Abstract
Purpose :
The accuracy of optical change induced by an excimer laser can be achieved by precise removal of corneal tissue with a scanning LASIK system. The precision is based on the depth and shape of the ablation crater for a single pulse and accurate lateral coordinate placement. The size and shape of a single ablation crater is determined by the diameter and energy. The lateral coordinate precision requirement can be easily achieved with galvanometer scanners. Accuracy of depth, programmable volume of ablation, and specific shape of the crater with methods of precise measurement are in the scope of this study for visual correction.
Methods :
A homogenous pulse is required for uniform corneal ablation. The raw excimer laser pulse is cut into 36 square pieces by a homogenizer. These squares are directed to overlap at a round aperture mask, allowing for uniform fluence with 4% error. Each pulse is rotated for further averaging. Each of 36 beamlets can be deflected at a specific angle to achieve programmable shape of ablation in plastic. For corneal measurements we used full field tissue profilometer using white light interferometric measurements. A flap of porcine cornea 100 micrometers in thickness was made on iFS device, J&J Vision, placed on mirror, and submerged in oil-based fluid with known refractive index 1.4700, and then covered with glass cover slip. White light interferometer measures the Optical Path Difference (OPD) in this cuvette with 30nm precision.
Results :
For given shape of the ablation in plastic, the programmable profile of fluence can be constructed to achieve the target shape with a few percent error, including the edge effects of the ablation influence on central island effect. Results for ablation rate in the center was 0.36 um/pulse, which is in remarkable agreement with published value 0.37 um/pulse, measured with shadow photography. Settings for iFS flap was 100 micrometers and measured on profilometer as 106 micrometers in thickness, 10 um peak to valley.
Conclusions :
Accuracy of the programmable ablation shape in plastic can be achieved with 2% error. Full field tissue profilometry with at least 300nm precision is demonstrated on porcine cornea. Profilometer can be used for characterization of LASIK and SMILE laser systems on tissue and have good agreement to independent measurements.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.