Yb:CaF2 - a new old laser crystal

Here is presented and discussed the history, the spectroscopic, the thermo-mechanical and the laser properties of the new old laser crystal Yb:CaF2, of its Sr and Ba isotypes as well as of an Yb and Na codoped compound. It will be shown that Yb:CaF2 is a very particular luminescent material, in which the laser-active center probably consists of a complex hexameric cluster, that it is the most promising Yb-doped fluoride material for large-scale, high-power and high-energy laser systems and that it can compete in several aspects with the currently used Yb-doped oxide crystals and glasses. The opportunity of operating the crystals at cryogenic temperatures and the recently achieved improvements in the field of femtosecond pulsed laser operation and high peak power laser amplification is highlighted and evaluated.