Abstract: A method of producing an optical fiber preform includes a silica glass body forming step of forming a silica glass body to be at least a portion of a core portion. The method includes an alkali-metal-doped silica glass body forming step of forming an alkali-metal-doped silica glass body doped with an alkali metal around the silica glass body such that the alkali-metal-doped silica glass body contacts the silica glass body. The method further includes a diffusing step of diffusing the alkali metal from the alkali-metal-doped silica glass body to the silica glass body by a heat treatment.

Abstract: A method for manufacturing an optical fiber preform includes: producing a core preform including a core portion made of transparent glass and a first cladding layer obtained by adding fluorine to the core portion; and forming, on an outer periphery of the first cladding layer, a second cladding layer made of glass having a refractive index higher than that of the first cladding layer. Further, a refractive index profile is formed in the first cladding layer due to a fluorine concentration profile, the refractive index profile being provided at least near a boundary surface with the second cladding layer and having a profile such that a refractive index difference between a refractive index of the first cladding layer and a refractive index of the second cladding layer decreases in accordance with a reduction in a distance from the boundary surface with the second cladding layer.

Abstract: A method for manufacturing an optical fiber preform includes: producing a core preform including a core portion made of transparent glass and a first cladding layer obtained by adding fluorine to the core portion; and forming, on an outer periphery of the first cladding layer, a second cladding layer made of glass having a refractive index higher than that of the first cladding layer. Further, a refractive index profile is formed in the first cladding layer due to a fluorine concentration profile, the refractive index profile being provided at least near a boundary surface with the second cladding layer and having a profile such that a refractive index difference between a refractive index of the first cladding layer and a refractive index of the second cladding layer decreases in accordance with a reduction in a distance from the boundary surface with the second cladding layer.

Abstract: A method of producing an optical fiber preform includes a silica glass body forming step of forming a silica glass body to be at least a portion of a core portion. The method includes an alkali-metal-doped silica glass body forming step of forming an alkali-metal-doped silica glass body doped with an alkali metal around the silica glass body such that the alkali-metal-doped silica glass body contacts the silica glass body. The method further includes a diffusing step of diffusing the alkali metal from the alkali-metal-doped silica glass body to the silica glass body by a heat treatment.

Abstract: A method of producing an optical fiber preform includes: an alkali-metal-doped silica glass body forming step of forming an alkali-metal-doped silica glass body doped with an alkali metal; a silica glass body forming step of forming a silica glass body to be at least a portion of a core portion around the alkali-metal-doped silica glass body such that the silica glass body contacts the alkali-metal-doped silica glass body; and a diffusing step of diffusing the alkali metal from the alkali-metal-doped silica glass body to the silica glass body by a heat treatment.

Abstract: A multicore fiber includes a plurality of unit multicore fibers each including: a plurality of core portions; and a clad portion which is formed in an outer circumference of the core portions and has a refractive index lower than a maximum refractive index of the core portions. The plurality of the core portions have substantially same refractive index profile and different group delays at same wavelength in same propagation mode. The core portions of the multicore fiber are configured so that the core portions of the plurality of the unit multicore fibers are connected in cascade, a maximum value of differential group delays between the core portions of the multicore fiber is smaller than a reduced value of a maximum value of differential group delays between the core portions of each unit multicore fiber as a value in terms of a length of the multicore fiber.

Abstract: A production method of an optical fiber preform includes first preparing a first preform having a plurality of glass preforms and a first cladding portion disposed between the plurality of glass preforms, and first arranging a second cladding portion to surround the first preform. At the first arranging, a material gas and a combustion gas are ejected from a burner to produce glass particles. The first preform and the burner are moved relative to each other in a longitudinal direction of the first preform. The glass particles are deposited on the first preform.

Abstract: A multicore fiber includes a plurality of unit multicore fibers each including: a plurality of core portions; and a clad portion which is formed in an outer circumference of the core portions and has a refractive index lower than a maximum refractive index of the core portions. The plurality of the core portions have substantially same refractive index profile and different group delays at same wavelength in same propagation mode. The core portions of the multicore fiber are configured so that the core portions of the plurality of the unit multicore fibers are connected in cascade, a maximum value of differential group delays between the core portions of the multicore fiber is smaller than a reduced value of a maximum value of differential group delays between the core portions of each unit multicore fiber as a value in terms of a length of the multicore fiber.

Abstract: A method of producing an optical fiber preform includes: an alkali-metal-doped silica glass body forming step of forming an alkali-metal-doped silica glass body doped with an alkali metal; a silica glass body forming step of forming a silica glass body to be at least a portion of a core portion around the alkali-metal-doped silica glass body such that the silica glass body contacts the alkali-metal-doped silica glass body; and a diffusing step of diffusing the alkali metal from the alkali-metal-doped silica glass body to the silica glass body by a heat treatment.

Abstract: A production method of an optical fiber preform includes: preparing a plurality of bar-shaped first preforms and a plurality of second preforms including through holes having substantially same shape with a shape of outer periphery of a cross section of the first preform, the cross section being orthogonal to a major axis of the first preform; and an assembly step of: matching the through holes of the second preforms to make communication holes; and inserting, through each of the communication holes, at least two of the first preforms arranged side by side in a direction of the major axis such that the second preforms and the first preforms are fitting each other. In at least one position in the direction of the major axis of the communication holes, a position where the second preforms contact with each other differs from a position where the first preforms contact with each other.

Abstract: A production method of an optical fiber preform includes first preparing a first preform having a plurality of glass preforms and a first cladding portion disposed between the plurality of glass preforms, and first arranging a second cladding portion to surround the first preform. At the first arranging, a material gas and a combustion gas are ejected from a burner to produce glass particles. The first preform and the burner are moved relative to each other in a longitudinal direction of the first preform. The glass particles are deposited on the first preform.

Abstract: An optical fiber module includes a polarization-maintaining highly-nonlinear optical fiber and a housing. The polarization-maintaining highly-nonlinear optical fiber includes a core, a cladding surrounding the core and having two stress applying portions formed to sandwich the core and extend along the core, and a coating covering the cladding. The housing houses the polarization-maintaining highly-nonlinear optical fiber which is wound in a ring coil shape. The polarization-maintaining highly-nonlinear optical fiber has a bending loss of 0.01 dB/turn or less at a wavelength of 1550 nm when bent to form a diameter of 20 mm, a nonlinear coefficient ? of 10 W1 km?1 or larger at the wavelength of 1550 nm, a cutoff wavelength ?c of 1530 nm or shorter, and a zero-dispersion wavelength of not shorter than 1400 nm and not longer than 1650 nm.