Patent ID: 8774590

Claim:
An ytterbium-doped optical fiber comprising a core containing at least ytterbium, aluminum and phosphorous and a clad surrounding the core, wherein a molar concentration of diphosphorus pentoxide with respect to phosphorus in the core is equal to a molar concentration of aluminum oxide with respect to aluminum in the core, wherein a ratio of a molar concentration of diphosphorus pentoxide with respect to phosphorus in the core to a molar concentration of ytterbium oxide with respect to ytterbium in the core is 10 to 30, wherein a relative refractive index difference between the core and the clad is 0.05 to 0.30%, wherein the molar concentration of ytterbium oxide in the core (α) is within the range 0.05≦α×0.5≦0.30, wherein a diameter of the core is 20 to 30 μm, and wherein the core does not contain germanium.