Diffractive optics for combined spatial- and mode- division demultiplexing of optical vortices: design, fabrication and optical characterization.

Gianluca Ruffato,Michele Massari,Filippo Romanato

doi:10.1038/srep24760

Abstract

During the last decade, the orbital angular momentum (OAM) of light has attracted growing interest as a new degree of freedom for signal channel multiplexing in order to increase the information transmission capacity in today’s optical networks. Here we present the design, fabrication and characterization of phase-only diffractive optical elements (DOE) performing mode-division (de)multiplexing (MDM) and spatial-division (de)multiplexing (SDM) at the same time. Samples have been fabricated with high-resolution electron-beam lithography patterning a polymethylmethacrylate (PMMA) resist layer spun over a glass substrate. Different DOE designs are presented for the sorting of optical vortices differing in either OAM content or beam size in the optical regime, with different steering geometries in far-field. These novel DOE designs appear promising for telecom applications both in free-space and in multi-core fibers propagation.

Highlights

An orbital angular momentum (OAM)-carrying beam is characterized by an azimuthally varying phase term exp(i θ), being θ the angular coordinate on a plane perpendicular to the optical axis and l the OAM content per photon in units of h/2π
The phase pattern of a diffractive optics intended for expanding the incident light field into different diffraction orders (Fig. 1) is given by the linear combination of n angular harmonics {ψi = exp(ilθ)} as it follows[20]: Figure 2. (a) Phase pattern of DOE performing OAM-mode division demultiplexing (OAM-MDM) of optical vortices in the range {− 2, − 1, 0, + 1, + 2}, 8 phase levels: 0, π /4, π /2, 3π /4, π, 5π /4, 3π /2, 7π /4
A custom code implemented in MATLAB is used to calculate the phase pattern for given sets of OAM values

Summary

Introduction

An OAM-carrying beam is characterized by an azimuthally varying phase term exp(i θ), being θ the angular coordinate on a plane perpendicular to the optical axis and l the OAM content per photon in units of h/2π. The maximum intensity radius increases linearly with l for Kummer beams and as for Laguerre-Gaussian beams[29] This property may create problems when coupling multiple OAM beams into a fiber with fixed annular index profile or under manipulation with finite-size optical elements. To overcome this limitation, Ostrovsky and coworkers first introduced the concept of “perfect vortex” proposing OAM beams whose ring-diameter and ring-width are both independent of the topological charge[30]. Perfect vortices are generated illuminating with a Gaussian beam a phase pattern loaded on a spatial light modulator that implements the combination of a spiral term and of an axicon contribution, as exploited elsewhere[28]. The directions of the demultiplexed beams in far-field can be arbitrarily controlled by properly designing the DOE phase pattern

Methods

Results

Conclusion