Orbital angular momentum (OAM), which describes the "phase twist" (helical phase pattern) of light beams, has recently gained interest due to its potential applications in many diverse areas. Particularly promising is the use of OAM for optical communications since: (i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal, (ii) inter-beam crosstalk can be minimized, and (iii) the beams can be efficiently multiplexed and demultiplexed. As a result, multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams, thereby potentially increasing the system capacity. In this paper, we review recent progress in OAM beam generation/detection, multiplexing/demultiplexing, and its potential applications in different scenarios including free-space optical communications, fiber-optic communications, and RF communications. Technical challenges and perspectives of OAM beams are also discussed.

One property of electromagnetic waves that has been recently explored is the ability to multiplex multiple beams, such that each beam has a unique helical phase front. The amount of phase front ‘twisting’ indicates the orbital angular momentum state number, and beams with different orbital angular momentum are orthogonal. Such orbital angular momentum based multiplexing can potentially increase the system capacity and spectral efficiency of millimetre-wave wireless communication links with a single aperture pair by transmitting multiple coaxial data streams. Here we demonstrate a 32-Gbit s−1 millimetre-wave link over 2.5 metres with a spectral efficiency of ~16 bit s−1 Hz−1 using four independent orbital–angular momentum beams on each of two polarizations. All eight orbital angular momentum channels are recovered with bit-error rates below 3.8 × 10−3. In addition, we demonstrate a millimetre-wave orbital angular momentum mode demultiplexer to demultiplex four orbital angular momentum channels with crosstalk less than −12.5 dB and show an 8-Gbit s−1 link containing two orbital angular momentum beams on each of two polarizations.

The capacity of free-space optical (FSO) communication links could potentially be increased by the simultaneous transmission of multiple orbital angular momentum (OAM) beams. For such an OAM multiplexing approach, one requires the collection of adequate power as well as proportion of the phase front for a system with minimal crosstalk. Here we study the design considerations for an OAM-multiplexed free-space data link, analyzing the power loss, channel crosstalk, and power penalty of the link in the case of limited-size receiver apertures and misalignment between the transmitter and the receiver. We describe the trade-offs for different transmitted beam sizes, receiver aperture sizes, and mode spacing of the transmitted OAM beams under given lateral displacements or receiver angular errors. Through simulations and some experiments, we show that (1) a system with a larger transmitted beam size and a larger receiver aperture is more tolerant to lateral displacement but less tolerant to the receiver angular error, and (2) a system with a larger mode spacing, which uses larger OAM charges, suffers more system power loss but less channel crosstalk; thus, a system with a small mode spacing shows a lower system power penalty when system power loss dominates (e.g., a small lateral displacement or receiver angular error), whereas that with a larger mode spacing shows a lower power penalty when channel crosstalk dominates (e.g., a larger lateral displacement or receiver angular error). This work could be beneficial to the practical implementation of OAM-multiplexed FSO links.

We propose a simple approach to broadband generation of orbital angular momentum (OAM) carrying vector beams based on compact metamaterials. It consists of two concentric rings in a gold film, where each ring is composed of subwavelength rectangular apertures with gradually varied orientation. The subwavelength rectangular aperture serves as a localized spatial polarizer. We show the generation of different OAM-carrying vector beams with OAM charge number and polarization order varying from -3 to +3 using a 11.2×11.2 μm device. The extinction ratio can exceed 20 dB, and the operation bandwidth (1500 nm) can cover from 1000 to 2500 nm (from near-infrared to mid-infrared). The device provides three degrees of freedom (polarization order l, polarization of input beam σ, and initial orientation angle α(0)) to flexibly generate different OAM-carrying vector beams. We can use a single device to generate two OAM-carrying vector beams with opposite charge sign of OAM by simply controlling the polarization of the input beam. We further study the performance dependence of the designed metamaterials on the offset of the initial orientation angle, length, and width of the rectangular apertures. The obtained results indicate favorable fabrication tolerance.