At a time when communications networks are scrambling for ways to transmit more data over limited bandwidth, a type of twisted light wave is gaining new attention. Called an “optical vortex,” or “vortex beam,” this complex beam resembles a corkscrew with waves that rotate as they travel, according to ScienceDaily.com. Now, applied physicists at the Harvard School of Engineering and Applied Sciences (SEAS) have created a new device that enables a conventional optical detector, which would normally only measure the light’s intensity, to pick up on that rotation. The device, described in the journal Nature Communications, has the potential to add capacity to future optical communications networks.
“Sophisticated optical detectors for vortex beams have been developed before but they have always been complex, expensive and bulky,” says principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. In contrast, the new device simply adds a metallic pattern to the window of a commercially available, low-cost photo detector. Each pattern is designed to couple with a particular type of incoming vortex beam by matching its orbital angular momentum: the number of twists per wavelength in an optical vortex.
Sensitive to the beam’s “twistiness,” this new detector can effectively distinguish between different types of vortex beams. Existing communications systems maximise bandwidth by sending many messages simultaneously, each a fraction of a wavelength apart; this is known as wavelength division multiplexing. Vortex beams can add an additional level of multiplexing and therefore should expand the capacity of these systems.
“In recent years, researchers have come to realise that there is a limit to the information transfer rate of about 100 terabits per second per fibre for communications systems that use wavelength division multiplexing to increase the capacity of single-mode optical fibres,” explains Capasso. “In the future, this capacity could be greatly increased by using vortex beams transmitted on special multicore or multimode fibres. For a transmission system based on this spatial division multiplexing to provide the extra capacity, special detectors capable of sorting out the type of vortex transmitted will be essential.”
The new detector is able to tell one type of vortex beam from another due to its precise nanoscale patterning. When a vortex beam with the correct number of coils per wavelength strikes the gold plating on the detector’s surface, it encounters a holographic interference pattern that has been etched into the gold. This nanoscale patterning allows the light to excite the metal’s electrons in exactly the right way to produce a focused electromagnetic wave, known as a “surface plasmon.” The light component of this wave then shines through a series of perforations in the gold and lands on the photodetector below. If the incoming light doesn’t match the interference pattern, the plasmon beam fails to focus or converge and is blocked from reaching the detector. Capasso’s research team has demonstrated this process using vortex beams with orbital angular momentum of −1, 0, and 1.
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