Temperature dispersion of refractive indices in β-BaB2O4 and LiB3O5 crystals for nonlinear optical devices

Abstract

The thermo-optic coefficients, i.e., the variation of refractive index with temperature (dn/dT), determine the temperature characteristics of nonlinear optical laser devices. A lack of proper dn/dT data prevents the assessment of the temperature-tuning behavior of various parametric three-wave interactions in barium borate and lithium triborate crystals. These dn/dT values are analyzed critically by use of a physical model based on the thermal expansion coefficient, the temperature coefficient of the excitonic band gap, and an isentropic band gap. The computed optical constants from the thermo-optic coefficients and the room temperature Sellmeier coefficients are used to calculate the refractive indices for any operating temperature and wavelength along the principal axes of both crystals. Temperature sensitivities are evaluated and thoroughly compared to the experimental values. Also, this study interprets the existing nonlinear optical devices, in particular, the recently introduced femtosecond traveling-wave parametric generations in a lithium triborate crystal, more accurately expressed in the range of −40 to 320 °C. A type I second-harmonic generation of a 1.48 μm laser diode with an output power of 100 mW is observed at a noncritical phase-matching temperature of 50 °C in this crystal. It is confirmed here that the thermo-optic coefficients are both wavelength and temperature dependent and the data at the band edge are especially important to account for the temperature-tuning phase-matched characteristics.