Ultra-Low Vibration Pulse-Tube Cryocooler Stabilized Cryogenic Sapphire Oscillator With ${\hbox {10}}^{-16}$ Fractional Frequency Stability

A low maintenance long-term operational cryogenic sapphire oscillator has been implemented at 11.2 GHz using an ultra-low-vibration cryostat and pulse-tube cryocooler. It is currently the world´s most stable microwave oscillator employing a cryocooler. Its performance is explained in terms of temperature and frequency stability. The phase noise and the Allan deviation of frequency fluctuations have been evaluated by comparing it to an ultra-stable liquid-helium cooled cryogenic sapphire oscillator in the same laboratory. Assuming both contribute equally, the Allan deviation evaluated for the cryocooled oscillator is σy ≈ 1 × 10^-15τ^-1/2 for integration times 1 <; τ <; 10 s with a minimum σy = 3.9 × 10^-16 at τ = 20 s. The long term frequency drift is less than 5×10^-14/day. From the measured power spectral density of phase fluctuations, the single-sideband phase noise can be represented by L_φ(f) = 10^-14.0/f⁴+10^-11.6/f³+10^-10.0/f²+10^-10.2/f+ 10^-11.0 rad²/Hz for Fourier frequencies 10^-3 <; f <; 10³ Hz in the single oscillator. As a result, L_φ ≈ -97.5 dBc/Hz at 1-Hz offset from the carrier.