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HIGH-ENERGY LASERS |
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Pulsed-laser-induced white-light generation in air is becoming an important tool for atmospheric studies. Applying time and spectrally resolved analysis to the backscatter data allows the remote sensing of many different atmospheric molecules at the same time (see Laser Focus World, November 1999, p. 36); in addition, highly intense light pulses create conductive channels that can be studied with respect to their lightning-conduction capabilities. |
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Lidar (light detection and ranging) applications of pulsed-laser-induced white-light generation in air were first demonstrated by a team of different German laboratories.1 In 1999, a German-French research project, the Teramobile project, was established, involving the Freie Universität (Berlin, Germany), the Friedrich-Schiller-Universität (Jena, Germany), the Université Claude Bernard (Lyon, France), and the Laboratoire Ondes et Acoustique (Palaiseau, France). In addition to other achievements, the setup of a mobile terawatt laser (the Teramobile) and associated laboratory experiments have been completed.2, 3, 4 |
 Laser pulses with terawatt-level peak power create a conductive pathway in air, guiding an artificially created lightning flash in a laboratory. An unguided flash takes a zigzag pathway (left), while a laser-guided flash takes a straight line (right).
Optical breakdown with isotropic white-light emission occurs at the focus of energetic laser pulses. However, the formation of white-light filaments in self-guided channels in air has become possible only with the generation of laser pulses of ultrahigh peak power emitted by femtosecond lasers. White-light generation is believed to originate predominantly from self-phase modulation. Self-focusing is due to the optical Kerr effect, becoming effective in air when the pulse intensity exceeds 1010W/cm2. It is balanced by diffraction and refraction from the diluted plasma, which is initiated at radiation intensities above 1013 W/cm2. |
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White-light filaments have been observed to be stable over tens of meters and can be initiated at a chosen distance from the emitting laser. The distance can be controlled by setting a pre-chirp of the laser so that group velocity dispersion becomes compensated when the pulse has traveled the chosen length. The spectral continuum generated by near-infrared terawatt pulses in air had been found to extend from 900 to 1500 nm; recently its extension into the mid-infrared up to 4.5 µm was observed. Thus, it is possible to speak of the emitted light, which propagates collinearly with the original laser beam, as a supercontinuum (SC). |
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For atmospheric investigations, a laboratory 100-fs, 220-mJ Ti:sapphire laser system emitting at 790 nm was used to make remote-sensing absorption measurements. Water vapor and oxygen were measured up to a height of about 800 m when positioning the SC generation zone at an altitude of 150 m. Using the mid-infrared portion of the SC, remote sensing is now possible for infrared-absorbing species such as hydrocarbons. |
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A test of the Teramobile |
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The laser system—including power supply, operating panel, and signal-processing electronics—weighs 9.5 tons and fits within a standard freight container of 6 x 2.5-m footprint. The first atmospheric lidar measurements using the Teramobile are to be performed late this year in cooperation with an astronomical observatory in Tautenburg, Germany. The first horizontal atmospheric measurements were already performed in spring 2001.5 |
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One potential application of lasers like the Teramobile is in lightning control. An experiment was held at the Technical University (Berlin, Germany) with the Teramobile in which lightning strikes were guided by the laser beam.6 The laser was set to emit 240-mJ pulses of 260-fs duration, with a peak power of 0.92 TW. Impressive results were obtained (see figure). The distance between the Teramobile container and the first high-voltage electrode was 20 m; the electrode gap was set at 2.5 m. The voltage necessary to ignite the unguided flash was 2.25 MV (negative polarity), which dropped to 1.6 MV when the laser beam was directed across the gap. Much longer electrically conductive filaments created in the atmosphere could someday guide lightning controllably and safely to a specified location. |
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Uwe Brinkmann |
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REFERENCES
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Laser Focus World January, 2002 |