Yb doping concentration and temperature influence on Yb:LuAG thermal lensing

The aim of this study was to investigate whether refractive power of thermal lens for Yb:LuAG crystal at cryogenic temperatures depends on Yb doping concentration. This problem has not been examined yet. The three measured Yb:LuAG laser rods samples (length of 3 mm, diameter 3 mm, AR @ 0.94 mu m and 1.03 mu m, doping concentration 5.4, 8.4 and 16.6 at. % Yb/Lu) were mounted in the temperature controlled copper holder of the liquid nitrogen cryostat. The samples were longitudinally pumped with fiber coupled CW laser diode at 0.930 mu m with the focal point 0.4 mm in diameter. The 38 mm long semi-hemispherical laser resonator consisted of a flat pump mirror (HR @ 1.03 mu m and HT 0.94 mu m) and curved output coupler (r=500 mm) of reflectivity 94 % @ 1.06 mu m. The refractive power of thermal lens was estimated indirectly by measuring of change in the position of focused laser beam focal point. The measurement was performed for constant absorbed power of 10 W in temperature range from 80 up to 240 K. It was observed that cryogenic cooling caused reduction of thermal lens power, which increased linearly with increasing temperature. For temperatures from 80 to 160 K refractive power was identical for all concentration. For higher temperature the refractive power of thermal lens increased with increasing Yb3+ concentration. Presented study shows that the application of cryogenic temperature leads to the reduction of thermal effect in Yb:LuAG crystal even for high dopant concentration. This is essential for reaching of high output power while maintaining high beam quality.