Inscription of type IIA Bragg reflectors in a highly non-linear microstructured optical fiber using deep ultraviolet laser radiation

Abstract

ABSTRACT The fabrication of Type IIA Bragg reflectors in a highly Ge-doped microstructured optical fiber using 193nm, 10ns laser radiation, is reported. The fiber exposed was manufactured in a five-air-ring design in order to ensure both a small central core cross-section and a large cladding area with high air fraction, while the maximum germanium content of the embedded core-socket is 36mol%. Finite-difference time-domain simulations revealed that the fiber supports two strongly guided modes. The refractive index evolution curves for both average and modulated index changes obtained for the two guiding modes, exhibited a Type IIA photosensitivity behaviour. Average refractive index changes of the order of 10 -3 were recorded, under exposures of 215mJ/cm 2 energy density per pulse. The refractive index evolution curves denoted that the two guiding modes do not exhibit the same index engineering behaviour due to sufficiently different overlap with the induced perturbations. The above finding was also confirmed during the thermal annealing of the gratings inscribed. The photosensitivity effects observed , as well as, post-exposure grating amplification effects are discussed with respect to the Type IIA stress and compaction model. Finally, preliminary data related to the inscription of Type IIA gratings in the same fibre utilizin g 248nm, 500fs radiation are also presented. Keywords: Microstructured Optical Fibres, Fibre Bragg Gratings, Type IIA Photosensitivity, Femtosecond Laser Photosensitivity