What is an Etalon?

A Fabry-Perot etalon is the simplest form of a Fabry-Perot interferometer. Its primary optical property is that if a monochromatic light ray travels back and forth between two mirrors and the back-and-forth optical distance between the mirrors equals an integral number of wavelengths (λ, 2λ, 3λ, etc.), then the light passes through the etalon.

An etalon is one of the following:

Two very flat, very parallel mirrors (planar etalon). Almost all etalons are planar etalons.
Two identical spherical mirrors with their concave sides facing each other and with the distance between the mirrors equal to each mirror’s radius of curvature (confocal or spherical etalon). These are much less common and not covered on this website.