Use of the Sellmeier dispersion formula for optical glasses and practical implications

Abstract

The Laurent series expansion of low order used hitherto to describe the dispersion of the refractive index, n((lambda) ), of optical glasses typically in the range from 0.365 01 micrometers to 1.013 98 micrometers is no longer useful, if the range of wavelength is increased. A more appropriate equation to describe n((lambda) ) is a Sellmeier-type equation with several oscillator terms. We applied a three-term Sellmeier equation to experimental data of n((lambda) ) of optical glasses for a large interval of wavelength ranging from the absorption edge in the UV to 2.325 4 micrometers . The data can be fitted in the full range with an accuracy of some units in the sixth decimal place corresponding to the accuracy of the measurements. The change of the refractive index with the temperature has also been investigated. A dispersion formula, which has been developed recently, has been applied to experimental data of the temperature coefficient of n((lambda) ) in the range of wavelength between the UV absorption edge and 1.013 98 micrometers for the temperature range from - 100 degree(s)C to + 140 degree(s)C. This dispersion formula can be integrated to give directly the increments or decrements of the absolute refractive index with respect to n((lambda) ) at a given reference temperature, e.g., 20 degree(s)C. Based on a one-term Sellmeier equation we also developed and successfully tested dispersion formulae for the stress-optical coefficient and the Verdet constant.