Thursday, October 23, 2014

What a real Star Wars laser "bullet" looks like

Credit: IPC PAS

A light pulse fired from a 10 terawatt** (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  at the University of Warsaw. 

Humorist S. J. Perelman once wrote, "there's little like a detailed report filled with decimal points and guarded generalizations to put a glaze on your eye like a Sung Vase."

Mr. Perelman never met the Terrawatt, an arcane measurement of size, time or something else, either incomprehensively small or unimaginably big, I don't remember which because my brain collapsed reading the definition, that a relatively normal person's brain tends to collapse trying to comprehend the definition.  (I remember when an IT guy used the word terraflop in mixed company.  I sobbed hysterically for nearly an hour.)

But trust me, the terrawatt, femtosecond and such tomfoolery are allegedly comprehensible to physicists and mathematicians and other people with too much time on their hands.

And then there are the Trekkies, who are going to be so thrilled by this bit of amazing photography that their drool will short out their iPads, rendering them helpless aboard public transportation resulting in the nerdification of the suburbs later today.  

Here's the story:

Action-packed science-fiction movies often feature colorful laser bolts. But what would a real laser missile look like during flight, if we could only make it out? How would it illuminate its surroundings? The answers lie in a film made at the Laser Centre of the Institute of Physical Chemistry of the Polish Academy of Sciences in cooperation with the Faculty of Physics at the University of Warsaw.

Tests of a new compact high-power laser have given researchers at the Laser Centre of the Institute of Physical Chemistry of the Polish Academy of Sciences and the Faculty of Physics, University of Warsaw  the opportunity to film the passage of an ultra-short laser pulse through the air. The film shows the journey of a light projectile at an extremely slow rate, similar to that watched on cinema screens by science-fiction aficionados.

"If you wanted to film a single light impulse to move as slowly on film as in our recording, you would have to use a camera operating at a speed of a billion frames per second," says Dr. Yuriy Stepanenko, leading the team responsible for the construction of the laser.

Cameras recording billions of frames per second in one sequence do not exist. In order to film the travelling laser pulse, researchers from the Laser Centre used an earlier known trick. A suitably adapted camera was synchronized with a laser generating laser pulses at a rate of approx. 10 shots per second. It was done in such a way that with every subsequent pulse the camera recorded an image minimally delayed than previous one.

"In fact, a different laser pulse can be seen in every frame of our film," explains Dr. Paweł Wnuk, and adds: "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."  (Are your eyes glazed over yet?  Just wait, we ain't finished.)

Suggested reading
The laser pulse, lasting a dozen or so femtoseconds* (millionths of a billionth of a second), was generated by a laser constructed at the Laser Centre. It was so powerful that it almost immediately ionized the atoms it encountered. As a result, a plasma fiber -- filament -- was formed alongside the pulse. By appropriately selecting the operating parameters of the laser, to permit a balance of the complex interactions between the pulse's electromagnetic field and the plasma filament, the laser light beam did not disperse in the air, conversely, it underwent self-focusing. This meant that the pulse could effectively move a much greater distance than low-power pulses, while maintaining its original parameters.

"It is worth noting that although the light we are shooting from the laser is in the near infrared range, a laser beam like this travelling through the air changes color to white. This happens since the interaction of the pulse with the plasma generates light of many different wavelengths. Received simultaneously, these waves give the impression of white," adds Dr. Stepanenko.

The ability of the light pulses from the new laser to penetrate the atmosphere over long distances is a feature that the Warsaw researchers made use of when demonstrating LIDAR, a device that can be used for the remote testing of atmospheric pollution. The fact that the pulses generate white light during passage is an important advantage in this context. Light at different wavelengths interacting with the atoms and molecules in the air is able to provide a far greater wealth of information. This means that LIDAR constructed using the new laser will be able to detect a larger number of elements and compounds polluting the atmosphere.

Photos and films of the ballistic laser pulse and plasma filaments were shot during testing of the compact laser generating femtosecond*** pulses with a power of over 10 terawatts, constructed at the Laser Centre of IPC PAS and FUW. The innovative device uses the direct transfer of energy from the pump laser beam to the reinforced beam. Due to the effects of nonlinear optics the light is amplified hundreds of millions of times on a distance of just a few centimetres with over 30% efficiency, which is outstanding among devices of this class.

The multi-pass optical parametric amplifier NOPCPA (Noncollinear Optical Parametric Chirped Pulse Amplifier) used in the laser was designed in-house and Prof. Czesław Radzewicz and his team have been developing it in the Laser Centre since 2005.

*  Femtowatt:  The femtowatt is equal to one quadrillionth (10−15) of a watt. Technologically important powers that are measured in femtowatts are typically found in reference(s) to radio and radar receivers.
** Terawatt: The terawatt is equal to one trillion (1012) watts. The total power used by humans worldwide (about 16 TW in 2006) is commonly measured in this unit. The most powerful lasers from the mid-1960s to the mid-1990s produced power in terawatts, but only for nanosecond time frames. The average lightning strike peaks at 1 terawatt, but these strikes only last for 30 microseconds.
***Femtosecond: The International System of Units measure of time equal to 10−15 of a second. That is one quadrillionth, or one millionth of one billionth, of a second.[1] For context, a femtosecond is to a second as a second is to about 31.7 million years. 
****(Now are your eyes glazed over?  My brain shut down about ten or eleven femtoseconds ago trying to wrap around these measurements.)


Related posts:
*  *  *  *  *
Story Source:  Materials provided by Institute of Physical Chemistry of the Polish Academy of Sciences.  "A 'Star Wars' laser bullet -- this is what it really looks like." ScienceDaily, 22 October 2014