Cryocooled sapphire oscillator with ultrahigh stability

We present test results and design details for the first short-term frequency standard to achieve ultrahigh stability without the use of liquid helium. With refrigeration provided by a commercial cryocooler, the compensated sapphire oscillator (10 K 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 10 K CSO include use of a tno-stage cryocooler with vibration isolation by helium gas at atmospheric pressure, and a new sapphire/ruby resonator design giving compensated operation at 8K to 10 K with Q = (1-2) x 10(9), Stability of the first unit shows an Allan deviation of sigma(y) 12.5 x 10(-15) for measuring times of 200 s less than or equal to tau less than or equal to 600 s. We also present results showing the capability of the 10 K CSO to eliminate local oscillator degradation for atomic frequency standards. Configured as local oscillator (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.