Research scientists have succeeded in using "twisted" light beams to carry multiple streams of data at very high speed within a frequency range currently needed to carry just one. The breakthrough is of tremendous future potential for wireless comms, satellite, fibre optics, radio and television, as Martyn Warwick reports.
The development exploits the characteristics of the "Orbital Angular Momentum" (OAM) of light to increase the number of data streams carried over a given frequency and, by this means, confer the ability to multiply the data transmission speed of the combined data streams to 2.5Tbit/s.
I have just spent an hour (with a strong coffee, an ice pack to my brow and a scientific dictionary at my fingertips) reading a description of the technique in a paper published in the New Journal of Physics. It's not an easy read, but it is fascinating.
The many millions of us who wear certain types of sunglasses or watch 3D movies and television are comfortable (or not comfortable at all, if you spend too much time watching a three-dimensional films) with certain applications of polarised light, even if we don't necessarily appreciate the workings of the physics behind the phenomenon.
Polarisation is also commonly used in electronic communications as a methodology by which to double the amount of information able to be carried by a particular bandwidth as the polarisation process ensures that longitudinal light waves are carried separately from, and therefore cannot interfere with, transverse light waves (and vice versa).
However, by exploiting orbital angular momentum, light waves can effectively be "twisted" to different degrees and multiplexed to flow in a particular frequency band. This "corkscrewing" of the light allows massively increased speed and capacity.
A group of researchers from the Swedish Institute of Space Physics, under the leadership of Bo Thide, demonstrated OAM in Venice (where, some 400 years ago, Galileo first showed his telescope to the disbelieving and contemptuous powers-that-be) and sent a twisted signal the 442 metres from the Island of San Giorgio across the lagoon and on to the Doge's Palace in St.
In the New Journal of Physics article, Bo Thide writes, "We have shown experimentally, in a real-world setting, that it is possible to use two beams of incoherent radio waves, transmitted on the same frequency but encoded in two different orbital angular momentum states, to simultaneously transmit two independent radio channels."
He continues,"This novel radio technique allows the implementation of, in principle, an infinite number of channels in a given, fixed bandwidth, even without using polarisation, multiport or dense coding techniques. This paves the way for innovative techniques in radio science and entirely new paradigms in radio communication protocols that might offer a solution to the problem of radio-band congestion."
Bo Thide concludes, "We have experimentally shown that by using helicoidal parabolic antennae, the use of OAM states might dramatically increase the capacity of any frequency band, allowing the use of dense coding techniques in each of these new vortex radio channels. This might represent a concrete proposal for a possible solution to the band saturation problem."
As is not unusual in the history of science, research into OAM is being conducted in several parts of the world simultaneously. For example, teams from the University of Southern California, the Nasa Jet Propulsion Laboratory in Pasadena, in the US are working together with researchers based at Tel Aviv University in Israel.
These groups have demonstrated the application of OAM techniques on a scale four times as great in terms of capacity but over a distance of just one metre. It is another confirmation that the approach works but also demonstrates that it will take a lot more research, and a lot more money, before OAM enters the mainstream. For as with many comms technologies atmospheric turbulence can severely affect direst airborne transmission and reception and it will be difficult to adapt the approach for fibre optic systems for long-distance applications.
Alan Wilner, team leader at the University of Southern California says, "For situations that require high capacity... over relatively short distances of less than 1kilometre, this approach could be appealing. There are also opportunities for long-distance satellite-to-satellite communications in space, where turbulence is not an issue."
That said, science has shown, time after time after time, that where there's a will there's usually a way, and given the saturation of existing bandwidth and the pressing need either to make more available from finite resources or to tweak/invent ways of cramming more capacity into existing spectrum, OAM holds a lot of promise.
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