Optical antennas promise ‘unlimited’ data capacity

Researchers at the University of California, Berkeley have outlined details of an optical antenna they claim could provide almost limitless bandwidth.

They suggest the key to the breakthrough is a method of being able to take full advantage of the orbital angular momentum (OAM) properties of a coherent light source, thus enabling multiplexing, or simultaneous transmission.

According to Boubacar Kante, the principal investigator of the Berkeley project  “it is the first time that lasers producing twisted light have been directly multiplexed.” He is an associate professor in the university’s Electronic Engineering and Computer Sciences Department, and the initial results of the work have just been published in Nature Physics.

Kante said the breakthrough is a ‘game changer’, with applications in biological imaging, quantum cryptography, high-capacity communications and sensors.  He added that till now, there had been fundamental limits to the number of coherent twisted lightwaves that could be directly multiplexed.

He also suggested the principle of the OAM could be likened to the vortex of a tornado.

“The vortex in light, with its infinite degrees of freedom, can, in principle, support an unbounded quantity of data. The challenge has been finding a way to reliably produce the infinite number of OAM beams. No one has ever produced OAM beams of such high charges in such a compact device before,” said Kante.

The researches created a topological antenna by etching a grid pattern into indium gallium arsenide phosphide, which they then bonded onto a surface of yttrium iron garnet. The grid was deigned to form quantum wells in a pattern of three concentric circles — the largest of which measured 50 microns in diameter — to trap photons.

This design created the necessary conditions for the so-called photonic quantum Hall effect, which describes the movement of photons when a magnetic field is applied , forcing the light to travel in only  one direction in the rings.

“People thought the quantum Hall effect with a magnetic field could be used in electronics but not in optics because of the weak magnetism of existing materials at optical frequencies. We are the first to show that the quantum Hall effect does work for light,” said Kante.

And by applying a magnetic field perpendicular to their two-dimensional microstructure, the researchers successfully generated three OAM laser beams travelling in circular orbits above the surface.

The studies also showed that the beams had quantum numbers as large as 276, referring to the number of times light twists around its axis in one wavelength.

Kante noted that the team demonstrated this capability at telecommunications wavelengths ad suggested that, in principle, it could be adapted to other frequency bands. “Even though we created three lasers, multiplying the data rate by three, there is no limit to the possible number of beams and data capacity.”

He also cautioned that existing methods of transmitting signals through electromagnetic waves are reaching their limit. “Frequency, for example, has become saturated, which is why there are only so many stations one can tune into on the radio. Polarization, where light waves are separated into two values –horizontal or vertical — can double the amount of information transmitted.”

The researchers said their next step would be to make quantum Hall rings that use electricity as a power source.