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 super(-5) cm super(2), have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30-100 mu m range from the excision edge at all fluences above the ablation threshold of F sub(th)) 60 J cm super(-2) (I sub(th)) 9 GW cm super(-2)). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterized by damage zones of less than 0.5 mu m were observed at all fluences above ablation threshold of F sub(th)) 1 J cm super(-2) or I sub(th)) 3 TW cm super(-2) (3x10 super(12) cm super(-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.