Hitherto, erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers have been available as optical amplifiers.
Raman amplifiers are utilized, for example, when an optical-fiber transmission path has a large length and a large amount of relay loss is likely. For example, a Raman amplifier that splits received light, monitors the power of the split received light, and controls the power of pump light on the basis of a result of the monitoring has been disclosed (for example, see Japanese Laid-open Patent Publication No. 2009-186599).
However, in the related art described above, since the power of light resulting from the splitting of the received light decreases, there are problems in that the power of the received light is not monitored with high accuracy and the amount of error in the Raman gain increases.