Effects of gas medium on femtosecond laser beam delivery

One obstacle for ultrafast laser materials processing is the nonlinear beam distortion due to changes in refractive index of the beam delivery gas medium, which are induced by the extremely high optical intensity of the focused femtosecond laser beam. The Kerr effect increases the refractive index and causes self-focusing, while plasma generated by multiphoton and tunneling ionization decreases the refractive index and defocuses the beam. The overall effects from these nonlinear mechanisms both spatially and temporally distort the original homogeneous beam profile, resulting in reduced feature quality for ultrafast laser materials processing. In this work, a theoretical model based on the nonlinear Schrödinger equation describing laser pulse propagation coupled with a rate equation for plasma evolution in a gas is used to simulate the beam distortion due to self-focusing and plasma defocusing. The resulted beam profile at the focal point in air exhibits ring-like structures, and the intensity distribution can significantly deviate from the original Gaussian profile. The model also shows that the nonlinear beam distortion can be dramatically reduced by delivering the beam using an inert gas, e.g., helium, rather than air, due to its unique physical properties, which suggests a novel beam delivery technique for high quality ultrafast laser-material processing.