Structured light, especially beams carrying orbital angular momentum (OAM), has gained much interest due to its unique amplitude and phase structures. In terms of communication systems, multiple orthogonal OAM beams can be potentially utilized for increasing link capacity in different scenarios. This review describes challenges, advances, and perspectives on different aspects of the OAM-based optical communications, including (a) OAM generation/detection and (de)multiplexing, (b) classical free-space optical communication links, (c) fiber-based communication links, (d) quantum communication links, (e) OAM-based communications in different frequency ranges, (f) OAM-based communications using integrated devices, and (g) novel structured beams for communications.

There is a growing interest in structured light, especially orbital angular momentum (OAM) beams, due to its unique amplitude and phase structure. Multiple orthogonal OAM beams can be utilized to enable high-capacity communication systems in various scenarios. This review will discuss the properties, challenges, advances, and perspectives of OAM-based optical communications, including the following (a) basic multiplexing and encoding links; (b) OAM-based communication in free space and fiber; (c) technical hurdles; (d) mitigation approaches; (e) encoded quantum communications; (f) advanced topics including different frequency ranges and novel beams; and (g) ecosystem and commercialization.

Structured electromagnetic (EM) waves have been explored in various frequency regimes to enhance the capacity of communication systems by multiplexing multiple co-propagating beams with mutually orthogonal spatial modal structures (i.e., mode-division multiplexing). Such structured EM waves include beams carrying orbital angular momentum (OAM). An area of increased recent interest is the use of terahertz (THz) beams for free-space communications, which tends to have: (a) larger bandwidth and lower beam divergence than millimeter-waves, and (b) lower interaction with matter conditions than optical waves. Here, we explore the multiplexing of THz OAM beams for high-capacity communications. Specifically, we experimentally demonstrate communication systems with two multiplexed THz OAM beams at a carrier frequency of 0.3 THz. We achieve a 60-Gbit/s quadrature-phase-shift-keying (QPSK) and a 24-Gbit/s 16 quadrature amplitude modulation (16-QAM) data transmission with bit-error rates below 3.8 × 10⁻³. In addition, to show the compatibility of different multiplexing approaches (e.g., polarization-, frequency-, and mode-division multiplexing), we demonstrate an 80-Gbit/s QPSK THz communication link by multiplexing 8 data channels at 2 polarizations, 2 frequencies, and 2 OAM modes.

In this paper, we experimentally demonstrate the mitigation of atmospheric turbulence effects using phase patterns that apply the inverse transmission matrix for pre-compensation in the 200-Gbit/s free-space orbital angular momentum (OAM) multiplexed optical link. In this link, two OAM channels are multiplexed, each carrying a 100-Gbit/s quadrature-phase shift keying (QPSK) signal. In the uni-directional (forward) link, a combination of two OAM modes is generated using a designed phase pattern in each of the two channels. After the two channels are multiplexed, each OAM mode will carry the combination of signals from both channels. Such beams could perform the inverse function of turbulence induced crosstalk. We explore the link performances including crosstalk and bit error rate (BER) under 6 different turbulence realizations with a Fried parameter of 1-mm. We find the following: (a) the inter-channel crosstalk is reduced by up to 21 dB; (b) the inter-channel crosstalk is below -10 dB for the 6 turbulence realizations when the compensation is applied; (c) a BER below the FEC limit could be realized for the 6 turbulence realizations with an optical signal-to-noise ratio (OSNR) of 16.8 dB for the uni-directional link with the compensation. Moreover, the post-compensation is also investigated for the two 100-Gbit/s OAM-multiplexed backward channels in the bi-directional link. For the backward channels, the phase patterns are used to mitigate turbulence induced crosstalk at the receiver for backward channels, and the inter-channel crosstalk is reduced by up to 16 dB.

In free-space optical (FSO) communications, there are scenarios (e.g., from a ground station to a drone/airplane) in which: (i) the transmitter (Tx) can have a relatively large aperture whereas the receiver (Rx) aperture should preferably be much smaller, and (ii) the distance between the Tx and Rx can vary such that beam divergence will cause a variation in the recovered signal power. In such cases, transmission using a fundamental Gaussian beam can be significantly degraded due to beam truncation caused by a limited-size Rx aperture. Here, we experimentally demonstrate a 1-Gbit/s on-off keying (OOK) FSO transmission link using a structured “pin-like” beam with a limited-size Rx aperture at various distances. The pin-like beam is generated by passing a Gaussian beam through an “Airy-type” phase pattern in the radial direction. When propagating, this structured beam first narrows and then tends to maintain its narrow beam size over a fairly wide range of distances. In comparison to its Gaussian counterpart, our experimental results show that the pin-like beam has ∼13 to 8 dB less power loss at distances ranging from 0.45 m to 0.8 m with an Rx aperture diameter of 1 mm. Moreover, we simulate the propagation of the pin-like beam and show its relatively lower power loss for a wide distance variation in a longer link (e.g., ∼1 km) with a limited-size Rx aperture. Furthermore, our results show that the pin-like beam can have a wider range of low-power-loss distances than a Gaussian beam that is focused to a given distance.