Active lma optical fiber with enhanced transverse mode stability

An active Large-Mode-Area (LMA) optical fiber for mitigating Transverse Mode Instability (TMI) effects, the active LMA optical fiber comprising a core and one or more claddings surrounding said core, the core having a refractive index and a temperature coefficient each having a radial profile, the core comprising:

• a center core region having a center core glass composition comprising one or more rare-earth center dopants and one or more center co-dopants, said rare-earth center dopants and center co-dopants having respective concentrations and distributions determining radial profiles of the refractive index and of the temperature coefficient within said center core region;
• and
• a peripheral core region contiguously surrounding said center core region, the peripheral core region having a peripheral core glass composition which is free of rare-earth dopants and comprises one or more peripheral dopants, said peripheral dopants having respective concentrations and distributions determining radial profiles of the refractive index and of the temperature coefficient within said peripheral core region,
• wherein the radial refractive index-profile of the core is generally continuous across a boundary between the center core region and the peripheral core region;
• and
• wherein a selection and the concentrations and distributions of the rare-earth center dopants, the center co-dopants and the peripheral dopants are such that the temperature coefficient is lower in the peripheral core region than in the center core region.