Direct generation of 3.5 optical-cycle pulses from a rare-earth laser

Unlike a conventional mode-locked laser where a cooling system is normally used for the gain medium, here we propose a strategy to originally extend the mode-locked bandwidth by increasing the crystal temperature. Mode locking of a thulium laser with super-broadband (>600 nm) centered at 2.05 µm has been realized, corresponding to a pulse duration of 26 fs (3.5 optical cycles), which we believe represents the shortest pulses ever achieved from Tm/Ho lasers in the 2-µm spectral range and the fewest cycles from all the present-day RE lasers. This work, as a proof of principle study, opens a new route for the mode-locked laser toward direct generation of extremely short pulses.