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We demonstrate 810-W fundamental mode beam from Yb-doped fiber laser operating at 1092 nm. Single-transverse-mode operation is achieved through distributed mode filtering in a coiled 20-µm diameter and 0.06 NA core fiber. OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (140.3070) Infrared and far-infrared lasersUse of multimode core fibers with diffraction-limited output beams and recent advances in high power and high brightness diode lasers have led to currently occurring dramatic power increase from Yb-doped fiber lasers [1, 2]. Large size core allows to significantly reduce detrimental nonlinear effects and to raise optical damage threshold as well as to facilitate pump power coupling and efficient pump absorption in a double clad structure. As a result, fiber lasers currently represent the solid-state laser technology with the highest available powers and the best beam quality.

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Two applications emerge as drivers for higher brightness fiber-coupled diode lasers: advanced solid-state pumping schemes and materials processing. In contrast to the well-established side-pumping schemes of laser rods, advanced pumping schemes for today's solid-state lasers make use of the high brightness of the pump sources to increase the performance and efficiency of the solid-state laser. Materials processing applications such as metal welding and cutting are commonly served with solid-state lasers or CO2 lasers. Lately, the increased lifespan, reduced systems costs and increased brightness of fiber-coupled diode laser systems make them a new alternative. In this work, a diode laser system is described that yields 250 watts in a 600 micrometer spot with a numerical aperture 0.2 of the focused beam, corresponding to a F/# of 2.4. The system is based on a single 15 bar stack that operates in cw-mode. For brightness enhancement, it incorporates a measure to increase t he fill factor of the emitting aperture and polarization multiplexing. The brightness in the focus spot is 105W/cm2 with a F/# of 2.4 focusing optic and 3.105 W/cm2 with a high speed F/# of 1.4. To achieve the required symmetry for fiber coupling, the system incorporates a beam transformer that assimilates the beam quality along the two main axes of the beam profile. A monolithic design is chosen to reduce alignment tolerances and to increase ruggedness