Fine structure of perturbed Laguerre-Gaussian beams: Hermite-Gaussian mode spectra and topological charge

TL;DR

This paper investigates how small perturbations in Laguerre-Gaussian beams affect their mode spectrum and proposes a method for measuring their topological charge, supported by theoretical and experimental results.

Contribution

It introduces a new understanding of the fine structure of perturbed LG beams and a practical measurement technique for their topological charge.

Findings

Perturbations create a fine Hermite-Gauss mode spectrum in LG beams.

The proposed method accurately measures topological charge with ring dislocations.

Experimental validation confirms theoretical predictions.

Abstract

We found that small perturbations of the optical vortex core in the Laguerre-Gaussian (LG) beams generate a fine structure of the Hermite-Gauss (HG) mode spectrum. Such perturbations can be easily simulated by weak variations of amplitudes and phases of the HG modes in the expansion of the LG beam field. We also theoretically substantiated and experimentally implemented a method for measuring the topological charge of LG beams with an arbitrary number of ring dislocations. Theoretical discussion and experimental studies were accompanied by simple examples of estimating the orbital angular momentum and the topological charge of perturbed LG beams.