A possible non-halide route to ultralow loss glasses.

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 rho(2)(2epsilon/2rho))(2), where rho is density and epsilon the high frequency dielectric constant, and measures the sensitivity of epsilon to small fractional variations of density. Since epsilon 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.