Smoke rings are similar to the vortices created in laser light
Yevhen Rychko / Alamy Stock Photo
Short pulses from a laser can be twisted into a series of doughnut-like vortices, offering a way to transmit more information using light.
We usually encounter vortices in water and air, but similar swirling structures can also be made to form in beams of light. One such structure is a vortex ring, in which particles of light – or photons – twist and swirl like particles of air do in a smoke ring. Jinping Yao at the Chinese Academy of Sciences and her colleagues have now worked out how to pack up to 28 vortex rings into one laser pulse.
To do so, the team precisely tailored the light’s intensity, and tweaked another property of light called phase. Light is an electromagnetic wave, and its phase specifies where in its repeated up-and-down oscillation each part of the wave is. The laser pulse, which lasted for less than a millionth of a second in the experiment, could only be given the doughnut-like shape and swirling character of a smoke ring when the researchers gave its phase specific values at certain parts of the pulse.
They sent the pulse through a series of lenses, special ridged crystals and a “spatial light modulator”, which is like a small projector. By the time it passed through them all to reach a detector, it was shaped like a chain of vortex rings. Yao and her colleagues called this a “spatiotemporal vortex string” and the largest they made consisted of 28 distinct vortex rings.
They then encoded an image of their institute’s logo into the properties of a string of vortex rings. As a proof of principle, they used strings of 16 vortices to transmit 16,384 pixels, but possible future experiments with strings containing 28 vortex rings would be an efficient way to transmit lots of information in a short burst of light.
Howard Milchberg at the University of Maryland, whose team pioneered experiments with vortex rings, says that techniques for making vortices in light have recently become more widely available, so there are many possible new applications. “The field is sort of exploding and people are thinking about different things to do with these structures [in light]. Information-carrying capacity is definitely something worth thinking about,” he says.
Making doughnut swirls out of light requires lots of care and getting all the details right, but the researchers also provided something like a recipe for making more and different vortices in the future, says Alan Willner at the University of Southern California. “Research groups around the world have tried to design interesting, structured light. They’re asking, ‘What kind of light can we even create?’ and this could be part of a toolkit we need to find out,” he says.
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