In this research, the effect of the H-2/O-2 ratio and the processing temperature parameters on the inducing and enhancement of the defect centers associated to the second-order optical non-linearity in SiO2:GeO2 glass preforms, prepared by vapor-phase axial deposition method, have been investigated. The formation of germanium oxygen deficient centers and the development of paramagnetic structures induced in the glass preforms after X-ray irradiation were investigated using UV-Vis absorption spectroscopy and electronic spin resonance. The results indicate that the concentration of germanium oxygen deficient centers increases exponentially when the H-2/O-2 ratio decreases, while the processing temperature increases, simultaneously. The electronic spin resonance spectra profiles, shows that defects of the electron trapped centers type [Ge(1), Ge(2)] are induced by the effect of X-ray irradiation. An efficient generation of defect centers associated to the second-order optical non-linearity in SiO2:GeO2 glass preforms, occurring in samples prepared with low H-2/O-2 ratios and high processing temperatures, have been observed. (c) 2005 Elsevier B.V. All rights reserved.2472285289