A possible non-halide route to ultralow loss glasses

Abstract

Intrinsic material low-loss limits in optical fiber materials are primarily set by Rayleigh scattering losses from frozen-in density fluctuations. The theoretical expression for this loss contains a factor ϱ2(∂ϵ/∂ϱ)2, where ϱ is density and ϵ the high-frequency dielectric constant, and measures the sensitivity of ϵ to small fractional variations of density. Since ϵ is the sum of all bond polarizabilities per unit volume (plus a vacuum contribution), small density increments affect it in two ways; firstly they increase the number of bonds per unit volume and secondly they perturb the individual bond polarizabilities themselves. Although the separate contributions are smallest for halides, this paper highlights the fact that for certain compositions and structures they can be of opposite signs and that the possibility of a significant cancellation can arise for certain multicomponent oxide glasses. Existing theory, coupled with an analysis of presently very limited experimental photoelastic data available in the literature, demonstrates that this mechanism can certainly lead to a dramatic decrease in Rayleigh scattering loss (over silica) for some alkali silicate and aluminosilicate compositions, and identifies a number of other oxide glasses for which a similar or possibly enhanced effect can be anticipated, particular focus being given to the calcium aluminate family.