JP2021113134A - Method for manufacturing optical fiber preform and method for manufacturing optical fiber using the same - Google Patents

Abstract

To provide a method, etc., for manufacturing an optical fiber preform, capable of forming an optical fiber having suppressed increase in transmission loss.SOLUTION: A method for manufacturing an optical fiber preform comprises a core material preparation step, a core part formation step and a cladding part formation step. The core material preparation step includes an alkali metal addition step of heating a glass tube by a heat source traversed in the longitudinal direction of the glass tube while supplying the vapor of an alkali metal compound together with a carrier gas including oxygen gas to a hollow part on the inner side of the glass tube to deposit alkali metal oxide on the inner surface of the glass tube and add alkali metal into the bulk of the glass tube. The alkali metal addition step includes a plurality of concentration increase steps of increasing the concentration of the alkali metal so that the final concentration of the alkali metal in the vicinity of the inner surface of the glass tube is higher than the initial concentration and a concentration reduction step performed between the concentration increase steps, of reducing the concentration of the alkali metal so that the final concentration of the alkali metal in the vicinity of the inner surface of the glass tube is lower than the initial concentration.SELECTED DRAWING: Figure 4

Description

The present invention relates to a method for producing an optical fiber base material and a method for producing an optical fiber using the same.

Conventionally, an optical fiber in which an alkali metal is added to a core has been known from the viewpoint of reducing transmission loss.

As a method for producing an optical fiber base material for forming such an optical fiber, for example, the production method described in Patent Document 1 below is known. In the same document, after the core portion is formed by performing an alkali metal addition step of adding an alkali metal to a glass tube, a diameter reduction step of reducing the diameter of the glass tube, a solidification step of solidifying the glass tube, and the like, the core is formed. A method of forming a clad portion around the portion to manufacture an optical fiber base material is disclosed. Further, in the same document, in the alkali metal addition step, the vapor of the alkali metal compound is allowed to flow together with oxygen gas into the hollow inside of the glass tube, and the alkali metal is diffused and added into the glass tube while heating the glass tube with an external heat source. Also disclosed that in the diameter reduction step, the supply of alkali metal vapor is stopped, the glass tube is continuously heated by an external heat source to reduce the diameter of the glass tube, and finally the solidification is performed in the solidification step. Has been done.

Japanese Unexamined Patent Publication No. 2013-136485

However, the method for manufacturing an optical fiber base material described in Patent Document 1 has room for improvement in terms of transmission loss in the obtained optical fiber when the optical fiber base material is drawn to manufacture the optical fiber. Was there.

The present invention has been made in view of the above circumstances, and uses a method for producing an optical fiber base material capable of producing an optical fiber base material capable of forming an optical fiber in which an increase in transmission loss is suppressed, and a method for producing the optical fiber base material. An object of the present invention is to provide a method for manufacturing an optical fiber.

The present inventors have made extensive studies to solve the above problems. As a result, in the method for producing an optical fiber base material described in Patent Document 1, when the alkali metal is added to the glass tube from the inner surface of the glass tube in the alkali metal addition step, the addition of the alkali metal is superior to the diffusion. The final concentration of alkali metal near the inner surface of the glass tube increases each time the external heat source is traversed, and when the alkali metal addition process is completed, the final concentration of alkali metal near the inner surface of the glass tube increases. The present inventors thought that the value would be very large. Further, as a result of the final concentration of the alkali metal becoming very large near the inner surface of the glass tube, crystallization is likely to occur in the glass tube, and these crystals remain in the core portion of the optical fiber base material, and further, light. The present inventors considered that the transmission loss of the optical fiber may increase as a result of remaining in the core of the optical fiber obtained by drawing the fiber base material. Therefore, as a result of further diligent studies, the present inventors have found that the above problems can be solved by the following inventions.

