Spatiotemporal cleaning of a femtosecond laser pulse through interaction with counterpropagating filaments in air

We demonstrate spatiotemporal cleaning of a femtosecond laser pulse impinging on two counterpropagating filaments in air. The retroreflected signal essentially has a perfect beam profile. Prepulses present in the incident pulse are also efficiently removed. The performance of the “filament mirror” is explained by a plasma-mediated wave mixing process that can be viewed as a transient three-dimensional hologram.

Figure 1

(a) Schematic setup: D: diaphragm; BS: beam splitter; L: lens; CCD: charge-coupled device camera; FM: filament mirror. (b) Photograph of the blue plasma luminescence emitted by the three overlapping beams; (c) and (d) schematics of the main diffraction process giving rise to signal beam 4.

Figure 2

Fluence profile of a femtosecond pulse (a) before and (b) after reflection on the filament mirror as recorded in a single shot with a CCD camera. Scan of the fluence profile along $x$ at $z$ = 0 (c) before and (d) after reflection. The red open circles represent a fit of reflected beam 4 by a Townes mode profile function.

Figure 3

(a) Reflectivity ($R=W_4/W_3$) of the filament mirror as a function of angle θ between pulses 1 and 3. Here $W_1$ = $W_2$ = 0.8 mJ; $W_3$ = 170 μJ. (b) Reflectivity as a function of pulses 1 and 2 energies. Pulse 3 has energies of 160 and 340 μJ and is incident at an angle of 90°. The three pulses are coincident in time.

Figure 4

(a) Signal of pulse 4 as a function of delay of filament pulse 2. Pulse 3 is incident at an angle of 90°. Pulses 1 and 3 are coincident in time. (b) The same but delaying pulse 3 with respect to pulses 1 and 2.

Figure 5

(a) Temporal profile of incident pulse 3 and (b) reflected pulse 4 measured with a FROG apparatus. (c) Contrast measurement performed with a 3ω cross correlator of laser pulse 3 before (black line) and after (red line) reflection by the filament mirror. The black arrow indicates an artifact of the correlator.