Mode control of cryogenic whispering-gallery mode sapphire dielectric-ring resonators

This paper presents a technique for solving the mode purity problem that exists in cryogenic microwave frequency discriminators using X-band sapphire whispering-gallery mode resonators. The sapphire cavity resonates in a pair of quasi-degenerate modes split by a few kHz due to small geometrical imperfections. Since the nature and location of the geometrical imperfections are difficult to determine, variable coupling of the modes to a fixed output port results. Resonator applications require reproducible coupling to only one of these modes. This objective can be achieved by adding a larger dominant perturbation to the resonator. Results of tests show that a noncircularly-symmetric sapphire piece suspended above the resonator can provide controllable mode coupling and simultaneous frequency tuning