Spectroscopy of rare earth doped glasses
An extensive investigation of the spectroscopy of rare earth doped glasses is presented. Such investigations are particularly important since they provide an insight into the physical processes affecting rare earth doped fibre devices. It is a central aim of this work to demonstrate how such devices can be improved by systematic changes to the host glass. Resonant fluorescence studies of thulium doped aluminosilicate and germanosilicate glass systems show that there are systematic variations in the Tm3+ site within the glass structure, indicating the non-random nature of the rare earth site. Fluorescence lifetime measurements in these silicate systems show a significant shortening of thulium energy level lifetimes when preforms are pulled into fibres, independent of the mechanism of decay. Furthermore, it is shown that multiphonon decay and therefore the vibrational properties of the host have a pivotal role in determining device performance. Raman spectra of a range of glass hosts are measured and analysed by new methods to enable accurate comparisons of vibrational properties to be made. Examination of the thulium 2µm laser system indicates that glasses with maximum vibrational energies of ~920cm-1 would give improved device performance. Based on these calculations the world's first lead-germanate based optical fibre was fabricated and fully characterised. Predicted improvements in thulium performance over fluoride or silicate systems are realised. Studies of the erbium-ytterbium 1.5µm amplifier system through Raman and lifetime measurements show that erbium site is determined purely by the ratio of phosphorus to aluminium in a silicate glass composition. It is shown that the type of erbium site determines the degree of vibrational coupling of the erbium to the highest energy vibrational modes of the glass. This coupling is linked to the performance of Er-Yb 1.5µm fibre amplifiers. Non-exponential decay of rare earth fluorescence frequently occurs and it is shown that such decays can easily result from purely single ion multiphonon decay processes when the rare earth is in a glassy host. Furthermore, a single stretched exponential function is shown to fit all observed and modelled non-exponential decays. Use of this function enables comparisons of the degree of non-exponentiality and time evolution of such decays to be made.