Important progress in the development of rare earth doped high power fiber lasers was possible by large-mode-area (LMA) fibers with increased core diameters and reduced core apertures as low as 0.05. In this way, the excellent beam quality is maintained, but the power density can be reduced below critical values despite of very high output powers beyond 1 kW. Sophisticated concepts had to be developed in order to maintain the low NA in the case of high doping, e.g. the codoping by index-decreasing components as boron or fluorine. Here we report on the progress in the preparation of microstructured LMA laser fibers, the core area of which is composed of parts with high doping and parts with refractive index lower than the silica pump cladding. In contrast to the direct codoping, in this way the atomic environment of the active atoms can be tailored and optimized independently on the mean refractive index of the core. The preparation was carried out by stacking different rods in a multistep process, leading to cores with up to more than hundred single elements. Both for ytterbium and erbium/ytterbium doped fibers, good optical properties concerning basic attenuation and rare earth fluorescence could be reached by introducing additional purification steps. Different fiber structures were characterized concerning mode field distribution, pump power absorption and laser behavior.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.