A double-clad optical fiber includes a core, a multimode inner cladding layer disposed about the core, and a second cladding layer disposed about the inner cladding layer. The core includes an active material for absorbing pump radiation received by the inner cladding. The multimode inner cladding includes truncated regions including a first material, where the first material has an index of refraction that is different than the material of the inner cladding that surrounds said truncated regions, for promoting the scattering of pump radiation propagating in the multimode inner cladding for increasing the absorption of the pump radiation by the active material of the core. Particles can be distributed in soot deposited via Outside Vapor Deposition for forming the truncated regions in an optical fiber drawn from the preform.