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|1. (WO2018042788) OPTICAL FIBER|
Provided is an optical fiber capable of sufficiently reducing the connection loss at a fusion splice point with a CSMF. The optical fiber is configured such that the refractive index of inner cladding (12) is substantially the same as the refractive index of outer cladding (13) while the concentration of the up-dopant in the inner cladding (12) is set so that the rate of increase of the refractive index due to the up-dopant is between 0.25% and 0.5%, inclusive.