Amplified 2-$\mu{\rm m}$ Thulium-Doped All-Fiber Mode-Locked Figure-Eight Laser

We report the first mode-locked, thulium-doped fiber figure-eight laser. The mode-locked oscillator produces 1.5-ps pulses with 63 pJ of pulse energy at a 10.4-MHz repetition rate with a 3-nm bandwidth at a center wavelength of 2034 nm. After amplification, the pulses are compressed to 370 fs with ~ 50 nJ of pulse energy. The oscillator can also operate in a square pulse regime, yielding stable pulses from ~ 100 ps to 20 ns long with ~ 100 nJ per pulse after amplification.