Fibre Bragg grating sensors

Three different fibre sensor systems using gratings are presented. Firstly, using an acousto-optic tunable filter and closed loop feedback, arrays of fibre Bragg gratings are being used to monitor small-scale perturbations in composite materials by mapping the strain field around a defect. Gratings are also used as distributed sensors by measuring the wavelength as a function of distance. Low-coherence interferometry selects the location under interrogation and a tunable filter measures the local wavelength. An open loop interrogation technique using a commercially available optical coherence domain reflectometer is demonstrated. The reflectivity of the sensor grating is measured as a function of both distance and wavelength. Gratings 40cm long are interrogated and several distributed grating sensors are multiplexed in an array. Thirdly, a new sensing concept using subcarrier fibre gratings (SFGs) has been proposed and modelled. The SFG is a periodic reflective array resonant at RF frequencies. The resonance may be measured using subcarrier interferometry. Modelling has demonstrated the SFG to have superior performance over other subcarrier sensors.