Patent ID: 7085043

Claim:
An optical amplifier comprising: a plurality of optical amplification sections connected in a cascade manner between an input terminal and an output terminal, which amplify wavelength multiplexed light which includes a plurality of signal lights arranged within a predetermined wavelength band; a constant output control section which has a variable optical attenuator arranged on at least one optical path which connects between said plurality of optical optical amplification sections, and controls said variable optical attenuator so that the power of wavelength multiplexed light output to said output terminal remains constant at a preset target level; and a constant gain control section which calculates the number of wavelengths in said wavelength multiplexed light based on the power of wavelength multiplexed light input to said input terminal, and according to the calculation result, controls each optical amplification section so that the overall gain of said plurality of optical amplification sections is constant, wherein said plurality of optical amplification sections each have: an amplification medium wherein the core is doped with a rare earth; a pumping light source which generates pumping light; a multiplexer which applies the pumping light emitted by said pumping light source to said amplification medium from a signal entry end; and a light blocking device inserted at a predetermined position on said optical amplification medium, which is capable of blocking amplified spontaneous emission light generated within said amplification medium which has a different wavelength band from the wavelength multiplexed light and which travels in the opposite direction to the wavelength multiplexed light, and said optical amplification sections are respectively constructed so that a part of the amplification medium which is on the signal exit end side of the insertion position of said light blocking device is pumped by pumping light applied from the signal entry end side from said pumping light source via said multiplexer, and said constant gain control section controls the pumping light power of each optical amplification section at a constant slope with respect to the number of wavelengths in the wavelength multiplexed light, so that the respective gains in said plurality of optical amplification sections is constant.