Spectroscopic assessment of novel rare earth doped glasses for active fibre devices
Spectroscopic studies of novel rare earth doped glasses are presented. Such studies provide an important insight into the physical interaction between the rare earth and the glass host. The aim of this thesis is to assess rare earth doped glasses as media for active fibre devices, particularly as components for the telecommunications industry. Through spectroscopic studies it is shown that by careful selection of the glass host, the rare earth properties can be modified to meet specific device requirements. An optical 1.3µm fibre amplifier has already been demonstrated in praseodymium doped ZBLAN. However, a greatly improved device performance has been predicted in praseodymium doped gallium-lanthanum-sulphide (GLS) glass. Oxide impurities are common in this glass. Therefore, in this study the effects of lanthanum oxide in GLS glass are identified. Oxide provides a second, preferentially occupied praseodymium site within the GLS glass which increases the inhomogeneity of the system. Further, the oxide site has a reduced 1.3µm quantum efficiency because of an increase in the maximum vibrational energy of the bonds surrounding the praseodymium ion. For 1.3µm fibre amplifiers, if oxide levels can be controlled, praseodymium doped GLS is a potentially useful and flexible host. Erbium-ytterbium codoped phosphoaluminosilicate fibres have been demonstrated to be efficient hosts for 1.5µm fibre amplifiers and lasers. In this study the role of aluminium in such fibres is identified. Through direct studies of energy transfer, aluminium is shown to have a two fold effect on the 1.5µm efficiency of the fibres. Firstly, the phosphorus to aluminium ratio determines the site of the rare earth and the non-radiative properties experienced. Secondly, an increasing aluminium concentration below that of phosphorus changes the average distance between the rare earth dopants. In both cases. the effects of aluminium are correlated with the performance of 1.5µm fibre lasers. Finally, the pairing of erbium ions is shown to limit the maximum 1.5µm erbium inversion. This has consequences for 1.5µm devices in erbium-ytterbium germanosilicate fibres. Cooperative luminescence is demonstrated in ytterbium-terbium codoped silica fibres for the first time. Time resolved fluorescence measurements confirm the presence of the phenomenon, and allow a direct measurement of the cooperative energy transfer probability. The effect of glass structure on the process is explored. Theoretical modelling shows that the probability of the process is dependent upon the strength of the dipole-dipole coupling between the rare earths. The effect of glass structure on the cooperative process is explained in terms of the influence of the individual glass constituents upon the rare earth distribution.