Self-Focusing and Self-Phase Modulation of a Focused Femtosecond Laser Beam in Fused Silica at Near-Critical Peak Power

Abstract

Self-focusing and self-phase modulation of focused femtosecond laser pulse were studied in bulk fused silica. Depending on pulse energy magnitude, three different regimes were observed in the experiment: 1) linear regime, when the self-focusing effect is insignificant, characterized by linear spectral broadening; 2) transition regime, when the self-focusing effect notably reduces a laser beam spot in a focal plane without considerable nonlinear absorption and beam collapse, characterized by quick increase of spectral broadening due to self-phase modulation; 3) nonlinear regime, filamentation, characterized by near-to- linear spectral broadening due to self-phase modulation. The slope for the linear and nonlinear regimes was the same within the measurement error bars. The numerical simulation, using the ray transfer matrix method, envisioned the self-phase modulation spectral broadening effect in all these regimes.

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

“…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 .…”

“…By comparing and analyzing these two figures, we can derive insights into the characteristics of laser pulse propagation as the input laser energy increases, as well as the underlying physical mechanisms. When the input laser energy is relatively low, the peak intensity of the laser pulse is very weak and the medium does not experience ionization 31 . Under this condition, the broadening of the laser spectrum only comes from the self-phase modulation.…”