Photokina 2014 highlights

"Air waveguides" used to send optical data through the air

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July 24, 2014

An illustration of an air waveguide, with the four laser-produced 'holes' (orange) and the...

An illustration of an air waveguide, with the four laser-produced 'holes' (orange) and the source of the transmitted light (green)

Efficient as fiber optic cables are at transmitting data in the form of light pulses, they do need to be physically supported, and they can only handle a finite amount of power. Still, what's the alternative ... just send those focused pulses through the air? Actually, that's just what scientists at the University of Maryland have already demonstrated in their lab.

In a traditional optical fiber, light travels along a transparent glass core. That core is surrounded by a cladding material with a lower refractive index than the glass. As a result, when the light tries to spread out (as it would if it were traveling through the air), the cladding reflects it back into the core, thus retaining its focus and intensity.

A team led by Prof. Howard Milchberg has created "air waveguides" that work on the same principle.

To make these waveguides, they start by using four lasers arranged in a square formation (one laser at each corner) to send short, powerful laser pulses through the air. As their beams collapse, they form into narrower beams known as filaments. The reason that these form is because in sufficiently powerful laser light, the light at the center of the beam has a higher refractive index than that at the outside.

Milchberg noted that the filaments heat the air as they pass through it, leaving a tunnel-like "hole" of low-density air in their wake. That air also has a lower refractive index than the surrounding air, so light doesn't travel through it as easily.

After the four filament-making lasers are fired, a fifth laser located in the middle of the square is used to create a spark in the air. The light from that spark travels down the space between the four holes, and is reflected back in by them whenever it starts to disperse.

The air waveguides only last for a matter of milliseconds, although that could still be plenty of time to both send and receive data.

So far, they've only been used to transmit light a distance of about one meter (3.3 ft). When that light reached a detector at the far end, it was found to be 1.5 times stronger than if a waveguide were not used. According to Milchberg, the relative advantage would be much larger over longer distances, in which the light would otherwise become considerably more dispersed.

To that end, he now hopes to use air waveguides to send a light signal over a distance of at least 50 meters (164 ft). Ultimately, he envisions the technology being used to transmit data across long spaces that aren't conducive to the stringing of cables, along with applications such as atmospheric pollution detection, inspection of hazardous locations such as nuclear reactors, and laser weapons.

A paper on his research was recently published in the journal Optica.

Source: University of Maryland

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away.   All articles by Ben Coxworth
7 Comments

"Milchberg noted that the filaments heat the air as they pass through it, leaving a tunnel-like "hole" of low-density air in their wake. That air also has a lower refractive index than the surrounding air, so light doesn't travel through it as easily."

Correct my basic understanding of optics, but isn't a higher refractive index on the conduit wall more desirable to facilitate internal reflection?

If anything, if the strong laser creates a partial low refractive index conduit, then you would use that to deliver the data, ie - strong leading pulse to carve out the low index, followed by a string of data, then another strong pulse and so on..

Nairda
24th July, 2014 @ 09:44 pm PDT

How much energy goes into making the waveguide compared to just transmitting data on a more powerful laser to get the same results.

Slowburn
25th July, 2014 @ 01:35 am PDT

Technically light does not propagate "through the air", unless the author meant that the light passes "through a tunnel of air". Also, in the statement that reads, "..That air also has a lower refractive index than the surrounding air, so light doesn't travel through it as easily...", the author confused the sentence's subject and object in the adverbial clause. Light has an "easier time", i.e. travels faster, through less dense or lower index of refraction air.

Wulfher
25th July, 2014 @ 05:33 am PDT

Would laser light be less absorbed by passing along a hollow, evacuated fibreglass filament?

windykites1
25th July, 2014 @ 06:36 am PDT

Nairda & Wulfher: Thank you. I kept reading that part over and over. It didn't make sense. If the air was excited by the laser energy, it would thin out, creating a less dense path, allowing easier passage for the center pulse.

Don Duncan
25th July, 2014 @ 05:38 pm PDT

it's cool from a theoretical POV that you can create a region where you get internal reflection like a waveguide.

But completely impractical.

You'll need vast laser power to cause the heating in the air required over longer distances.

Also there's still the issue of birds, and rain that have plagued point-to-point laser links for decades.

Adrien
27th July, 2014 @ 03:32 pm PDT

not to mention the waveguide would be disrupted by wind, so no use for outdoors.....

curiosity value only I think.

Adrien
27th July, 2014 @ 03:41 pm PDT
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