Nonlinear refractive index of porcine cornea studied by z-scan and self-focusing during femtosecond laser processing

Abstract

We have investigated the nonlinear refractive index of ex-vivo pig cornea by a combined approach using the standard z-scan technique on extracted corneas or corneal slices, as well as studying the deviations caused by self-focusing during femtosecond laser processing of the pig eyes. The experiments yield consistently an upper limit of 1.2 MW for the critical power of self-focusing in porcine cornea, and a value of 2 x 10(-19) m(2)/W for its nonlinear refractive index. We also demonstrate that due to this nonlinear refraction the cutting depth of typical fs-laser surgery processing in cornea may depend considerably, albeit in a well controllable way, on the laser parameters.

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].…”

Assessing the phase retardation in corneal tissues using a femtosecond laser

“…These high intensities are capable of changing the optical properties of the tissue, including birefringence. 21 Therefore, it is important to measure the birefringence using a clinically relevant FSL. Additionally, our intention is not to map or image the corneal birefringence, but rather to sample the central cornea to generate a reference point for the laser parameters associated with this FSL.…”

Applanation-free femtosecond laser processing of the cornea

“…It has recently been shown that the pig cornea has a nonlinear refractive index of 2·10 −19 m 2 /W, which is relatively high, compared to water [9]. This means that a precise calculation of the focal position can only be done by taking into account self-focusing effects.…”