Fluorotellurite glasses with improved mid-infrared transmission

Thermal properties: glass transformation temperature (Tg); onset crystallization temperature (Tx) and the liquidus (Tm), obtained from differential thermal analysis (DTA); infrared (IR) spectra and refractive indices of glasses in the ternary fluorotellurite system (90−x)TeO2 · xZnF2 · 10Na2O (x=5, 10, 15, 20, 25, and 30 mol%) are reported here with a view to making fluorotellurite optical fibres. Glass stability (Tx−Tg) is shown to increase, and Tg decrease, with ZnF2 addition. Refractive index also decreases substantially and linearly with fluoride addition from 2.02 (5 mol%) to 1.85 (30 mol%). OH absorption, which occurs in the mid-IR for tellurite glasses, is significantly reduced with fluoride addition, and virtually indistinguishable from the background absorption for short optical path lengths of high fluoride containing glasses. Volatilization during glass melting of the series is shown to increase, as expected, with ZnF2 addition and melting time, resulting in lower optical loss. A core/clad glass pair is also proposed from the series giving a suitable numerical aperture for guiding light. Identification of OH bands and other mid-infrared bands are discussed here.