Multicore fiber

A multicore fiber comprising:

• a center core configured to propagate an LP01 mode light beam, an LP11 mode light beam, an LP21 mode light beam, and an LP02 mode light beam;
• a first core disposed at a position overlapped with a first line segment extending from a center of the center core in a radial direction;
• a second core disposed at a position overlapped with a second line segment extending from the center of the center core in a radial direction at an angle of 90 degrees to the first line segment;
• a third core disposed at a position overlapped with a third line segment extending from the center of the center core in a radial direction at an angle of 67.5 degrees to the first line segment and at an angle of 157.5 degrees to the second line segment;
• a fourth core disposed at a position overlapped with a fourth line segment extending from the center of the center core in a radial direction at an angle of 67.5 degrees to the second line segment and at an angle of 135 degrees to the third line segment;
• and
• a fifth core disposed at a position overlapped with a fifth line segment extending from the center of the center core in a radial direction at an angle of 67.5 degrees to the third line segment and at an angle of 67.5 degrees to the fourth line segment, wherein
• a different mode interaction section and a different mode non-interaction section are provided along a longitudinal direction,
• in the different mode interaction section, a propagation constant of the LP11 mode light beam propagated through the center core is matched with propagation constants of LP01 mode light beams propagated through the first core and the second core such that crosstalk occurs between the LP11 mode light beam propagated through the center core and the LP01 mode light beams propagated through the first core and the second core, a propagation constant of an LP21 mode light beam propagated through the center core is matched with propagation constants of LP01 mode light beams propagated through the third core and the fourth core such that crosstalk occurs between the LP21 mode light beam propagated through the center core and the LP01 mode light beams propagated through the third core and the fourth core, and a propagation constant of an LP02 mode light beam propagated through the center core is matched with a propagation constant of an LP01 mode light beam propagated through the fifth core such that crosstalk occurs between the LP02 mode light beam propagated through the center core and the LP01 mode light beam propagated through the fifth core, and
• in the different mode non-interaction section, propagation constants of the LP mode light beams propagated through the center core are not matched with propagation constants of LP mode light beams propagated through the first core, the second core, the third core, the fourth core, and the fifth core,
• wherein the first core, the second core, the third core, the fourth core, and the fifth core are separated from one another, from one longitudinal end of the multicore fiber to an opposite longitudinal end of the multicore fiber opposite to the one longitudinal end, and
• wherein a refractive index of the center core is varied in a radial direction of the center core such that a center part of the center core has a refractive index lower than a refractive index of an outer circumferential part of the center core, the outer circumferential part of the center core being located on an outer circumferential side of the center part in the radial direction.