Distinguishing the nonlinear propagation regimes of vortex femtosecond pulses in fused silica by evaluating the broadened spectrum

Abstract

The nonlinear propagation dynamics of vortex femtosecond laser pulses in optical media is a topic with significant importance in various fields, such as nonlinear optics, micromachining, light bullet generation, vortex air lasing, air waveguide and supercontinuum generation. However, how to distinguish the various regimes of nonlinear propagation of vortex femtosecond pulses remains challenging. This study presents a simple method for distinguishing the regimes of nonlinear propagation of femtosecond pulses in fused silica by evaluating the broadening of the laser spectrum as the input pulse power gradually increases. The linear, self-focusing and mature filamentation regimes for Gaussian and vortex femtosecond pulses in fused silica are distinguished. The critical powers for self-focusing and mature filamentation of both types of laser pulses are obtained. Our work provides a rapid and convenient method for distinguishing different regimes of nonlinear propagation and determining the critical powers for self-focusing and mature filamentation of Gaussian and structured laser pulses in optical media.

Numerical simulation study on distinguishing nonlinear propagation regimes of femtosecond pulses in fused silica

“…It is evident that the cut-off wavelength undergoes a gradual extending with increasing incident laser energy, followed by a rapid transition towards saturation. This trend is in excellent agreement with the results obtained in experiment 21 . Based on this trend, three distinct regimes can be easily distinguished, corresponding to linear propagation, self-focusing, and filamentation processes.…”

“…The intersections of the fitted lines provide the deviation positions of 247.7 nJ and 289.6 nJ, which correspond to the self-focusing critical power of 3.8 MW and the mature filamentation critical power of 4.5 MW, respectively. These values of self-focusing and filamentation critical powers are slight smaller than the experimental ones 21 , which may be attributed to differences in the initial parameters used for the simulation.

Figure 1 ( a ) The far-field spectrum of a Gaussian laser beam for different input laser energies, ( b ) the evolution of the cut-off wavelength as a function of input laser energy.

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“…It also helps in determining the critical powers for self-focusing and mature filamentation of both femtosecond Gaussian and structured beams. Additionally, an independent study conducted by Kinyaevskiy et al has presented compelling evidence that supports the effectiveness and validity of the S-scan method 21 , 31 .…”

“…Moreover, directly determining the exact nominal critical power in experiment can be challenging 26 . However, for the purpose of providing clear and referenceable measurements for experimental investigations and practical applications of femtosecond filamentation, in this study, we adopt the crossover power as the critical power for self-focusing, as done in our recent work and relevant literatures 10 , 11 , 20 , 21 , 24 , 28 , 29 .…”

“…Subsequently, if the incident laser power exceeds the critical power for self-focusing, these hot spots collapse into multiple filaments. In our recent study, the nonlinear propagation regimes of self-focusing and filamentation of femtosecond vortex laser pulses in fused silica have been experimentally distinguished by measuring the spectral broadening of the laser beam 21 . This technique, which we refer to as the S-scan method, has proven to be effective in characterizing the propagation dynamics of femtosecond beams.…”