Long-range optical communications constitute a challenging topic with important applications. Besides the classical beam shaping techniques, alternative solutions have been explored to overcome the scattering of light through aerosols and dense fogs. For example, laser filamentation, which generates a tenuous plasma along extended paths, can launch a fog-clearing cylindrical acoustic wave. However, the achieved dimensions of the optical channel and plasma non-uniformities still prevent opening of efficient transmission lines.

In this study, Schroeder et al. instead propose to monitor the rotational wave packets of air nitrogen and excite acoustic waves clearing fogs and clouds without filamentation. Their key idea is to pump a quantum rotational wavepacket of nitrogen using trains of ultrashort sub-pulses each having a duration shorter than the rotational coherence time and separated from each other by the rotational revival time. Rotational wavepacket decoherence and thermalization over 100 ps time scales causes an air pressure spike that launches the fog-clearing acoustic wave by ~100 ns. Using an experimental setup measuring both the optical transmission and induced shockwaves, the authors prove that this technique can efficiently eject water droplets out of the beam area and create clear channels over ~mm diameters inside a cloud chamber. This discovery opens the perspective to clear even broader channels by using mid-infrared lasers.

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