The invention relates to the field of amplifying or emitting optical fibers. It comprises an amplifying optical fiber doped with a rare earth comprising a plurality of successive segments (1, 2, 3, 4, 5), presenting a monomode core and a multimode core, presenting at the periphery of the multimode core a peripheral segment (4) of low index so as to increase the numerical aperture of the optical fiber, outer cladding (5) being situated at the periphery of the low index peripheral segment (4), the multimode core being at least in part above the outer cladding (5), the peripheral segment (2) presenting a decreasing gradient shape.