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Ren, Y. et al. (2016) Experimental characterization of a 400 Gbit/s orbital angular momentum multiplexed free-space optical link over 120 m. Optics Letters, 41(3), pp. 622-625.There may be differences between this version and the published version. You are advised to consult the publisher's version if you wish to cite from it.http://eprints.gla.ac.uk/115873/ Free-space optical (FSO) communications has attracted much attention for a variety of applications, such as back-haul and data centers [1, 2]. Given the rapid growth of bandwidth demand for these applications, there is increased interest in utilizing advanced multiplexing of multiple data streams to increase the data capacity and spectral efficiency of an FSO system [3]. Multiplexing in wavelength and polarization, known as wavelength division multiplexing (WDM) and polarization division multiplexing (PDM) respectively, have previously been used for FSO transmission [3, 4]. Another potential approach is to use space division multiplexing (SDM), for which multiple beams each carrying an independent data stream are transmitted through a common medium [5, 6]. Provided these spatially-overlapping beams can be properly demultiplexed with tolerable crosstalk, the total capacity and spectral efficiency of the communication system is increased by a factor equal to the number of transmitted orthogonal modes. An orthogonal spatial modal basis set for SDM that has gained interest is orbital angular momentum (OAM) [5][6][7][8]. OAM beams with different ℓ values (ℓ is an unbounded integer) are mutually orthogonal [8, 9], so that beams carrying different OAM can act as independent data channels for efficiently multiplexing multiple information-bearing signals in an SDM-based communication system [5]. Moreover, similar to any SDM approach, OAM multiplexing is in principle compatible with existing WDM and PDM techniques [6]. We note that compared to other modal sets, such as Hermite-Gaussian (HG) modes that could also be used for SDM, OAM modes might offer the potential advantage of being conveniently matched to many optical subsystems due to their circular symmetry.It is known that the amount of phase change per unit area for an OAM beam is greatest in the beam center, and that collecting sufficient phase changes is critical for ensuring orthogonality among OAM beams [9]. As a result, OAM multiplexing might be more sensitive to system alignment as it relies more critically on a common optical axis to achieve low inter-modal crosstalk [10]. OAM multiplexing has been employed to demonstrate high-capacity FSO transmission links in laboratory settings [6, 7]. These experiments were generally

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.

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We explore the potential of combining the advantages of multiple-input multiple-output (MIMO)-based spatial multiplexing with those of orbital angular momentum (OAM) multiplexing to increase the capacity of free-space optical (FSO) communications. We experimentally demonstrate an 80 Gbit/s FSO system with a 2×2 aperture architecture, in which each transmitter aperture contains two multiplexed data-carrying OAM modes. Inter-channel crosstalk effects are minimized by the OAM beams' inherent orthogonality and by the use of 4×4 MIMO signal processing. Our experimental results show that the bit-error rates can reach below the forward error correction limit of 3.8×10(-3) and the power penalties are less than 3.6 dB for all channels after MIMO processing. This indicates that OAM and MIMO-based spatial multiplexing could be simultaneously utilized, thereby providing the potential to enhance system performance.

We experimentally investigate the potential of using ‘self-healing’ Bessel-Gaussian beams carrying orbital-angular-momentum to overcome limitations in obstructed free-space optical and 28-GHz millimetre-wave communication links. We multiplex and transmit two beams (l = +1 and +3) over 1.4 metres in both the optical and millimetre-wave domains. Each optical beam carried 50-Gbaud quadrature-phase-shift-keyed data, and each millimetre-wave beam carried 1-Gbaud 16-quadrature-amplitude-modulated data. In both types of links, opaque disks of different sizes are used to obstruct the beams at different transverse positions. We observe self-healing after the obstructions, and assess crosstalk and power penalty when data is transmitted. Moreover, we show that Bessel-Gaussian orbital-angular-momentum beams are more tolerant to obstructions than non-Bessel orbital-angular-momentum beams. For example, when obstructions that are 1 and 0.44 the size of the l = +1 beam, are placed at beam centre, optical and millimetre-wave Bessel-Gaussian beams show ~6 dB and ~8 dB reduction in crosstalk, respectively.

We experimentally demonstrate spatial multiplexing of an orbital angular momentum (OAM)-encoded quantum channel and a classical Gaussian beam with a different wavelength and orthogonal polarization. Data rates as large as 100 MHz are achieved by encoding on two different OAM states by employing a combination of independently modulated laser diodes and helical phase holograms. The influence of OAM mode spacing, encoding bandwidth, and interference from the co-propagating Gaussian beam on registered photon count rates and quantum bit error rates is investigated. Our results show that the deleterious effects of intermodal crosstalk effects on system performance become less important for OAM mode spacing Δ≥2 (corresponding to a crosstalk value of less than -18.5  dB). The use of OAM domain can additionally offer at least 10.4 dB isolation besides that provided by wavelength and polarization, leading to a further suppression of interference from the classical channel.

Electromagnetic waves carrying orbital angular momentum (OAM) have been used for mode division multiplexing in free-space communication systems to increase both the capacity and the spectral efficiency. In the case of conventional wireless communication links using non-OAM beams, multipath effects caused by beam spreading and reflection from the surrounding objects affect the system performance. This paper presents the results of analysis, simulations, and measurements of multipath effects in a millimetre-wave communication link using OAM multiplexing at 28 GHz. Multipath-induced intra- and inter-channel crosstalk, which are caused by specular reflection from a plane parallel to the propagation path, are analysed and measured. Both the simulation and the experimental results show that an OAM channel with a high OAM number ℓ tends to suffer from both strong intra-channel crosstalk and strong inter-channel crosstalk with other OAM channels. Results of the analysis show that this observation can be explained on the basis of both the properties of OAM beam divergence and the filtering effect at the receiver, which is associated with the spiral wavefront of OAM beams.