Metrology of frequency comb sources: Assessing the coherence, from multimode to mode-locked operation

Since the beginning of this millennium, frequency comb generators have reshaped frequency metrology and related areas. After more than two decades since their first realization, several other ways to generate frequency combs, in any spectral region, have been demonstrated, each way with its peculiar features. This trend has triggered the need to quantitatively assess how close the new comb realizations are to an ideal comb, a feature that will be called combness throughout this paper. We will briefly review the very dynamic area of novel frequency comb sources and we will describe the techniques that have been recently developed to quantitatively assess the key parameters of old and new frequency combs, in view of the specific applications. Finally, we will try to sketch future steps in this recently born research area.