1.1 kW monolithic thulium-doped fiber laser in quasi-continuous-wave operation

A thulium-doped fibre laser in (quasi-continuous-wave) QCW and CW operation is investigated at 2031 nm wavelength. In QCW operation, a pump power limited peak power of 1142 W is reached at 100 µs pulse width at 1 kHz repetition rate. By overdriving the pump diodes, the peak power could be increased up to 1287 W at 10 µs pulse width. In CW operation, an average power of 770 W is reached. The beam quality is measured to be < 1.1 for 1142 W peak power in QCW operation and for 500 W average power in CW operation. During power scaling in QCW operation, modulation instabilities caused by intermodal FWM are observed and mitigation strategies are discussed. No indications of TMI (transverse mode instabilities) are observed during peak power and average power scaling.