[en] The results of studies of optical reflection in the far- and mid-infrared spectral regions are reported. The reflectance of five Bi_2Se_3 topological insulator films grown by molecular-beam epitaxy on Si(111) substrates is measured. The characteristic parameters of phonons and plasmons are determined by means of dispersion analysis for multilayer structures. It is found that the plasma frequency in a layer close to the Si–film interface is noticeably higher than that in the film bulk. Calculations of the loss function show that plasmon–phonon coupling plays an important role in Bi_2Se_3 films. The attenuated total internal reflection method is used to determine the frequency of the surface plasmon–phonon mode.