Optimizing photoablation parameters in the mid IR: predictive model for the description of experimental data

Abstract

An ablation model for pulsed laser ablation, based upon Lambert's law is presented. Ablation is assumed to be entirely the result of heating and subsequent evaporation of tissue water. The model predicts the ablation depth per pulse as well as the collateral damage beyond the surface as a function of the target material's absorption coefficient and allows variation of irradiance exposure per pulse to be taken into account. As a function of the wavelength of the applied laser light the ablation model was used to describe ablation data from the FEL at Vanderbilt University in Nashville, TN between the wavelengths of 2.7 micrometers and 6.5 micrometers and from the FEL at Rijnhuizen, NL (FELIX) between the wavelengths of 8 micrometers and 17 micrometers . Gelatine as well as procine cornea was used as target material. As a function of the absorption coefficient of the target material the ablation model was used to describe ablation data obtained with an Er:YAG laser (2.94 micrometers ) in corneas with different absorption coefficients. The corneas absorption coefficient was varied by substituting heavy water (D<SUB>2</SUB>O) for light water (H<SUB>2</SUB>O). It was found that the ablation depth per pulse decreases and the collateral damage beyond the surface increases with a decreasing absorption coefficient of the target material. Comparison of the experimental data with the ablation model indicates that around 3 micrometers heat conduction during pulse duration (FEL 4 microseconds; Er:YAG 250 microseconds), and between 6.5 micrometers and 10.5 micrometers the absorption not only by water but also e.g. by proteins must be taken into account.