Chapter 3 - Thermo-Optic Coefficients

Abstract

The refractive index of the optical materials is not a constant parameter over the temperature region in which the materials—such as crystals, semiconductors, and glasses are used in different optical devices or systems. The variation of the refractive index with the temperature at a constant pressure is called the “thermo-optic coefficient.” Its unit is per degree centigrade or Kelvin. Normally, the value of thermo-optic coefficient is very small. Despite the value being small, it is however possible to measure it with sufficient accuracy. The analyses of thermo-optic coefficients are essential to characterize the temperature dependent nonlinear optical devices, the optical fiber communication systems, semiconductor technology, and the ultrafast femtosecond technology. The measurements of the change in refractive index as a function of temperature for crystals and other optical materials are reported by various groups. In ionic materials with a low melting point, thermal expansion is high and the thermo-optic coefficient is negative. Whereas when the thermal expansion is small for some nonlinear crystals and has a high melting point, the thermo-optic coefficient is positive and dominated by the volume change in polarizability.