Added optical fiber and optical device using the same

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PROBLEM TO BE SOLVED : To provide a rare earth element-doped optical fiber where the confinement efficiency of signals and excitation light has been improved, and to provide an optical device using the optical fiber. SOLUTION : Since a hollow section 13 is formed at the center of a core 11 doped with rare earth elements, relative index difference can be increased greatly, and signal light and excitation light are confined at a core region doped with the rare earth elements for propagation and amplification, thus obtaining a high-efficiency and high-gain amplifier. Since light can be distributed into each core 11 for propagation and amplification, nonlinear effect cannot occur easily even if the signal light with large power is inputted. Not only the signal light but also the excitation light are efficiently coupled into the core 11 and are confined and propagated, thus possibly achieving further efficient optical amplifier.