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 times 10<sup>-16</sup> at 10 000 s averaging, which is sufficiently lower than the uncertainty of UTC.
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