Laser made plasma filament transmits radio waves in first antenna demonstration

Researchers at North Carolina State University have used a laser to create a beam of plasma in air that works as an antenna and transmits radio waves, the first demonstration that a plasma-filament antenna can function.

The work is described in the paper "Laser-Induced-Plasma-Filament Antenna Transmitting 30 MHz VHF," published open access in the IEEE Journal of Microwaves.

"The plasma beam antenna looks like a lightsaber and is tunable, meaning we should be able to transmit across a broad range of frequencies," said Prya Darshni, corresponding author of the paper and a Ph.D. student at North Carolina State University. "And while we have not demonstrated its ability to serve as an antenna that can receive radio signals, there's no reason to believe it wouldn't also work as a receiver."

"This is an exciting new concept that enables one to be able to have a customized antenna without complex mechanical deployment mechanisms," said Paul Franzon, co-author of the paper and the Cirrus Logic Distinguished Professor of Electrical and Computer Engineering at NC State.

The length of an antenna controls the frequencies at which it can transmit and receive radio waves. Changing that length to sweep a desired range of frequencies is difficult in some applications, including space exploration technologies.

"One of the questions we wanted to explore with this work was whether it would be possible to create plasma antennas using lasers, which would allow us to generate antennas at whatever length was needed," Darshni said. "And we have now shown that it is possible."

The researchers fire a laser beam of a specific power and diameter to ionize a thin column of air. The result is a defined shaft of plasma called a plasma filament.

To make the filament useful, the team also had to find a way to connect it to radio equipment so that it could carry a signal. They built and demonstrated a contactless antenna feed made of a metal ring that serves as a capacitor. The laser passes through the ring, so the plasma filament it creates is surrounded by the capacitor. By generating an electromagnetic field with the capacitor, the researchers can interact with the plasma beam without touching it.

In operation, a radio frequency generator feeds a signal into the capacitor, the capacitor generates the appropriate electromagnetic field, and the field causes the plasma filament to transmit radio waves at the appropriate frequency.

"By controlling the parameters of the laser, you can control the characteristics of the plasma filament - including its length," Darshni said. "This is valuable for applications where you need an antenna that can sweep all frequencies. But there's another benefit as well.

"There are also applications where it is important to be able to control the angle of the antenna, in order to target the direction of radar sweeps or to improve the strength of a signal you want to pick up," Darshni said. "The technique we've demonstrated here would allow users to control the angle of the plasma filament antenna via beam steering - simply shifting the direction of the laser."

The researchers see possible long-term uses in a range of fields. Satellites and space exploration systems are one area of interest, because payload and the ability to scan across a wide range of frequencies are both important considerations there.

"In low earth orbit, there is sufficient air to form a plasma," Franzon said.

"This is the first step, but it is a big step - it is the first time anyone has ever demonstrated that plasma-filament antennas can work," Darshni said. "Now that we've shown it is possible, we can begin improving its performance."

The paper was co-authored by Arthur Dogariu of Texas A&M University and Princeton University, who helped with measurements.

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