KR20000028947A - Preform for Optical fiber and manufacturing method thereof - Google Patents

Abstract

PURPOSE: Quartz glass preform for optical fiber and the manufacturing method are provided to provide high quality capable of minimizing the clearance between a quartz glass tube and a core glass rod without air bubbles at the welded inter surface. CONSTITUTION: Preform for optical fiber after heating the synthetic quartz glass tube and the core glass rod for the preform of optical fiber is characterized by follows: the OH-radical concentration in that tube for the materials, below 1ppm; chlorine content, below 3000ppm; hydrogen gas and water vapor releasing amount, below 1.0 x1018 molecule/cm¬3 and 2.0 x1017 molecule/cm¬3, respectively.

Description

Base material for optical fiber and manufacturing method thereof

The present invention provides an optical fiber base material and a method for manufacturing the fiber, and more particularly, an optical fiber base material and a method of manufacturing the same, in which the quartz glass tube and the core glass rod are welded well and there are no bubbles in the welding interface. It is about.

In recent years, the use of optical fibers, especially single-mode optical fibers, has led to the use of a large number of optical fibers. However, it is expected that a larger amount of optical fibers will be required as the use range of the optical fibers is extended from the long distance trunk to the general subscriber system. Mass production and low cost of optical fibers are indispensable for the expansion of such an application range. Therefore, it is the easiest way to prepare a base material for large and long optical fibers and wire draw them. However, in the method of manufacturing the optical fiber base material by the shaft attachment method (VAD method) or the external attachment method (OVD), which has been put to practical use, both core and clad parts are made by the VAD method or the OVD method. In other words, there is a drawback that the production cost is increased due to the relationship between raw materials, combustion gas, and equipment. In addition, in order to produce a large and long base material for an optical fiber, a soot body corresponding to a precursor of the optical fiber base material (a porous body in which silica fine particles have been deposited, refers to a silica body before transparent vitrification, hereinafter referred to as a porous suit body) Since it is assumed that the formation of the porous suit body itself is large, there is a concern that the productivity will be remarkably reduced due to cracks or troubles such as falling of the porous suit body. . As a manufacturing method of an optical fiber which eliminates these drawbacks, a tube for cladding, which occupies 80% or more of the cross-sectional area, is made by a high-performance and low-cost method, and a core glass prepared by the cladding tube and the VAD method or the OVD method. A method for producing an optical fiber base material by a so-called rod in tube method, in which a rod is heated and welded integrally, has been proposed in JP-A-7-109136.

However, in the manufacturing method described in the above publication or the like, unbonded parts and bubbles remain on the welding interface at the time of integral welding between the quartz glass tube for the optical fiber base material and the core glass rod for the optical fiber base material. Or defects in the connection of the optical fiber after wire drawing. However, since the manufacturing method of this publication is the manufacturing method which is easy to enlarge and mount | attach the base material for optical fibers, and is the optimal manufacturing method for mass production and low cost, the present inventors earnestly researched about the improvement, It was found that the occurrence of undesired portion at the interface and the presence of bubbles were caused by the OH group concentration, chlorine concentration, hydrogen gas emission amount and water vapor emission amount in the quartz glass tube for the optical fiber base material constituting it. The OH-based concentration in the quartz glass tube is 1 ppm or less, chlorine content is 3000 ppm or less, hydrogen gas emission is 1.0 × 10 ¹⁸ molecules / cm 3 or less, and water vapor emission is 2.0 × 10 ¹⁷ molecules or less. Finding that can be obtained, to complete the present invention. In other words,

An object of the present invention is to provide an optical fiber base material in which the welding of the synthetic quartz glass tube for the optical fiber base material and the core glass rod for the optical fiber base material is good and there are no bubbles in the welding interface.

Moreover, an object of this invention is to provide the manufacturing method of the said base material for optical fibers.

