Resonant Wavelength Thermal Stability of Fiber Bragg Gratings Produced by Femtosecond Laser

This study is focused on the fiber Bragg gratings central resonant wavelength thermal stability, fabricated by the point-by-point inscription method, using an infrared femtosecond laser. A new methodology is proposed to analyze the refractive index decay using the central wavelength shift of fiber Bragg gratings, executing stepped and continuous annealing regimes up-to a maximum temperature of 800 °C. The obtained thermal refractive index decay follows a typical power-law function, and from the theoretical model fit to the experimental data was possible to simulate and predict the refractive index decay over a large time period, up-to 1000 years. With the proposed approach it is possible to adjust the fiber Bragg gratings pre-treatment conditions to ensure effective operation during a certain time period, at a specific working temperature, without any degradation. An experimental method to mitigate this decay is proposed, and the results demonstrate the high potential of using infrared femtosecond inscribed fiber Bragg gratings for high temperature monitoring.