Let there be (twisted) light

Unlimited wireless spectrum? Braiding light or radio waves is a promising approach to a 117-year-old stalemate
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First Published: Thu, Oct 18 2012. 09 23 PM IST
Every modern communications device uses spectrum; that includes television, mobile phones, radio, GPS devices. Photo: Pradeep Gaur/Mint
Every modern communications device uses spectrum; that includes television, mobile phones, radio, GPS devices. Photo: Pradeep Gaur/Mint
Updated: Fri, Oct 19 2012. 08 27 AM IST
Are you waiting for the iPhone5?
It’s really not going to make that much of a difference to your telephone calls or Internet experience. Sure, you will get smart, new applications, and you will get the “oohs” and “aahs” you crave.
Every modern smartphone or tablet is rendered primitive because it faces a nasty block on the great telecommunications highway—the lack of wireless spectrum, or the radio frequencies that carry voice calls and data. So, calls drop and files buffer.
3G, or third-generation networks, speeded up things a bit, but not by much. Now, we wait for 4G, or fourth-generation networks, which will speed up things just a bit more.
Like land and coal, spectrum is a scarce resource, as the many billions paid periodically by telecom giants for it indicate, as indeed do the scams that hover over it. Every modern communications device uses spectrum; that includes television, mobile phones, radio, GPS devices.
The problem is that the basic concept of wireless communication has not changed substantively since Guglielmo Marconi’s first wireless transmission in 1895, riding a single frequency.
There are many research groups across the world trying to find the next holy grail of mobile telecommunications—infinite wireless spectrum. One of the most innovative, promising and difficult approaches is something that braids light, or radio waves (both behave similarly), so that a variety of signals can be zapped across the same frequency. Imagine digital television, mobile calls and GPS signals riding the same airwave.
These twisted waves of light and radio waves were created for the first time this year, tapping into a property of theirs called orbital angular momentum (OAM). To use a crude metaphor, this is akin to the earth orbiting the sun, as against the earth spinning on its own axis, technically called spin angular momentum, which is the characteristic normally used to transmit light and radio waves.
The concept of OAM was first evolved by Bo Thidé, a Swedish physicist, in a 2007 paper. “This enables new types of experiments that go beyond what is possible in optics,” Thidé’s group of Swedish and Russian researchers wrote in the journal Physical Review Letters.
In March 2012, Thidé and Italian researchers added OAM to a radio signal and, for the first time, demonstrated that the capacity of radio waves can, in theory, be infinitely increased.
“This novel radio technique allows the implementation, of, in principle, an infinite number of channels in a given, fixed bandwidth,” the team reported in the New Journal of Physics.
Less than three months later, a group of US, Israeli and Chinese researchers did the same thing with light waves, reporting in Nature Photonics that they sent a wireless transmission at 2.5 terabits per second, perhaps the fastest wireless link ever created on the planet.
How fast is that?
It’s enough to download 66 high-definition movies the length of Sholay—the 1975 Bollywood blockbuster, more than three hours long—in one second. It would take about seven hours to do that on the home broadband, and days, if not weeks, to do it on a mobile-phone’s 3G network.
But these are very early days for the technology. The June breakthrough using eight light beams, in bundles of four, indicates just how early. Four beams in one bundle were transmitted like threads in a rope, corkscrewed together. The other bundle formed a sheath on the outside. If eight beams can be strapped together, it should be possible to do more, although there are other formidable obstacles.
The 2.5 terabit light wave transmission travelled only a metre, and the earlier radio-wave transmission 442 metres, across a canal in Venice. For perspective, in 1895, Marconi’s wireless transmissions reached no more than 2.5km.
A major problem is that atmospheric disturbances degrade OAM transmissions. Presently, this means the best applications could be in space—where there is no atmosphere—between satellites, or as technology improves, between two points. And using OAM in light waves may not make much sense because traditional fibre-optic cables have surplus bandwidth, and laboratory work with fibre has reached the petabit range, or nearly a thousand times faster.
Equipment to catch up with the potential of twisted light will need to be faster, smaller and many orders more sophisticated than anything in the market or laboratory today. Those efforts are underway, such as recent attempts to produce chips that use twisted light.
Yesterday, a team of British and Chinese researchers announced the smallest-ever chip of the type.“Our contribution is a very small and compact emitter that is 1,000 times smaller than any previous device,” Siyuan Yu, a photonics engineer and professor at the University of Bristol, UK, told me over email. Yu said his team fabricated an array of these chips to demonstrate practicality.
The iPhone5 may not find a wireless network to realise its potential, but it is entirely conceivable that the iPhone10 might.
Samar Halarnkar is a Bangalore-based journalist. This is a fortnightly column that explores the cutting edge of science and technology. Comments are welcome at frontiermail@livemint.com
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First Published: Thu, Oct 18 2012. 09 23 PM IST
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