Optical amplifier

An optical amplifier comprising:

• a first excitation light source that outputs a first excitation light;
• a second excitation light source that outputs a second excitation light;
• and
• an amplifying optical fiber doped with a rare-earth element and excited from an upstream side by the first excitation light and excited from a downstream side by the second excitation light to amplify light input thereto, wherein
• a rate between the intensity of the first excitation light and the intensity of the second excitation light is controlled according to the number of signal lights wavelength-multiplexed in the light input.

An optical amplifier comprising:

• an excitation light source that outputs excitation light;
• a branching unit that branches the excitation light output from the excitation light source according to a variable branching rate;
• a first amplifying optical fiber doped with a rare-earth element and excited by a first excitation light branched from the excitation light by the branching unit to amplify light input to the first amplifying optical fiber;
• and
• a second amplifying optical fiber doped with a rare-earth element and excited by a second excitation light branched from the excitation light by the branching unit to amplify the light output from the first amplifying optical fiber, wherein
• the branching rate of the branching unit is controlled according to the number of signal lights wavelength-multiplexed in the light input.