Glasses

Glasses form an important class of host materials for some of the rare earths, particularly Nd³⁺. The outstanding practical advantage compared to crystalline materials is the tremendous size capability for high-energy applications. Rods up to 1m in length and over 10 cm in diameter, and disks up to 90 cm in diameter and several centimeters thick have been produced. The optical quality is excellent, and glass, of course, is easily fabricated and takes a good optical finish. Laser ions placed in glass generally show a larger fluorescent linewidth than in crystals as a result of the lack of a unique and well-defined crystalline field surrounding the individual active atoms. Therefore, the laser thresholds for glass lasers have been found to run higher than their crystalline counterparts. Also, glass has a much lower thermal conductivity than most crystalline hosts. The latter factor leads to a large thermally induced birefringence and optical distortion in glass laser rods when they are operated at high average powers.
For bulk solid-state lasers, glass doped with Nd³⁺ or Er³⁺ is important. In fiber oscillators and amplifiers, Er³⁺- and
Yb³⁺-doped glass provides the active medium.