Lithium niobate (${\mathrm{LiNbO}}_3$) is a key anisotropic material for midinfrared photonics, nonlinear optics, and acousto-optic applications, where accurate dielectric function (DF) models are required for optical simulations under arbitrary crystallographic orientations. In this work, an anisotropic DF model for commercial X-, ${\text{Y-128}}^{\circ }$, and Z-cut ${\mathrm{LiNbO}}_3$ substrates is developed in the infrared (IR) spectral range using a factorized transverse optical and longitudinal optical (TO-LO) formalism. The model is obtained through spectroscopic ellipsometry (SE) supported by polarized Raman spectroscopy and IR reflectance measurements. These complementary techniques enable the identification and cross-validation of phonon modes associated with the in-plane $E\left(x\right)$ and out-of-plane $A_1\left(z\right)$ components of the dielectric tensor (DT). A single parametrized DT consistently reproduces the optical response of all analyzed crystal cuts through appropriate Euler-angle rotations, including different azimuthal configurations that exhibit distinct birefringent responses. The proposed approach provides a Kramers-Kronig consistent description of the dielectric response through a physically constrained parametrization, reducing parameter correlations during optical modeling. The resulting DF is presented in analytical form over the $230-1200{\mathrm{cm}}^{-1}$ spectral range and is directly applicable to integrated photonics, acousto-optic, and nonlinear optical device simulations in the IR.

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• Dielectric properties
• Ellipsometry
• Raman spectroscopy