A photonic-bandgap fiber includes a photonic crystal lattice with a material having a first refractive index and a pattern of regions formed therein. Each of the regions has a second refractive index lower than the first refractive index. The photonic-bandgap fiber further includes a core and a core ring surrounding the core and having an inner perimeter, an outer perimeter, and a thickness between the inner perimeter and the outer perimeter. The thickness is sized to reduce the number of ring surface modes supported by the core ring.