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Terawatt laser extends LIBS range

10 Nov 2004

Remote material analysis that exploits a mobile terawatt laser could have a range of up to 1 kilometer.

Researchers from France and Germany say that they have found a way to extend the detection range of laser-induced breakdown spectroscopy (LIBS) by an order of magnitude.

Relying on filaments of light generated by the mobile terawatt-class laser "Teramobile", the team believes its technique could operate in the kilometer range instead of just a few tens of meters. (Applied Physics Letters 85 3977)

LIBS uses a strongly focused, pulsed laser beam to generate a plasma. Analyzing the emission spectrum of the plasma reveals what elements are present. To date, portable systems have been used to analyze everything from archaeological artefacts to food.

Applications in hostile environment however, such as monitoring molten alloys or identifying radioactive nuclear waste, require remote analysis. The problem is that diffraction limits the delivery of high laser intensities to less than 100 meters.

The Teramobile team believes that self-guided filaments of light, which arise from nonlinear propagation of ultrashort laser pulses, can overcome diffraction and deliver high laser intensities without focusing over longer distances. It calls this approach remote filament-induced breakdown spectroscopy, or R-FIBS.

In initial tests with the Teramobile firing 250 mJ pulses centered at 800 nm, the R-FIBS system successfully detected copper and steel samples located 90 m away. The laser's repetition rate was 10 Hz with a minimum pulse duration emitted of 80 femtoseconds. The researchers say a beam profile containing around 30 filaments was typically incident on the sample giving an intensity of around 1013W/cm2.

"Unambiguous spectra could be obtained with only 1000 shots - less than 2 minutes at 10 Hz, allowing real-time monitoring," say the authors in their paper. "Under the same conditions, uncompressed (200 ps) and unseeded (typically 5 ns) laser pulses yield neither ablation nor a LIBS signal."

By improving the coupling into the detector system, the team believes R-FIBS measurements can be made at distance up to a kilometer. "This technique, which only requires reasonable laser energies, can be practical for versatile remote analysis on hazardous and unreachable spots, such as polluted sites, nuclear plants or chemical leakages," conclude the Teramobile team.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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