A method for manufacturing an optical fiber preform includes: producing a core preform including a core portion made of transparent glass and a first cladding layer obtained by adding fluorine to the core portion; and forming, on an outer periphery of the first cladding layer, a second cladding layer made of glass having a refractive index higher than that of the first cladding layer. Further, a refractive index profile is formed in the first cladding layer due to a fluorine concentration profile, the refractive index profile being provided at least near a boundary surface with the second cladding layer and having a profile such that a refractive index difference between a refractive index of the first cladding layer and a refractive index of the second cladding layer decreases in accordance with a reduction in a distance from the boundary surface with the second cladding layer.

A method of forming an optical fiber preform includes the steps: igniting a burner having a fume tube assembly to produce a first spray size of silicon dioxide particles; depositing the silicon dioxide particles on a core cane to produce a soot blank; and adjusting an effective diameter of an aperture of the fume tube assembly to produce a second spray size of the silicon dioxide particles. The second spray size is larger than the first spray size.

Apparatuses and methods for processing optical fiber preforms are disclosed. According to one aspect, an apparatus generally includes a furnace body and a muffle disposed within the furnace body. A space between the muffle and the furnace body defines a first interior volume. The muffle defines a second interior volume sealed from the first interior volume. An annulus gas is supplied to the first interior volume and a process gas is supplied to the second interior volume. A differential pressure gauge is coupled to the interior volumes. A flow controller is coupled to at least one of the gas sources and to the differential pressure gauge. The flow controller receives a differential pressure signal from the differential pressure gauge and adjusts a flow of a gas such that the pressure differential between the first interior volume and the second interior volume is minimized.

A glass body manufacturing apparatus includes: a first heating furnace including a furnace core tube accommodating the soot and a first heater, to supply a dehydration gas into the furnace core tube and heat the soot at a first treatment temperature lower than a softening point of the porous portion by the first heater; a second heating furnace including a structural body accommodating the soot and a second heater, to heat the soot at a second treatment temperature equal to or higher than the softening point by the second heater; and a conveyance container, connectable to each of the first and second heating furnaces while keeping airtightness with respect to the atmosphere, to accommodate and hold the soot, and convey the soot between the first and second heating furnaces.

A co-doped optical fiber is provided having an attenuation of less than about 0.17 dB/km at a wavelength of 1550 nm. The fiber includes a core region in the fiber having a graded refractive index profile with an alpha of greater than 5. The fiber also includes a first cladding region in the fiber that surrounds the core region. Further, the core region has a relative refractive index of about −0.10% to about +0.05% compared to pure silica. In addition, the core region includes silica that is co-doped with chlorine at about 1.2% or greater by weight and fluorine between about 0.1% and about 1% by weight.

A co-doped optical fiber is provided having an attenuation of less than about 0.17 dB/km at a wavelength of 1550 nm. The fiber includes a core region in the fiber having a graded refractive index profile with an alpha of greater than 5. The fiber also includes a first cladding region in the fiber that surrounds the core region. Further, the core region has a relative refractive index of about −0.10% to about +0.05% compared to pure silica. In addition, the core region includes silica that is co-doped with chlorine at about 1.2% or greater by weight and fluorine between about 0.1% and about 1% by weight.

According to some embodiments method for making an optical fiber preform comprises the steps of: (i) placing a plurality of rods with an outer surface having a coefficient of friction 0.02≦COF≦0.3 into an inner cavity of an apparatus; (ii) placing particulate glass material in the inner cavity between the rods and an inner wall of the mold cavity; and (iii) applying pressure against the particulate glass material to press the particulate glass material against the plurality of rods.

According to some embodiments method for making an optical fiber preform comprises the steps of: (i) placing a plurality of rods with an outer surface having a coefficient of friction 0.02≦COF≦0.3 into an inner cavity of an apparatus; (ii) placing particulate glass material in the inner cavity between the rods and an inner wall of the mold cavity; and (iii) applying pressure against the particulate glass material to press the particulate glass material against the plurality of rods.

A method of manufacturing a porous glass preform includes depositing glass particles on an outer periphery of a target rod while a burner for synthesizing glass particles and the target rod that is rotating are relatively reciprocated, wherein V and r are gradually reduced while a variation in sweeping pitch P [mm] expressed as V/r is caused to be within a range of a central value±10% when a glass particle deposition layer of a portion satisfying a relation 0.5 L≤R≤0.8 L is synthesized; where a final outer diameter of the manufactured porous glass preform for an optical fiber is L [mm], an outer diameter of a glass particle deposition body in the middle of the manufacture is R [mm], a rotating speed of the target rod is r [rpm], and a relative moving speed between the target rod and the burner is V [mm/min.].

An optical fiber includes: a central core portion; an intermediate layer; a trench layer; and a cladding portion. Further, Δ1>Δ2>Δ3 and 0>Δ3 are satisfied, where Δ1 is a relative refractive-index difference of the central core portion, Δ2 is a relative refractive-index difference of the intermediate layer, and Δ3 is a relative refractive-index difference of the trench layer with respect to the cladding portion, respectively, and (c−b) is smaller than 4.5 μm when Δ1 is equal to or larger than 0.36% and equal to or smaller than 0.40%, Δ2 is equal to or larger than −0.05% and equal to or smaller than 0.05%, |Δ3| is equal to or smaller than 0.25%, Δ1×|Δ3| is equal to or smaller than 0.08%.sup.2, an inner diameter of the trench layer is 2b, and an outer diameter of the trench layer is 2c.