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Dispersion and temperature dependence of thermo-optic coefficients of optical materials over their whole transparency range: vectorial formalism and application to KTiOPO4

The temperature-dependent dispersion of the refractive index of optical materials is analyzed by introducing an appropriate formulation of what we call their "normalized thermo-optic coefficients" (NTOC). These parameters are obtained experimentally by performing accurate interferometric measurements of both thermal expansion and changes in optical thickness of parallelepipedic shaped samples of millimetric size which are submitted to a linear ramp of temperature. Changes in optical thickness are recorded as function of temperature at a few discrete laser wavelengths; they are expressed as power series of temperature and the use of a simple vectorial formalism allows then to determine the thermal behavior of the dispersion law over the whole transparency range of the material. The validity and reliability of the proposed method has been confirmed by modeling the temperature dependence of the type II second harmonic generation 1.064 μm - 0.532 μm in a KTiOPO4 single crystal, recorded up to 120°C.