The present invention, which achieves the above object, is an optical fiber base material obtained by heating and integrating a composite quartz glass tube for an optical fiber base material and a core glass rod for an optical fiber base material, wherein the OH group concentration in the synthetic quartz glass tube for the base material is 1 ppm or less, and the chlorine content The present invention relates to a base material for an optical fiber and a method of manufacturing the same, wherein the amount of released hydrogen gas is 1.0 × 10 ¹⁸ molecules / cm 3 or less and the amount of water vapor released is 2.0 × 10 ¹⁷ molecules / cm 3 or less.

The synthetic quartz glass tube for optical fiber base material is formed by depositing silica fine particles produced by flame hydrolysis of volatile silicon compounds such as high purity silicon tetrachloride and organosilicon compounds on a heat resistant gas to form a porous suit body. After the dehydration treatment, the quartz glass ingot obtained by vitrification is mechanically ground and manufactured by mechanical grinding as necessary. The OH group concentration in the synthetic quartz glass tube is controlled to 1 ppm or less, the chlorine content to 3000 ppm or less, the hydrogen gas emission to 1.0 × 10 ¹⁸ molecules / cm 3 or less, and the steam content to 2.0 × 10 ¹⁷ molecules / cm 3 or less, respectively. The control is performed by appropriately selecting the supply amount of oxygen and hydrogen as the combustion gas and the supply amount of the source gas, and dehydrating the porous soot body. The supply amount of oxygen is selected from the range of 1 to 10 m 3 / h, the supply amount of hydrogen to 1 to 20 m 3 / h, and the supply amount of volatile silicon compound to 1000 to 3000 g / h. The dehydration treatment is performed by heating to chlorine or a mixed gas atmosphere of chlorine and nitrogen at 1000 占 폚 or higher. When the OH group concentration, chlorine content, hydrogen gas emission and water vapor emission in the obtained quartz glass tube for the base material exceed the above ranges, bubbles are present in the welding interface between the quartz glass tube and the core glass rod of the base material for the optical fiber, and wire drawing It will adversely affect itself or the optical fiber after wire drawing.

When the base glass obtained by the above manufacturing method is heat treated at atmospheric or inert gas or in a vacuum atmosphere at 800 to 2500 ° C., the amount of hydrogen gas emitted and the amount of vapor released can be reduced, and the quartz glass tube of the base material for optical fibers And the welding state between the core glass rod is further improved. In the case of heating under the air or inert gas, the amount of hydrogen gas released is 5.0 × 10 ¹⁷ molecules / cm 3 or less, and the amount of water vapor released is 1.0 × 10 ¹⁷ molecules / cm 3. In the case of heat treatment at 800 to 2500 ° C in a vacuum atmosphere, the amount of hydrogen gas released is 2.0 × 10 ¹⁶ molecules / cm 3 or less, and the amount of water vapor released is 5.0 × 10 ¹⁶ molecules / cm 3 or less. In particular, in the vacuum atmosphere and the heat treatment of 800-2500 degreeC, the amount of hydrogen gas emission and water vapor emission can be reduced to the detection limit, and it is suitable. The vacuum degree in the vacuum treatment is 10 Pa or less, preferably 0.5 Pa or less.

As described above, since the quartz glass tube for a base material used in the present invention is required to control the combustion gas and the source gas to form a porous soot body, and then dehydrate it, the silica fine particles conventionally known as a method for producing a quartz glass tube. Is deposited on a heat-resistant gas and then manufactured by the so-called soot method, which is heated and melted and vitrified in an electric furnace, and a direct method or a transparent crystalline silicon dioxide is deposited on a heat-resistant substrate and transparent vitrification is used. Bernoulli law cannot be adopted.

As the mechanical grinding in the production of the quartz glass tube for the base metal, an external grinding device, a core drill hole drilling device and the like can be used, and as the mechanical grinding, a precision honing device or the like can be used.

The concentration of OH in the quartz glass tube is determined by DM DODD and DB FRASER Optical determination of OH in fused silica, Jounal of Applied Physics, Vol. By the measuring method described in 37 (1966), the chlorine content is also measured by the silver nitrate turbidity method. The amount of hydrogen gas and water vapor is measured as the amount of gas released under 1000 ° C. vacuum, and Y. MORIMOTO. et. al., Analysis of gas release from vitreous silica, Jounal of Non-Crystalline Solids, Vol. It is measured by the measuring method as described in 139 (1992). In the above measuring method, the hydrogen gas emission amount is 2.0 × 10 ¹⁶ molecules / cm 3, and the water vapor emission amount is 5.0 × 10 ¹⁶ molecules / cm 3.

