Self-focusing and Self-phase Modulation of 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.