Accelerated recording of negative index gratings in Ge-doped optical fibers using 248-nm 500-fs laser radiation

The fabrication of Bragg gratings in commercial B-Ge and high-Ge optical fibers using 248-nm 500-fs laser radiation is demonstrated. The underlying photosensitivity process is that of Type IIA, where negative index change gratings are recorded, following a nonmonotonic index change growth. Refractive index evolution curves for both average and modulated index changes are presented. Index changes of the order of 10 ^-3 are obtained for the fibers exposed, under 2.4 kJ/cm² accumulated energy density. For comparative purposes grating exposures using 248-nm nanosecond excimer laser radiation are also appended. Thermal annealing studies performed revealed that Type IIA negative index gratings fabricated using femtosecond pulses exhibit enhanced thermal stability, enduring temperatures up to 650degC