That is, the present invention is a method for manufacturing an optical fiber base material for producing an optical fiber base material having a core portion and a clad portion surrounding the core portion, the core material preparation step for preparing the core material, and the core material preparation step. The core material preparation step includes a core portion forming step of forming the core portion using the core material and a clad portion forming step of forming the clad portion so as to surround the core portion. While supplying vapor of an alkali metal compound to the inner hollow portion together with a carrier gas containing oxygen gas, the glass tube is heated by a heat source traversing along the longitudinal direction of the glass tube to reach the inner surface of the glass tube. The alkali metal addition step of depositing an alkali metal oxide and adding the alkali metal into the bulk of the glass tube is included, and the alkali metal addition step is the first final concentration of the alkali metal in the vicinity of the inner surface of the glass tube. Is performed at at least one of a plurality of concentration increasing steps for increasing the concentration of the alkali metal so as to be higher than the first initial concentration and between the concentration increasing steps, and the inner surface of the glass tube. This is a method for producing an optical fiber base material, which includes a concentration reducing step of reducing the concentration of the alkali metal so that the second final concentration of the alkali metal in the vicinity becomes lower than the 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 to the hollow portion inside the glass tube together with a carrier gas containing oxygen gas. , An alkali metal addition step is performed in which an alkali metal oxide is deposited on the inner surface of the glass tube and the alkali metal is added into the bulk of the glass tube. Then, 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 in the vicinity of the inner surface of the glass tube becomes higher than the first initial concentration, and the concentration is increased. At least one of the points between the increasing steps is a concentration lowering step of lowering the alkali metal concentration so that the second final concentration of the alkali metal in the vicinity of the inner surface of the glass tube is lower than the second initial concentration. It is done and the core material is prepared. Then, the core portion is formed using the core material thus prepared. Therefore, when the total amount of alkali metals added in the bulk of the glass tube is the same, the alkali metals are formed by performing the above concentration decreasing step at at least one of the locations between the concentration increasing steps. The final concentration of the alkali metal in the vicinity of the inner surface of the glass tube in the concentration increasing step finally performed in the addition step can be reduced as compared with the case where the concentration decreasing step is performed after all the plurality of concentration increasing steps are performed. can. Therefore, it is possible to make it difficult for crystallization to occur in the glass tube, and it is possible to reduce the number of crystals in the core portion. Therefore, when an optical fiber is manufactured by drawing after surrounding such a core portion with a clad portion, it is possible to manufacture an optical fiber base material capable of forming an optical fiber in which an increase in transmission loss is suppressed. Can be done.

In the method for producing an optical fiber base material, 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 step is performed after the alkali metal addition step, the wall thickness of the glass tube is relatively thinner than that when the alkali metal addition step is performed after the diameter reduction step. It can be diffused to the outer peripheral side of the tube. Therefore, the alkali metal can be more widely distributed in the core portion of the obtained optical fiber base material.

In the method for producing an optical fiber base material, the temperature on the outer surface of the glass tube due to the heat source is made lower or higher than the temperature on the outer surface of the glass tube in the concentration increasing step in the concentration lowering step. It is preferable to reduce the concentration of the alkali metal so that the second final concentration of the alkali metal in the vicinity of the inner surface of the glass tube is lower than the second initial concentration.

In this case, when the temperature on the outer surface of the glass tube due to the heat source is made lower than the temperature on the outer surface of the glass tube in the concentration increasing step in the concentration lowering step, the amount of alkali metal oxide produced in the hollow portion inside the glass tube is increased. It can be reduced, and as a result, the amount of deposit on the inner surface of the glass tube can be reduced. On the other hand, by making the temperature on the outer surface of the glass tube by the heat source higher than the temperature on the outer surface of the glass tube in the concentration increasing step, the alkali metal oxide deposited on the inner surface of the glass tube is put into the bulk of the glass tube. It can promote diffusion. Therefore, the concentration of the alkali metal in the vicinity of 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 temperature on the outer surface of the glass tube by the heat source can be changed extremely easily, and the concentration decreasing step after the concentration increasing step is immediately performed. This can be done, or the concentration increasing step after the concentration decreasing step can be performed immediately.

In the method for producing an optical fiber base material, in the concentration lowering step, the traverse rate of the heat source is made smaller or larger than the traverse rate of the heat source in the concentration increasing step, thereby in the vicinity of the inner surface of the glass tube. It is preferable to reduce the concentration of the alkali metal so that the second final concentration of the alkali metal is lower than the second initial concentration.

