Abstract
The past two decades have seen rapid development of optical atomic clocks with fractional performance at 10−18 level or even below. Dissemination of optical atomic clocks into microwave domain and their intercomparisons using optical frequency combs as clockwork requires precise measurements of their carrier-envelope offset frequencies (fceo). To this end, passive stabilization to zero offset frequency has been considered as a promising approach. However, previous studies often indicate imperfections in passive stabilization schemes. Therefore, in this Letter, aiming to unveil the in-depth noise performance and origin of fceo of passively stabilized frequency combs, a frequency-shifted f-2f interferometer is presented to characterize the fceo frequency stability, frequency shift, frequency uncertainty, linewidth, and phase noise. The stability observed amounts to 2.46 × 10−17 and 4.05 × 10−20 in 1 and 10 000 s, respectively. The linewidth of fceo is 3.7 mHz, with resulting phase noise of − 40 dBrad2/Hz and −64 dBrad2/Hz at 1 and 10 Hz offset frequency, respectively. The phase noise at > 1 kHz is confirmed to be of stochastic origin through correlation analysis. The measured fceo is conclusively determined as −75(14) μHz.
Published Version
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