Optical properties of heavily ytterbium- and fluorine-doped aluminosilicate core fibres

An improved MCVD process has been proposed which allows one to fabricate ytterbium-doped fibre preforms with a fluorine-doped aluminosilicate glass-based core. Using this process, we have demonstrated the possibility of codoping the glass with high Yb³⁺ concentration (about 2 wt %), high fluorine concentration (2 wt %) and aluminium oxide concentration above 1.5 mol % (to ensure high ytterbium solubility), resulting in a record low refractive index of the glass (Δn ≈ 0.000–0.003). A series of fibre with a SiO₂ – Al₂O₃ – GeO₂ – Yb₂O₃ – F core have been fabricated and their optical characteristics have been investigated. In particular, we have demonstrated the possibility of reaching the highest possible lasing efficiency (82.7% with respect to absorbed pump power). We have studied the photodarkening level in the fibres as a function of ytterbium concentration and demonstrated the feasibility of making ytterbium-doped fibre amplifiers based on the described fibres, with an extremely low output power degradation rate (less than 20% after 10 000 h of continuous operation).