JP7397679B2 - Method for manufacturing optical fiber base material and method for manufacturing optical fiber using the same - Google Patents

Description

The present invention relates to a method of manufacturing an optical fiber preform and a method of manufacturing an optical fiber using the same.

2. Description of the Related Art Optical fibers in which an alkali metal is added to the core have been known from the viewpoint of reducing transmission loss.

As a method for manufacturing an optical fiber preform for forming such an optical fiber, for example, the manufacturing method described in Patent Document 1 below is known. In this document, after forming the core part by performing an alkali metal addition process of adding an alkali metal to the glass tube, a diameter reduction process of reducing the diameter of the glass tube, a solidification process of solidifying the glass tube, etc. A method of manufacturing an optical fiber preform by forming a cladding part around the optical fiber part is disclosed. The same document also states that in the alkali metal addition step, the vapor of the alkali metal compound is flowed into the hollow interior of the glass tube together with oxygen gas, and while the glass tube is heated by an external heat source, the alkali metal is diffused and added into the glass tube. , it is also disclosed that in the diameter reduction process, the supply of alkali metal vapor is stopped, the glass tube is continued to be heated by an external heat source to reduce the diameter of the glass tube, and finally solidification is performed in the solidification process. has been done.

Japanese Patent Application Publication No. 2013-136485

However, the method for manufacturing an optical fiber preform described in Patent Document 1 has room for improvement in terms of transmission loss in the resulting optical fiber when an optical fiber is manufactured by drawing an optical fiber preform. was.

The present invention has been made in view of the above circumstances, and provides a method for manufacturing an optical fiber preform that can produce an optical fiber preform that can suppress an increase in transmission loss, and a method using the same. The purpose of this invention is to provide a method for manufacturing optical fibers.

The present inventors have made extensive studies to solve the above problems. As a result, in the method for manufacturing an optical fiber preform described in Patent Document 1, when adding an alkali metal to the glass tube from the inner surface of the glass tube in the alkali metal addition step, addition of the alkali metal is superior to diffusion. The final concentration of alkali metal near the inner surface of the glass tube increases with each traversal of the external heat source, and at the end of the alkali metal addition step, the final concentration of alkali metal near the inner surface of the glass tube increases. The inventors thought that this would become very large. Furthermore, as a result of the final concentration of alkali metal becoming very large near the inner surface of the glass tube, crystallization tends to occur within the glass tube, and these crystals remain in the core of the optical fiber base material, and furthermore, light The present inventors thought that as a result of remaining in the core of the optical fiber obtained by drawing the fiber preform, the transmission loss of the optical fiber may increase. Therefore, as a result of further intensive studies, the present inventors discovered that the above-mentioned problem could be solved by the following invention.

That is, the present invention provides a method for manufacturing an optical fiber preform that includes a core portion and a cladding portion surrounding the core portion, the method comprising: preparing a core material; The core material preparation step includes a core part forming step of forming the core part using the core material, and a clad part forming step of forming the clad part so as to surround the core part, and the core material preparation step includes the step of forming the core part using the core material. While supplying vapor of an alkali metal compound to the inner hollow part together with a carrier gas containing oxygen gas, the glass tube is heated by a heat source that traverses along the longitudinal direction of the glass tube, and the inner surface of the glass tube is heated. an alkali metal addition step of depositing an alkali metal oxide to add an alkali metal into the bulk of the glass tube, the alkali metal addition step including a first final concentration of the alkali metal near the inner surface of the glass tube; a plurality of concentration increasing steps for increasing the concentration of the alkali metal such that the alkali metal concentration is higher than the first initial concentration; and at least one of the locations between the concentration increasing steps, and and a concentration lowering step of lowering the concentration of the alkali metal so that a second final concentration of the alkali metal in the vicinity is lower than a second initial concentration.

According to the above manufacturing method, the glass tube is heated by a heat source that traverses along the longitudinal direction of the glass tube while supplying vapor of an alkali metal compound together with a carrier gas containing oxygen gas to the hollow part inside the glass tube. , an alkali metal addition step is performed in which an alkali metal oxide is deposited on the inner surface of the glass tube to add the alkali metal into the bulk of the glass tube. In the alkali metal addition step, a plurality of concentration increasing steps are performed to increase the concentration of the alkali metal so that the first final concentration of the alkali metal near the inner surface of the glass tube is higher than the first initial concentration. a concentration reducing step of reducing the concentration of the alkali metal such that the second final concentration of the alkali metal near the inner surface of the glass tube is lower than the second initial concentration at at least one of the locations between the increasing steps; The core material is prepared. Then, a core portion is formed using the core material prepared in this way. Therefore, if the total amount of alkali metal added to the bulk of the glass tube is the same, the concentration decreasing step described above is performed at at least one location between the concentration increasing steps, so that the alkali metal is added to the bulk of the glass tube. The final concentration of the alkali metal near the inner surface of the glass tube in the concentration increasing step performed last in the addition step can be lowered compared to the case where the concentration decreasing step is performed after all the multiple concentration increasing steps. can. Therefore, crystallization can be made less likely to occur in the glass tube, and crystals within the core can be reduced. Therefore, it is desirable to manufacture an optical fiber preform that can suppress an increase in transmission loss when manufacturing an optical fiber by drawing after surrounding such a core part with a cladding part. I can do it.

