Optical Solitons and Frequency Comb Generation - Abstract

Steinmeyer, Günter

Pseudo mode-locking

Today the generation of femtosecond laser pulses nearly exclusively relies on passive mode-locking, which requires 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 necessary to include an effective saturable absorption mechanism in the laser cavity. However, there exists a number of experimental demonstrations of mode-locking 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.