In this case, when the traverse rate of the heat source in the concentration lowering step is smaller than the traverse rate of the heat source in the concentration increasing step, the heating time per unit time of the glass tube becomes long, so that the alkali metal oxide becomes an alkali metal oxide. It becomes easy to diffuse from the inner surface of the glass tube toward the outer diameter direction before the amount of metal deposited is small or the glass tube does not crystallize. On the other hand, when the traverse rate of the heat source in the concentration lowering step is higher than the traverse rate 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 the alkali metal in the vicinity of 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 rate of the heat source can be changed extremely easily, and the concentration decreasing step after the concentration increasing step can be immediately performed. It is also possible to immediately perform the concentration increasing step after the concentration decreasing step.

In the method for producing an optical fiber base material, when switching from the concentration lowering step to the concentration increasing step in the alkali metal addition step, conditions for lowering the alkali metal concentration in the concentration lowering step are set in advance. It is preferable to return to the condition for increasing the concentration of the alkali metal in the concentration increasing step. Further, in the method for producing the optical fiber base material, when switching from the concentration increasing step to the concentration decreasing step in the alkali metal addition step, the condition for increasing the alkali metal concentration in the concentration increasing step is set. It is preferable to return to the condition for reducing the concentration of the alkali metal in advance in the concentration lowering step.

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 the alkali metal added is the same, the concentration of the alkali metal near the inner surface of the glass tube is high. Since the temporary increase is suppressed, the glass tube is less likely to crystallize.

At this time, when switching from the concentration lowering step to the concentration increasing step in the alkali metal addition step, the condition for lowering the alkali metal concentration in the concentration lowering step is set in advance immediately before the concentration lowering step. It is preferable to return to the condition for increasing the concentration of the alkali metal in the concentration increasing step. Further, when switching from the concentration increasing step to the concentration decreasing step in the alkali metal addition step, the condition for increasing the alkali metal concentration in the concentration increasing step is previously set to the concentration immediately before the concentration increasing step. In the lowering step, it is preferable to return to the condition for lowering 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 the alkali metal added is the same, the concentration of the alkali metal oxide near the inner surface of the glass tube is temporary. The glass tube is less likely to crystallize because it is more suppressed from becoming too high.

Further, in the present invention, the base material preparation step for preparing the optical fiber base material produced by the above-described optical fiber base material manufacturing method and the optical fiber base material prepared in the base material preparation step are drawn. It is a method of manufacturing an optical fiber including a drawing step of obtaining an optical fiber.

According to this method for manufacturing an optical fiber, it is possible to make it difficult for crystallization to occur in a glass tube and reduce the number of crystals in the core portion by the above-mentioned method for manufacturing an optical fiber base material. Therefore, when the optical fiber base material produced by surrounding such a core portion with a clad portion is drawn to produce an optical fiber, it is possible to produce an optical fiber in which an increase in transmission loss is suppressed.

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

Further, in the present invention, the "hollow portion inside the glass tube" means a hollow portion surrounded by the inner surface of the glass tube.

Further, in the present invention, the "near the inner surface" is the bulk of the glass tube, the region from the inner surface of the glass tube to the radial outer side of the glass tube, and the region where the added alkali metal diffuses. Yes, 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 producing an optical fiber base material capable of producing an optical fiber base material capable of forming an optical fiber in which an increase in transmission loss is suppressed, and a method for producing an optical fiber using the same. NS.

It is an end view which shows an example of the optical fiber base material manufactured by the manufacturing method of the optical fiber base material of this invention. It is an end view which shows the core material used for forming the core part in the manufacturing method of the optical fiber base material of this invention. It is a figure which shows the alkali metal addition process included in the manufacturing method of the optical fiber base material of this invention. It is a graph which shows the time change of the concentration of alkali metal in the vicinity of the inner surface of a glass tube.

