  The thermal lensing effects in the lasing crystal appear to be dominant when
the pump laser focused on the lasing crystal which causes the local heating
effect. The spatial refractive index variations observed if the crystal has
non-uniform temperature distribution. In the present study, the demonstration
of thermal lensing effect of Brewster-cut Ti: Sapphire crystal carried out when
pumped by 25 ps pulses at 80 MHz repetition rate with 532 nm central
wavelength, while the average pump power controlled by the attenuator. The
thermal lensing effects measured first at the ~293 K temperature and later at
~40 K by using the cryogenic cooling system. The thermal lensing variations at
different average pump power were calculated and measured by using the HASO4
Shack-Hartmann wavefront sensor.
