Optical Vortex Beam for Gentle and Ultraprecise Intrastromal Corneal Dissection in Refractive Surgery

Abstract

Purpose: We introduce a novel focus shaping concept for intrastromal corneal dissection that facilitates cleavage along corneal lamellae, and we analyze laser-tissue interactions governing cutting effectiveness and mechanical side effects.

Methods: Focus shaping was achieved by a spiral phase plate that converts an incident Gaussian beam into a Laguerre-Gaussian beam with a helical phase. Such vortex beams have zero intensity at their center, are propagation invariant, and possess a ring focus equal in length to the Gaussian focus but with a larger diameter. Cutting precision and the required absorbed energy for flap dissection were compared for Gaussian and vortex beams on ex vivo porcine corneal specimens at pulse durations between 480 fs and 9 ps. Cutting quality and bubble formation were characterized by scanning electron microscopy and macro photography.

Results: With the vortex beam, the cuts were much smoother. Bubble formation was markedly reduced because cutting can be performed close to the bubble threshold, whereas with the Gaussian beam energies well above threshold are needed. Although the incident energy at the flap dissection threshold was slightly larger for the vortex beam, the absorbed energy was much smaller and contributed more effectively to cutting. This reduced plasma-induced pressure more than sevenfold.

Conclusions: The vortex beam approach for corneal dissection is a simple, versatile, and cost-effective way of improving the precision of refractive surgery while reducing bubble formation and pressure-related mechanical side effects.

Translational relevance: Phase plates for propagation invariant vortex beams are easily implemented in the beam path of next-generation clinical devices.

Keywords: flap cutting; focus shaping; laser dissection; lenticule dissection; refractive surgery.

Figure 1.

Schematic drawing of the elongated laser plasma of a Gaussian beam in side view, roughly indicating the direction of mechanical forces leading to cavity formation within the corneal lamellae (A). To improve cutting precision, it would be ideal to have a disk-like plasma orientated parallel to the corneal surface along the cutting direction and the cleavage lines given by corneal lamellae (B). This feature can be approximated by introducing a spiral phase mask with 2π phase shift (C) into the beam path that converts the linear polarized Gaussian beam exiting the laser into a vortex beam with helical phase (D). The foci of the Gaussian and vortex beam are presented in (E), with the vortex beam focus shown in the top view. It has a ring shape, with the same length in the axial direction as the focus of the Gaussian beam, but with a diameter two times larger.

Figure 2.

Experimental setup for the investigation of intrastromal laser dissection.

Figure 3.

Classification of flap cuts at different laser pulse energies for Gaussian (dots) and vortex (circles) beams at laser pulse durations of (A) 480 fs, (B) 3 ps, and (C) 8.8 ps. Cutting attempts where no flap lifting was possible are marked in red, hard flap lifting in orange, and easy flap lifting in green. For each measurement series, the lowest energy value for which easy flap lifting could be achieved is indicated next to the corresponding data point.

Figure 4.

SEM images of 6-mm flaps in porcine cornea that were cut using Gaussian (left column) and vortex (right column) beams with 1030-nm wavelength and 480-fs laser pulse duration. Pulse energies were slightly above the threshold for easy flap lifting.

Figure 5.

Photographs of the laser-induced gas bubble layer after flap dissection with Gaussian (left column) and vortex (right column) beams with 1030-nm wavelength and 480-fs laser pulse duration. Photographs in the upper row were taken through the focusing objective less than 1 minute after flap cutting. Photographs in the lower row were taken 10 minutes after flap cutting using a stereo microscope.

Figure 6.

Thresholds for flap cutting (green) and bubble formation in water (blue) for Gaussian (dots) and vortex (circles) beams at 1030-nm wavelength and pulse durations of 480 fs, 3 ps, and 8.8 ps. NA = 0.38 in cornea; NA = 0.4 in water.

Figure 7.

Laser energy transmission during intrastromal flap cutting for Gaussian and vortex beams at laser pulse durations of (A) 480 fs, (B) 3 ps, and (C) 8.8 ps. For each parameter set, two different porcine corneas were investigated (red and black dots). The thresholds for visible bubble formation in cornea are shown as gray dots (Gaussian) and gray circles (vortex). The energy values required for flap cutting are marked with green arrows, and the corresponding transmission values are indicated.

Figure 8.

Absorbed laser pulse energy at E_th,cut during flap cutting in porcine cornea with Gaussian (dots) and vortex (circles) beams.

Figure 9.

Temperature (A) and pressure (B) as a function of internal energy for a mass density ρ₀ = 1000 kg m^–3, according to the IAPWS-95 formulation of the EOS of water.