Original interrogation system for quasi-distributed FBG-based temperature sensor with fast demodulation technique

A fast demodulation method for a quasi-distributed sensor based on an original interrogating system for concatenated identical low reflective Fibre Bragg Gratings (FBGs) is proposed. The system is based on Optical Time Domain Reflectometry (OTDR) for which a commercial device has been extended to a wavelength-tuneable device, within an automated experimental set-up. This interrogation system has the advantage of allowing a large number of gratings to be addressed simultaneously. In our application, all gratings are low reflective (about 10%) and have a Bragg wavelength of about 1535 nm (at room temperature and without any strain applied). The demodulation scheme is based on the optimization of the least square differences between reference and measured data. This process requires only two measurement points within the central lobe of the reflection spectrum of the sensing FBG for its correct reconstruction. The measurement time is therefore drastically reduced. Repeatability measurements and related precision of the sensor are presented, for different wavelength sampling rates of the interrogating process.