A femtosecond laser direct writing system was developed to explore the fabrication of periodic structures in optical fibers.The possibility to write type I first- and second-order Bragg gratings in the same single-mode fiber (SMF-28e), withreflectivities of 99.6 % and 59.3 %, respectively, is presented. The fabrication of structures (waveguides and grating) in acoreless and in a SMF-28e fiber was first demonstrated, and the gratings were then exposed to a thermal annealing up to1000°C. The FBG inscribed in the SMF-28e fiber presents thermal stability at temperatures of 800 °C and a temperaturesensitivity of 14.34 pm/°C was determined.
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