In the optical fiber preform manufacturing method, it is preferable that the core material preparation step further includes a diameter reduction step of reducing the outer diameter of the glass tube after the alkali metal addition step.

In this case, if the diameter reduction process is performed after the alkali metal addition process, the wall thickness of the glass tube will be relatively thinner compared to the case where the alkali metal addition process is performed after the diameter reduction process, so the alkali metal will be removed from the glass. It can be diffused further to the outer periphery of the tube. Therefore, the alkali metal can be more widely distributed in the core portion of the resulting optical fiber preform.

In the method for manufacturing an optical fiber preform, the temperature at the outer surface of the glass tube caused by the heat source in the concentration lowering step is made lower or higher than the temperature at the outer surface of the glass tube in the concentration increasing step. Preferably, the concentration of the alkali metal is reduced so that the second final concentration of the alkali metal near the inner surface of the glass tube is lower than the second initial concentration.

In this case, if the temperature at the outer surface of the glass tube caused by the heat source in the concentration lowering step is lower than the temperature at the outer surface of the glass tube in the concentration increasing step, the amount of alkali metal oxide produced in the hollow part inside the glass tube can be reduced. As a result, the amount of deposits on the inner surface of the glass tube can be reduced. On the other hand, by making the temperature at the outer surface of the glass tube by the heat source higher than the temperature at the outer surface of the glass tube during the concentration increase step, the alkali metal oxide deposited on the inner surface of the glass tube can be absorbed into the bulk of the glass tube. Diffusion can be promoted. Therefore, the concentration of alkali metal near the inner surface of the glass tube can be effectively reduced. In addition, when the concentration decreasing step is completed and the next concentration increasing step is performed, the temperature at the outer surface of the glass tube due to the heat source can be changed extremely easily, and the concentration decreasing step is immediately performed after the concentration increasing step. Alternatively, the concentration increasing step can be performed immediately after the concentration decreasing step.

In the method for manufacturing an optical fiber preform, the traverse speed of the heat source is made smaller or larger than the traverse speed of the heat source in the concentration increase step in the concentration lowering step, so that the traverse speed of the heat source near the inner surface of the glass tube is reduced. Preferably, the concentration of the alkali metal is reduced such that the second final concentration of the alkali metal is lower than the second initial concentration.

In this case, if the traverse speed of the heat source in the concentration reduction step is lower than the traverse speed of the heat source in the concentration increase step, the heating time per unit time of the glass tube becomes longer, so that the alkali metal oxide When the amount of deposited is small or before crystallization of the glass tube occurs, it becomes easier to diffuse from the inner surface of the glass tube toward the outer diameter. On the other hand, if the traverse speed of the heat source in the concentration lowering step is higher than the traverse speed of the heat source in the concentration increasing step, the amount of alkali metal oxide deposited on the inner surface of the glass tube per unit time can be reduced. Therefore, the concentration of alkali metal near the inner surface of the glass tube can be effectively reduced. Further, when the concentration decreasing step is completed and the next concentration increasing step is performed, the traverse speed of the heat source can be changed very easily, and the concentration decreasing step can be performed immediately after the concentration increasing step. The concentration increasing step can also be carried out immediately after the concentration decreasing step.

In the above method for manufacturing an optical fiber preform, when switching from the concentration decreasing step to the concentration increasing step in the alkali metal addition step, conditions for decreasing the concentration of the alkali metal in the concentration decreasing step are set in advance. In the concentration increasing step, it is preferable to return to conditions that increase the concentration of the alkali metal. Further, in the method for manufacturing an optical fiber preform, when switching from the concentration increasing step to the concentration decreasing step in the alkali metal addition step, conditions for increasing the concentration of the alkali metal in the concentration increasing step are set. It is preferable that the conditions are returned to lowering the concentration of the alkali metal in the concentration lowering step in advance.

In this case, even if the total time of the concentration increase step and the concentration decrease step is the same, that is, the amount of alkali metal added is the same, the concentration of the alkali metal near the inner surface of the glass tube is Since a temporary increase in temperature is suppressed, the glass tube is less likely to crystallize.

