Enhanced Temperature $({\sim}{800}^{\circ}{\rm C})$ Stability of Type-IIa FBG Written by 255 nm Beam

This letter reports a first time study on enhanced temperature (~800°C) stability of type-IIa as compared with type-I fiber Bragg gratings (FBGs) written by high repetition rate (5.5 kHz) 255 nm UV beam. Both the FBGs were written in high photosensitive Ge doped (~10 mole%) single mode fiber without hydrogen loading. Both the FBGs survived 800°C over 9 h, however, time rate of fall of type-IIa FBG reflectivity was ~3 times lower as compared with type-I. The high temperature stability of 255 nm written FBGs is attributed to enhanced core-cladding stress release in the strong seed grating written by highly stabilized interference fringes and high cumulative fluence of 255 nm, 5.5 kHz UV pulses.