Putting a new twist on optical communications
Jun 25, 2012 10 comments
If the lacklustre speed of your Internet connection is getting you down, help could soon arrive from the orbital angular momentum of light. That is because an international team of researchers has developed a prototype system that uses this previously unexploited property of electromagnetic radiation to boost the amount of information that can be transmitted using a given amount of bandwidth. Although the test transmission was done across just a few metres in a vacuum, the technology developed in this proof-of-principle application could find wider application in optical telecommunications.
The rate at which data can be transmitted using electromagnetic radiation is normally limited by how much of the electromagnetic frequency spectrum is used – a quantity referred to as the bandwidth of the system. However, electromagnetic radiation has other degrees of freedom in addition to frequency and researchers are keen to use these to develop multiplexing schemes that boost the amount of data that can be sent over a link. For example, photons have an intrinsic spin angular momentum that manifests itself in the polarization of light. This property has already been used to increase data transmission rates – one stream of data is transmitted using photons with vertical polarization, for example, and another stream using photons with horizontal polarization.
Orbital angular momentum
It turns out that light can also carry orbital angular momentum. This is a result of the phase fronts of the waves rotating relative to their direction of propagation to create a pattern resembling a corkscrew. Whereas spin angular momentum can take only two values, orbital angular momentum can, in principle, take an infinite number of values. This could, in theory, allow a large number of data channels to be created using a finite amount of bandwidth.
This orbital angular momentum was first considered as a possible means of quantum communication in 2001 by the Austrian quantum physicist Anton Zeilinger. The idea that classical information could also be encoded in the orbital-angular-momentum states of photons was then demonstrated in 2004 by Miles Padgett and colleagues at the University of Glasgow in the UK. However, while Padgett's group proved that the principle could work, there was much to be done to produce a practical system.
The challenge has been taken up by Alan Willner and team at the University of Southern California, who, together with colleagues elsewhere in the US and in Israel, are the first to use orbital-angular-momentum states for multiplexing. Each data stream is encoded in the usual way using a series of on/off laser pulses. Then, separate streams of data are given a different orbital angular momentum before the beams are combined and transmitted. Finally, the different streams are separated in a process called "demultiplexing".
The different orbital-angular-momentum states are orthogonal, which means that there is no "crosstalk" between the beams. As a bonus, since quantum mechanics allows you to know both the orbital and the spin angular momentum of a photon at the same time, the researchers managed to perform both polarization multiplexing and orbital-angular-momentum multiplexing on their beams of light. This doubled the number of states available and allowed the transmission to reach terabit speeds.
"What impresses me most about the [research] is that it goes beyond a proof of principle to the point where the researchers' results show meaningful amounts of speed," comments Padgett. "It's not just 'let me prove the basic physics' – they're also putting in place lots of the supporting technology that would be needed in practice to build a runnable system."
There is still much to be done, however. The test was conducted in a vacuum, with a transmission distance of only a few metres. Willner explains that the presence of an atmosphere can cause problems. "In the atmosphere there is turbulence," he explains, "which tends to create crosstalk. As a result of clouds, wind or warm air, some of the energy from one twisted beam might appear on another twisted beam." Absorption of the signal is another problem associated with transmitting through the atmosphere. Nevertheless, Willner remains optimistic. "We're trying to increase the capacity and explore the limitations of propagation," he says.
Using multiplexed transmission over an optical fibre is another possibility, according to Willner, who points out that researchers working at Boston University have already shown that orbital-angular-momentum modes can be transmitted over 1 km.
The research is published in Nature Photonics.
About the author
Tim Wogan is a science writer based in the UK