Fig. 1. Block diagram of an optical frequency synthesizer. An OFS includes an internal oscillator, an optical frequency comb, and output lasers. The internal oscillator is a cavity-stabilized laser at 1064 nm with a frequency instability of 6×10−16 at 1 s averaging time. The comb is phase locked to the RF signals referenced to a Rb microwave clock. Using the comb combined with comb frequency noise-reduction techniques, the frequencies of the output laser and a 578 nm laser are accurately set. The phase-locked 578 nm laser is used to probe the clock transition of Yb atoms and then make the internal oscillator further referenced to the Yb atomic clock as νo=νYb×R1.

Fig. 2. Performance of optical frequency synthesis using a comb phase locked to a Rb clock. (a) Frequency fluctuation of fr and f0 when phase locked to RF signals referenced to the Rb clock. (b) Spectrum of the beat note fb1 between a comb line at 1064 nm and the cavity-stabilized laser at 1064 nm (left panel, RBW of 30 kHz). The spectrum of fb (right panel, RBW of 1 mHz and acquisition time of 1000 s). It shows a combined noise of optical frequency synthesis and SHG. (c) Fractional instability of a comb line (blue triangles), the most stable optical clocks (blue solid line), and Rx−m (optical frequency synthesis noise) in terms of Allan deviation (i) when the comb is phase locked to the Rb clock, and optically self-referenced time base ftime is used as the time base of ftune and the counters (green dots); (ii) when the comb is phase locked to a cavity-stabilized laser, and the optically self-referenced time base ftime is used (purple filled squares); and (iii) when the comb is phase locked to the Rb clock, and the Rb clock is used as the time base of ftune and the counters (red open squares). (d) Experimental diagram to test the performance of optical frequency synthesis. The ratio Rx is measured against RSHG set by the second-harmonic generation. PLL, phase-locked loop.

Fig. 3. OFS referenced to an Yb optical clock. (a) Experimental diagram. With a comb phase locked to a Rb clock, the coherence of the internal oscillator at 1064 nm is transferred to 578 nm for detecting the clock transition of Yb atoms. Any frequency deviation of the 578 nm laser from the atomic transition is fed back to νo to assure νo=νYb×R1. A tunable single-frequency laser is employed as the output of the OFS, whose frequency is controlled to be νout=νYb×R1/Rx. (b) Rabi spectrum of Yb clock transition (no averaging). Fourier-limited Rabi spectra with spectral linewidth of 4.1 Hz and 2 Hz (inset) are observed at interrogation times of 200 ms and 400 ms, respectively. (c) Fractional frequency instability of ν578 when referenced to the Yb clock based on an interleaved measurement.