Knife-Edge Diffraction of Scalar and Vector Vortex Light

TL;DR

This paper demonstrates that simple knife-edge diffraction patterns can effectively identify the orbital angular momentum of optical vortex beams, providing a quick and illustrative measurement method.

Contribution

The study introduces a novel, straightforward technique using knife-edge diffraction to determine the OAM of light, applicable to both scalar and vector vortex beams.

Findings

Diffraction patterns reveal OAM through fork dislocations.

Vector vortex beams without net OAM show standard Fresnel fringes.

Results align with simulations and diffraction as interference.

Abstract

Various methods have been introduced to measure the orbital angular momentum (OAM) of light, from fork holograms to Dove prism interferometers, from tilted lenses to triangular apertures - each with their own benefits and limitations. Here we demonstrate how simple knife-edge diffraction can be used to identify the OAM of an optical phase vortex from the formation of fork dislocations within the Fresnel diffraction pattern. For vector vortex beams without net OAM, the conventional Fresnel fringes are recovered, whereas the polarization in the geometric shadow is changed in its ellipticity. The observed diffraction patterns agree with simulations and their features can be explained by considering diffraction as an interference phenomenon. Knife-edge diffraction provides not only an instructive illustration of various properties of phase and polarization vortices, but can also serve as an ideal method for the quick determination of optical OAM, with potential applications beyond optics, where alternative detection measurement methods may be harder to realize.