On the other hand, as a core glass rod for optical fibers, a quartz glass rod or the quartz glass rod in which the optical clad part was formed in the circumference | surroundings of light is mentioned. That is, in the present invention, "core glass rod" refers to the core rod and the clad core rod. The core rod which does not have a clad part can be formed by a well-known VAD method, an OVD method, etc. Moreover, as a means of producing a cladding core rod, the method of covering a core rod with a quartz glass tube, or a core rod The method of forming a clad part by OVD method etc. around is mentioned.

In the production of an optical fiber base material using the optical fiber base material quartz glass tube, the core glass rod for the base material is carefully inserted in the quartz glass for the base material so as not to come into contact with the inner circumferential surface of the base material, and the core glass rod for the base material and the base material are used. The centers of the quartz glass tubes are fixed and aligned. Preferably, both ends are connected with the dummy quartz material. Then, the entire surface is rotated to correct the bends and twists caused by the connecting process. It is good to integrally weld by heating at 1900-2800 degreeC in strip shape one by one. The heating in the sequential band shape is referred to as so-called zone melt, and refers to heating in which the heating zone moves gradually.

Examples of the present invention will be described below, but the present invention is not limited thereto.

Example 1

The VAD method was used to vaporize high-purity silicon tetrachloride, flame hydrolyze in an oxyhydrogen flame, and deposit silica silica particles on a quartz glass rod rotating at 10 rpm to form a porous suit body in the axial direction. The tetrahydrochloride burner was supplied with 1500 g / h of silicon tetrachloride, 3.0 m 3 / h of hydrogen gas, and 2.3 m 3 / h of oxygen gas, respectively. The obtained porous suit body had an outer diameter of about 250 mm and a length of about 2000 mm. The porous soot body was placed in an electric furnace and heated and dewatered at 1100 ° C. in a mixed gas atmosphere of nitrogen gas 0.5 m 3 / h and chlorine gas 0.05 m 3 / h, taking into account the conditions such as the refractive index of the core glass rod. Cylindrical quartz glass ingots were prepared by transparent vitrification at the following vacuum atmosphere and 1600 degreeC. The obtained cylindrical quartz glass ingot was about 120 mm in outer diameter and about 1500 mm in length. The both ends of the ingot are cut, the outer diameter is ground to 90 mm by a cylindrical grinding device, the center of the outer diameter is obtained, and the hole is drilled by the core drill boring device according to the dual center, and 25 mm by the precision honing processing device. Polished to the dimensions of. The obtained quartz glass tube had an outer diameter of 90 mm, an inner diameter of 25 mm, and cut into a length of 1000 mm.

For the samples of the quartz glass tube, the amount of OH groups and chlorine and the gas discharged under vacuum at 1000 ° C. were measured. The residual OH group concentration was 0.1 ppm, the chlorine concentration was 1500 ppm, and the hydrogen gas emission was 5.5 × 10 ¹⁷ molecules /. Cm 3 and the amount of water vapor released was 1.8 × 10 ¹⁷ molecules / cm 3.

On the other hand, the cladding core rod for an optical fiber base material was created by the VAD method, and it heated and extended to the outer diameter of 20 mm and the length of 1000 mm in a vertical electric furnace. Carefully insert the core glass rod into the quartz glass tube so as not to contact the inner circumferential surface of the core glass rod, fix the core glass rod and the center of the center of the quartz glass tube, and fix both ends to the dummy quartz material. After the upper end was welded to the vertical electric furnace at 90 mm / min, and the lower end was welded, the inside of the quartz glass tube was decompressed by a vacuum pump, heated in sequential order to be integrally welded, and thus a glass fiber base material for optical fibers was prepared. The obtained quartz glass base material for optical fibers was cut in units of 1000 mm, and one of them was exposed to white light from the end face in the dark room. The minimum number of visible bubbles of 0.1 mm or more was 7 per 1000 mm in length. It was a dog.

