JP7023802B2 - Method for manufacturing base material for optical fiber, method for manufacturing optical fiber, and method for doping silica glass - Google Patents

Description

The present invention relates to a method for producing an optical fiber base material, a method for producing an optical fiber, and a method for doping silica glass, and particularly to a method suitable for producing an optical fiber capable of reducing transmission loss.

In an optical fiber communication system, the optical signal / noise ratio must be increased in order to increase the optical transmission distance and the optical transmission speed. Therefore, it is required to reduce the transmission loss of the optical fiber. Now that the manufacturing method of optical fibers is highly sophisticated, it is considered that the transmission loss due to impurities contained in optical fibers is reduced to almost the limit. The main cause of the remaining transmission loss is the scattering loss due to the fluctuation of the structure and composition of the glass constituting the optical fiber. This is unavoidable because the optical fiber is made of glass.

As an optical fiber capable of reducing the scattering loss due to fluctuations in the structure and composition of the glass as described above, those provided with a core made of silica glass doped with an alkali metal oxide or an alkaline earth metal oxide in a small amount. Are known. Hereinafter, alkali metal oxides and alkaline earth metal oxides may be referred to as alkali oxides.

By doping the silica glass with an alkali oxide, the softening point of the silica glass is greatly reduced. That is, when compared with silica glass not doped with alkali oxide at the same temperature, the silica glass doped with alkali oxide has a low viscosity, so that structural relaxation is likely to be promoted. Therefore, when a base material for an optical fiber is manufactured so that silica glass doped with an alkali oxide becomes a core, and the base material for the optical fiber is spun to manufacture an optical fiber, the silica glass constituting the core is formed. Structural fluctuations are quickly reduced. As a result, it is possible to manufacture an optical fiber whose transmission loss can be reduced.

In Patent Document 1 below, a glass rod is inserted into a glass tube, and while heating the glass tube and the glass rod, an alkali metal vapor is circulated between the inner peripheral surface of the glass tube and the outer peripheral surface of the glass rod. A method for producing a base material for an optical fiber doped with an alkali oxide is disclosed. Further, in Patent Document 2 below, an alkaline compound is vaporized in a silica glass tube, then cooled to be finely divided, and this is circulated from one end to the other end of the silica glass tube with a carrier gas to obtain silica. A method of doping a glass tube with an alkaline compound is described. Further, Patent Document 3 below describes a method of doping a silica glass tube with an alkali compound by circulating fine particles of the alkali compound from one end of the silica glass tube toward the other end and then heating the silica glass tube. ing.

By the way, as a method of doping silica glass with an alkaline compound, a method of immersing a silica glass rod in a melt of the alkaline compound, as in the method described in Patent Document 4 below, is also conceivable. In this method, since the high concentration alkaline compound can be brought into contact with the silica glass rod, the alkaline compound can be doped into the silica glass rod in a short time. Further, by immersing the entire silica glass rod in the melt of the alkaline compound, the concentration deviation of the alkaline compound doped in the silica glass rod can be reduced over the entire silica glass rod.

Special Table 2005-537210 Japanese Patent No. 5656469 Japanese Patent No. 5586388 Japanese Patent No. 5894828

However, in the methods described in Patent Documents 1 to 3, it is difficult to control the amount of vapor of the alkaline compound, and it is difficult to control the concentration deviation of the alkaline compound doped in the longitudinal direction of the silica glass tube. Further, the present inventor has found that the silica glass rod doped with the alkaline compound may be crystallized by the method described in Patent Document 4.

Therefore, the present invention uses a method for doping silica glass, which can reduce the concentration deviation of the alkali metal compound or alkaline earth metal compound doped in the silica glass while suppressing the crystallization of the silica glass. It is an object of the present invention to provide a method for manufacturing an optical fiber base material and a method for manufacturing an optical fiber using the optical fiber base material.

In order to solve the above problems, the method for producing a base material for an optical fiber of the present invention comprises a preparation step of preparing a silica glass tube and a first silica glass rod having an outer diameter smaller than the inner diameter of the silica glass tube, and the above-mentioned. In the insertion step of inserting the first silica glass rod into the silica glass tube and the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod, the inner peripheral surface of the silica glass tube. A melt of an alkali metal compound or an alkaline earth metal compound is brought into contact with a part of the above and a part of the outer peripheral surface of the first silica glass rod, and the compound constituting the melt is put into the silica glass tube and the first silica. After the alkaline dope step of doping one silica glass rod and the alkali dope step, the silica glass tube is heated and reduced in diameter to integrate the silica glass tube and the first silica glass rod. The alkali dope step comprises a solidification step of actualizing and producing a second silica glass rod, and an external layer forming step of forming a silica glass layer on the outer peripheral surface of the second silica glass rod. In the silica glass tube and the first contact position between the inner peripheral surface of the silica glass tube and the melt, and the contact position between the outer peripheral surface of the first silica glass rod and the melt. It is characterized in that it is moved along the circumferential direction and the longitudinal direction of the silica glass rod.

In the method for producing a base material for an optical fiber, the silica glass tube is doped with the alkali compound by contacting the inner peripheral surface of the silica glass tube with the melt of the alkaline compound. Further, when the melt of the alkaline compound comes into contact with the outer peripheral surface of the first silica glass rod, the alkali compound is doped into the first silica glass rod. Therefore, compared to the case where either one of the silica glass tube and the first silica glass rod is doped with the alkali compound, the second silica glass rod composed of the silica glass tube and the first silica glass rod is doped with the alkali compound. It becomes easier to do. Further, the position where the melt of the alkaline compound comes into contact with the silica glass tube and the first silica glass rod moves along the circumferential direction and the longitudinal direction of the silica glass tube and the first silica glass rod. By adjusting the moving speed of the contact position of the melt and the like, the concentration deviation of the doped alkaline compound can be reduced in the circumferential direction and the longitudinal direction of the silica glass tube and the first silica glass rod.

Further, the present inventor makes the silica glass tube and the first silica glass tube and the first one by contacting the melt of the alkaline compound only with a part of the inner peripheral surface of the silica glass tube and a part of the outer peripheral surface of the first silica glass rod. It has been found that the crystallization of silica glass rods can be suppressed. The reason for this is not clear, but it is thought to be as follows. When the silica glass is doped with an alkaline compound, an addition reaction occurs with respect to the Si—O—Si bond of the silica glass, and the alkali metal ion or the alkaline earth metal ion breaks the Si—O—Si bond while SiO ₄ Get into the gaps in the network. Therefore, when silica glass is doped with an alkaline compound, the volume of silica glass generally shrinks and the density increases. However, when the concentration of the alkaline compound doped in the silica glass becomes higher than a certain amount, it is considered that the alkali metal ion or the alkaline earth metal ion cannot enter the gap of the SiO ₄ network, and the volume of the silica glass starts to expand. Be done. It is considered that the morphology of the silica glass causes a difference in the generation and relaxation of stress caused by the above-mentioned density change and morphology change, and a difference in the progress of crystallization of the silica glass. Compared to the case where the entire silica glass is immersed in the melt of the alkaline compound, the stress is easily relieved and the crystallization of the silica glass is suppressed by contacting the melt of the alkali compound with only a part of the silica glass. it is conceivable that.

