The refractive indices of the potassium titanyl phosphate (KTP) crystal have been measured in the temperature interval 20- 550 °C. It is shown that their values increase as temperature increases, and this is because the lattice contribution predominates. The curves showing the temperature dependence of the refractive indices for various wavelengths are not parallel to each other, and, as a consequence, the thermooptical coefficients Dn/Dt for a violet wavelength are almost a factor of two greater than for red. Dispersion curves are presented for the refractive indices and thermooptical coefficients.
I. T. Bodnar' and A. U. Sheleg, "Refractive indices of potassium titanyl phosphate: Dispersion and temperature dependence," J. Opt. Technol. 70, 673-675 (2003)