Anomalies and peculiarities of radiation-induced light absorption in pure silica optical fibers at different temperatures

Abstract

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.

Pulsed X‐Ray Radiation Responses of Solarization‐Resistant Optical Fibers

“…Always in previous study, the NBOHC band peaked at 1.97 eV was observed, its contribution is not resolved in this work, probably because its oscillator strength is too low (1.9 × 10 −4 ). Following the STH kinetics of both fibers in Figure , it is evident that the annealing processes of the corresponding defects differ, highlighting that the generation and the lifetime of STHs strongly depend on the material properties, as discussed in literature …”

“…At LNT, the UV‐Vis part of the RIA spectrum is well reproduced considering the sole STH contribution. Both types of STHs are then present: the ones already observed at RT, and new absorption bands linked to the strain‐assisted STHs: strain‐assisted STH 1 , peaked at 1.88 eV, FWHM = 0.3 eV; strain‐assisted STH 2 , peaked at 1.63 eV, FWHM = 0.4 eV. The strain‐assisted STHs are probably bleached at RT at time shorter than a few milliseconds.…”

“…To deeper analyze the defects at the origin of the transient RIA and their dynamics after the X‐ray pulse, a quantitative analysis has been performed. In details, we used previously published investigations to identify the possible contributors to the RIA: we considered alternative sets of Gaussian curves proposed in literature and associated to various structures of color centers (Table ) …”

“…The strain‐assisted STHs are probably bleached at RT at time shorter than a few milliseconds. A good fit of the spectra cannot be achieved without considering the contribution of the two absorption bands peaking at 2.16 and 2.61 eV, respectively, associated with the so‐called inherent STHs . Although these defects are known to be unstable at RT, Lancry and coworkers found experimentally that STH precursors are present still at RT.…”

“…As an example the attenuation of a germanosilicate optical fiber at 1550 nm increases from 0.2 dB km −1 before the X‐ray pulse up to 2000 dB km −1 during the pulse . For such transient irradiation response, the most cause of RIA in PSC optical fibers is associated to self‐trapped holes (STHs), which are unstable at RT (lifetime about tens of seconds). Two types of STHs have been identified by D.L.…”