This Week

28 January 2015

Laser flight path caught on camera for the first time


PEW pew! That’s the welcome sound of the first video of a laser beam in flight.

Watching lasers fly through the air makes for dramatic battles in sci-fi films, but they’re not so easy to see in real life. To observe a laser, photons from it must directly hit your eyes. But laser photons travel in a tightly focused beam, all heading in the same direction. This means you can only see them when the laser hits something that reflects a portion of the light and produces a visible dot.

“Normally you can only see a laser when it hits something that reflects a portion of the light”


A few photons scatter off air molecules, but normally these are too faint to see. You can get around this by firing a laser through smoke, giving the photons more molecules to scatter off – but that’s not the effect we see in the movies.

“The challenge was to have a movie of light moving directly in air,” says Genevieve Gariepy of Heriot-Watt University in Edinburgh, UK.

She and her colleagues constructed a camera sensitive enough to pick up those few scattering photons. It is built from a 32 by 32 grid of detectors that log the time a photon arrives at them with incredible precision, equivalent to snapping around 20 billion frames a second.

The team arranged the camera to film a side-on view of a green laser firing at an arrangement of mirrors. By firing 2 million pulses over a 10-minute period and subtracting background noise, they were able to build up enough air-scattered photons in the camera to track the laser’s path as it bounced.

“What comes out is a frame by frame of the light moving through our system,” says Gariepy (Nature Communications, DOI: 10.1038/ncomms7021).

The experiment started as a pure research challenge, but Gariepy thinks the camera could have practical applications. In another experiment, the team filmed a focused laser that ionised air molecules to produce a plasma. Gariepy says a similar setup could help people studying the properties of such plasmas.

Precise timing data could also be used to measure the distance photons have travelled, an effect previously exploited to take pictures around corners. Normally this takes about an hour, but the ability to take multiple images rapidly could generate movies from around corners. “With our camera this can be done in seconds.”