Multicore fiber, optical device, and method for manufacturing multicore fiber

A multi-core fiber comprising:

• a cladding;
• cores extending in an extending direction inside the cladding;
• a marker inside the cladding;
• and
• an end surface inclined in an inclined direction that is not orthogonal to the extending direction, wherein
• the cores at the end surface are line-symmetrically arranged with respect to a virtual axis orthogonal to the inclination direction,
• the virtual axis virtually divides the end surface into a first area and a second area,
• the cores include:
• a first core disposed farthest from the virtual axis in the first area;
• and
• a second core disposed farthest from the virtual axis in the second area, and
• a center of the marker at the end surface is disposed in an area between:
• a straight line, passing through the first core or a mode field of the first core, parallel to the virtual axis, and
• a straight line, passing through the second core or a mode field of the second core, parallel to the virtual axis.