Creating optical cables out of thin air

Imagine being able to instantaneously run an optical cable or fibre to any point on earth, or even into space. That’s what Howard Milchberg, professor of physics and electrical and computer engineering at the University of Maryland, wants to do.

Milchberg and his team recently used an ‘air waveguide’ to enhance light signals collected from distant sources. These air waveguides could have many applications, including long-range laser communications, detecting pollution in the atmosphere, making high-resolution topographic maps and laser weapons.

Because 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 worse, due to interactions with gases in the air. Fibre-optic cables can trap light beams and guide them like a pipe, preventing loss of intensity or focus.

Typical fibres 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 inwards. But solid optical fibres 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. Now, Milchberg’s team has found a way to make air behave like an optical fibre, guiding light beams over long distances without loss of power.

Milchberg’s air waveguides consist of a ‘wall’ of low-density air surrounding a core of higher density air. The wall has a lower refractive index than the core - just like an optical fibre. The researchers broke down the air with a laser to create a spark. An air waveguide conducted light from the spark to a detector about a metre away. The researchers collected a strong enough signal to analyse the chemical composition of the air that produced the spark.

The signal was 1.5 times stronger than a signal obtained without the waveguide. That may not seem like much, but over distances that are 100 times longer, where an unguided signal would be severely weakened, the signal enhancement could be much greater.

Milchberg creates his air waveguides using very short, very powerful laser pulses. A sufficiently powerful laser pulse in the air collapses into a narrow beam, called a filament. This 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.

Because the waveguides are so long-lived, Milchberg believes that a single waveguide could be used to send out a laser and 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,” says Milchberg.

First, though, he needs to show that these waveguides can be used over much longer distances - 50 metres at least. If that works, it opens up a world of possibilities. Air waveguides could be used 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.