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June 25, 2012

Adding a Twist dimension to communication for 2.5 terabits per second now and hundreds of times more later

Extreme Tech - American and Israeli researchers have used twisted, vortex beams to transmit data at 2.5 terabits per second. This twist encoding technique is likely to be used in the next few years to vastly increase the throughput of both wireless and fiber-optic networks.

These twisted signals use orbital angular momentum (OAM) to cram much more data into a single stream. In current state-of-the-art transmission protocols (WiFi, LTE, COFDM), we only modulate the spin angular momentum (SAM) of radio waves, not the OAM. If you picture the Earth, SAM is our planet spinning on its axis, while OAM is our movement around the Sun. Basically, the breakthrough here is that researchers have created a wireless network protocol that uses both OAM and SAM.

According to Thide, OAM should allow us to twist together an “infinite number” of conventional transmission protocols without using any more spectrum. In theory, we should be able to take 10 (or 100 or 1000 or…) WiFi or LTE signals and twist them into a single beam, increasing throughput by 10 (or 100 or 1000 or…) times.

The next task for Willner’s team will be to increase the OAM network’s paltry one-meter transmission distance to something a little more usable. “For situations that require high capacity… over relatively short distances of less than 1km, this approach could be appealing. Of course, there are also opportunities for long-distance satellite-to-satellite communications in space, where turbulence is not an issue,” Willner tells the BBC. In reality, the main limiting factor is that we simply don’t have the hardware or software to manipulate OAM. The future of wireless networking is very bright indeed.

Nature - Terabit free-space data transmission employing orbital angular momentum multiplexing


Concept and Principle - a, Generation of an information-carrying OAM beam with a helical phase front. b, Recovery of an information-carrying beam with a planar phase front. c, Multiplexing/demultiplexing of information-carrying OAM beams together with polarizatio



The recognition in the 1990s that light beams with a helical phase front have orbital angular momentum has benefited applications ranging from optical manipulation to quantum information processing. Recently, attention has been directed towards the opportunities for harnessing such beams in communications. Here, we demonstrate that four light beams with different values of orbital angular momentum and encoded with 42.8 × 4 Gbit s−1 quadrature amplitude modulation (16-QAM) signals can be multiplexed and demultiplexed, allowing a 1.37 Tbit s−1 aggregated rate and 25.6 bit s−1 Hz−1 spectral efficiency when combined with polarization multiplexing. Moreover, we show scalability in the spatial domain using two groups of concentric rings of eight polarization-multiplexed 20 × 4 Gbit s−1 16-QAM-carrying orbital angular momentum beams, achieving a capacity of 2.56 Tbit s−1 and spectral efficiency of 95.7 bit s−1 Hz−1. We also report data exchange between orbital angular momentum beams encoded with 100 Gbit s−1 differential quadrature phase-shift keying signals. These demonstrations suggest that orbital angular momentum could be a useful degree of freedom for increasing the capacity of free-space communications.

Experimental results of the multiplexing/demultiplexing of four pol-muxed 10.7X4 Gbit s-1 16-QAM-carrying OAM beams (OAM+4, OAM+8, OAM-8, OAM+16). a1-a4, Four computer-generated spiral phase masks with charges of -4, +8, -8 and -16 loaded into SLM4, SLM3, SLM2 and SLM1.


Implementation details of the experimental setup. A,C,D,E, multiplexing/demultiplexing of information-carrying OAM beams; B,C,F, data exchange between OAM beams. (D)QPSK, (differential) quadrature phase-shift keying; 16-QAM, quadrature amplitude modulation; PC, polarization controller; EDFA, erbium-doped fibre amplifier; BPF, band-pass filter; DGD, differential group delay; Pol., polarizer; TDL, tunable delay line; AM, amplitude modulator; OC, optical coupler; Col., collimator; HWP, half-wave plate; SLM1-6, spatial light modulator; BS1-3, non-polarizing beamsplitter; BS4, BS5, polarizing beamsplitter; M1-M4, mirror; PM, power metre; EAM, electroabsorption modulator; Att, attenuator; DLI, delay-line interferometre; Rx, receiver; LO, local oscillator; ADC, analog-to-digital converter; DSP, digital signal processing.



Experimental (b) and theoretical results (c1-c5,d1-d3,e1-e4,f1-f3,g1-g3) of the multiplexing/demultiplexing of four OAM beams (OAM-8, OAM+10, OAM+12, OAM-14).

12 pages of supplemental information


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