Patent ID: 8750663

Claim:
A coupled multi-core fiber comprising a plurality of cores and a clad surrounding the plurality of cores, wherein the plurality of cores are arranged in such a way that periphery surfaces of adjacent cores among the cores contact with each other, and each of the cores is made to have a refractive index higher than the clad and includes: an outer region having a predetermined thickness from a corresponding one of the periphery surfaces; and an inner region made to have a higher refractive index than the outer region and surrounded by the outer region, wherein a refractive index distribution of the inner region is α power distribution, and the α power distribution is expressed by an equation of n(r)=n 1 [[1−2Δ(r/α) α ] 1/2 , where “r” represents a distance from the center of the inner region, “n 1 ” represents the refractive index of the inner region at the center thereof, “Δ” represents a relative refractive index difference of the inner region from the outer region, “a” is a diameter of the inner region, and “α” is set to 1.5 to 5.