Ultra-stability Yb-doped fiber optical frequency comb with 2 × 10^-18/s stability in-loop

We demonstrate a full control ultra-stability Yb-doped fiber optical frequency comb (OFC). The carrier-envelop offset frequency (f_ceo) and the repetition rate (f_r) are locked with the standard phase locked loop (PLL) technique. The f_ceo is locked to the radio frequency (RF) synthesizer, and the Allan deviation is 1.2 × 10^-17/s. The f_r is locked to an ultra-stability continuous wave (CW) laser at 972 nm. The beat signal (f_beat) between the Yb-doped fiber OFC and CW laser is obtained with the signal to noise ratio (SNR) of 43 dB at 300 kHz resolution bandwidth (RBW). The time jitter of the f_beat signal is 278 as, which is integrated from 1 Hz to 10 MHz. The long-term stability is 575 μHz in 3 hours, and the corresponding Allan deviation is 2 × 10^-18/s, which is the best stability result in Yb-doped fiber OFC. The linewidth is narrowed from 200 kHz to subhertz magnitude limited by the instrument resolution bandwidth.