Contribution of thermoelastic stresses to dn/dT of hexagonal and trigonal crystals heated by laser radiation

Analytic expressions are obtained for thermoelastic stresses. A calculation is given of the contribution of these stresses to the refractive index of hexagonal and trigonal crystals heated locally by a laser beam traveling along the optic axis. Numerical estimates are given for z-cut LiNbO₃ crystals, which are frequently used as optical modulators. It is shown that the thermoelastic stresses in LiNbO₃ increase the temperature coefficient of the refractive index dn/dT by 47% compared with the homogeneous heating case.