Optical fibres have already played a huge part in ground based astronomical instrumentation, however, with the revolution in photonics currently taking place new fibre technologies and integrated optical devices are likely to have a profound impact on the way we manipulate light in the future. The Anglo Australian Observatory, along with partners at the Optical Fibre Technology Centre of the University of Sydney, is investigating some of the developing technologies as part of our Astrophotonics programme2. In this paper we discuss the advances that have been made with infrared transmitting fibre, both conventional and microstructured, in particular those based on fluoride glasses. Fluoride glasses have a particularly wide transparent region from the UV through to around 7μm, whereas silica fibres, commonly used in astronomy, only transmit out to about 2μm. We discuss the impact of advances in fibre manufacture that have greatly improved the optical, chemical resistance and physical properties of the fluoride fibres. We also present some encouraging initial test results for a modern imaging fibre bundle and imaging fibre taper.
Heavy metal fluoride glasses have many photonic applications because of their wide spectral window and lasing properties when doped with rare earths. Before fluorozirconate glasses were discovered almost all known glasses were oxide based and glass formation in fluoride compositions was not predicted. The usual theoretical approach considers the thermodynamics of solidification from the melt. The theory presented here assumes that almost any material can be solidified as a glass if cooled fast enough and considers conditions necessary to devitrify the glass formed. Nucleation and crystal growth parameters can be defined which depend only on the composition of the glass and the thermodynamic and atomic properties of the constituents and which are independent of time and thermal history of the glass. These give a quantitative expression corresponding to experimental glass-stability which can be used to plot the entire glass forming phase diagram of any fluoride system. The theoretical glass-forming phase diagrams closely match the experimental diagrams. The nucleation and crystal growth parameters can be used to define glass-forming limits which are universal for all glasses, whether fluoride, chalcogenide, oxide, nitride or even metallic glasses. Silica is confirmed as the most stable of all possible glasses.