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.

An optical fiber includes a core region of silica glass doped with an alkali metal oxide. A depressed-index cladding region surrounds the core region and comprises silica glass doped with a first concentration of fluorine. The depressed-index cladding region has a minimum relative refractive index Δ.sub.3min in a range from −0.80% to −0.30%. An outer cladding region comprises silica glass doped with a second, lesser concentration. The outer cladding region has a relative refractive index Δ.sub.4, where Δ.sub.4−Δ.sub.3min>0.05%. The optical fiber has a time-to-peak hydrogen aging value at 23° C. of less than 100 hours upon exposure to an atmosphere having a total pressure of 1 atm and containing a partial pressure of 0.01 atm H.sub.2 and a partial pressure of 0.99 atm N.sub.2. The optical fiber exhibits an attenuation <0.16 dB/km.

The present invention relates to a method of forming an optical fiber precursor including: forming an alkali metal doped tube; inserting an optical fiber core rod within the alkali metal doped tube; forming a cladding jacket around the alkali metal doped tube; and diffusing an alkali metal from the alkali metal doped tube through a surface of the optical fiber core rod. The present invention further relates to an optical fiber preform having: an optical fiber core rod; an alkali metal doped tube surrounding the optical fiber core rod; and a cladding jacket surrounding the alkali metal doped tube.

According to some embodiments a method of processing an optical fiber comprises the steps of: (i) drawing the fiber at a drawing rate of at least 30 m/sec; and (ii) cooling the drawn fiber in a gas at an average cooling rate less than 5000° C./s, such that said cooling reduces the temperature of the fiber from an entering temperature in the range between 1500° C. and 1700° C. to another temperature in the range between 1200° C. and 1400° C., the gas being at a temperature between 800° C. and 1500° C.; and the thermal conductivity κ of the gas being not greater than 1.5×10.sup.−4 cal/cm-s-K for at least one temperature within a range of 800° C. to 1500° C. at one atm (atmosphere) pressure absolute.

A method for manufacturing an optical fiber preform including a core part and a cladding part is disclosed. The method includes: adding an alkali metal to an inner surface of a silica-based glass pipe; etching the inner surface of the silica-based glass pipe to which the alkali metal is added; making a glass rod by collapsing the silica-based glass pipe after the etching; and making an optical fiber preform using the glass rod. The silica-based glass pipe is heated in the adding such that a surface temperature of the silica-based glass pipe falls within a temperature range of 1500° C. or higher to lower than 2000° C.

A method of producing a glass preform including: forming a porous glass soot configured by an inner deposition soot deposited on a start material and an outer deposition soot deposited outside the inner deposition soot; and sintering, after the forming, the porous glass soot while doping with fluorine to form a glass body including an inner glass portion and an outer glass layer. An amount of the fluorine, with which the inner deposition soot is doped at the sintering, is equal to or more than 0 g/cm.sup.3 and less than an amount of the fluorine with which the outer deposition soot is doped.

An optical fiber preform includes: a core formed of silica glass which does not contain Ge, wherein the core has at least one of characteristics in spectrometry of (1) an absorption peak is present at a wavelength of 240 nm to 255 nm, and (2) a wavelength at which an ultraviolet transmittance is 50% or lower is longer than 170 nm.

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.