1.1 kW nanosecond fiber amplifier based on spectral beam combination

High average power and high repetition-rate nanosecond pulsed lasers with considerable pulse parameters and diffraction limited beam quality are needed for many industrial high-speed material processing applications like cutting, welding and drilling. Continuous-wave as well as pulsed Yb-doped fiber lasers have been established as a very power-scalable solid state laser concept with unique properties like single transverse mode operation, high power capability coupled with a high optical efficiency. Nonetheless the strong confinement of laser radiation within the guiding core over several meters of fiber unavoidably leads to the appearance of nonlinearities that limit the performance of such single emitters. One of the most promising technique to scale further beyond these limitations is the spectral beam combining method where a number of single beams get combined on a wavelength-selective element e.g. a grating. This paper reports on scaling average power with the technique of spectral beam combining of four 5 MHz repetition rate 5 ns pulsed Yb-doped fiber amplifier systems operating around 1040 nm wavelength. Superposing the beams spatially and temporally by use of an all-reflective diffraction grating 1.1 kW average power and 220 muJ pulse energy are extracted with a combining efficiency of-99 %. At maximum output power the beam quality factor M² is 1.46 and 2.7, respectively. This represents the highest average output power and the most efficient combining process ever presented in a spectrally combined pulsed fiber amplifier configuration.