Long distance laser filamentation using Yb:YAG kHz laser

Abstract

In the framework of the Laser Lightning Rod project, whose aim is to show that laser-induced filaments can guide lightning discharges over considerable distances, we study over a distance of 140 m the filaments created by a laser system with J-range pulses of 1 ps duration at 1 kHz repetition rate. We investigate the spatial evolution of the multiple filamentation regime using the fundamental beam at 1030 nm or using combination with the second and third harmonics. The measurements were made using both a collimated beam and a loosely focused beam.

Figure 1

Evolution of the number of filaments N along the laser propagation axis z of a 500 mJ 1030 nm laser pulse, for pulse durations of (a) 1 ps, (b) 3 ps, (c) 5 ps and (d) 7 ps. Examples of photosensitive paper measurements performed at the distances (e) z = 19 m, (f) z = 25 m, (g) z = 29 m for a pulse duration of 1 ps. (h) Comparison between the microphone signal (orange dashed line) and the simulated signal S (blue solid line) along the laser propagation axis z for a laser pulse of 500 mJ at 1030 nm with a duration of 1 ps.

Figure 2

Experimental distribution of the filament widths produced by (a) 500 mJ 1 ps laser pulses at 1030 nm and (b) 500 mJ 1 ps laser pulses at 1030+515 nm measured on photosensitive paper. Insert: Example of impacts of single filaments on photosensitive paper.

Figure 3

Evolution of the number of filaments N at 1030 nm (red solid line) and at 515 nm (green blue dashed line) along the laser propagation, created by (a) the fundamental 1030 nm only, (b) 1030+515 nm pulses, (c) the isolated fundamental part of 1030 + 515 nm pulses and (d) the isolated second harmonic part of the 1030 + 515 nm pulses. The dotted line corresponds to the total number of filaments at 1030 nm and 515 nm.

Figure 4

Evolution of the spectrum of the triple frequency-converted pulse along its propagation. (a–c) being the spectral components around 343, 515 and 1030 nm, respectively. (d) Comparison between the measured and calculated redshift along the laser propagation, for the pulse spectral component at 515 nm.

Figure 5

Spatial evolution of the surface occupied by filaments created by the 500 mJ 1 ps fundamental beam (red solid line) or the frequency doubled beam (green dashed line), focused at (a) $z=55$ m or (b) $z=115$ m. Examples of photosensitive paper measurements performed at the respective focal point (c) z = 55 m and (d) z = 115 m.

Figure 6

(a) Phase shift $\mathrm{\Delta }\varphi$ induced by the frequency doubled 500 mJ, 1 ps pulse, focused at $z=55$ m (red solid line) and $z=115$ m (blue solid line), measured at the geometrical focus of the beam 1 $\mathrm{\mu }$s after the last laser pulse. (b) Phase shift $\mathrm{\Delta }\varphi$ induced by the 500 mJ, 1 ps frequency doubled beam (blue solid line) and the 500 mJ, 1 ps fundamental beam (red dashed line), focused at $z=55$ m, measured at the focal distance 1 $\mathrm{\mu }$s after the last laser pulse.