Frequency chirp and pulse width control of internal modification of transparent material using tailored femtosecond laser pulse

Abstract

To experimentally investigate the frequency-chirp- and pulse-width-dependent modification of silica glass, we use the chirped pulse amplification (CPA) system with the femtosecond automatic pulse compressor. When a high-intensity laser pulse is focused into silica glass, laser-induced solid-density plasma and small-scale filaments are produced at an intensity of 1 TW/cm/sup 2/ approximately. The solid-density plasma formation is due to both mechanisms of self-focusing due to the intensity-dependent refractive index change and of self-defocusing by laser-induced plasma. The other nonlinear phenomena in which ultrashort laser propagating in a silica glass, are self-phase modulation (SPM), self-channeled plasma formation, and permanent bulk modification. All processes are also strongly dependent on the input intensity, pulse width and chirp. Although detailed simulations of femtosecond pulse propagation are difficult tasks we simulate pulse propagation with different input pulse parameters (Fourier transform limited, down-/up-chirped pulse). We consider, here, only the SPM and material dispersion in silica glass and use the split step Fourier method. It is clearly seen that the pulse-profile is less stretched using an optimal chirped pulse as the input pulse. In addition, longer Fourier transform-limited pulses are less stretched after propagation.