Patent ID: 8655131

Claim:
A multi-core optical fiber, comprising: a plurality of core regions in a same cross-section, each of the core regions extending along an optical axis; a cladding region provided on an outer periphery of the each core region; and a leakage reduction portion provided on a straight line connecting the adjacent core regions among the plurality of core regions, wherein: the multi-core optical fiber is a silica-based optical fiber, the leakage reduction portion has a refractive index profile such that a confinement factor of propagating light in a region surrounded by the leakage reduction portion is raised, each of the core regions allows light with a wavelength to be used to propagate in a single mode therein, the cladding region comprises an optical cladding, and a physical cladding provided on an outer periphery of the optical cladding, the optical cladding being provided on the outer periphery of each of the core regions as a region which influences transmission characteristics of light propagating in each of the core regions, and in the physical cladding, the leakage reduction portion is provided outside a position at which a distance from a center of each of the core regions is 5/2 times a mode field diameter at 1.55 μm of each of the core regions, or outside a position at which an electric field amplitude is 10 −4 of a peak value thereof.