Microwave characterisation of calcium fluoride in the temperature range 15-300 K

Abstract

Calcium Fluoride crystals are extensively used in many optical applications. In order to test the feasibility of using CaF2 crystal at microwave frequencies, a rod shaped CaF2 crystal is characterised using two types of dielectric resonators; namely Hakki-Coleman dielectric resonator and dielectric post resonator. The precise measurement results of the perpendicular component of complex permittivity of CaF2 as a function of temperature (from 15 K to 300 K) at frequencies 17 GHz and 29 GHz is presented in this paper. We have used the multifrequency S-parameter measurement technique and the Transmission Mode Q-Factor technique to improve the accuracy of measurements and hence precise values of permittivity and loss tangent. The typically used microwave parameters such as Q0×f0, temperature coefficient of frequency and temperature coefficient of permittivity are also analysed for both measurement techniques. The Qo×fo and temperature coefficient of frequency depends on the measurement technique where as the temperature coefficient of permittivity remains independent of measurement technique. This implies that temperature coefficient of permittivity data represents a material property.