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Researchers Demonstrate A Terahertz Multiplexer

Multiplexers are essential components of any wireless communication system

2 min read
Researchers Demonstrate A Terahertz Multiplexer
Illustration: Mittleman Lab/Brown University

Terahertz waves, which span a frequency range of 300 to 3000 gigahertz, promise a hundred times faster data transmission than today’s cellular and wireless networks. Researchers now face the daunting task of designing and building a communication system with all new components that can work with terahertz radiation. Many efforts have focused on building compact terahertz sources, transmitters and detectors.

Now a group at Brown University in Providence, R.I., has built another key component of any wireless communication system: a multiplexer. Multiplexers combine separate data streams, typically each at a different frequency, into a single combined stream that is sent over optical fibers, TV cables, or telephone lines, making it possible for that medium to carry thousands of phone calls or tens of TV channels at the same time. A demultiplexer separates the signals at the receiver end.

The new device, which acts as both a multiplexer and demultiplexer, is reported in the journal Nature Photonics. It consists of two smooth aluminum plates (any metal should work) placed in parallel, with a few-millimeter gap between them. The plates act like a waveguide for terahertz waves, which travel between them.

To explain how the multiplexer works, it’s helpful to first describe how signals are separated at the receiving end. As the waves travel through the waveguide, some of the radiation leaks out of the slit. The angle at which it emerges depends on the frequency of the wave. So if an entering wave contains multiple frequencies, each carrying a separate data channel, they would each come out a different angle, effectively separated or demultiplexed.

The converse occurs with a multiplexer; the device accepts a signal of a certain frequency only if it comes in at a certain angle.

“We’re envisioning free-space wireless terahertz communication networks,” says Daniel Mittleman, a professor of engineering at Brown University. “But the important difference between a terahertz network and the existing cellular network is that the antenna is broadcasting in all directions. With terahertz, there’s 100 times less diffraction so it’s much more reasonable to think of it as a beam with some directionality.”

The researchers demonstrated a simple multiplexing scheme in which they sent two independent THz-frequency channels to the multiplexer at two different angles. For the source, they used a commercial femtosecond laser, which delivers short pulses of terahertz radiation and is used for spectroscopy. In a practical multiplexer, specially-shaped waveguides and optics could send hundreds of different channels into the multiplexer at different angles.

The device is completely passive right now. But Mittleman says he and his team could make it dynamic by varying the spacing between the plates. The researchers found that this spacing affects the frequency, angle, and bandwidth of the signal that leaks out of the slit. This could, for instance, be used to tune the bandwidth of the channels, Mittleman says.

“In the waveguide, imagine that the lower plate has a trench dug into it parallel to the slot and directly underneath,” he says. “You could use a silicon microelectromechanical switch to dynamically tune the depth of the trench at any location, so that the plate separation varies underneath slot. That would change the bandwidth of that channel.”

The researchers hope to make and demonstrate an active device of that type that changes positions on the timescale of a few milliseconds.

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Metamaterials Could Solve One of 6G’s Big Problems

There’s plenty of bandwidth available if we use reconfigurable intelligent surfaces

12 min read
An illustration depicting cellphone users at street level in a city, with wireless signals reaching them via reflecting surfaces.

Ground level in a typical urban canyon, shielded by tall buildings, will be inaccessible to some 6G frequencies. Deft placement of reconfigurable intelligent surfaces [yellow] will enable the signals to pervade these areas.

Chris Philpot

For all the tumultuous revolution in wireless technology over the past several decades, there have been a couple of constants. One is the overcrowding of radio bands, and the other is the move to escape that congestion by exploiting higher and higher frequencies. And today, as engineers roll out 5G and plan for 6G wireless, they find themselves at a crossroads: After years of designing superefficient transmitters and receivers, and of compensating for the signal losses at the end points of a radio channel, they’re beginning to realize that they are approaching the practical limits of transmitter and receiver efficiency. From now on, to get high performance as we go to higher frequencies, we will need to engineer the wireless channel itself. But how can we possibly engineer and control a wireless environment, which is determined by a host of factors, many of them random and therefore unpredictable?

Perhaps the most promising solution, right now, is to use reconfigurable intelligent surfaces. These are planar structures typically ranging in size from about 100 square centimeters to about 5 square meters or more, depending on the frequency and other factors. These surfaces use advanced substances called metamaterials to reflect and refract electromagnetic waves. Thin two-dimensional metamaterials, known as metasurfaces, can be designed to sense the local electromagnetic environment and tune the wave’s key properties, such as its amplitude, phase, and polarization, as the wave is reflected or refracted by the surface. So as the waves fall on such a surface, it can alter the incident waves’ direction so as to strengthen the channel. In fact, these metasurfaces can be programmed to make these changes dynamically, reconfiguring the signal in real time in response to changes in the wireless channel. Think of reconfigurable intelligent surfaces as the next evolution of the repeater concept.

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