Luminescence and photoinduced absorption in ytterbium-doped optical fibres

Photochemical reactions induced in the glass network of an ytterbium-doped fibre core by IR laser pumping and UV irradiation have been investigated by analysing absorption and luminescence spectra. We have performed comparative studies of the photoinduced absorption and luminescence spectra of fibre preforms differing in core glass composition: Al2O3 : SiO2, Al2O3 : Yb2O3 : SiO2, and P2O5 : Yb2O3 : SiO2. The UV absorption spectra of unirradiated preform core samples show strong bands peaking at 5.1 and 6.5 eV, whose excitation plays a key role in photoinduced colour centre generation in the glass network. 'Direct' UV excitation of the 5.1- and 6.5-eV absorption bands at 244 and 193 nm leads to the reduction of some of the Yb3+ ions to Yb2+. The photodarkening of ytterbium-doped fibres by IR pumping is shown to result from oxygen hole centre generation. A phenomenological model is proposed for the IR-pumping-induced photodarkening of ytterbium-doped fibres. The model predicts that colour centre generation in the core glass network and the associated absorption in the visible range result from a cooperative effect involving simultaneous excitation of a cluster composed of several closely spaced Yb3+ ions.