Researchers have demonstrated a technique that uses a laser to produce a plasma beam antenna capable of transmitting radio waves.
"The plasma beam antenna looks like a lightsaber and is tunable, meaning we should be able to transmit across a broad range of frequencies," says Prya Darshni, corresponding author of a journal article on the work 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," says 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 is important because it controls the frequencies at which radio waves can be transmitted and received. But manipulating the length of antennas to sweep a desired range of frequencies can be challenging in some applications – such as 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," says Darshni. "And we have now shown that it is possible."
By firing a laser beam of a specific power and diameter, the researchers are able to ionize a thin beam of air, creating a defined shaft of plasma called a plasma filament.
But in order to make the plasma antenna a practical tool, the researchers also needed to develop a way to connect the antenna to radio technology in order to transmit a radio signal.
To solve that problem, the researchers also created and demonstrated a contactless antenna-feed, which consists of a metal ring that serves as a capacitor. The laser passes through the ring, creating a plasma filament that is surrounded by the capacitor. By generating an electromagnetic field with the capacitor, the researchers are able to interact with the plasma beam without touching it.
Altogether, the process works like this: a radio frequency generator feeds a signal into the capacitor; this generates the appropriate electromagnetic field; which then causes the plasma filament antenna 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," says Darshni. "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," says Darshni. "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."
In the long term, the technology holds promise for use in a variety of applications. Satellites and space exploration technologies are one potential area of interest, because payload and the ability to scan across a wide range of frequencies are both important considerations.
"In low earth orbit, there is sufficient air to form a plasma," says Franzon.
"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," says Darshni. "Now that we've shown it is possible, we can begin improving its performance."
The article, " Laser-Induced-Plasma-Filament Antenna Transmitting 30 MHz VHF ," is published open access in the IEEE Journal of Microwaves. The paper was co-authored by Arthur Dogariu of Texas A&M University and Princeton University, who helped with measurements.
The researchers thank Chris Hewett, Byron Goode, and Joe McElveen in NC State's Instrument Shop for their ingenuity in manufacturing the experimental components.
