Mode Conversion of Gaussian Beams at Dielectric Interfaces

TL;DR

This paper studies how Gaussian beams change mode when passing through dielectric interfaces, showing that polarization-dependent effects cause coupling into higher-order modes and reduce mode fidelity near the diffraction limit.

Contribution

It introduces a vector angular spectrum approach to quantify polarization-dependent mode conversion at dielectric interfaces, revealing the impact on beam fidelity.

Findings

Higher-order Laguerre-Gaussian modes are generated during transmission.

Mode fidelity decreases as the beam waist approaches the diffraction limit.

Polarization-dependent filtering acts as a spatial filter causing mode coupling.

Abstract

We investigate mode conversion of $\mathrm{TEM}_{00}$ Gaussian beams upon transmission through planar dielectric interfaces. We show that the angle-dependent Fresnel coefficients act as a spatial filter, inevitably generating higher-order spatial modes. Using a vector angular spectrum formulation and numerical simulations, we reveal that this polarization-dependent filtering induces a coupling from $\mathrm{TEM}_{00}$ into higher-order Laguerre-Gaussian modes, yielding a quadrupolar field pattern. We quantify the associated amplitude and phase deviations, showing that the mode fidelity decreases significantly as the beam waist approaches the diffraction limit.