A rare-earth-doped optical fiber having small numerical aperture

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The invention relates to an amplifying optical fiber comprising a central core adapted to convey and amplify an optical signal, and an cladding surrounding the central core and adapted to confine the conveyed optical signal in the central core. The central core is formed of a core matrix containing nanoparticles, the nanoparticles comprising a nanoparticle matrix and rare-earth-dopant elements. The core matrix also includes an additional dopant. Furthermore, the concentration by weight of rare-earth-dopant elements in the central core lies in the range 200 ppm to 1000 ppm, the concentration by weight of the nanoparticle matrix in the central core lies in the range 0.5 wt% to 5 wt%, preferably in the range 1.5 wt% to 4 wt%, and the concentration by weight of the additional dopant in the central core lies in the range 1 wt% to 10 wt%.