The present invention relates to a method of fabricating Bragg grating in an optical fibre, characterised by the steps of: photosensitising the optical fibre by soaking the optical fibre in high pressure hydrogen for a minimum of a week at room temperature; and writing Bragg grating on the photosensitised optical fibre using argon fluoride (ArF) excimer laser irradiation at a pulse energy of 4-7 mJ/pulse; wherein the Bragg grating writing step is performed within an hour after the photosensitisation step; wherein the photosensitised optical fibre is being written using phase mask technique.