Intratissue Refractive Index Shaping (IRIS) of the Cornea and Lens Using a Low-Pulse-Energy Femtosecond Laser Oscillator

Abstract

To assess the optical effect of high-repetition-rate, low-energy femtosecond laser pulses on lightly fixed corneas and lenses. Eight corneas and eight lenses were extracted postmortem from normal, adult cats. They were lightly fixed and stored in a solution that minimized swelling and opacification. An 800-nm Ti:Sapphire femtosecond laser oscillator with a 27-fs pulse duration and 93-MHz repetition rate was used to inscribe gratings consisting of 20 to 40 lines, each 1-microm wide, 100-microm long, and 5-microm apart, 100 mum below the tissue surface. Refractive index changes in the micromachined regions were calculated immediately and after 1 month of storage by measuring the intensity distribution of diffracted light when the gratings were irradiated with a 632.8-nm He-Ne laser. Periodic gratings were created in the stromal layer of the corneas and the cortex of the lenses by adjusting the laser pulse energy until visible plasma luminescence and bubbles were no longer generated. The gratings had low scattering loss and could only be visualized using phase microscopy. Refractive index changes measured 0.005 +/- 0.001 to 0.01 +/- 0.001 in corneal tissue and 0.015 +/- 0.001 to 0.021 +/- 0.001 in the lenses. The gratings and refractive index changes were preserved after storing the micromachined corneas and lenses for 1 month. These pilot experiments demonstrate a novel application of low-pulse-energy, MHz femtosecond lasers in modifying the refractive index of transparent ocular tissues without apparent tissue destruction. Although it remains to be verified in living tissues, the stability of this effect suggests that the observed modifications are due to long-term molecular and/or structural changes.