Fabry-perot etalon with evaporated schott glass as a spacer-layer.
Fabry-Perot etalons have long been in use and consist principally of two reflectors placed parallel to each other with an error smaller than λ/30 (see Ref. 1). Such an ar­ rangement results in an optical resonator with definite frequency eigenvalues. If the reflecting surfaces are, as is usual, only slightly transparent, e.g., a few percent, the light falling onto either surface passes the optical resona­ tor (i.e., the etalon) almost undiminished in the case of resonance. Light of other wavelengths is reflected. The spectral distance of two transmission maxima ∆λ is deter­ mined by