Self-focusing and spherical aberrations in corneal tissue during photodisruption by femtosecond laser

Abstract

Abstract. The use of ultrashort pulse lasers is current in refractive surgery and has recently been extended to corneal grafting ͑kerato-plasty͒. When performing keratoplasty, however, permanent degradation of the optical properties of the patient's cornea compromises the penetration depth of the laser and the quality of the incisions, therefore causing unwanted secondary effects. Additionally, corneal grafting needs considerably higher penetration depths than refractive surgery. Little data are available about the interaction processes of the femtosecond pulses in the volume of pathological corneas-i.e., in the presence of spherical aberrations and optical scattering. We investigate the influence of the focusing numerical aperture on the lasertissue interaction. We point out that at low numerical apertures ͑NAs͒, tissue damage is produced below and above the focal region. We attribute this phenomenon to nonlinear self-focusing effects. On the other hand, at high NAs, spherical aberrations become significant when focusing at high depths for posterior surgeries, which also limit the cutting efficiency. As high NAs are advisable for reducing unwanted nonlinear effects and ensure accurate cutting, particular attention should be paid to aberration management when developing clinical femtosecond lasers.

Determination of scattering properties and damage thresholds in tissue using ultrafast laser ablation

“…ThermoFisher) in an agar phantom. The NA is chosen to reduce the effects of spherical aberration, which can decrease the Strehl ratio as ablation depth is increased, 21 and to avoid self-focusing, which can cause damage before the intended focus or create a tighter focus, and lead to an underestimation of the ablation fluence threshold. 22,23 We ablate a 40 × 40 μm 2 FoV by scanning 512 lines at 1/16 frames per second, causing ∼0.078 μm separation between lines and ∼1.28 μm separation between pulses on the same line, resulting in a ∼20 pulse overlap.…”

Optical side-effects of fs-laser treatment in refractive surgery investigated by means of a model eye

“…Although it has been shown that solid model systems such as gelatin or fused silica [34,35] are better suited than water [36], for example, they do not account for inhomogeneities of the corneal tissue and inter-subject variations [36,37]. Especially, nonlinear phenomena such as self-focusing depend critically on actual tissue properties and focusing conditions [36,38]. Furthermore, a recent study has shown that there is a difference in visual outcome between laser patterns which are centrifugally or centripetally applied [39].…”

Interface quality of endothelial keratoplasty buttons obtained with optimised femtosecond laser settings

“…The optical aberrations accumulate proportionally as the beam is focused deeper through the stromal layers,13 by nonlinear self-focusing effects 14. This jeopardises the laser efficacy at cutting and probably initiated the adherences observed in groups 1 to 4.…”