Frequency comparison of two fiber-based frequency combs at 633 nm

Two fiber-based frequency combs were compared at 633 nm. An offset laser was locked to a frequency comb, and the locked laser was measured with another comb. Consequently, the frequency uncertainty of the combs was less than 3 x 10
⁻¹⁶
at 10 000 s averaging, which is sufficiently lower than the uncertainty of UTC.