Diffractive smoothing of excimer laser ablation using a defocused beam

OBJECTIVE: To determine if excimer laser myopic ablation with a defocused laser image produces a smoother ablation profile than does focused laser light. METHODS: An ArF excimer laser was used to ablate a 5.00-diopter myopic correction in test blocks using both a contracting and expanding iris aperture. Defocused ablation was performed using a contracting iris aperture by translating the target away from the laser source. A confocal laser scanning microscope was used to analyze the surface smoothness at 55x and 275x magnifications. RESULTS: The confocal laser scanning micrographs revealed a series of sharply demarcated concentric ridges in the focused ablation, and less prominent, slightly wavy lines in the defocused ablation performed with a contracting aperture. The focused ablation with an expanding aperture also created concentric ridges toward the periphery, but with slightly smoother edges. CONCLUSION: Micrographic analysis of the surface profile of excimer laser myopic ablation using a variable iris aperture revealed the presence of concentric ridges following focused ablation and smooth curvilinear lines when ablation was performed with a defocused image.