Topological charge and angular momentum of light beams carrying optical vortices

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.

The key role of off-axis singularities in free-space vortex transmutation

“…Scalar coherent light beams can present phase singularities, that is, zeros of the complex scalar wave which are recognized by a vanishing intensity around which the phase whirls [1,2,3]. Orbital angular momentum (OAM) can be associated to these wave fields, commonly termed as optical vortices [4,5].…”

The curious arithmetic of optical vortices

“…The same rule holds for the multiplication of finite beams. Clues that suggest the failure of such simple arithmetic rules in the sum of vortex beams are given by the superposition of vortices nested in coaxial beams with different shapes, where the number of existing vortices and their net topological charge is found to depend on the beam shapes and their relative widths and amplitudes, 6 by the superposition of laser cavity modes, 7 or by the explosions of perturbed higher-order dislocations. 8 Our goal in this Letter is to show that the superposition of noncoaxial beams opens the door to a fascinating new range of possibilities.…”

Direct generation of optical vortices

“…It is already known how to create the necessary adaptations to the formulation we have provided; the dipole-dipole interaction tensor in the above calculations may be replaced with the medium-modified form introduced by Juzeliūnas [72]. Experimentally, the feasibility of such a setup also requires additional consideration, with the stability of higher l-value states having been questioned in recent work on the decay of high-order vortex modes into groups of more stable unit vortices [73][74][75]. Such problems have nonetheless been shown to be surmountable: The propagation of OAM modes in a single mode fiber can preserve OAM structure [76].…”