High-Resolution Slow-Light Fiber-Bragg-Grating Microphone and Hydrophone

Abstract

This paper reports a slow-light fiber-Bragg-grating (FBG) pressure sensor that functions both as a microphone and a hydrophone and pushes the limit of acoustic pressure resolution of conventional FBG sensors operated without external diaphragms. The sensor utilizes a narrow slow-light resonance to enhance its optical response (greater transmission change per wavelength shift), a soft polymeric coating to enhance its acoustic response (greater wavelength shift per pressure), and an ultra-narrow linewidth laser to reduce its laser frequency noise (which often dominates in such sensors). The coated fiber microphone exhibits an average resolution of ~210 μPa/√Hz (100 Hz to 10 kHz), which is ~6 times better than the previous record. When used as a hydrophone, the resolution is ~880 μPa/√Hz (1 to 100 kHz), which is better than the previous record for a passive FBG hydrophone by a factor of ~23 over the same acoustic frequency range. The wavelength shift per unit pressure is larger for the coated FBG than for the bare FBG by a measured factor of ~16 in air and ~3.7 in water. This last value differs markedly from a published ~300-fold enhancement using a conventional FBG hydrophone coated with the same polymer. It is shown that most of this discrepancy is due to a referencing error in the publication that reported this improvement. With further improvement, slow-light FBGs could be utilized for high-resolution applications such as in the deep sea and seismic research.