An optical fiber includes a cladding 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 optical fiber further includes a core region and a core ring
surrounding the core region 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.