Gallium lanthanum sulphide glasses for near-infrared photonic applications
This thesis investigates gallium lanthanum oxysulphide (GLS) glasses for potential fibre device applications. GLS glasses have low phonon energy and high rare earth solubility, which makes them candidates for rare earth-doped devices, and in particular for the 1.3 µm praseodymium-doped optical fibre amplifier. In addition, they have one of the highest nonlinear figures of merit among optical glasses, and therefore have potential for all-optical switching and other nonlinear devices. Practical applications, however, have been prevented by the impossibility, to date, of achieving single mode fibres with low attenuation. Previous efforts were concentrated on fabrication and especially tried to address the known issues of glass purity and thermal stability. The present work is focussed on assessing the fundamental glass transparency and on the mechanisms affecting the transmission loss of GLS glasses. The optical absorption at 1.55 and 1.7 µm was measured for the first time by laser absorption calorimetry using a tunable free electron laser source. To this end, a calorimeter was designed and commissioned and an improved model for the heat flow analysis of laser calorimetry was also developed. Our measurements identified optical absorption as the principal near-IR loss mechanism for GLS glasses. The extrinsic absorption due to transition metal ion impurities was also measured, and the presence of the weak absorption tail, due to absorption from gap states, was investigated. The occurrence of photoinduced effects and their impact on the material?s transmission wak also analysed. A thorough characterisation of the photodarkening and photoinduced absorption due to bandgap illumination, including its dependence on the excitation wavelength, and the kinetics of its formation, decay and reversibility, was achieved. Finally, the response of GLS glasses to high-intensity irradiation in the 1 µm wavelength region was studied and photoinduced darkening was observed to occur in low oxide undoped and Pr3+-doped GLS. All these topics are of great practical relevance. The present work clearly demonstrates that, in addition to the known fabrication challenges of monomode GLS fibres, careful consideration of the intrinsic transparency and photoinduced effects is essential for their successful application in fibre devices.