Topological charge and angular momentum of light beams carrying optical vortices

TLDR

The study investigates light beams with optical vortices, revising topological charge conservation and showing that vortices can appear or annihilate during free propagation. The authors theoretically and experimentally analyze how topological charge transforms in a Gaussian beam carrying multiple vortices, and derive a rule for angular‑momentum density distribution from coaxial vortex interference. They find that, despite changes in vortex number, the total angular momentum of the combined beam remains constant during propagation.

Abstract

We analyze the properties of light beams carrying phase singularities, or optical vortices. The transformations of topological charge during free-space propagation of a light wave, which is a combination of a Gaussian beam and a multiple charged optical vortex within a Gaussian envelope, are studied both in theory and experiment. We revise the existing knowledge about topological charge conservation, and demonstrate possible scenarios where additional vortices appear or annihilate during free propagation of such a combined beam. Coaxial interference of optical vortices is also analyzed, and the general rule for angular-momentum density distribution in a combined beam is established. We show that, in spite of any variation in the number of vortices in a combined beam, the total angular momentum is constant during the propagation.