Femtosecond laser pulse multifilamentation induced by small scale air turbulence

Abstract

We report on experimental and numerical investigations of femtosecond pulse propagation locally disturbed by the turbulent flow field of a hot-air blower. The experiments show that turbulence may shorten the collapse/filamentation distance and induce the onset of multiple filaments. This is supported by numerical simulations indicating that the high frequency part of the turbulence spectrum (inner scale below three millimeters) plays a significant role. The experiments were carried out with a Ti:Sapphire CPA laser system delivering 10 mJ, 80 fs pulses with a repetition rate up to 100 Hz. The corresponding peak power is 125 GW which is about 20 times the critical power for self-focusing in air. The perpendicular flow of a hot-air blower was used to generate strong local turbulence in the laser optical path with a structure parameter of the refractive index Cn2 of ~ 5  10-10 m-2/3. The transverse intensity profile was measured at various distances from the compressor output using a CCD camera that records the light scattered by a glass diffuser. For each position along the laser propagation axis, 100 shots were recorded with and without turbulence in the optical path. The number of filaments and the filament positions in the transverse plane were determined for each shot and statistically analyzed. The measurements are in good agreement with numerical simulations of the pulse propagation starting from the measured intensity profile right behind the laser. Our nonlinear propagation equation comprises diffraction and self-focusing due to the optical Kerr effect. The defocusing effect of plasma generated by tunnel/multiphoton ionization is taken into account. The flow field of the hot-air blower is represented by a Kolmogorov phase screen capable of consistently handling high and very low frequencies of the turbulence spectrum. Thus, turbulence scales reaching from the millimeter range to centimeters or, if necessary, even up to several tens of meters can be simultaneously considered.