Multi-core optical fiber and method of manufacturing the same

A multi-core optical fiber comprising:

• a plurality of core portions;
• and
• a cladding portion positioned so as to surround an outside of each of the core portions,
• wherein
• each of the core portions includes a center core portion that is positioned at a center of that core portion and that has a refractive index which is greater than that of the cladding portion, a second core portion that is formed so as to surround an outside of the center core portion and that has a refractive index which is less than that of the center core portion, and a depressed portion that is formed so as to surround an outside of the second core portion and that has a refractive index which is less than those of the second core portion and the cladding portion,
• a relative refractive-index difference of the center core portion with respect to the cladding portion is Δ1, a relative refractive-index difference of the second core portion with respect to the cladding portion is Δ2, a relative refractive-index difference of the depressed portion with respect to the cladding portion is Δ3, a diameter of the center core portion is 2A, an outside diameter of the second pore portion is 2B, an outside diameter of the depressed portion is 2C, B/A is Ra2, and C/A is Ra3,
• Δ1 is from 0.05% to 1.2%, Δ2 is almost 0%, Δ3 is equal to or greater than −0.7%, 2A is from 4 μm to 14 μm, Ra2 is 1 to 3, (Ra3−Ra2) is equal to or less than 2, a cut-off wavelength is from 1 μm to 1.53 μm, and an effective core area at a wavelength of 1.55 μm is from 30 μm2 to 180 μm2, and
• an interval distance between each of the core portions and another one of the core portions positioned adjacent thereto is set such that optical cross-talk between the core portions for a total length of the multi-core optical fiber is equal to or less than −30 dB at a wavelength of 1.55 μm.