Nonlinear Propagation and Filamentation on 100 Meter Air Path of Femtosecond Beam Partitioned by Wire Mesh

Abstract

High-intensity (∼1 TW/cm2 and higher) region formed in the propagation of ∼60 GW, 90 fs Ti:Sapphire laser pulse on a ∼100 m path in air spans for several tens of meters and includes a plasma filament and a postfilament light channel. The intensity in this extended region is high enough to generate an infrared supercontinuum wing and to initiate laser-induced discharge in the gap between the electrodes. In the experiment and simulations, we delay the high-intensity region along the propagation direction by inserting metal-wire meshes with square cells at the laser system output. We identify the presence of a high-intensity region from the clean-spatial-mode distributions, appearance of the infrared supercontinuum wing, and occurrence of the laser-induced discharge. In the case of free propagation (without any meshes), the onset of the high-intensity zone is at 40-52 m from the laser system output with ∼30 m extension. Insertion of the mesh with 3 mm cells delays the beginning of the high-intensity region to 49-68 m with the same ∼30 m extension. A decrease in the cell size to 1 mm leads to both delay and shrinking of the high-intensity zone to 71-73 m and 6 m, respectively. Three-dimensional simulations in space confirm the mesh-induced delay of the high-intensity zone as the cell size decreases.

Keywords: beam regularization; femtosecond filamentation; infrared supercontinuum; remote discharge triggering.

Figure 1

(a) Photo of wire meshes used for the beam regularization. Scheme of (b) experimental setup and (c) portable registration system. The latter allowed us to record the transverse fluence distributions (left), check the discharge triggering (center), and record the pulse spectrum (right). The symbol “#” in (b) indicates the mesh inserted into the beam. In (c), PL is a plane-parallel plate with a single matted surface, HV is a high-voltage supply, Osc is an oscilloscope.

Figure 2

Normalized transverse fluence distributions measured at different distances z in the range from 50 m to 94 m for the free propagation without a mesh (left column) as well as in the case of regularization by the mesh with 3 mm cells (middle column) and 1 mm cells (right column).

Figure 3

Transverse fluence profiles (normalized to the maximum value) at various propagation distances z without modulation (left column, (a–d)), when modulated by meshes with 3-mm (middle column, (e–h)) and 1-mm (right column, (i–l)) cells. Mesh amplitude masks are shown by the blue grids at (e,i).

Figure 4

Dependencies of minimal $D_{\min }$ and maximal $D_{\max }$ measured out of 16 laser shots beam diameters (circles) on the propagation distance z in comparison with the beam diameter obtained from simulations (solid curves) for different modulation regimes: without modulation ((a), blue), using 3 mm ((b), green) and 1 mm ((c), red) meshes. Sixteen measured at the certain propagation distance z diameters lie between $D_{\min }$ and $D_{\max }$ inside the filled area in (a–c). Insets in the left column show images of burns on the photographic paper at $z\approx 73$ m. (d–f) Dependencies of beam diameter (solid curves) and maximal intensity (dotted curves) on propagation distance z obtained in simulations in the range of z from 0 to 100 m for all three cases. Colored rectangles highlight the regions shown in left column. Color bands in lower part of (d–f) demonstrate the range where the laser pulse triggered the electric discharge.

Figure 5

Normalized pulse spectra measured at $z=94$ m for different modulation regimes: without modulation ((a), blue), using 3 mm ((b), green) and 1 mm ((c), red) meshes. (d) Dependencies of the position ${\lambda }_{\mathrm{Stokes}}$ of the longest-wavelength maximum in the spectrum on the propagation distance z for all three cases. Dashed line in (d) indicates pulse central wavelength ${\lambda }_0$. Color bands in lower part of (d) show the range where the laser pulse triggered the electric discharge.

Figure 6

Dependencies of (a) the beam diameter and (b) the position ${\lambda }_{\mathrm{Stokes}}$ of the longest-wavelength maximum in the spectrum on the propagation distance z. Filled blue circles and open gray circles stand for the free beams with the pulse energies of 6.2 and 5.2 mJ, respectively. Red triangles represent data in the case of beam regularization by the 1 mm mesh.