Understanding the beam self-cleaning behavior of ultrashort laser pulse filamentation

Abstract

In this paper we report a recent study on the beam self-cleaning behavior occurred during the ultrashort laser pulse filamentation process. The propagation of a Gaussian beam with distorted beam profile is numerically simulated based on the nonlinear wave equation. Our results demonstrate that when the power is not too high so that multiple filaments are not yet induced, the intensity perturbation contained in the initial beam profile could be treated as high order spatial modes superpositioning on a fundamental mode. Then the self-focusing of the laser beam acts as a spatial filter. It focuses the fundamental mode toward the propagation axis, and produces a fundamental mode profile at the self-focus. While the strong diffraction of higher order modes could not be counteracted by the self-focusing. Therefore their propagation is mainly governed by the divergence without destroying the high profile quality at the self-focal region. These lead to the observation of beam profile self-cleaning behavior. filamentation, ultrashort pulse, self-cleaning

Focused beam self-cleaning during laser filamentation

“…2d). We did not reach multifilamentation regime and the corresponding beam break-up, reported in previous papers [11,12] due to limitations of laser pulse energy and the propagation path length (13 m path appeared to be not enough for multifilamentation to develop even at 20P cr ).…”

“…The further analysis showed even more complicated behavior: the red-shifted supercontinuum components propagating on the pulse front were perfectly self-cleaned, while the central wavelength along with the blue-shifted components still had notable inhomogeneities caused by the refraction on plasma [12]. Moreover, at pulse powers much higher than the self-focusing critical power (P cr ), self-cleaning was breached due to multifilamentation [11,12]. Therefore, the pulse powers, at which self-cleaning takes place, appear to be bounded from below by approximately P cr and from above by the multifilamentation threshold.…”

“…Since the first observation in 1995 [3], a number of outstanding properties making filaments a prominent radiation source for a variety of applications has been revealed [4,5]. Indeed, intrinsically nonlinear nature of the filamentation phenomenon manifests in spectrum enrichment and supercontinuum generation in frequency domain [6,7], pulse self-shortening in time [8,9] and beam mode improvement also known as mode self-cleaning [10][11][12]. In this paper we discuss the latter property, which is of importance for such filament applications as a white-light laser [4,13] and pulse self-compression [14].…”

“…All the aforementioned papers examined self-cleaning either in collimated [11,12,17] or in extremely close to collimated [10] geometry. However, the multifilamentation threshold strongly depends on focusing conditions, decreasing with the increase of a numerical aperture (NA) [18].…”

Focused beam self-cleaning during laser filamentation

“…Since the first observation in 1995 [3], a number of outstanding properties making filaments a prominent radiation source for a variety of applications has been revealed [4,5]. Indeed, intrinsically nonlinear nature of the filamentation phenomenon manifests in spectrum enrichment and supercontinuum generation in frequency domain [6,7], pulse self-shortening in time [8,9] and beam mode improvement also known as mode self-cleaning [10][11][12]. In this paper we discuss the latter property, which is of importance for such filament applications as a white-light laser [4,13] and pulse self-compression [14].…”