Rare earth doped silica based fiber lasers and amplifiers with very high output power and excellent beam quality are efficient devices for a variety of applications in industry, science and medicine. During the last years, important progress was possible by new design concepts but also by carefully tailoring the material properties. Aspects of material and technology development concerning the interaction of different dopants and co-dopants will be discussed in the following. Our discussion concentrates on the optical properties of the laser fiber as refractive index, absorption and emission of the rare earths and especially on phenomena concerning the background loss of the fibers. We have found a strong rare earth specific loss component which remarkably depends on the kind and the ratio of the co-dopants. The relations of this background loss to material composition and fabrication technology are demonstrated and discussed on an empirical base.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.