At this time, in the alkali metal addition step, when switching from the concentration reduction step to the concentration increase step, the conditions for reducing the concentration of the alkali metal in the concentration reduction step are set in advance to the conditions immediately before the concentration reduction step. In the concentration increasing step, it is preferable to return to conditions that increase the concentration of the alkali metal. Further, in the alkali metal addition step, when switching from the concentration increasing step to the concentration decreasing step, the conditions for increasing the concentration of the alkali metal in the concentration increasing step are set in advance to the concentration immediately before the concentration increasing step. In the lowering step, it is preferable to return to conditions that lower the concentration of the alkali metal.

In this case, even if the total time of the concentration increasing step and the concentration decreasing step is the same, that is, even if the amount of alkali metal added is the same, the concentration of the alkali metal oxide near the inner surface of the glass tube will temporarily change. Since the glass tube is further suppressed from increasing in temperature, the glass tube becomes more difficult to crystallize.

Further, the present invention includes a preform preparation step of preparing an optical fiber preform manufactured by the method for producing an optical fiber preform described above, and a step of drawing the optical fiber preform prepared in the preform preparation step. This is a method of manufacturing an optical fiber, including a drawing step to obtain an optical fiber.

According to this optical fiber manufacturing method, the above-described optical fiber preform manufacturing method can make it difficult for crystallization to occur in the glass tube, and can reduce crystals in the core portion. Therefore, when manufacturing an optical fiber by drawing an optical fiber preform manufactured by surrounding such a core part with a cladding part, it is possible to manufacture an optical fiber in which an increase in transmission loss is suppressed.

In the present invention, the "bulk of the glass tube" refers to the portion between the inner surface and the outer surface of the glass tube.

Furthermore, in the present invention, the term "hollow part inside the glass tube" refers to a hollow part surrounded by the inner surface of the glass tube.

Furthermore, in the present invention, the term "near the inner surface" refers to the bulk of the glass tube, a region radially outward from the inner surface of the glass tube, where the added alkali metal diffuses. For example, a region where the concentration of the added alkali metal is particularly high is a region thinner than 1/10 of the wall thickness of the glass tube from the inner surface of the glass tube.

According to the present invention, there is provided a method for manufacturing an optical fiber preform that can manufacture an optical fiber preform that can form an optical fiber with suppressed increase in transmission loss, and a method for manufacturing an optical fiber using the same. Ru.

1 is an end view showing an example of an optical fiber preform manufactured by the method for manufacturing an optical fiber preform of the present invention. FIG. FIG. 2 is an end view showing a core material used to form a core portion in the method for manufacturing an optical fiber preform of the present invention. FIG. 3 is a diagram showing an alkali metal addition step included in the method for manufacturing an optical fiber preform of the present invention. It is a graph showing the time change of the concentration of alkali metal in the vicinity of the inner surface of the glass tube.

<Manufacturing method of optical fiber base material>
Hereinafter, embodiments of the method for manufacturing an optical fiber preform of the present invention will be described in detail.

First, an embodiment of the optical fiber preform of the present invention will be described with reference to FIG. FIG. 1 is an end view showing an example of an optical fiber preform manufactured by the optical fiber preform manufacturing method of the present invention.

As shown in FIG. 1, the optical fiber preform 10 is a solid preform and includes a core portion 1 and a clad portion 2 surrounding the core portion 1.

Next, a method for manufacturing the optical fiber preform 10 will be described with reference to FIGS. 2 to 4. FIG. 2 is an end view showing the core material used to form the core part in the method for manufacturing an optical fiber preform of the present invention, and FIG. FIG. 4 is a diagram showing a metal addition process, and FIG. 4 is a graph showing changes over time in the concentration of alkali metal near the inner surface of the glass tube.

As shown in FIG. 2, first, the core material 11 is prepared (core material preparation step).

In order to form the core material 11, as shown in FIG. The tube 22 and the second dummy tube 23 are rotatably supported by a lathe (not shown). The glass tube 21, the first dummy tube 22, and the second dummy tube 23 are each made of, for example, silica glass.

Inside the first dummy tube 22, an alkali metal compound 30 serving as a raw material is placed in a solid state at a position away from the connection between the first dummy tube 22 and the glass tube 21. As the alkali metal compound 30, for example, alkali metal halides (chlorides, bromides, fluorides), sulfides, carbonates, hydrogen carbonates, etc. can be used. Examples of the alkali metal include potassium, lithium, sodium, rubidium, and cesium.

