Redes de Bragg em fibras óticas nas bandas do visível e infravermelho

Abstract

This thesis presents the production, thermal treatment, and comprehensive characterization of fiber Bragg grating (FBG)-based optical sensors fabricated in standard optical fibers and tapered optical fibers, operating in both the infrared (IR) and visible (VIS) spectral bands. The study focuses on the development of conventional FBGs and regenerated fiber Bragg gratings (RFBGs), aiming to evaluate their thermal and strain sensitivities, long-term stability, and suitability for high-temperature and high-deformation sensing applications. Initially, saturated seed FBGs were inscribed in standard single-mode optical fibers using ultraviolet excimer laser irradiation and phase-mask techniques, targeting operation in the infrared region around 1550 nm. These gratings were thermally characterized and subsequently subjected to controlled high-temperature treatments, reaching temperatures close to 1000 °C, in order to promote the regeneration process. The evolution of the spectral response during thermal annealing was monitored in real time, enabling the analysis of grating degradation, regeneration dynamics, wavelength shift, and reflected signal stability. The resulting regenerated Bragg gratings demonstrated enhanced thermal endurance and spectral robustness when compared to conventional FBGs. In parallel, FBGs operating in the visible spectral band were fabricated in photosensitive optical fibers and characterized under thermal loading. Their spectral behavior, sensitivity coefficients, and regeneration feasibility were investigated and compared to those obtained for infrared gratings. The results highlight the intrinsic differences between IR and VIS FBGs, particularly regarding thermal sensitivity, fabrication complexity, and interrogation system cost, reinforcing the potential of visible-band FBGs for low-cost sensing solutions. Additionally, this work explores the fabrication of tapered optical fibers with diameters ranging from 30 µm to 125 µm and the inscription of FBGs within their uniform waist regions. These tapered FBG sensors were experimentally characterized in terms of temperature and axial strain. The influence of fiber diameter reduction on the effective refractive index, Bragg wavelength, and sensitivity coefficients was systematically analyzed. The results demonstrate a significant enhancement in strain sensitivity as the fiber diameter decreases, confirming the suitability of tapered FBGs for applications requiring high deformation resolution. Overall, this thesis provides a detailed comparative analysis of FBG-based sensors operating in different spectral bands and fiber geometries. The findings contribute to the understanding of thermal regeneration mechanisms, sensitivity optimization, and practical implementation of optical fiber sensors for harsh environments, including high-temperature monitoring and structural health sensing in industrial and automotive applications.