Fabrication of low-OH GeS<inf>2</inf> glasses and multimode fibres for mid-IR applications

Abstract

The fabrication properties of ternary and quaternary GeS2-based chalcogenide glasses are discussed in view of the removal impurities and control of crystallization by controlling the starting composition, raw materials purification and fabrication steps. The two specific compositions discussed for glass and fibre fabrication are: GeS2-Ga 2S3-Cs I and GeS2-Ga 2S3-Cs I-SnS which were optimised for multimode fibre fabrication by investigating the thermal and viscosity properties. For chemical process control in food, pharmaceutical and petrochemical industry, optical fibre sensors operating above 200 o C in chemical process environment are required. For food and pharmaceutical it is also safer to use materials which are non-toxic. With these aims in mind we focussed on designing the glass structure by raising the Tg from 350 o C in ternary glass composition to 450 o C. By modifying the glass structure with the addition of SnS, the probability of near-Tg and sub-Tg structural relaxation in optical fibres, derived from quaternary compositions.For the analysis of structural modification in glass, we have systematically carried out Raman spectroscopy using 514 and 633 nm sources. We report changes in the structure as a function of composition. The Raman spectroscopic analysis is also complemented by the IR reflection and transmission spectroscopy for a more comprehensive characterisation the vibrational structures. The glass compositions were also analysed for their devitrification and near Tg relaxation behaviour. On the basis of the devitrification characteristics we have also characterised the thermal stabilities of various compositions under thermal cyclic loading in the transformation region. Th e structural relaxation behaviour above Tg was also characterised by using a differential mechanical analyser by measuring the viscosity. INTRODUCTIONGermanium disulphide (GeS2) based glasses are modelled on the structure of [SiO4] 4-structure in silicates. The main structural building block is [GeS4] 4-tetrahedron unit which forms the 3D-random network. In such a structure, the cation-anion bond length (R,Å) due to Ge 4+ (0.53Å)-S 2-(1.84Å) pair (2.37Å) is larger than that in [SiO4] 4-(R= 1.74Å). The glass formation ability and composition in GeS2 system has been very well known, as an extended IR material. However, little is known about the glass-forming system for fibre fabrication and its transmission properties. The larger bond length and lower value of electronegativity of sulphur atoms than that for oxygen in [SiO4] 4-and heavier Ge and S atoms than Si and O, makes GeS2 based glass an ideal material for engineering extended mid -IR fibres. The structural approach, discussed above, is consistent with the linear harmonic oscillator model equation 1 for calculating the fundamental frequency (o) of molecular vibration and estimation of IR absorption edge. In equation 1: