Radiation Effects on Silica-Based Preforms and Optical Fibers—I: Experimental Study With Canonical Samples

Prototype samples of preforms and associated fibers have been designed and fabricated through MCVD process to investigate the role of fluorine (F) and germanium (Ge) doping elements on the radiation sensitivity of silica-based glasses. We characterized the behaviors of these canonical samples before, during and after 10 keV X-ray irradiation through several spectroscopic techniques, to obtain global information (in situ absorption measurements, electron paramagnetic resonance) or spatially-resolved information (confocal microscopy, absorption and luminescence on preform). These tests showed that, for the Ge-doped fiber and in the 300-900 nm range, the radiation-induced attenuation (RIA) can be explained by absorption bands associated with the following radiation-induced point defects: Ge(l); Ge-NBOHC and GeX. Other defects such as GeE´ Ge(2); and Ge-ODC are generated but do not contribute in this spectral domain. For the F-doped sample, the different point defects identified, SiE´, Si-NBOHC and Si-ODC(II), are unable to reproduce the RIA spectra for energies lower than 4 eV. We suggest that the radiation-induced absorption in this part of the spectrum is due to chlorine-related species, probably CI⁰ radiolytic groups that absorb at around 3.5 eV. The comparison between the sensitivities of the preform and the fiber reveals the influence of the drawing process on the glass response. Its effect is strongly dose-dependent for the germanosilicate glass. The drawing process seems to be responsible for the main part of the defects generated at low doses (<1 Mrad).