Multi-core optical fiber and multi-core optical fiber cable

A four-core multi-core optical fiber comprising:

• only four cores, each extending along a central axis;
• and
• a common cladding covering each of the four cores, wherein
• the common cladding has an outer periphery that is circular on a cross-section of the multi-core optical fiber, the cross-section being orthogonal to the central axis,
• on the cross-section, the four cores are respectively arranged at positions to be line symmetric with respect to a straight line that intersects with the central axis and that intersects with none of the four cores, and
• on the cross-section, a core arrangement defined by the four cores has rotational symmetry at most once with any point being a center of rotation, wherein
• a diameter CD of the common cladding is 126 μm or less,
• in each of the four cores, a mode field diameter MFD at a wavelength of 1310 nm and a cable cutoff wavelength λcc measured on a 22 m length of fiber satisfy a following Formula (1):
• dcoat≥2.88MFD/λcc+5.36 (1),
• in each of the four cores, the mode field diameter MFD is 8.2 μm or more and 9.6 μm or less, and a difference between a maximum value and a minimum value is 0.8 μm or less,
• in each of the four cores, a zero-dispersion wavelength is 1300 nm or more and 1352 nm or less, and a difference between a maximum value and a minimum value is 24 nm or less,
• in each of the four cores, a dispersion slope at the zero-dispersion wavelength is 0.092 ps/(nm2·km) or less,
• in each of the four cores, the cable cutoff wavelength λcc is 1260 nm or less,
• either a first condition or a second condition is satisfied,
• the first condition is defined that a crosstalk between cores having an adjacent relationship for a fiber length of 10 km at a wavelength of 1360 nm is −10 dB or less, in each of the four cores, a relationship of a following Formula (2):
• CDnominal≥13.31MFD/λcc+24.47 (2)
• is satisfied,
• and in each of the four cores, MFD/λcc and a center-to-center interval Λ between the cores having the adjacent relationship satisfy a following Formula (3):
• 6.5≤MFD/λcc≤9.5≤0.443Λ−5.33 (3), and
• the second condition is defined that the crosstalk between the cores having the adjacent relationship for the fiber length of 10 km at the wavelength of 1360 nm is −20 dB or less, in each of the four cores, a relationship of a following Formula (4):
• CDnominal≥9.37MFD/λcc+31.73 (4)
• is satisfied,
• and in each of the four cores, MFD/λcc and the center-to-center interval Λ between the cores having the adjacent relationship satisfy a following Formula (5):
• 6.5≤MFD/λcc≤9.5≤0.392Λ−4.88 (5), and wherein
• an optical cladding having a depressed type refractive index profile without a trench layer is provided in the periphery of each of the four cores.