Further, according to the method for manufacturing a base material for an optical fiber, a base material core portion made of a second silica glass rod and a base material clad portion made of a silica glass layer formed on the outer peripheral surface of the second silica glass rod are formed. A base material for an optical fiber to have is produced. The base metal clad portion is a portion that becomes a clad portion of an optical fiber. The base metal core portion is a portion that becomes the core of the optical fiber, and is composed of a silica glass tube doped with an alkali compound and a first silica glass rod as described above. As described above, the silica glass tube and the first silica glass rod are made of silica glass which can suppress crystallization and reduce the concentration deviation of the alkaline compound doped in the circumferential direction and the longitudinal direction. That is, according to the method for producing a base material for an optical fiber, the concentration deviation of the alkali metal compound or alkaline earth metal compound doped in the silica glass while suppressing the crystallization of the silica glass constituting the base material core portion. Can be reduced.

Further, in the alkali doping step, it is preferable to move the heat source for heating the alkali metal compound or the alkaline earth metal compound along the longitudinal direction of the silica glass tube and the first silica glass rod.

As the heat source that heats the alkaline compound moves, so does the melt of the alkaline compound. By moving the heat source as described above, the melt of the alkaline compound in contact with the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod is melted in the longitudinal direction of the silica glass tube and the first silica glass rod. It becomes easy to move along.

Further, it is preferable that the powder of the alkali metal compound or the alkaline earth metal compound is adhered to the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod before the alkali doping step.

Before the alkaline doping step, that is, before the alkaline doping step or at the same time as the alkaline doping step, the powder of the alkaline compound is adhered as described above. By melting the powder, the melt of the alkaline compound can be brought into contact with the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod. Therefore, by adjusting the position and amount to which the powder of the alkaline compound adheres, a desired amount of the alkaline compound can be doped at a desired position on the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod. ..

Further, it is preferable to have an additional heating step of further heating the silica glass tube and the first silica glass rod without supplying the alkali metal compound and the alkaline earth metal compound after the alkali dope step.

By further heating the silica glass tube and the first silica glass rod in this way, the alkali remaining on the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod after the alkali doping step. The compound can be removed. Further, when the alkali compound doped in the silica glass tube or the first silica glass rod is unevenly distributed, the concentration of the alkali compound can be leveled by heating as described above. Further, by performing such an additional heating step, the internal stress generated in the silica glass tube and the first silica glass rod can be alleviated by doping the silica glass tube and the first silica glass rod with the alkaline compound.

Further, in the alkali doping step, a gas is formed in the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod from one end to the other end of the silica glass tube. It is preferable to distribute.

By allowing the gas to flow through the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod in this way, the moving direction of the melt or powder of the alkaline compound is limited to the flowing direction of the gas. Therefore, the alkaline compound can be effectively used.

Further, it is preferable that the gas contains oxygen.

In the alkali doping step, oxygen flows through the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod, so that the alkali compound doped in the silica glass tube and the first silica glass rod is heated. Can cause an oxidation reaction. Further, by flowing oxygen in this way, the generation of the following oxygen-deficient type defects can be suppressed on the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod. For example, when potassium chloride is used as an alkaline compound, when potassium chloride diffuses into a silica glass tube, the Si—O—Si bond is cleaved and a Si—OK bond or a Si—Cl bond is formed by an addition reaction. .. Moisture in the gas flowing through the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod, and a slight hydroxyl group (Si—OH) contained in the silica glass tube and the first silica glass rod. Reacts, Cl is discharged out of the system as hydrochloric acid HCl. Therefore, defects may occur on the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod due to lack of oxygen. Further, when the silica is heated to a very high temperature, a part of the silica is volatilized, and even if it is desorbed as SiOx (x> 2), an oxygen-deficient type defect may occur.

Further, it is preferable that the gas contains chlorine.

In the alkali dope step, chlorine flows through the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod, so that the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod are circulated. The alkali compound arranged above and a part of the impurities contained therein are regarded as chloride. Most of the main group and transition metal chlorides that are assumed to be impurities volatilize preferentially over alkali metal chlorides by heating at temperatures near the melting point of alkali metal chlorides. Therefore, in the alkali doping step, chlorine flows through the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod, and the alkaline compound is heated, so that the purification of the alkaline compound can be promoted. .. When chlorine is present in the gas phase, chlorine is unlikely to affect the crystallization of silica glass as described above.

Further, it is preferable that the alkali metal compound contains an alkali metal halide.

Alkali metal halides exhibit melting points without decomposition and are therefore suitable for use as melts as described above.

Further, it is preferable that the alkali metal is potassium.

When a halide of potassium is used, it is easy to set the diffusion rate into silica glass as an appropriate rate mainly due to the atomic weight of potassium.

Further, it is preferable that the first silica glass rod is a solid glass rod.

Since the first silica glass rod is a solid glass rod, it becomes easy to manufacture the second silica glass rod in the solidification step.

Further, the method of doping silica glass of the present invention comprises a preparatory step of preparing a silica glass tube and a first silica glass rod having an outer diameter smaller than the inner diameter of the silica glass tube, and the first method of doping the silica glass tube. In the insertion step of inserting the silica glass rod and the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod, a part of the inner peripheral surface of the silica glass tube and the first A melt of an alkali metal compound or an alkaline earth metal compound is brought into contact with a part of the outer peripheral surface of the silica glass rod, and the compound constituting the melt is doped into the silica glass tube and the first silica glass rod. In the alkaline dope step, the contact position between the inner peripheral surface of the silica glass tube and the melt, and the outer peripheral surface of the first silica glass rod and the melt are provided. It is characterized in that the contact position is moved along the circumferential direction and the longitudinal direction of the silica glass tube and the first silica glass rod.

As described above, the alkali metal compound or alkali that is doped into the silica glass tube and the first silica glass rod while suppressing the crystallization of the silica glass tube and the first silica glass rod by going through the alkali doping step. It is possible to reduce the concentration deviation of earth metal compounds. That is, according to the above-mentioned doping method for silica glass, it is possible to reduce the concentration deviation of the alkali metal compound or alkaline earth metal compound doped in the silica glass while suppressing the crystallization of the silica glass.

Further, the method for manufacturing an optical fiber of the present invention includes a step of manufacturing an optical fiber base material by any of the above methods for manufacturing an optical fiber base material, and a drawing step of drawing the optical fiber base material. It is characterized by having.

According to the method for producing a base material for an optical fiber, a base material core portion made of a second silica glass rod doped with an alkali compound and a silica glass layer formed on the outer peripheral surface of the second silica glass rod are used. A base material for an optical fiber having a base material clad portion is obtained. By drawing the base material for the optical fiber, the core portion of the base material doped with the alkaline compound becomes the core, and an optical fiber having a core capable of reducing the transmission loss can be obtained.

As described above, according to the present invention, a method for doping silica glass, which can reduce the concentration deviation of the alkali metal compound or alkaline earth metal compound doped in the silica glass while suppressing the crystallization of the silica glass. A method for manufacturing an optical fiber base material using the doping method and a method for manufacturing an optical fiber using the optical fiber base material are provided.

