Intra-tissue Refractive Index Shaping (IRIS) of the cornea and lens using a low-pulse-energy femtosecond laser oscillator

¹ Institute of Optics, University of Rochester, Rochester, New York, United States
² Ophthalmology, University of Rochester, Rochester, New York, United States
³ Ophthalmology, University of Rochester Med Ctr, 601 Elmwood Ave, Box 314, Rochester, New York, 14642, United States

^* To whom correspondence should be addressed. E-mail: huxlin{at}cvs.rochester.edu.

	Abstract

PPurpose: To assess the optical effect of high-repetition-rate, low energy femtosecond laser pulses on lightly-fixed corneas and lenses. Methods: Eight corneas and eight lenses were extracted post-mortem from normal, adult cats. They were lightly fixed and stored in a solution that minimized swelling and opacification. An 800nm Ti:Sapphire femtosecond laser oscillator with a 27fs pulse duration and 93MHz repetition rate was used to inscribe gratings consisting of 20-40 lines, each 1 micron wide, 100 microns long, 5 microns apart and 100 microns below the tissue surface. Refractive index changes in the micromachined regions were calculated immediately and after one month of storage by measuring the intensity distribution of diffracted light when the gratings were irradiated with a 632.8nm He-Ne laser. Results: Periodic gratings were created into 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 one month. Conclusions: 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.

Key Words: corneal stroma, lens ultrastructure, optics, refractive surgery, femtosecond laser

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