6 kW single mode monolithic fiber laser enabled by effective mitigation of the transverse mode instability

The transverse mode instability (TMI) has been one of the main limitations for the power scaling of single mode fiber lasers. In this work, we report a 6 kW single mode monolithic fiber laser enabled by effective mitigation of the TMI. The fiber laser employs a custom-made wavelength-stabilized 981 nm pump source, which remarkably enhanced the TMI threshold compared with the wavelength of 976 nm. With appropriately distributing bidirectional pump power, the monolithic fiber laser is scaled to 6 kW with single mode beam quality (M²<1.3). The stability is verified in a continuous operation for over 2 hours with power fluctuation below 1%.