It is sectional drawing which shows the optical fiber which concerns on embodiment of this invention. It is a flowchart which shows the process of the manufacturing method of the optical fiber which concerns on embodiment of this invention. It is a perspective view which shows the silica glass tube prepared in the preparation process of FIG. It is a perspective view which shows the 1st silica glass rod prepared in the preparation process of FIG. It is a perspective view which shows the state of one scene of the insertion process of FIG. It is sectional drawing which shows the state of one scene of the alkali doping process of FIG. It is sectional drawing which shows the state of another scene of the alkali doping process of FIG. It is a perspective view which shows the state of the solidification process of FIG. It is a perspective view which shows the 2nd silica glass rod obtained through the solidification process. It is a perspective view which shows the state of the external layer forming process by the suit method. It is a perspective view which shows the appearance that the required amount of silica glass soot containing fluorine adhered on the outer peripheral surface of a silica glass rod. It is sectional drawing which shows the base material for an optical fiber. It is a figure which shows the state of the line drawing process of FIG. It is sectional drawing which shows the state of one scene of the alkali doping process which concerns on the modification of this invention. It is sectional drawing which shows the state of one scene of the alkali doping process which concerns on other modification of this invention.

Hereinafter, drawings will be drawn on preferred embodiments of a method for doping silica glass according to the present invention, a method for producing an optical fiber base material using the doping method, and a method for producing an optical fiber using the optical fiber base material. It will be explained in detail with reference to it.

FIG. 1 is a cross-sectional view showing an optical fiber according to an embodiment of the present invention. As shown in FIG. 1, the optical fiber 1 of the present embodiment includes a core 11, a clad 12 that surrounds the outer peripheral surface of the core 11 without gaps, an inner protective layer 13 that covers the outer peripheral surface of the clad 12, and an inner protective layer 13. The outer protective layer 14 that covers the outer peripheral surface of the above is provided. The core 11 is made of silica glass doped with at least an alkali metal oxide or an alkaline earth metal oxide. Further, the clad 12 of the present embodiment is made of, for example, fluorine-doped silica glass.

Next, a method for manufacturing the optical fiber 1 will be described.

FIG. 2 is a flowchart showing a process of a method for manufacturing an optical fiber according to an embodiment of the present invention. The method for manufacturing an optical fiber shown in FIG. 2 includes a preparation step P1, an insertion step P2, an alkali doping step P3, an additional heating step P4, a solidification step P5, an external layer forming step P6, and a drawing step P7. ..

<Preparation process P1>
This step is a step of preparing a silica glass tube and a first silica glass rod. FIG. 3 is a perspective view showing the silica glass tube 20 prepared in the preparation step P1, and FIG. 4 is a perspective view showing the first silica glass rod 25 prepared in the preparation step P1.

The silica glass tube 20 has a hollow portion 20h having a predetermined inner diameter d1. The hollow portion 20h is formed along the central axis of the silica glass tube 20, and the wall thickness of the silica glass tube 20 is substantially constant. As the silica glass tube 20, for example, a commercially available synthetic silica glass tube for optical fiber can be used. The silica glass tube 20 may be made of pure silica glass to which no dopant is added, or a dopant other than the alkaline compound may be added. Examples of such dopants include chlorine, fluorine, germanium and the like. A plurality of types of dopants may be doped in the silica glass tube 20, and the dopants may be doped so as to cause a concentration distribution in the thickness direction of the silica glass tube 20. However, since the silica glass tube 20 becomes a part of the core 11 as described later, the concentration of the dopant added to the silica glass tube 20 is preferably low from the viewpoint of reducing the transmission loss of the optical fiber 1. The size of the silica glass tube 20 is not particularly limited, but for example, the inner diameter d1 may be 27 mm and the wall thickness may be 2.5 mm.

Further, in the preparation step P1 of the present embodiment, as will be described later, a silica glass tube 20 having an annular recess formed outward at one end is prepared.

The first silica glass rod 25 of the present embodiment is a solid glass rod having an outer diameter d2 smaller than the inner diameter d1 of the silica glass tube 20. Further, the first silica glass rod 25 becomes a part of the core 11 as described later, and is made of silica glass similar to the silica glass tube 20. The size of the first silica glass rod 25 is not particularly limited as long as it can be inserted into the hollow portion 20h of the silica glass tube 20, but for example, the outer diameter d2 can be 22 mm.

<Insert step P2>
This step is a step of inserting the first silica glass rod 25 into the silica glass tube 20. FIG. 5 is a perspective view showing a scene of the insertion step P2.

As shown in FIG. 5, by inserting the first silica glass rod 25 into the hollow portion 20h of the silica glass tube 20, the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface of the first silica glass rod 25 are formed. A gap 28 is formed between the 25s and the 25s. It is preferable that the silica glass tube 20 and the first silica glass rod 25 are fixed so that the gap 28 is formed substantially uniformly in the circumferential direction and the longitudinal direction of the first silica glass rod 25.

<Alkaline dope step P3>
In this step, in the gap 28 between the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25, a part of the inner peripheral surface 20s of the silica glass tube 20 and the first silica glass rod 25. This is a step of simultaneously contacting a part of the outer peripheral surface 25s of 25 with a melt of an alkaline compound. When the melt of the alkali metal compound or the alkaline earth metal compound comes into contact with the inner peripheral surface 20s of the silica glass tube 20, the alkali metal compound or the alkaline earth metal compound is silica from the inner peripheral surface 20s side of the silica glass tube 20. It is doped in the glass tube 20. Further, when the melt of the alkali metal compound or the alkaline earth metal compound comes into contact with the outer peripheral surface 25s of the first silica glass rod 25, the alkali metal compound or the alkali is formed from the outer peripheral surface 25s side of the first silica glass rod 25. The earth metal compound is doped into the first silica glass rod 25.

The alkali dope step P3 is carried out by attaching the silica glass tube 20 and the first silica glass rod 25 to a glass forming lathe used for improved chemical vapor deposition (MCVD). FIG. 6 is a cross-sectional view showing a scene of the alkali doping step P3. FIG. 7 is a cross-sectional view showing a state of another scene of the alkali doping step P3.

As shown in FIG. 6, the silica glass tube 20 has an annular recess 20a recessed outward at one end. In the alkali dope step P3 of the present embodiment, the compound 30 of an alkali metal or an alkaline earth metal is arranged in the recess 20a in the hollow portion 20h of the silica glass tube 20. In this embodiment, potassium chloride (KCl) is used as the compound 30. When potassium chloride is used as the compound 30, it is easy to set the diffusion rate into silica glass as an appropriate rate mainly due to the atomic weight of potassium.

