Analyses of Self-Focusing Phenomenon and Temperature Rise in Fused Silica by Ultrashort Pulse Laser Irradiation

When an ultrashort pulse laser is irradiated into the inside of a permeable medium, the refractive index changes dependently on the electric field strength. As a result, a self-focusing phenomenon occurs and self-focusing can lead to a filamentation. In the light absorption medium, the absorbed light energy changes to the thermal energy after ultrashort pulse laser irradiation. Then melting or partial ablation phenomena occur near the focusing area in the medium. Therefore, internal modification, lap welding of permeable materials and so on can be achieved. In this study, a paraxial wave equation including light absorption term was solved by numerical calculation using the fast Fourier transform for irradiation of a Kerr medium by an ultrashort pulse laser. Using the absorbed energy distribution as the initial condition, heat transfer analysis in the material was conducted. The influences of the nonlinear index intensity coefficient and the absorption coefficient on the temperature distribution in the material were investigated.