Influence of Drawing Conditions on the Properties and Radiation Sensitivities of Pure-Silica-Core Optical Fibers

The structure and radiation sensitivities of three different pure-silica-core fluorine-doped-cladding optical fibers, made from the same original preform, were investigated by several experimental techniques. The fibers were obtained by changing the drawing speed and tension in the typical ranges of values used for the radiation-tolerant waveguides. The Raman spectroscopy revealed no significant difference among the fibers before irradiation. At variance, the comparison between the fibers and their associated preform highlighted an increase in the amplitude of the D2 band that is related to the concentration of three-member rings. Moreover, in the zones where the D2 increase is larger, we observed that the R band peak shifts to higher energy values. The larger shifts are detected in the fluorine-doped cladding. These data suggest higher values of fictive temperature and density in the fibers with respect to the associated preform. The study of the concentrations of gamma-ray induced nonbridging oxygen hole centers and E'Si centers provided no evidence for significant dependence on the investigated drawing conditions. Under gamma-rays, a linear relation between the concentrations of these two defects is observed. These concentrations increase sublinearly with the dose suggesting the breaking of the Si-O-Si strained bonds as the main generation channel. Finally, under X-rays, we observed that Cl-related defects are mainly responsible for the induced absorption at about 3.5 eV. These defects are unstable at room temperature and a bleaching of their related attenuation is observed 1 h after the end of the irradiation.