The invention relates to an optical link comprising N optical fibers, with N?2. Each optical fiber comprises an optical core and an optical cladding surrounding the optical core, the optical core having a single ?.sub.i graded-index profile with ?.sub.i?1, and the optical core having a radius R.sub.1i, where i E [1; N] is an index designating said optical fiber. Said optical cladding comprises a region of depressed refractive index n.sub.trenchi, called a trench, surrounding the optical core. According to embodiments of the invention, for all optical fibers in said link, said optical core radius R.sub.1i and said length L.sub.i are chosen such that R.sub.1i?13.5 ?m and so as to satisfy a criterion C of quality. Thus, the invention provides a few-mode optical fiber link, which allow guiding an increased number of LP modes as compared to prior art FMF links, while reaching low Differential Mode Group Delay.

An object of the present invention is to provide a structure of an optical fiber capable of satisfying desired requirements of an output power, a propagation distance, and a beam quality. In the design of the PCF of the present invention, the PCF has air holes having diameters d and intervals in an overlapping region where a region of A.sub.eff of a desired value or more and a cutoff region in a desired higher-order mode overlap each other on a graph where the horizontal axis represents d/ and the vertical axis represents , so that it is possible to sufficiently cut off the mode which is the desired higher-order mode or more, and thus, it is possible to select a region where the A.sub.eff is large.

A higher-order mode (HOM) fiber is configured as a polarization-maintaining fiber by including a pair of stress rods at a location within the cladding layer that provides for a sufficient degree of birefringence without unduly comprising the spatial mode profile of the propagating higher-order modes. Long-period gratings are used as mode couplers at the input and output of the PM-HOM fiber, where the gratings are formed by exposing areas of the core region orthogonal to the position of the stress rods. The diameter of the stress rods (D) and displacement of the rods from the center of the core region (R1) are controlled to yield a configuration with an acceptable birefringence and polarization extinction ratio (PER) within the HOM fiber, even in situations where the fiber is bent (a bend radius less than 50 cm).

A multicore fiber communicates using light up to an xth-order LP mode (where x is an integer of 1 or more) in a communication band. The multicore fiber includes: a plurality of cores; a clad that surrounds the plurality of cores and has a refractive index lower than refractive indexes of the plurality of cores; and a cover layer that covers the clad and has a refractive index higher than the refractive index of the clad. Each of the plurality of cores propagates light up to an (x+1)th-order LP mode. A core pitch is set to a distance where crosstalk of the light up to the xth-order LP mode becomes less than or equal to 40 dB/km and crosstalk of light of the (x+1)th-order LP mode becomes greater than or equal to 30 dB/km.

A multi-core fiber performs communication using light up to an xth-order LP mode (where x is an integer of 1 or more) in a communication band. The multi-core fiber includes: a plurality of signal light propagation cores that propagate light up to an (x+1)th-order LP mode; and at least one high-loss core that has a higher loss of propagated light than the signal light propagation cores. Crosstalk occurs between light of the (x+1)th-order LP mode propagated through at least one signal light propagation core and light of a primary LP mode propagated through at least one high-loss core.

An HOM-based optical fiber amplifier is selectively doped within its core region to minimize the presence of dopants in those portions of the core where the unwanted lower-order modes (particularly, the fundamental mode) of the signal reside. The reduction (elimination) of the gain medium from these portions of the core minimizes (perhaps to the point of elimination) limits the amount of amplification impressed upon the backward-propagating Stokes wave. This minimization of amplification will, in turn, lead to a reduction in the growth of the Stokes power that is generated by the Brillouin gain, which results in increasing the amount of power present in the desired, forward-propagating HOM amplified optical signal output.

The invention relates to an optical fiber comprising an optical core and an optical cladding surrounding the optical core, the optical core having a single graded-index profile with 1, and the optical core having a radius R1 and a maximal refractive index n.sub.0, said optical cladding having a refractive index n.sub.Cl. Said optical cladding comprises a region of depressed refractive index n.sub.trench, having an inner radius R.sub.2, with R.sub.2R.sub.1, and an outer radius R3, with R3>R2. According to embodiments of the invention, the -value of said graded index profile and the optical core radius R.sub.1 are chosen such that R.sub.113.5 m and so as to satisfy a criterion C of quality. Thus, the invention provides a few-mode optical fiber, which allow guiding an increased number of LP modes as compared to prior art FMFs, while reaching the lowest Differential Mode Group Delay. The system reach is thus increased over prior art.

The specification describes multimode optical fibers with specific design parameters, i.e., controlled refractive index design ratios and dimensions, which render the optical fibers largely immune to moderately severe bends. The modal structure in the optical fibers is also largely unaffected by bending, thus leaving the optical fiber bandwidth essentially unimpaired. Bend performance results were established by DMD measurements of fibers wound on mandrels vs. measurements of fibers with no severe bends. Additional embodiments of the present invention describe an improved optical link when the inventive multimode fiber is connected to standard or conventional multimode fibers.