Relaxation Phenomena Of The Refractive Index Caused By Thermal Treatment Of Optical Glasses Below Tg

Abstract

Optical glasses are fine annealed with annealing rates of about 7 to less than 1 K/hour in order to reduce residual stresses and to avoid inhomogeneities induced by the stress in the glass, such as birefringence and small variations of the density and the refractive index. Upon annealing in a temperature range from the glass transition temperature Tg to Tg-150 K the properties of optical glasses are often assumed to be essentially independent on the subsequent annealing procedure at temperatures below that range. In contrast to this expectation, however, it has been observed that the refractive index can be changed by heat treatment pro-cedures far below that range. These effects of heat treatment can occur during cementing, polishing and coating of optical components made from these glasses. In a recent study, samples of different types of optical glasses have been heated to tempera-tures 100 K, 200 K or as low as 300 K below Tg and subsequently these samples have been coarse or fast annealed to room tem-perature. Depending on the type of glass we have observed changes of the refractive index even as large as some 10-4 with respect to its former value. An overview will be given on the behaviour of those different types of glass investigated until now.