Author Topic: Twisted Light Could Dramatically Boost Data Rates - Part 1  (Read 290 times)

Offline najnin

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Twisted Light Could Dramatically Boost Data Rates - Part 1
« on: August 21, 2016, 05:41:03 PM »
A remarkable invention about light: Certain beams, traveling through space with a spiraling pattern suggestive of a corkscrew, carry a form of momentum called orbital angular momentum.

OAM waves with different “twists” don’t interfere with one another. That means they can be overlaid one on top of another to carry a theoretically unlimited number of different data streams at the same time. Hardware that can transmit and receive even a few such OAM beams could dramatically boost the capacity of optical and radio transmissions without placing any more demands on the crowded electromagnetic spectrum than we do today.

One of the most curious aspects of light is that it has energy and momentum, just like an ordinary physical object moving through space, even though it doesn’t have mass. And just like an ordinary physical object, when light strikes something, it exerts a force. A solar sail, for example, takes advantage of this property as it accelerates through space, pushed by sunlight alone. Light carries that “pushing” capacity—its linear momentum—along the direction it’s moving. But light can also have angular momentum.

For a long time, the only commonly discussed form of such momentum was spin angular momentum. To understand how it works requires a bit of background on polarization. A ray of light has an electric field associated with it that oscillates perpendicularly to the direction of the ray. In light that is linearly polarized, the electric field oscillates along a fixed line. Light that is circularly polarized has an oscillating electric field that rotates around the direction the light moves in. Such circularly polarized waves have spin angular momentum, the electromagnetic equivalent of a spinning top or a planet rotating on its axis. Remarkably, this form of momentum can also impart a torque: Shine light with spin angular momentum on a microscopic object and you can make it rotate.

Not all light has OAM. To have it, a beam must have a particular kind of phase front. Phase is the component of an electromagnetic wave that governs the arrival times of its peaks and troughs.

Consider a beam of light --the beam as a collection of a very large number of “miniwaves.” But there is no rule demanding that different parts of a light beam all have the same phase. In the case of a helical wave, the sort that carries OAM, the miniwaves in the cross section of the beam aren’t uniform. Instead, the phase of each miniwave depends on its angular location around the center of the beam.

To give a more intuitive feel for how this translates into a “twisting” beam, think of those miniwaves as they move through space. At first, all the miniwaves that are at their peak will lie along some angle, like the hand on a clock. A short time later, those miniwaves will no longer be at their peak; instead, the peak will have advanced, like the clock hand, to another set of miniwaves at another angle around the circle. This process continues, so that if you track the angular location of the miniwave peaks, the wave appears to twist as it moves.

But the exploitation of OAM may have the biggest impact in field of communications. OAM waves with different “twists” don’t interfere with one another. That means they can be overlaid one on top of another to carry a theoretically unlimited number of different data streams at the same time. Hardware that can transmit and receive even a few such OAM beams could dramatically boost the capacity of optical and radio transmissions without placing any more demands on the crowded electromagnetic spectrum than we do today. Indeed, my team at the University of Southern California, in Los Angeles, and others have performed experiments to test this idea, and they worked just as the theory predicted.

If OAM communications sounds familiar, you may have noticed the news a few years ago when one of the first OAM radio demonstrations was performed. At the time, some engineers argued that the approach wasn’t new and was instead just a version of another strategy for sending multiple waves at the same time. But since then, it’s become clear that OAM transmission really is a novel and powerful technology, one that could allow us to transmit much more information along wireless connections and dramatically speed up parts of the networks that underpin the Internet. The technological challenge is finding good ways to harness OAM. We are finally starting to do just that.

Fig. 1

One of the most curious aspects of light is that it has energy and momentum, just like an ordinary physical object moving through space, even though it doesn’t have mass. And just like an ordinary physical object, when light strikes something, it exerts a force. A solar sail, for example, takes advantage of this property as it accelerates through space, pushed by sunlight alone. Light carries that “pushing” capacity—its linear momentum—along the direction it’s moving. But light can also have angular momentum.

