SPECTROSCOPY: White-light pulsed lidar checks atmosphere

France's Centre National de la Recherche Scientifique and the Deutsche Forschungsgemeinschaft are providing DM 2.5 million ($1.35 million) over the next two years to support a research project to develop a mobile terawatt white-light lidar system (see figure). The device will combine the height-resolution advantage of differential-absorption lidar and the multicomponent-analysis capability of differential optical absorption spectroscopy.

Project participants include Roland Sauerbrey and his research team at the Friedrich-Schiller-Universität Jena (Jena, Germany), Ludger Wöste and colleagues at the Freie Universität Berlin (Berlin, Germany), André Mysyrowicz and his research team at the Ecole National Supérieure de Techniques Avancées (Paris, France), and Jean-Pierre Wolff and associates at the Université Claude Bernard de Lyon (Lyon, France).

The femtosecond-pulse technique will generate white-light pulses with a chirped-pulse-amplification Ti:sapphire laser (790 nm) having pulse energy up to 250 mJ, 100-fs pulse duration, and a 10-Hz repetition rate. This light will be directed into the sky through a plasma channel developed by the system. The light backscattered from the channel will then be analyzed spectrally to simultaneously trace the water-vapor distribution, ozone, and other trace gases such as methane (CH₄), sulphur dioxide (SO₂), or nitrous oxide (NO_x) in the atmosphere.

In preliminary laboratory experiments at the Universität Jena, in collaboration with the Berlin project group, researchers investigated the electrical properties of the plasma channels. They reported the detection of free laser-induced charges along the light filament produced by the laser beam.¹ During tests of the system, the scientists positioned a channel-like source of white light in the atmosphere by adjusting the arrival time of the spectrally dispersed laser pulse in the focus. The system detected backscatter signals from as high as 13 km; recorded spectra indicated that the white-light source can be favorably used for absorption measurements.

The channel produced extended along the laser beam across a considerably longer distance than the short geometrical focal region. Even the free-propagating beam contracted and finally self-focused—a fact explained by the interplay of refractive-index modulation due to the intensity-dependent optical Kerr effect and diffraction.

For lidar applications, the white-light source can be shifted along the optical axis of the laser beam by changing both the focal length and the overlap of the spectral components of the laser pulse (control of group-velocity dispersion). This shifting opens the possibility of performing absorption measurements of different optical lengths and positioning the white-light source behind an atmospheric layer of interest.

The researchers plan to investigate the principal propagation properties of intense laser pulses in air, the plasma channeling process, and the application of multicomponent white-light lidar at large facilities where no high-power laser is available but is needed. The system may also be combined with larger astronomical telescopes to enhance the detection sensitivity of the system or act as guide star for the astronomers. Other possible uses include atmospheric research and pollution detection. In addition, the researchers believe the conductive properties of the laser-induced channel could be used for lightning control near airports, power plants, or other sites.

REFERENCE

1. Appl. Phys. B 68, 753 (1999).