Dependence of H2/O2 ratio and GeO2 content on the enhancement of second-order non-linearity related defects in Ge-doped optical fiber preforms

In this research, we studied the influence of the process conditions through the H2/O2 ratio and the GeO2 content, on the formation of germanium oxygen deficient centers and paramagnetic defects center induced by X-ray irradiation in SiO2:GeO2 glass preform prepared by the vapor-phase axial deposition method. The absorption band at around 5.1 eV, characteristic of germanium oxygen defect center was observed in all glass preform sample absorption spectra. The absorption coefficient increased when the GeO2 concentration increased, however, the absorption was higher in samples prepared with high H2/O2 ratio. The electronic spin resonance spectrum profiles suggest that paramagnetic structures, such as, electron trapped centers and positively charged germanium oxygen deficient centers, assigned as Ge(1) and Ge(2), respectively, are induced in all samples after being X-ray irradiated for 1 h. It was found that the concentration of induced paramagnetic centers is linearly proportional to germanium oxygen defect center concentration and that the efficiency in the enhancement of paramagnetic defect centers associated with second-order optical non-linearity increased as H2/O2 ratio used in the fabrication process was decreasing. It was concluded that SiO2:GeO2 glass preform with low H2/O2 ratio and higher GeO2 content must be prepared for the enhancement of the second-order non-linearity.