Optical amplifier

An optical amplifying method comprising:

• amplifying an input light by a first excitation light and thereby outputting a first amplified light;
• amplifying the first amplified light by a second excitation light and thereby outputting an output light;
• and
• controlling a level of the first excitation light and the second excitation light based on a detected absorption of the first excitation light in amplifying the input light and a detected absorption of the second excitation light in amplifying the first amplified light.

An optical apparatus comprising:

• a first optical amplifier to amplify an input light by a first excitation light and to thereby output a first amplified light;
• a second optical amplifier to amplify the first amplified light by a second excitation light and to thereby output an output light;
• and
• a controller to control a level of the first excitation light and the second excitation light based on detected absorption of the first excitation light by the first optical amplifier and detected absorption of the second excitation light by the second optical amplifier.