Self-focusing and maximum ablation depth in ultrafast laser surgery of tissue (Conference Presentation)

Ultrafast lasers are used to precisely ablate tissue below the surface. The maximum depth of ablation is ultimately limited by scattering and absorption by the tissue. As the depth of ablation is increased, higher laser powers are required to reach the ablation threshold at the laser focus, which leads to nonlinear self-focusing or surface damage. Here, we investigate self-focusing and the maximum depth of ablation in tissue experimentally and computationally. We find the maximum ablation depth in a model porcine tissue for a variety of focusing conditions and pulse widths by imaging ablation voids with third-harmonic generation imaging. The effect of self-focusing is measured by the shift in the focal plane of the ablation void and by the presence of self-focusing induced filaments. Computational models simulate laser pulse propagation and free-electron generation in tissue. Using our experimental data, we fit a nonlinear index to tissue. We then use the model to predict the role of self-focusing and the maximum ablation depth for a range of laser parameters.