Abstract
UV laser pulses at 266 nm with 170 ps duration and up to 300 mJ energy create UV filaments. The effects of different beam preparation scenarios on UV filamentation are discussed. Beam preparation by focusing in vacuum and launching into the atmosphere through an aerodynamic window establishes the existence of filaments as self-guided non-diffracting entities. In addition, focusing through the aerodynamic window converts the initial super-Gaussian into a parabolic beam profile at the edge of the nonlinear medium (air). The shock-wave generated by the UV filament is investigated via shadowgraphy techniques. The shock-wave is cylindrically symmetric, leaving a low density channel behind, which is well-sustained for 1 ms, as it propagates outward. This low density channel is a waveguide that can be used for guiding purposes. Applications of UV filaments including guiding, remote sensing, and drag reduction for supersonic aircrafts are discussed.
Highlights
Laser filamentation is a phenomenon that has attracted much attention in the past few decades and has many applications, including remote sensing,1–3 laser guided electrical discharges,4–8 and air lasing.9–13 Filamentation was defined by Akhmanov14 as a selfguiding phenomenon that can occur in transparent media with an intensity-dependent index of refraction
If the non-linearity of the medium is positive at moderate intensities and saturates or turns over at higher intensity, a stable waveguide can be formed, which can exist for distances exceeding the Rayleigh range
Such stable waveguides were observed in liquids at very low optical powers, exploiting the nonlinearity of microemulsions
Summary
Laser filamentation is a phenomenon that has attracted much attention in the past few decades and has many applications, including remote sensing, laser guided electrical discharges, and air lasing. Filamentation was defined by Akhmanov as a selfguiding phenomenon that can occur in transparent media with an intensity-dependent index of refraction. If the non-linearity of the medium is positive at moderate intensities (self-focusing) and saturates or turns over (self-defocusing) at higher intensity, a stable waveguide can be formed, which can exist for distances exceeding the Rayleigh range Such stable waveguides were observed in liquids at very low optical powers, exploiting the nonlinearity of microemulsions.. Since the critical power for self-focusing is much lower in the UV (13 MW at 250 nm), filamentation in air was observed with sub-mJ ps pulses at 248 nm.. Since the critical power for self-focusing is much lower in the UV (13 MW at 250 nm), filamentation in air was observed with sub-mJ ps pulses at 248 nm.20 Another requirement for filamentation is that the pulse duration be shorter than the time required to heat the photo-excited electrons by inverse bremsstrahlung to the ionization potential. In the case of less intense UV filaments, a loss of 60 μJ/m was measured, which is a very small fraction of tens of mJ energy of the 170 ps UV pulse
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