Abstract: An amplification optical fiber operable to propagate light beams in a plurality of modes in a predetermined wavelength range through a core doped with a rare earth element, wherein Expression (1) is satisfied, where a cutoff wavelength of a propagated highest mode light beam is defined as ?max, under conditions in which the cutoff wavelength of the highest mode light beam is defined as ?c, a shortest wavelength of the wavelength range is defined as ?min, and a cutoff wavelength of a second-highest mode light beam to the highest mode light beam is ?min. ?c>0.5 ?min+0.

Abstract: A plurality of cores 51 is disposed around the center axis of a first cladding 52 in a state in which an inter-core distance ? of cores adjacent to each other is equal, a refractive index n1 of the core 51 is provided higher than a refractive index n2 of the first cladding 52, and the refractive index n2 of the first cladding 52 is provided higher than a refractive index n3 of a second cladding 53. Moreover, 5.8??/MFD(2?c/(?c+?op))?8 is satisfied, where the inter-core distance is defined as ?, a mode field diameter of the core is defined as MFD, a cutoff wavelength is defined as ?c, and a wavelength of communication light incident on the core 51 is defined as ?op.

Abstract: An optical device includes: multiple cores each including an inner core and an outer core surrounding an outer circumferential surface of the inner core without any gap therebetween; and cladding surrounding an outer circumferential surface of the cores without any gap therebetween and having a refractive index lower than that of the outer core, wherein each of the cores has a tapered portion that is tapered from one side toward the other side thereof in a longitudinal direction, each of the inner cores includes a low-refractive-index portion, and a high-refractive-index portion surrounding an outer circumferential surface of the low-refractive-index portion without any gap therebetween and having a refractive index higher than that of the low-refractive-index portion, and the outer core has a refractive index lower than that of the high-refractive-index portion.

Abstract: An optical fiber for amplification includes a core having an inner core and an outer core surrounding the outer circumferential surface of the inner core. The relative refractive index difference of the inner core to a cladding is smaller than the relative refractive index difference of the outer core to the cladding. The outer core is entirely doped with erbium. The theoretical cutoff wavelength of an LP11 mode light beam is a wavelength of 1,565 nm or more. The theoretical cutoff wavelength of an LP21 mode light beam is a wavelength of 1,530 nm or less. The theoretical cutoff wavelength of the LP02 mode light beam is a wavelength of 980 nm or less.

Abstract: 0.5377×r2?7.7?V2/V1?0.5377×r2?5.7, 3?r2/r1?5 are satisfied, where a radius of the inner core before tapered in diameter is defined as r1, a radius of the outer core before tapered in diameter is defined as r2, a refractive index volume formed of a product of a cross sectional area of the inner core and a relative refractive index difference of the inner core to the cladding before tapered in diameter is defined as V1, and a refractive index volume formed of a product of a cross sectional area of the outer core and a relative refractive index difference of the outer core to the cladding before tapered in diameter is defined as V2.

Abstract: A multicore fiber for communication 10 which allows propagation of an optical signal includes: a clad 12; a core 11a which is arranged in a center of the clad 12; and seven to ten cores 11b which are arranged at equal intervals surrounding the core 11a, and the cladding diameter is 230 ?m, distances between centers of the mutually neighboring cores 11a and 11b are 30 ?m or more, distances between the centers of the cores 11b and an outer peripheral surface of the clad 12 are 35 ?m or more and a mode field diameter of light propagating in the cores 11a and 11b is 9 ?m to 13 ?m.

Abstract: A multicore fiber for communication 10 which allows propagation of an optical signal includes: a clad 12; a core 11a which is arranged in a center of the clad 12; and seven to ten cores 11b which are arranged at equal intervals surrounding the core 11a, and the cladding diameter is 230 ?m, distances between centers of the mutually neighboring cores 11a and 11b are 30 ?m or more, distances between the centers of the cores 11b and an outer peripheral surface of the clad 12 are 35 ?m or more and a mode field diameter of light propagating in the cores 11a and 11b is 9 ?m to 13 ?m.

Abstract: 0.5377×r2?7.7?V2/V1?0.5377×r2?5.7, 3?r2/r1?5 are satisfied, where a radius of the inner core before tapered in diameter is defined as r1, a radius of the outer core before tapered in diameter is defined as r2, a refractive index volume formed of a product of a cross sectional area of the inner core and a relative refractive index difference of the inner core to the cladding before tapered in diameter is defined as V1, and a refractive index volume formed of a product of a cross sectional area of the outer core and a relative refractive index difference of the outer core to the cladding before tapered in diameter is defined as V2.

Abstract: A multicore fiber 1 includes: a small diameter portion 33 in which a propagation constant of light of an x1-th order LP mode of the first core 11 (here, x1 is an integer of “2” or more and x or less, x is an integer of “2” or more) and a propagation constant of light of a y1-th order LP mode of the second core 12 (here, y1 is an integer of “1” or more and y or less other than x1, y is an integer of “1” or more) coincide with each other and a large diameter portion in which a propagation constant of light of each LP mode of the first core 11 and a propagation constant of light of each LP mode of the second core 12 are configured not to coincide with each other are arranged.

