Spectral analysis of type III femtosecond fiber Bragg gratings at high temperatures

Fiber Bragg gratings are key components for optical fiber sensing applications in harsh environments. This study investigates the spectral response of type III fiber Bragg gratings, fabricated using femtosecond lasers and the point-bypoint technique, at very high temperatures (800°C-1200°C). During a 30-minute isochronal annealing process, we observed significant degradation in the reflected peak amplitude and a pronounced wavelength shift (red shift) at temperatures above 1125°C. These changes in wavelength could be explained by a structural relaxation in the glass, occurring above the glass transition temperature. These findings provide valuable insights into the physical transformations in type III FBGs under extreme thermal conditions, informing their potential for high-temperature sensing applications.