  Q-switched mode-locking (QML) has been widely observed in various lasers, but
its generation mechanism in passive mode-locking remains unclear. In this
paper, we build up a dual-color QML Erbium-doped fiber laser and find a
bound-state-like envelope on the optical spectrum for the first time.
Theoretically, the formation mechanism of QML is numerically investigated using
the coupled Ginzburg-Landau equations. In addition, we demonstrated the
existence of two QML pulse evolution patterns with gain or polarization state
variations in simulation. Our results deepen the understanding of QML pulses in
mode-locked fiber lasers and provide a foundation for studying mode-locking
nonlinear evolutionary paths.
