Influence of the Overlap of a 1.0 mm Gaussian Beam Flying Spot Excimer Laser on Surface Roughness

Abstract

Abstract: : Purpose: The use of flying spot Excimer lasers may create surfaces with a significantly increased surface roughness and waviness compared to formerly used broad beam lasers. Goal of this study was to assess the roughness for different overlaps of a 1.0 mm Gaussian beam flying spot Excimer laser in order to create surfaces with a minimally increased roughness and waviness. Methods: With a flying spot Excimer laser (Schwind ESIRIS, SCHWIND, Germany) PTK treatments up to 250 µm in depth were performed on PMMA plates, as well as on porcine corneas. Overlap was varied between 12.5% and 92.5% in steps of 10% for laser fluences between 0.37 and 0.6 and different local ablation frequencies with an ablation rate of 0.15 µm per pulse in PMMA and 0.53 µm per pulse in porcine cornea. Results: The surface roughness for a PTK ablation in PMMA plates (porcine cornea) varied between 0.26 and 0.49 µm (2.75 and 5.15 µm) for a 50 µm deep ablation and 0.65 to 1.12 µm for a 250 µm deep ablation. Waviness in PMMA plates varied between 4 and 12 µm for a 50 µm deep ablation (overlap 42.5%). In PMMA the minimal surface roughness was found for an overlap of 72.5%. Increased fluence was associated with decreased surface roughness as well as decreased surface waviness. Local ablation frequencies above 100 Hz changed the shape of the ablation (less ablation in the center of the PTK).The surface roughness between ablated and non-ablated PMMA-balls, as well as porcine corneas was increased by a factor between 0.9 and 2.0 (PMMA) and 1.9 to 2.7 (porcine cornea). Conclusions: The theoretically smoothest surface is expected for an overlap of close to 1. A potential problem occurs when the local ablation frequency gets in the order of or over 100 Hz, as this is associated with an increased surface waviness due to an additive effect of the ablation rate. An overlap of close to 1 would also bring the need for a different calibration of the laser system, switching from calibrating per layer to calibrating per pulse, as with an overlap of close to 1 the ablation rate per layer is higher as the ablation rate per pulse.

Keywords: refractive surgery: optical quality • laser • cornea: basic science