Abstract: An optical fiber preform includes: a cladding glass body that is a cladding of an optical fiber, is cylindrical, and comprises an inner hole along an axial direction; a glass rod accommodated in the inner hole; and a dummy silica rod selected from either one of a first solid dummy silica rod fixed to a first end of the cladding glass body that closes a first end of the inner hole positioned at the first end of the cladding glass body, or a second solid dummy silica rod accommodated and integrated in a connecting glass tube fixed to the first end to close a first tip opening end of the connecting glass tube. A tip seal that closes a second end of the inner hole at a second end of the cladding glass body is provided in the second end of the cladding glass body.

Abstract: An optical fiber preform production method includes: inserting at least one glass rod into at least one through-hole that penetrates a cladding glass body that is a cladding of an optical fiber; integrating a dummy rod by either integrating a solid dummy silica rod with a first end of the cladding glass body by heating the first end to close a first opening of the through-hole that opens in the first end, or forming a base end seal that closes the first opening in the first end and integrating the solid dummy silica rod with the base end; and closing a second opening of the through-hole that opens in a second end of the cladding glass body by heating and deforming the second end.

Abstract: An optical fiber preform production method includes: inserting at least one glass rod into at least one through-hole that penetrates a cladding glass body that is a cladding of an optical fiber; integrating a dummy rod by either integrating a solid dummy silica rod with a first end of the cladding glass body by heating the first end to close a first opening of the through-hole that opens in the first end, or forming a base end seal that closes the first opening in the first end and integrating the solid dummy silica rod with the base end; and closing a second opening of the through-hole that opens in a second end of the cladding glass body by heating and deforming the second end.

Abstract: A multicore fiber includes a plurality of cores including a first core and a cladding surrounding the plurality of cores. The first core includes: an inner core; and an outer core surrounding the inner core with no gap and having a refractive index higher than a refractive index of the inner core and a refractive index of the cladding. The core is not doped with any rare earth element. At least two LP mode light beams at a predetermined wavelength propagate through the first core at an attenuation of 0.3 dB/km or less.

Abstract: A multicore fiber includes a plurality of cores including a first core and a cladding surrounding the plurality of cores. The first core includes: an inner core; and an outer core surrounding the inner core with no gap and having a refractive index higher than a refractive index of the inner core and a refractive index of the cladding. The core is not doped with any rare earth element. At least two LP mode light beams at a predetermined wavelength propagate through the first core at an attenuation of 0.3 dB/km or less.

Abstract: No core is disposed at the lattice point of a triangular lattice of a first layer LY1. First cores 11a and 11b of the core elements 10a and 10b are disposed at the lattice points of a second layer LY2. A first core 11c of the core element 10c and the second core 21 are alternately disposed at the lattice points of a third layer LY3. In a fourth layer LY4, no core is disposed at six lattice points, and the first cores 11a and 11b of the core elements 10a and 10b are disposed at the other lattice points. The second cores 21 are adjacent to the lattice points of the fourth layer LY4, at which no core is disposed. The effective refractive indexes of the core elements adjacent to each other are different from each other.

Abstract: No core is disposed at the lattice point of a triangular lattice of a first layer LY1. First cores 11a and 11b of the core elements 10a and 10b are disposed at the lattice points of a second layer LY2. A first core 11c of the core element 10c and the second core 21 are alternately disposed at the lattice points of a third layer LY3. In a fourth layer LY4, no core is disposed at six lattice points, and the first cores 11a and 11b of the core elements 10a and 10b are disposed at the other lattice points. The second cores 21 are adjacent to the lattice points of the fourth layer LY4, at which no core is disposed. The effective refractive indexes of the core elements adjacent to each other are different from each other.

Abstract: A method of connecting multicore fibers 2 includes a shaping step S2 and a fusing step S3. The multicore fibers 2 satisfy Y?20, where a distance from a center of a core 11 located on the outer periphery side of a clad 20 to a side surface of the clad 20 is defined as Y ?m. The shaping step S2 includes heating a end surfaces 50 to satisfy 0<X/Y?0.0054Y+0.268, where a distance in a longitudinal direction from a portion of each multicore fiber 2 located at an end in the longitudinal direction on each shaped end surface 50 to a position at which the end surface 50 and the side surface of the clad 20 meet each other is defined as X ?m.

Abstract: In a C band and an L band, the effective refractive indices of light propagating through the cores 11 and 21 adjacent to each other are different from each other such that a magnitude of crosstalk of light of a highest-order LP mode commonly propagating through the cores 11 and 21 adjacent to each other between the cores 11 and 21 adjacent to each other becomes a peak at a bending diameter smaller than a diameter of 100 mm, and the core has a higher refractive index in a center portion than in an outer circumferential portion such that a differential mode group delay of the cores 11 and 12 is 700 picoseconds/km or less.

Abstract: A method of connecting multicore fibers 2 includes a shaping step S2 and a fusing step S3. The multicore fibers 2 satisfy Y?20, where a distance from a center of a core 11 located on the outer periphery side of a clad 20 to a side surface of the clad 20 is defined as Y ?m. The shaping step S2 includes heating a end surfaces 50 to satisfy 0<X/Y?0.0054Y+0.268, where a distance in a longitudinal direction from a portion of each multicore fiber 2 located at an end in the longitudinal direction on each shaped end surface 50 to a position at which the end surface 50 and the side surface of the clad 20 meet each other is defined as X ?m.

Abstract: In a C band and an L band, the effective refractive indices of light propagating through the cores 11 and 21 adjacent to each other are different from each other such that a magnitude of crosstalk of light of a highest-order LP mode commonly propagating through the cores 11 and 21 adjacent to each other between the cores 11 and 21 adjacent to each other becomes a peak at a bending diameter smaller than a diameter of 100 mm, and the core has a higher refractive index in a center portion than in an outer circumferential portion such that a differential mode group delay of the cores 11 and 12 is 700 picoseconds/km or less.