09 May 2007
Researchers have shot a 32 TW laser pulse some 20 km into the sky.
A team of French and Swiss researchers has used the Alisé beamline in France to generate an ultrahigh power white-light laser beam. "Our laser source launched 32 TW, 26 J pulses centered on 1053 nm into the atmosphere," Jérôme Kasparian of the French laser research center CNRS-LASIM told optics.org. (Applied Physics Letters, 90, 151106).
"Such levels represent large increases in peak power and pulse energy compared to earlier experiments, as well as a conceptual step forward. It was quite unknown whether the beam would show normal propagation behavior at such extreme levels, or if it would collapse at some relatively short distance."
Propagation of intense laser pulses in the atmosphere has previously been achieved at the subjoule and terawatt levels, where self-guided laser filaments have been created over many tens of meters.
The researchers found that their ultraintense beams did indeed propagate well into the stratosphere, beyond 20 km altitude. The beam generated up to 400 individual filaments and a broad supercontinuum of light from 300 to 850 nm in the same way, although not quite as efficiently, as subjoule pulses do.
The Alisé laser source was employed in a chirped-pulse amplification mode with six stages of Nd:phosphate amplifiers. High-power pulses are normally produced in cleanroom conditions, compressed and focused onto targets in a vacuum, so the team had to modify both the design of the laser system and the architecture of their building to launch the beam out of an open window in the building's roof.
Indirect observation from the ground was the principal means of beam characterization. Diagnostics included a beam profile analyser, a single-shot autocorrelator and a streak camera. A frequency-doubled Nd:YAG laser, co-linear to the Alisé beam, was used as a reference to estimate the conversion efficiency into the white-light supercontinuum.
"Detection issues were made all the more difficult by the fact that our experiment involved the laser emitting one pulse every hour," commented Kasparian. "The atmosphere fluctuates significantly in this time scale, so all our measurements had to be recorded in a single shot to be worthwhile."
Such filamentation of laser beams at high altitudes opens up some intriguing applications. "Filaments generate conducting plasma channels, which can trigger and guide high-voltage discharges over some meters," noted Kasparian. "The plasma channel left behind by our laser pulse could act as a lightning rod, placed anywhere in the sky to attract a lightning discharge."
Developing such novel applications will need longer and more intense filaments. Going to even higher energy beams is impractical, as suitable lasers are few and far between, so Kasparian's team intends to optimize the filamentation of the 30 TW source they already have. Detectors positioned all along the beam path will help them to understand the formation, propagation, merging and ending of the filaments. But since this would be tricky 20 km up, their next experiments will be aimed horizontally.