Self-starting coherent mode locking in a two-section laser with identical gain and absorber media

Abstract

Coherent mode-locking (CML) represents a method for ultrashort pulse generation in lasers based on the self-induced transparency (SIT) phenomena, which allows us to overcome the principal limitations on the pulse duration imposed by the active medium's gain bandwidth in standard passively mode-locked lasers. So far, this regime was only studied in lasers with transition dipole moments in the absorber medium being twice larger than in the gain medium. This fact ensures that the $2\ensuremath{\pi}$-SIT-soliton in the absorber also forms the stable $\ensuremath{\pi}$-soliton in the gain but can be barely realized in experiments. In this paper we demonstrate theoretically that the self-starting stable coherent mode-locking regime can also arise if the same medium is used both in the gain and absorber laser sections, i.e., the transition dipole moments are equal. These results are both obtained analytically using the area theorem and confirmed by the numerical solution of the Maxwell-Bloch equations. Our findings can enable new opportunities in the area of ultrafast lasers.