Trivalent rare earth ions in crystalline or fiber hosts are among the most successful of laser materials, but new dopant-host combinations and more detailed understanding of existing materials continue to be needed. This paper presents a few examples from the work of our team at the Army Research Laboratory, highlighting the interrelation between spectroscopic properties and laser behavior. It focuses on bulk solids, though rare-earth-doped fiber lasers are also extremely important. One system discussed is Nd:YAG, particularly concentration quenching in heavily doped ceramic YAG. Spectroscopic properties of Yb:Y2O3 and Yb:Sc2O3 help to elucidate their laser performance. Spectra indicate that Er:YAG is more promising than Er:Sc2O3 for room temperature laser operation, but that the reverse is true for operation at and somewhat above liquid nitrogen temperature.

A novel colloidal co-casting process was developed to fabricate laser quality, multisegment composite ceramic laser gain materials. The approach was demonstrated for a three segment transparent composite rod 62 mm long by 3 mm diameter consisting of undoped yttrium aluminus garnet (YAG), 0.25% Er:YAG, and 0.5% Er:YAG. The Er concentration profile in the composite has steep, controllable gradients at the segment interfaces, while maintaining constant dopant concentrations within each segment. The composite rod has 84% transmittance at 1645 nm (the lasing wavelength) with a scatter loss of 0.4% cm−1. Laser operation of such a composite Er:YAG ceramic rod was demonstrated for the first time, with nearly equivalent lasing behavior to an Er:YAG single crystal rod.

Mn5+:Sr5(VO4)3F is a new laser material with spectroscopic and laser properties similar to those of other recently reported Mn doped vanadates, but with the advantage of easier crystal growth. It has strong, broad absorption bands from 600–1000 nm and a sharp emission peak at 1164 nm. The splittings and polarization dependence of the spectra are consistent with the approximate C3v site symmetry reported for other apatites. Despite relatively large distortion from tetrahedral symmetry, the 1E and 3T2 states remain sufficiently separate that the emission is almost purely from the 1E state. As a result, laser operation is of the three-level type. Single pass gain/loss measurements indicate that any excited state absorption at the laser wavelength must be significantly smaller than the stimulated emission.