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 two-stage cryocooler with vibration isolation by helium gas at atmospheric pressure, and a new sapphire/ruby resonator design giving compensated operation at 8 K to 10 K with Q=(1-2)×10⁹. Stability of the first unit shows an Allan deviation of σ_y ⩽2.5×10^-15 for measuring times of 200 s⩽τ⩽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 3.0×10^-14/τ ^1/2, the lowest value measured to date for a passive atomic frequency standard, and virtually identical to the value calculated from photon statistics