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Picosecond CO2 Laser Filamentation in Air


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Very-long-range guiding and distant projection of high-energy laser pulses and high-average power beams are of interest for many potential DoD applications. Earth's atmospheric trasmittance favors the long-wavelength infrared (LWIR) window from 8 to 14 m, however diffraction proportional to the wavelength results in strong beam spreading in space in this spectral range. We have performed proof-of-concept experiments on nonlinear filamentation at ~10 m by propagating ~1TW 3 ps CO2 laser pulses in laboratory air. The main experimentalobservation is that an intense CO2 laser beam is self-guided in a macroscopic single filament of </= 1 cm diameter over many Rayleigh lengths. We call such a single filament a megafilament because it confines an ~1TW laser pulse with several Joules of energy and its cross-section is 10^4 times larger than a typical near-IR single filament. In this study we demonstrated self-guiding of an ~1TW CO2 laser pulse in air over at least 32 meters (~20 Rayleigh ranges). We have measured the temporal dynamics of the laser pulse self-guided in air. It wasfound that an initial 10 m macropulse consisted of a leading ~1 TW picosecond pulse and one or more weaker postpulses is modified during nonlinear propagation in the atmosphere. The leading self-channeled pulse is shortened from 3.5 ps to 1.8 ps and the postpulses separated by25 ps are strongly screened by plasma produced via the avalanche ionization mechanism. Taking into account such pulse shortening, the clamped intensity in the megafilament was measured to be ~1TW/cm2 much smaller that the tunnel ionization threshold of O2/N2. By comparing these results with simulations, we find that in addition to Kerr self-focusing and diffraction, the self-channeling of LWIR pulses with an intensity of 1 TW/cm2 can only be explained by considering plasma formation below the tunnel ionization threshold due to many-body Coulomb induced ionization in air.



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