Er : Glass

Erbium has been made to lase at 1.54μm in both silicate and phosphate glasses. Owing to the three-level behavior of erbium, and the weak absorption of pump radiation, codoping with other rare earth ions is necessary to obtain satisfactory system efficiency. In fact, erbium must be sensitized with ytterbium if the material is to lase at all at room temperature. Pump radiation is absorbed by ytterbium, which has an absorption band between 0.9 and 1μm, and transferred to the erbiumions. This transfer is illustrated in Fig. 1. In the co-doped erbium, Yb : glass radiation from a flashlamp or laser diode pumps the strong ²F_7/2 − ²F_5/2 transition in Yb³⁺. Because of the good overlap between the upper states of Yb3+ and Er³⁺, the excited Yb³⁺ ions transfer energy to the ⁴I_11/2 level of Er³⁺. The erbium ions then relax to the upper laser level ⁴I_13/2 and lasing occurs down to the 4I15/2 man-

FIGURE 1. Energy level diagram in codoped Er, Yb : glass.

ifold. For flashlamp pumping, the efficiency of Er : glass can be further improved by adding Cr³⁺ in addition to Yb³⁺. The Cr³⁺ ions absorb flashlamp radiation in two broad bands centered at 450 and 640μm, and emit radiation in a broad band centered at 760 nm. This allows energy to be transferred from Cr³⁺ to Yb³⁺ and Er³⁺. With the development of strained layer InGaAs laser diodes, which have emission between 0.9 and 1μm, it is now possible to pump the strong Yb³⁺ transition ²F_7/2−²F_5/2. Diode pumping at 940 nm has dramatically improved the efficiency and average power achievable from Er : glass lasers.