US Patent 7653311 Optical wavelength add-drop multiplexer

Part of an inputted optical add signal 118 is reflected by a mirror 117, and is thereby inputted into an optical wavelength multiplexer 105 in the reverse direction so that the optical add signal is returned to paths 115-1 through 115-16 corresponding to wavelengths λ1 through λ16. If the returned optical add signal is an optical add signal having a correct wavelength, the optical signal enters its corresponding backward direction optical detector 113-16. Accordingly, it is possible to check whether or not a wavelength of the optical add signal is correct.