24 October 2012—Researchers in the United Kingdom and China have made a microchip capable of emitting laser beams with a peculiar property known as orbital angular momentum (OAM).
Today’s optical-fiber communication systems use different wavelengths of laser light to squeeze multiple channels of data through the same “light pipe” simultaneously, offering much speedier data transfer rates. But taking advantage of photons with orbital—sometimes called “twisted” or “corkscrew”—angular momentum is a less-studied way to encode data channels in light. Each wavelength, for example, could carry different values of orbital angular momentum. “It’s another dimension,” says Siyuan Yu, a researcher at the University of Bristol, in England, who notes that twisted light has an infinite number of states. “OAM appears to be the last parameter of light that we haven’t explored so much,” observes Yu, who is part of the team that created the orbital angular momentum beam emitter.
While researchers have been aware of OAM as a physical property of photons for quite some time, it was only in the 1990s that they figured out the first ways of manipulating it. Even then OAM use was confined to physics experiments. But by the mid-2000s, the wider scientific community started to think about twisted light for communications.
The importance of the advance—reported last week in Science by Yu and collaborators at the University of Glasgow, Fudan University, and Sun Yat-sen University—becomes clear when it is compared with separate research reported earlier this year. A device for multiplexing and demultiplexing twisted light beams was reported in a paper presented in March at the Optical Fiber Communication Conference and Exposition, in Los Angeles. And in June, a team of researchers at the University of Southern California (USC) reported in Nature Photonics that they had used OAM to transfer data at 2.5 terabits per second over a distance of a about a meter.
Making twisted light requires shifting a laser’s phase in a particular way, says Yu. The multiplexer described at the March conference accomplished this with multiple waveguides carved onto a chip; the new device created by Yu’s group requires only one waveguide. This improvement allowed the researchers to shrink the emitter, which is made from CMOS-compatible silicon photonic integrated circuits, by several orders of magnitude.
Yu and company also created integrated arrays that can emit multiple optical vortices. According to Alan Willner, a member of the USC team, creating a hundred beams using the kind of devices from his group’s terabit demonstration would call for “a large number of expensive, discrete spatial light modulators.”
The improvements reported in the Science paper this week should be welcome news to companies such as Intel and Luxtera, which have been racing to find ways to replace the expensive exotic semiconductors and separate components in most optical communications systems with cheap integrated chips made of silicon.
According to Yu, twisted light arrays could someday allow communication channels between chips in a computer. Another potential application the University of Bristol researcher foresees is an imaging tool that makes it possible to “shine a light” to see the difference in chirality—left- or right-handedness—in molecules.
For the moment, emission efficiency, the optical power coming out of a waveguide compared with that going in, ranges between 3 and 13 percent. But Yu thinks it can be pushed well past 50 percent. USC’s Willner concurs, saying that the British and Asian researchers could greatly improve the twisted beam emitter with some clever engineering. Key to that will be improving the design of the coupling between the waveguide and another component, something Yu and company are confident they can accomplish.