A team of optical scientists from the University of Arizona (UA) and the University of Central Florida in the United States have developed a technology capable of sending high-intensity laser pulses through the atmosphere much farther than was previously possible. In the future, the technology, which was described in Nature Photonics at the end of March, could be used to divert strikes of lightning away from buildings or highly populated areas.
High-intensity lasers are not capable of propagating laser pulses over distances more than a couple of metres because of diffraction. Therefore, they are too short-ranged for applications such as diverting lightning.
The team of scientists managed to overcome this by embeding the primary, high-intensity laser beam inside a second beam of lower intensity (see figure 1). As the primary beam travels through the air, the second beam, or dress beam, refuels it with energy and sustains the primary beam over much greater distances.
‘Think of two airplanes flying together, a small fighter jet accompanied by a large tanker,’ said Maik Scheller, an assistant research professor in the UA College of Optical Sciences, who led the experimental work. ‘Just like the large plane refuels the fighter jet in flight and greatly extends its range, our primary, high-intensity laser pulse is accompanied by a second laser pulse – the "dress" beam – which provides a constant energy supply to compensate for the energy loss of the primary laser beam as it travels farther from its source.’
Figure 1: Top image shows the case of the intense central beam alone - the beam dissipates. The bottom figure shows the central beam accompanied by the dress beam - both beams are extended. Credit: University of Arizona
The development was supported by a five-year, $7.5 million US Department of Defense grant. The project was led by Jerome Moloney, a UA mathematics and optical sciences professor, who is now stepping up the research to investigate the effects of ultra-short laser pulses in the atmosphere and ways to improve their propagation over many kilometres.
Improved understanding of the pulses would create the groundwork for a new class of robust laser beams that are more effective in overcoming scattering caused by atmospheric turbulence, water droplets in clouds, mist and rain, according to Moloney. Such beams could be used in detection systems reaching over long distances.
Simulations performed by Matthew Mills at the University of Central Florida demonstrated that by scaling the new laser technology to atmospheric proportions, the range of the laser filaments could reach 50 metres or farther.
As the filaments travel through the air, they leave a channel of plasma in their wake, consisting of ionised molecules stripped of their electrons. Such plasma channels could be used as a path of least resistance to attract and channel lightning bolts. Therefore, the laser could be used to control lightning bolts during a thunderstorm and guide them away from buildings.
The laser pulses used in the university project had extremely high energy and very short (femtosecond) durations. ‘Usually, if you shoot a laser into the air, it is limited by linear diffraction. But if the energy is high enough and condensed into a few femtoseconds, creating a burst of light of extremely high intensity, it propagates through the air in a different way due to self-focusing,’ Scheller said. ‘The problem is that as it also ionises the air and creates plasma, so the laser loses energy.’ The filament therefore doesn't travel far because of the energy loss that ultimately causes the laser to dissipate.
The two-beam approached used by the research team overcomes this limitation. Similar to the principle of noise-cancelling headphones, the energy loss of the primary laser beam and the energy supply from the dress laser beam cancel each other out. ‘We use two different kinds of beams: One is a focused central beam of high intensity that creates the filament,’ explained Scheller. ‘The other that surrounds it has a long range of almost constant intensity. As a result, the dress beam propagates in a nearly linear manner.’
