Abstract: A rare-earth doped fiber includes: a core having a radius R and having a doped region to which a rare-earth element has been doped, the doped region ranging from a center of the core to a radius Rre; and a cladding surrounding a circumference of the core and having a refractive index lower than a refractive index of the core. A normalized frequency V at an operating wavelength is greater than or equal to 2.4 and less than or equal to 4.5. The radius Rre is in a range of 0<Rre<(0.0247 V2?0.3353 V+1.6474)R.

Abstract: A fiber laser device includes: a first pumping light source that outputs a first pumping light of a first wavelength; a second pumping light source that outputs a second pumping light of a second wavelength that is shorter than the first wavelength; an amplifying fiber that includes a core including an active element that is configured to be excited by the first pumping light and the second pumping light; an HR-FBG (High Reflectivity-Fiber Bragg Grating) on a side of a first end of the amplifying fiber; an OC-FBG (Output Coupler-Fiber Bragg Grating) disposed on a side of a second end of the amplifying fiber and that has a reflectance lower than a reflectance of the HR-FBG; and a first coupler that couples the first pumping light to the amplifying fiber from the side of the first end.

Abstract: An optical fiber includes a core that propagates a light that includes a wavelength equal to or larger than 1000 nm and equal to or smaller than 1100 nm. The light propagates in the core at least in an LP01 mode and an LP11 mode. A difference between a propagation constant of the light in the LP01 mode and a propagation constant of the light in the LP11 mode is 1735 rad/m or larger and 4000 rad/m or smaller.

Abstract: An optical fiber includes a core and a cladding. An effective area Aeff of light of a fundamental mode, having a wavelength of 1070 nm and propagating through the core, is 500 ?m2 or more. A numerical aperture NA of the core satisfies the following formula: NA?(1.3×10?11×a4/b6)1/6 where a (m) is a radius of the core and b (m) is a radius of the cladding. A V value, that is a waveguide parameter of the optical fiber, satisfies the following formula: V?1.3583×b?0.2555.

Abstract: An optical fiber connector includes: amplifying fibers in which an active element activated by excitation light is added to a core of each of the amplifying fibers. The amplifying fibers are connected together such that an absorption amount of excitation light per unit length increases with an increase of a distance from an incident end of the excitation light. A mode field diameter of laser light propagating through the core is same among the amplifying fibers.

Abstract: An active element added-optical fiber includes a core, having a radius d and including a first region and a second region, and a cladding that surrounds an outer peripheral surface of the core without a gap and propagates light in a few mode. The first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element. The second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium. The active element is not added to a region within the second region from a radius rc to the radius d. The germanium is not added to a region within the first region from the central axis to a radius rb, and a concentration of the germanium is highest among the plurality of dopants.

Abstract: An active element-doped optical fiber includes: a core that includes first and second regions. The first region ranges from a central axis to a predetermined radius, and is doped with an active element excited by excitation light. The second region surrounds the first region with no gap, extends to an outer peripheral surface of the core, and is not doped with the active element. The core satisfies 0.1 d<ra<d, where ra is a radius of the first region and d is a radius of the core. The core has, in a region of 0.2 d<r?0.9 d, a maximum value position at which a refractive index becomes maximum, where r is a distance from a central axis of the core in a radial direction.

Abstract: An active element-doped optical fiber includes a core that includes a first region and a second region. The first region satisfies 0?r?0.65d, and the second region surrounds the first region and satisfies 0.65d<r?d, where d is a radius of the core and r is a distance from a central axis of the core in a radial direction. At least a part of the first region is doped with an active element excited by excitation light, the second region is not doped with the active element, and a shape index is 0.99 or more and less than 1.

Abstract: An amplification optical fiber includes: a core; an inner cladding having a refractive index lower than a refractive index of the core, wherein an active element pumped by pumping light is entirely doped to the core, and a relative effective refractive index difference of light in an LP01 mode is greater than or equal to 0.05% and a relative effective refractive index difference of light in an LP21 mode is less than 0.05% in light propagating through the core.

Abstract: An optical fiber includes a core that propagates a light that includes a wavelength equal to or larger than 1000 nm and equal to or smaller than 1100 nm. The light propagates in the core at least in an LP01 mode and an LP11 mode. A difference between a propagation constant of the light in the LP01 mode and a propagation constant of the light in the LP11 mode is 1735 rad/m or larger and 4000 rad/m or smaller.