Next, while rotating the silica glass tube 20 and the first silica glass rod 25 around the axis, the carrier is directed from one end to the other end of the silica glass tube 20 and the first silica glass rod 25. The gas CG is circulated in the gap 28. As the carrier gas CG, for example, a gas containing dry oxygen heated to room temperature or a temperature of about 80 ° C. to 120 ° C. can be used. Further, the compound 30 is dried by the hydrogen acid burner 31 which is a heating means. For example, compound 30 is heated at 150 ° C. for 15 minutes or longer. After the compound 30 is dried in this way, the compound 30 is heated again by the oxyhydrogen burner 31 and melted while the carrier gas CG is circulated in the gap 28 as described above. The heating temperature at this time can be, for example, 780 ° C. When the compound 30 is melted in this way, the vapor of the compound 30 is generated according to the vapor pressure. The generated vapor of the compound 30 is flowed by the carrier gas CG to the downstream side in the distribution direction of the carrier gas CG from the recess 20a. Further, the vapor of the compound 30 thus flowed is cooled and condensed to form powder 30P, which adheres to the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. .. In this way, the powder 30P adheres to the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 on the downstream side of the recess 20a in the flow direction of the carrier gas CG. At this time, the powder 30P may be easily adhered by cooling the portion of the silica glass tube 20 and the first silica glass rod 25 to which the powder 30P is to be adhered.

Next, as shown in FIG. 7, the hydrogen acid hydrogen burner 31 is moved relative to the silica glass tube 20 and the first silica glass rod 25 from the upstream side to the downstream side in the distribution direction of the carrier gas CG. That is, the hydrogen acid hydrogen burner 31 is moved along the longitudinal direction of the silica glass tube 20 and the first silica glass rod 25. At least a part of the powder 30P is heated and melted by the oxyhydrogen burner 31 that moves in this way, and becomes a melt 30L. The heating temperature of the hydrogen acid burner 31 at this time may be any temperature as long as the powder 30P is melted, and may be, for example, 800 ° C.

As described above, the powder 30P adhering to the inner peripheral surface 20s of the silica glass tube 20 is melted, so that a part of the inner peripheral surface 20s of the silica glass tube 20 and the melt 30L come into contact with each other. Then, the alkaline compound is doped into the silica glass tube 20 from the inner peripheral surface 20s side of the silica glass tube 20. Further, as described above, the powder 30P adhering to the outer peripheral surface 25s of the first silica glass rod 25 is melted, so that a part of the outer peripheral surface 25s of the first silica glass rod 25 and the melt 30L are melted. Come in contact with. Then, the alkaline compound is doped into the first silica glass rod 25 from the outer peripheral surface 25s side of the first silica glass rod 25. As shown in FIG. 7, the melt 30L tends to accumulate below the gap 28 due to gravity. Therefore, even if the melt 30L fills a part of the gap 28, the carrier gas CG passes above the gap 28, and the carrier gas CG can prevent the melt 30L from flowing too much to the downstream side. Further, in order to secure the flow path of the carrier gas CG in this way, it is preferable to control the size of the gap 28 and the amount of the powder 30P adhered.

Further, in the process of heating the melt 30L by the oxyhydrogen burner 31, a part of the melt 30L is vaporized. The vapor of the compound 30 generated in this way is cooled again and condensed to form powder 30P, which adheres to the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. At this time, due to the circulation of the carrier gas CG, the powder 30P is contained in the inner peripheral surface 20s of the silica glass tube 20 and the first silica glass on the downstream side of the melt 30L in the flow direction of the carrier gas CG. It reattaches to the outer peripheral surface 25s of the rod 25. Further, since the hydrogen acid hydrogen burner 31 moves along the longitudinal direction of the silica glass tube 20 and the first silica glass rod 25 as described above, the inner peripheral surface 20s of the silica glass tube 20 and the first silica glass rod 25 The powder 30P reattached to the outer peripheral surface 25s of the above is melted again by the hydrogen acid burner 31.

When the hydrogen acid hydrogen burner 31 is moved along the longitudinal direction of the silica glass tube 20 and the first silica glass rod 25, the melt 30 L also matches the movement of the hydrogen acid hydrogen burner 31 with the silica glass tube 20 and the first silica glass. It moves along the longitudinal direction of the rod 25. Further, the powder 30P repeats melting and condensation while moving in accordance with the movement of the oxyhydrogen burner 31 as described above. Therefore, the melt 30L in contact with the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 moves along the longitudinal direction of the silica glass tube 20 and the first silica glass rod 25. do. Further, the silica glass tube 20 and the first silica glass rod 25 rotate around the axis. Therefore, the melt 30L in contact with the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 is the inner peripheral surface 20s of the silica glass tube 20 and the first silica glass rod 25. It moves spirally between the outer peripheral surface 25s and the outer peripheral surface 25s. That is, in the alkali dope step P3, the contact position between the inner peripheral surface 20s of the silica glass tube 20 and the melt 30L and the contact position between the outer peripheral surface 25s of the first silica glass rod 25 and the melt 30L are silica. It moves along the circumferential direction and the longitudinal direction of the glass tube 20 and the first silica glass rod 25. In this way, the melt 30L partially and briefly contacts the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25, and the contact position of the melt 30L moves continuously. do. By appropriately adjusting the moving speed of the hydrogen acid burner 31 and the rotation speed of the silica glass tube 20 and the first silica glass rod 25, the inner peripheral surface 20s of the silica glass tube 20 and the outer periphery of the first silica glass rod 25 are appropriately adjusted. The contact position and contact time of the melt 30L in contact with the surface 25s can be appropriately adjusted.

<Additional heating step P4>
This step is a step of further heating the silica glass tube 20 and the first silica glass rod 25 without supplying the compound 30 after the alkali dope step P3.

By further heating the silica glass tube 20 and the first silica glass rod 25 in this way, after the alkali doping step P3, the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 s. The alkaline compound remaining on the top can be removed. In this step, the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 are heated to such an extent that they are not fused.

Further, when the alkaline compound doped in the silica glass tube 20 or the first silica glass rod 25 is unevenly distributed, the concentration of the alkali compound can be leveled by performing the additional heating step P4. That is, by performing the additional heating step P4, the concentration deviation of the alkaline compound doped in the silica glass tube 20 and the first silica glass rod 25 can be reduced. If the alkaline compound is excessively present in the silica glass, crystallization of the silica glass can occur with a slight change in the thermal history. It is considered that this is because the structural change is likely to occur as the viscosity of the silica glass decreases. Such crystallization tends to be remarkable when potassium and chlorine coexist. It is considered that this is because the nucleus of the crystal of silica glass is formed in the place where potassium chloride is unevenly distributed. By leveling the concentration of the alkaline compound and suppressing uneven distribution as described above, crystallization of the silica glass constituting the silica glass tube 20 and the first silica glass rod 25 can be suppressed.

Further, by performing the additional heating step P4, the internal stress generated in the silica glass tube 20 and the first silica glass rod 25 is relaxed by doping the silica glass tube 20 and the first silica glass rod 25 with the alkaline compound. obtain.

The alkali doping step P3 and the additional heating step P4 can be repeated until the concentration of the alkali compound doped in the silica glass tube 20 and the first silica glass rod 25 reaches a desired value.

<Practicalization process P5>
In this step, after the alkali dope step P3, the silica glass tube 20 is heated to reduce the diameter, and the silica glass tube 20 and the first silica glass rod 25 are integrated and solidified, and the second silica glass rod is solidified. Is the process of producing. FIG. 8 is a diagram showing a state of the solidification step P5. Further, FIG. 9 is a perspective view showing a second silica glass rod 40 obtained through the solidification step P5.

