An optical fiber includes a core portion having a radius r.sub.C and a graded refractive index profile .sub.C having an alpha value greater than or equal to 1 and less than or equal to 8. The core portion includes a silica-based glass and a down-dopant, where a concentration of the down-dopant is graded such that the concentration of the down-dopant decreases from the radius r.sub.C towards the center of the core portion. The optical fiber comprises a cladding portion surrounding the core portion and having a relative refractive index .sub.OC that is less than a maximum refractive index .sub.Cmax of the core portion.

Described herein are coated optical fibers including an optical fiber portion, wherein the optical fiber portion includes a glass core and cladding section that is configured to possesses certain mode-field diameters and effective areas, and a coating portion including a primary and secondary coating, wherein the primary coating is the cured product of a composition that possesses specified liquid glass transition temperatures, such as below 82 C., and/or a viscosity ratios, such as between 25 C. and 85 C., of less than 13.9. Also described are radiation curable coating compositions possessing reduced thermal sensitivity, methods of coating such radiation curable coating compositions to form coated optical fibers, and optical fiber cables comprising the coated optical fibers and/or radiation curable coating compositions elsewhere described.

An optical fiber includes a core that propagates a light that includes a wavelength equal to or larger than 1000 nm and equal to or smaller than 1100 nm. The light propagates in the core at least in an LP01 mode and an LP11 mode. A difference between a propagation constant of the light in the LP01 mode and a propagation constant of the light in the LP11 mode is 1735 rad/m or larger and 4000 rad/m or smaller.

An optical fiber comprising a core region doped with a first alkali dopant and a second alkali dopant. The first alkali dopant has a first average core concentration of C1 and a first diffusivity D1. The second alkali dopant has a second average core concentration of C2 and a second diffusivity D2. An outer cladding region surrounds the core region. The diffusivities D1, D2 of the first and second alkali dopants satisfy the relation D1>D2. The average core concentrations C1, C2 of the first and second alkali dopants satisfy the relation 0.1C2/C11.

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 .sub.1. Further, the primary trench region has a relative refractive index .sub.3. Furthermore, the primary trench region has a curve parameter .sub.trench-1. Moreover, the secondary trench region has a relative refractive index .sub.4. Also, the secondary trench region has a curve parameter .sub.trench-2.

Described herein are coated optical fibers including an optical fiber portion, wherein the optical fiber portion includes a glass core and cladding section that is configured to possesses certain mode-field diameters and effective areas, and a coating portion including a primary and secondary coating, wherein the primary coating is the cured product of a composition that possesses specified liquid glass transition temperatures, such as below 82 C., and/or a viscosity ratios, such as between 25 C. and 85 C., of less than 13.9. Also described are radiation curable coating compositions possessing reduced thermal sensitivity, methods of coating such radiation curable coating compositions to form coated optical fibers, and optical fiber cables comprising the coated optical fibers and/or radiation curable coating compositions elsewhere described.

A few mode optical fiber that includes an optical core and an optical cladding surrounding the optical core. The FMF has a step-index profile. The optical core has a core outer radius R.sub.17.5 m and a core refractive index difference n.sub.1 such that 14.510.sup.3<n.sub.1<2410.sup.3. The optical cladding comprises: an index ring with: a ring inner radius R.sub.r1 between 12 m and 19 m; a ring refractive index difference n.sub.r such that n.sub.1/n.sub.r is between 2 and 4; a ring volume V.sub.ring=n.sub.r(Rr.sub.2.sup.2Rr.sub.1.sup.2) between 1.8 m.sup.2 and 4.1 m.sup.2 where R.sub.r2 is the ring outer radius; an inner cladding between the optical core and the index ring, with an inner cladding inner radius R.sub.i1 and an inner cladding outer radius R.sub.i2, the inner cladding having an inner cladding refractive index difference n.sub.clad1 between 1.010.sup.3 and 1.010.sup.3.

The optical fiber has an effective area that is greater than or equal to 110 m.sup.2 and less than or equal to 180 m.sup.2 at a wavelength of 1550 nm and a cable cut-off wavelength of less than or equal to 1530 nm. An average value of a glass outer diameter in a longitudinal direction is 1250.5 m. When is a standard deviation of the glass outer diameter in the longitudinal direction, 3 is greater than or equal to 0.1 m and less than or equal to 0.5 m.

The optical fiber according to the present disclosure is an optical fiber having a pure silica core which is a step index core through which an LPm1 mode with an effective area of 250 m.sup.2 or more is propagated by setting a relative refractive index difference to 0.60% or less, considering an effective cutoff condition, and only 2 or more modes or the LPm1 mode are propagated, with the optical fiber as the transmission line.

An optical fiber according to an embodiment has a structure for enabling determination of improvement in transmission loss at a preform stage. The optical fiber includes: a core containing Cl and having an average refractive index lower than a refractive index of pure silica glass; a first cladding containing F; a second cladding; and a resin coating, in which an effective area at a wavelength of 1550 nm is 135 m.sup.2 or more and 170 m.sup.2 or less, a ratio of the effective area to a cutoff wavelength .sub.C is 85.0 m or more, a bending loss of an LP01 mode at a wavelength of 1550 nm and at a bending radius of R15 mm is less than 4.9 dB per 10 turns, and the resin coating includes a primary resin layer having a Young's modulus of 0.3 MPa or less.