Spectroscopic Study of $\gamma$ -Ray and Pulsed X-Ray Radiation-Induced Point Defects in Pure-Silica-Core Optical Fibers

We investigate the radiation-induced effects on pure-silica-core (PSC) optical fibers. For this, we measured the radiation-induced attenuation (RIA) growth and decay kinetics in four fibers with different hydroxyl and chorine contents. Our results show that PSC fibers exhibit different transient and continuous radiation responses depending of the silica-glass composition. Self-trapped charges [self-trapped excitons (STEs) and self-trapped holes (STHs)] seem mainly responsible for the fiber transient responses (times shorter than 1 s after the X-ray pulse) in the ultraviolet to near infrared part of the spectrum (300-900 nm). As these defects are unstable at room temperature, the contribution of other defects, like non-bridging oxygen hole centers (NBOHC) increases with time. Finally, these stable defects mainly explain the fiber permanent responses. To complete our online RIA measurements, we also studied by confocal microscopy luminescence (CML) the spatial distribution of the stable radiation-induced emitting centers in both pristine and irradiated fibers. Our CML results, obtained with 514 and 633 nm laser excitation, showed that several optically-active centers are non-uniformly generated in the fiber cross-sections. The generation of NBOHC by breakage of Si-O-Si strained bonds is clearly enhanced in high-OH fibers, especially at their core and cladding interfaces. The generation mechanisms of other unidentified defects emitting around 600 nm seem to be affected by both the hydroxyl content in the fiber core and by another factor that is not related to the silica glass composition.