Scrutinizing the Equidistance of Frequency Comb Sources

Frequency combs have revolutionized frequency metrology, enabling optical clockworks with unprecedended precisions approaching the 10
^-18
range [1]. Early frequency comb measurements exclusively relied on mode-locked lasers, which provide a periodic waveform as a matter of working principle. In turn, the resulting frequency comb is guaranteed to be absolutely equidistant. While this expectation is ultimately based on theoretical mode-locking models, experimental tests have scrutinized deviations from equidistance of mode-locked laser combs down to the 10
^-15
level. Experimental tests of equidistance are rather elaborate and typically require beating of different spectral parts of the comb with a reference comb. Such tests have also been performed with microresonator combs [2], which are not a priori guaranteed to be equidistant from theoretical considerations. Finally, there are rather recent reports of quantum cascade laser combs; yet careful investigations of the equidistance of such sources still appear to be outstanding. Here we propose and demonstrate a novel experimental test for the equidistance of frequency combs.