Ultra-Low Vibration Pulse-Tube Cryocooler Stabilized Cryogenic Sapphire Oscillator With 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 sigma(y) approximate to 1 x 10(-15)tau(-1/2) for integration times 1 < tau < 10 s with a minimum sigma(y) = 3.9 x 10(-16) at tau = 20 s. The long term frequency drift is less than 5 x 10(-14)/day. From the measured power spectral density of phase fluctuations, the single-sideband phase noise can be represented by L-phi(f) = 10(-14.0)/f(4) + 10(-11.6)/f(3) + 10(-10.0)/f(2) + 10(-10.2)/f + 10(-11.0) rad(2)/Hz for Fourier frequencies 10(-3) < f < 10(3) single oscillator. As a result, L-phi approximate to -97.5 dBc/Hz at 1-Hz offset from the carrier.