OPTICAL AMPLIFIER

PROBLEM TO BE SOLVED : To provide an optical amplifier which can detect the interruption of an inputted signal light easily. SOLUTION : The optical amplifier has a first terminal and a second terminal, a rare earth doped fiber 2 in which the signal light is guided from the first terminal to the second terminal, a pumping light source 6 which outputs a pumping light, an optical coupling means 4 which is connected with the doped fiber and the pump light source optically, and guides the pumping light to the dope fiber, a sensing means 10, 14 which detect whether the light outputted from the second terminal of the dope fiber contains the amplified signal light or not, and an optical output power controller 22 which attenuates or cuts off the amplified signal light temporarily, when it detects that the light outputted from the dope fiber does not contain the amplified signal light.