Preform for producing an optical fiber and method therefor

A glass preform for providing a drawn optical fiber, comprising:

• a plurality of first and second glass rods having a chemical composition comprising silica, said first lass rods further comprising a plurality of groups of glass rods each comprising a rare-earth dopant, wherein each of said groups of glass rods comprise a concentration of said rare-earth dopant which is different from every other group, and wherein each of said groups are concentrically disposed with respect to each other to provide a core region having an rare-earth-dopant concentration increasing radially outward from said longitudinal axis, said second glass rods further comprising one of two or more different refractive indices, wherein said first glass rods are bundled together to form a substantially contiguous core bundle around a longitudinal axis, and said second glass rods uniformly grouped around said core bundle in the form of a substantially cylindrical annulus, wherein said cylindrical annulus has an refractive index falling within a range spanned by said two or more refractive indices substantially equal to a predetermined average refractive index, and wherein said core bundle and said cylindrical annulus form a composite preform bundle;
• and
• a glass tube surrounding and containing said composite preform bundle, said glass tube and said composite preform bundle fused at a glass fusion temperature to form a solid glass preform, wherein said chemical compositions of each of said glass rods is maintained in a location proximate or about coincident with a position of each said glass rods within said glass preform bundle.