Disclosed is an optical fiber having a silica-based core and a silica-based cladding, the core comprising germania, and oxides of an alkali metal and phosphorous. By appropriately selecting the concentration of alkali metal and phosphorous oxides, fibers exhibiting low attenuation and low hydrogen aged attenuation may be obtained. In a preferred embodiment, the alkali metal oxide is potassium oxide (K.sub.2O).