Femtosecond Laser Direct Writing of Diffraction Gratings for Modifying the Refractive Index of Intraocular Lenses

Abstract

An advanced femtosecond laser experimental platform with high precision was developed for the reconstruction of the refractive index of intraocular lenses (IOLs), and its accuracy was rigorously evaluated. Diffraction gratings were inscribed on the surface of an acrylate sample utilizing a fiber femtosecond laser operating at a wavelength of 515 nm with a repetition rate of 40 MHz. The samples were subsequently measured using an Abbe refractometer to assess the alterations in their refractive index induced by the femtosecond laser scanning process. Scanning electron microscopy, confocal Raman microscopy, and X-ray photoelectron spectroscopy were employed to examine the morphology of the diffraction grating on the sample surface following femtosecond laser scanning. Additionally, these techniques were utilized to investigate the alterations in molecular structure within the material postlaser scanning, as well as to elucidate the underlying mechanisms responsible for changes in refractive index. Furthermore, the parameters of the femtosecond laser utilized in this study were compared with those of lasers commonly employed in clinical settings.

Figure 1

Schematic diagrams of one photon absorption and two photon absorption.

Figure 2

Schematic diagram of femtosecond laser engraving diffraction grating.

Figure 3

Zemax simulating femtosecond laser scanning to form a new lens within an IOL. Femtosecond lasers are incorporated into ophthalmic devices designed to emit visible light that sequentially traverses the anterior corneal surface, the posterior corneal surface, the aqueous humor, and the anterior surface of the IOL, ultimately reaching the center of the IOL, where the femtosecond laser creates a new diffraction lens.

Figure 4

Optical schematic diagram of the femtosecond laser direct writing processing system. The wavelength of 1030 nm emitted by the femtosecond laser is converted to 515 nm by the frequency doubling module for experimental processing.

Figure 5

Surface morphology observed by SEM. Each dot represents the result of the interaction between the pulse emitted by the femtosecond laser and the material surface, and the pulse energy is set the same in the same row, and the top-down femtosecond laser energy is gradually increased.

Figure 6

Bland–Altman plots for the comparisons between the values of point and line spacing. (a): point spacing, (b): line spacing.

Figure 7

Formation of laser-induced periodic surface structure on acrylic disc surface with a line spacing of 10 μm (Top view).

Figure 8

Microscopic Raman spectra of experimental samples before and after processing. The excitation wavelength was 532 nm. No new peaks appeared, indicating that the structure of the sample itself was not damaged by the femtosecond laser.

Figure 9

X-ray photoelectron spectroscopy (XPS) of experimental samples in the processed area (a) and the unprocessed area (b) showed that the content of C=O bonds and C–O single bonds increased after processing, suggesting that the hydrophilicity of the processed area increased.