High-capacity millimetre-wave communications with orbital angular momentum multiplexing

Abstract

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.

Figure 1. Concept of utilizing OAM and polarization multiplexing in a free-space mm-wave communication link.

This technique could have potential applications in places, such as data centres, where large bandwidth links between computer clusters are required.

Figure 2. Experiment of OAM and polarization multiplexing in a free-space mm-wave communication link.

(a) Schematic diagram of a millimetre-wave link based on OAM and polarization multiplexing. (b) Polarization-multiplexed (pol-muxed) OAM beams (ℓ=−3, −1, +1 and +3) are generated by dual-polarization lensed-horn antennae, followed by SPPs. These beams are multiplexed by a 1 × 4 combiner (three beamsplitters in our experiment). (c) SPP-based OAM demultiplexer. The received OAM beams are split into four copies by a 1 × 4 splitter (three beamsplitters), for each of which (ℓ) is demultiplexed and converted into a Gaussian beam by a reverse SPP of (−ℓ). (d) OAM mode demultiplexer. The received OAM beams are demultiplexed by an OAM mode demultiplexer, which can spatially separate the four OAM beams without power-splitting loss. Pol: polarization, MUX: multiplexing, DeMUX: demultiplexing, Ch: channel, BS: beamsplitter.

Figure 3. Normalized intensity and interferogram of mm-wave OAM beams.

(a) Normalized measured intensity of four mm-wave OAM beams of charge ℓ=±1 and ℓ=±3. (b) Interferogram images of a Gaussian beam and OAM beams combined by a beam splitter. (c) Normalized measured and simulated intensity distribution of the multiplexed OAM beams after beam splitters.

Figure 4. Results of 32 Gbit s⁻¹ data transmission using eight pol-muxed mm-wave OAM channels.

(a) Constellations of the received 1-Gbaud 16-QAM signals for OAM channel ℓ=+3 under single Y-pol and dual-pol conditions. The SNR is 19 dB. (b) Measured BER curves of 1-Gbaud 16-QAM signals for (i) a single OAM channel (no crosstalk), (ii) four OAM channels on Y-pol (with crosstalk, Y-pol) and (iii) eight OAM channels on both X-and Y-pol (with crosstalk, X-pol and Y-pol).

Figure 5. Results of 8 Gbit s⁻¹ data transmission using mm-wave OAM mode demultiplexer for demultiplexing.

(a) The normalized intensity distributions of four input OAM beams (ℓ=±1 and ℓ=±3) in the OAM mode sorting direction in the detection plane. (b) The received constellations of the 1-Gbaud QPSK signals for OAM channel ℓ=−1 in single Y-pol and dual-pol cases with an SNR=11 dB. (c) Measured BER curves of 1-Gbaud QPSK signals for (i) a single OAM channel (no crosstalk), (ii) two OAM channels on Y-pol (with crosstalk, Y-pol) and (iii) four OAM channels on both X-and Y-pol (with crosstalk, X-pol and Y-pol).