Undoped-silica-core F-doped-silica-cladding optical fibers (“undoped fibers”) are an important fiber type for applications requiring resistance to ionizing radiation (e.g., the nuclear industry, space, and military applications), the most important fundamental radiation-induced color centers arising in such fibers being self-trapped holes (STH). Despite the previous in-depth STH investigations, there have remained a few not-fully understood issues, such as the relationship between the radiation-induced absorption (RIA) bands due to STH in undoped fibers, on the one hand, and in bulk silica samples, on the other, the role of strain in the silica network in the STH occurrence, and possible peculiarities of short-lived STH-like radiation-induced color centers at temperatures above RT. To address these issues, we investigate the RIA spectra in undoped fibers with different frozen-in strain in their silica network immediately in the process of γ-irradiation to a dose of 1 kGy, the irradiation temperature being in the range ±60 °C or liquid nitrogen temperature (LNT). Gaussian decomposition of the RIA spectra measured at LNT has yielded STH bands at 2.6 and 2.16 eV together with the “classical” STH bands at 1.88 and 1.63 eV observed in fibers more frequently than the former bands. Based on this observation, it is proposed that all the STH bands observable in fibers fall into two classes: those inherent in silica and those strain-assisted, which can adjoin each other in the fiber silica network. The inherent STH include the well-known low-temperature infrared absorption and the bands at 2.6 and 2.16 eV; the strain assisted STH, the 1.88- and 1.63-eV bands. The 1.88-eV band is argued to be due to STH₁, the 1.63-eV one, due to STH₂. Anomalously high RIA at T = 0 and +60 °C is revealed and explained for the first time. The former effect is found to be caused by extreme compression of silica at T ∼ 0 °C enhancing the strain-assisted STH bands. The anomaly at T = +60 °C is found to be due to a previously unknown broad RIA band at ∼1.08 eV, which is likely to be associated with STH or self-trapped electrons and to result from either large network strain at the compression phases of enhanced thermal oscillation, or large expansion at the opposite phases. The RIA enhancement at T = +60 °C observed in this paper for the first time can influence fiber applications in the nuclear industry associated with high temperatures and high dose rates.

Radiation-induced attenuation (RIA) at wavelength λ=1.55 μm as well as the RIA spectra were investigated in optical fibers during and after irradiation at the IVG.1M nuclear reactor (Republic of Kazakhstan). The fibers studied were single-mode at λ=1.55 μm, had an undoped-silica-core and an-F-doped-silica cladding (”pure-silica fibers, PSFs”), and were provided by different manufacturers. In addition to five PSFs, a single-mode revolver hollow-core fiber was also studied. The PSFs had different coating materials: polyimide, copper, or aluminum. All fibers were exposed to three runs of reactor irradiation at enhanced temperature (155-355 °C) so that by the end of the 3rd run, the fast-neutron fluence amounted to Φ=4.46∙1017n/cm2 and γ-dose to D=2.91∙107 Gy. The total induced loss at λ=1.55 μm was found to be composed of 1) RIA, 2) bending loss (BL), caused by thermal expansion of the copper cylinder the PSFs were wound on, and 3) thermo-induced microbending loss (ML) in Cu-coated PSFs. At the end of the irradiation campaign, the net RIA in PSFs proved to lie in the interval of 0.12-0.16 dB/m, which appears to be admissible to intrareactor transport fibers. It was confirmed that long-wavelength RIA tail is the only RIA origin at λ=1.55 μm in PSFs in such strong radiation fields. The hollow-core fiber was found to feature zeroth RIA throughout the campaign.

Radiation-induced absorption (RIA) of light in five isotropic optic fibers (OFs) with a core of undoped silica glass (SiO2) and a fluorosilicate cladding and one birefringent OF of the...

Radiation-induced absorption (RIA) is investigated at $ \lambda = {1.55}~ \mu \text{m}$ in technologically improved pure-silica-core (PSC) PANDAs, in which we minimized drawing-induced silica network strain. The results of the steady-state $\gamma $ - and pulsed-X-ray irradiations are compared with those obtained earlier on former, non-optimized PSC PANDAs. It is found that the technological improvements have resulted in nearly complete suppression of the most deleterious RIA band at ~1 eV to drastically improve the radiation resistance of the PSC PANDAs.