Further, a first heat source 41 for heating the alkali metal compound 30 serving as a raw material is provided so as to surround the first dummy tube 22 . A second heat source 42 is provided outside the glass tube 21 to heat the glass tube 21 from the outside. The second heat source 42 can traverse along the longitudinal direction A of the glass tube 21. As the first heat source 41 and the second heat source 42, for example, an oxyhydrogen burner, an electric furnace, etc. can be used.

Further, carrier gas C can be supplied to the composite glass tube 20 from the first dummy tube 22 side, and exhaust gas E can be exhausted from the dummy tube 23 side. Carrier gases include O ₂ gas, inert gases, or mixtures thereof. Examples of the inert gas include helium and argon.

Next, while rotating the composite glass tube 20 around its central axis along the longitudinal direction A using a lathe, the alkali metal compound 30 is heated and melted by the first heat source 41 to generate vapor of the alkali metal compound 30. let

On the other hand, a carrier gas containing O ₂ gas is supplied from the first dummy tube 22 side of the composite glass tube 20 . Then, the vapor of the alkali metal compound 30 is flowed downstream by the carrier gas, and the vapor of the alkali metal compound 30 is flowed downstream in the region R1 between the connection part of the first dummy tube 22 and the glass tube 21 and the part where the alkali metal compound 30 serving as the raw material is arranged. It is cooled, becomes fine particles, becomes an aerosol, and is supplied to the hollow part inside the glass tube 21.

On the other hand, the second heat source 42 is moved back and forth along the longitudinal direction A of the glass tube 21. At this time, the alkali metal compound undergoes an oxidation reaction with the O ₂ gas in the carrier gas C in the hollow part inside the glass tube 21 to become an alkali metal oxide, which is deposited on the inner surface of the glass tube 21 . At this time, the alkali metal oxide is deposited on the inner surface of the glass tube 21 downstream of the heating section by the second heat source 42 due to the thermophoresis effect. Then, when the second heat source 42 passes through the deposited alkali metal oxide later, the alkali metal oxide is diffused into the bulk of the glass tube 21 from the inner surface of the glass tube 21 toward the outer diameter direction. . In this way, an alkali metal is added to the glass tube 21 (alkali metal addition step).

In the alkali metal addition step, first, as shown in FIG. 4, the alkali metal concentration is increased so that the first final concentration C2 of the alkali metal is higher than the first initial concentration C1 near the inner surface of the glass tube 21. (concentration increasing step). At this time, the alkali metal concentration starts from a first initial concentration C1, reaches a maximum, and then reaches a first final concentration C2. The reason why the alkali metal concentration changes in this way is as follows.

That is, first, the alkali metal oxide is deposited on the inner surface of the glass tube 21 due to the thermophoresis effect, and some of it begins to dissolve into the bulk of the glass tube 21, so that the alkali metal oxide near the inner surface of the glass tube 21 is deposited on the inner surface of the glass tube 21. concentration increases. Next, as time passes, the temperature of the glass tube 21 increases as the second heat source 42 approaches, and the amount of deposited and dissolved alkali metal oxides increases, so that the alkali metal concentration near the inner surface of the glass tube 21 increases. It gets even higher. However, due to restrictions on the supply amount of the alkali metal compound 30, the amount of deposited alkali metal oxide becomes saturated, and the concentration of the alkali metal becomes maximum. Next, as the second heat source 42 approaches further, the temperature of the glass tube 21 increases and the alkali metal oxide begins to diffuse from the inner surface side of the glass tube 21 to the outer surface side, so that the inner surface of the glass tube 21 The concentration of alkali metals in the vicinity begins to decrease. When the second heat source 42 approaches further, the temperature of the glass tube 21 becomes higher, the diffusion rate of the alkali metal oxide increases, and the alkali metal concentration near the inner surface of the glass tube 21 decreases. When the second heat source 42 is removed, the temperature of the glass tube 21 decreases, so that the diffusion of the alkali metal oxide almost stops, and the alkali metal concentration near the inner surface of the glass tube 21 approaches a constant level.

This concentration increasing step is completed when the second heat source 42 is traversed at a constant speed along the longitudinal direction A of the glass tube 21 and made one reciprocation (first traverse). At this time, the second heat source 42 heats the outer surface of the glass tube 21 to, for example, 1500° C. or more and 2100° C. or less. Further, the traverse speed of the second heat source 42 is, for example, 25 to 150 mm/min in the downstream direction, and the traverse speed in the upstream direction affects the concentration distribution of the alkali metal near the inner surface of the glass tube 21. The speed is set at such a high speed, for example, 100 to 400 mm/sec, that the temperature of the outer surface of the glass tube 21 is 1000° C. or less, so as not to cause any damage.

