The aims of the research is to study the thermal lensing induced by end-pumped 1.5 at % Nd:YVO4 Laser crystal. Diode laser centered at 808 nm was employed as an optical pumping source. Using CCD camera, the laser beam spot was recorded. And it was found that the beam spot diameter linearly increased as the pumping power .The crystal has poor thermal conductivity compared to the other laser crystal. Neglecting the thermal birefringence effect on the gain medium, and using the optical characteristics of the a–cut of the Nd: YVO4, thermal lensing due to heat deposited in diode end-pumped a 1.5 at % Nd:YVO4 crystal was investigated. The focal length of the thermal lens was obtained to be exponentially decreasing with respect to pumping power, implies that the higher the pumping power the shorter the focal length thus the greater the aberration effect which degrade the beam quality.M2 factor technique was used to measure the beam quality. The results proved that the beam quality become poor as indicates by the increasing of M2 factor.
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