Monolithic thulium fiber laser with 567  W output power at 1970  nm

We report on a monolithic thulium fiber laser with 567 W output power at 1970 nm which, to the best of our knowledge, is the highest power reported so far directly from a thulium oscillator. This is achieved by optimization of the splice parameters for the active fiber (minimizing signal light in the fiber cladding) and direct water cooling. Dual transverse mode operation is visible from the optical spectrum and can be deduced from the measured beam quality of M²=2.6.