The unit cell of high temperature superconductor YBa 2 Cu 3 O 7 is presented as two thin highly conducting CuO 2 layers separated by Y and Ba insulator layers. Neglecting electron hopping between conducting layers the dielectric function ϵ(q, ω) for arbitrary wave vector q and frequency ω is calculated in the random phase approximation. Interaction between collective electron excitations and lattice vibrations is taken into account and mixed polar phonon-plasmon modes dispersion relations are deduced. In the framework of developed model the anomalous infrared, the inelastic neutron scattering and Cu nuclear magnetic relaxation data are discussed.