Annealing of gratings photowritten in Tin-codoped germanosilicate preform plates and fibers

Bragg gratings were written in the cores of tin codoped germanosilicate preform plates through exposure to cw UV light (244 nm). The experiments were performed using unloaded and H-2 loaded samples. The diffraction efficiencies of the holograms were measured at a probe wavelength of 633 nm. The photoinduced refractive index modulations were calculated from these data. Microscopic inspections of the plate surfaces have shown that UV exposure induced corrugations at the grating places. A phase shift interferometric microscope was used to measure the depths of the corrugations. Afterwards, both the refractive index modulations and the depths of the corrugations were measured in the course of 30 min isochronal step annealing experiments. The heating induced refractive index modulation changes were calculated from the changes in the corrugation depths by means of a strain densification model and then compared to the experimental ones. The results are compared to the ones obtained from fibers drawn from these preforms.