Chromium-doped silica optical fibres: influence of the core composition on the Cr oxidation states and crystal field

We have made chromium-doped, silica-based preforms and optical fibres by modified chemical vapour deposition (MCVD) and have studied the influence of the chemical composition of the doped region on the Cr-oxidation states and the spectroscopic properties of the samples. Chromium, introduced initially as Cr3+, is partially or totally oxidized during the fabrication process, and is stabilized in the core of the preforms and fibres as Cr3+ in octahedral coordination, and/or as Cr4+ in distorded tetrahedral coordination (C-s). Small concentrations (similar to1-2 mol%) of codopants in silica, such as germanium or aluminium, suffice to promote a particular oxidation state. Particularly, 1 mol% of aluminium stabilizes all the present chromium into Cr4+. The ligand field parameters, Dq, B and Dq/B, for Cr3+ and Cr4+ are derived. It is shown that the chromium ions in our samples are in low-to-intermediate ligand field and that Dq and Dq/B decrease when the aluminium content increases. The absorption cross-sections of Cr3+ are similar to those reported in glasses and crystals, whereas Cr4+ has lower values than the reference laser materials. (C) 2001 Elsevier Science B.V. All rights reserved.