Inscription of Bragg reflectors in all-silica microstructured optical fibres using 248nm, picosecond, and femtosecond laser radiation

Abstract

The inscription of Bragg reflectors in commercial, all-silica microstructured optical fibres using picosecond and femtosecond 248nm laser radiation, will be presented. The research results reported herein aim to the investigation of the optimum photosensitivity regime and index engineering route, and its dependence upon the wavelength and intensity of the laser source, for the high yield inscription of Bragg reflectors in all-silica microstructured optical fibres. The ultraviolet laser source used was a 248nm, 5ps/500fs hybrid dye/excimer; while hydrogenated Blazephotonics ESM-12 and Crystal Fiber LMA-10 microstructured optical fibres were exposed. Refractive index evolution curves for both average and modulated index changes, as well as, thermal annealing results are presented and discussed. Refractive index changes of the order of 10^-4 were obtained for relatively low accumulated energy density doses (<18KJ/cm²). For the case of the 248nm femtosecond radiation, the underlying photosensitivity process was speculated to be that of two-photon absorption, however, significant contribution from single-photon processes related with hydrogen generated and oxygen pre-existing defects is also possible. The index engineering and thermal annealing results presented for the case of 248nm ps and fs radiation, are compared to Bragg grating inscriptions using 193nm, 10ns excimer laser radiation. Further, issues related to the spatial distribution of the ultraviolet laser energy density inside the fibre core for side-illumination are presented and discussed in conjunction with the refractive index growth data.