For a long time, the only commonly discussed form of such momentum was spin angular momentum. To understand how it works requires a bit of background on polarization. A ray of light has an electric field associated with it that oscillates perpendicularly to the direction of the ray. In light that is linearly polarized, the electric field oscillates along a fixed line. Light that is circularly polarized has an oscillating electric field that rotates around the direction the light moves in. Such circularly polarized waves have spin angular momentum, the electromagnetic equivalent of a spinning top or a planet rotating on its axis. Remarkably, this form of momentum can also impart a torque: Shine light with spin angular momentum on a microscopic object and you can make it rotate.

In 1992, physicist Les Allen, working with Han Woerdman and colleagues at Leiden University, in the Netherlands, pointed out that a certain spiraling beam carries another form of angular momentum—orbital angular momentum. If light with spin angular momentum is like a spinning planet, the physical analogue of OAM light could be a planet orbiting the sun. OAM light can also impart a torque, a “twist” that, depending on where the beam hits, can cause a small object to rotate or move in an orbit around the center of the beam.

Not all light has OAM. To have it, a beam must have a particular kind of phase front. Phase is the component of an electromagnetic wave that governs the arrival times of its peaks and troughs. To picture what a phase front looks like, consider a beam of light. If you look at its cross section, it’s easy to view the beam as a collection of a very large number of “miniwaves.” If all these miniwaves oscillate in unison as they propagate—as they do in a common laser beam—the beam is a plane wave, and it has a flat phase front. At any given point along the beam’s propagating direction, the entire cross section has one phase value—that is, all the minibeams are at the peak of a crest, the bottom of a trough, or more likely, somewhere in between.

But there is no rule demanding that different parts of a light beam all have the same phase. In the case of a helical wave, the sort that carries OAM, the miniwaves in the cross section of the beam aren’t uniform. Instead, the phase of each miniwave depends on its angular location around the center of the beam. If you were to trace a circle around the center, the phase would either steadily increase or decrease as you go around.

A plane wave, such as an ordinary laser beam, has the same phase at each point in cross section; the value, which corresponds to where each miniwave in the beam is in its oscillation, changes as the beam propagates. In an OAM beam, the phase can take on a variety of values, and the resulting phase profile rotates around the center of the beam as it moves.       

To give a more intuitive feel for how this translates into a “twisting” beam, think of those miniwaves as they move through space. At first, all the miniwaves that are at their peak will lie along some angle, like the hand on a clock. A short time later, those miniwaves will no longer be at their peak; instead, the peak will have advanced, like the clock hand, to another set of miniwaves at another angle around the circle. This process continues, so that if you track the angular location of the miniwave peaks, the wave appears to twist as it moves. To get a better sense of how this phase behavior translates into what looks like movement, consider a neon sign with individual bulbs timed to turn on and off in sequence. With the right program, the neon light can look as if it’s moving in one direction, even though none of the bulbs move. The same is true for OAM. Each miniwave in a beam oscillates steadily, but the time sequence at which each miniwave peaks makes the phase front twist, describing a helix as the beam moves.

Crucially, the more dramatic the phase shift as you move in a circle around the cross section of the beam, the bigger the twist and the higher the amount of OAM. Phase changes around the entire circle can come in integer multiples of 360 degrees.

Fig. 2.

Such a twisting, helical wave is hard to visualize, but it does produce a clear visible effect. Unlike a conventional beam, which is brightest at the center, the cross section of a helical beam has a ringlike shape, with a dark center. This happens because the center of the beam is full of miniwaves with every possible phase, and a miniwave at its peak is very likely to overlap at least in part with a miniwave at its trough. The opposing pairs cancel one another out through destructive interference.

« Last Edit: August 21, 2016, 05:44:36 PM by najnin »