Temperature compensated sapphire resonator for ultra-stable oscillator capability at temperatures above 77 kelvin

We report on the design and test of a whispering gallery sapphire resonator for which the dominant (WGH_n11) microwave mode family shows frequency-stable, compensated operation for temperatures above 77 kelvin. The resonator makes possible a new ultra-stable oscillator (USO) capability that promises performance improvements over the best available crystal quartz oscillators in a compact cryogenic package. A mechanical compensation mechanism, enabled by the difference between copper and sapphire expansion coefficients, tunes the resonator to cancel the temperature variation of sapphire´s dielectric constant. In experimental tests, the WGH₈₁₁ mode showed a frequency turn-over temperature of 87 K in agreement with finite element calculations. Preliminary tests of oscillator operation show an Allan deviation of frequency variation of 1.4-6×10^-12 for measuring times 1 second ⩽τ⩽100 seconds with unstabilized resonator housing temperature and a mode Q of 2×10⁶. We project a frequency stability 10^-14 for this resonator with stabilized housing temperature and with a mode Q of 10⁷