| Purpose: Potential applications of the femtosecond laser to corneal surgery are very promising. The goal of this study is to describe femtosecond laser ablation thresholds of different corneal layers and nonbiological transparent materials, such as poly-methacrylates (PMA) – used for soft contact lens fabrication – for various water concentration, as a function of laser pulse duration and wavelength.
Methods: The INRS Titanium : Sapphire laser system with a pulse duration of 60 fs (femtosecond), a center wavelength of 800 nm, a repetition rate of 10 Hz, a maximum energy of 600 mJ, corresponding to a peak intensity of 10 TW – was used for this investigation. The laser pulse was focused using a lens (focal length of 88 mm) on the surface of the sample – a freshly enucluated pig eye or a plastic sample rehydrated in a saline solution. A silicon photodiode (Thorlabs) with appropriate filters to cut-off the laser primary wavelength was placed in front of the sample to collect photons emitted from plasma recombination. The ablation threshold was measured by varying the laser energy to obtain full photodiode signal.
Results: For pulse duration of 80 fs, the ablation thresholds of the endothelium, epithelium and stroma were constant (a mean value of 0.4 J/cm2). For longer pulses, the ablation threshold of the endothelium remained constant for 500 fs pulses and increased for 3 ps pulses (0.8 J/cm2). For the epithelium, the ablation threshold increased for 500 fs (0.6 J/cm2) and 3 ps pulses (1.6 J/cm2). For PMA, the ablation threshold for 80 fs pulses was 0.5 J/cm2 for dry material and increased respectively to 1.5 J/cm2 and 1.6 J/cm2 for hydrated materials (42 and 55% of water). At 3ps, the ablation thresholds were 1.0 J/cm2, 1.0 J/cm2 , and 1.5 J/cm2 for water concentrations of 0%, 42%, and 55%, respectively. For energy values in the same range but longer pulse duration (270 ps), small cavities were induced under the surface of PMA instead of surface damage. Conclusions: Ablation threshold of cornea and PMA were investigated for two pulse durations: 80 fs and 3 ps. The ablation threshold of PMA increases with the concentration of water. Ultrashort pulses (80 fs) are shown to reduce corneal damage because the ablation threshold is lower than for longer pulses (3 ps). However, the ablation threshold for hydated PMA does not seem to increase with the pulse duration as for the cornea. More investigations are needed to determine to what extent PMA can be used as a useful model for the cornea.
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