The influence of β-radiation exposure (the total dose up to 41.1 MGy and dose rate of 2.5 kGy/s) on the spectral characteristics of high and low reflective FBGs inscribed using femtosecond laser radiation in Ge-doped and pure-silica core fibers with polyimide and metal coating is presented. The largest Bragg wavelength induced shift (BWS) of +55 pm is observed in the case of weak reflective FBG (type I) inscribed in Ge-doped fiber. A comparable red shift in wavelength of + 50 pm is observed in the case of high reflective FBG (type II), which is explained by an increase in the concentration of GeE’-centers and, accordingly, an increase in the effective refractive index. Moreover, a significantly smaller BWS of -10 pm was obtained in the case of high reflective FBGs inscribed in pure-silica core fibers.
Radiation-induced attenuation (RIA) of light is studied in a pure-silica-core Panda fiber under γ-irradiation in the dose range 1-106 Gy at the dose rates in the range 0.015-5 Gy/s. A 5-mW erbium fiber superluminescent source or a white-light lamp (integral power of <0.5 μW) are used as the probe light source. Both RIA dose dependences at separate wavelengths in the near-IR range and RIA spectra in the near-IR and visible ranges at fixed doses are investigated. The spectra are Gaussian deconvolved and analyzed. 1.55-μm light (500 μW) is found to produce photobleaching not only of the 0.95- and 1.12-eV RIA bands, the effect observed previously, but also of the 2.6- and 1.63-eV RIA bands associated with inherent and strain-assisted self-trapped holes (STHs), respectively. RIA at λ=1.55 μm is found not to depend on dose rate at those below 0.15 Gy/s, which opens up the possibility to simply estimate the upper bound of RIA gained by the end of space mission. In particular, the RIA upper bound at λ=1.55 μm of ~1.5 dB/km is predicted for a 1-kGy space mission. RIA at λ=1.55 μm in pure-silica-core Pandas is also found to cease to depend on dose rate and probe light power at high doses (~105 Gy). The reason is that all the short-lived RCCs sensitive to the former parameters completely disappear at high doses, whereas RIA at λ=1.55 μm becomes wholly determined by the long-wavelength RIA band peaking at λ<1.7 μm and known to be dose-rate-independent and photobleaching-insensitive.
Radiation-induced absorption (RIA) of light in the visible spectral region is studied in three differently coated puresilica- core, F-doped-silica cladding optical fibers ("pure-silica fibers", PSFs) drawn from the same preform under γ-irradiation up to 1 kGy, the irradiation temperature being varied in the range ±60 °C. The coating types include aluminum, copper and polyimide. It is found out that the extremum of the temperature dependence of RIA due to strainassisted self-trapped holes (STHs) in Al- and polyimide-coated PSFs is shifted towards higher temperatures (≥+60 °C) as compared to acrylate-coated PSFs, of which the extremum temperature was previously found to be around 0 °C. Additional heat treatment in the process of application of the above coatings is argued not to the main factor influencing RIA, which increases primarily to the very presence of a metal layer on the silica surface. The latter, in turn, increases strain in the silica network and the STH population. Photobleaching of the STH-associated bands and UV-tail produced by the probe white-light halogen lamp under γ-irradiation at a dose of ~1 kGy and dose rate of 1.31 Gy/s is assessed quantitatively to be in the range from 20% to 90%.
Three single-mode fibers have been drawn from each of four MCVD-made pure-silica-core F-doped-silica-cladding preforms, the drawing parameters (temperature, speed, and tension) being varied among the fibers in a controlled fashion. The initial optical loss spectra in the range 200-1700 nm as well as radiation-induced attenuation (RIA) spectra in the near-IR range under γ-irradiation to 82 kGy (7.6 Gy/s) are measured in the fibers. RIA is found to increase significantly with increasing the drawing temperature and to increase much less with increasing the speed and tension. The mechanism of the strong drawing temperature effect on RIA is argued to be associated with a rise in the fiber silica fictive temperature, which, in turn, enhances the concentration of strain-assisted radiation-induced self-trapped holes.
Pavel Kashaykin, Alexander Tomashuk, Mikhail Salgansky, Alexey Abramov, Lyudmila Iskhakova, Nikolay Lobanov, Konstantin Nishchev, Alexey Gurynov, Eugeny Dianov
The technology, initial properties, and the value of radiation-induced attenuation (RIA) of light in the optical communication spectral range ~1.1–1.7 μm are discussed of the novel MCVD-produced undoped-silica-core F-dopedsilica- cladding fibers, of which the core is synthesized in high O2 excess (HOE) conditions (HOE-fibers). The RIA mechanisms are analyzed and compared in the HOE-fibers and in the F-doped-silica-core fibers previously commonly considered as the most radiation-resistant. The measured RIA values in the HOE-fibers and the literature data on the RIA in the commercial radiation-resistant F-doped-silica-core fibers of Fujikura are compared at λ=1.31 and 1.55 μm. Based on this consideration, the HOE-fibers are argued to be potentially superior to the F-doped-silica-core fibers as to radiation resistance especially at long wavelengths (in particular, at λ~1.55 μm). It is also argued that the fiber drawing tension reduction can further lower RIA in the HOE-fibers. A direct experimental comparison of RIA under γ-radiation from a 60Co-source at a dose rate of 8.7 Gy/s up to a dose of 94 kGy is carried out in two HOE-fibers and a commercial radiation-resistant fiber of European make. RIA in the HOE-fibers is found to be many times lower than that in the commercial fiber throughout the optical communication spectral range ~1.1–1.7 μm.
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