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Molecular quantum wakes for clearing fog

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Abstract

High intensity laser filamentation in air has recently demonstrated that, through plasma generation and its associated shockwave, fog can be cleared around the beam, leaving an optically transparent path to transmit light. However, for practical applications like free-space optical communication (FSO), channels of multi-centimeter diameters over kilometer ranges are required, which is extremely challenging for a plasma based method. Here we report a radically different approach, based on quantum control. We demonstrate that fog clearing can also be achieved by producing molecular quantum wakes in air, and that neither plasma generation nor filamentation are required. The effect is clearly associated with the rephasing time of the rotational wave packet in N2.Pump excitation provided in the form of resonant trains of 8 pulses separated by the revival time are able to transmit optical data through fog with initial extinction as much as −6 dB.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Supplementary Material (2)

NameDescription
Visualization 1       Shock wave produced in a cloud by a sequence of 8 pulses resonant with the rotational revival time
Visualization 2       Lack of shock wave in a cloud when a sequence of 8 pulses is detuned from resonance resonant with the rotational revival time

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Figures (5)

Fig. 1.
Fig. 1. Quantum wake clearing of fog: a series of $8$ pulses of $56$ fs are generated by a nested Michelson interferometer (pulse stacker [32]) and softly focused into a fog chamber. The interval between the pulses can be tuned on- or off-resonance with the rotational revival times $T_R$ of the nitrogen molecules in air. Clearing of the fog is assessed with a counter-propagating CW laser at telecom wavelength ($1.55$$\mu$m) and a photodiode that measures the transmission. In addition a synchronized Nd:YAG laser is used for imaging the induced shockwave by shadowgraphy and interferometry
Fig. 2.
Fig. 2. Coherent control of the rotational quantum wake induced shockwave in fog. Visualized through shadowgraphic measurements. Left: train with pulse intervals slightly detuned from resonance: $8.66$ ps; Right: train of $8$ pulses tuned in resonance with the full revival time for N$_2$: $8.36$ ps . Delay between TiSa pump and Nd:YAG probe: $4$$\mu$s. Total energy of the pump pulse train: $3.8$ mJ
Fig. 3.
Fig. 3. Change in air refractive index due to rotational gas heating, measured by interferometry. Left: detuned pulse train: $8.66$ ps; Right: resonant pulse train at full revival time for N$_2$: $8.36$ ps . Delay between TiSa pump and Nd:YAG probe: $500$$\mu$s. The insets show the shift in the refractive index for time delays of $200$, $400$, $600$ and $800$ $\mu$s after the pump, in order of deepest to shallowest.
Fig. 4.
Fig. 4. Fog clearing induced by the rotational quantum wake. Transmission gain in the case of (a) No fog with the $8$ pulse train pump laser, (b) Fog ($1.3\times 10^5$ cm$^{-3}$ droplet concentration) with the $8$ pulse train tuned on-resonance with the quantum wake rephasing time, and (c) Fog ($0.7\times 10^5$ cm$^{-3}$ droplet concentration) with the $8$ pulse train tuned off-resonance with the quantum wake rephasing time. Total energy of the pump: $3.8$ mJ. Transmission is normalized to 1 in each case, corresponding to the situation where no pump is present ($t < 0$).
Fig. 5.
Fig. 5. Fog clearing induced by the rotational quantum wake as a function of the initial signal attenuation caused by different fog densities 5(a). Clearing is efficient for all densities in the case of a pulse train in resonance with the rephasing time, and the efficiency increases with the initial density (see text). Clearing is strongly reduced for the off-resonant case. 5(b) comparison with the plasma induced shockwave - Purple: single pulse of $3.8$ mJ (entire energy of the train in one pulse), Blue : single pulse of $0.48$ mJ (one pulse of the train). Transmission is normalized to $1$ in each case, corresponding to the situation where no pump is present ($t < 0$).

Equations (1)

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| Ψ R ( t ) = J , m c J , m | J , m e i E R t / ,
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