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PURPOSE : A rare-earth doped fiber for optical fiber amplifiers and the optical fiber amplifier using the same are provided to enhance gain efficiency of an erbium doped fiber amplifier by simply changing an addition region of the erbium doped fiber amplifier. CONSTITUTION : A rare-earth doped fiber(110) is used as a gain medium of an optical fiber amplifier and includes a rare-earth addition region(112). The rare-earth doped fiber(110) is designed such that a ratio of a signal light overlap index, obtained for the addition region structure, to a pumping light overlap index lies in a range between 0.8 and 5.0. The overlap index is defined according to a mathematical expression 1, using variables such as r, Φ, and N. The r and Φ represent a radius and an angle variable in a cylindrical coordinate, respectively. The N represents an average density of a rare-earth element. I(r, Φ) represents a normalized mode distribution function of a pumping or a signal light. n(r, Φ) represents a density distribution function of the rare-earth element. © KIPO 2006