Multicore optical fiber, method of designing multicore optical fiber, and optical transmission method

A multicore optical fiber comprising:

• four cores having a radius a1 disposed in a longitudinal direction in square lattice shape;
• first cladding regions disposed around each of the cores and having a radius a2 with a refractive index lower than a refractive index of the cores, a specific relative refractive index difference to the cores being Δ1;
• and
• a second cladding region disposed on an outer periphery of the first cladding regions and having a refractive index higher than the refractive index of the first cladding regions and lower than the refractive index of the cores, a specific relative refractive index difference to the core being Δ2, wherein
• an outer diameter of the second cladding region is 125±1 μm,
• an effective cross-sectional area at a wavelength of 1550 nm is 80 μm2 or more,
• bending loss at a wavelength of 1625 nm and a bending radius of 30 mm is 0.1 dB/100 turns or less, and
• a cutoff wavelength is 1530 nm or less.