As shown in FIG. 8, the silica glass tube 20 is heated from the outer peripheral surface side to about 2000 ° C. by, for example, an acid hydrogen burner 31 while rotating the silica glass tube 20 and the first silica glass rod 25 around the axis. By heating the silica glass tube 20 in this way, the diameter of the silica glass tube 20 is reduced to close the gap between the silica glass tube 20 and the first silica glass rod 25, and the inside of the silica glass tube 20 is closed. The peripheral surface 20s and the outer peripheral surface 25s of the first silica glass rod 25 can be fused and solidified. By heating the silica glass tube 20 while relatively moving the hydrogen acid hydrogen burner 31 along the longitudinal direction of the silica glass tube 20, the entire silica glass tube 20 is gradually reduced in diameter and solidified. In this way, a second silica glass rod 40 made of silica glass doped with an alkaline compound is obtained.

In the solidification step P5, it is preferable to etch the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 before solidifying the silica glass tube 20. The compound 30 may contain impurities such as transition metals, and the alkali oxide diffuses deep inside the silica glass tube 20 and the first silica glass rod 25, whereas the impurities are more than the alkali oxide. Difficult to diffuse in silica glass. Therefore, impurities tend to remain on the inner peripheral surface 20s side of the silica glass tube 20 and the outer peripheral surface 25s side of the first silica glass rod 25. Therefore, the impurities can be removed by etching the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25.

<External layer forming step P6>
This step is a step of forming a silica glass layer on the outer peripheral surface of the second silica glass rod 40 obtained through the solidification step P5. In the present embodiment, a fluorine-containing silica glass layer is formed on the outer peripheral surface of the second silica glass rod 40. In the external layer forming step P6, for example, a silica glass layer containing fluorine can be formed on the outer peripheral surface of the second silica glass rod 40 by the soot method. That is, by attaching silica glass soot to the outer peripheral surface of the second silica glass rod 40 and then sintering it in a fluorine-containing compound-containing atmosphere, fluorine-containing silica glass is attached to the outer peripheral surface of the second silica glass rod 40. Layers can be formed.

FIG. 10 is a diagram showing a state of the external layer forming step P6 by the soot method. In the external layer forming step P6, for example, a silica glass soot to be a fluorine-containing silica glass layer is attached to the outer peripheral surface of the second silica glass rod 40 by the Outside Vapor Deposition (OVD) method. First, the second silica glass rod 40 is fixed to a chuck of a lathe (not shown) and rotated about the axis. Then, as shown in FIG. 10, while rotating the second silica glass rod 40, the silica glass soot to be the fluorine-containing silica glass layer is adhered to a predetermined thickness. Note that FIG. 10 shows a state in which the silica glass soot, which is the fluorine-containing silica glass layer, has not yet adhered to the second silica glass rod 40.

First, vaporized SiCl ₄ is introduced into the flame of the hydrogen acid hydrogen burner 31 by a carrier gas whose flow rate is controlled, and SiCl ₄ is made into SiO ₂ (silica glass). The SiO ₂ is a silica glass suit that adheres to the outer peripheral surface of the second silica glass rod 40. Then, while the acid hydrogen burner 31 is relatively moved in the longitudinal direction of the second silica glass rod 40 a plurality of times, the silica glass soot is attached so as to cover the outer peripheral surface of the second silica glass rod 40. By moving the oxyhydrogen burner 31 as many times as necessary in this way, as shown in FIG. 11, a required amount of silica glass soot 22a is attached to the outer peripheral surface of the second silica glass rod 40.

After the silica glass soot 22a adheres to the outer peripheral surface of the second silica glass rod 40 as described above, dehydration is performed as necessary. Dehydration is performed by aging in a furnace provided with a heater and filled with a gas such as argon (Ar) or helium (He) for a predetermined time. Further, as the dehydrating agent, a chlorine-containing compound such as chlorine (Cl ₂ ) and thionyl chloride (SOCl ₂ ) may coexist.

Next, the concentration is controlled to introduce a fluorine-containing compound such as silicon tetrafluoride (SiF ₄ ), methane tetrafluoride (CF ₄ ), and ethane hexafluoride (C ₂ F ₆ ) into the furnace. Then, the temperature inside the furnace is further raised and the silica glass soot 22a is sintered until it becomes a transparent glass body, whereby a fluorine-containing silica glass layer is formed. The furnace used at this time may be the furnace used for the above dehydration, or may be a different furnace from the furnace used for the above dehydration. However, by continuously forming the fluorine-containing silica glass layer in the above dehydration step, the readsorption of water to the silica glass soot 22a can be suppressed, and the fluorine-containing silica glass layer having a low water content can be obtained. be able to.

By performing the external layer forming step P6 a plurality of times, the fluorine-containing silica glass layer may have a desired thickness. In this way, as shown in FIG. 12, the base material 1P for an optical fiber has a base material core portion 11P derived from the second silica glass rod 40 and a base material clad portion 12P derived from the fluorine-containing silica glass layer. ..

<Line drawing process P7>
This step is a step of drawing a line from the preparation step P1 to the optical fiber base material 1P produced by the method for manufacturing an optical fiber base material including the external layer forming step P6. FIG. 13 is a diagram showing a state of the line drawing process P7. First, as a preparatory step for performing this step, the base material 1P for an optical fiber is installed in the spinning furnace 110.

Next, the heating unit 111 of the spinning furnace 110 is heated to heat the base material 1P for an optical fiber. At this time, the lower end of the base material 1P for an optical fiber is heated to, for example, 2000 ° C. and becomes a molten state. Then, the glass is melted from the base material 1P for the optical fiber, and the glass is drawn. Then, when the drawn molten glass comes out of the spinning furnace 110, it immediately solidifies, and the base metal core portion 11P becomes the core 11 and the base metal clad portion 12P becomes the clad 12, so that the core 11 becomes the core 11. It is an optical fiber strand composed of a clad 12. After that, the optical fiber wire passes through the cooling device 120 and is cooled to an appropriate temperature. When entering the cooling device 120, the temperature of the optical fiber wire is, for example, about 1800 ° C., but when exiting the cooling device 120, the temperature of the optical fiber wire is, for example, 40 ° C. to 50 ° C.

The optical fiber wire emitted from the cooling device 120 passes through the coating device 131 containing the ultraviolet curable resin serving as the inner protective layer 13 and is coated with the ultraviolet curable resin. Further, by passing through the ultraviolet irradiation device 132 and being irradiated with ultraviolet rays, the ultraviolet curable resin is cured and the inner protective layer 13 is formed. Next, the optical fiber coated with the inner protective layer 13 passes through the coating device 133 containing the ultraviolet curable resin to be the outer protective layer 14, and is coated with the ultraviolet curable resin. Further, by passing through the ultraviolet irradiation device 134 and being irradiated with ultraviolet rays, the ultraviolet curable resin is cured to form the outer protective layer 14, and the optical fiber 1 shown in FIG. 1 is obtained. Further, after coating the ultraviolet curable resin to be the inner protective layer 13, the ultraviolet curable resin to be the outer protective layer 14 is coated without passing through the ultraviolet irradiation device, and then the ultraviolet rays pass through the ultraviolet irradiation device. By being irradiated, two UV curable resins may be cured at once, or the UV curable resin serving as the inner protective layer 13 and the UV curable resin serving as the outer protective layer 14 may be collectively cured in one coating device. After coating with UV rays, the two UV curable resins may be cured at once by passing through an ultraviolet irradiation device and being irradiated with ultraviolet rays.

