Patent ID: 7990608

Claim:
An optical transmission system comprising: an add/drop multiplexer adapted to demultiplex input signal light, drop and permit to be through the demultiplexed light, and add signal light; a plurality of photodiodes adapted to monitor signal light at each wavelength which may be an output from said add/drop multiplexer, and output an electric signal; and an optical amplifier provided at a rear stage of said add/drop multiplexer, said optical amplifier comprising: a first-stage optical amplifier adapted to amplify input signal light; an optical attenuator adapted to attenuate signal light from said first-stage optical amplifier; a second-stage optical amplifier adapted to amplify signal light from said optical attenuator; an automatic gain controller adapted to control said first-stage optical amplifier and said second-stage optical amplifier so that a gain of signal light output from said second-stage optical amplifier to the signal light input to said first-stage optical amplifier is constant; and an attenuation amount controller adapted to produce a first gain slope related to a spectral hole burning (SHB) generated at said first-stage and said second-stage optical amplifiers from a wavelength arrangement and an input level obtained on the basis of an electric signal from each of said photodiodes for each wavelength of the signal light input to said first-stage optical amplifier, produce a second gain slope at said first-stage and second-stage optical amplifiers obtained from a change amount of an attenuation amount by said optical attenuator, and control the attenuation amount at said optical attenuator to adjust the second gain slope so that the first gain slope is substantially canceled.