We report on a novel type of h-shaped pulse with femtosecond substructures from a linear-cavity semiconductor saturable-absorber mirror mode-locked thulium-doped fiber laser. It yields a stable nanosecond h-shaped pulse-train with a pulse envelope width ranging from 880 ps to 1.41 ns with a typical substructure pulse duration around 440 fs. The obtained h-shaped pulse is a noise-like pulse with strong robustness to pump power changes and environmental perturbation. It can also operate in the harmonic mode-locking regime. The scheme makes the structure compact and stable, and can obtain a MHz repetition rate, which is advantageous for real-life applications 

Experimental setup of the 2 μm SESAM mode-locked laser oscillator