Fig. 2: Optical spectra.
From: Vernier microcombs for integrated optical atomic clocks
a,b, The main comb (a) and the Vernier comb (b), each measured after pump suppression using a coarse wavelength-division multiplexing filter. c, The short-wavelength main comb line (dispersive wave) (blue) and the SFG lines at 1 μm (green) for f-2f beat detection. d, The 871 nm LO laser (light blue), the FC lines (grey) and the SFG lines at 871 nm (purple and yellow) generated by summing two lines from the same comb (purple) and by summing two lines from different combs (yellow). Detailed operation conditions for obtaining these two SFG curves can be found in Supplementary Section 5.2. The FC lines are spaced by ~250 MHz, which is smaller than the resolution of the optical spectrum analyser (~1 GHz). Hence, individual comb lines are not resolved here, and the trace appears featureless. The other spectra are comb-line resolved. c and d are measured at 0.01 nm resolution.
