Diffraction of combined optical vortices

The diffraction of light on the lens of a Gaussian beam past a uniaxial crystal is equivalent to the beam diffraction on a helicoidal phase grating of two partial beams with different curvatures of the wave front. This diffraction significantly distorts the Gaussian beam profile and an extended region with three beam waists is formed near the focal plane. The beam waist region can be rectified by varying the radii of the primary beam waist and the lens pupil.