Diffraction effects during materials treatment with laser beams

A method for the fast calculation of three-dimensional diffraction patterns of different kinds of diaphragms was developed for a high power laser system and laser-matter interaction problems. The method, based on the Hopkins algorithm, computes the Fresnel diffraction at elliptical diaphragms. It is shown that the intensity maxima move towards the focal points with decreasing axis ratio. The calculation of the diffraction field at focussed annular diaphragms is discussed. Previous results were used for the calculation of the focal shift and the position of the intensity maximum on the optical axis. Theoretical predictions were experimentally verified using an He-Ne laser and a pulsed Nd-YAG laser.