Patent ID: 7006751

Claim:
An optical fiber, doped at least in part with a rare earth so as to be capable of transforming light energy at a pump wavelength into light energy over a signal wavelength band that does not include the pump wavelength, comprising in succession, from the center of the optical fiber towards its periphery: a central segment ( 1 ) of maximum index n 1 and of radius r 1 ; a peripheral segment ( 2 ) of maximum index n 2 and of radius r 2 ; a peripheral segment ( 4 ) of minimum effective index n 4 and of radius r 4 ; and a peripheral segment ( 5 ) of maximum index n 5 and of radius r 5 ; where n 1 >n 2 >n 4 and n 5 >n 4 and r 1 <r 2 <n 4 <r 5 , the indices and the radii being determined in such a manner that: firstly, over the signal wavelength band, the optical fiber presents a monomode core extending from the center of the optical fiber to radius r 1 ; and secondly at the pump wavelength, the optical fiber presents a multimode core extending from the center of the optical fiber to radius r 2 ; the minimum effective index n 4 being small enough to ensure that the numerical aperture of the optical fiber at the pump wavelength is greater than 0.25, the optical fiber being characterized in that n 2 >n 5 ; and in that the peripheral segment ( 2 ) of maximum index n 2 presents a decreasing graded shape so as to recenter the light energy of at least some of the guided modes at the pump wavelength so as to increase overlap between said recentered guided modes and the portion ( 6 ) of the optical fiber which is doped with the rare earth.