Contrary to what's portrayed in scifi, short bursts of laser pulses travel at speeds far too fast for the human eye to see. But what would it look like if we could film it in slow motion? Thanks to a team of physicists from the Polish Academy of Sciences, we now have the video evidence.
A laser pulse only lasts for a dozen or so femtoseconds (we're talking millionths of a billionth of a second). So, to capture it on film, we'd need to have a camera operating at a speed of a billion frames per second! Obviously, no such camera exists. To get over this limitation, researchers at the University of Warsaw's Laser Centre used an old camera trick in order to test a new compact high-power laser.
The laser in this video is actually a composite of many, many laser shots. To capture it, an adapted camera was synchronized with laser generating pulses at a rate of approximately 10 shots per second. It was done in such a way that with every subsequent pulse the camera recorded, an image was minimally delayed from the previous one.
A light pulse fired from a 10 TW laser, dispersing into water vapor. The blue glow is laser light. The source of the other colors is mainly plasma fiber (filament) arising as a result of ionized matter, located in the air in the path of the light pulse. Caption and image: IPC PAS.
"In fact, a different laser pulse can be seen in every frame of our film," noted researcher Paweł Wnuk in a statement. "Luckily, the physics always stays the same. So, on the film one can observe all the effects associated with the movement of the laser pulse in space, in particular, the changes in ambient light depending on the position of the pulse and the formation of flares on the walls when the light passes through the dispersing cloud of condensed water vapor."
The laser was so powerful that it almost immediately ionized the atoms it encountered. Also, although the light being shot from the laser is in the near infrared range, a laser beam like this traveling through the air changes color to white.
"This happens since the interaction of the pulse with the plasma generates light of many different wavelengths," added team leader Yuriy Stepanenko. "Received simultaneously, these waves give the impression of white."
This is a cross-section of light pulse beam with formed plasma filament. At the top is the appearance of several laser pulses passing through a cloud of condensed water vapor. Caption and image: IPC PAS.
So what's it all for? Because the new laser can penetrate the atmosphere over long distances, the technology could be used for the remote testing of atmospheric pollution. The fact that it generates white light is an added advantage; light at different wavelengths interacting with atoms and molecules are able to provide more information.
The work was commissioned by the Institute of Physical Chemistry of the Polish Academy of Sciences.