Optical fiber amplifier for reducing differential modal gain of an optical fiber

An optical fiber, comprising:

• a core region for guiding a propagation of a multimode optical signal, the core region defining a circular cross-section, the core region comprising:
• a central core having a first refractive index;
• at least one core layer laterally surrounding the central core, each of the at least one core layer having:
• a corresponding internal radius, an internal radius of an innermost core layer corresponding to a radius of the central core;
• a corresponding external radius, an external radius of an outermost core layer corresponding to a radius of the core region;
• and
• a corresponding refractive index below the first refractive index, the refractive indexes of the at least one core layer decreasing as the internal radius of the at least one core layer increases to form a step refractive index profile;
• a cladding region laterally surrounding the core region, the cladding region having a second refractive index below any refractive index of the at least one core layer;
• and
• at least two doped rings, each of the at least two doped rings having a corresponding rare-earth or transition metal dopant concentration,
• wherein the at least two doped rings are doped with erbium ions and ytterbium ions, a concentration of the ytterbium ions in the at least two doped rings being between 10 and 30 times higher than a concentration of the erbium ions therein.

An optical fiber, comprising:

• a core region for guiding a propagation of a multimode optical signal, the core region defining a circular cross-section, the core region comprising:
• a central core having a first refractive index;
• at least one core layer laterally surrounding the central core, each of the at least one core layer having:
• a corresponding internal radius, an internal radius of an innermost core layer corresponding to a radius of the central core;
• a corresponding external radius, an external radius of an outermost core layer corresponding to a radius of the core region;
• and
• a corresponding refractive index below the first refractive index, the refractive indexes of the at least one core layer decreasing as the internal radius of the at least one core layer increases to form a step refractive index profile;
• a cladding region laterally surrounding the core region, the cladding region having a second refractive index below any refractive index of the at least one core layer;
• and
• at least two doped rings, each of the at least two doped rings having a corresponding rare- earth or transition metal dopant concentration,
• wherein the at least two doped rings comprise:
• a first doped ring having erbium ion concentration of between 1.3×1025 m−3 and 1.7×1025 m−3;
• and
• a second doped ring having erbium ion concentration of between 1.8×1025 m−3 and 2.2×1025 m−3.