Model of a defect as a source of red luminescence in the formation of refractive-index gratings in germanosilicate glass optical fibres

The MNDO quantum-mechanical method and the framework of a model of an isolated molecular cluster were used in a study of the properties of a defect representing a doubly-negative charged ≡ Ge – O – Ge ≡ bridge in glassy silica. It was found that such a defect has optical absorption bands near 600 and 400 nm, and that absorption in these bands excites fluorescence near 650 nm. The defect also has an anomalously high polarisability, which is at least two orders of magnitude higher than the local polarisability of the glass network. A model describing a change in the refractive index of glass under the action of UV radiation is proposed on the basis of the calculated properties of the defect.