Lens effect in an open resonator with an inhomogeneity in the form of a plane parallel layer

The focusing effect of an inhomogeneity in the form of a plane parallel layer inside an open resonator is investigated, and it is shown that such an inhomogeneity leads to changes in the resonance beam parameters. Depending on its parameters and its position within the resonator, such an inhomogeneity can behave like a focusing or a defocusing lens. In particular, it can lead to a rarefaction of the resonator spectrum and give rise to new modes that are not typical of an empty (no inhomogeneities) resonator. It is also shown that such an inhomogeneity can be used to control the energy characteristics of the resonator, particularly its diffraction losses.