'Optical fibre' made out of thin air
Air waveguides use differences in density to keep light beams focused
Scientists say they have turned thin air into an "optical fibre" that can transmit and amplify light signals without the need for any cables.
In a proof-of-principle experiment they created an "air waveguide" that could one day be used as an instantaneous optical fibre to any point on earth, or even into space.
The findings, reported in the journal Optica, have applications in long range laser communications, high-resolution topographic mapping, air pollution and climate change research, and could also be used by the military to make laser weapons.
"People have been thinking about making air waveguides for a while, but this is the first time it's been realized," said Howard Milchberg of the University of Maryland, who led the research, which was funded by the U.S. military and National Science Foundation.
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.
Light and air
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.
Once Milchberg and colleagues created their air waveguide, they used a second laser to spark the air at one end of the waveguide turning it into plasma.
An optical signal from the spark was transmitted along the air waveguide, over a distance of a metre to a detector at the other end.
The signal collected by the detector was strong enough to allow Milchberg and colleagues to analyze the chemical composition of the air that produced the spark.
The researchers found the signal was 50 per cent stronger than a signal obtained without an air waveguide.
The findings show the air waveguide can be used as a "remote collection optic," says Milchberg.
"This is an optical fibre cable that you can reel out at the speed of light and place next to [something] that you want to measure remotely, and have the signal come all the way back to where you are."
Australian expert Ben Eggleton of the University of Sydney says this is potentially an important advance for the field of optics.
"It's sort of like you have an optical fibre that you can shine into the sky, connecting your laser to the top of the atmosphere," says Eggleton.
"You don't need big lenses and optics, it's already guided along this channel in the atmosphere."