An etalon is an optical interferometer in which a beam of light undergoes multiple reflections between two reflecting surfaces, and the resulting optical transmission (or reflection) is periodic in wavelength. Etalons can be considered a narrowband wavelength filter and can be used as precise wavelength references in telecommunication applications where the periodicity of the signal provides an array of reference frequencies for the telecommunication frequency grid. When used with active feedback electronics, etalons form the basis of wavelength locking systems that can be used to stabilize the wavelength of a laser.

The typical etalon types CRYLIGHT offers are air-spaced and solid etalons made from either optical glass or silicon material.

Etalons are widely used in telocommunication, lasers and spectroscopy to control and measure the wavelengths of light.