A novel method to calculate a near field of widely divergent laser beams

A new method to calculate the near field of widely divergent laser beams based on a given far field is proposed. It is assumed that the field of radiation consists of several transversal modes and the far field is determined by the Huygens principle. The idea consists in employing the least squares method to find a minimum of the difference between the measured far field and the far field calculated under the assumption that is consists of several transversal modes with assumed weights. To find this minimum the Levenberg–Marquardt algorithm is used. Thus the problem is dealt beyond the paraxial approximation. The method is illustrated by its results for experimental mid-infrared quantum cascade laser beams and for the beams obtained by simulations.