Then, the direction of the optical fiber 1 is changed by the turn pulley 141, and the optical fiber 1 is wound by the reel 142.

In this way, the optical fiber 1 shown in FIG. 1 is manufactured.

As described above, the method for manufacturing the base material 1P for an optical fiber of the present embodiment includes a preparation step P1, an insertion step P2, an alkali doping step P3, an additional heating step P4, a solidification step P5, and an external attachment. The layer forming step P6 is provided. Then, in the alkali doping step P3, the contact position between the inner peripheral surface 20s of the silica glass tube 20 and the melt 30L is moved along the circumferential direction and the longitudinal direction of the silica glass tube 20. Further, in the alkali doping step P3, the contact position between the outer peripheral surface 25s of the first silica glass rod 25 and the melt 30L is moved along the circumferential direction and the longitudinal direction of the first silica glass rod 25.

In the method for producing the base material 1P for an optical fiber of the present embodiment, the alkali compound is doped in the silica glass tube 20 by contacting the melt 30L of the alkali compound with the inner peripheral surface 20s of the silica glass tube 20. Further, when the melt 30L of the alkaline compound comes into contact with the outer peripheral surface 25s of the first silica glass rod 25, the alkali compound is doped in the first silica glass rod 25. Therefore, the second silica glass rod 40 composed of the silica glass tube 20 and the first silica glass rod 25 is compared with the case where either one of the silica glass tube 20 and the first silica glass rod 25 is doped with the alkaline compound. It becomes easy to dope an alkaline compound. The contact between the melt 30L and the inner peripheral surface 20s of the silica glass tube 20 and the contact between the melt 30L and the outer peripheral surface 25s of the first silica glass rod 25 are performed at the same time. The position where the melt 30L of the alkaline compound comes into contact with the silica glass tube 20 and the first silica glass rod 25 moves along the circumferential direction and the longitudinal direction of the silica glass tube 20 and the first silica glass rod 25. .. By adjusting the moving speed of the contact position of the melt 30L and the like, the concentration deviation of the alkali compound to be doped can be reduced in the circumferential direction and the longitudinal direction of the silica glass tube 20 and the first silica glass rod 25.

Further, the present inventor suppresses the crystallization of silica glass by limiting the contact point between the melt of the alkaline compound and the silica glass to only a part of the silica glass, although the reason is not clear as described above. Found that it could be. In the method for producing the base material 1P for an optical fiber of the present embodiment, the melt 30L of the alkaline compound comes into contact with a part of the inner peripheral surface 20s of the silica glass tube 20. Therefore, the crystallization of the silica glass tube 20 can be suppressed. Further, the melt 30L of the alkaline compound comes into contact with a part of the outer peripheral surface 25s of the first silica glass rod 25. Therefore, the crystallization of the first silica glass rod 25 can be suppressed.

Further, the silica glass layer formed on the outer peripheral surface of the base material core portion 11P made of the second silica glass rod 40 and the second silica glass rod 40 by the method of manufacturing the base material 1P for an optical fiber of the present embodiment. An optical fiber base material 1P having a base material clad portion 12P made of the same material is produced. The base material clad portion 12P is a portion that becomes the clad 12 of the optical fiber 1. The base material core portion 11P is a portion to be the core 11 of the optical fiber 1, and includes a silica glass tube 20 doped with an alkali compound and a first silica glass rod 25 as described above. As described above, the silica glass tube 20 and the first silica glass rod 25 are made of silica glass capable of suppressing crystallization and reducing the concentration deviation of the alkali compound doped in the circumferential direction and the longitudinal direction. That is, according to the method for producing the base material 1P for optical fiber of the present embodiment, the alkali metal compound or alkaline earth doped in the silica glass while suppressing the crystallization of the silica glass constituting the base material core portion 11P. The concentration deviation of the metal compound can be reduced.

Further, as described above, the alkaline compound is doped from the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. Therefore, in the cross section perpendicular to the longitudinal direction of the second silica glass rod 40, the concentration of the alkaline compound consists of the boundary between the outer peripheral surface 25s of the first silica glass rod 25 and the inner peripheral surface 20s of the silica glass tube 20. The site is likely to be the highest and the alkaline compound can be doped in a donut shape. Therefore, the alkaline compound can be donut-shaped in a cross section perpendicular to the longitudinal direction of the core 11 made of the second silica glass rod 40. By the way, in the optical fiber 1, the power distribution of the light propagating through the core 11 can be a distribution in which a peak is formed on the outer peripheral side from the center of the core 11. By doping the core 11 with the alkaline compound in a donut shape as described above, the concentration of the alkaline compound can be increased in the portion of the core 11 where the power of light propagating relative to the other portions is high. Therefore, the transmission loss of the light propagating through the core 11 can be reduced.

Further, in the method for producing the base material 1P for an optical fiber of the present embodiment, in the alkali doping step P3, the hydrogen acid hydrogen burner 31, which is a heat source for heating the alkali metal compound or the alkaline earth metal compound, is attached to the silica glass tube 20 and the first. Is moved along the longitudinal direction of the silica glass rod 25 of. As the heat source for heating the alkaline compound moves, 30 L of the alkaline compound melt also moves. By moving the heat source as described above, 30 L of the melt of the alkaline compound in contact with the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 is applied to the silica glass tube 20 and the first silica glass tube 20. It becomes easy to move the silica glass rod 25 along the longitudinal direction. Further, by adjusting the moving speed of the heat source, the position and time at which the melt 30L comes into contact with the silica glass tube 20 and the first silica glass rod 25 can be adjusted, so that the silica glass tube 20 and the first silica glass rod 25 can be adjusted. It is possible to reduce the concentration deviation of the alkaline compound doped in 25.

Further, in the method for producing the base material 1P for an optical fiber of the present embodiment, in the alkali doping step P3, the powder 30P of the alkaline compound is applied to the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface of the first silica glass rod 25. Adhere to 25s. By adhering the powder 30P in this way, the powder 30P is melted, and the melt 30L of the alkaline compound is melted on the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. Can be contacted. Therefore, by adjusting the position and amount to which the powder 30P of the alkaline compound adheres, a desired amount of alkali is placed at a desired position on the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. The compound can be doped.

Further, in the method for producing the base material 1P for an optical fiber of the present embodiment, in the alkali doping step P3, the inner peripheral surface 20s of the silica glass tube 20 is directed from one end to the other end of the silica glass tube 20. The carrier gas CG is circulated in the gap 28 between the first silica glass rod 25 and the outer peripheral surface 25s of the first silica glass rod 25. By passing the carrier gas CG through the gap 28 between the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 in this way, the melt 30L and the powder 30P of the alkaline compound move. Since the direction can be limited to the flow direction of the carrier gas CG, the alkaline compound can be effectively used.

