Theoretical study of mode-locked lasers with nonlinear loop mirrors

Abstract

We analyze the properties of a unidirectional class-A ring laser containing a nonlinear amplifying loop mirror (NALM). The NALM is a Sagnac interferometer consisting of an amplifier and a Kerr-type nonlinear element, and has a reflectivity that periodically varies with the intra-cavity power. To model the dynamics of these lasers, we use the approach based on Delay Differential Equations (DDEs) that has been successfully applied to describe the properties of passively mode-locked semiconductor lasers. The proposed model allows us to investigate mode locking operation in this laser. The analysis of this DDE model for mode-locked operation was performed numerically and analytically in the limit of large cavity round trip times. We demonstrate that mode-locked pulses are born though a modulational instability of the steady state solutions when the pseudo- continuous branch crosses the imaginary axis. These asymmetric pulses always co-exist with the stable laser-off solution. Hence, they can be viewed as temporal cavity solitons having similar properties with localized structures observed in bistable spatially-extended systems.