OPTICAL AMPLIFIER

An optical amplifier for amplifying light includes a light source for emitting pump light in accordance with current amount; a rear earth element doped optical fiber doped rear earth element, the rear earth element doped optical fiber pumped by the pump light from the light source; a detector for detecting upconversion light leaked from the rear earth element doped optical fiber; a memory for storing correspondence relationship data of the current amount for the light source and an intensity of the upconversion light in normal state of the light source; a difference calculator for calculating a difference between the intensity of the upconversion light being detected by the detector and a converted amount being converted the current amount for the light source by the use of the correspondence relationship data; and a discriminator for discriminating whether the difference calculated at the difference calculator exceeds a predetermined value.