  We have made chromium-doped, silica-based preforms and optical fibres by
Modified Chemical Vapour Deposition 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 (Cs). Small
concentrations (~1-2 mol%) of codopants in silica, such as germanium or
aluminium, suffice to promote a particular oxidation state. Particularly, 1 mol
% of aluminium stabilizes all 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 that reported in glasses and crystals, whereas
Cr4+ has lower values than reference laser materials.
