Active element added-optical fiber, resonator, and fiber laser device

An active element-doped optical fiber comprising:

• a core that includes a first region and a second region, wherein
• the first region ranges from a central axis to a predetermined radius, and is doped with an active element excited by excitation light,
• the second region surrounds the first region with no gap, extends to an outer peripheral surface of the core, and is not doped with the active element,
• the core satisfies 0.1 d<ra<d, where ra is a radius of the first region and d is a radius of the core, and
• the core has, in a region of 0.2 d<r≤0.9 d, a maximum value position at which a refractive index becomes maximum at a refractive index higher than an average value of a refractive index in a region of 0≤r≤0.9 d exists, where r is a distance from a central axis of the core in a radial direction.