An all-silica-fiber thulium-doped fiber laser emitting at 0.82 μm on the transition from 3H4 to the ground state 3H6 outputs 105 W continuous-wave power and 555 W quasi-continuous-wave instantaneous power with 0.96% duty cycle in 240 μs rectangular pulses. The system comprises a double-clad thulium-doped fiber designed and fabricated in-house, incorporated into an all-fiber cavity and cladding-pumped by diode lasers at 0.79 μm. Co-lasing at 1.9 μm counteracts population trapping in 3F4. The slope efficiency reaches 64% and 77.5% under quasi-continuous-wave and continuous-wave operations, respectively. Under quasi-continuous-wave conditions, the beam quality M2 becomes 2.2 (beam parameter product: 0.57 mm mrad) and 2.45 (0.64 mm mrad) in orthogonal directions at approximately 250 W of instantaneous output power. In addition, a modified quasi-continuous-wave setup is continuously wavelength-tunable from 812 to 835 nm. We believe this is the first reported demonstration of high-power laser operation of the 3H4 → 3H6 transition in a thulium-doped fiber.
