Fig. 1: Overview of Vernier dual-microcomb scheme for OFD.

a, An optical frequency comb system is analogous to a gear set, transferring the stability from optical frequency to RF. The large (small) size gears represent radio (optical) frequencies. The Vernier dual comb can further downconvert the RF by beating against each other. The optical reference can be a Yb⁺ ion trap (as our system is designed for) or a stabilized FC (as a stable frequency proxy we adopted). b, An illustration of a Vernier dual-microcomb system for OFD. The dual combs excited by a shared pump at 1,550 nm generate broadband spectra spanning from ~1 μm to ~2 μm. The dashed boxes indicate the four spectral regions being used, and the zoom-in views are shown above. The first sidebands around the pump (near 1550nm) are used for RF clock output. The ~2 μm light is used for Vernier beat detection and the SFG process (green arrow line) for f-2f. The dual comb is related to the 871 nm LO laser through summing the pump and one of the Vernier comb lines at ~2 μm (purple arrow line), and the 871 nm laser is locked to a stabilized frequency comb. The dotted black lines indicate the feedback of the two locks to the pump intensities of the two combs.