We explore the use of orbital-angular-momentum (OAM)-multiplexing to increase the capacity of free-space data transmission to moving platforms, with an added potential benefit of decreasing the probability of data intercept. Specifically, we experimentally demonstrate and characterize the performance of an OAM-multiplexed, free-space optical (FSO) communications link between a ground transmitter and a ground receiver via a moving unmanned-aerial-vehicle (UAV). We achieve a total capacity of 80 Gbit/s up to 100-m-roundtrip link by multiplexing 2 OAM beams, each carrying a 40-Gbit/s quadrature-phase-shift-keying (QPSK) signal. Moreover, we investigate for static, hovering, and moving conditions the effects of channel impairments, including: misalignments, propeller-induced airflows, power loss, intermodal crosstalk, and system bit error rate (BER). We find the following: (a) when the UAV hovers in the air, the power on the desired mode fluctuates by 2.1 dB, while the crosstalk to the other mode is −19 dB below the power on the desired mode; and (b) when the UAV moves in the air, the power fluctuation on the desired mode increases to 4.3 dB and the crosstalk to the other mode increases to −10 dB. Furthermore, the channel crosstalk decreases with an increase in OAM mode spacing.

Novel forms of beam generation and propagation based on orbital angular momentum (OAM) have recently gained significant interest. In terms of changes in time, OAM can be manifest at a given distance in different forms, including: (1) a Gaussian-like beam dot that revolves around a central axis, and (2) a Laguerre-Gaussian (LG ';p) beam with a helical phasefront rotating around its own beam center. Here we explore the generation of dynamic spatiotemporal beams that combine these two forms of orbital-angular-momenta by coherently adding multiple frequency comb lines. Each line carries a superposition of multiple LG ';p modes such that each line is composed of a different ' value and multiple p values. We simulate the generated beams and find that the following can be achieved: (a) mode purity up to 99%, and (b) control of the helical phasefront from 2π-6π and the revolving speed from 0.2-0.6 THz. This approach might be useful for generating spatiotemporal beams with even more sophisticated dynamic properties.

A light beam’s frequency can blueshift when the beam travels through a medium that exhibits a time-dependent decrease in the refractive index. Here we show that a metasurface made of a plasmonic antenna array on a thin indium tin oxide (ITO), which exhibits epsilon-near-zero (ENZ) response, can behave as a time-varying medium and change the frequency of a sufficiently strong light beam through self-action effect. Specifically, we observe that a near-resonant optical excitation of the 92 nm thick metasurface leads to an intensity-dependent blueshift of the excitation pulse. We measured a maximum blueshift of ∼1.6 THz with ∼4 GW/cm2 incident intensity. The observed effect using an ITO-based ENZ metasurface has an energy requirement that is up to 200× lower than implementations using ITO alone.

Beams carrying orbital-angular-momentum (OAM) have gained much interest due to their unique amplitude and phase structures. In terms of communication systems, each of the multiple independent data-carrying beams can have a different OAM value and be orthogonal to all other beams. This paper will describe the use of multiplexing and the simultaneous transmission of multiple OAM beams for enhancing the capacity of communication systems. We will highlight the key advances and technical challenges in the areas of (a) free-space and fiber communication links, (b) mitigation of modal coupling and channel crosstalk effects, (c) classical and quantum systems, and (d) optical and radio frequency beam multiplexing.

Light beams can be characterized by their complex spatial profiles in both intensity and phase.Analogous to time signals, which can be decomposed into multiple orthogonal frequency functions, a light beam can also be decomposed into a set of spatial modes that are taken from an orthogonal basis. Such a decomposition can provide a tool for spatial spectrum analysis, which may allow the stable, accurate and robust extraction of physical object information that may not be readily achievable using traditional approaches. As an example, we measure the opening angle of an object using the complex spectrum of orbital angular momentum (OAM) modes as the basis, achieving a >15 dB signal-to-noise ratio. We find that the dip (i.e., notch) positions of the OAM intensity spectrum are dependent on an object's opening angle but independent of the object opening's angular orientation, whereas the slope of the OAM phase spectrum is dependent on the object opening's orientation but independent on the opening angle.2