Further, in the method for producing the base material 1P for an optical fiber of the present embodiment, the carrier gas CG contains oxygen. In the alkali dope step P3, the silica glass tube 20 and the first silica glass rod 25 are circulated through the gap 28 between the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. Alkaline compounds doped with glass can cause a thermal oxidation reaction. Further, by flowing oxygen in this way, the generation of the following oxygen-deficient type defects can be suppressed on the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. For example, when potassium chloride is used as the alkaline compound, when potassium chloride diffuses into the silica glass tube 20 and the first silica glass rod 25, the Si—O—Si bond is cleaved and Si—O— is subjected to an addition reaction. K bond and Si—Cl bond are formed. Moisture in the gas flowing through the gap 28 between the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 and a slight hydroxyl group contained in the silica glass tube 20 and the first silica glass rod 25. When (Si—OH) reacts, Cl is discharged to the outside of the system as hydrochloric acid HCl. Therefore, defects may occur on the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 due to lack of oxygen. Further, when the silica is heated to a very high temperature, a part of the silica is volatilized, and even if it is desorbed as SiOx (x> 2), an oxygen-deficient type defect may occur.

Further, the doping method for silica glass of the present embodiment includes a preparation step P1, an insertion step P2, and an alkali doping step P3. According to the method of doping silica glass of the present embodiment, the silica glass tube 20 and the first silica glass tube 20 and the first silica glass tube 20 and the first silica glass tube 20 are suppressed from crystallization by undergoing the alkali doping step P3. The concentration deviation of the alkali metal compound or alkaline earth metal compound doped in the silica glass rod 25 can be reduced. That is, according to the method of doping silica glass of the present embodiment, it is possible to reduce the concentration deviation of the alkali metal compound or alkaline earth metal compound doped in the silica glass while suppressing the crystallization of the silica glass.

Further, the method for manufacturing the optical fiber 1 of the present embodiment includes a step of manufacturing the optical fiber base material 1P by the manufacturing method of the optical fiber base material 1P of the present embodiment and a line for drawing the optical fiber base material 1P. A pulling step P7 is provided. As described above, according to the method for producing the base material 1P for an optical fiber of the present embodiment, the base material core portion 11P made of the second silica glass rod 40 doped with an alkali compound and the second silica glass rod 40 An optical fiber base material 1P having a base material clad portion 12P made of a silica glass layer formed on the outer peripheral surface of the above can be obtained. By drawing the base material 1P for an optical fiber, the core portion 11P of the base material doped with an alkaline compound becomes the core 11, and the optical fiber 1 having the core 11 whose transmission loss can be reduced can be obtained.

Although the present invention has been described above by exemplifying the above embodiments, the present invention is not limited thereto. For example, in the above embodiment, the case where the additional heating step P4 is provided has been described as an example, but the additional heating step P4 is not an essential step.

Further, in the above embodiment, as shown in FIG. 4, an example in which the first silica glass rod 25 is a solid glass rod has been described, but the first silica glass rod 25 may be a hollow glass tube. However, since the first silica glass rod 25 is a solid glass rod, it becomes easy to manufacture the second silica glass rod 40 in the solidification step P5.

Further, in the above embodiment, in the alkali doping step P3, the powder 30P of the alkali metal compound or the alkaline earth metal compound is attached to the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. I explained by citing. However, the powder 30P may be attached to the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 before the alkali dope step P3, and the silica glass may be attached before the alkali dope step P3. It may be attached to one of the inner peripheral surface 20s of the tube 20 and the outer peripheral surface 25s of the first silica glass rod 25, and the inner peripheral surface 20s of the silica glass tube 20 and the first silica glass rod may be attached before the alkali doping step P3. It does not have to be attached to the outer peripheral surface 25s of 25.

FIG. 14 is a diagram showing a scene of the alkali doping step P3 according to the modified example of the present invention. In FIG. 14, the same components as those in the above embodiment are designated by the same reference numerals, and the description thereof will be omitted. As shown in FIG. 14, the powder 30P may be attached to the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25 in advance before the alkali doping step P3. By melting the powder 30P with the hydrogen acid hydrogen burner 31, the melt 30L can be brought into contact with the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25. Further, by moving the hydrogen acid burner 31 along the longitudinal direction of the silica glass tube 20 while rotating the silica glass tube 20 and the first silica glass rod 25 around the axis, the melt is melted in the same manner as in the above embodiment. The contact position of 30 L can be moved.

Further, in the above embodiment, an example in which the recess 20a is formed in the silica glass tube 20 and the compound 30 is arranged in the recess 20a has been described, but the inside of the silica glass tube 20 is not provided with the recess 20a in the silica glass tube 20. The compound 30 may be arranged on the peripheral surface 20s. FIG. 15 is a diagram showing a scene of the alkali doping step P3 according to another modification of the present invention. In FIG. 15, the same reference numerals are given to the same configurations as those of the above embodiment, and the description thereof will be omitted. As shown in FIG. 15, even when the compound 30 is arranged on the inner peripheral surface 20s of the silica glass tube 20 without providing the recess 20a in the silica glass tube 20, the compound 30 is heated by the hydrogen acid hydrogen burner 31. The melt 30L can be brought into contact with the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25.

Further, in the above embodiment, the case where oxygen is used as the carrier gas CG has been described as an example, but as the carrier gas CG, the gases exemplified below or a mixture of these gases can be used. The carrier gas CG may be, for example, an inert gas such as argon, helium, or nitrogen. Further, the carrier gas CG may be a mixed gas of silicon tetrachloride and oxygen. In this case, since silicon tetrachloride and oxygen hardly react at a temperature near the melting point of potassium chloride used as the compound 30 in the above embodiment, silicon dioxide is difficult to be produced. Therefore, substantially the same result as when the inert gas is used as the carrier gas CG can be obtained. If silicon dioxide is generated on the inner peripheral surface 20s of the silica glass tube 20, the surface area of the inner peripheral surface 20s of the silica glass tube 20 increases, so that the melt 30L comes into contact with the inner peripheral surface 20s of the silica glass tube 20. The area can be increased. Similarly, when silicon dioxide is produced on the outer peripheral surface 25s of the first silica glass rod 25, the surface area of the outer peripheral surface 25s of the first silica glass rod 25 increases, so that the melt 30 L and the first silica glass rod The contact area of the 25 with the outer peripheral surface 25s can be increased.

Further, the carrier gas CG may contain chlorine. In the alkali dope step P3, chlorine is circulated in the gap 28 between the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25, so that chlorine is circulated on the inner peripheral surface 20s of the silica glass tube 20 and the first. An alkaline compound arranged on the outer peripheral surface 25s of one silica glass rod 25 and a part of impurities contained therein are regarded as chlorides. Most of the main group and transition metal chlorides that are assumed to be impurities volatilize preferentially over alkali metal chlorides by heating at temperatures near the melting point of alkali metal chlorides. Therefore, in the alkali dope step P3, chlorine is circulated in the gap 28 between the inner peripheral surface 20s of the silica glass tube 20 and the outer peripheral surface 25s of the first silica glass rod 25, and the alkaline compound is heated to obtain the alkaline compound. May accelerate purification. When chlorine is present in the gas phase, chlorine is unlikely to affect the crystallization of silica glass as described above.

Also, carrier gas CG is not essential. When the carrier gas CG is not used, the powder 30P spreads on both ends of the silica glass tube 20 and easily adheres to them. However, by tilting the silica glass tube 20, the moving direction of the powder 30P or the melt 30L can be controlled to some extent.

