Evaluation of the Laguerre-Gaussian mode purity produced by three-dimensional-printed microwave spiral phase plates.

D Isakov,B Allen,C J Stevens,P S Grant,G J Gibbons,Y Wu

doi:10.1098/rsos.200493

Abstract

Computer-aided design software and additive manufacturing provide flexibility for the direct fabrication of multi-material devices. This design and fabrication versatility has been investigated for the manufacture of dielectric spiral phase plates (SPP) that generate electromagnetic waves with helical wavefronts. Three types of SPPs designed to produce an orbital angular momentum (OAM) mode number l = |1| were additively manufactured using material extrusion and polyjet fabrication methods. The OAM mode characteristics of the transformed helical microwaves as a function of the SPP geometrical features were investigated experimentally in the 12–18 GHz frequency range. The SPPs were further combined with an additively manufactured dielectric lens that provided a marked improvement in OAM mode purity. Finally, multiplexing and de-multiplexing of two OAM modes were demonstrated successfully using an optimum SPP geometry and arrangement.

Highlights

Spiral phase plates (SPPs) are optical devices that transform a plane electromagnetic wave to a Laguerre–Gaussian (LG) beam
The LG beam carries orbital angular momentum (OAM) with a nonzero LG mode number that is characterized by a helical phase front with the axis of rotation along the boresight, or direction of propagation
We investigate how different designs of the spiral phase plates (SPP) can affect the quality of the phase transformation of a plane wave into a wave with a helical front

Summary

Introduction

Spiral phase plates (SPPs) are optical devices that transform a plane electromagnetic wave to a Laguerre–Gaussian (LG) beam. The LG beam carries orbital angular momentum (OAM) with a nonzero LG mode number that is characterized by a helical phase front with the axis of rotation along the boresight, or direction of propagation. 2 The helicoid shape of the wavefront has a step λ/l, where λ is the wavelength and l 1⁄4 +1, +2, +3 determines the rotational mode (topological charge of the phase vortex) with an azimuthal structure related to exp (ilw), where w is the azimuthal coordinate [1,3]. The general principle for the generation of a helical wave with OAM is an induced azimuthal phase delay proportional to exp (ilw) in the Gaussian beam. OAM can be obtained using a spatial light modulator or forked diffraction grating generating an interference pattern hologram that converts a plane wave into a beam with a pre-defined phase and amplitude structure [1]. OAM modes can be used to enhance the security of a transmission channel [4] and spectral efficiency [5]

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