Optical Fibre Bragg Gratings for Acoustic Sensors

In this paper, we give a short review of Fibre Bragg Grating (FBG) sensors for the detection of acoustic signals, in particular ultrasound. The primary advantage of FBGs as sensing elements is their spectral encoding of the measurand, which can be either strain or temperature. However, spectral decoding methods cannot be utilized to detect high frequency signals due to their inherent low speed. We review the interrogation method required for the high speed detection of high frequency signals, in addition to discussing the theory behind FBGs as sensors. A number of applications of FBGs will be outlined for these FBG acoustic sensors, including in-vivo biomedical sensing, acoustic hydrophones, non-destructive evaluation and structural health monitoring. In addition to this introduction to the field of FBG acoustic sensing, we also present recent results on the implementation of a novel cost effective detection system. The FBG detection system developed to convert the strain induced spectral shift of the FBG into an intensity modulation is called a Transmit Reflect Detection System (TRDS). The TRDS is an extension to the standard power detection method for FBGs. In conventional power detection schemes, the reflected portion of the incident spectrum is monitored to determine the change in the measurand. In the TRDS, both the transmitted and reflected portions of the input spectrum, from a narrow band light source, are utilised. The optical power of the transmitted and reflected signals are measured via two separate photoreceivers. As the spectral response of the FBG shifts due to the measurand, the transmitted power will increase, and the reflected power will decrease, or vice versa. By differentially amplifying the transmitted and reflected components, the overall signal is increased. This results in improved sensitivity and efficiency of the photonic sensor. We show results for the sensitivity and dynamic resolution of the detection system.