Influence of 3D helical microstructure shape deviations on the properties of a vortex beam generated in the near diffraction zone

Subject of study. The effect of various deviations of the helical microstructure shape and position associated with manufacturing and alignment errors on the properties of vortex beams formed in the near diffraction zone (at a distance of about a dozen wavelengths) has been studied. Aim of study. The aim is to determine the influence of different types of shape deviations and the spiral microstructure position on the formed vortex beam properties. Method. Numerical simulation was performed using the finite-difference time-domain method with both linear and nonlinear spiral phase plates, allowing the real features of the 3D structure of the element under study to be taken into account with respect to reflection and refraction on a complex surface. The height of the microrelief, the radius of the illuminating beam, and its displacement were varied in a series of numerical experiments. Main results. It was shown that 3D shape deviations from the helical microstructure, for example, nonlinearities of the relief, lead to a distortion of the vortex dependence of the phase and break the annular intensity of the formed beam. However, in this case, the overall stability of the singular beam structure is preserved, which is completely destroyed in the case of misalignment of the illuminating beam and the optical element. A change in the height of the microrelief leads to a change in both the topological charge and the shape of the beam. As for the influence of the aperture radius of the input Gaussian beam, it is possible to scale the formed vortex beams by changing the aperture radius. Misalignment in the optical system leads to the loss of the annular structure of the vortex beam and its invariant properties. Practical significance. The obtained results can be useful in applications involving adjustable optical elements, as well as microstructures formed in photosensitive media. The main reasons for the distortion of the formed beam structure include both technological inaccuracies during etching, such as the height mismatch and changes in the structure of the zones of the diffractive optical element, and alignment errors of the optical system, such as the misalignment of the illuminating beam and the optical element. It should be noted that the characteristics of the 3D optical element structure most noticeably affect the diffraction pattern in the near zone.