Type IIA Grating Inscription in a Highly Nonlinear Microstructured Optical Fiber

The fabrication of Type IIA Bragg reflectors in a highly Ge-doped microstructured optical fiber using 193-nm 10-ns laser radiation is reported. Refractive index evolution curves for both average and modulated index changes are presented, for all the guided modes. Average refractive index changes of the order of 10^-3 were obtained, under exposures of 215 mJ/cm² energy density per pulse. The refractive index evolution curves denote that the two guiding modes do not exhibit the same index engineering behavior due to different overlap with the induced perturbations. The above finding was also confirmed during the thermal annealing performed, where the spectral notches corresponding to these two modes, exhibit a substantially different erasing trend.