Next, as shown in FIG. 4, the second final concentration C3 of the alkali metal near the inner surface of the glass tube 21 becomes the second initial concentration (that is, the first final concentration of the alkali metal in the concentration increasing step performed immediately before). The concentration of the alkali metal is lowered to be lower than C2 (concentration lowering step). At this time, the concentration of the alkali metal starts from the second initial concentration C2, decreases over time, and reaches the second final concentration C3. Further, the concentration lowering step, like the concentration increasing step, is completed when the second heat source 42 is traversed at a constant speed along the longitudinal direction A of the glass tube 21 and reciprocated once (second traverse).

Then, before switching from the concentration decreasing step to the concentration increasing step, the conditions for decreasing the alkali metal concentration in the concentration decreasing step are returned to the conditions for increasing the alkali metal concentration in the concentration increasing step. Specifically, the second heat source 42 causes the temperature of the outer surface of the glass tube 21 and the traverse speed of the second heat source 42 to be the same as in the concentration increasing step performed previously.

Next, in the vicinity of the inner surface of the glass tube 21, alkali metal is Increase the concentration (concentration increasing step). At this time, the alkali metal concentration starts from a first initial concentration C3, reaches a maximum, and then reaches a first final concentration C4. This concentration increasing step is completed when the second heat source 42 is traversed along the longitudinal direction A of the glass tube 21 and made one reciprocation (third traverse).

Then, before switching from the concentration increasing step to the concentration decreasing step, the conditions for increasing the alkali metal concentration in the concentration increasing step are returned to the conditions for decreasing the alkali metal concentration in the concentration decreasing step. Specifically, the temperature of the outer surface of the glass tube 21 caused by the second heat source 42 and the traverse speed of the second heat source 42 are set to be the same as in the concentration lowering step performed previously.

Next, as shown in FIG. 4, the second final concentration C5 of the alkali metal near the inner surface of the glass tube 21 becomes the second initial concentration (that is, the first final concentration of the alkali metal in the concentration increasing step performed immediately before). The concentration of the alkali metal is lowered to be lower than C4 (concentration lowering step). At this time, the concentration of the alkali metal starts from the second initial concentration C4, decreases over time, and reaches the second final concentration C5. At this time, the concentration lowering step, like the concentration increasing step, is completed when the second heat source 42 is traversed along the longitudinal direction A of the glass tube 21 and reciprocated once (fourth traverse).

Next, the outer diameter of the glass tube 21 to which the alkali metal has been added is reduced (diameter reduction step). The diameter reduction step is usually carried out under conditions in which heating of the alkali metal compound 30 by the first heat source 41 is stopped, thereby stopping the supply of the alkali metal compound to the hollow portion inside the glass tube 21 . In the diameter reduction step, the glass tube 21 is heated while the internal pressure of the glass tube 21 is lower than the external pressure, or the temperature of the outer surface of the glass tube 21 is higher than the temperature of the outer surface of the glass tube 21 in the alkali metal addition step. This can be done by heating the glass tube 21 so as to achieve this. The added alkali metal continues to diffuse from the inner surface to the outer surface of the glass tube 21 even during the diameter reduction process.

Subsequently, the diameter-reduced glass tube 21 is finally solidified to obtain the core material 11 (solidification step). The added alkali metal continues to diffuse from the inner surface of the glass tube 21 to the outer surface, or from the center of the core material 11 to the outer periphery, even during the solidification process.

Subsequently, the first dummy tube 22 and the second dummy tube 23 that constituted the composite glass tube 20 are removed from the core material 11 by fusing.

Then, the core material 11 is subjected to stretching and outer circumferential grinding as necessary to form the core portion 1 (core portion forming step).

Next, the cladding part 2 is formed so as to surround the core part 1 (cladding part forming step). The cladding part 2 is formed to have a lower refractive index than the core part 1.

Since the cladding part 2 usually has a larger volume than the core part 1, it may be formed in layers in multiple steps. The refractive index difference and radius ratio between the core portion 1 and the cladding portion 2 may be selected as appropriate depending on the required characteristics of the optical fiber. Further, the cladding portion 2 may be formed from a plurality of layers having different refractive indexes. The optical fiber preform 10 is manufactured as described above.

