Optical fiber for a fiber laser, fiber laser, and production method for optical fiber for a fiber laser

An optical fiber for a fiber laser including a core to which a rare-earth element is added, a first cladding formed around the core, and a second cladding formed around the first cladding, in which excitation light is guided from at least one end of the first cladding to excite the rare-earth element to output a laser oscillation light, wherein

• an addition concentration of the rare-earth element to the core is different in a longitudinal direction of the optical fiber for a fiber laser,
• a core diameter and a numerical aperture of the optical fiber for a fiber laser are constant in the longitudinal direction of the optical fiber for a fiber laser, and
• a numerical aperture adjustment element that changes a refractive index of the first cladding is added to the first cladding so that a numerical aperture of the optical fiber for a fiber laser is maintained to be constant in the longitudinal direction of the optical fiber for a fiber laser with respect to change in a numerical aperture of the optical fiber for a fiber laser occurring due to change in a refractive index of the core due to change in the addition concentration of the rare-earth element to the core.