Patent ID: 7466478

Claim:
An optical-level control method, comprising: demultiplexing a wavelength division multiplexed signal into optical signals corresponding to each wavelength of the wavelength division multiplexed signal, and supplying each optical signal to a respective variable optical attenuator of a plurality of variable optical attenuators, corresponding to the wavelength of the optical signal, the respective variable optical attenuator outputting an output optical signal having an optical level of a constant value; detecting a loss of light of an optical signal when an optical level of said optical signal directly input to said respective variable optical attenuator for the wavelength corresponding to the wavelength of the optical signal, is lower than a predetermined threshold; setting an attenuation quantity of said variable optical attenuator for the wavelength for which said loss of light has been detected, at a fixed value determined in advance, when said loss of light is detected; and controlling said variable optical attenuator for the wavelength for which said loss of light has been detected, to return to said constant value of the optical level of the output optical signal, after a predetermined period of time has lapsed.