According to the above manufacturing method, the second heat source traverses along the longitudinal direction of the glass tube 21 while supplying the vapor of the alkali metal compound 30 to the hollow part inside the glass tube 21 together with the carrier gas containing O ₂ gas. An alkali metal addition step is performed in which the glass tube 21 is heated by 42, an alkali metal oxide is deposited on the inner surface of the glass tube 21, and the alkali metal is added into the bulk of the glass tube 21. In the alkali metal addition step, two concentration increasing steps are performed to increase the alkali metal concentration so that the first final concentration of the alkali metal near the inner surface of the glass tube 21 is higher than the first initial concentration. , a concentration lowering step for lowering the concentration of the alkali metal so that the second final concentration of the alkali metal near the inner surface of the glass tube 21 is lower than the second initial concentration at a location between the two concentration increasing steps. Further, after the concentration decreasing step, a concentration increasing step is performed. That is, the steps consisting of the concentration increasing step and the concentration decreasing step are repeated multiple times. In this way, the core material 11 is prepared, and the core part 1 is formed using the core material 11. Therefore, if the total amount of alkali metal added to the bulk of the glass tube 21 is the same, the above concentration decreasing step is performed at two of the locations between the concentration increasing steps, so that the alkali metal is added to the bulk of the glass tube 21. The final concentration of the alkali metal near the inner surface of the glass tube 21 in the concentration increasing step performed last in the addition step can be lowered compared to the case where the concentration decreasing step is performed after all the plurality of concentration increasing steps. can.

For example, in FIG. 4, in the second traverse of the second heat source 42, the concentration decreasing step is changed to a concentration increasing step as shown by a broken line, and in the third traversing, the concentration increasing step is changed to a concentration decreasing step as shown by a broken line. If this is changed, the concentration decreasing step will be performed twice in succession after all the two concentration increasing steps have been performed. In this case, the first final concentration of the alkali metal near the inner surface of the glass tube 21 in the second concentration increasing step (the last concentration increasing step) is C6. On the other hand, in this embodiment, the first final concentration of the alkali metal on the inner surface of the glass tube 21 in the second concentration increasing step (the last concentration increasing step) is C4. Then, C4 will be lower than C6 by ΔC.

Since the concentration of alkali metal can be further reduced near the inner surface of the glass tube 21 in this way, crystallization can be made less likely to occur in the glass tube 21, and crystals in the core portion 1 can be reduced. . Therefore, if the optical fiber preform 10 is manufactured by surrounding the core part 1 with the cladding part 2, it is possible to form an optical fiber in which an increase in transmission loss is suppressed when manufacturing the optical fiber by drawing. It is possible to manufacture an optical fiber preform 10 that allows for the following.

Further, in the above manufacturing method, a diameter reduction step is performed after the alkali metal addition step. For this reason, the wall thickness of the glass tube 21 becomes relatively thinner than when the alkali metal addition step is performed after the diameter reduction step, so that the alkali metal can be diffused further to the outer circumferential side of the glass tube 21. Therefore, the alkali metal can be more widely distributed in the core portion 1 of the optical fiber preform 10 obtained.

Furthermore, in the above manufacturing method, when switching from the concentration decreasing step to the concentration increasing step, the conditions for decreasing the alkali metal concentration in the concentration decreasing step are set in advance so that the alkali metal concentration is changed in the concentration increasing step immediately before the concentration decreasing step. When returning to the conditions for increasing the concentration and switching from the concentration increasing step to the concentration decreasing step, the conditions for increasing the alkali metal concentration in the concentration increasing step are changed in advance by reducing the alkali metal concentration in the concentration decreasing step immediately before the concentration increasing step. We are returning to the conditions that allow us to do so. Therefore, even if the total time of the concentration increasing step and the concentration decreasing step is the same, that is, even if the amount of alkali metal added is the same, the concentration of the alkali metal near the inner surface of the glass tube 21 Since the temporary increase in the temperature is further suppressed, the glass tube 21 becomes more difficult to crystallize.

Next, the concentration lowering step will be explained in detail.

Specifically, the concentration lowering step can be realized by performing the following steps (1) to (4).
(1) A step of reducing the amount of alkali metal compound 30 supplied than the amount of alkali metal compound 30 supplied in the concentration increasing step.
(2) A step of decreasing the amount of O ₂ gas supplied than the amount of O ₂ gas supplied in the concentration increasing step.
(3) A step of making the temperature of the second heat source 42 lower or higher than the temperature of the second heat source 42 in the concentration increasing step.
(4) A step of making the traverse speed of the second heat source 42 smaller or larger than the traverse speed of the second heat source 42 in the concentration increasing step.

Specifically, the step (1) includes reducing the heating temperature of the alkali metal compound 30 by the first heat source 41 to a temperature lower than the melting point of the alkali metal compound 30, stopping the heating by the first heat source 41, or This can be done by providing a shutter (not shown) in the hollow part inside the glass tube 20 to prevent the alkali metal compound 30 from being supplied from the first dummy tube 22 to the hollow part inside the glass tube 21. .

Specifically, the step (2) involves reducing the amount of carrier gas supplied than the amount of carrier gas supplied during the concentration increasing step, or reducing the proportion of O ₂ gas in the carrier gas during the concentration increasing step. This can be done by lowering the proportion of O ₂ gas (for example, from 100% to 0%).

