Fan-in/fan-out device for multicore fiber

A fan-in/fan-out device for multicore fiber that is connected to a first end portion of a multicore fiber which comprises a plurality of first cores and a first cladding covering the plurality of the first cores,

• the fan-in/fan-out device comprising a plurality of single-core fibers which are connected to the plurality of the first cores of the multicore fiber and which comprise an elongated portion extending in a longitudinal direction so as to reduce a diameter and being connected to the first end portion of the multicore fiber at a second end portion in an extending direction of the elongated portion, wherein:
• a refractive index distribution of each of the single-core fibers has a single peak;
• a relative refractive index difference of a second core with respect to a second cladding in each of the single-core fibers is 0.8% or more;
• and
• a second mode field diameter of the second end portion of the elongated portion is greater than a first mode field diameter of the first end portion of the multicore fiber.