Calculating the three-dimensional structure of the near-focus diffraction field in the caustic zone of a convergent aberrational laser beam
Abstract
A structurally stable optical caustic, called the diffraction catastrophe, is formed in the focal region of a laser beam focused by means of a two-component crystal-optic element (TCE) made from uniaxial crystals and distorted as a result of spherical aberration and astigmatism. The source of the primary astigmatic distortions is the refractive-index anisotropy for the extraordinary wave in one of the components of the TCE. An additional astigmatism coefficient, caused by the oblique incidence of the laser beam on the entrance face of the TCE, depends on the angle of incidence α in a power form. The calculated amplitude and phase distributions of the diffraction field coincide with the experimentally observed features of the three-dimensional diffraction field both for α=0 and at small values of α (up to 13°). Closed, compactly localized screw dislocations of the phase are detected in the diffraction field investigated here. The latter are located in the interior region of the caustic, with a characteristic “crystalline” cellular structure.