A compact diffractive sorter for high-resolution demultiplexing of orbital angular momentum beams

Gianluca Ruffato,Michele Massari,Filippo Romanato,Erfan Mafakheri,Bereneice Sephton,Marcello Girardi,Andrew Forbes,Pietro Capaldo

doi:10.1038/s41598-018-28447-1

Abstract

The design and fabrication of a compact diffractive optical element is presented for the sorting of beams carrying orbital angular momentum (OAM) of light. The sorter combines a conformal mapping transformation with an optical fan-out, performing demultiplexing with unprecedented levels of miniaturization and OAM resolution. Moreover, an innovative configuration is proposed which simplifies alignment procedures and further improves the compactness of the optical device. Samples have been fabricated in the form of phase-only diffractive optics with high-resolution electron-beam lithography (EBL) over a glass substrate. A soft-lithography process has been optimized for fast and cheap replica production of the EBL masters. Optical tests with OAM beams confirm the designed performance, showing excellent efficiency and low cross-talk, with high fidelity even with multiplexed input beams. This work paves the way for practical OAM multiplexing and demultiplexing devices for use in classical and quantum communication.

Highlights

During the last decade, Space Division Multiplexing (SDM) has experienced an upsurge of research interest, both in academia and industry, as a possible means to address the ever increasing worldwide demand for bandwidth[1]
We designed and fabricated a compact and high-resolution sorter of optical beams carrying orbital angular momentum, by combining the well-known method based on conformal mapping with a fan-out optical operation
The unitary optical transformation is traditionally performed by means of two optical elements, i.e. unwrapper and phase-corrector, which efficiently convert the azimuthal phase gradients of orbital angular momentum (OAM) beams into linear phase gradients, focused at different positions with a Fourier lens

Summary

Introduction

The mapping is executed by two optical elements in sequence: the first performing a log-pol coordinate transformation and the second correcting the introduced phase distortion. This method has been widely used as a sorting technique, for example, in recent telecom experiments both in the classical[18] and quantum[19,20] regimes. The measurement bandwidth of the sorter, which is proportional to the Fresnel number of the optics[16], should be increased in order to provide a sufficient number of modes after channel selection This is achievable for instance by either decreasing the focal length or increasing the size of the first element performing optical transformation[21]. Applications in optical fibers could prescribe severe limitations to the number of supported OAM modes, i.e. the maximum l value, and the selection of non-consecutive OAM values could dramatically reduce the number of available channels

Methods

Results

Conclusion