An optical fiber has a core to which chlorine is added and a clad to which fluorine is added, chlorine of 9000 to 13000 ppm is added to the core, a relative refractive index difference Δ1 of the core to a pure silica glass is 0.09 to 0.13%, a relative refractive index difference Δ2 of the clad to a pure silica glass is −0.36 to −0.17%, a difference (Δ1-Δ2) between the relative refractive index difference Δ1 of the core and the relative refractive index difference Δ2 of the clad is larger than or equal to 0.30%, a mode field diameter at wavelength 1.31 μm is 8.8 to 9.6 μm, and a stress difference occurring at an interface between the core and the clad is lower than or equal to 60 MPa.

A production method and others according to the present embodiment are provided with a structure for effectively preventing occurrence of accidental spiking during drawing of a preform. In order to control the residual He-concentration in the center part of the preform, a transparent glass rod that has a predetermined outer diameter and is already sintered but is not doped with an alkali metal yet is annealed in in the atmosphere not containing He gas for an annealing time determined by referring to result data in which the relationship between the annealing time and the residual He-concentration is previously recorded for each outer diameter. In the result data, actually measured data of the residual He-concentration in a produced optical fiber preform and the annealing time are accumulated as annealing treatment results.

A method of manufacturing a glass core preform for optical fibres including providing a porous soot core preform having a central longitudinal hole extending axially therethrough and an a/b ratio of from 0.20 to 0.40; simultaneously dehydrating and doping with fluorine the soot core preform at a temperature of from 1000 C. to 1350 C. by exposing it to an atmosphere containing a chlorine-containing gas and a fluorine-containing gas, the content of the fluorine-containing gas in the atmosphere being of from 0.01% to 0.50% by volume, and simultaneously consolidating the soot core preform and closing the central longitudinal hole by exposing the soot core preform to an atmosphere substantially devoid of fluorine and of chlorine at a consolidation temperature of from 1500 C. to 1650 C., while reducing the pressure down the central hole, thereby forming a glass core preform.

Preparation of halogen-doped silica is described. The preparation includes doping silica with high halogen concentration and sintering halogen-doped silica to a closed-pore state. The sintering includes a high pressure sintering treatment and a low pressure sintering treatment. The high pressure sintering treatment is conducted in the presence of a high partial pressure of a gas-phase halogen doping precursor and densifies a silica soot body to a partially consolidated state. The low pressure sintering treatment is conducted in the presence of a low partial pressure of gas-phase halogen doping precursor and transforms a partially consolidated silica body to a closed-pore state. The product halogen-doped silica glass exhibits little foaming when heated to form fibers in a draw process or core canes in a redraw process.

According to embodiments, a method of making a micro structured glass article 100 includes bundling M bare optical fibers in a fiber bundle, wherein M is an integer greater than 100. Thereafter, the fiber bundle may be inserted in a cavity of a soot preform. The soot preform may have a density of less than or equal to 1.5 g/cm3 and comprise silica-based glass soot. The soot preform and inserted fiber bundle may then be consolidated to form a microstructured glass article preform. The micro structured glass article preform may then be drawn into the microstructured glass article 100 comprising M core elements 102 embedded in a cladding matrix 104.

An improved process for preheating and doping a preform having a consolidated glass core and a silica soot cladding surrounding core involves waveguiding millimeter wavelength electromagnetic radiation into the preform to cause heating of the preform within the interior via absorption of the electromagnetic radiation by silica in the preform while the preform is exposed to a gas phase dopant.

Bromine doping of silica glass is demonstrated. Bromine doping can be achieved with SiBr.sub.4 as a precursor. Bromine doping can occur during heating, consolidation or sintering of a porous silica glass body. Doping concentrations of bromine increase with increasing pressure of the doping precursor and can be modeled with a power law equation in which doping concentration is proportional to the square root of the pressure of the doping precursor. Bromine is an updopant in silica and the relative refractive index of silica increases approximately linearly with doping concentration. Bromine can be used as a dopant for optical fibers and can be incorporated in the core and/or cladding regions. Core doping concentrations of bromine are sufficient to permit use of undoped silica as an inner cladding material in fibers having a trench in the refractive index profile. Co-doping of silica glass with bromine and chlorine is also demonstrated.

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.

Methods for manufacturing fluorine-doped glass preforms for optical fibers are disclosed. An exemplary method includes exposing a soot preform to an atmosphere containing a fluorine-containing gas in a first elongated chamber of a first furnace. The first elongated chamber typically has a single isothermal hot zone, which may be maintained at a doping temperature of about 800 C. to 1200 C., to obtain a fluorine-doped soot preform. The exemplary method further includes dehydrating the fluorine-doped soot preform by exposing it to an atmosphere containing a chlorine-containing gas in a second elongated chamber of a second furnace. The second elongated chamber typically has an upper hot zone, which may be maintained at a dehydration temperature of about 1000 C. to 1350 C., and a lower hot zone, which may be maintained at a consolidation temperature of about 1500 C. to 1650 C. Dehydration of the fluorine-doped soot preform typically occurs in the upper hot zone of the second furnace. The exemplary method further includes consolidating the fluorine-doped soot preform within the lower hot zone of the second furnace to form a fluorine-doped glass preform.

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 in the fiber having a graded refractive index profile with an alpha of greater than 5. The fiber also includes a cladding in the fiber that surrounds the core addition, the core includes silica that is co-doped with two or more halogens.