Further, in the above embodiment, potassium chloride is mentioned as an example of the compound 30, but the compound 30 is not limited to this. Examples of the compound 30 include halides (chlorides, bromides, fluorides, iodides) of alkali metals such as lithium, sodium, potassium, rubidium and cesium, and alkaline earth metals such as beryllium, magnesium, calcium, strontium and barium. Iodide), sulfides, carbonates, hydrogen carbonates and the like can be used. A suitable compound as the compound 30 is appropriately selected depending on the physical characteristics peculiar to the substance such as the melting point of these compounds, the vapor pressure at each temperature, and the heat capacity of the steam. Two or more of these compounds may be mixed and used. In particular, halides are preferable because they show a melting point without being decomposed. Further, the hydrogen carbonate is decomposed at a low temperature to become a carbonate, and the carbonate is preferable because it has a melting point. Hydroxides, hydrides, salts of organic acids, etc. can also be used, but since these compounds contain hydrogen that causes the formation of OH groups in the molecule, these compounds are used. In this case, it is preferable to carry out an additional dehydration treatment after the alkali doping step P3 and at least before the solidification step P5.

Further, in the above embodiment, an example of forming a silica glass layer on the outer peripheral surface of the second silica glass rod 40 by the soot method has been described, but the method for forming the silica glass layer is not limited to this. For example, the silica glass layer can also be formed by the jacket method. That is, by covering the second silica glass rod 40 with a silica glass tube doped with fluorine and fusing the inner peripheral surface of the silica glass tube and the outer peripheral surface of the second silica glass rod 40, the second is also possible. A silica glass layer containing fluorine can be formed on the outer peripheral surface of the second silica glass rod 40.

Further, in the above embodiment, the example in which the clad 12 is doped with fluorine has been described, but the clad 12 may not be doped with fluorine. When the core 11 is doped with a dopant such as germanium that increases the refractive index, the clad 12 may be pure silica glass without any dopant doped. Further, in order to lower the refractive index of the clad 12, the clad 12 may be doped with boron or the like.

Further, in the above embodiment, the case where the oxyhydrogen burner 31 is used as the heating means has been described as an example, but the heating means may be an electric furnace, a plasma furnace, or the like.

As described above, according to the present invention, a method for doping silica glass, which can reduce the concentration deviation of the alkali metal compound or alkaline earth metal compound doped in the silica glass while suppressing the crystallization of the silica glass. , A method for manufacturing an optical fiber base material using the doping method, and a method for manufacturing an optical fiber using the optical fiber base material are provided. These methods can be used in the field of optical fiber communication. It can also be used in the manufacture of optical fibers used in fiber laser devices and other devices using optical fibers.

1 ... Optical fiber 1P ... Base material for optical fiber 11 ... Core 12 ... Clad 20 ... Silica glass tube 20s ... Inner peripheral surface 25 ... First silica glass rod 25s・ ・ ・ Outer peripheral surface 28 ・ ・ ・ Gap 30 ・ ・ ・ Compound 30P ・ ・ ・ Powder 30L ・ ・ ・ Melt 31 ・ ・ ・ Hydrogen acid burner 40 ・ ・ ・ Second silica glass rod P1 ・ ・ ・ Preparation step P2 ... Insertion process P3 ... Alkaline doping process P4 ... Additional heating process P5 ... Solidification process P6 ... External layer forming process P7 ... Drawing process

Claims (12)

A preparatory step for preparing a silica glass tube and a first silica glass rod having an outer diameter smaller than the inner diameter of the silica glass tube, and
The insertion step of inserting the first silica glass rod into the silica glass tube and
In the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod, a part of the inner peripheral surface of the silica glass tube and a part of the outer peripheral surface of the first silica glass rod. An alkali dope step in which a melt of an alkali metal compound or an alkaline earth metal compound is brought into contact with the glass, and the compound constituting the melt is doped into the silica glass tube and the first silica glass rod.
After the alkali doping step, the silica glass tube is heated to reduce the diameter, and the silica glass tube and the first silica glass rod are integrated and solidified to prepare a second silica glass rod. The realization process and
An external layer forming step of forming a silica glass layer on the outer peripheral surface of the second silica glass rod, and
Equipped with
In the alkali dope step, the melt is moved while rotating the silica glass tube and the first silica glass rod around the axis, and the contact position between the inner peripheral surface of the silica glass tube and the melt. The contact position between the outer peripheral surface of the first silica glass rod and the melt is spirally moved along the circumferential direction and the longitudinal direction of the silica glass tube and the first silica glass rod, respectively. A method for manufacturing a base material for an optical fiber, which comprises. Claim 1 is characterized in that in the alkali doping step, a heat source for heating the alkali metal compound or the alkaline earth metal compound is moved along the longitudinal direction of the silica glass tube and the first silica glass rod. The method for manufacturing a base material for an optical fiber according to. A claim characterized by adhering a powder of the alkali metal compound or the alkaline earth metal compound to the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod before the alkali doping step. Item 2. The method for producing a base material for an optical fiber according to Item 1 or 2. A claim comprising, after the alkali doping step, an additional heating step of further heating the silica glass tube and the first silica glass rod without supplying the alkali metal compound and the alkaline earth metal compound. Item 6. The method for producing a base material for an optical fiber according to any one of Items 1 to 3. In the alkali doping step, gas flows from one end of the silica glass tube to the other end in the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod. The method for producing a base material for an optical fiber according to any one of claims 1 to 4, wherein the base material is made to be optical fiber. The method for producing an optical fiber base material according to claim 5, wherein the gas contains oxygen. The method for producing an optical fiber base material according to claim 5 or 6, wherein the gas contains chlorine. The method for producing a base material for an optical fiber according to any one of claims 1 to 7, wherein the alkali metal compound contains a halide of the alkali metal. The method for producing a base material for an optical fiber according to claim 8, wherein the alkali metal is potassium. The method for producing a base material for an optical fiber according to any one of claims 1 to 9, wherein the first silica glass rod is a solid glass rod. A preparatory step for preparing a silica glass tube and a first silica glass rod having an outer diameter smaller than the inner diameter of the silica glass tube, and
The insertion step of inserting the first silica glass rod into the silica glass tube and
In the gap between the inner peripheral surface of the silica glass tube and the outer peripheral surface of the first silica glass rod, a part of the inner peripheral surface of the silica glass tube and a part of the outer peripheral surface of the first silica glass rod. An alkali dope step in which a melt of an alkali metal compound or an alkaline earth metal compound is brought into contact with the glass, and the compound constituting the melt is doped into the silica glass tube and the first silica glass rod.
Equipped with
In the alkali dope step, the melt is moved while rotating the silica glass tube and the first silica glass rod around the axis, and the contact position between the inner peripheral surface of the silica glass tube and the melt. The contact position between the outer peripheral surface of the first silica glass rod and the melt is spirally moved along the circumferential direction and the longitudinal direction of the silica glass tube and the first silica glass rod, respectively. A method for doping silica glass. A step of producing an optical fiber base material by the method for producing an optical fiber base material according to any one of claims 1 to 10.
The wire drawing process for drawing the base material for the optical fiber and
A method for manufacturing an optical fiber, which comprises.

Images