Ytterbium-doped optical fiber, fiber laser and fiber amplifier

An ytterbium-doped optical fiber comprising:

• a core which consists of ytterbium, aluminum, phosphorus, fluorine, and at least one element selected from a group consisting of transition metal elements and rare earth elements other than ytterbium and erbium;
• and
• a cladding which encircles the core, wherein
• an aluminum oxide equivalent concentration of the aluminum in the core is 0.2 mol % or more;
• a diphosphorus pentaoxide equivalent concentration of the phosphorus is higher than the aluminum oxide equivalent concentration;
• and
• the core either does not contain germanium or contains less than 1.1 mol % of germanium in a germanium dioxide equivalent concentration,
• wherein the diphosphorous pentaoxide equivalent concentration in the core is greater than a summation of an ytterbium oxide equivalent concentration of the ytterbium and the aluminum oxide equivalent concentration in the core, and the ytterbium oxide equivalent concentration in the core is in a range of 0.01 to 1.0 mol %.