Femtosecond thulium fiber laser utilizing a gain-switched laser diode.

We present an ultrafast thulium fiber laser based on nonlinear pulse compression of a gain-switched laser diode operating at 1.87 µm. Seeded by a 40-ps gain-switched laser diode (GSLD), thulium-doped fiber amplifiers (TDFA) are used to increase the pulse peak power, and a normal-dispersion highly nonlinear fiber (HNF) is employed to generate a positive chirp through self-phase modulation (SPM). A bandpass filter is introduced to remove the nonlinear chirp components induced by both SPM and gain switching. A two-stage compression process is employed to maximize pulse compression and suppress the pulse pedestal. The process begins with linear compression in a passive single-mode fiber, resulting in a 1.2-ps pulse duration, followed by soliton compression in a thulium fiber amplifier. This method produces pulses with an energy of 4.7 nJ and a near transform-limited compressed pulse width of 509 fs. The all-fiberized system shows great potential as a compact, affordable, and robust alternative to mode-locked lasers for ultrafast applications.