Patent ID: 8406595

Claim:
A multi-core fiber comprising: a plurality of types of cores respectively having different effective refractive indexes in design and extending along a predetermined axis; and a cladding region integrally surrounding the plurality of types of cores, the cores being arranged such that a minimum core spacing D min between the same type of cores, servicing as a center-to-center spacing, becomes not shorter than the minimum spacing that causes no problem with crosstalk, wherein the multi-core fiber has at least a pair of cores of different types with a core spacing shorter than D min , and wherein when, in all pairs of cores of different types with the core spacings shorter than D min , the core spacing and a bending radius in a cross-section of the multi-core fiber orthogonal to the predetermined axis are respectively defined as D and R, a relative refractive index difference Δ eff between effective refractive indexes of the core of a certain type and the core of the other type satisfies the following condition: Δ eff ⁢ ⁢ ( % ) > 2 ⁢ D R + ( D R ) 2 2 ⁢ ( 1 + D R ) 2 · 100.