An active optical device comprises a glass, waveguiding structure disposed on a substantially planar principal surface of a substrate. The structure includes a silica-based, erbium-doped active core. The active core has an absolute erbium concentration of at least about 0.5.times.10.sup.20 atoms per cubic centimeter, and a radiative lifetime of the erbium lasing level of at least about 5 milliseconds. The active core does not contain significant amounts of alkali metals or alkaline earth metals but does contain at least two modifier metals. Also disclosed is a method for forming an active optical device, including the step of depositing an erbium-doped active core by sputtering.