Whispering gallery mode thermometry

This paper presents a state-of-the-art whispering gallery mode (WGM) thermometer system, which could replace platinum resistance thermometers currently used in many industrial applications, thus overcoming some of their well-known limitations and their potential for providing lower measurement uncertainty. The temperature-sensing element is a sapphire-crystal-based whispering gallery mode resonator with the main resonant modes between 10 GHz and 20 GHz. In particular, it was found that the WGM around 13.6 GHz maximizes measurement performance, affording sub-millikelvin resolution and temperature stability of better than 1 mK at 0 °C. The thermometer system was made portable and low-cost by developing an ad hoc interrogation system (hardware and software) able to achieve an accuracy in the order of a few parts in 109 in the determination of resonance frequencies. Herein we report the experimental assessment of the measurement stability, repeatability and resolution, and the calibration of the thermometer in the temperature range from −74 °C to 85 °C. The combined standard uncertainty for a single temperature calibration point is found to be within 5 mK (i.e., comparable with state-of-the-art for industrial thermometry), and is mainly due to the employed calibration setup. The uncertainty contribution of the WGM thermometer alone is within a millikelvin

Low-Cost Instrument for Whispering Gallery Mode Thermometry up to 19 GHz

partially_open5In this paper, a portable low-cost system for the accurate measurement of complex resonance frequencies up to 19 GHz for whispering gallery mode thermometry is presented and validated. The instrument performs vectorial transmission measurements for the determination of resonance frequencies with accuracy in the order of a few parts in in 109 over two different RF/microwave bands: 1) 800 MHz-7 GHz and 2) 13-19 GHz. The former band is suitable for quasi-spherical resonator (QSR)-based experiments, currently of great interest in different metrological applications, while the latter is expressly supported for whispering gallery thermometry. The capability of the instrument to meet the performance required for whispering gallery thermometry is first verified using an extremely stable setup based on a QSR. Afterward, the calibration of the whispering gallery thermometer composed of the developed instrument and an existing resonator is performed. The obtained results prove the suitability of the instrument to replace vector network analyzers in accurate resonance-based experiments and also its capability to enable the use of whispering gallery thermometers as transfer standards in the industrial field.partially_openCORBELLINI S., RAMELLA C., PIROLA M., FERNICOLA V., CAPPELLA A.Corbellini, S.; Ramella, C.; Pirola, M.; Fernicola, V.; Cappella, A

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 thermometer

Nano-Kelvin thermometry and temperature control: beyond the thermal noise limit

We demonstrate thermometry with a resolution of 80 $\mathrm{nK} / \sqrt{\mathrm{Hz}}$ using an isotropic crystalline whispering-gallery mode resonator based on a dichroic dual-mode technique. We simultaneously excite two modes that have a mode frequency ratio very close to two ($\pm0.3$ppm). The wavelength- and temperature-dependence of the refractive index means that the frequency difference between these modes is an ultra-sensitive proxy of the resonator temperature. This approach to temperature sensing automatically suppresses sensitivity to thermal expansion and vibrationally induced changes of the resonator. We also demonstrate active suppression of temperature fluctuations in the resonator by controlling the intensity of the driving laser. The residual temperature fluctuations are shown to be below the limits set by fundamental thermodynamic fluctuations of the resonator material

Applications of Microwave Resonators to Thermal Metrology

L'abstract è presente nell'allegato / the abstract is in the attachmen

Accurate Characterization of High-Q Microwave Resonances for Metrology Applications

Microwave resonators are widely adopted as high sensitivity sensors in both applied and fundamental metrology, to measure a number of different physical quantities, such as temperature, humidity, pressure, length and material properties. High sensitivity, and thus potential high measurement precision and accuracy, can be achieved by resorting to high-quality-factor ( $Q$ ) resonators. Nonetheless, in order to accurately measure a high- $Q$ resonance and obtain low measurement uncertainty, as required by metrology applications, the entire measurement set-up must be carefully designed. This papers presents an overview of resonance frequency measurements for metrology applications, illustrating the various aspects and issues to be dealt with when pursuing highly accurate measurements, as well as of the most relevant achievements in this field

Whispering Gallery Mode Resonators for Precision Temperature Metrology Applications

In this work, the authors exploited the whispering gallery mode (WGM) resonator properties as a thermometer. The sensor is made of a cylindrical sapphire microwave resonator in the center of a gold-plated copper cavity. Two coaxial cables act as antennas and excite the WGM standing waves in the cylindrical sapphire at selected resonance frequencies in the microwave range. The system affords a high quality factor that enables temperature measurements with a resolution better than 15 mu K and a measurement standard uncertainty of 1.2 mK, a value approximately three times better than that achieved in previous works. The developed sensor could be a promising alternative to platinum resistance thermometers, both as a transfer standard in industrial applications and as an interpolating instrument for the dissemination of the kelvin