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

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.

Non-paraxial design and fabrication of a compact OAM sorter in the telecom infrared

“…The area of the camera was divided into rectangular regions of interest, centered on each elongated spot in far-field and with the width given by the minimum distance between any two adjacent channels. By integrating the total intensity, the relative modal power and modal cross-talk XT j of the jth OAM channel could be determined using the definition [29]: ,/ 10 , 10 log j ALL j j j ALL…”

Nano-fabrication and characterization of silicon meta-surfaces provided with Pancharatnam-Berry effect

“…The phase pattern of a diffractive optics designed for analyzing the orbital angular momentum spectrum of the incident light field in the OAM set {ℓ j } is given by the linear combination over a set of n orthogonal modes {ψ j =R j (ρ,ϑ)exp(iℓ j ϑ)} as it follows [29]: (11) where ϑ=arctan(v/u), {(α j , β j )} are the n vectors of carrier spatial frequencies and {c j } are complex coefficients whose modulus is given arbitrarily, usually unitary, and their phases are free parameters of the task, fitted in such a manner that Eq. (11) becomes an exact equality [30]. The positions {(x j , y j )} of the corresponding signal spots in far-field ( Fig.…”

“…As a consequence of the polarization-dependent optical response, a beam carrying OAM equal to ℓ and left-handed circular polarization generates a bright spot in the far-field at a position which is symmetric to the spot produced by the right-handed circularly-polarized beam with opposite OAM. A custom code implemented in MATLAB® has been used to calculate the phase pattern for the desired set of OAM values and the corresponding carrier spatial frequencies, taking into account definite limitations, as explained in [30], in particular the phase quantization into 16 equally-spaced levels. We limited the choice to OAM values in the set from -3 to +3 for a total of 7 OAM channels ( Fig.…”

Pancharatnam–Berry Optical Elements for Spin and Orbital Angular Momentum Division Demultiplexing

“…In fact, according to Equation (6), the far-field intensity peaks are expected to be at the following angular positions: A custom code implemented in MATLAB ® is used to calculate the phase pattern for the desired set of OAM values { j } and corresponding carrier spatial frequencies {(β j , ϑ j )}. The implemented algorithm is based on a successive computation of the sum in Equation (1) and integrals in Equation (2), using the fast Fourier transform algorithm and considering definite limitations, as explained in Ruffato et al [9], in particular the quantization of phase into 16 equally-spaced levels.…”

“…Industry requirements favor technological solutions that are efficient, suitable to miniaturization and integration, compatible with mass production, and potentially encompassing many multiplexing techniques. In the quest for miniaturization, we have recently demonstrated the design and fabrication of 3D multi-level diffractive optics [9][10][11][12] in the visible range performing OAM mode generation and demultiplexing, based either on OAM mode projection [13] or log-pol optical transformation [14]. With respect to bulky refractive optical elements, the diffractive counterpart offers a flat and integrable solution, especially when short focal lengths are required.…”