Bragg grating formation and germanosilicate fiber photosensitivity

Abstract

ABSTRACT The refractive index of germanosiicate and germano—aluminosilicate fiber can be increased by a few parts in iO toupwards of five parts in i04 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 arewritten with a specified period by using a pair ofintersecting coherent beams. We have investigated the fiber photosensi-tivity by monitoring the grating reflectivity and wavelength spectrum during the exposure. The Bragg wavelength shifts asthe grating is written due to a small increase in the average refractive index of the core. From these measurements, onecan 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 ofgratings and the transient and permanent changes in fiber absorption.