Air Waveguides

Wednesday, August 6th, 2014

Lasers lose intensity and focus with increasing distance as photons naturally spread apart and interact with atoms and molecules in the air.

Fibre optics solves this problem by beaming the light through glass cores with a high refractive index, which is good for transmitting light.

The core is surrounded by material with a lower refractive index that reflects light back in to the core, preventing the beam from losing focus or intensity.

Fibre optics, however, are limited in the amount of power they can carry and the need for a physical structure to support them.

Air waveguides may get around some of these limitations:

Milchberg and colleagues’ made the equivalent of an optical fibre out of thin air by generating a laser with its light split into a ring of multiple beams forming a pipe.

They used very short and powerful pulses from the laser to heat the air molecules along the beam extremely quickly.

Such rapid heating produced sound waves that took about a microsecond to converge to the centre of the pipe, creating a high-density area surrounded by a low-density area left behind in the wake of the laser beams.

“A microsecond is a long time compared to how far light propagates, so the light is gone and a microsecond later those sound waves collide in the centre, enhancing the air density there,” says Milchberg.

The lower density region of air surrounding the centre of the air waveguide had a lower refractive index, keeping the light focused.

“Any structure [even air] which has a higher density will have a higher index of refraction and thereby act like an optical fibre,” says Milchberg.

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