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Ultrashort pulses span wide spectrum

8 October 2002

A single laser system that creates a multi colored stripe of light could detect many atmospheric compounds simultaneously.

An international research collaboration has come up with an efficient way of generating broadband UV pulses, using near IR laser pulses. A lidar system making use of this technique could potentially monitor virtually all the compounds, including chemical pollutants and many active biological species in the atmosphere. (Physical Review Letters 89 143901)

This discovery relies on a process known as self-channeling, which occurs when high-power, subpicosecond pulses propagate through air. This process generates a so-called filament of light containing both a fundamental pulse and a third harmonic. Critically, the team says this is the first time that the two pulses within the filament remain in-phase and retain both their peak intensity and energy over distances far greater than the coherence length.

To generate the filament, the researchers take a Ti:Sapphire laser operating at 800 nm and emitting 45 fs pulses with energies around 486 µJ. The pulses are focused through a 100 cm focal length lens producing a filament about 10 cm in length with a diameter of 100 µm.

The high intensities within the filament induce a nonlinear effect that spectrally broadens these pulses resulting in a supercontinuum of light containing wavelengths ranging between 200 and 2000 nm. German and French Scientists have already demonstrated the potential use of white-light continuum generation to probe the atmosphere using a mobile laser system called Teramobile. The in-phase propagation of the third harmonic together with the fundamental pulse extends the spectral range of the probing pulse further in the UV, allowing the detection of a large number of important biological species.

"Filament formation is due to a dynamic balance between self-focusing and plasma defocusing," team member Neset Aközbek from US-based Time Domain Corporation told Optics.org. "A nonlinear phase-locking mechanism allows the third-harmonic pulse to co-propagate with the pump over distances much longer than the coherence length."

The team now hopes to extend its current setup to study UV pulse generation in longer filaments on the order of tens of meters. Aközbek says the pulse can be collimated or weakly focused by using a longer focal length lens and may be used to probe the atmosphere.

"Higher pulse energies are also required," he added. "Our current laser can produce terawatt laser pulses. Using such a femtosecond terawatt laser system, we have already shown that such pulses propagate over hundreds of meters in air." He also says that studying the phenomena in other media, such as condensed matter, is not out of the question.

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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