Sub-10-nJ femtosecond mode-locked oscillator based on a double-clad thulium-doped fluoride fiber

We demonstrate a high-energy 2 µm femtosecond mode-locked oscillator utilizing a double-clad thulium-doped fluoride fiber. The active fiber was pumped by a multi-mode semiconductor laser, and stable mode-locked operation was achieved via nonlinear polarization rotation. The maximum pulse energy reached 7.8 nJ, representing one of the highest energies reported for a stable femtosecond mode-locked fiber laser oscillator at 2 µm. While the direct output pulse duration was 1.3 ps, we obtained 72 fs pulses through nonlinear self-compression in a 48 cm single-mode silica fiber. Furthermore, we demonstrate the generation of an octave-spanning supercontinuum by launching the pulse into a highly nonlinear fiber.