According to Google Scholar, more than 5,000 scientific papers that discuss OAM were published in 2015. Communications is a powerful property of OAM.
Overlapping beams with different values of OAM essentially behave as if they can’t “see” each other. To use the parlance of communications engineers and physicists, those OAM beams are orthogonal to one another. This means that each beam is distinct. It can’t be constructed using beams with other values of OAM, and it isn’t intrinsically capable of interacting with those other beams.
This orthogonality means that OAM beams can occupy the same space without interfering with one another. In fact it’s theoretically possible to use an infinite number of beams, each with a different value of OAM, to carry signals. In practice, there are limitations, as there always are.
A major step forward for OAM communications occurred in 2004, when physicist Miles Padgett and his colleagues showed that OAM waves could be used to encode data, by using different values of OAM to represent information. Later, it became clear that a beam with a single fixed OAM value could act as a data channel—that is, it could be modulated in a variety of conventional ways to carry information. The most straightforward of such modulation methods is on-off keying, which uses the presence and absence of a beam to represent “1” and “0” data bits.
There are a variety of ways to create and transmit helical beams.The transmitter generates regular laser beams, which are then passed through a spatial light modulator, based on liquid crystal, in order to impart a twist to the beam. At the receiving end, each OAM beam was converted back into a regular plane wave by passing it through a spatial modulator with the inverse pattern. The data could then be recovered by a conventional optical receiver.
In 2012, the author published the first journal article on this approach. Our experiment sent 32 different optical beams of the same frequency, each carrying 80 gigabits per second of data, over a modest distance—just 1 meter—in the laboratory. But the total transmission rate, some 2.5 terabits per second, was actually quite high for free-space communications. And it held out the promise for longer-distance transmissions and, because we used only one frequency, much higher data rates.
The research can be roughly divided into three fields: free-space optical links, traditional radio-frequency wireless transmissions, and fiber-optic communications.
The first two, free-space optical and radio, are the farthest along. In the optical category, in 2014, Anton Zeilinger of the University of Vienna and colleagues reported that they had used OAM light to send data, including a grayscale image of Mozart, between two sites in Vienna separated by 3 kilometers.
Quantum communication, which can be used to send information far more securely than traditional systems do, can also take advantage of OAM. Today quantum bits, or qubits, can be made of photons that are constructed from a superposition of two possible polarization states—vertical and horizontal. But OAM isn’t just a property of electromagnetic waves; it’s also a quantum property of single photons. A single photon can have many possible values of OAM, which can be used to increase the capacity of a quantum link. Robert Boyd of the University of Rochester, in New York, has demonstrated quantum communication systems that can carry more than one bit per photon by using OAM.
About the Author
Alan E. Willner is a professor of electrical engineering at the University of Southern California.
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