Pseudo Combs — the Liquid State of Mode-Locking

Today the generation of frequency combs nearly exclusively relies on passive mode-locking, requiring a phase lock between the longitudinal modes of a laser. In order to overcome the non-equidistance of the cold cavity modes, it is generally considered mandatory to include an effective saturable absorption mechanism in the laser cavity. However, there exist a number of experimental demonstrations of mode-locking or comb formation in which saturable absorption was clearly absent. Here we show that four-wave mixing may equally well lead to a mode-locking effect. However, the resulting pulse trains are only partially coherent, and the comb structures lack perfect equidistance. Operation of lasers in the pseudo mode-locked regime can easily be confused with traditional mode-locking. We discuss indications and characterization approaches for unveiling pseudo mode-locking as well as limitations for application of pseudo combs.