An optical structure containing a pair of mirrors at opposite ends of a cavity. Light reflects back and forth between the mirrors, and one or both transmit a fraction of the resonant frequency. The resonance is created by making the distance of one round trip between mirrors equal to an integral number of wavelengths of the cavity material. Optically speaking, this is an interferometer, because it relies on the interference of light for its operation. A Fabry-Perot device becomes a light generator when a laser medium is used in the cavity, otherwise it is a passive filter. See distributed feedback laser and etalon.