Abstract: As a critical material system for infrared detectors, T2SL effectively mitigates non-radiative recombination by spatially separating electrons and holes. However, this separation also reduces the overlap between electron and hole wave functions, typically resulting in a diminished absorption coefficient. In this context, thin-film interference effects become highly significant in T2SL multilayer structures, yet analytical studies on such optical systems remain relatively scarce. In this study, we conduct a systematic investigation of T2SL-based multilayer stacks using reflectance measurements obtained with a Fourier-transform infrared spectrometer. The experimental findings reveal that the homoepitaxial buffer layer and the substrate do not form an optically abrupt interface, indicating that the buffer layer does not exhibit substantial interference coupling with the T2SL absorber layer. This observation not only supports the reasonable simplification of the buffer layer in optical simulations of T2SL devices but also provides an experimental foundation for analyzing the influence of thin-film interference on material and device performance. Furthermore, by leveraging the thin-film interference phenomenon, we have successfully extracted the average refractive index of In_0.8Ga_0.2As/In_0.2Ga_0.8Sb T2SL grown on an InAs substrate within the 5000-1000 cm⁻¹ range, yielding an approximate value of 3.7.