An optical fiber includes a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. A mean relative refractive index difference Δ1 of the core, a mean relative refractive index difference Δ2 of the inner cladding, and a mean relative refractive index difference Δ3 of the outer cladding satisfy a relationship of Δ1>Δ3≥Δ2. A ratio r2/r1 of an inner cladding radius r2 to a core radius r1 is 4.5 or higher and 5.5 or lower. A minimum value Δmin of a relative refractive index difference is −0.030% or higher and −0.010% or lower. A radius rmin at which the relative refractive index difference is the minimum value Δmin satisfies a relationship of r1<rmin<r2. (Δmin−Δ(r1))/(rmin−r1) is −0.002%/μm or lower, where Δ(r1) denotes the relative refractive index difference with the core radius r1.

The present disclosure provides an optical fibre. The optical fibre includes a core extended from a central longitudinal axis to a first radius r1. Further, the optical fibre includes a first trench region extended from a second radius r2 to a third radius r3, a second trench region extended from the third radius r3 to a fourth radius r4 and a cladding region extended from the fourth radius r4 to a fifth radius r5.

A method for manufacturing an optical fibre includes placing the powdery substance compactly in the fluorine doped tube to form a core section. The core section of the glass preform is defined along a longitudinal axis of the glass preform. In particular, the fluorine doped tube is sintered to solidify the powdery substance. Moreover, the glass preform is heated at high temperature to draw the optical fibre.

A gain flattening filter includes a first optical fiber that has a core, a first cladding, and a second cladding and that has a uniform composition in a length direction; and a pair of second optical fibers fused to both ends of the first optical fiber. The first optical fiber has a first section in which a slanted refractive index grating is formed and a pair of second sections connecting both ends of the first section to the pair of second optical fibers. The first cladding contains a photosensitive material whose refractive index increases upon irradiation with light having a specific wavelength. In the core, a tensile stress remains in the first section. An average MFD of the second sections is larger than an average MFD of the second optical fibers and smaller than an average MFD of the first section.

The refractive index of the inner core part 11 in a region in contact with the boundary of the outer core part 12 is higher than the refractive index of the outer core part 12. The refractive index of the outer core part 12 is gradually decreased from the inner circumferential side to the outer circumferential side. The refractive index of the inner cladding part 21 is equal to the refractive index of the outermost circumferential part of the outer core part 12 and not greater than the refractive index of the outer cladding part 22.

The present disclosure provides an optical fibre. The optical fibre includes a core region, a primary trench region and a secondary trench region. The core region has a radius r.sub.1. In addition, the core region has a relative refractive index Δ.sub.1. Further, the primary trench region has a relative refractive index Δ.sub.3. Furthermore, the primary trench region has a curve parameter α.sub.trench-1. Moreover, the secondary trench region has a relative refractive index Δ.sub.4. Also, the secondary trench region has a curve parameter α.sub.trench-2.

A power over fiber system includes an optical fiber cable. The optical fiber cable includes a core, a first cladding and a second cladding. The core transmits signal light. The first cladding is positioned in contact with periphery of the core and transmits feed light. The second cladding is positioned in contact with periphery of the first cladding. Radial refractive index distribution of the first cladding is distribution in which refractive index gradually decreases from a local maximum at an internal point toward points where the first cladding is in contact with the core and the second cladding, respectively. The internal point is away from the core and the second cladding. The refractive index of the core is higher than the refractive index of the first cladding at the point where the first cladding is in contact with the core.

A composite optical fiber is provided for permitting sensing of multiple parameters. The optical fiber is for incorporation into a sensing system, the optical fiber comprising: a single mode optical fiber core, a multimode optical fiber core, and an optical fiber cladding layer surrounding the single mode optical fiber core and the multimode optical fiber core. The optical fiber provided preferably enables multiple sensing and/or measurements to take place at a single location and at a single time.

An optical fiber includes a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. A mean relative refractive index difference Δ1 of the core, a mean relative refractive index difference Δ2 of the inner cladding, and a mean relative refractive index difference Δ3 of the outer cladding satisfy a relationship of Δ1>Δ3≥Δ2. A ratio r2/r1 of an inner cladding radius r2 to a core radius r1 is 4.5 or higher and 5.5 or lower. A minimum value Δmin of a relative refractive index difference is −0.030% or higher and −0.010% or lower. A radius rmin at which the relative refractive index difference is the minimum value Δmin satisfies a relationship of r1<rmin<r2. (Δmin−Δ(r1))/(rmin−r1) is −0.002%/μm or lower, where Δ(r1) denotes the relative refractive index difference with the core radius r1.

A composite optical fiber is provided for permitting sensing of multiple parameters. The optical fiber is for incorporation into a sensing system, the optical fiber comprising: a single mode optical fiber core, a multimode optical fiber core, and an optical fiber cladding layer surrounding the single mode optical fiber core and the multimode optical fiber core. The optical fiber provided preferably enables multiple sensing and/or measurements to take place at a single location and at a single time.