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

TL;DR

This study investigates how the core composition of chromium-doped silica optical fibers affects chromium oxidation states and their spectroscopic properties, revealing how dopants like aluminium influence oxidation and ligand field parameters.

Contribution

It provides new insights into how core composition and codopants control chromium oxidation states and spectroscopic characteristics in silica fibers, with detailed ligand field analysis.

Findings

Chromium exists as Cr3+ and Cr4+ in the fibers.

Aluminium stabilizes chromium as Cr4+.

Ligand field parameters vary with composition.

Abstract

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.