Example 2

In the same manner as in Example 1, a porous soot body was prepared by the VDA method, followed by dehydration to perform transparent vitrification, and the obtained quartz glass tube was ground to an outer diameter of 100 mm by the same grinding and polishing method as in Example 1. Polished to 25 mm. Subsequently, the quartz glass tube was degassed and surface-hardened, and placed in a horizontal electric furnace heated at 2000 ° C., and subjected to heat treatment while flowing an inert gas into and out of the quartz glass tube. The outer diameter of the quartz glass tube after the treatment was 90 mm, the inner diameter was 25 mm, and cut to a length of 1000 mm. For the samples of this quartz glass tube, the amount of OH groups and chlorine and gas discharged under vacuum at 1000 ° C. was measured. The residual OH group concentration was about 0.1 ppm, the chlorine concentration was 1500 ppm, and the hydrogen gas emissions were 2.5 × 10 ¹⁷ molecules. / Cm 3 or less, and the amount of water vapor released was 8.0 × 10 ¹⁶ molecules / cm 3 or less.

On the other hand, the cladding core rod for an optical fiber base material was created by the VAD method, and it heated and extended to the outer diameter of 20 mm and the length of 1000 mm in a vertical electric furnace. Carefully insert the core glass rod into the quartz glass tube so as not to contact the inner circumferential surface of the core glass rod, fix the core glass rod and the center of each circle of the quartz glass tube, and connect both ends to the dummy quartz material. After the upper end was welded to the vertical electric furnace at 90 mm / min, and the lower end was welded, the inside of the quartz glass tube was depressurized, heated in a continuous band shape, and welded to form a base material for the optical fiber. The obtained quartz glass base material was exposed to white light from its end face in the dark room, and the number of bubbles having a minimum unit of 0.1 mm or more, which was visible to the eye, was 4 per 1000 mm in length.

Example 3

In the same manner as in Example 1, a porous soot body was prepared by the VAD method, followed by dehydration treatment, followed by transparent vitrification, and grinding and polishing in the same manner as in Example 1 to obtain a synthetic quartz glass tube having an outer diameter of 90 mm, an inner diameter of 25 mm, and a length of 1000 mm. . In order to remove this gas and surface-mirror, it inserted into the horizontal type electric furnace heated at 1000 degreeC, and heat-degassing-processing was carried out by making furnace interior into the vacuum atmosphere below 1 Pa. For the samples of this quartz glass tube, the amount of OH groups, chlorine and gas discharged under vacuum at 1000 ° C was measured. The residual OH group concentration was about 0.1 ppm, the chlorine concentration was 1500 ppm, and the hydrogen gas emission amount was 2.0 ×. 10 ¹⁶ molecules / cm 3 or less, and the amount of water vapor released was 5.0 × 10 ¹⁶ molecules / cm 3 or less, which is a detection limit.

On the other hand, the cladding core rod for an optical fiber base material was created by the VAD method, and it heated and extended to the outer diameter of 20 mm and the length of 1000 mm in a vertical electric furnace. Carefully insert the core glass rod into the quartz glass tube so as not to come into contact with the inner circumferential surface of the core glass rod, fix the core glass rod and the center of the center of the quartz glass tube, and fix both ends to the dummy quartz material. After the upper end was welded to the vertical electric furnace at 90 mm / min, and the lower end was welded, the inside of the quartz glass tube was decompressed, heated in a sequential band shape, and integrally welded to prepare a base material for the optical fiber. The obtained quartz glass base material was exposed to white light from its end face in the dark room, and no bubbles of 0.1 mm or more, the smallest visible unit, were observed.