According to the step (1) or (2) above, the amount of alkali metal oxide produced is smaller than the amount produced in the concentration increasing step, so the amount of alkali metal oxide deposited on the inner surface of the glass tube 21 is reduced. Therefore, the concentration of alkali metal on the inner surface of the glass tube 21 can be efficiently reduced.

According to step (3), when the temperature at the outer surface of the glass tube 21 caused by the second heat source 42 in the concentration lowering step is lower than the temperature at the outer surface of the glass tube 21 in the concentration increasing step, The amount of alkali metal oxide produced on the inner surface can be reduced. On the other hand, by making the temperature on the outer surface of the glass tube 21 by the second heat source 42 higher than the temperature on the outer surface of the glass tube 21 in the concentration increasing step, the alkali metal oxide glass deposited on the inner surface of the glass tube 21 Diffusion into the tube 21 can be promoted. Therefore, the concentration of alkali metal near the inner surface of the glass tube 21 can be effectively reduced. Further, when the concentration increasing step is completed and the concentration decreasing step is performed, or conversely, when the concentration decreasing step is completed and the next concentration increasing step is performed, when an oxyhydrogen flame is used as the second heat source 42, The temperature on the outer surface of the glass tube 21 caused by the second heat source 42 can be changed very easily, the concentration decreasing step can be performed immediately after the concentration increasing step, and the concentration increasing step can be performed immediately after the concentration decreasing step. It can also be done immediately.

According to step (4), if the traverse speed of the second heat source 42 in the concentration decreasing step is smaller than the traverse speed of the second heat source 42 in the concentration increasing step, the heating time of the glass tube 21 per unit time is longer. Therefore, the alkali metal oxide easily diffuses from the inner surface of the glass tube 21 toward the outer diameter direction when the amount of the alkali metal oxide deposited is small or before crystallization of the glass tube 21 occurs. On the other hand, if the traverse speed of the second heat source 42 in the concentration decreasing step is higher than the traverse speed of the second heat source 42 in the concentration increasing step, the amount of alkali metal oxide deposited on the inner surface of the glass tube 21 per unit time can be reduced. Therefore, the concentration of alkali metal on the inner surface of the glass tube 21 can be effectively reduced. Furthermore, when the concentration decreasing step is completed and the next concentration increasing step is performed, the traverse speed of the second heat source 42 can be changed very easily, and the concentration decreasing step can be performed immediately after the concentration increasing step. Alternatively, the concentration increasing step can be performed immediately after the concentration decreasing step.

Note that the steps (1) to (4) above can be performed individually, but it is also possible to perform them in combination of two or more steps.

<Optical fiber manufacturing method>
Next, a method for manufacturing an optical fiber according to the present invention will be explained. The optical fiber manufacturing method of the present invention includes a preform preparation step for preparing an optical fiber preform 10 manufactured by the above-described method for producing an optical fiber preform; and an optical fiber preform 10 prepared in the preform preparation step. and a drawing step of drawing an optical fiber to obtain an optical fiber.

According to this optical fiber manufacturing method, crystallization can be made difficult to occur in the glass tube 21 by the above-described optical fiber preform manufacturing method, and crystals in the core portion 1 can be reduced. Therefore, when manufacturing an optical fiber by drawing the optical fiber preform 10 manufactured by surrounding the core part 1 with the cladding part 2, it is possible to manufacture an optical fiber in which an increase in transmission loss is suppressed. I can do it.

<Line drawing process>
In the drawing process, the optical fiber preform 10 is set in a drawing device, heated in a heating furnace, and drawn. As a result, an optical fiber having a core doped with an alkali metal and a cladding is manufactured.

Here, the drawing temperature (furnace temperature) is, for example, 2000 to 2200°C.

The present invention is not limited to the above embodiments. For example, in the above embodiment, the concentration increasing step is performed twice in the alkali metal addition step, but the concentration increasing step may be performed multiple times, and may be performed three or more times. Further, in this case, there are two or more locations between the concentration increasing steps, but in this case, the concentration decreasing step only needs to be performed at at least one location. Therefore, the concentration lowering step may be performed at all locations. For example, in the alkali metal addition step, if the concentration increasing step is performed four times, there will be three locations between the concentration increasing steps, but the concentration decreasing step may be performed at all three locations, or one or two locations. A concentration lowering step may be performed at certain locations.

Further, in the above embodiment, the alkali metal addition step is performed before the diameter reduction step in the core material preparation step, but the alkali metal addition step may be performed after the diameter reduction step, or the alkali metal addition step may be performed multiple times. When performing a diameter reduction step, an alkali metal addition step may be performed at a location between the diameter reduction steps.

