SENSITIVITY AND OPTIMIZATION OF A HIGH-Q SAPPHIRE DIELECTRIC MOTION-SENSING TRANSDUCER

A high-Q sapphire dielectric motion sensing transducer that operates a t microwave frequencies has been developed. The device uses cylindrica l whispering gallery modes of quality factor greater than 10(5) at roo m temperature and greater than 10(8) at 4 K. The tuning coefficient of the transducer resonance frequency with respect to displacement was m easured to be of the order of a few MHz/mu m. An electromagnetic model that predicts the resonant frequency and tuning coefficient has been developed and was verified by experiment. We implemented the model to determine what aspect ratio and what dielectric mode is necessary to m aximize the sensitivity. We found that the optimum mode type was a TM whispering gallery mode with azimuthal mode number of about 7 for a re sonator of 3 cm in diameter. Also, we determined that the tuning coeff icients were maximized by choosing an aspect ratio that has a large di ameter with respect to the height. By implementing a microwave pump os cillator of SSB phase noise -125 dBc/Hz at 1 kHz offset, we have measu red a sensitivity of order 10(-16) m/root Hz. We show that this can be improved with existing technology to 10(-18) m/root Hz and that in th e near future this may be further improved to 10(-19) m/root Hz.