Scientists report first data transmission through terahertz multiplexer

The researchers demonstrated the transmission of two different video signals by a terahertz multiplexer with a data rate of more than 100 times faster than today's fastest cellular data networks.

PROVIDENCE, R.I. [Brown University] multiplexing, the ability to send multiple signals over a single channel is a basic feature of any voice or data communication system. An international research team has demonstrated for the first time a method for multiplexing data on terahertz waves, high-frequency radiation, which can enable the next generation of high-bandwidth wireless networks.

In the journal Nature Communications, the researchers report on the transmission of two real-time video signals via a terahertz multiplexer with a total data rate of 50 gigabits per second, about 100 times the optimal cell - today.

"We have shown that we can transmit separate data streams on terahertz waves at very high speeds and with very low error rates," said Daniel Mittleman, professor at Brown's School of Engineering and the author of the document. "This is the first time someone has characterized a terahertz multiplex system with real data, and our results show that our approach could be viable in future terahertz fun networks."

Current voice and data networks use microwaves to transport wireless signals. But the demand for data transmission is rapidly growing faster than those who can manage microwave networks. Terahertz waves have higher frequencies than microwaves and thus a much larger capacity to transport data. However, the scientists have only begun to experiment with terahertz frequencies, and many of the necessary components necessary for terahertz communication do not yet exist.

A multiplexing and demultiplexing system (also called Mux / Demux) is one of these basic components. It is the technology that allows a cable to transport multiple TV channels or hundreds of users to access a wireless Wi-Fi network.

The Mux / Demux approach Mittleman and his developed colleagues use two metal plates arranged parallel to one another to form a waveguide. One of the plates has a cut slot. When the terahertz waves pass through the waveguide, some of the radiation comes out of the slot. The angle at which the radiation beams escape depends on the frequency of the wave.

"We can put a lot of waves on different frequencies - everyone carrying a data stream - into the waveguide, and they will not interfere because they are different frequencies, it is multiplexing," said Mittleman. "Each of these frequencies escapes the slot from a different angle, separates the data streams, it is demultiplexing."

Due to the nature of terahertz waves, signals in terahertz communication networks spread as directional beams, not omnidirectional emissions as in existing wireless systems. This directional relationship between the propagation angle and the frequency is the key to enable the Mux / Demux in the Terahertz systems. A user at a particular location (and thus at a certain angle from the multiplex system) communicates at a particular frequency.

In the year 2015, the Mittleman laboratory first published an article describing its concept of the waveguide. For this initial work, the team used a broadband terahertz light source to confirm that different frequencies actually originated from the device at different angles.

Although this was an effective proof of the concept, Mittleman said this latest work took the crucial step of testing the device with real data.

In collaboration with Guillaume Ducournau at the Institute of Microelectronic and Nanotechnology, CNRS / University of Lille, France, the researchers coded two high-resolution television programs on terahertz waves with two different frequencies: 264.7 GHz and 322.5 GHz. They then passed the two frequencies in the multiplex system, with a television receiver sensing the signals as they emerged from the device. When the researchers tuned their receiver to the angle from which 264.7 GHz waves were emitted, they saw the first channel. When they were aligned with 322.5 GHz, they saw the second.

Other experiments showed that transmissions up to 10 gigabits per second were error-free, which is much faster than today's standard Wi-Fi speed. The error rates rose slightly as the speed was increased to 50 gigabits per second (25 gigabits per channel), but still in the range that can be corrected with the direct error correction that is commonly used in today's communications networks.

In addition to the demonstration that the device has worked, Mittleman said that the research revealed surprising details about the transmission of data on terahertz waves. When a terahertz wave is modulated to encode data - that is, to turn on and off to produce zeros and ones - the main wave is accompanied by sideband frequencies, which must also be recognized by a receiver to be transmitted, all the data. Research has shown that the angle of the detector with respect to the sidebands is important to reduce the error rate.

"When the angle is a bit off, we could see the overall signal strength, but we get a sidebar a little better than the other, which increases the error rate," Mittleman said. "So it's important to have the right angle."

Basic details like these will be essential, said Mittleman when it comes to designing the architecture for complete terahertz data systems. "This is something we did not expect and it shows how important it is to characterize these systems by using data and not just a source of unmodulated radiation."

The researchers plan to further develop these and other terahertz components. Mittleman was recently licensed by the FCC to conduct open-air tests at Terahertz frequencies on the Brown University campus (see sidebar).

"We believe we have the highest frequency license currently being released by the FCC and hopefully it signals that the agency is seriously thinking about terahertz communications," Mittleman said. "Enterprises will hesitate to develop terahertz technologies until regulators have made serious efforts to assign frequency bands to specific specifications, so this is a step in the right direction."

This work was supported by the US National Science Foundation, the US Army Research Office, the WM Keck Foundation and the French National Research Agency in the COM'TONIQ research fellowships. TERALINKS and within the framework of the CPER "Photonics for Society" in the Hauts-de-France region.