Abstract: An optical fiber includes a core that propagates a light that includes a wavelength of 1060 nm. The light propagates in the core at least in an LP01 mode and an LP11 mode. A difference ?? between a propagation constant of the light in the LP01 mode and a propagation constant of the light in the LP11 mode is 2000 rad/m or smaller. The expression L ? ? ? M 2 - 2 ? 3 ? 7 ? 2 × 1 ? 0 - 6 × ? ? ? + 4 . 8 ? 1 ? 9 × 1 ? 0 - 3 is satisfied, where L is a length, M2 is a beam quality of light, ?M2 is a deterioration amount of the beam quality of light due to propagation in the optical fiber.

Abstract: An optical fiber, used in a laser device, propagates light having a wavelength of 1060 nm through a core in at least an LP01 mode and an LP11 mode. A difference between a propagation constant of light in the LP01 mode and a propagation constant of light in the LP11 mode is 1850 rad/m or more and 4000 rad/m or less.

Abstract: An amplification optical fiber according to the present invention includes: a core doped with an active element, through which multi-mode light propagates; an inner cladding that surrounds the core and has a refractive index lower than that of the core; and an outer cladding that surrounds the inner cladding and has a refractive index lower than that of the inner cladding. The inner cladding has a polygonal outline in a cross section perpendicular to the longitudinal direction, and the inner cladding has a permanent twist applied by turning around the central axis of the core.

Abstract: An amplification optical fiber includes: a core; an inner cladding having a refractive index lower than a refractive index of the core, wherein an active element pumped by pumping light is entirely doped to the core, and a relative effective refractive index difference of light in an LP01 mode is greater than or equal to 0.05% and a relative effective refractive index difference of light in an LP21 mode is less than 0.05% in light propagating through the core.

Abstract: An amplification optical fiber includes: a core doped with an active element; an inner cladding; and an outer cladding. The inner cladding is twisted about a center axis of the core, an outer circumference of the inner cladding has a polygon with rounded corners; and an angularity c defined by Expression (1) and Expression (2) is 0.15 or more and 0.8 or less, A=cos(?/n) . . . (1), c={1?(d1/d2)}/(1?A) . . . (2), where a number of vertices of the polygon is defined as n, a diameter of a circumcircle of the outer circumference of the inner cladding is defined as d2, and a diameter of an inscribed circle of the outer circumference of the inner cladding is defined as d1.

Abstract: An optical fiber, used in a laser device, propagates light having a wavelength of 1060 nm through a core in at least an LP01 mode and an LP11 mode. A difference between a propagation constant of light in the LP01 mode and a propagation constant of light in the LP11 mode is 1850 rad/m or more and 4000 rad/m or less.

Abstract: Provided is an optical coupler configured to cause an NA of light, which exits a taper fiber, to be smaller as compared with a conventional optical coupler. A taper fiber has a high refractive index part which is provided inside a core of the taper fiber and which has a refractive index smaller than a refractive index ncore of the core. An exit end surface of each GI fiber is bonded to an entrance end surface of the taper fiber so that at least a part of the exit end surface of the each GI fiber overlaps with a section of the high refractive index part. A relative refractive index difference of the taper fiber is smaller than 0.076%.

Abstract: An amplification optical fiber according to the present invention includes: a core doped with an active element, through which multi-mode light propagates; an inner cladding that surrounds the core and has a refractive index lower than that of the core; and an outer cladding that surrounds the inner cladding and has a refractive index lower than that of the inner cladding. The inner cladding has a polygonal outline in a cross section perpendicular to the longitudinal direction, and the inner cladding has a permanent twist applied by turning around the central axis of the core.

Abstract: Provided is an optical coupler configured to cause an NA of light, which exits a taper fiber, to be smaller as compared with a conventional optical coupler. A taper fiber has a high refractive index part which is provided inside a core of the taper fiber and which has a refractive index smaller than a refractive index ncore of the core. An exit end surface of each GI fiber is bonded to an entrance end surface of the taper fiber so that at least a part of the exit end surface of the each GI fiber overlaps with a section of the high refractive index part. A relative refractive index difference of the taper fiber is smaller than 0.076%.