FabryPerot

The Fabry–Pérot interferometer, commonly called an etalon, is an optical cavity formed by two parallel, highly reflective mirrors separated by a fixed distance. Light entering the cavity undergoes multiple reflections, and the transmitted or reflected light results from interference among the multiple beams. The device can act as a tunable filter or as a high-resolution spectroscopic instrument, depending on mirror reflectivity and spacing.

Named for Charles Fabry and Alfred Pérot, who devised and described the device in the late 19th century, the Fabry–Pérot interferometer became a widely used tool for resolving closely spaced spectral lines. Its ability to produce very sharp transmission peaks makes it valuable in spectroscopy and wavelength selection.

Constructive interference occurs when the round-trip phase is an integer multiple of 2π. In a uniform medium of index n and at normal incidence, the resonance condition is 2 n d = m λ, where d is the plate separation and m is an integer. The separation between successive transmission peaks, the free spectral range, is roughly FSR ≈ λ^2 /(2 n d). The sharpness of the peaks is quantified by the finesse F ≈ π sqrt(R) /(1−R), with R the mirror reflectivity. Higher reflectivity yields narrower peaks but lower peak transmission. The overall spectral response is a comb of narrow peaks separated by the FSR.

Variants include air-spaced etalons and solid or dielectric-coated mirrors, including fiber-coupled versions for sensing. The Fabry–Pérot cavity is used in high-resolution spectroscopy, laser stabilization, optical filtering in telecommunications, and precision metrology. It also serves as a component in laser cavities and in scanning tunable filters and cavity-based sensors.