Development of Mid-infrared Fluoroindate Glasses

Abstract

Heavy metal fluoride glasses are promising materials for active optical fibres. The initial systems reported in the literatures were fluorozirconate glasses. However, new materials are required to achieve higher and extended transmission in the IR spectrum (or equivalently lower phonon energies). This motivates the investigation of fluoroindate glasses in this project, as their lower phonon energies relative to fluorozirconate glasses extends their infrared transmission up to 8 μm (~3 mm thickness). In addition, fluoroindate glasses offer higher fibre drawing ability and resistance against devitrification. A fluoroindate glass with the specific composition of 32InF₃-20ZnF₂-20SrF₂-18BaF₂-8GaF₃-2CaF₂ (IZSBGC) has been developed in this project. This material demonstrates high transmission in the IR and high thermal stability against crystallisation. This thesis presents an approach for reducing the loss of optical fiber made using this material by optimising glass ₂melting, preform extrusion and fiber drawing conditions. The optimised fibre fabrication procedures enable us to produce fibres with reduced surface crystallization, fibre loss and enhanced fibre bending strain. Glass flow analysis provides insights into the glass temperature-viscosity behaviour and the relationship between preform surface roughness and extrusion temperature, which enables the fabrication of preforms with low surface roughnesses and eventually reduced fiber scattering loss. This thesis also presents an investigation into the chemical durability of the IZSBGC fluoroindate glass, by comparing its leaching behaviour (in deionized water) against that of the widely studied fluorozirconate (ZBLAN) system. The work presented in this thesis advances our understanding of the corrosion behaviour between the two types of fluoride glasses in deionized water and demonstrates the introduction of Na reduces the chemical stability of fluoride glasses. Based on IZSBGC glass composition, work has also been carried out in the preparation of rare earth elements doped fluoroindate glass and step index (core/clad) optical fibre for the development of a fibre laser at ~4 μm. We investigated the feasibility of techniques for fabrication including stacked billet and rod-in-tube methods. While further optimisation of fabrication procedure is required to enhance transmission, preliminary results suggest that core and clad glasses with similar fibre drawing temperature/viscosity are desirable for step-index fibre fabrication.