Optical amplifier and method of controlling the same

An optical amplifier comprising:

• a first-stage optical amplifying unit adapted to amplify inputted signal light;
• an optical attenuator adapted to attenuate signal light from said first-stage optical amplifying unit;
• a second-stage optical amplifying unit adapted to amplify signal light from said optical attenuator;
• an automatic gain controller adapted to control said first-stage optical amplifying unit and said second-stage optical amplifying unit so that a gain of signal light outputted from said second-stage optical amplifying unit to the signal light inputted to said first-stage optical amplifying unit is constant;
• and
• an attenuation amount controller adapted to control an attenuation amount at said optical attenuator on the basis of information on wavelength arrangement and an input level of the signal light inputted to said first-stage optical amplifying unit to adjust a gain-versus-wavelength characteristic at said first-stage and second-stage optical amplifying units automatic-gain-controlled so that a gain slope characteristic at said first-stage and second-stage optical amplifying units due to the wavelength arrangement and the input level is flattened.