Provided is a method of manufacturing a porous glass deposition body for optical fiber comprising depositing silica powder on a starting member being raised and rotated by using burners with different deposition positions. With a glass raw material flow rate supplied to a core deposition burner represented by F.sub.1 and a total flow rate of glass raw material supplied to a cladding deposition burner adjacent to the core deposition burner represented by F.sub.2, during an initial deposition stage occurring before gas conditions reach a stable state, glass raw material is supplied to points at the same longitudinal position of the deposition body such that a glass raw material flow rate ratio F.sub.2/F.sub.1 is no less than 0.69 and no greater than 1.03.

A method for producing a glass particulate deposit, said method comprising using siloxane as a raw material for glass, discharging the siloxane gasified in a vaporizer and a combustion gas from a burner and combusting, and thus forming a glass particulate deposit in a reaction vessel, wherein: after producing a good section of the glass particulate deposit, the supply of the siloxane that is the raw material for glass to the burner is ceased while continuously supplying the combustion gas to the burner; then the glass particulate deposit is taken out from the reaction vessel; a raw material gas port from the vaporizer to the burner is purged by flowing an inert gas therethrough; and, when a color derived from the combustion of the siloxane gas is not observed any more in the flame of the burner, then the supply of the combustion gas is ceased.

A flow rate fluctuation of the liquid raw material of the organic siloxane supplied to the vaporizer is suppressed and a deposition density of the silica fine particles is uniformizes. The method of manufacturing the porous glass base material according to the present invention, a liquid organic siloxane raw material stored in a raw material tank of internal pressure P.sub.1 is controlled by a mass flow controller at a predetermined flow rate and pumped through pipe of internal pressure P.sub.2 to a vaporizer, the liquid raw material is vaporized in the vaporizer and supplied as a gas raw material to a burner, and the silica fine particles formed by burning the gas raw material in the burner are deposited to form a porous glass base material. The present invention is characterized by the method of manufacture described above, wherein P.sub.1≤P.sub.2 is satisfied.

A manufacturing method of a porous glass base material for optical fiber includes: supplying an organic siloxane raw material supplied from a raw material tank is fed to a vaporizer; mixing and vaporizing the raw material and carrier gas in the vaporizer; and externally depositing SiO.sub.2 fine particles through combustion reaction by supplying the mixed gas of raw the material and the carrier gas to the burner in the manufacturing apparatus of porous glass base material for optical fiber. Before starting to supply the raw material to a raw material gas supply pipe of the burner, the burner and the inside of a manufacturing apparatus of porous glass base material for optical fiber are pre-heated by flowing purge gas of 60° C. or higher into the raw material gas supply pipe and supplying combustible gas and combustion supporting gas to the burner.

A method for manufacturing a glass preform, the method having: a depositing step for installing a starting rod and a burner for generating glass fine particles in a reaction container, introducing a siloxane as a glass raw material to the burner, oxidizing the glass raw material in a flame formed by the burner and generating glass fine particles, depositing the generated glass fine particles on the starting rod and fabricating a glass fine particle deposited body; and a transparentizing step for heating the glass fine particle deposited body and manufacturing a transparent glass preform, wherein, after the depositing step, the transparentizing step is performed after the glass fine particle deposited body is heated for a time range of one to eight hours in an oxygen-containing atmosphere at a temperature lower than the temperature of the transparentizing step.

A manufacturing apparatus of a porous glass base material for optical fiber includes: a liquid mass flow controller for controlling a flow rate of a raw material liquid of an organic siloxane; a vaporizer for mixing the raw material liquid and a carrier gas to vaporize the raw material liquid to form a mixed gas in which a raw material gas and the carrier gas are mixed; a raw material liquid nozzle for ejecting the raw material liquid into the vaporizer; a carrier gas supply pipe for supplying the carrier gas into the vaporizer; a raw material liquid pipe for introducing the raw material liquid into the raw material liquid nozzle; a burner for combusting the mixed gas together with a combustible gas and a combustion supporting gas to produce SiO.sub.2 fine particles; a mixed gas pipe; an open/close valve; and a purge gas supply pipe.

A method for manufacturing a glass ingot includes preparing a supply system including a gasifier that gasifies a raw material compound and a burner that combusts the gasified raw material compound; adding an oxygen-containing gas to the raw material compound at a plurality of addition places including an upstream addition place located in the gasifier or on an upstream side of the gasifier and a downstream addition place located on a downstream side of the gasifier in which locations of the raw material compound in a flow direction are different in the supply system so as to form a raw material mixture; and adding the oxygen-containing gas at the upstream addition place so that a concentration of oxygen or a concentration of the raw material compound in the raw material mixture is not in a combustible range of the raw material mixture.

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 gas branching apparatus that branches and supplies a gas to first to N-th supply targets, includes first to N-th pipes wherein the first to N-th pipes are each branched into first to N-th branch pipes on a downstream end side, and wherein the i-th branch pipes of the respective first to N-th pipes are connected in common to the i-th supply target, and the i-th branch pipes of the respective first to N-th pipes are provided with valves, respectively, where i denotes each of integers of 1 to N.

A method of producing an optical fiber preform includes a silica glass body forming step of forming a silica glass body to be at least a portion of a core portion. The method includes an alkali-metal-doped silica glass body forming step of forming an alkali-metal-doped silica glass body doped with an alkali metal around the silica glass body such that the alkali-metal-doped silica glass body contacts the silica glass body. The method further includes a diffusing step of diffusing the alkali metal from the alkali-metal-doped silica glass body to the silica glass body by a heat treatment.