An etalon is an optical interferometer in which a beam of light undergoes multiple reflections between two reflecting surfaces, and whose resulting optical transmission (or reflection) is periodic in wavelength. Actually it is narrowband wavelength filters. Etalons 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 Photop can offer are Air-space and Solid.