Multicore optical fiber

A multicore optical fiber comprising:

• a glass optical fiber having a plurality of cores each extending along a central axis, and a cladding surrounding the plurality of cores, the plurality of cores including a first core with a high refractive index and a second core with a refractive index lower than that of the first core;
• and
• a resin coating provided on an outer peripheral surface of the glass optical fiber, wherein
• on a cross-section of the multicore optical fiber orthogonal to the central axis, an arrangement of centers of a plurality of portions that has a refractive index different from a refractive index of the cladding has no rotational symmetry about a center of the cross-section,
• the multicore optical fiber further comprises a mode leakage adjustment structure for reducing a difference between a cutoff wavelength of a residual higher-order mode in the second core and a cutoff wavelength of a residual higher-order mode in the first core, wherein
• the mode leakage adjustment structure includes:
• a double cladding structure of the cladding comprising an inner cladding surrounding the plurality of cores and having an outer diameter D, and an outer cladding on an outer peripheral surface of the inner cladding and having a refractive index higher than that of the inner cladding;
• and
• a core arrangement in which centers of the first core and the second core are shifted by Δx along a line perpendicular to the central axis so that the first core is closer to an inner surface of the outer cladding than the second core, with respect to a reference positions of the first core and the second core whose centers are equidistant from the inner surface of the outer cladding in the cross-section.