Femtosecond Laser Ablation Threshold Dependence On Corneal Depth And Laser Pulse Width

Abstract

Purpose: : Diode pumped all–solid–state ultrafast lasers are promising for mini–invasive refractive surgery and keratoplasty. A fundamental systematical study concerning ultrafast laser tissue interactions is crucial before applying the state of the art laser technology to ophthalmic applications.

Methods: : Porcine cornea samples were treated with three diode pumped all–solid–state ultrafast lasers, namely, a Nd:glass femtosecond laser, a Yb:KYW femtosecond laser, and a Nd:YAG picosecond laser. The pulse width of the ultrafast laser pulses ranges from 800 fs to 20 ps. The corneal ablation threshold as a function of the laser pulse width and the depth in the corneal stroma was precisely determined by simultaneous monitoring of the intensity of the laser–induced plasma and the second harmonic signals.

Results: : This study confirmed in a new way that the plasma mediated ablation in the cornea is a function of laser pulse width. Namely the plasma spark and the second harmonic generation signal were detected at the same time. The ablation threshold is proportional to the square root of the laser pulse width, which is consistent with previous studies. The dependence of the corneal ablation threshold on the depth of the stroma is not significant in the first 200 µm.

Conclusions: : We proposed a reliable and accurate method to determine the ablation threshold. Based on our experimental finding, an 800 fs laser pulse is optimal for intrastromal corneal surgery due to minimized thermal and nonlinear effects. Within the typical flap cutting depth (150 – 200 µm) for microkeratom–free LASIK, the corneal ablation threshold is nearly independent of the corneal depth.

Keywords: cornea: basic science • cornea: stroma and keratocytes • refractive surgery