Optical Cables, from Thin Air!
It’s a project that would make Tesla proud. Just imagine being able to instantaneously run an optical cable or fiber to any point on earth, or even into space. That’s what researchers are trying to do. Did I mention it was instantaneous and involved no connection other than the air around us? Well if you are as excited as I am, then you should read on! If not, two words, laser weapons!!
In a new paper published, researchers report using an “air waveguide” to enhance light signals collected from distant sources. Besides being super cool, air waveguides could have many applications, including long-range laser communications, detecting pollution in the atmosphere, making high-resolution topographic maps and [when we are talking cool futuristic stuff, why not include…] laser weapons.
So most of us know, light loses intensity with distance, which means that the range over which such tasks can be done is limited. Even lasers, which produce highly directed beams, lose focus due to their natural spreading, or worse, due to interactions with gases in the air, which fun fact, is why we don’t have death laser technology… yet. Fiber-optic cables can trap light beams and guide them like a pipe, preventing loss of intensity or focus.
Typical fibers consist of a transparent glass core surrounded by a cladding material with a lower index of refraction. When light tries to leave the core, it gets reflected back inward. But solid optical fibers can only handle so much power, and they need physical support that may not be available where the cables need to go, like the upper atmosphere. Now, the team has found a way to make air behave like an optical fiber, guiding light beams over long distances without loss of power.
The air waveguides consist of a “wall” of low-density air surrounding a core of higher density air. The wall has a lower refractive index than the core—just like an optical fiber. In the paper, the team broke down the air with a laser to create a spark. An air waveguide conducted light from the spark to a detector about a meter away. The researchers collected a strong enough signal to analyze the chemical composition of the air that produced the spark.
The signal was “only” 1.5 times stronger than a signal obtained without the waveguide. That may not seem like much, but over distances that are 100 times longer, where an unguided signal would be severely weakened, the signal enhancement could be much, much greater.
The group creates air waveguides using very short, very powerful laser pulses. When they create a sufficiently powerful laser pulse, the air collapses into a narrow beam, called a filament. This happens because the laser light increases the refractive index of the air in the center of the beam, sort of like the pulse is carrying its own lens with it.
The team showed previously that these filaments heat up the air as they pass through, causing the air to expand and leaving behind a “hole” of low-density air in their wake. This hole has a lower refractive index than the air around it. While the filament itself is very short lived [less than one-trillionth of a second], it takes a billion times longer for the hole to appear.
“It’s like getting a slap to your face and then waiting, and then your face moves,” according to Milchberg, lead researcher at the Institute for Research in Electronics and Applied Physics at UMD.
On Feb. 26, 2014, the lab reported that if four filaments were fired in a square arrangement, the resulting holes formed the low-density wall needed for a waveguide. When a more powerful beam was fired between these holes, the second beam lost hardly any energy when tested over a range of about a meter.
Not only that, but the “pipe” produced by the filaments lasted for a few milliseconds, a million times longer than the laser pulse itself. For many laser applications, Milchberg says, “milliseconds is infinity.”
Because the waveguides are so long-lived, the team believes that a single waveguide could be used to send out a laser and collect a signal.
“It’s like you could just take a physical optical fiber and unreel it at the speed of light, put it next to this thing that you want to measure remotely, and then have the signal come all the way back to where you are,” says Milchberg.
Of course the team needs to show that these waveguides can be used over much longer distances—50 meters at least. But if that works, it opens up a world of possibilities. Air waveguides could be used to conduct chemical analyses of places like the upper atmosphere or nuclear reactors, where it’s difficult to get instruments close to what’s being studied. The waveguides could also be used for LIDAR, a variation on radar that uses laser light instead of radio waves to make high-resolution topographic maps, oh and don’t forget the laser weapons!
Want to build laser death ray weapons? Well to get you started, you can find the full study —here!!
Rosenthal, E., Jhajj, N., Wahlstrand, J., & Milchberg, H. (2014). Collection of remote optical signals by air waveguides Optica, 1 (1) DOI: 10.1364/OPTICA.1.000005
Jhajj, N., Rosenthal, E., Birnbaum, R., Wahlstrand, J., & Milchberg, H. (2014). Demonstration of Long-Lived High-Power Optical Waveguides in Air Physical Review X, 4 (1) DOI: 10.1103/PhysRevX.4.011027