Cr4+-doped silica optical fibres: absorption and fluorescence properties

Chromium-doped silica-based optical fibres emit infrared fluorescence at 77 K near 1250 nm (500 nm band-width) under 860-980 nm excitation. Visible and near-infrared absorption spectra of fibres were analysed using the Tanabe-Sugano formalism. It is shown that although the fibre core is codoped with only 1 mol% of aluminium. chromium is preferentially stabilized as Cr4+ in Al-rich regions of the glassy matrix in distorted tetrahedrally coordinated sites. The infrared fluorescence is assigned to Cr4+ along a transition from the T-3(2) state down to the (3)A(2) ground state.