14-MeV Neutron, $\gamma$-Ray, and Pulsed X-Ray Radiation-Induced Effects on Multimode Silica-Based Optical Fibers

In this paper, we investigate the mechanisms of absorbing defect generation by pulsed X-rays (~1 MeV), gamma-rays (~1.2 MeV) and 14-MeV neutrons in silica-based optical fibers. We measure the spectral and temporal dependence of the radiation-induced attenuation (RIA) from the ultraviolet to the near-infrared part of the spectrum (300-900 nm). The choice of the dopants incorporated in the fiber core and cladding strongly affects the fiber radiation tolerance. The structure and generation mechanisms of the point defects responsible for the fiber degradation are discussed on the basis of Gaussian decomposition of the RIA spectra with previously characterized absorption bands. In this spectral range, the absorption bands at 2.2, 2.5, and 3.1 eV (570, 510, and 400 nm) of phosphorus(P)-oxygen hole centers (POHC) explain the increase of losses in P-codoped fibers. For P-free fibers, our measurements showed that the RIA can be attributed to different germanium-related defects with absorption bands centered at 4.41, 2.61, and 1.97 eV (280, 475, and 630 nm) respectively, named Ge(1), GeX, and Ge-NBOHC centers. In the case of pulsed X-rays, another transient defect has been evidenced with an absorption band at 3.0 eV (400 nm, FWHM=0.60 eV) that has been associated in the literature with the [GeO4]- structure. Gamma-rays and 14-MeV neutrons globally lead to the same defect generation mechanisms and the same class of defects contributes to the induced losses. For a high dose rate environment and high-speed transmission measurements, very unstable defects, like self-trapped charges, have to be considered for the evaluation of the transient vulnerability of optical fibers