Interaction of Femtosecond Pulses with Transparent Media for Application of Corneal Microsurgery

Femtosecond (10-15s) lasers are used for high precision microsurgery because energy deposition of ultrashort pulses occurs with minimal heat transfer and damages. Consequently, potential applications of femtosecond lasers to corneal surgery are very promising. The INRS Ti:Sapphire laser was used for investigation of the ablation threshold (which is the minimal energy per unit surface to create damage) of freshly enucleated pig eyes and hydrogel buttons. The laser pulse was focused using different lenses, with focal lengths from 125 mm to 30 mm, on the surface of the sample. A silicon photodiode with appropriate filters to cut-off the laser fundamental wavelength was placed in front of the sample to collect the integrated optical signal emitted from electron ionization and recombination in the plasma. For pulse duration of 200 fs, the ablation thresholds of three corneal layers, the endothelium, epithelium and stroma, were found to be nearly the same and increased with the pulse duration. The integrated optical signal was found to be strongly affected by the hydratation level during the experimentation; which infers that the ablation threshold change with corneal dehydratation. On another hand, for energy values in the same range but longer pulse duration (270 ps), small cavities were induced under the surface of hydrogel instead of the surface limited damage induced by 200 fs pulses. Results will be compared to theoretical calculation for light-cornea interaction using a model developed at INRS. These experiments constitute an initial step toward the development of a new type of high precision surgical tool for corneal microsurgery.