Abstract Rare earth doped glasses are of increasing importance for laser applications and for optical communication. Especially Er 3+ doped glasses with their transition at 1530 nm are a key component for optical telecommunication applications. The optical transitions within the 4f states of the rare earth ions are only weakly dependent on the surrounding crystal field. This weak dependence of the spectral properties can be observed as a function of glass composition. Judd–Ofelt theory uses the deviation from spherical symmetry of the Er 3+ site as a perturbation. Applying a Judd–Ofelt analysis the glass composition can be systematically correlated with spectral properties, Judd–Ofelt parameters and properties of an optical amplifier. We show that a detailed tailoring of spectroscopic properties via the glass composition is possible and identify key components to selectively influence amplifier performance.