WAVELENGTH MULTIPLEXER

WDM apparatus which can take corrective action by quickly and accurately detecting a wavelength deviation. To detect a wavelength deviation accurately and quickly, the detection of a wavelength deviation is performed in the wavelength locker (26) situated in the transponder and before the variable attenuator VAT (10). More specifically, the absolute value of difference between the optical power of the light transmitted through the optical filter (36) and that of the light transmitted through the optical filter (38), represents the magnitude of the wavelength deviation. The apparatus controller (60) receives this value from the wavelength locker (26) and, if this value exceeds a predetermined value, then determines that the wavelength deviation has exceeded the allowable level and instructs the SAU (16) to set the amount of attenuation in the corresponding VATT (12) to a maximum value.