Thermometry based on Whispering Gallery Mode resonators

Whispering gallery (WG) mode resonators were studied since 1980s for precision clock oscillators and for cavity quantum electrodynamics studies. They are a kind of stable, high Q, microwave resonators where a symmetric dielectric medium, such as a cylinder or a disk, is suspended in the centre of a metal cavity. A coaxial cable or a waveguide are used to couple the EM field in the microwave region and thus to excite the system resonant frequencies. WG modes are resonant modes of higher-order azimuthal number (m) having most of the EM energy concentrated on the dielectric surface. Within the temperature range of -196 °C to 500 °C the most commonly used industrial thermometer is platinum resistance thermometer (PRT) with the uncertainties of 10 mK. The PRT offers high accuracy, low drift, a wide operating range; however, it is very sensitive to mechanical shock in handing and shipping. Besides, an AC resistance bridge which is typically required as a readout device for PRT is very expensive. Accordingly, there is a great need for a stability-improved, resistant to mechanical shock, potential lower uncertainty and cost-effective industrial thermometer. WGMR thermometer (WGMRT) is a new kind of thermometer which offers greater vibration immunity, improved stability, smaller uncertainty in temperature measurement and potential lower cost than platinum resistance thermometry. An innovative sapphire whispering gallery thermometer (SWGT) was first explored at the National Institute of Standards and Technology (NIST) in 2007 by Strouse [1] with the uncertainty less than 10 mK. Five WGMs with nominal resonant frequencies ranging from 14.4 GHz to 19.1 GHz and with Q-factors, respectively, ranging from 20,000 to 90,000 were measured within the temperature range of 0 °C to 100 °C. The accuracies of his WGMTs were in the range of ± 0.02 °C and ice point repeatability was better than 2 mK. The thesis reports the tests performed on several WGMR thermometers which have different shapes of crystals to evaluate their stability, resolution and repeatability in the temperature range of -40 °C to 85°C. Thermal cycle experimental results IV showed a Q in excess of 100000 for the mode with the highest azimuthal number, making it possible to achieve a potential temperature resolution of 0.1 mK. Besides, different specimens of crystals with the same nominal specification and reassemble for the same specimen were both tested to check the reproducibility of the thermometer. The birefringence of the sapphire was also studied to make an innovative thermometer. The ratios of two doublet frequencies are sensitive to the temperature-dependent birefringence of the crystal and relatively insensitive to surface contamination and changes in the shape of the cavity. Besides, it can have an external shape that closely approximates the shape of conventional platinum resistance thermometers.