Effect of laser beam truncation (pinhole), (ordered) dithering, and jitter on residual smoothness after poly(methyl methacrylate) ablations, using a close-to-Gaussian beam profile

Smoother surfaces after laser vision correction have been widely accepted as a factor for improving visual recovery regardless of the used technique (PRK, LASIK, or even SMILE). We tested the impact of laser beam truncation, dithering (expressing a continuous profile on a basis of lower resolution causing pixels to round up/down the number of pulses to be placed), and jitter (a controlled random noise (up to ±20 µm in either direction) added to the theoretical scanner positions) on residual smoothness after Poly(methyl methacrylate) (PMMA) ablations, using a close-to-Gaussian beam profile. A modified SCHWIND AMARIS system has been used providing a beam profile with the following characteristics: close-to-Gaussian beam profile with full width at half maximum (FWHM) of 540 µm, 1050 Hz. Laser parameters have been optimized following Invest. Ophthalmol. Vis. Sci., vol. 58, no. 4, pp. 2021–2037, 2017, the pulse energy has been optimized following Biomed. Opt. Express vol. 4, pp. 1422–1433, 2013. For the PMMA ablations, two configurations (with a 0.7 mm pinhole and 0.75 mJ and without pinhole and 0.9 mJ (for fluences of 329 mJ/cm2 and 317 mJ/cm2 and corneal spot volumes of 174 and 188 pl)) were considered, along with two types of lattices (with and without ordered dithering to select the optimum pulse positions), and two types of spot placement (with and without jitter). Real ablations on PMMA (ranging from −12D to +6D with and without astigmatism of up to 3D) completed the study setup. The effect of the 2 × 2 × 2 different configurations was analyzed based on the roughness in ablation estimated from the root mean square error in ablation. Truncation of the beam is negatively associated to a higher level of residual roughness; ordered dithering to select the optimum pulse positions is positively associated to a lower level of residual roughness; jitter is negatively associated to a higher level of residual roughness. The effect of dithering was the largest, followed by truncation, and jitter had the lowest impact on results. So that: Dithering approaches help to further minimize residual roughness after ablation; minimum (or no) truncation of the beam is essential to minimize residual roughness after ablation; and jitter shall be avoided to minimize residual roughness after ablation. The proposed model can be used for optimization of laser systems used for ablation processes at relatively low cost and would directly improve the quality of results. Minimum (or no) truncation of the beam is essential to minimize residual roughness after ablation. Ordered dithering without jitter helps to further minimize residual roughness after ablation. Other more complex dithering approaches may further contribute to minimize residual roughness after ablation.