According to the present invention the optical fiber includes a core with a first refractive index (n.sub.1) and the innermost core region with the refractive index n.sub.0, a cladding surrounding the core, the cladding having a third refractive index (n.sub.3), wherein n.sub.1>n.sub.3 and n.sub.0n.sub.2. It is preferable that at least one of the core, innermost core region and/or moat has a non-circular shape.