Optical fibre amplifier

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An optical fiber includes : a first core portion (1) doped with rare earth ions; a second core portion (2) having a lower refractive index than that of the first core portion (1), provided along an outer circumference of the first core portion (1), and doped with the rare earth ions; and a clad portion (3) having a lower refractive index than that of the second core portion (2) and provided along an outer circumference of the second core portion (2), and is configured such that a concentration of the rare earth ions added to the second core portion (2) is higher than that to the first core portion (1). With this configuration, it is possible to suppress an amount of FWM crosstalk in an optical amplification by decreasing the length of a fiber while alleviating efficiency deterioration due to concentration quenching.