To provide a light amplifier which controls the output level by detecting a change in wavelength or a gain gradient of signal light based on a change in signal light.
In the light amplifier, a measurement point is set on the input side in an intermediate stage, or on the output side. The light amplifier is equipped with a specific wavelength measuring means 31 for measuring a specific wavelength at the measurement point, and a total light energy measuring means 32 for measuring a total light energy at the measurement point. When there is no change in output measured by the specific wavelength measuring means 31 and there is a change in output measured by the total light energy measuring means 32, a change in the number of wavelength multiplexing of a received wavelength multiplexing optical signal is detected based on the outputs measured by the specific wavelength measuring means 31 and the total light energy measuring means 32.
SHIMANO YOSHIO
IIZUKA HIROSHI
ITO HIROYUKI
