Nima Ashrafi

Optical communications using orbital angular momentum beams

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.

Experimental characterization of a 400 Gbit/s orbital angular momentum multiplexed free-space optical link over 120 m

We experimentally demonstrate and characterize the performance of a 400-Gbit/s orbital angular momentum (OAM) multiplexed free-space optical link over 120 m on the roof of a building. Four OAM beams, each carrying a 100-Gbit/s quadrature-phase-shift-keyed channel are multiplexed and transmitted. We investigate the influence of channel impairments on the received power, intermodal crosstalk among channels, and system power penalties. Without laser tracking and compensation systems, the measured received power and crosstalk among OAM channels fluctuate by 4.5 dB and 5 dB, respectively, over 180 s. For a beam displacement of 2 mm that corresponds to a pointing error less than 16.7 μrad, the link bit error rates are below the forward error correction threshold of 3.8×10(-3) for all channels. Both experimental and simulation results show that power penalties increase rapidly when the displacement increases.

Experimental demonstration of 16-Gbit/s millimeter-wave communications link using thin metamaterial plates to generate data-carrying orbital-angular-momentum beams

We present the design and performance characterization of a thin metamaterial plate for generation of orbital angular momentum (OAM) modes of a millimeter-wave beam, which can carry independent data streams over the same physical medium. The plate has a thickness of 1.56 mm, and consists of 3.06 × 0.68 mm rectangular apertures with spatial variant orientations. It generates OAM beams l = +1 and l = +3 with mode purity larger than 77.5% over a bandwidth of 6 GHz (25-31 GHz). We then use these streams to experimentally demonstrate a 16-Gbit/s millimeter-wave wireless communications link using two multiplexed OAM modes, each carrying a 2-Gbaud 16-QAM signal. A channel crosstalk less than -20 dB over a bandwidth of 4 GHz (26-30 GHz) and biterror-rates (BER) less than 3.8 × 10-3 are achieved.

Experimental measurements of multipath-induced intra- and inter-channel crosstalk effects in a millimeter-wave communications link using orbital-angular-momentum multiplexing

This paper reports on an experimental measurement and analysis of multipath-induced intra- and interchannel crosstalk effects in a mm-wave communications link using orbital angular momentum multiplexing at 28 GHz. The reflection is from an ideal reflector parallel to the propagation path. The intra-channel crosstalk effect is measured when a single OAM beam is transmitted, and inter-channel crosstalk effect is measured when 2 multiplexed OAM beams are transmitted. Both simulation and experimental results show that OAM channels with larger OAM number ℓ tend to have stronger intra-channel crosstalk because less power is received from the direct path and more power is received from the reflected path. This effect is caused by OAM beam divergence, as OAM beams with larger ℓ spread into a larger beam size and have less power in the beam center. For the same reason, OAM beams of larger ℓ lead to stronger inter-channel crosstalk with the other OAM channels.

Design challenges and guidelines for free-space optical communication links using orbital-angular-momentum multiplexing of multiple beams

In this paper, recent studies on the potential challenges for an orbital angular momentum (OAM) multiplexing system were reviewed. The design guideline for a practical OAM multiplexing system were investigated in term of (i) the power loss due to the beam divergence and limited-size receiver, and (ii) the channel crosstalk due to the misalignment between the transmitter and receiver.

Patch Antenna Array for the Generation of Millimeter-Wave Hermite–Gaussian Beams

This letter introduces a method of using patch antennas to generate millimeter-wave Hermite-Gaussian (HG) beams at E-band. An HG11 beam is formed using four inset-fed microstrip patch elements arranged with a microstrip corporate feeding network. The designed antennas are fabricated on a high-performance FR4 circuit board with a relative permittivity of 3.75 and a loss tangent of 0.018. The overall size of the antenna array is 8 × 8 × 0.125 mm3. A full-wave electromagnetic simulator, HFSS, is used to design the array. Radiation pattern measurements were taken on an NSI 700S-360 spherical near-field system at 73 GHz together with an Agilent vector network analyzer. The simulations and measurements are in good agreement.

Performance metrics and design parameters for an FSO communications link based on multiplexing of multiple orbital-angular-momentum beams

We investigate the design parameters for an OAM multiplexing-based free-space optical data link by analyzing power loss, channel crosstalk, and power penalty of the link, considering the misalignment between the transmitter and receiver (displacement, angular error, or their combination). Given a specific link distance, a transmitted beam size design approach is suggested. In addition, we also provide a trade-off approach to design proper transmitter and receiver aperture sizes and mode spacing of the transmitted OAM beams under a certain displacement or angular error. Our simulation indicates that (1) system with a larger beam size and a larger aperture seems to be more tolerant to the displacement but less tolerant to the receiver angular error. (2) system with a small mode spacing shows lower system power penalty under a small displacement or receiver angular error, while a larger mode spacing shows lower power penalty when the displacement or receiver angular error is above a certain threshold.

Orbital-angular-momentum-multiplexed free-space optical communication link using transmitter lenses

In this paper, we explore the potential benefits and limitations of using transmitter lenses in an orbital-angular-momentum (OAM)-multiplexed free-space optical (FSO) communication link. Both simulation and experimental results indicate that within certain transmission distances, using lenses at the transmitter to focus OAM beams could reduce power loss in OAM-based FSO links and that this improvement might be more significant for higher-order OAM beams. Moreover, the use of transmitter lenses could enhance system tolerance to angular error between transmitter and receiver, but they might degrade tolerance to lateral displacement.

Atmospheric turbulence mitigation in an OAM-based MIMO free-space optical link using spatial diversity combined with MIMO equalization.

We explore the mitigation of atmospheric turbulence effects for orbital angular momentum (OAM)-based free-space optical (FSO) communications with multiple-input multiple-output (MIMO) architecture. Such a system employs multiple spatially separated aperture elements at the transmitter/receiver, and each transmitter aperture contains multiplexed data-carrying OAM beams. We propose to use spatial diversity combined with MIMO equalization to mitigate both weak and strong turbulence distortions. In a 2×2 FSO link with each transmitter aperture containing two multiplexed OAM modes of ℓ=+1 and ℓ=+3, we experimentally show that at least two OAM data channels could be recovered under both weak and strong turbulence distortions using selection diversity assisted with MIMO equalization.

Physical phaseplate for the generation of a millimeter-wave hermite-Gaussian beam

This paper demonstrates the design, fabrication and characterization of a physical phaseplate, which transforms plane waves to Hermite-Gaussian mode beams. The phaseplate is designed for a HG11 mode, working at E-band from 71 to 76 GHz. Simulations were run in ANSYS HFSS, a full-wave electromagnetic simulator, and compared to measured results for phaseplates fabricated with laminate substrates and polymer sheets. The simulations and measurements are in good agreement and demonstrate the designed Hermite-Gaussian beam.