Ultra-low vibration pulse-tube cryocooler stabilized cryogenic sapphire oscillator with 10⁻¹⁶ fractional frequency stability

Abstract

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 Lφ(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, Lφ ≈ -97.5 dBc/Hz at 1-Hz offset from the carrier.