We propose a new design of microstructured fiber combining large doped area (500 mum(2)), high rare earth concentration and single mode propagation despite the high core refractive index (n(Si) + 0.01). Actually, original guiding properties, based on a total internal reflection guidance regime modified by coupling between core and resonant cladding modes (close to the ARROW model) ensure single mode propagation. Moreover spectral properties which are largely governed by characteristics of high index cladding rods can be adjusted by properly choosing diameter and refractive index of the rods.