An optical fiber is described in which the cladding is provided with a refractive index that increases in a radially outward direction. In particular embodiments the refractive index of the cladding increases monotonically from a low value to a value close to or higher than the refractive index of the core. Such a fiber can be formed that can be operated in an effective single mode manner or in multimode operation and which is very suitable for use in high-bit-rate communication systems.