Comparison of ND:YAG laser beam quality between free running and Q-switched modes

In this paper, we present the comparison result of M factor between free running and Q-switched mode of Nd:YAG laser. The system consists of sensitive infrared IR detector, lenses, distance scanner and a single array detector. Developed photographic paper was used to detect the invisible of IR beam which instantly and permanently records the beam spot. The beam spot was scanned and send to computer for analysis via Matrox Inspector version 5 software. A homemade scanner was utilized to ease the mobility of the detector. It moves in two directions; parallel and perpendicular to the optical axis. The beam width of near field and far field along the Rayleigh region were measured. The results showed that the M factor for free running of Nd:YAG laser are 7.68 and 7.38 and for Q-switch mode are 25.62 and 19.48 corresponding to both lenses with focal distance of 15 and 20 cm respectively. The free running operation is found to have better beam quality in comparison to Q-switch mode. The reason for the poor quality of Q-switch mode is due to the large beam width of near field induced by giant pulse, losses of energy and shorter duration.