Imagine being able to run an optical fibre to any point on Earth or in space.
That is what one physicist says could be possible in the future, opening up the possibility of communicating with colonies of people on Mars via a kind of broadband, for example.
Professor Howard Milchberg believes that an ‘air waveguide’ could enhance light signals collected from distant sources, making long-distance communication possible as never before.
The professor of physics at the University of Maryland believes waveguides could have many applications, including long-range laser communications, detecting pollution in the atmosphere, making high-resolution topographic maps and even laser weapons.
As light loses intensity with distance, 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 due to interactions with gases in the air.
But fibre optic cables trap light beams and guides them like a pipe, preventing loss of intensity or focus.
They typically 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.
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, such as the upper atmosphere.
To solve these problems, Professor Milchberg and his team have found a way to make air behave like an optical fibre, which could guide beams of light over long distances without loss of power, according to the study in the journal Optica.
The air waveguides consist of a ‘wall’ of low-density air surrounding a core of higher density air.
Just like a conventional optical fibre, the wall has a lower refractive index than the core, guiding light along a ‘pipe’.
The physicists broke down the air with a laser to create a spark and used the air waveguide to conduct light from the spark to a detector a three feet (1 metre) away.
The signal was strong enough so that they could analyse the chemical composition of the air that produced the spark.
In fact, the signal was one-and-a-half times stronger than a signal obtained without the waveguide.
While this may not seem a lot, over distances that are 100 times longer – where an unguided signal would be severely weakened – the signal enhancement could be much greater, the scientists explained.
The air waveguides were made using very short, powerful laser pulses.
A laser pulse transforms into a narrow beam called a filament, which happens because the laser light increases the refractive index of the air in the centre of the beam, as if the pulse is carrying its own lens with it.
Professor Milchberg has previously shown that the filaments heat up the air as they pass through, causing it to expand and leave a ‘hole’ of low-density air in their wake.
This hole has a lower refractive index than the air around it and while the filament itself lasts for just one trillionth of a second, it takes a billion times longer for the hole to appear.
The physicists have previously showed that four filaments were fired in a square arrangement, produce holes forming the low-density wall needed for a waveguide.
The ‘pipe’ produced by the filaments lasted a few milliseconds, a million times longer than the laser pulse, which for many laser applications is ‘infinity,’ Professor Milchberg said.
Because the waveguides are relatively long-lived, he believes a single guide could be used to send out a laser can collect a signal.
‘It’s like you could just take a physical optical fibre 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,’ he said.
The next step is for the scientists to show that the waveguides can be used over distances of at least 164ft (50 metres).
They could then be honed 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.