Start main page content

Re-inventing the Doppler effect

- Wits University

Wits and HUST researchers report a new form of the Doppler effect.

The research was published this weekend in Nature Communications, a prestigious on-line journal of the Nature group.

Contributing author Professor Andrew Forbes from the Wits School of Physics explains how their findings paves the way for enhanced metrology and measurement of moving objects with structured light.

“Measuring the speed of a moving object would seem a simple task, but surprisingly, we can’t measure speeds directly. GPS systems, popular with runners and cyclists, measure a distance and a time to work out the speed, while laser speed traps used by traffic police use a frequency shift in light to infer the speed.”

The latter exploits what is known as the “Doppler effect”, that change in tone/pitch of a siren that you hear as an ambulance zooms past. Discovered in 1842, the Doppler effect is a universal wave phenomenon that has been widely applied to acoustic and optical metrology in astronomy, oceanography, medicine, engineering, and many other fields. In particular, as for light wave, because of its ultra-high velocity, large bandwidth, and perfect directionality, the Doppler effect of light has spurred a myriad of applications, from cooling atoms to monitoring traffic flow. This effect originates from the relative motion between a wave source and an observer, resulting in a shift of the wave frequency.

The problem is that the Doppler effect only tells you how fast the object is moving, but not in what direction it is moving in.

“It only measures speeds of things coming towards or away from you, not objects moving sideways or on some weird trajectory, for example, a rocket tracing a curve in space. This is why the traffic police officers are always facing you head-on when they take the measurement. There are many applications where the direction matters, such as telling the difference between spinning, orbiting and lateral movements of objects,” says Forbes.

The HUST-Wits team added a vectorial twist to the tale, structuring the light in how it looks so that it can extract more information from the moving object.

“What we have done is to re-invent this old idea and add more functionality to it, so that now we have a Doppler approach that can measure the speed in all directions,” says Forbes.

The team realised that to know the “directions of the speeds”, what physicists call a velocity vector, the light itself would have to be a vector, imbued with lots of local directions in the electric field. The team showed that they could track a micro-particle moving across the beam, at each instant knowing both its position and speed, crucial for monitoring flow in micro-fluidic systems used in health care and for fluid flow in 3D.

The work is a collaboration between the top optics university in China, the Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology (HUST) and the Wits School of Physics.

Forbes is a Distinguished Professor in the School of Physics at Wits and holds an honorary professorship at HUST. He is also heads up the Structured Light Laboratory at Wits and has recently been appointed as Director of WitsQ, a strategic initiative of Wits to be a driver of quantum technologies in Africa.