Femtosecond-comb based 10 MHz-to-optical frequency link with uncertainty at the 10−18 level

The International System of Units (SI) definition of the ‘second’ is likely to be redefined in the optical frequency region in the near future. Therefore, uncertainty reduction in the frequency link between 10 MHz, which is used in the frequency link with an artificial satellite, and the ultrastable laser for observing clock transitions in optical clocks, is an important issue for numerical determination in the new definition and for International Atomic Time operation after the redefinition. We have evaluated all the possible uncertainty factors in the frequency link with a femtosecond comb between 10 MHz and an optical frequency and obtained the lowest uncertainty yet reported of 3.9 × 10−18 by temperature-stabilizing RF multipliers and by using temperature insensitive coaxial cables. Furthermore, we have validated the uncertainty of the frequency link by simultaneously measuring an optical frequency of an identical laser using two femtosecond combs referenced to a common 10 MHz source and confirmed to be consistent with the evaluated uncertainty.