The microwave down converted from ultra-stable laser by an optical frequency comb

An ultra-stable microwave down converted from the ultra-stable laser directly by an optical frequency comb (OFC) is built at the National Institute of Metrology China. The repetition rate (f
_r
) of OFC is stabilized by stabilizing the sum of the carrier envelope offset frequency (f
_ceo
) of OFC and the beating frequency between the ultra-stable laser (USL) and OFC. A current feedback with 30 kHz bandwidth and a piezoelectric transducer with 1 kHz bandwidth are applied for the frequency stabilization. The frequency jitter of the stabilized sum frequency is about 4 Hz. Though the counting of locked f
_r
, the shifted frequency of the USL is about 2087 Hz/day. As the frequency jitter of the mixing frequency of f
_ceo
and the beating frequency between this OFC and another USL is about 10Hz, the short time stability of f
_r
is calculated to be about 5e-14. This ultra-stable microwave will be applied as the local oscillator for NIM5 Cs fountain to improve its shorter term instability.