10−20 Level of Accuracy for Optical Frequency Ratio Measurements

Optical frequency combs give access to the fast oscillation of optical clocks' transitions, nowadays reaching 10
^-17
/√τ level of stability [1] and 10
^-18
level of accuracy [2]-[4]. Besides allowing a phase-coherent down-conversion to the electronically accessible radio frequencies for generating a time scale, they represent a gear in the optical domain enabling comparisons between optical frequency standards based on different atomic species [5]. Due to the need of high signal-to-noise ratio of the optical beat notes between the clocks' local oscillators and the frequency comb teeth, dedicated non-linear amplifiers with subsequent frequency conversion to the spectral region of interest is desired. Such approach allows to speed up measurement time for performing clocks' inter-comparisons and increase the robustness of the experimental setup.