The role of beam waist in Laguerre-Gauss expansion of vortex beam

TL;DR

This paper investigates how the beam waist parameter affects the expansion of vortex beams into Laguerre-Gauss modes, deriving new formulas and optimizing the expansion process.

Contribution

It derives the scalar product between LG modes with different waists and applies this to optimize beam expansions, including for Circular Beams.

Findings

Derived the scalar product for LG modes with different waists

Provided new expansion formulas for generic beams

Identified optimal waist parameter for beam expansion

Abstract

Laguerre-Gauss (LG) modes represent an orthonormal basis set of solutions of the paraxial wave equation. LG are characterized by two integer parameters $n$ and $\ell$ that are related to the radial and azimuthal profile of the beam. The physical dimension of the mode is instead determined by the beam waist parameter $w_0$: only LG modes with the same $w_0$ satisfy the orthogonality relation. Here, we derive the scalar product between two LG modes with different beam waists and show how this result can be exploited to derive different expansions of a generic beam in terms of LG modes. In particular, we apply our results to the recently introduced Circular Beams, by deriving a previously unknown expansion. We finally show how the waist parameter must be chosen in order to optimize such expansion.