Furthermore, in the above embodiment, when switching from the concentration reduction step to the concentration increase step, the conditions for reducing the alkali metal concentration in the concentration reduction step are set in advance so that the alkali metal concentration is changed in the concentration increase step immediately before the concentration reduction step. Although we have returned to the condition where it increases, it does not necessarily have to be returned.

In addition, in the above embodiment, when switching from the concentration increase step to the concentration decrease step, the conditions for increasing the alkali metal concentration in the concentration increase step are set in advance so that the alkali metal concentration is changed in the concentration decrease step immediately before the concentration increase step. Although the condition is restored to lower conditions, it does not necessarily have to be restored.

1... Core part 2... Clad part 10... Optical fiber base material 11... Core material 21... Glass tube 42... Second heat source (heat source)

Claims (9)

A method for manufacturing an optical fiber preform, comprising: a core portion; and a cladding portion surrounding the core portion;
a core material preparation step for preparing a core material;
a core part forming step of forming the core part using the core material;
a cladding part forming step of forming the cladding part so as to surround the core part,
The core material preparation step
The glass tube is heated by a heat source that traverses along the longitudinal direction of the glass tube while supplying vapor of an alkali metal compound together with a carrier gas containing oxygen gas into the hollow space inside the glass tube. an alkali metal addition step of adding an alkali metal into the bulk of the glass tube by depositing an alkali metal oxide on the inner surface;
The alkali metal addition step is
a plurality of concentration increasing steps of increasing the concentration of the alkali metal such that a first final concentration of the alkali metal near the inner surface of the glass tube is higher than a first initial concentration;
Concentration of the alkali metal is carried out at at least one of the locations between the concentration increasing steps such that the second final concentration of the alkali metal near the inner surface of the glass tube is lower than the second initial concentration. A method for manufacturing an optical fiber preform, the method comprising: reducing the concentration of the preform. The method for manufacturing an optical fiber preform according to claim 1, wherein the core material preparation step further includes a diameter reduction step of reducing the outer diameter of the glass tube after the alkali metal addition step. In the concentration lowering step, the alkali in the vicinity of the inner surface of the glass tube is reduced by making the temperature at the outer surface of the glass tube by the heat source lower or higher than the temperature at the outer surface of the glass tube in the concentration increasing step. 3. The method of manufacturing an optical fiber preform according to claim 1, wherein the concentration of the alkali metal is reduced so that the second final concentration of metal is lower than the second initial concentration. In the concentration lowering step, by making the traverse speed of the heat source smaller or larger than the traverse speed of the heat source in the concentration increasing step, the second final concentration of the alkali metal near the inner surface of the glass tube is reduced to a second concentration. The method for manufacturing an optical fiber preform according to any one of claims 1 to 3, wherein the concentration of the alkali metal is lowered to be lower than an initial concentration. In the alkali metal addition step, when switching from the concentration reduction step to the concentration increase step, the conditions for reducing the alkali metal concentration in the concentration reduction step are set in advance so that the alkali metal concentration in the concentration increase step is changed. The method for producing an optical fiber preform according to any one of claims 1 to 4, wherein the method is returned to increasing conditions. In the alkali metal addition step, when switching from the concentration reduction step to the concentration increase step, the conditions for reducing the alkali metal concentration in the concentration reduction step are set in advance in the concentration increase step immediately before the concentration reduction step. 6. The method for manufacturing an optical fiber preform according to claim 5, wherein conditions are returned to increase the concentration of the alkali metal. In the alkali metal addition step, when the concentration decreasing step is performed multiple times and the concentration increasing step is switched to the second or subsequent concentration decreasing step, conditions for increasing the concentration of the alkali metal in the concentration increasing step. The method for producing an optical fiber preform according to any one of claims 1 to 6, wherein the alkali metal is returned to conditions that reduce the concentration of the alkali metal in the concentration lowering step. In the alkali metal addition step, when switching from the concentration increasing step to the second or subsequent concentration decreasing step, the conditions for increasing the concentration of the alkali metal in the concentration increasing step are set in advance to the conditions immediately before the concentration increasing step. 8. The method for manufacturing an optical fiber preform according to claim 7, wherein the concentration of the alkali metal is returned to a condition that lowers the concentration of the alkali metal in the concentration lowering step. A preform preparation step of preparing an optical fiber preform manufactured by the method for manufacturing an optical fiber preform according to any one of claims 1 to 8;
A method for manufacturing an optical fiber, comprising a drawing step of obtaining an optical fiber by drawing the optical fiber preform prepared in the preform preparation step.

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