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The role of coherence during comb formation

Kolloquium der Abteilung 4

Mode‐locked laser combs are an indispensable tool for modern frequency metrology. While the cold‐cavity modes of a laser are not necessarily equidistant, the mode‐locking process automatically forces the modes into this favorable condition, as has been experimentally verified to the 10‐15 level. Recently, a number of new frequency comb sources have been proposed that do not rely on established mode‐locking mechanisms, e.g., passive microring resonators or quantum cascade lasers. The alleged source of comb formation in these devices is four‐wave mixing, which, however, can be shown to lead to partial coherence and results in non‐equidistant combs. Moreover, we also investigated coherence degradation processes in mode‐locked laser combs. This much weaker process is driven by Schawlow‐Townes noise. The resulting 1/f noise is expected to start limiting optical frequency measurements at the 10‐20 level or below.