In situ Determination of the Laser Attenuation and Ablation Threshold by Second Harmonic Generation in Human Cornea for Femtosecond Laser Eye Surgery

Abstract

Purpose: : By measuring the second harmonic signal emitted by the cornea, when illuminated by a Nd:glass femtosecond laser (=1.06 µm), we are able to quantify the attenuation of the laser intensity in the tissue. Complementary analyses were performed on the samples by ultrastructural histology.

Methods: : The femtosecond laser is a CPA system with a regenerative amplifier delivering pulses at a wavelength of 1.06 µm, pulse durations of 400 fs and a maxi[[Unsupported Character – Codename ­]]mum energy of 60 µJ. The repetition rate is adjustable from single shot up to 10 kHz. The delivery system provides spot sizes down to the micron range and peak power densities well above 10¹⁵W/cm². The human corneas are mounted on an anterior chamber system, installed on a computer controlled step motor positioning system. This set–up permits to treat cornea in situ, reproducing the in vivo environment of the eye. A photomultiplier tube captures photons generated by non–linear processes. A lock–in amplifier tuned to the laser repetition rate and recorded by a computer filters the signal.

Results: : The predominant non–linear effect generated by collagen, under the action of fs pulses, is the SH light. 2D graphs, acquired by scanning the sample, showed changes in the SHG intensity depending on the cornea spot lighted. Data indicated that laser beam is attenuated exponentially in the sample and permitted us to measure the attenuation parameters varying with the optical properties of the cornea. A histological study of the samples revealed that the laser penetrates less deep and the ablation threshold is higher when the tissue has lost his transparency and presents oedemas.

Conclusions: : The SH generated by the collagen when exposed to a femtosecond laser can be used to better understand the interaction of the laser with the corneal tissue. The in situ quantification of the laser attenuation permits to tune the laser parameters to the ablation threshold for keratoplasty.

Keywords: laser • optical properties • microscopy: electron microscopy