Air can be considered as a nonlinear optical medium for sufficiently high laser intensities. Short pulses of high peak intensity have been seen to create their own waveguide and propagate through the atmosphere while maintaining a diameter of the order of 100 μm over distances far in excess of the Rayleigh range. It is generally believed that the waveguiding results from a balance between self-focusing (Kerr effect) and self-defocusing due to a low desnity electron plasma created by multiphoton ionization of air. This delicate balance is destroyed after a short time because of inverse Bremstrahlung, which leads generally to avalanche ionization. Observation of filaments has therefore been limited to femtosecond pulses. At wavelengths shorter than 306 nm, ionization of oxygen is only a 3 photon process, and therefore the intensity in UV filaments is 20 to 3 orders of magnitude smaller than in the IR. Furthermore, the time required for inverse Bremstrahlung to lead to avalanche ionization is 1000 times longer, i.e. of the order of a nanosecond. As a consequence, it should be possible to channel up to 1 Joule of energy in the UV filaments, as opposed to a few mJ.
To create such high energy, we have developed a compact frequency quadrupled Nd:YAG laser oscillator-amplifier, compressed from 3 ns down to 200 ps by stimulated Brillouin scattering in FC75. The oscillator is seeded by a stabilized semiconductor laser to ensure the narrow band operation required for the stimulated Brillouin scattering. Efficient transfer of power from the beam to the filaments is achieved by focusing the larger beam issued from the Brillouin cell in vacuum, onto a supersonic flow of air serving of window between vacuum and atmosphere.