Investigation of sapphire-based whispering-gallery mode resonators as transfer standard thermometers

An investigation on the metrological performance of thermometers based on spherical-sapphire whispering-gallery mode (WGM) resonators was carried out and reported in this work. In view of its use in industrial applications as a transfer standard with full sensor interchangeability, several thermometers were built with different sapphires having the same, nominal, specifications and a series of tests were performed in the temperature range from −40 to 85 °C in order to explore the repeatability and reproducibility of the resulting temperature measurement systems. The repeatability, stability and measurement accuracy of each thermometer were assessed and compared. A single calibrated WGM thermometer affords a measurement uncertainty of few millikelvins. An overall temperature accuracy of 20 mK can be routinely achieved.