Light-based optical fiber internet services from providers such as Google and AT&T are already replacing slower, outdated technologies based on electrons such as cable and DSL.
Much of the difference in bandwidth, however, does not come from light’s ability to outrace electricity. Instead it comes from “multiplexing” techniques—the ability to encode data into multiple aspects of a light wave, such as its amplitude, wavelength and polarization. But the aspects that scientists are able to control are beginning to run out, as is the amount of room left to store data in the conventional properties of light.
To break through this barrier, engineers are exploring some of light’s harder-to-control properties, such as its orbital angular momentum (OAM). The results can be imagined as a swirling vortex of light.
“I explain OAM to my classes by thinking about planets,” said Litchinitser. “Imagine a particle trapped by an optical beam with both spin and orbital momentum. Spin angular momentum makes a particle spinning on its axis like the Earth, which gives us night and day. At the same time, the particle rotates around the axis of the beam like the Earth rotates around the moving sun.”
The first ultracompact “vortex” laser source was demonstrated in 2016 by Litchinitser and Liang Feng, associate professor of materials science and engineering and electrical and systems engineering at the University of Pennsylvania, while the two were both still at the University at Buffalo. Until recently, however, the technology lacked the ability to switch between multiple different vortex modes, making it impossible to multiplex with them.
In a recent paper, however, Feng discovered a method to switch between three different orbital angular momentum (OAM) modes. Litchinitser devised a metamaterial filter that enabled further dynamical tunability of this microlaser, allowing it to demonstrate at least five different OAM modes. This new integrated optical source is likely to help optical communications devices carry five times as much data, or even more if the researchers can create versions of the technology with additional modes.
The ability to dynamically tune OAM values could also enable a photonic update to a classic encryption technique: frequency hopping. By rapidly switching between OAM modes in a pre-defined sequence known only to the sender and receiver, optical communications could be made impossible to intercept.
The sky is the limit for the improvements a dynamically tunable vortex laser could provide fields ranging from communications to computing. And Litchinitser is excited about pursuing them with her colleagues—just as long as the media doesn’t mistake the technology for a magic wand.