Bragg grating formation and germanosilicate fiber photosensitivity

The refractive index of germanosilicate and germano-aluminosilicate fiber can be increased by a few parts in 10⁵ to upwards of five parts in 10⁴ by photobleaching the oxygen vacancy defect band of germania. Typically, the fiber core is exposed through the side of the cladding to UV radiation at a wavelength from 240 - 250 nm. Permanent phase gratings are 'written' with a specified period by using a pair of intersecting coherent beams. We have investigated the fiber photosensitivity by monitoring the grating reflectivity and wavelength spectrum during the exposure. The Bragg wavelength shifts as the grating is 'written' due to a small increase in the average refractive index of the core. From these measurements, one can determine the growth and saturation characteristics of the photoinduced changes and study the effects of composition and high temperature hydrogen annealing. We have also measured the polarization sensitivity and thermal stability of gratings and the transient and permanent changes in fiber absorption.