Example 4

The OVD method was used to vaporize high-purity silicon tetrachloride, flame hydrolyze in an oxyhydrogen flame, and deposit it around a gas having an outer diameter of 50 mm rotating at 50 rpm to prepare a porous suit body. The burner for flame hydrolysis was supplied with 1500 g / h of silicon tetrachloride, 1.8 m 3 / h of hydrogen gas, and 0.9 m 3 / h of oxygen gas, respectively. The obtained porous suit body had an outer diameter of about 400 mm and a length of about 3500 mm. The porous soot body was placed in an electric furnace and heated and dewatered at 1100 ° C. in a mixed gas atmosphere of nitrogen gas 0.5 m 3 / h and chlorine gas 0.05 m 3 / h, taking into account the conditions such as the refractive index of the core glass rod. After transparent vitrification at the following vacuum atmosphere and 1600 degreeC, the gas was degassed and the cylindrical quartz glass ingot was produced. This cylindrical quartz glass ingot was about 200 mm in outer diameter, about 50 mm in inner diameter, and about 3500 mm in length. Both ends of this quartz glass ingot were cut, and the inner and outer circumferences were mechanically ground and polished to prepare a synthetic quartz glass tube having an outer diameter of 195 mm, an inner diameter of 55 mm, and a length of 3000 mm.

About the sample obtained from the said quartz glass tube, OH group and chlorine concentration, and the amount of gas discharge | emission under the vacuum of 1000 degreeC were measured. As a result, the residual OH group concentration was 0.1 ppm, the chlorine concentration was 2000 ppm, the hydrogen gas emission amount was 6.0 × 10 ¹⁷ molecules / cm 3 and the water vapor emission amount was 2.0 × 10 ¹⁷ molecules / cm 3.

On the other hand, the cladding core rod for an optical fiber base material was produced by the VAD method, and it heated and extended to the outer diameter 50mm and length 3000mm in the vertical electric furnace. Carefully insert the core glass rod into the quartz glass tube so as not to come into contact with the inner circumferential surface of the core glass rod, fix the core glass rod and the center of the center of the quartz glass tube, and fix both ends to the dummy quartz material. After putting into a vertical electric furnace of 25 mm / min from upper part, and welding a lower end part, the inside of a quartz glass tube was pressure-reduced, it heated in strip shape one by one, and integrally welded, and produced the quartz glass base material for optical fibers. The obtained quartz glass base material for optical fibers was cut in units of 1000 mm, and one of them was exposed to white light from the end face in a dark room. As a result, the number of bubbles having a minimum size of 0.1 mm or more was 9 per 1000 mm in length. It was possible to obtain a large-scale quartz glass base material for large optical fibers with small bubbles from a large-diameter quartz glass tube.

Comparative Example 1

A porous soot body was prepared in the same manner as in Example 1, and was heated to 1000 ° C. in a furnace inside a nitrogen gas of 0.5 m 3 / h without dehydration treatment, followed by transparent glass at 1600 ° C. The obtained quartz glass ingot was about 120 mm in outer diameter and about 1500 mm in length. The both ends of the ingot are cut, the outer periphery is ground to 90 mm by a cylindrical grinding device, the center of the outer diameter is obtained, and the hole is drilled by the core drill boring device in accordance with the center of gravity, and the precision honing processing device is used. Polished to a dimension of 25 mm. The outer diameter of the quartz glass tube was 90 mm, the inner diameter was 25 mm, and cut into a length of 1000 mm.

For the samples of the quartz glass tube, the amount of OH groups and chlorine and gas discharged under vacuum at 1000 ° C. was measured. The residual OH group concentration was 300 ppm, the chlorine concentration was 10 ppm, and the hydrogen gas emission was 1.2 × 10 ¹⁸ molecules / cm 3. The amount of water vapor released was 2.3 × 10 ¹⁷ molecules / cm 3.

On the other hand, the cladding core rod for an optical fiber base material was created by the VAD method, and it heated and extended to the outer diameter of 20 mm and the length of 1000 mm in a vertical electric furnace. Carefully insert the core glass rod into the quartz glass tube so as not to come into contact with the inner circumferential surface of the core glass rod, fix the core glass rod and the center of the center of the quartz glass tube, and fix both ends to the dummy quartz material. After the upper end was welded to the vertical electric furnace at 90 mm / min, and the lower end was welded, the inside of the quartz glass tube was depressurized, heated in a continuous band shape, and welded to form a base material for the optical fiber. The obtained quartz glass base material was exposed to white light from its end face in the dark room, and the number of bubbles of 0.1 mm or more, which is the smallest visible unit, was 19 per 1000 mm.

