Coupled irradiation-temperature effects on induced point defects in germanosilicate optical fibers

We investigated the combined effects of temperature and X-rays exposures on the nature of point defects generated in Ge-doped multimode optical fibers. Electron paramagnetic resonance (EPR) results on samples X-ray irradiated at 5 kGy(SiO2), employing different temperatures and dose rates, are reported and discussed. The data highlight the generation of the Ge(1), Ge(2), E'Ge and E'Si defects. For the Ge(1) and Ge(2), we observed a decrease in the induced defect concentrations for irradiation temperatures higher than similar to 450 K, whereas the E' defects feature an opposite tendency. The comparison with previous post-irradiation thermal treatments reveals peculiar effects of the temperature increase during the irradiation. Such difference, confirmed also by online radiation-induced attenuation measurements, has to be considered for practical use of these fibers in a mixed environment. Importantly, even if post-irradiation fading should be considered, the Ge(1) and Ge(2) concentrations measured by postmortem EPR experiments in room-temperature-irradiated samples are quite representative of the concentrations induced in the temperature range 230-450 K regardless of the investigated dose rate. The enhancement of the E' content can be related to the simultaneous generation of this defect with non-bridging oxygen hole center from strained bonds implying a relevant modification of the defects generation/formation processes in the host glass matrix.