Investigations on instability effects in a sapphire-based whispering gallery mode thermometer

Whispering gallery mode thermometers (WGMTs) represent an attractive alternative to platinum resistance thermometers (PRTs) as transfer standards in industrial applications. Recent studies on the WGMT performance are also prompting its use as interpolating thermometers for the dissemination of the thermodynamic temperature scale. A low-cost and portable sapphire-based WGMT was recently developed and characterized by the authors, showing very promising results in selected temperature ranges. However, when characterized over a larger temperature range (e.g. below -20 °C), the thermometer exhibited unexpected response instability. This work explores the root cause and reports the experimental tests carried out to investigate the phenomenon, which is attributed to the resonators mechanical instability under large temperature variations, in particular to the displacement of the sapphire support. The study results are expected to lead to an improved WGMT system which, overcoming the current limitations, could achieve optimized performance over a very wide temperature range.