OPTICAL AMPLIFIER AND OPTICAL AMPLIFICATION METHOD

An optical amplifier that amplifies a wavelength multiplexing optical signal includes a first rare-earth-doped fiber, a second rare-earth-doped fiber connected in series with the first rare-earth-doped fiber, an excitation light combiner that inputs core excitation light to any of a core of the first rare-earth-doped fiber and a core of the second rare-earth-doped fiber and inputs clad excitation light to a clad of the first rare-earth-doped fiber, the clad excitation light having a wavelength different from that of the core excitation light, and an optical filter that is arranged between the first rare-earth-doped fiber and the second rare-earth-doped fiber, and transmits the wavelength multiplexing optical signal and the core excitation light and blocks the clad excitation light.