Comparative Example 2

Transparent quartz glass tubes were prepared by the so-called direct method of depositing silica glass fine particles produced by flame hydrolysis of high-purity silicon tetrachloride on a glass rod rotating at 20 rpm and transparent vitrification. The flame hydrolysis burner was supplied with 2,000 g / h of silicon tetrachloride, 20 m 3 / h of hydrogen gas, and 10 m 3 / h of oxygen gas, respectively. The obtained transparent glass ingot had an outer diameter of about 120 mm and a length of about 1300 mm. The both ends of the quartz glass ingot are cut, the outer periphery is ground to 90 mm by a cylindrical grinding device, the center of the outer diameter is obtained, and the hole is drilled by the core drill hole kneeling device according to the center of the precision, and a precision honing processing device Was polished to a size of 25 mm. The obtained quartz glass tube had an outer diameter of 90 mm, an inner diameter of 25 mm, and cut into a length of 1000 mm.

For the samples of the quartz glass tube, the amount of OH groups and chlorine and gas discharged under vacuum at 1000 ° C. was measured. The residual OH group concentration was about 1000 ppm, the chlorine concentration was 100 ppm, and the hydrogen gas emission was 4.0 × 10 ¹⁸ molecules /. Cm 3 and water vapor release were 2.2 × 10 ¹⁷ molecules / cm 3.

On the other hand, the cladding core rod for an optical fiber base material was created by the VAD method, and it heated and extended to the outer diameter of 20 mm and the length of 1000 mm in a vertical electric furnace. Carefully insert the core glass rod into the quartz glass tube so as not to contact the inner circumferential surface of the core glass rod, fix the core glass rod and the center of the center of the quartz glass tube, and fix both ends to the dummy quartz material. After the upper end was welded to the vertical electric furnace at 90 mm / min, and the lower end was welded, the inside of the quartz glass tube was decompressed, heated in a sequential band shape, and integrally welded to prepare a base material for the optical fiber. The obtained quartz glass base material was exposed to white light from its end face in the dark room, and the number of bubbles having a minimum unit of 0.1 mm or more was 28 per 1000 mm.

Comparative Example 3

Using quartz produced naturally, a transparent quartz glass ingot having an outer diameter of about 120 mm and a length of about 1300 mm was produced in an oxyhydrogen flame of hydrogen gas 30 m 3 / h and oxygen gas 15 m 3 / h. The both ends of the ingot are cut, the outer periphery is ground to 90 mm by a cylindrical grinding machine, the center of the outer diameter is obtained, and the hole is drilled by the core drill boring machine in accordance with the center of gravity, and the precision honing processing device is used. Polished to a dimension of 25 mm. The outer diameter of the quartz glass tube was 90 mm, the inner diameter was 25 mm, and the length was 1000 mm.

For the sample of the quartz glass tube, the amount of OH groups and chlorine and gas discharged under vacuum at 1000 ° C. was measured. The residual OH group concentration was about 180 ppm, chlorine was not detected, and the hydrogen gas emission amount was 1.3 × 10 ¹⁹ molecules. / Cm 3, and the amount of water vapor released was 6.0 × 10 ¹⁷ molecules / cm 3.

On the other hand, the cladding core rod for an optical fiber base material was created by the VAD method, and it heat-stretched by 20 mm of external diameters and 1300 mm in length in a vertical electric furnace. Carefully insert the core glass rod into the quartz glass tube so as not to contact the inner circumferential surface of the core glass rod, fix the core glass rod and the center of the center of the quartz glass tube, and fix both ends to the dummy quartz material. After the upper end was welded at 90 mm / min in the vertical electric furnace at 0 ° C., the lower end was welded, and the inside of the quartz glass tube was decompressed, and subsequently heated in a strip shape to be integrally welded to prepare a base material for the optical fiber. The obtained quartz glass base material was exposed to white light from its end face in the dark room, and the number of bubbles of 0.1 mm or more, which is the smallest visible unit, was 48 per 1000 mm.

