FEMTOSECOND-PULSE LASER-ABLATION OF HUMAN CORNEAS

A femtosecond pulse laser in the visible spectral region shows promise as a potentially new powerful corneal sculpting tool. It combines the clinical and technical advantages of visible wavelengths with the high ablation quality observed with nanosecond-pulse excimer lasers at 193 nm. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs and 7 ns, and centre wavelengths at 615 nm and 600 nm, respectively, both focused to an area of the order of 10(-5) CM2, have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30-100 mum range from the excision edge at all fluences above the ablation threshold of F(th) almost-equal-to 60 J CM-2 (I(th) almost-equal-to 9 GW cm-2). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterised by damage zones of less than 0.5 mum were observed at all fluences above ablation threshold of F(th) almost-equal-to 1 J cm-2 or I(th) almost-equal-to 3 TW cm-2 (3 x 10(12) W cm-2). The transparent cornea material can be forced to absorb ultrashort pulses of extremely high intensity. The fs laser generates its own absorption by a multiphoton absorption process.