Optical amplifier and optical amplification method

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An optical amplifier for amplifying light in a longer wavelength band. The optical amplifier includes first and second optical fibers doped with a rare earth element and optically connected so that a signal light travels through the first optical fiber and then through the second optical fiber. Excitation light causes the signal light to be amplified in both the first and second optical fibers. Spontaneous emission lights are generated in the first optical fiber. An oscillation generator causes the spontaneous emission lights to oscillate in the first optical fiber, to thereby generate laser oscillation. Light generated by the laser oscillation is supplied to the second optical fiber as excitation light in the second optical fiber. The oscillation generator can be formed by fiber gratings along the first optical fiber and which reflect spontaneous emission light at a predetermined wavelength, thereby causing spontaneous emission lights to oscillate between the fiber gratings in the first optical fiber. Alternatively, the oscillation generator can be a traveling-wave type optical oscillator. In an additional embodiment, an optical amplifier includes first and second optical amplifiers amplifying first and second signal lights, respectively, the first and second signal lights being in different wavelength bands. An excitation light providing device branches a portion of the signal light amplified by the first optical amplifier, and provides the branched portion to the second optical amplifier as excitation light in the second optical amplifier.