Rotational Doppler shift spotted in twisted light
Aug 5, 2013 6 comments
"Twisted light" has been used by researchers in the UK to develop a new way of measuring the angular velocity of a remote spinning object. The team fired two beams of light carrying orbital angular momentum at a rotating surface and showed that the resulting interference pattern in the reflected light is related to the surface's angular velocity. The researchers hope that the phenomenon can be used to develop systems to carry out a range of practical measurements, from monitoring industrial equipment to calculating rotation rates of astronomical objects.
The Doppler shift – a shift in the frequency of waves emitted or reflected by an object moving relative to the observer – is a well-understood phenomenon with numerous uses in science and engineering. These include determining the speed at which distant galaxies are approaching or receding and making it easier for the police to catch speeding motorists. It can also be used to study objects that are rotating when some of the object is rotating towards the observer and some is rotating away. However, it cannot be used to work out how fast an object is rotating about the axis pointing along the direct line of sight between the object, light source and observer.
This latest work was done using beams of light that carry orbital angular momentum. This involves the wavefronts of the light's electric and magnetic fields rotating around the direction of the propagation vector. The fields trace out fusilli-like spirals and the faster the rotation, the greater the orbital angular momentum. This twisted light is of great interest to those working in the telecommunications industry and researchers have already shown that orbital angular momentum can be used to boost the amount of information that can be transmitted using light and other electromagnetic radiation.
The study was done by Martin Lavery and colleagues at the University of Glasgow, together with researchers at the University of Strathclyde. The team's rotating surface is simple – a piece of aluminium foil stuck to a wheel that is spun by a motor taken from a remote-controlled car. The back of the foil (the matt side) is illuminated by two superposed light beams of the same frequency and intensity but with equal and opposite angular momenta.
The researchers found that when the light hit the rotating surface, the two beams are affected slightly differently. This is because although the angular momenta of the two beams are equal and opposite in the laboratory reference frame, the angular momenta relative to the spinning surface are different. The frequency of the scattered beam with orbital angular momentum in the same direction as the surface is raised slightly (blue-shifted), while the frequency of the beam with angular momentum in the opposite direction is lowered (red-shifted) by the same amount (see figure).
When the scattered light is detected, it is therefore a mixture of two slightly different frequencies, which move repeatedly in and out of phase. When the frequencies are in phase, this causes constructive interference; while when they are out of phase, the interference is destructive. This results in a regular pulsation in the light intensity that is detected. From the rate of this pulsation, the researchers can calculate the rotation rate of the spinning disc.
Lorenzo Marrucci, the leader of the Laboratory of Nonlinear Optical Spectroscopy at the University of Naples in Italy, is intrigued by the work. "I think it's quite unexpected and might be surprising that you have this Doppler effect even though there is nothing that is moving closer or farther from the detector," he says. "Of course, you can understand it with hindsight by reasoning about the effect, but without this work you would not expect it to occur."
Bo Thidé, an expert in electromagnetic radiation from space at the Swedish Institute of Space Physics at Uppsala University in Sweden, is more sceptical. He says that while the beating between two reflected waves is a new observation, "the whole concept of the rotational Doppler shift is, as the researchers say, not new. It's inherent in the laws of nature and there are many articles that have discussed it theoretically and described how you can perform this experiment."
Protecting wind turbines
Lavery is keen to explore the possible applications of the technology in engineering, suggesting that, among other things, it could potentially help prevent turbulence from damaging wind turbines. "Up here on the west coast of Scotland, there was a big fire on a turbine last year," he explains. "With this effect we would potentially be able to make a head-on measurement of the scatter coming back off the atmosphere and determine how fast that atmosphere is rotating. You could use that to make a feedback system that can follow the wind turbines and make sure they can cope with the amount of wind that's coming onto them."
Marrucci, meanwhile, is looking beyond windmills and believes that the phenomenon should be further investigated to see if it could be used to study spinning astronomical objects such as stars. "I would really like to see an analysis of whether or not there can be an application in astronomical settings, because if something like that should come out, that would be very interesting," he says.
The research is published in Science.
About the author
Tim Wogan is a science writer based in the UK