An optical fiber includes: a first core portion capable of transmitting first light; a second core portion formed on an outer periphery of the first core portion in a structure different from that of the first core portion and capable of transmitting second light different from the first light. The second core portion is formed around the outer periphery of the first core portion, and a center of the second core portion is positioned in a region of the first core portion.

A mode loss difference compensator of the present disclosure includes a main waveguide configured to allow propagation of N or more modes (where N is an integer of 3 or more), a first auxiliary waveguide having, at one end thereof, a first coupling portion configured to mode-convert an LP0n mode (where n is an integer of 2 or more) propagating in the main waveguide into a fundamental mode in the first auxiliary waveguide and transfer the fundamental mode from the main waveguide to the first auxiliary waveguide and having, at the other end thereof, a second coupling portion configured to mode-convert the fundamental mode propagating in the first auxiliary waveguide into the LP0n mode (where n is an integer of 2 or more) in the main waveguide and transfer the LP0n mode from the first auxiliary waveguide to the main waveguide, and a second auxiliary waveguide having, at one end thereof, a third coupling portion configured to convert a higher-order mode, other than any LP0n mode (where n is an integer of 2 or more), propagating in the main waveguide into a fundamental mode in the second auxiliary waveguide and transfer the fundamental mode from the main waveguide to the second auxiliary waveguide and having, at the other end thereof, a terminal end portion configured to eliminate the fundamental mode propagating in the second auxiliary waveguide from the second auxiliary waveguide, wherein the main waveguide includes a loss imparting portion configured to impart a loss to a fundamental mode propagating in the main waveguide between the first and second coupling portions.

An optical fiber includes a core and a cladding surrounding an outer periphery of the core and has a refractive index profile in which a relative refractive index difference with respect to a distance r from a center of the core is represented by Δ(r), where a value of A represented by

A=−∫.sub.0.sup.0.22MFD.sup.1.31(Δ(r)−Δ.sub.ref(r))dr+∫.sub.0.22MFD.sub.1.31.sup.0.44MFD.sup.1.31(Δ(r)−Δ.sub.ref(r))dr  (Formula 1)

is 0.3%.Math.μm or less, where a unit of r is μm, a unit of a relative refractive index difference Δ(r) is %, Δ.sub.ref(r)=−0.064r+0.494, and MFD.sub.1.31 is a mode field diameter at a wavelength of 1.31 μm.

In an optical fiber, a plurality of individual cores extend through a common cladding. Each individual core supports at least one local transverse spatial mode. The individual cores and surrounding cladding are structured to support propagation of plurality of desired signal-carrying modes, while suppressing undesired modes, thereby supporting the propagation of one or more spatially multiplexed signals. The core-to-core spacing of the fiber is configured to maintain an acceptably low level of mode-coupling between cores.

An MCF cable according to an embodiment contains a plurality of MCFs each including at least one coupled core group and a common cladding. Λ is set such that κ at a wavelength of 1550 nm is falls within a range of from 1×10.sup.−1 [m.sup.−1] to 1×10.sup.3 [m.sup.−1], and (βΛC.sub.avg)/(2κ) or (βΛC.sub.f)/(2κ) is set in a specific range in a wavelength band of from 1530 nm to 1625 nm, where C.sub.avg [m.sup.−1], C.sub.f [m.sup.−1], and f.sub.twist [turn/m] represent the average curvature, the pseudo-curvature, and the average torsion, respectively, for each MCF, and κ [m.sup.−1], β [m.sup.−1], and Λ [m] represent the coefficient of mode coupling between adjacent cores, the average of propagation constants, and the core center-to-center distance, respectively.

The invention relates to an optical assembly for producing such. The invention also relates to the use of the optical assembly. Laser radiation received via a bundle of individual optical feed fibers is guided to a fiber laser fiber. Each feed fiber has a cladding layer surrounding the core of the fiber to provide total internal reflection in said core, and the cladding layers of the fibers are fused at least partially together to form a zone containing the cores of the feed fibers arranged in a cylindrical configuration inside said zone This configuration provides the shaping of an annular laser beam that can be fed into a fiber laser fiber having an annular light guiding zone and to present the annular laser beam e.g. to a workpiece.

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 n.sub.core 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%.

An optical fiber includes: a first core portion capable of transmitting first light; a second core portion formed on an outer periphery of the first core portion in a structure different from that of the first core portion and capable of transmitting second light different from the first light. The second core portion is formed around the outer periphery of the first core portion, and a center of the second core portion is positioned in a region of the first core portion.

In some implementations, a waveguide may comprise an inner core to receive a first beam and an outer core surrounding the inner core to receive a second beam that is displaced from the first beam by an offset. The outer core may comprise a beam guiding region that rotationally expands over a length of the waveguide into an annulus that concentrically surrounds the inner core or a partial annulus that partially surrounds the inner core. For example, the beam guiding region may be defined by one or more low refractive index features that have a varied orientation and/or a varied shape over the length of the waveguide such that the second beam enters the waveguide as an offset beam and exits from the waveguide as a ring-shaped beam or a partial ring-shaped beam.

An optical fiber according to an embodiment includes a core and a cladding. The average value n1_ave of the refractive index of the core, the minimum value nc_min of the refractive index of the cladding, and the refractive index n0 of pure silica glass satisfy relationships of n1_ave>nc_min and nc_min<n0. The cladding contains fluorine. The fluorine concentration in the cladding is adjusted to be minimum in the outermost portion of the cladding including the outer peripheral surface of the cladding.