Spectral Beam Combining of kW-class Thulium-doped Fiber Lasers

Thulium-doped fiber amplifiers have been limited to average output powers of around 1 kW for over a decade. To achieve multi-kW powers around 2 mu m wavelength, we propose using dual-grating spectral beam combining (SBC). Three customized kW-class Tm-doped fiber amplifiers operating in the 2030-2050 nm range were developed. The amplifiers consist of three stages and are pumped with non- stabilized, fiber-coupled diode lasers at 790 - 795 nm. Singlemode, TMI-free output powers exceeding 800 W, with narrow linewidths of FWHM < 115 pm, were achieved and subsequently combined using highly efficient in-house fabricated reflection gratings. With an overall combining efficiency of 90 % and a thermal slope of the combining grating measured as 6.8 K/kW, scalability to kW-level powers is enabled. The combined output power achieved a record- breaking 1.91 kW with good beam quality (M-2 < 2) and potential for further optimization. Finally, the potential power scalability of this non-coherent combining approach to power levels exceeding 20 kW is discussed.