New radiation colour centre in germanosilicate glass fibres

We have investigated radiation-induced absorption (RIA) of light in five optical fibres with a germanosilicate core and a lightreflecting cladding made of undoped silica glass in the visible and near-IR ranges under γ-irradiation to a dose of 1 kGy at temperatures from –60 to +60 °C. The concentration of GeO₂ in the cores of the studied fibres ranged from 3.5 to 50 mol %. It is found that the RIA dependences on temperature for optical fibres having been lightly or heavily doped by germanium are different. An increase in RIA with increasing temperature for heavily doped fibres is associated with an increase in the absorption intensity of the GeX centre due to thermal decay of Ge(1) centre. It is found that, in fibres lightly doped by germanium, RIA in the near-IR range is determined mainly by a previously unknown radiation colour centre (RCC) called the GeY centre, with a maximum of the absorption band at a wavelength of ~900 nm (1.38 eV), a half-width of 495 nm (0.71 eV), and an activation energy of 0.15 eV. It is established that the concentrations of GeX and GeY centres differently depend on the irradiation temperature and the content of germanium in silica glass: the concentration of GeX centres increases and the concentration of GeY centers, on the contrary, decreases with increasing temperature and GeO₂ concentration in the core. Thus, in accordance with its spectral position, the GeY centre represents the main RCC limiting the radiation resistance of standard telecommunication fibres with a small addition of germanium (3.5 mol % of GeO₂).