Femtosecond-laser-based optical clockwork with instability ≤63 × 10^–16 in 1 s

Two octave-spanning optical-frequency combs (750-MHz comb spacing) are phase locked to a common continuous-wave laser diode. The measured instability of the heterodyne beat between the two combs demonstrates that the intrinsic fractional frequency noise of a comb is $\le 6.3\times {10}^{-16}$ in 1 s of averaging across the $\sim 300‐\mathrm{THz}$ bandwidth. Furthermore, the average frequencies of the elements of the two combs are found to agree within an uncertainty of $4\times {10}^{-17}$ across the entire octave. We demonstrate the possibility of transfering the stability and accuracy of the best current optical standards to $\sim \mathrm{500,000}$ individual oscillators across the visible and near-infrared spectrum.