An article comprising a microstructured optical fiber having an axial direction and a cross section perpendicular to said axial direction, the optical fiber comprising a core region surrounded by a cladding region that comprises a multiplicity of spaced apart cladding features that are elongate in the axial direction and disposed in a first cladding material, the core region having an effective refractive index N.sub.o, the cladding features having a refractive index that differs from a refractive index of the first cladding material, and the cladding region having an effective refractive index that is less than N.sub.o ;
- CHARACTERIZED IN THAT
- the cladding region comprises an inner cladding region surrounding the core region and an outer cladding region surrounding the inner cladding region, said inner and outer cladding regions having effective refractive indices N.sub.ci and N.sub.co, respectively, with N.sub.ci <N.sub.co ;
- the cladding features in the inner cladding region having a larger size in cross section than the cladding features in the outer cladding region.