We report on an ytterbium-doped fiber laser (FL) pumped by an Yb:YAG thin disk laser . The FL essentially acts as a brilliance converter suitable for applications requiring a diffraction limited kilowatt-class laser source. The FL can be positioned close to the application at the end of a multimode delivery fiber, which solves the length limitation of highpower single-mode delivery caused by non-linear effects in the fibers. The 11 m long fiber used in the experiments has a highly doped (1.3•1026 ions/m3) core with a diameter of 30 μm surrounded by a circular cladding with a diameter of 100 μm. The core has a N.A. of 0.06 and a V-parameter of 5.2. A commercial thin disk laser with a BPP of 4 mm•mrad is used as pump source. At the wavelength of 1030 nm, the pump light absorption is 1.4 dB/m, which is 77 times lower than the peak value at 976 nm. So far, a maximum cw output power of 925 W has been achieved at a pump power of 1203 W . The output power was limited by the incomplete conduction cooling of the fiber and the temperature stability of the acrylate coating. With single pass pumping, an absolute efficiency of 77 % was achieved in good agreement with the simulation result of 81 %. Double-pass pumping will further increase the efficiency. The M2 was measured to be 2.6 with the fiber coiled on a 120 mm diameter cylinder. With a fully optimized fiber, having a smaller V-number and an octagonal pump cladding, an efficiency of 85 % and a beam quality of M2 < 1.5 are expected. We present the preliminary results achieved with a commercially available fiber as well as the recent results obtained with an improved active fiber as described above.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.