New results on the temperature coefficient of the refractive index for various optical materials

The refractive index of optical materials varies with changes in temperature. This, in turn, leads to temperature dependent wavefront distortions in the application. However, temperature-dependent variations in the refractive index can be offset by the thermal expansion coefficient, which leads to what is called athermal glass behavior. A crucial element for designing athermal optics is a dependable database of the thermal coefficients of the refractive index for the optical materials.For many years, reliable data on these temperature coefficients have been a key component of the data sheets for optical glass. ISO has released two standards, ISO 6760-1 and 6760-2, focused on measuring the temperature coefficient of the refractive index. More and more applications need broadband dn/dT evaluations up to 2325 nm. Current measurement capabilities are limited to 1060 nm wavelength. Questions arise if extrapolation of dn/dT data from 1060 nm to 2325 nm is a valid approach. This presentation showcases measurement results between 1060 nm and 2325 nm in comparison to datasheet extrapolations for several different glasstypes.