It was found that, at high and low strains, the growth kinetics of type-IIa gratings in 1.8μm core diameter fibers of SiO2 which was doped with 28mol%Ge could be fitted by using the known growth kinetics of Ge E' centers. It was therefore suggested that there was an association between Ge E' centers and the index modulation of type-IIa gratings; in agreement with their reported similarity with regard to thermal stability. The fact that type-IIa gratings were observed in Ge-SiO2 fibres with small core diameters, but not in pre-forms, could also be explained. A positive Bragg wavelength-shift was attributed to the growth and bleaching kinetics of Ge(1) and Ge(2) centers which contributed to the uniform index of short-period gratings.

Stress-Dependent Growth Kinetics of Ultraviolet-Induced Refractive Index Change and Defect Centers in Highly Ge-Doped SiO2 Core Fibers. T.E.Tsai, T.Taunay, E.J.Friebele: Applied Physics Letters, 1999, 75[15], 2178-80