Microwave Photonics Filtering Interrogation Technique Under Coherent Regime For Hot Spot Detection on a Weak FBGs Array

An interrogation technique of cascaded FBG sensors based on a microwave photonics filtering technique under coherent regime is presented. The sensing information of a 5-m fiber coil containing 500 weak FBG with a similar central wavelength is retrieved. The FBGs are 9-mm long and the spatial separation between consecutive FBGs is 1 mm. The principle of operation is based on the calculation of the impulse response by recording the electrical frequency response of the system. Hot spot detection, location, and temperature measurement are demonstrated by experimental measurements with a temperature sensitivity of 0.6 dB/°C in a 10 °C range. Fifteen measurements are recorded in order to average them to reduce noise and smooth the saw tooth appearance typical of coherent measurements. The resolution of the proposed interrogation technique is related to the bandwidth of the electrical measurement. We have obtained a resolution of 20 cm for an electrical bandwidth of 1 GHz. The SNR was larger than 16 dB despite the fact that no amplification was used within the system.