Generation of coherent frequency-tunable radiation in $F$ centers

An investigation was made of lasers utilizing the $F_2$, $F^+_2$ and $F^-_2$ centers emitting in the spectral range $0.6-1.26\,\mu$. Quasi-cw stimulated emission from the $F^-_2$ centers was obtained as a result of pumping with a cw YAG laser ($\lambda=1.06\,\mu$). Thermal tuning of the emission frequency was observed and a qualitative explanation of this effect was found. The time stability of the centers and the influence of the pump radiation were studied. A fairly simple method for restoring the $F^+_2$ centers was found.