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: Provided is a method of producing a preform 10P for a coupled multi-core fiber including: an arranging process P1 for arranging a plurality of core glass bodies 11R and a clad glass body 12R in such a way that the plurality of core glass bodies 11R are surrounded by the clad glass body 12R; and a collapsing process P2 for collapsing a gap between the core glass bodies 11R and the clad glass body 12R, wherein the respective core glass bodies 11R have outer regions 16 having a predetermined thickness from the periphery surfaces and made of silica glass undoped with germanium, and the clad glass body 12R is made of silica glass having a refractive index lower than a refractive index of the outer regions of the core glass bodies 11R.

Abstract: The first cladding 52 has a two-layer structure formed of a solid inner layer 62A passed through the center axis of the first cladding 52 and an outer layer 62B enclosing the inner layer 62A and the plurality of cores 51 with no gap. A refractive index n1 of the core 51 is provided higher than refractive indexes n2A and n2B of the inner layer 62A and the outer layer 62B, the refractive indexes n2A and n2B of the inner layer 62A and the outer layer 62B are provided higher than a refractive index n3 of the second cladding 53, and the refractive index n2A of the inner layer 62A is provided lower than the refractive index n2B of the outer layer 62B.

Abstract: A multi-core fiber (1) is a multi-core fiber including 10 or greater of even numbered cores and a cladding surrounding the core. In the even numbered cores, a half of cores (11a) are disposed in such a manner that centers are located on the apexes of a regular polygon (RP) whose center is at an origin point (O) in a cladding (20). In the even numbered cores, other cores (11b) are disposed in a manner that centers are located on perpendicular bisectors (LV) of the edges of a regular polygon on the inner side of the regular polygon (RP). The other cores (11b) are disposed in a specific range in the regular polygon (RP).

Abstract: A solid photonic band gap fiber includes: a core area located at a central portion of a cross-section with respect to a longitudinal direction of the fiber, the core area being formed of a solid substance having a low refractive index; cladding areas having base portions formed of a solid substance having a low refractive index, the cladding areas surrounding the core area; and a plurality of fine high refractive index scatterers provided in the cladding areas, and disposed in a dispersed manner so as to surround the core area, the number of fine high refractive index scatterers being formed of a solid substance having a high refractive index, wherein in a state that the solid photonic band gap fiber is held at a predetermined bending radius, propagation in a high-order mode is suppressed by using a difference in a bending loss between a fundamental mode and the high-order mode, and only the fundamental mode is substantially propagated, the fundamental mode and the high-order mode being caused by bending.

Abstract: A butting step S1 of butting end surfaces of multi-core fibers against each other by aligning central axes CA of clads 20 of the multi-core fibers to cause each core 11 of one multi-core fiber 1a and each core 11 of the other multi-core fiber 1b to face each other, and a fusing step S2 of fusing the multi-core fibers to each other by carrying out discharge by a pair of discharge electrodes 50a and 50b that sandwich a butted position of the multi-core fibers and face each other are provided. The fusing step S2 causes tips 51a and 51b of the discharge electrodes to perform reciprocating motion such that a straight line SL that connects the tips 51a and 51b of the discharge electrodes moves while describing a surface perpendicular to the central axes CA.

Abstract: The first cladding 52 has a two-layer structure formed of a solid inner layer 62A passed through the center axis of the first cladding 52 and an outer layer 62B enclosing the inner layer 62A and the plurality of cores 51 with no gap. A refractive index n1 of the core 51 is provided higher than refractive indexes n2A and n2B of the inner layer 62A and the outer layer 62B, the refractive indexes n2A and n2B of the inner layer 62A and the outer layer 62B are provided higher than a refractive index n3 of the second cladding 53, and the refractive index n2A of the inner layer 62A is provided lower than the refractive index n2B of the outer layer 62B.

Abstract: A plurality of cores 51 is disposed around the center axis of a first cladding 52 in a state in which an inter-core distance ? of cores adjacent to each other is equal, a refractive index n1 of the core 51 is provided higher than a refractive index n2 of the first cladding 52, and the refractive index n2 of the first cladding 52 is provided higher than a refractive index n3 of a second cladding 53. Moreover, 5.8??/MFD(2?c/(?c+?op))?8 is satisfied, where the inter-core distance is defined as ?, a mode field diameter of the core is defined as MFD, a cutoff wavelength is defined as ?c, and a wavelength of communication light incident on the core 51 is defined as ?op.

Abstract: Each of a first clad region (12) and a second clad region (13) has holes (12a, 13a) which have identical diameters and are periodically formed so that the first clad region (12) and the second clad region (13) each have an effective refractive index lower than a refractive index of a core region (11), the effective refractive index of the first clad region (12) being lower than that of the second clad region (13).

Abstract: An optical device includes: multiple cores each including an inner core and an outer core surrounding an outer circumferential surface of the inner core without any gap therebetween; and cladding surrounding an outer circumferential surface of the cores without any gap therebetween and having a refractive index lower than that of the outer core, wherein each of the cores has a tapered portion that is tapered from one side toward the other side thereof in a longitudinal direction, each of the inner cores includes a low-refractive-index portion, and a high-refractive-index portion surrounding an outer circumferential surface of the low-refractive-index portion without any gap therebetween and having a refractive index higher than that of the low-refractive-index portion, and the outer core has a refractive index lower than that of the high-refractive-index portion.

Abstract: A multi-core fiber includes a plurality of cores, a marker which is disposed to be parallel to the cores, and a clad which surrounds outer peripheral surfaces of the cores and the marker. The marker may propagate light having a wavelength which is the same as a wavelength of light which propagates in the core as single mode light.