Methods of manufacturing multi-mode optical fiber, and multi-mode optical fiber produced thereby, are disclosed. According to embodiments, a method for forming an optical fiber may include heating a multi-mode optical fiber preform and applying a draw tension to a root of the multi-mode optical fiber preform on a long axis of the multi-mode optical fiber preform thereby drawing a multi-mode optical fiber from the root of the multi-mode optical fiber preform. The draw tension may be modulated while the multi-mode optical fiber is drawn from the root of the multi-mode optical fiber preform. Modulating the draw tension introduces stress perturbations in the multi-mode optical fiber and corresponding refractive index perturbations in a core of the multi-mode optical fiber.

In some implementations, a substrate tube in a modified chemical vapor deposition process may rotate while glass precursors flow into the substrate tube at a fixed rate. Dopants may be delivered into the substrate tube while heat is applied to the substrate tube to deposit, on an inner wall of the substrate tube, a layer of material including the glass precursors and the dopants. A lateral position of an exit of an injection tube used to deliver the dopants may be adjusted while the substrate tube is rotated and heat is applied to the substrate tube such that the material deposited on the inner wall of the substrate tube has an azimuthally non-uniform doping concentration. Alternatively, a rotation of the substrate tube may be adjusted to create opposing temperature gradients within the substrate tube, causing non-uniform layer deposition to occur on different sides of the substrate tube in alternating passes.

The invention relates to a method of manufacturing an active optical fiber having a cladding and a doped core, as well as the active optical fiber equipped with the cladding and the doped core. The active optical fiber according to the invention is adapted to conduct and generate radiation having a wavelength and is provided with a cladding and a core containing at least one active dopant, characterized in that the core comprises elongate elements made of a first type of glass having a first refractive index n.sub.1 and elongate elements of a second type of glass having a second refractive index n.sub.2, oriented along the optical fiber and forming a compact bundle, wherein transverse dimensions of the elongate core elements are smaller than of the wavelength . Such optical fibers are used in laser generation and in amplification techniques.

According to some embodiments, a multimode optical fiber comprises a graded index glass core with refractive index 1, a maximum refractive index delta 1.sub.MAX, and a core radius between 10 and 40 microns; and cladding region surrounding the core comprising refractive index 4, wherein the fiber exhibits an overfilled bandwidth exhibits an overfilled bandwidth of at least 3 GHz-km at a wavelength of 850 nm and an overfilled bandwidth of at least 1.2 GHz-km at one or more wavelengths between 980 and 1060 nm.

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 (r), where a value of A represented by
A=.sub.0.sup.0.22MFD.sup.1.31((r).sub.ref(r))dr+.sub.0.22MFD.sub.1.31.sup.0.44MFD.sup.1.31((r).sub.ref(r))dr(Formula 1)
is 0.3%.Math.m or less, where a unit of r is m, a unit of a relative refractive index difference (r) is %, .sub.ref(r)=0.064r+0.494, and MFD.sub.1.31 is a mode field diameter at a wavelength of 1.31 m.

A method for a defined deposition of a glass layer on an inner wall of a preform for an optical fiber and/or for setting a refractive index profile of the preform for a multi-mode fiber. The method includes providing the preform having a cavity and an inner wall which defines an inner diameter of the preform, and spreading a deposition gas at a flow speed (v) in the cavity of the preform so as to provide the defined deposition of the glass layer. The defined deposition is performed at a reduced change in the flow speed a*v, where a<1. Based on the defined deposition, a change in the flow speed (v):

[00001]$.Math..Math.v=\frac{4.Math..Math.Q}{}.Math.\left(\frac{1}{d_i^2}-\frac{1}{d_{i+1}^2}\right)$

forms at a volume flow (Q), a first diameter (d.sub.i), and a second diameter (d.sub.i+1).

The invention relates to a method of manufacturing an active optical fibre having a cladding and a doped core, as well as the active optical fibre equipped with the cladding and the doped core. The active optical fibre according to the invention is adapted to conduct and generate radiation having a wavelength and is provided with a cladding and a core containing at least one active dopant, characterised in that the core comprises elongate elements made of a first type of glass having a first refractive index n.sub.1 and elongate elements of a second type of glass having a second refractive index n.sub.2, oriented along the optical fibre and forming a compact bundle, wherein transverse dimensions of the elongate core elements are smaller than of the wavelength . Such optical fibres are used in laser generation and in amplification techniques.

A method of making an optical fiber preform comprising in order: (i) manufacturing a glass preform with at least one porous layer; (ii) exposing the glass preform with at least one porous layer to a fluorine precursor at temperature below 1295 C. to make a fluorine treated preform, and (iii) exposing the fluorine treated glass preform with at least one porous silica based layer the temperatures above 1400 C. to completely sinter the preform. Preferably, the porous silica based layer of the glass preform exposed to fluorine precursor has average density of at least 0.7 g/cm.sup.3 but less than 1.9 g/cm.sup.3.

A method of making a multi-mode optical fiber that includes: depositing a porous germania-doped silica soot to form a germania-doped porous soot preform; depositing a porous silica layer over the porous soot preform; doping the porous soot preform and the porous silica layer with a fluorine dopant to form a co-doped soot preform having a core region and a fluorine-doped trench region; consolidating the co-doped soot preform to form a sintered glass, co-doped core preform having a refractive index alpha profile between 1.9 and 2.2 measured at 850 nm; depositing a cladding comprising silica over the sintered glass, co-doped preform to form a multi-mode optical fiber preform; drawing the optical fiber preform into a multi-mode optical fiber. Further, the step of doping the germania-doped soot preform and the porous silica layer is conducted according to a doping parameter () that is set between 20 and 300, and given by: $=\frac{1{10}^{14}{R}_{\mathrm{prc}}^2\exp \left(-E/{\mathrm{RT}}_{\mathrm{dop}}\right)T_{\mathrm{dop}}^{1/2}}{x^{3/4}}.$

Methods of forming a bandwidth-tuned optical fiber for short-length data transmission systems include establishing a relationship between a change in a modal delay , a change T in a draw tension T and a change in a BM wavelength of light in a BM wavelength range from 840 nm and 1100 nm for a test optical fiber drawn from a preform and that supports BM operation at the BM wavelength. The methods also include drawing from either the preform or a closely related preform the bandwidth-tuned optical fiber by setting the draw tension based on the established relationships of the aforementioned parameters so that the bandwidth-tuned optical fiber has a target bandwidth greater than 2 GHz.Math.km at a target wavelength within the BM wavelength range.