Single-shot measurement of the orbital-angular-momentum spectrum of light

TL;DR

This paper introduces a single-shot interferometric method to accurately measure the true orbital-angular-momentum spectrum of optical fields, overcoming efficiency and stability issues of previous techniques.

Contribution

The authors develop and demonstrate a novel interferometric approach that encodes the OAM spectrum in the azimuthal intensity profile, allowing full spectrum measurement with minimal acquisitions.

Findings

Successfully measured the OAM spectrum in a single shot.

Demonstrated measurement of the angular Schmidt spectrum of entangled photons.

Reported a broad spectrum with an angular Schmidt number of 82.1.

Abstract

The existing methods for measuring the orbital-angular-momentum (OAM) spectrum suffer from issues such as poor efficiency, strict interferometric stability requirements, and too much loss. Furthermore, most techniques inevitably discard part of the field and measure only a post-selected portion of the true spectrum. Here, we propose and demonstrate an interferometric technique for measuring the true OAM spectrum of optical fields in a single-shot manner. Our technique directly encodes the OAM-spectrum information in the azimuthal intensity profile of the output interferogram. In the absence of noise, the spectrum can be fully decoded using a single acquisition of the output interferogram, and, in the presence of noise, acquisition of two suitable interferograms is sufficient for the purpose. As an important application of our technique, we demonstrate measurements of the angular Schmidt spectrum of the entangled photons produced by parametric down-conversion and report a broad spectrum with the angular Schmidt number 82.1.