Provided is an optical fiber including a low-refractive-index layer that is disposed between a first cladding and a second cladding and that has a refractive index between those of the first and second claddings. The first cladding has a cladding diameter that is 2.5 or more times the MFD of the fundamental mode at a fluorescence wavelength of an active element in a core, and the low-refractive-index layer has a thickness that is equal to or more than an absorption wavelength of the active element.