A single mode optical fiber having a core made from silica and less than or equal to about 11 weight % germania and having a maximum relative refractive index Δ.sub.1MAX. The optical fiber also has an inner cladding surrounding the core and having a minimum relative refractive index Δ.sub.2MIN, a first outer cladding surrounding the inner cladding and a second outer cladding surrounding the first outer cladding. The viscosity at 1650° C. of the second outer cladding minus the viscosity at 1650° C. of the first outer cladding is greater than 0.1e.sup.7 Poise, and Δ.sub.1MAX>Δ.sub.2MIN. The single mode optical fiber may also have an outer cladding surrounding the inner cladding made from silica or SiON. The first outer cladding has a maximum relative refractive index Δ.sub.3MAX, and Δ.sub.3MAX>Δ.sub.2MIN.

A method for manufacturing an optical fiber preform includes a deposition step and an introduction step. In the deposition step, glass fine particles are generated from a glass raw material gas in a flame obtained by burning a flammable gas supplied to a burner, and the glass fine particles are deposited to produce a hollow porous glass preform. In the introduction step, a first gas is introduced into an inside of a hollow of the porous glass preform, and a second gas is introduced to an outside of the porous glass preform. In the method, at least one of the first gas and the second gas is a gas containing halogen. In the gas introduction step, the gas containing halogen is introduced so that a first partial pressure of the first gas and a second partial pressure of the second gas are different from each other.

An optical fiber having a core comprising silica and greater than 1.5 wt % chlorine and less than 0.5 wt % F, said core having a refractive index Δ.sub.1MAX, and an inner cladding region having refractive index Δ.sub.2MIN surrounding the core, where Δ.sub.1MAX>Δ.sub.2MIN.

A method of producing bi-modal particles includes the steps of igniting a first precursor gas using a primary burner thereby producing a first plurality of particles of a first size, fluidly transporting the first plurality of particles down a particle tube, igniting a second precursor gas using a secondary burner thereby producing a second plurality of particles of a second size, flowing the second plurality of particles into the first plurality of particles, and capturing the first and second plurality of particles.

A method of manufacturing a quartz glass fibre includes producing a quartz glass primary preform by modified chemical vapor deposition (MCVD) in a quartz glass substrate tube and inserting the quartz glass primary preform into a glass jacketing tube. Defect-generating UV radiation is irridiated into the cross-sectional area of the glass jacketing tube while combining the quartz glass primary preform with the glass jacketing tube in the jacketing process to form a cladding layer to a secondary preform. A quartz glass fibre is pulled from the secondary 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 in a gas-phase environment that has a low partial pressure of impermeable gases. Impermeable gases are difficult to remove from halogen-doped fiber preforms and lead to defects in optical fibers drawn from the preforms. A low partial pressure of impermeable gases in the sintering environment leads to a low concentration of impermeable gases and a low density of gas-phase voids in densified halogen-doped silica. Preforms with fewer defects result.

The present invention relates to a method of manufacturing an optical fiber preform for obtaining an optical fiber with low transmission loss. A core preform included in the optical fiber preform comprises three or more core portions, which are each produced by a rod-in-collapse method, and in which both their alkali metal element concentration and chlorine concentration are independently controlled. In two or more manufacturing steps of the manufacturing steps for each of the three or more core portions, an alkali metal element is added. As a result, the mean alkali metal element concentration in the whole core preform is controlled to 7 atomic ppm or more and 70 atomic ppm or less.

One embodiment of the present disclosure relates to an optical fiber having lower transmission loss. The optical fiber is an optical fiber comprised of silica-based glass and includes a core including a central axis and a cladding. The cladding surrounds the core and has a refractive index lower than a refractive index of the core. The core contains phosphorus, chlorine, and fluorine. The core further includes an alkali metal element or an alkaline earth metal element. In a cross section of the optical fiber orthogonal to the central axis, a ratio Rp/Ra of a radius Rp of a phosphorus-containing region with respect to a radius Ra of the core is 0.3 or more.

An optical fiber is formed from silica-based glass. The optical fiber includes a core including a central axis and a cladding surrounding the core. A refractive index of the core is greater than a refractive index of the cladding. The core contains chlorine, and one or more kinds of elements selected from an element group consisting of alkali metal elements and alkaline earth metal elements. A relative refractive index difference of the core based on a refractive index of pure silica is 0.00% or greater and 0.15% or less. An average concentration of fluorine in the cladding is 1.2% or less in a mass fraction.

Provided are such a method of manufacturing a hollow synthetic quartz glass porous preform and method of manufacturing a synthetic quartz glass cylinder as described below: even a soot body having an outer diameter of more than 300 mm can be produced without significantly increasing a load on an apparatus, such as a centrifugal force generated during growth; even when manufactured at low-speed rotation, the soot body is free of any crack or rupture; and a target can be easily extracted. Specifically, provided is a method of manufacturing a hollow porous quartz glass preform by an OVD method, wherein the rotation peripheral speed of the soot body is controlled so as to be practically constant by fluctuating the rotation number of the soot body on the basis of a fluctuating outer diameter of the soot body during growth, and wherein a frequency factor γ calculated by the following equation is set so as to fall within the range of 0.13≤γ<1.0 in a range in which the outer diameter of the soot body is more than 250 mm: γ=S/(L.Math.N.sub.m), where S represents the moving speed (mm/min) of the burners, L represents the moving distance (mm) of the burners, and N.sub.m represents the lowest value (rpm) of the rotation number of the soot body, which is fluctuated.