Abstract
A method for calculating the average refractive index of a sample using transmission terahertz time‐domain spectroscopy in a single measurement is presented. The refractive index is derived from the analysis of the frequency‐domain interference caused by Fabry‐Pérot reflections in the sample through the discrete Fourier transform of the spectrum module. This approach is simple and fast (not requiring a reference measurement), improves sensitivity over direct time‐domain analysis and allows the derivation of the refractive index in a specific frequency window. The method can also be used to identify unwanted reflections in the experimental setup.