 160 dB with a low harmonic distortion of better than -30 dB. A method for suppressing cross coupling between sensors in the same hydrophone head is also proposed. A prototype was fabricated, assembled, and tested. The sensitivity was measured from 100 Hz to 100 kHz, demonstrating a minimum detectable pressure down to 12 {\mu}Pa (1-Hz noise bandwidth), a flatband wider than 10 kHz, and very low distortion.