Design and fabrication of customized ﬁber gratings to improve the interrogation of optical ﬁber sensors
Amelia Lavinia Ricchiuti
Fiber grating sensors and devices have demonstrated outstanding capabilities in both telecommunications and sensing areas, due to their well-known advantageous characteristics. Therefore, one of the most important motivations lies in the potential of customized ﬁber gratings to be suitably employed for improving the interrogation process of optical ﬁber sensors and systems.
This Ph.D. dissertation is focused on the study, design, fabrication and performance evaluation of customized ﬁber Bragg gratings (FBGs) and long period gratings (LPGs) with the double aim to present novel sensing technologies and to enhance the response of existing sensing systems. In this context, a technique based on time-frequency domain analysis has been studied and applied to interrogate diﬀerent kind of FBGs-based sensors. The distribution of the central wavelength along the sensing structures has been demonstrated, based on a combination of frequency scanning of the interrogating optical pulse and optical time-domain reﬂectometry (OTDR), allowing the detection of spot events with good performance in terms of measurand resolution. Moreover, diﬀerent customized FBGs have been interrogated using a technology inspired on the operation principle of microwave photonics (MWP) ﬁlters, enabling the detection of spot events using radio-frequency (RF) devices with modest bandwidth. The sensing capability of these technological platforms has been fruitfully employed for implementing a large scale quasi-distributed sensor, based on an array of cascaded FBGs. The potentiality of LPGs as ﬁber optic sensors has also been investigated in a new fashion, exploiting the potentials of MWP ﬁltering techniques. Besides, a novel approach for simultaneous measurements based on a half-coated LPG has been proposed and demonstrated. Finally, the feasibility of FBGs as selective wavelength ﬁlters has been exploited in sensing applications; an alternative approach to improve the response and performance of Brillouin distributed ﬁber sensors has been studied and validated via experiments. The performance of the reported sensing platforms have been analyzed and evaluated so as to characterize their impact on the ﬁber sensing ﬁeld and to ultimately identify the use of the most suitable technology depending on the processing task to be carried out and on the ﬁnal goal to reach.