Patent ID: 7843631

Claim:
A gain control apparatus which controls a gain of an optical amplifier made to amplify a wavelength-multiplexed signal light including a plurality of signal lights at different wavelengths that are wavelength division multiplexed, comprising: a target gain calculating unit to calculate, as a target gain for said optical amplifier, a substantially constant gain value in a case in which the number of signal lights at different wavelengths that are wavelength division multiplexed in the wavelength-multiplexed signal light is larger than a threshold set in advance, and to calculate, as a target gain for said optical amplifier, and based on characteristics of said optical amplifier, a value obtained by one of: increasing a gain value of the wavelength-multiplexed signal light according to the number of signal lights at different wavelengths that are wavelength division multiplexed together in the wavelength-multiplexed signal light in a case in which the number of signal lights at different wavelengths that are wavelength division multiplexed in the wavelength-multiplexed signal light is less than the threshold, and decreasing a gain value of the wavelength-multiplexed signal light according to the number of signal lights at different wavelengths that are wavelength division multiplexed together in the wavelength-multiplexed signal light in the case in which the number of signal lights at different wavelengths that are wavelength division multiplexed in the wavelength-multiplexed signal light is less than the threshold; and a control signal outputting unit to output a control signal to said optical amplifier so that the wavelength-multiplexed signal light is amplified with the target gain calculated by said target gain calculating unit, wherein said target gain calculating unit includes a number-of-wavelengths information acquiring unit to demultiplex the wavelength-multiplexed signal light inputted to said optical amplifier or outputted from said optical amplifier into the plurality of signal lights at different wavelengths, to monitor a power of each of the demultiplexed plurality of signal lights at different wavelengths and, based on the monitored power, to acquire the number of signal lights at different wavelengths that are wavelength division multiplexed in the wavelength-multiplexed signal light, and a calculation unit to calculate the target gain based on the number of signal lights at different wavelengths that are wavelength division multiplexed in the wavelength-multiplexed signal light acquired by said number-of-wavelengths information acquiring unit, wherein the threshold is equal to or smaller than a number of signal lights at different wavelengths corresponding to 25% of a total number of signal lights at different wavelengths transmittable in the wavelength-multiplexed signal light in a wavelength multiplex optical transmission system.