Optical fiber for amplification or for laser emission

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.