Spherical-sapphire-based whispering gallery mode resonator thermometer

A novel microwave whispering gallery mode (WGM) resonator based on a spherical sapphire crystal was developed at INRIM and its use as a thermometer was tested. The temperature dependence of the WGM frequencies was studied and the most promising resonance near to 13.6 GHz, with a loaded quality factor as large as 82 000, was carefully investigated. Its potential use in thermometry was evaluated through a study of its main metrological characteristics, such as the temperature sensitivity, the frequency stability, the repeatability, and the resolution at several temperatures over the temperature range -40 °C to 85 °C. Finally, the INRIM spherical sapphire thermometer was compared with the NIST SWGT, a dielectric thermometer based on a cylindrical sapphire resonator [V. B. Braginsky, V. S. Ilchenko, and Kh. S. Bagdassarov, Phys. Lett. A 120(3), 300 (1987)].

Measurement and analysis of a microwave oscillator stabilized by a sapphire dielectric ring resonator for ultra-low noise

Phase-noise measurements are presented for a microwave oscillator whose frequency is stabilized by a whispering gallery mode sapphire ring resonator with Q of 2x10(5). The nature of the mode, which involves little metallic conduction, allows nearly full use of the very low dielectric loss in sapphire. Several mode families have been identified with good agreement with calculated frequency predictions. Waveguide coupling parameters have been characterized for the principal (lowest frequency) mode family, for n=5 to n =10 full waves around the perimeter. For a 5-cm wheel resonator in a 7.6-cm container, Q-values of above 10(5) were found at room temperature for all of the modes in this sequence. Coupling Q-values for the same modes ranged from 10(4) (n =5) to 10(5) (n=10) for a WR112 waveguide port at the center of the cylinder wall of the containing can. Phase noise measurements for a transistor oscillator locked to the n=10 (7.84-GHz) mode showed a 1/f(3) dependence for low offset frequencies, and a value of L(f)=-55 dB/Hz at an offset of 10 Hz from the carrier. The oscillator shows phase noise below the previously reported for any X-band source.