Yb:CaGdAlO4 thin disk laser with 70% slope efficiency

In the last decades high power ultrafast lasers have attracted considerable attention for many applications. The most promising approach to get high power ultrashort pulses directly from an oscillator is a modelocked thin disk laser (TDL) using a broadband gain material. For this purpose, Yb-doped materials are very suitable since their simple energy-level scheme allows for highly efficient laser operation and the strong electron-phonon coupling results in broad spectra. In addition, to maintain efficient cooling a high thermal conductivity is required which often stands in contradiction to a very broad spectrum. This paper reports on improved crystal growth and highly efficient thin disk laser operation of Yb:CALGO, using a 5.9% Yb-doped CALGO in a TDL setup with 24 pump passes through the gain medium and a pump spot diameter of 1.2 mm. A slope efficiency of 70% and 57% of optical-to-optical efficiency with respect to the incident pump power could be obtained with an output coupler transmission of 1.6%, representing the highest efficiency obtained with this material so far. The laser output power of 30 W was limited by the available pump power from the 50 W fiber coupled pump diode laser.