Femtosecond laser Fiber Bragg Grating fabrication and in-line interferometric optical fiber structures for advanced sensing

Fiber Optic Sensors (FOSs) successfully suite critical sensing applications like in biomedicine or harsh environments. They combine robustness to noise and interference with minimal invasive impact, real-time measurement capabilities, and remote interrogation. FOSs are all-dielectric devices that are immune to electromagnetic interference and their small size allows embedding into various structures including biological tissues, overcoming the major limitations of conventional electronic sensors. Fiber Bragg Gratings (FBG)s are the most widespread FOS because of their high rejection to intensity noise and the ability to achieve quasi-distributed sensing through multiplexing. However, their fabrication requires a complex and expensive setup to induce a periodic refractive index perturbation in the fiber core that produces light reflection at a given wavelength dependent on temperature and applied strain. Another FOS analyzed is the interferometric sensor, which consists of splicing a multi-mode fiber section between two single-mode sections. The sensor is characterized by low fabrication costs and higher sensitivity, which can be up to 10 times that of FBGs. The thesis explores the optimization of fabrication process of these two types of FOSs and presents a combination of them for better performance. The FBGs in this work are written using a femtosecond laser, which is a smart and programmable solution, alternative to the more common phase-mask technique, to fabricate custom sensors, specific for a particular application. The optimization of the gratings required many runs of the fabrication-characterization processes, using different kinds of fibers (standard single-mode, bend-insensitive, double cladding, etc.) and led to high repeatable FBGs with different characteristics, but all with optimized spectral response to be used both in 2 mm long single-point configuration and multipoint sensor array. As an example of application, some FBG sensor array have been used to monitor the temperature increase during simulated laser ablation of tumors. As for the SMS sensors, a novel inline interferometric fiber structure made by offset- splicing a 10 cm long G.657A2 Bend-Insensitive Fiber (BIF) acting as the multi-mode section between two single mode pigtails at 1550 nm is also investigated. At the considered wavelength the BIF can behave like a two-mode fiber given its short length. The first junction excites both the fundamental and the first higher order mode and the two combine again at the second junction originating the interference spectrum, whose shift can be used to detected temperature and strain variations. Different samples are fabricated and characterized as temperature sensors showing a large sensitivity up to 100 pm/°C. The structure is tested realizing the optical version of the hot wire flow-meter, in which the flow of air is measured through the change in temperature induced by the flowing air in the BIF section. The optical heating is obtained by exploiting the absorption of a laser by a graphite layer deposited on the BIF section. A blue shift in the interference spectrum is well measured as air flows in the tube and dissipates heat. The behavior of the optical flow-meter is compared by connecting a commercial flow-meter in series in the same pipe, showing repeatability and sensitivity of 0.33 nm/slm. In conclusion by leveraging the distinct sensitivities of SMS and FBG it is possible to effectively mitigate cross-sensitivity issues and achieve accurate measurements