Low cost photodector for optical vortex beams
1 min read
Applied physicists at the Harvard School of Engineering and Applied Sciences (SEAS) have created a device that enables a conventional optical detector to pick up on the rotation in an optical vortex.
Optical vortices, in which the beam is twisted like a corkscrew, are said to have the potential to boost the capacity of future optical communications networks.
"Sophisticated optical detectors for vortex beams have been developed before, but they have always been complex, expensive and bulky," said principal investigator Professor Federico Capasso.
In the new device, a metallic pattern is added to the window of a commercially available, low cost photodetector. 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. Because it is sensitive to the beam's 'twistiness', the detector can effectively distinguish between different types of vortex beams.
"In recent years, researchers have come to realise that there is a limit to the information transfer rate of about 100Tbit/s per fibre for communication systems that use wavelength division multiplexing to increase the capacity of single mode optical fibres," explained Prof 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 can discriminate between vortex beams 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. The light then excites the metal's electrons in exactly the right way to produce 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.
Patrice Genevet, lead author for the paper published in Nature Communication, said: "In principle, an array of many different couplers and detectors could be set up to read data transmitted on a very large number of channels."
