Thermal and population lensing of Yb:YLF at cryogenic temperature

We present what is, to our knowledge, the first detailed study of thermal and population lensing in cryogenically cooled Yb:YLF. Using a rod-shaped a-cut Yb:YLF crystal, the thermal lens strength is measured as a function of absorbed pump power for both E//a and E//c polarizations for pump power levels up to 600 W. Our experiments revealed that the thermal lensing behavior in Yb:YLF is quite asymmetric. For the E//a polarization, the thermal lens is rather strong and always converging for both horizonal and vertical axes. For the E//c configuration, thermal lensing is rather weak, and its sign could be different in different axis, and furthermore, the sign of thermal lensing is observed to be dependent on the absorbed pump power level. Simulations and detailed experimental measurements show that the observed peculiar behavior is a result of competition between the photo-elastic effect and thermo-optic effect: as their relative strength varies at different pump power levels. We have also measured the contribution of surface bulging into thermal lensing and found it to be relatively small in our crystals with un-doped end caps. Lastly, the population lensing is measured in Yb:YLF for the first time, and it is found to be quite weak as well, and the polarizability difference parameter of Yb:YLF is determined as (3 +/- 1.5) x 10-34 m3.