The slit diffraction of circular OV beams is studied both theoretically and by experiment, with explicit involvement of the incident beam convergence or divergence (finite value of the wavefront curvature radius). Based on the example of Laguerre-Gaussian mode with zero radial index and non-zero azimuthal index m we confirm that the far-field diffraction pattern contains exactly |m| bright lobes elongated orthogonally to the slit (which was reported previously) and show that the far-field profile possesses an asymmetry with respect to the slit axis depending on the wavefront curvature (which is a new result). Being combined, these features enable simple and efficient means for the simultaneous express diagnostics of the magnitude and the sign of the OV topological charge, which can be useful in many OV applications, including the OV-assisted metrology and information processing.
A double-phase-ramp (DPR) converter is known as an element introducing the screw dislocation into an input beam with smooth wavefront, or increasing (decreasing) the topological charge of the input beam with optical vortex (OV). We study the features of the set of phase singularities (singular skeleton, SS) formed within the Laguerre-Gaussian beam LG<sub>0</sub><sup>4</sup> after passage through the chargelowering DPR converter. The main attention is paid to the SS sensitivity to small transverse misalignments in horizontal, vertical and diagonal directions of the DPR converter with respect to the nominal centered position. The obtained results are compared with the SS characteristics typical for the case of perfect DPR alignment. It is shown that different OVs in the transformed field have different misalignment sensitivity. Conditions enabling especially high sensitivity of the separate OVs are revealed and discussed.
We present the experimental and numerical study of the transverse profile for a beam obtained by the screen-edge diffraction of optical-vortex (OV) Kummer beams with topological charges 1, 2 and 3, generated with the help of a “fork” hologram. The main results concern the behavior of the secondary OVs formed in the diffracted beam due to splitting of the incident multicharged OV into a set of single-charged ones. When the screen edge moves across the incident beam, OVs in every cross section of the diffracted beam describe complicated spiral-like trajectories, which distinctly manifests the screw-like nature and the energy circulation in the OV beam. At certain conditions, positions of the separate OVs as well as their mutual configuration (singular skeleton of the diffracted beam) shows high sensitivity to the screen edge dislocation with respect to the incident beam axis. This can be used for remote measurements of small displacements and deformations.