Abstract

Filamentation of Mid-IR pulses in ambient air is significantly more complicated and challenging as compared to near-IR filaments, due the changing sign of dispersion in the vicinity of 3.6 μm, substantially lower ionization rates and hence plasma density [1], as well as because of the presence of molecular resonances around the atmospheric transparency windows. Experimental observations reveal that despite its low concentration in ambient air, which is only 400-600 ppm (0.04-0.06%), CO₂ strongly affects filamentation of 3.9-μm pulses by influencing spectral and temporal dynamics, as well as spatial beam transformations and propagation losses. As a main loss mechanism during filamentation of loosely focused 3.9-μm pulses in ambient air, dynamic absorption by CO₂ is identified. Since in the case of loose focusing plasma density in the filament is negligible, pulses during filamentation in both main constituents, O₂ and N₂, experience pronounced red-sided broadening (Fig.1a), which is governed by stimulated Raman scattering. In ambient air, newly generated spectral components roll over CO₂ resonant absorption band situated in the vicinity of 4.2-μm and are effectively absorbed, which results in up to 50% filamentation losses. To our surprise, an increase of CO₂ concentration to ~5% level (by exhaling air [3] into 2-m long open-ended tube) leads to complete disappearance of losses.

© 2017 IEEE