We investigate the use of PbS quantum-dot doped glasses as saturable absorbers for ultrashort-pulse lasers by means of absorption bleaching experiments and numerical analysis of the pulse shaping process using Haus' master equation. We explain the mode-locking mechanism and the limitations of these absorbers. The generation of transform-limited fs-pulses is predicted by soliton mode-locking initiated by the absorption saturation of higher excited states of the quantum-dot saturable absorber.