The quartz glass base material for optical fibers of the present invention is an excellent quartz glass base material for optical fibers in which no unbonded portion or bubbles are present at the welding interface between the quartz glass tube for the base material and the core glass rod. In addition, it is possible to enlarge and lengthen the quartz glass base material for the optical fiber. By wire drawing the base material for the optical fiber, a high quality optical fiber can be produced with low productivity.

Claims (5)

An optical fiber base material obtained by heating and integrating a composite quartz glass tube for an optical fiber base material and a core glass rod for an optical fiber base material, wherein the OH group concentration in the synthetic quartz glass tube for the base material is 1 ppm or less, the chlorine loading is 3000 ppm or less, and the hydrogen gas emission amount is 1.0. A base material for an optical fiber, wherein × 10 ¹⁸ molecules / cm 3 or less and water vapor emission amount are 2.0 × 10 ¹⁷ molecules / cm 3 or less. Silica fine particles obtained by flame hydrolysis of high-purity silicon compounds are deposited on a heat-resistant gas to form a porous soot body, dehydrated, and transparent vitrified, and the OH group concentration produced by mechanical grinding is 1 ppm or less, and chlorine content is 3000 ppm. Hereinafter, the core glass rod for optical fibers is inserted into a synthetic quartz glass tube for an optical fiber base material having a hydrogen gas emission amount of 1.0 × 10 ¹⁸ molecules / cm 3 or less and a water vapor emission amount of 2.0 × 10 ¹⁷ molecules / cm 3 or less so as to be integrally welded. Method for producing an optical fiber base material. Silica fine particles obtained by flame hydrolysis of high-purity silicon compounds are deposited on a heat-resistant gas to form a porous soot body, dehydrated, and transparent vitrified, and the OH group concentration produced by mechanical grinding is 1 ppm or less, and chlorine content is 3000 ppm. Hereinafter, a synthetic quartz glass tube for optical fiber base material having a hydrogen emission amount of 1.0 × 10 ¹⁸ molecules / cm 3 or less and a water vapor emission amount of 2.0 × 10 ¹⁷ molecules / cm 3 or less is further heated at 800 to 2500 ° C. in an air or an inert gas. A synthetic quartz glass tube for an optical fiber base material having a hydrogen gas emission amount of 5.0 × 10 ¹⁷ molecules / cm 3 or less and a water vapor emission amount of 1.0 × 10 ¹⁷ molecules / cm 3 or less, and the synthetic quartz glass tube for an optical fiber base material is a core glass for an optical fiber base material. A method of manufacturing a base material for an optical fiber, comprising inserting a rod and heating the film to integrally weld the rod. Silica fine particles obtained by flame hydrolysis of high-purity silicon compounds are deposited on a heat-resistant gas to form a porous soot body, dehydrated, and transparent vitrified, and the OH group concentration produced by mechanical grinding is 1 ppm or less, and chlorine content is 3000 ppm. Hereinafter, a synthetic quartz glass tube for optical fiber base material having a hydrogen gas emission amount of 1.0 × 10 ¹⁸ molecules / cm 3 or less and a water vapor emission amount of 2.0 × 10 ¹⁷ molecules / cm 3 or less is further heated in a vacuum atmosphere at 800 to 2500 ° C. to obtain hydrogen. A synthetic quartz glass tube for an optical fiber base material having a gas discharge amount of 2.0 × 10 ¹⁶ molecules / cm 3 or less and a vapor discharge amount of 5.0 × 10 ¹⁶ molecules / cm 3 or less, and a core glass rod for an optical fiber is inserted into the optical fiber base synthetic quartz glass tube. A method for producing an optical fiber base material, characterized in that the welding is integrated by heating. The method for producing the base material for an optical fiber according to any one of claims 2 to 4, wherein the dehydration treatment is a treatment in which the chlorine-based atmosphere is heated at 1000 ° C or higher.