Ytterbium-doped optical fiber, fiber laser, and fiber amplifier

An ytterbium-doped optical fiber comprising:

• a core which contains ytterbium, aluminum, and phosphorus, and does not contain germanium;
• and a cladding which surrounds this core, wherein:
• the ytterbium concentration in the core in terms of ytterbium oxide is 0.09 to 0.68 mole percent;
• a molar ratio between the phosphorus concentration in the core in terms of diphosphorus pentoxide and the ytterbium concentration in the core in terms of ytterbium oxide is 3 to 30;
• a molar ratio between the aluminum concentration in the core in terms of aluminum oxide and the ytterbium concentration in the core in terms of ytterbium oxide is 3 to 32;
• a molar ratio between the aluminum concentration in terms of aluminum oxide and the phosphorus concentration in terms of diphosphorus pentoxide is 1 to 2.5, and
• the relative refractive index difference between the core and the cladding is 0.1 to 0.25%.