Fabry-Pérot interferometer is a multi-beam interferometer composed of two parallel glass plates, in which the opposite inner surfaces of two glass plates have high reflectivity. When two glass plates are separated and fixed by a hollow spacer with a fixed length, it is also called Fabry-Perot etalon or simply referred to as etalon.

F-P etalon is a high-resolution spectroscopic instrument because of its high reflectivity and narrow oblique interference fringes such as multiple beams. It can be used for high-resolution spectroscopy and studying spectral lines with very close wavelengths, such as isotope spectra of elements, hyperfine structure of spectra, slight frequency shift during light scattering, Doppler shift of spectral lines caused by atomic movement, and internal structure and shape of spectral lines. It can also be used as a high-resolution optical filter to construct a precise wavemeter. In laser system, it is often used for intracavity narrow spectrum or single mode operation of laser system. It can be used as bandwidth control and tuner device in broadband picosecond laser to analyze and detect spectral components (longitudinal mode and transverse mode) in laser.