By making light beams curve in mid-air around obstacles, researchers hope to help make blazingly fast 6G data networks a reality, a new study finds.

The next generation of wireless communications, 6G, will likely rely on terahertz waves to help reach unprecedented speeds. Terahertz rays (also called submillimeter radiation or far-infrared light) lie between optical waves and microwaves on the electromagnetic spectrum. Ranging in frequency from 0.1 to 10 terahertz, terahertz rays could be key to future high-speed wireless networks, which will transmit data at terabits (trillions of bits) per second.

“The data can be successfully delivered to a target even if there is an obstruction that partially blocks the view of the target from the transmitter.” —Daniel Mittleman, Brown University

One major problem terahertz signals face is how they can be blocked by most solid objects. This means that, unlike Wi-Fi, terahertz signals generally require a direct line of sight between transmitters and receivers.

In the new study, researchers at Brown University in Providence, R.I., and Rice University in Houston sought to avoid this problem by creating terahertz signals that follow a curved trajectory around obstacles, instead of getting blocked by them.

To be clear, “we are not making photons travel along curved trajectories,” says Daniel Mittleman, a professor of engineering at Brown University. “That’s not what is going on here, although it might look that way.”

Photons generally move in straight lines, unless they pass through areas where the fabric of space and time is warped by powerful gravitational fields, such as one created by a black hole, Mittleman explains.

“Instead, what we are doing is making a very carefully tailored pattern of straight light rays that collectively interfere to produce an intensity pattern that follows a curved trajectory,” Mittleman says.

Previous research first produced such curving beams with visible light in 2007. Subsequent work also generated curving beams of terahertz light.

“What we did was to show that one can load those beams with digital data and send the signal around obstacles,” Mittleman says. “The data can be successfully delivered to a target even if there is an obstruction that partially blocks the view of the target from the transmitter.”

How do terahertz signals (seem to) curve?

The scientists developed a transmitter that can produce a variety of patterns of terahertz rays. When an obstacle blocks one pattern, the transmitter adjusts to send data using another pattern to keep the communications link intact.

“When we started working on this project, it was not obvious to me that it would work,” Mittleman says. “So maybe the most surprising thing was that it did.”

In order for this new technique to work, the distance from the transmitter to the receiver has to be small enough so that the receiver is in the near field of the transmitter, Mittleman says. For a typical current wireless data system operating at roughly 3 gigahertz frequencies with an antenna about 10 centimeters across, the near field extends only a few dozen centimeters, too short a range to prove very useful, he explains.

“But things are very different if we use higher frequencies,” such as terahertz frequencies, Mittleman notes. “For the same 10-cm transmitter antenna, the near field at a frequency of 300 GHz extends tens of meters away. This means that it becomes relevant and useful for typical Wi-Fi scenarios.”

This new strategy does not solve all blockage problems terahertz signals may face, cautions Hichem Guerboukha, who led the study as a postdoctoral researcher at Brown University and is now an assistant professor at the University of Missouri—Kansas City’s School of Science and Engineering.

“There are some real physical limits to what can be done here,” Mittleman explains. “For instance, the amount of curvature is limited by the size of the transmitter, so for a given transmitter, you cannot make any curve you want.”

Future work can explore how much terahertz signals may curve and how far away they may reach. In addition, the researchers want to see what effects curvature might have on bandwidth.

“When sending data at high data rates, one requires a broad bandwidth—that means your signal is composed of many different frequencies,” Mittleman says. “What if each frequency follows a slightly different curved trajectory? We call this effect ‘curvature dispersion.’ In the worst case, this means that your receiver will miss some of the transmitted frequencies because they curve the wrong amount. We need to understand the impact of this effect in more detail.”

Demetrios Christodoulides, who with his colleagues was the first to produce curving light beams in 2007, suggests this new research might have imaging applications as well.

“When you want to avoid regions that are opaque, I can see this approach providing advantages,” says Christodoulides, a professor of electrical and computer engineering and physics and astronomy at the University of Southern California in Los Angeles, who did not participate in this study.

The scientists detailed their findings online on 30 March in the journal Communications Engineering.

Source: IEEE Spectrum Telecom Channel