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 high-density FAU comprises a support substrate having a grooved front-end section that supports glass end sections of the small diameter low-attenuation optical fibers. A cover is disposed on the front-end section and secured thereto to hold the glass end sections in place. The substrate and the cover can be made of the same glass or glasses having about the same CTE. The glass end sections have a diameter d4 so that the pitch P2 of the fibers at the front end of the FAU can be equal to or greater than d4, wherein d4=2r.sub.4, with r.sub.4 being the radius of the glass end section as defined by the optical fiber cladding. The glass end section has a radius r.sub.4 less than 45 microns, allowing for a high-density FAU and a high-density optical interconnection device.

An optical fiber includes: a central core portion; an intermediate layer; a trench layer; and a cladding portion. Further, relationships Δ1>Δ2>Δ3 and 0>Δ3 are satisfied, where Δ1, Δ2, and Δ3 are a relative refractive-index difference of the central core portion, the intermediate layer, and the trench layer, respectively, with respect to the cladding portion, Δ1 is equal to or larger than 0.34% and equal to or smaller than 0.37%, |Δ3| is equal to or larger than 0.1% and equal to or smaller than 0.25%, Δ1×|Δ3| is equal to or smaller than 0.08%.sup.2, a mode field diameter at a wavelength of 1310 nm is equal to or larger than 8.8 μm, and a transmission loss at a wavelength of 1550 nm is equal to or smaller than 0.195 dB/km.

A rollable optical fiber ribbon utilizing low attenuation, bend insensitive fibers and cables incorporating such rollable ribbons are provided. The optical fibers are supported by a ribbon body, and the ribbon body is formed from a flexible material such that the optical fibers are reversibly movable from an unrolled position to a rolled position. The optical fibers have a large mode filed diameter, such as ≥9 microns at 1310 nm facilitating low attenuation splicing/connectorization. The optical fibers are also highly bend insensitive, such as having a macrobend loss of ≤0.5 dB/turn at 1550 nm for a mandrel diameter of 15 mm.

Embodiments of the current disclosure include small diameter single-mode optical fibers having gratings and methods of forming thereof. In some embodiments, methods of forming a small diameter single-mode optical fibers having gratings include providing an optical fiber having a core and cladding with a combined outer diameter of 100 μm to 125 μm and a coating having a thickness of less than or equal to 20 μm, wherein the coating comprises one of: (i) a high-modulus coating layer surrounding the cladding region; or (ii) a low-modulus coating layer surrounding the cladding region and a high-modulus coating layer surrounding the low-modulus coating layer; and exposing the core, through the coating, to a pattern of ultraviolet radiation to form an optical grating within the core.

A single mode optical fiber is provided that includes a core region having an outer radius r.sub.1 and a maximum relative refractive index Δ.sub.1max. The single mode optical fiber further includes a cladding region surrounding the core region, the cladding region includes a depressed-index cladding region, a relative refractive index Δ.sub.3 of the depressed-index cladding region increasing with increased radial position. The single mode optical fiber has a bend loss at 1550 nm for a 15 mm diameter mandrel of less than about 0.75 dB/turn, a bend loss at 1550 nm for a 20 mm diameter mandrel of less than about 0.2 dB/turn, and a bend loss at 1550 nm for a 30 mm diameter mandrel of less than 0.005 dB/turn. Additionally, the single mode optical fiber has a mode field diameter of 9.0 microns or greater at 1310 nm wavelength.

A coupled-core multicore optical fiber has a plurality of cores that are doped with alkali metals or chlorine to achieve low attenuation and a large effective area. The cores may be embedded in a common cladding region that may be fluorine doped. The cores may also be doped with chlorine, either with the alkali metals described above or without the alkali metals.

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 multimode optical fiber having a core region. The core region includes silica, has an outer radius r.sub.1, and has a maximum relative refractive index of about 1.5% or less. Additionally, the multimode optical fiber is configured to have an effective bandwidth of about 4.7 GHz-Km or greater for an excited portion of the core region that has a diameter greater than 50 microns, the effective bandwidth being at a wavelength that is within a range of between about 800 and about 1370 nm.

An optical device includes a core, a first cladding, a second cladding, a slanted fiber Bragg grating, and a high refractive index material. The first cladding covers the core and has a lower refractive index than the core. The second cladding covers the first cladding and has a lower refractive index than the first cladding. The slanted fiber Bragg grating is formed in the core and couples stimulated Raman scattering light, propagating through the core, to the first cladding. The high refractive index material has a higher refractive index than the second cladding and covers an outer peripheral surface of a removal portion where the second cladding is removed and a portion of the first cladding that covers the region where the slanted fiber Bragg grating is formed in the core.