Generation and dynamics of stepped h-shaped noise-like pulses in a mode-locked thulium-doped fiber ring laser

New h-shaped pulses with stepped flattops are observed in a mode-locked thulium-doped fiber laser (251 m long cavity) by the nonlinear polarization rotation technique. The step at the end of an h-shaped pulse has a lower intensity and an almost fixed duration (similar to 5 ns) when it is tuned in the range of several tens of ns. By incorporating simple gain dynamics and a model of saturable absorption, the h-shaped pulses are simulated from a retrieval of noise-like pulse solutions. The change regularity of pulses in experiment is basically verified and explained by the numerical results. An analysis reveals that gain depletion and mode-locking threshold are key factors to form the 'h' shape. It is also proposed that the step tail is a kind of noise-like square wave pulse generated at the trailing edge.