AN INNOVATIVE FIBRE BRAGG GRATING BASED SENSING SYSTEM

Fiber Bragg grating (FBG) based sensors are one of the most promising devices for optical sensing. FBGs have been used in an increasing number of applications in the past 20 years: most notable examples are in health monitoring of large structures, such as bridges, dams, buildings, and planes, but they are also employed for sensing of chemicals and in medicine. The reasons for their popularity in such a large variety of fields are the well-mastered and repeatable fabrication technology, combined with high sensitivity, light weight and intrinsic electromagnetic immunity. Moreover, FBGs can be easily multiplexed along a single fiber to realize a quasi-distributed multipoint sensing system. Another advantage of FBGs is that the measurement signal is encoded into the wavelength, so it is very robust to intensity noise, a property highly appreciable in conditions that require long fiber spans to remote the measurement system or when sharp bends are necessary to reach the measurement point. However, despite these excellent properties, their use is still confined to specific niche applications so their diffusion is not matching the expectations. Based on the survey conducted, it emerged that the most critical factor limiting their further application comes from the interrogation system. Many different approaches have been proposed in the literature, and some of them led to the development of commercial products. Still, the price/performance and performance/complexity trade-offs have not been solved yet. The thesis presents the development and some applications of an innovative interrogation system for FBG sensors that is aimed at overcoming the previously outlined limitation by shifting the complexity, and thus the cost, from the optical components to the electronics. The interrogator makes use of a portable and lowresolution but fast spectrum analyser; then, thanks to high speed data processing algorithm, the resolution is improved to state-of-the-art values. The final outcome is a demonstrator able to reach a 2.5 kHz processing speed with a 1 pm precision for a 30dB signal to noise ratio. This high speed and precision enable the real-time and precise measurement of both temperature and strain in many practical cases. Therefore, to assess the interrogation system performance, some applications are explored, mainly in real-time temperature monitoring for solid tumor thermal ablation and real-time vibration detection in structural health monitoring. Theexperimental results showed a good potential of application for the developed sensing system. Additionally, the thesis introduces two theoretical new ideas about sensors based on FBG. One is about a novel displacement FBG sensor with tunable sensitivity and multiple working range; the other one is an innovative FBG based sensor which can realize simultaneous measurement of temperature and strain.