Wavelength division multiplexing apparatus

The present invention relates to a wavelength division multiplexing (WDM) apparatus that combines a plurality of optical signals of different wavelengths and optically amplifies the combined signal. WDM apparatuses are classified into two types: a non-transponder type which takes as inputs a plurality of narrowband optical signals of different wavelengths, combines them together, and optically amplifies the combined signal; and a transponder type which has at its front end a plurality of transponders that respectively convert a plurality of wideband optical signals of the same wavelength into a plurality of narrowband optical signals of different wavelengths. The present invention concerns both types of WDM apparatus. Disclosed is a wavelength division multiplexing apparatus comprising a plurality of variable attenuators () which respectively attenuate a plurality of optical signals of different wavelengths with variable amounts of attenuation, an optical combiner () which combines optical outputs of the plurality of variable attenuators, and an optical amplifier () which optically amplifies an optical output of the optical combiner, wherein in order to prevent light leaking from an unused wavelength from affecting the optical amplification of wavelengths in use, an optical switch () is provided in front of each variable attenuator and, for the unused wavelength, the optical switch is switched to the attenuation film side to secure a sufficient amount of attenuation.