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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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Commercially available high power diode lasers (HPDLs) with output powers of up to 6 kW have been recognized as an interesting tool for industrial applications. In certain fields of application they offer many advantages over Nd:YAG and CO2 lasers because of their low maintenance, compact design and low capital costs. Examples of successful industrial implementation of HPDLs include plastic welding, surface hardening and heat conduction welding of stainless steel and aluminum. The joining of plastics with an HPDL offers the advantages of producing a weld seam with high strength, high consistency and superior appearance. One example is the keyless entry system introduced with the Mercedes E-class where the microelectronic circuits are embedded in a plastic housing. Other applications include instrument panels, cell phones, headlights and tail lights. Applications in the field of surface treatment of metals profit from the HPDL's inherent line-shaped focus and the homogen eous intensity distribution across this focus. An HPDL system is used within the industry to harden rails for coordinate measurement machines. This system contains a customized zoom optic to focus the laser light onto the rails. With the addition of a temperature control, even complex shapes can be hardened with a constant depth and minimum distortion

Monolithic linear arrays of diode lasers, also known as diode laser bars, are the basic elements for most high-power laser applications such as solid-state laser pumping or material processing. Cylindrical microlenses used as fast-axis collimators for 10-mm diode bars require very high angles of aperture (up to 100 degree FWI/e2, equivalent to a numerical aperture of approx. 0.8) to capture most of the emitted laser power. For the efficient longitudinal pumping of laser rods, or the narrow focusing of the diode laser radiation (fiber coupling, material processing), high-quality microlenses with small lens aberrations are necessary to avoid power losses and beam quality degradation A technique for coupling the output of high-power diode laser bars into one multimode fiber with high efficiency, easy alignment requirements and low manufacturing costs is demonstrated using a single fiber with core diameter down to 400 jim. This technique comprises two micro step-mirrors for beam shaping. The overall efficiency from one diode-laser bar to fiber is 71% with 20 W cw laser power through the fiber. Coupling of 12 diode laser bars and power of 200 W out of a fiber with core diameter of 0.8 mm and NA 0.2 is achievable with this technique.