Compact temperature-compensated cryogenic whispering gallery mode resonator operated at 63 K in a closed cycle cooler

Single crystal dielectric materials such as sapphire demonstrate remarkably low values for the dielectric loss tangent, even at rather modest cryogenic temperatures (40-80 K) which are readily achieved using closed cycle coolers. We have taken our previously reported novel design for a composite dielectric resonator incorporating both sapphire and rutile single crystal elements and further improved the Q value while raising the frequency versus temperature compensation point to greater than 60 K. At the same time the geometrical size of the dielectric pucks has been increased while still using the same closed cycle cooler so that a compact loop-oscillator system is realised at a frequency close to the Cs atomic clock hyperfine frequency (~10 GHz) and an operating temperature of 63.5 K. Unloaded Q values as high as 10⁷ have been achieved under these operating conditions for this composite resonator. In addition we have investigated a compensated sapphire/rutile whispering gallery mode system at 17 GHz with a view to reducing the phase noise of oscillator based on this technology. Such oscillators can be used both as low phase noise sources and as long term stable frequency standards. The following sections describe some of the issues to be addressed to optimise performance