Extraction of more than 10kW from a single ytterbium-doped MM-fiber

High-power fiber lasers have become one of the most important tools for material processing in the last decade. Laserline GmbH, which is primarily known for its multi-kilowatt direct-diode lasers, also introduced a fiber-laser platform as a brightness converter for its direct-diode lasers a few years ago. Currently, output powers up to 6 kW at beam qualities down to 4 mm mrad are commercially available. The platform is based on a single directly water-cooled ytterbium doped XLMA (extra-large mode area) fiber in an end-pumped configuration, which can easily be combined in series with a standard diode laser. In this paper, we present the latest progress in power scaling of our fiber-laser system. By improving the material properties of the active fiber, the thermal management and the thermo-mechanical stability of the resonator, up to 10 kW output power from a single, unidirectionally pumped fiber-laser oscillator with a beam parameter product in the range of 4 mm mrad to 8 mm mrad is demonstrated. Further power scaling up to 17.5 kW with 8 mm mrad was achieved by bidirectional pumping of the active fiber. In both cases, a rather high optical conversion efficiency of 75-77 % leads to a wall-plug efficiency of ~35 % for the whole laser system. Currently, we do not observe any physical limits, such as nonlinear effects for example.