Nonlinear optical properties of human cornea measured by spectral domain Z-scan method

In the myopia correction surgery by femtosecond laser, such as Laser in Situ Keratomileusis (LASIK) and Small Incision Lenticule Extraction (SMILE), the nonlinear refractive index of the cornea may cause the deviation of cutting depth. In order to improve the cornea cutting's accuracy and reduce the possibility of undercorrection, the nonlinear refractive index coefficient n₂ of the human cornea must be measured with high accuracy. The spectral domain Z-scan technique can measure n₂ of the highly scattering biological tissues with much better signal to noise ratio and thus better accuracy than the conventional methods. In this paper, the n₂ coefficient of one ex-vivo human corneal sample was measured by the spectral domain Z-scan technique. Experimental results show that as this corneal sample gradually dehydrates, its n₂ coefficients are 1.1 ± 0.1×10^-19 m²/W, 1.4 ± 0.2×10^-19 m²/W and 1.6 ± 0.2 ×10^-19 m²/W respectively for the corneal sample with water contents of 89%, 82%, and 78%. The increase of the water content reduces the value of n₂, which is reasonable since the nonlinear refractive index coefficient of water is one order of magnitude smaller than that of the cornea.