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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 corecladding 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.

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