Pulse dynamics and solitons in mode-locked lasers

A model system is presented that describes the physical properties of mode-locked lasers. This distributed system incorporates gain and filtering saturated with energy while loss is saturated with power. It is found that general initial pulses evolve to stable localized solutions which exist for wide choices of the parameters, the only requirement being sufficient gain. Moreover, these pulses are essentially solitons of the classical nonlinear Schrödinger (NLS) equation. In the anomalous regime, the additional terms present in the system serve to provide the mode locking mechanism. Consequently, these pulses are approximated by the classical NLS soliton, given by hyperbolic secant functions, in agreement with recent experiments.