By bending beams of light through the air to make them avoid obstacles, researchers hope to help make superfast 6G data networks a reality, according to a new study.
The next generation of wireless communication, 6G, will likely use terahertz waves to achieve unprecedented speeds. Terahertz radiation (also known as submillimeter or far-infrared waves) lies between light waves and microwaves on the electromagnetic spectrum. With frequencies ranging from 0.1 to 10 terahertz, terahertz radiation could be the key to future high-speed wireless networks transmitting data at terabits (trillions of bits) per second.
“Even if there is an obstruction that partially blocks the transmitter’s view of the target, the data will still be successfully delivered to the target.” —Daniel Mittleman, Brown University
One of the big problems facing terahertz signals is that they can be blocked by most solid objects, which means that unlike Wi-Fi, terahertz signals typically require a direct line of sight between the transmitter and receiver.
In the new study, researchers from Brown University in Providence, Rhode Island, and Rice University in Houston tried to get around this problem by generating terahertz signals that travel in a curved trajectory around obstacles, rather than being blocked by them.
To be clear, “we’re not sending photons along curved trajectories,” says Daniel Mittelman, a professor of engineering at Brown University. “It may seem that way, but it’s not actually the case.”
Mittelman explains that photons normally travel in straight lines, unless they pass through a region where the fabric of space and time is distorted by a strong gravitational field, such as those produced by a black hole.
“Instead, what we do is create very carefully calibrated patterns of straight light beams that collectively interfere to produce an intensity pattern that follows a curved trajectory,” Mittelman says.
Previous research first produced such curved beams with visible light in 2007. Subsequent research also produced curved beams with terahertz light.
“What we did was show that we could load digital data onto these beams and send a signal around obstacles,” Mittelman says, “and even if there’s an obstacle that partially blocks the transmitter’s view of the target, the data still gets to the target successfully.”
How does a terahertz signal bend?
The scientists developed a transmitter that can generate different patterns of terahertz light: if one pattern is blocked by an obstacle, the transmitter adjusts to send data using a different pattern, keeping the communications link intact.
“When we started working on this project, we didn’t know if it would work,” Mittelman said, “so the biggest surprise was that it worked.”
For this new technology to work, the distance from the transmitter to the receiver must be short enough that the receiver is in the transmitter’s near field, Mittelman said. For typical wireless data systems today, which operate at frequencies of about 3 gigahertz with antennas about 10 centimeters in diameter, the near field extends only a few tens of centimeters, too far to be very useful, he explained.
“But things are very different when you use higher frequencies, such as terahertz frequencies,” Mittelman points out. “For the same 10 cm transmitting antenna, the near field at 300 GHz frequencies extends out to tens of meters, meaning it becomes relevant and useful for typical Wi-Fi scenarios.”
The new strategy won’t solve all the interference problems that terahertz signals might face, cautions Hichem Gherbouka, who led the work as a postdoctoral researcher at Brown University and is now an assistant professor in the School of Science and Engineering at the University of Missouri-Kansas City.
“There are real physical limits to what we can do here,” Mittelman explains. “For example, the amount of curvature is limited by the size of the transmitter, so with any given transmitter you can’t get the curves you want.”
Future studies could explore how much the terahertz signals bend and how far they can travel. Additionally, the researchers would like to see how the curvature affects their bandwidth.
“When transmitting data at high data rates, you need a wide bandwidth, which means that the signal is made up of many different frequencies,” Mittelman says. “What if each frequency follows a slightly different curved trajectory? We call this effect ‘curvature dispersion.’ In the worst case, the receiver will not receive some of the transmitted frequencies because it follows the wrong amount of curve. We need to understand the impact of this effect in more detail.”
Demetrios Christodoulides, who and his colleagues first generated bending beams of light in 2007, suggests that the new research may also have imaging applications.
“We think this approach could be advantageous if you want to avoid opaque regions,” said Christodoulides, a professor of electrical and computer engineering and of physics and astronomy at the University of Southern California in Los Angeles, who was not involved in the research.
The scientists published detailed findings in an online journal on March 30. Communication Engineering.
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