A Practical method for determining the beam profile near the focal spot

In laser material processing, knowledge of the beam profile and beam diameter are fundamental in achieving acceptable results. The beam profile is affected by errors in the focusing optics, namely spherical aberration. This causes the minimum beam diameter to be larger than that predicted by beam propagation equations. Current methods of beam profiling use electronic equipment to measure the relative intensity of the beam at different locations along the beam, which is time consuming and involves high cost. Furthermore, the measurement near the focus spot usually is not feasible, due to excessive power density in this region. A novel one-step technique is developed to determine the beam profile near the minimum spot and the associated effect of spherical aberration. The effect of spherical aberration on the beam profile is investigated for three plano-convex lenses and compared to the ideal beam profile based on the beam propagation model. It was found that the technique is useful in determining the beam profile without using expensive equipment. It was also found that using a beam expander in conjunction with a focusing lens introduced errors in the form of spherical aberration that caused the diameter of the laser beam to increase as well as the beam quality. Removing the beam expander removed the spherical aberration effect, but the beam diameters were increased because of decreased focusing ability.