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The answer to faster wireless is blowing in the wind

Get out your hair-dryer to make an optical waveguide that boosts laser comms

Internet Security Threat Report 2014

If you make enough hot air, it'll act as a waveguide for optical signals and behave something like an optical fibre.

Don't, however, stick your eye in the beam. To create the “air waveguide” described in this paper at Arxiv, the boffins from the University of Maryland were letting loose at least enough heat to burn paper, because that's one test they applied to check that the beams were holding their shape.

The problem with free space optics is that air and dust scatters laser signals, so the signal-to-noise ratio degrades over distance (and in the presence of dust, smoke, rain, and fog).

To get over this, the researchers used femtosecond pulses to try and recreate conditions inside an optical fibre. In an optical fibre, the lower refractive index of the cladding reflects light back along the fibre path.

The pulsing laser created “filaments” in the air – heated air expands, leaving a wake of low-density air that has a lower refractive index than the surrounding air. This has a lensing effect, refocussing the optical beam.

The pulse was then split with a segmented mirror to create four or eight lobes of the filaments, as shown in the image below.

Heating air to create optical waveguide

Hot air creates a waveguide. Image: Howard Milchberg

Four or eight of these filaments, as shown in the image, create a region inside through which another laser can carry communications, and while the filaments can be created by a femtosecond pulse, they last hundreds of microseconds (after which another pulse can re-create the filaments).

Apart from communication, the technique would, if it can be scaled up, be useful for LIDAR imaging, because it would improve the collection of reflected light as well.

The researchers report that their technique improves free-space optical transmission by 50 per cent, but has only been tested over a distance of one metre. They predict that they should be able to get 10,000 times better signal-to-noise ratio over 100 metres, but that's for a future test.

The university's release is here. ®

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