Silica based, rare-earth doped fiber lasers become strong competitors for other solid state lasers thanks to their compactness, inherent stability and excellent beam quality. For high output power and maximum conversion efficiency, composition and geometric structure of the laser fibers must be optimized. Problems arise especially due to the limited rare-earth solubility in the silica matrix. With Nd-doped fibers as an example, we describe, how fluorescence measurement techniques can be used to find optimal compositions giving high lifetimes of the upper laser level. For that, we not only use measurements at fibers, but also nondestructive spatially resolving methods at fiber preforms.