Method for exploring the topological charge and shape of an optical vortex generated by a spiral phase plate

A spiral phase plate (SPP) represents one of the most efficient and cost-effective optical devices used for vortex beam generation. In this work, we studied, both experimentally and theoretically, the effect of illumination of an SPP with a laser beam of a different wavelength than the one for which it was designed. We visualized the transversal intensity profile without any additional focusing element and the spatial phase of the beam was investigated using a Sagnac interferometer. SPPs with topological charges from 1 to 8 were used in these investigations. The results of the study revealed that the characteristic diffraction patterns which show rotational symmetry of order m, but no inversion symmetry, can be used to determine the absolute value m and the sign of the topological charge of the SPP. In addition, the same technique has the potential to be used for singularity manipulation and shaping the intensity distribution of vortex beams.