An optical amplifier comprising: an optical fiber including: a first core portion doped with rare earth ions; a second core portion having a lower refractive index than that of the first core portion and provided along an outer circumference of the first core portion, the second core portion being doped with rare earth ions of a higher concentration than that of the first core portion; and a clad portion having a lower refractive index than that of the second core portion and provided along an outer circumference of the second core portion; a forward pump unit (6a, 7) for inputting first forward pump light in the 980-nm band and second forward pump light in the 1480-nm band to an input end of the optical fiber; and a backward pump unit for inputting backward pump light in the 1480-nm band to an output end of the optical fiber, wherein the optical amplifier is configured such that the first forward pump light is more strongly confined to the first core portion than the second pump light, such that an amount of leak of the first forward pump light to the second core portion is smaller than an amount of leak of the second forward pump light to the second core portion, and such that the population inversion in the second core portion is dominantly determined by the second forward pump light; and the optical amplifier is configured such that, in use, population inversion ratios in the first and second core portions are controlled by controlling an intensity ratio between the first forward pump light and the second forward pump light. an optical fiber including: a first core portion doped with rare earth ions; a second core portion having a lower refractive index than that of the first core portion and provided along an outer circumference of the first core portion, the second core portion being doped with rare earth ions of a higher concentration than that of the first core portion; and a clad portion having a lower refractive index than that of the second core portion and provided along an outer circumference of the second core portion; a forward pump unit (6a, 7) for inputting first forward pump light in the 980-nm band and second forward pump light in the 1480-nm band to an input end of the optical fiber; and a backward pump unit for inputting backward pump light in the 1480-nm band to an output end of the optical fiber, wherein the optical amplifier is configured such that the first forward pump light is more strongly confined to the first core portion than the second pump light, such that an amount of leak of the first forward pump light to the second core portion is smaller than an amount of leak of the second forward pump light to the second core portion, and such that the population inversion in the second core portion is dominantly determined by the second forward pump light; and the optical amplifier is configured such that, in use, population inversion ratios in the first and second core portions are controlled by controlling an intensity ratio between the first forward pump light and the second forward pump light. a first core portion doped with rare earth ions; a second core portion having a lower refractive index than that of the first core portion and provided along an outer circumference of the first core portion, the second core portion being doped with rare earth ions of a higher concentration than that of the first core portion; and a clad portion having a lower refractive index than that of the second core portion and provided along an outer circumference of the second core portion; a forward pump unit (6a, 7) for inputting first forward pump light in the 980-nm band and second forward pump light in the 1480-nm band to an input end of the optical fiber; and a backward pump unit for inputting backward pump light in the 1480-nm band to an output end of the optical fiber, wherein the optical amplifier is configured such that the first forward pump light is more strongly confined to the first core portion than the second pump light, such that an amount of leak of the first forward pump light to the second core portion is smaller than an amount of leak of the second forward pump light to the second core portion, and such that the population inversion in the second core portion is dominantly determined by the second forward pump light; and the optical amplifier is configured such that, in use, population inversion ratios in the first and second core portions are controlled by controlling an intensity ratio between the first forward pump light and the second forward pump light.