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

TL;DR

This paper reports the development of an ultra-low vibration pulse-tube cryocooler stabilized cryogenic sapphire oscillator achieving a fractional frequency stability of 10^-16, making it the most stable microwave oscillator with cryocooler cooling.

Contribution

The authors present a long-term operational cryogenic sapphire oscillator stabilized with an ultra-low-vibration cryocooler, demonstrating unprecedented stability and low phase noise compared to previous cryocooled oscillators.

Findings

Achieved fractional frequency stability of 10^-16.

Long-term frequency drift less than 5 x 10^-14/day.

Phase noise characterized with a spectral density L_phi approx -97.5 dBc/Hz at 1 Hz offset.

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 sigma_y = 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 side band 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 for Fourier frequencies 10^-3<f<10^3 Hz in the single oscillator. As a result L_phi approx -97.5 dBc/Hz at 1 Hz offset from the carrier.