T/C, which is a ratio of an area T of a skirt part outside the boundary to an area C of the core region in a refractive index distribution, is 4% or more and 30% or less, the boundary is defined at a position where an absolute value of a change amount of the index becomes maximum between the center of the core region and the outer peripheral part of the first clad region, the area C of the core region is defined in a range from the center of the core region to the boundary in the radial direction, the area T of the skirt part is defined in a range from the boundary to the outer peripheral part of the first clad region.

Small-radius coated optical fibers having large mode field diameter and low bending losses. The coated fiber may have an outer radius of 110 μm or less, while providing a mode field diameter of 9.0 μm or greater and a bending loss when wrapped about a 15 mm mandrel of 0.5 dB/km or less at wavelength of 1550 nm. The coated fiber may have a mode field diameter of 9.2 μm or greater and may have a bending loss at 1550 nm of 0.25 dB/km or less when wrapped about a 20 mm mandrel or a bending loss at 1550 nm of 0.02 dB/km or less when wrapped about a 30 mm mandrel.

An optical fiber includes (i) a chlorine doped silica based core having a core alpha (Core.sub.α)≥4, a radius r.sub.1, and a maximum refractive index delta Δ.sub.1max % and (ii) a cladding surrounding the core. The cladding surrounding the core includes a) a first inner cladding region adjacent to and in contact with the core and having a refractive index delta Δ.sub.2, a radius r.sub.2, and a minimum refractive index delta Δ.sub.2min such that Δ.sub.2min<Δ.sub.1max, b) a second inner cladding adjacent to and in contact with the first inner cladding having a refractive index Δ.sub.3, a radius r.sub.3, and a minimum refractive index delta Δ.sub.3min such that Δ.sub.3min<Δ.sub.2, and c) an outer cladding region surrounding the second inner cladding region and having a refractive index Δ.sub.5, a radius r.sub.max, and a minimum refractive index delta Δ.sub.3min such that Δ.sub.3min<Δ.sub.2. The optical fiber has a mode field diameter MFD at 1310 of ≥9 microns, a cable cutoff of ≤1260 nm, a zero dispersion wavelength of 1300 nm≤zero dispersion wavelength≤1324 nm, and a macrobending loss at 1550 nm for a 20 mm mandrel of less than 0.75 dB/turn.

An optical fiber with low attenuation and methods of making same are disclosed. The optical fiber has a core, an inner cladding surround the core, and an outer cladding surrounding the inner cladding. The outer cladding is chlorine-doped such that the relative refractive index varies as a function of radius. The radially varying relative refractive index profile of the outer cladding reduces excess stress in the core and inner cladding, which helps lower fiber attenuation while also reducing macrobend and microbend loss. A process of fabricating the optical fiber includes doping an overclad soot layer of a soot preform with chlorine and then removing a portion of the chlorine dopant from an outermost region of the overclad soot layer. The soot preform with the modified chlorine dopant profile is then sintered to form a glass preform, which can then be used for drawing the optical fiber.

An aim is to provide an optical coupler that contributes increasing pump-efficiency in an optical amplifier, and the optical amplifier. The optical amplifier includes: a main optical fiber that includes a core transmitting signal light, an inner cladding portion formed around an outer periphery of the core and having a refractive index lower than a refractive index of the core, and an outer cladding portion formed around an outer periphery of the inner cladding portion and having a refractive index lower than the refractive index of the inner cladding portion, a part of the outer cladding portion of which in a longitudinal direction being removed; and at least one pump-light input-output optical fiber that is fusion-spliced to the inner cladding portion of the main optical fiber at the portion where the outer cladding portion is removed, an average refractive index of which in a contact region where the pump-light input-output optical fiber is in contact with the inner cladding portion being larger than the refractive index of the inner cladding portion. Pump-light propagating in the pump-light input-output optical fiber is coupled to the inner cladding portion from the contact region and propagates in the inner cladding portion, or the pump-light propagating in the inner cladding portion is coupled to the pump-light input-output optical fiber from the contact region and propagates in the pump-light input-output optical fiber.

The present disclosure provides optical fibers with an impact-resistant coating system. The fibers feature low attenuation. The coating system includes a primary coating and a secondary coating. The primary coating and secondary coating have reduced thickness to provide low-diameter fibers without sacrificing protection. The primary coating has high tear strength and is resistant to damage caused by mechanical force. The secondary coating has high puncture resistance. The outer diameter of the optical fiber is less than or equal to 190 μm.

The present disclosure provides optical fibers with an impact-resistant coating system. The fibers feature low attenuation. The coating system includes a primary coating and a secondary coating. The primary coating and secondary coating have reduced thickness to provide low-diameter fibers without sacrificing protection. The primary coating has high tear strength and is resistant to damage caused by mechanical force. The secondary coating has high puncture resistance. The outer diameter of the optical fiber is less than or equal to 190 μm.

Small-radius coated optical fibers having large mode field diameter and low bending losses. The coated fiber may have an outer radius of 110 μm or less, while providing a mode field diameter of 9.0 μm or greater and a bending loss when wrapped about a 15 mm mandrel of 0.5 dB/km or less at wavelength of 1550 nm. The coated fiber may have a mode field diameter of 9.2 μm or greater and may have a bending loss at 1550 nm of 0.25 dB/km or less when wrapped about a 20 mm mandrel or a bending loss at 1550 nm of 0.02 dB/km or less when wrapped about a 30 mm mandrel.

Small-radius coated optical fibers having large mode field diameter and low bending losses. The coated fiber may have an outer radius of 110 μm or less, while providing a mode field diameter of 9.0 μm or greater and a bending loss when wrapped about a 15 mm mandrel of 0.5 dB/km or less at wavelength of 1550 nm. The coated fiber may have a mode field diameter of 9.2 μm or greater and may have a bending loss at 1550 nm of 0.25 dB/km or less when wrapped about a 20 mm mandrel or a bending loss at 1550 nm of 0.02 dB/km or less when wrapped about a 30 mm mandrel.

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.