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

Abstract

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.

Optimization of the Spot Spacings for Reducing Roughness in Laser-Induced Optical Breakdown Processes for Corneal Laser Vision Correction

“…This has been also empirically demonstrated for femtosecond systems [17,18]. Both correspond to our simulation findings and also shared in laboratory experiences with excimer ablation procedures [19]. But it is not only the energy that is responsible for roughness; spot placement (spacings) also plays an important role, as demonstrated for excimer based corneal ablations [20,21].…”

Method for assessing the impact of residual roughness after corneal ablation simulated as random and filtered noise in polychromatic vision

“…The lasers used in micromachining are normally pulsed excimer lasers, where the duration of the pulses is very short compared to the time period between the pulses. Despite attempts for achieving high levels of ablation smoothness 18,19,20,21 laser corneal refractive surgery still presents some challenges in terms of the residual roughness associated with the cornea postoperatively (Figure 1 20 ). This residual roughness may influence the epithelium to respond in a way that produces haze, regression and reduced accuracy of refractive correction, to degrade overall visual performance 22 .…”

“…Finally, in the third frame, the residual roughness remains that is a combination of the residual roughness of the previous frames, limited by the maximum ablation depth at each location. In this manner, the residual roughness depends on several factors like the laser beam characteristics (like spot energy, spot diameter, super Gaussian order, truncation radius, spot geometry), spot overlap and the lattice geometry used in ablation 20 . Vinciguerra et al used a Nidek Eas-1000 Anterior Eye Segment Analysis System to examine the ablated surface or the interface regularity immediately after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) 23 .…”