Rare-earth-doped optical fiber, optical fiber amplifier, and method of manufacturing a preform for

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A first step, in which P 2 O 5 -containing glass is deposited inside a silica glass pipe, and a second step, in which a Cl 2 -containing gas is introduced into the pipe and the P 2 O 5 -containing glass is dehydrated by heating the pipe, are repeated alternately A third step, in which glass that does not contain P 2 O 5 is deposited on the inside of the silica glass pipe, may further be provided such that the first step, the second step, and the third step are repeatedly performed in this order. A rare-earth-doped optical fiber, which has a attenuation of 15 dB/km or less at a wavelength of 1200 nm, comprises a core region and a cladding region enclosing the core region, wherein the core region includes phosphorus of 3 wt% or more, aluminum of 0.3 wt% or more, a rare-earth element of 500 wtppm or more, and chlorine of 0.03 wt% or more, and the cladding region has a refractive index that is lower than the refractive index of the core region.