Cryo-cooled sapphire oscillator with ultra-high stability

We present test results and design details for the first short-term frequency standard to achieve ultra-high stability without the use of liquid helium. With refrigeration provided by a commercial cryocooler, the Compensated Sapphire Oscillator (10K CSO) makes available the superior short-term stability and phase noise performance of cryogenic oscillators without periodic interruptions for cryogen replacement. Technical features of the 10K CSO include use of a a-stage cryocooler with vibration isolation by helium gas at atmospheric pressure, and a new sapphire/ruby resonator design giving compensated operation at 8-10K with Q = 1 - 2 x 10(9). Stability of the first unit shows an Allan Deviation of sigma(y) less than or equal to 2.5 x 10(-15) for measuring times of 200 seconds less than or equal to tau less than or equal to 600 Seconds. We also present results showing the capability of the 10K CSO to eliminate local oscillator degradation for atomic frequency standards. Configured as L.O. for the LITS-7 trapped mercury ion frequency standard, the CSO/LITS combination demonstrated a limiting performance of 3.0 x 10(-14)/tau(1/2), the lowest value measured to date for a,passive atomic frequency standard, and virtually identical to the value calculated from photon statistics.