Cryogenic sapphire oscillator using a low- vibration design pulse-tube cryocooler: first results

A cryogenic sapphire oscillator (CSO) has been implemented at 11.2 GHz using a low-vibration design pulse-tube cryocooler. Compared with a state-of-the-art liquid helium cooled CSO in the same laboratory, the square root Allan variance of their combined fractional frequency instability is ??_y = 1.4 ?? 10^-5??^-1/2 for integration times 1 < ?? < 10 s, dominated by white frequency noise. The minimum ??_y = 5.3 ?? 10^-16 for the two oscillators was reached at ?? = 20 s. Assuming equal contributions from both CSOs, the single oscillator phase noise S_?? ?? -96 dB.rad²/Hz at 1 Hz set from the carrier.