A comparative study on the use of the extended-Cauchy dispersion equation for fitting refractive index data in crystals

This study attempts a comparative evaluation of chromatic dispersion equations in 61 crystalline solids. Based on previously published data, it is demonstrated that the extended-Cauchy equation (which in the past has been mainly used with glasses) outperforms Sellmeier models and other alternative approaches in the cases of 43 crystals, including BBO, BiBO, KTP, etc. Accumulated Cauchy coefficients for these 43 materials are presented. Performance characteristics of the extended-Cauchy model are related to specific experimental conditions, such as the number of available refractive index data, as well as the spectral location and bandwidth of measurement. The number of Cauchy coefficients required for reaching practically maximum fitting accuracy is determined. It is shown that typically, extended-Cauchy equations constructed by use of only five experimental data may be used for precise modeling of two-octave spanning spectral bandwidths.