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Telescopes and space missions

Telescopes and space missions

Physicists put a new twist on radio

07 Mar 2012
The writing on the wall in Venice


Physicists in Italy have shown that, like light, radio waves can have their wavefronts twisted so that they take on a corkscrew shape. The researchers have successfully transmitted twisted beams several hundred metres across the lagoon in Venice using a specially shaped antenna. They believe that such beams could dramatically enhance the information capacity of wireless communications by multiplying the number of channels that can be encoded in a given frequency range.

Physicists have known for many years that a beam of light can be twisted so that its wavefront rotates around its direction of propagation in a spiral shape. The twisting is achieved by controlling the orbital angular momentum of the light. This property is associated with the shape of a light beam’s wavefront – the imaginary line or plane joining the points on a wave that have the same phase.

The phenomenon was first seen inside laser cavities and then researchers worked out how to tune a beam’s orbital angular momentum to create “optical tweezers” that can move tiny objects. The orbital angular momentum should not be confused with the more familiar spin angular momentum of light, which is associated with a wave’s polarization or direction of oscillation.

Inspired by black holes

Now, Fabrizio Tamburini of the University of Padova in Italy, Bo Thidé of the Swedish Institute of Space Physics in Uppsala and colleagues have manipulated the orbital angular momentum of radio waves. This work stems from earlier research by Tamburini and Thidé in which they calculated that a spinning black hole should distort space–time in such a way as to leave a noticeable twist in the wavefront of electromagnetic radiation that passes close by.

The researchers believe it should be possible to create a similar twisting effect on Earth, without the need for an ultra-massive object. To do this they set up a spiral-staircase-like structure inside a room insulated against sound and electromagnetic waves at the University of Uppsala. They bounced radio waves off the structure and used a pair of receiving antennas in a plane some metres from the structure to record the variation in phase across the plane. They found that the phase varied as would be expected from a twisted wavefront.

Now, the researchers have transmitted radio waves with a well-defined orbital angular momentum in noisy, real-world conditions – 442 m across St Mark’s Basin in Venice. A specially adapted satellite-dish-style antenna on the lighthouse on St George’s Island was used to create radio waves with a frequency of 2.4 GHz and an orbital angular momentum of 1 – the latter meaning that the wavefront is rotated 360° in the space of one wavelength. A standard “Yagi” antenna was also used to send waves of the same frequency but without orbital angular momentum across the same stretch of lagoon.

A “revolution” in radio technology

By varying the distance between the two receiving antennas on the balcony of the Doge’s Palace, the researchers were able to tune in to either the twisted or the untwisted beam. This proves, they say, that they were able to transmit two channels simultaneously using just a single frequency.

According to Tamburini, this demonstration could lead to a “revolution” in radio technology, since, he says, it means in principle being able to create an infinite number of channels in a given bandwidth, with each channel encoded using a different orbital angular momentum. He points out that a vast expansion of wireless capacity would be highly prized, given the huge and rapidly growing demand for wireless services, estimating that 11 new channels per frequency band (corresponding to five orbital angular momentum states – five clockwise, five anticlockwise and one untwisted) should prove “economically reasonable” in the short term.

Michael Berry of Bristol University, who with John Nye showed in the 1970s that phase singularities are pervasive features of waves of all kinds, was present at the demonstration. He says the event was “a splendid occasion” that caused him to change his mind about the potential of twisted radio waves. “I initially thought that this was a clever but unsurprising example of wave interference,” he admits, “but then I witnessed the intense interest of executives from the satellite broadcasting industry.” However, he adds that the practical importance of the technology “remains to be explored by communications engineers”.

Real-world demonstration

Taco Visser of the Delft University of Technology in the Netherlands is also enthusiastic about the latest research, describing it as “a very elegant real-world demonstration of encoding information by using orbital angular momentum”. But he too cautions that more work must be done to prove its practical utility. “How many beams with different angular momentum can you still distinguish when they have travelled through, say, two miles of atmospheric turbulence?” he asks.

In fact, Tamburini and colleagues hope to perform new tests of the twisted radio waves over distances of several kilometres within the next few months. These tests will be carried out with new kinds of antenna designed to minimize the “singularity” that is created at the centre of a twisted wave and that reduce the intensity of the transmitted radio signal. “The modified parabolic dish was a brutal approach that we know works,” he says. The new approach will involve generating the beam electronically using a set of dipole antennas rather than a dish.

Tamburini adds that the group is also continuing with its astronomical research. It is hoping to use the Very Large Array radio telescope (and possibly the planned Square Kilometre Array) to make measurements of the black hole at the centre of the Milky Way to prove that it is indeed rotating.

The work is described in New Journal of Physics.

Fabrizio Tamburini describes the research in great detail in the video below, which also contains images from the test in Venice.

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