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

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

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

Next, a method of manufacturing the optical fiber base material 10 will be described with reference to FIGS. 2 to 4. FIG. 2 is an end view showing a core material used for forming a core portion in the method for producing an optical fiber base material of the present invention, and FIG. 3 is an alkali contained in the method for producing an optical fiber base material of the present invention. It is a figure which shows the metal addition process, and FIG. 4 is a graph which shows the time change of the concentration of alkali metal in the vicinity of the inner surface of a 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. 3, first, a composite glass tube 20 in which the first dummy tube 22 and the second dummy tube 23 are connected to both ends of the glass tube 21 is prepared, and the first dummy is formed. 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, the alkali metal compound 30 as a raw material is arranged in a solid state at a position away from the connection portion between the first dummy tube 22 and the glass tube 21. As the alkali metal compound 30, for example, alkali metal halides (chlorides, bromines, fluorides), sulfides, carbonates, hydrogen carbonates and the like can be used. Examples of the alkali metal include potassium, lithium, sodium, rubidium, cesium and the like.

Further, a first heat source 41 for heating the alkali metal compound 30 as a raw material is provided so as to surround the first dummy tube 22. A second heat source 42 for heating the glass tube 21 from the outside is provided on the outside of the glass tube 21. 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, or the like can be used.

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

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

On the other hand, the carrier gas containing _{the 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 to the downstream side by the carrier gas, and in the region R1 between the connecting portion of the first dummy tube 22 and the glass tube 21 and the portion where the alkali metal compound 30 as a raw material is arranged. It is cooled, becomes fine particles, becomes an aerosol, and is supplied to the hollow portion inside the glass tube 21.

On the other hand, the second heat source 42 is reciprocated along the longitudinal direction A of the glass tube 21. _{At this time, the alkali metal compound oxidizes with the O 2} gas in the carrier gas C in the hollow portion inside the glass tube 21 to form 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 on the downstream side of the heating portion by the second heat source 42 due to the thermophoresis effect. Then, when the second heat source 42 later passes through the deposited alkali metal oxide, the alkali metal oxide is diffused into the bulk of the glass tube 21 from the inner surface of the glass tube 21 in the outer radial direction. .. In this way, the 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 becomes higher than the first initial concentration C1 in the vicinity of the inner surface of the glass tube 21. (Concentration increasing step). At this time, the concentration of the alkali metal starts from the first initial concentration C1 and reaches the maximum, and then reaches the 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 first deposited on the inner surface of the glass tube 21 due to the thermophoresis effect, and a part of the alkali metal oxide begins to dissolve in the bulk of the glass tube 21, so that the alkali metal in the vicinity of the inner surface of the glass tube 21 Concentration increases. Next, as the second heat source 42 approaches with the passage of time, the temperature of the glass tube 21 rises, and the accumulated amount and the dissolved amount of the alkali metal oxide increase, so that the alkali metal concentration in the vicinity of the inner surface of the glass tube 21 increases. It will be even higher. However, due to the limitation of the supply amount of the alkali metal compound 30, the accumulated amount of the alkali metal oxide becomes saturated, and the concentration of the alkali metal becomes maximum. Next, when the second heat source 42 approaches further, the temperature of the glass tube 21 rises and the alkali metal oxide starts to diffuse from the inner surface side to the outer surface side of the glass tube 21, so that the inner surface of the glass tube 21 starts to diffuse. Alkali metal concentration in the vicinity begins to decrease. Then, when the second heat source 42 is further approached, the temperature of the glass tube 21 becomes higher, the diffusion rate of the alkali metal oxide increases, and the alkali metal concentration in the vicinity of the inner surface of the glass tube 21 decreases. Then, when the second heat source 42 is separated, the temperature of the glass tube 21 is lowered, so that the diffusion of the alkali metal oxide is almost stopped, and the alkali metal concentration in the vicinity of the inner surface of the glass tube 21 approaches a constant value.

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 reciprocated once (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 higher and 2100 ° C. or lower. The traverse speed of the second heat source 42 is, for example, 25 to 150 mm / min for the speed toward the downstream side, and the speed toward the upstream side affects the concentration distribution of alkali metal in the vicinity of the inner surface of the glass tube 21. The outer surface of the glass tube 21 is set to a high speed such that the temperature is 1000 ° C. or lower, for example, 100 to 400 mm / sec.

Next, as shown in FIG. 4, the second final concentration C3 of the alkali metal is the second initial concentration (that is, the first final concentration of the alkali metal in the concentration increasing step performed immediately before) in the vicinity of the inner surface of the glass tube 21. The concentration of the alkali metal is lowered so as to be lower than C2 (concentration lowering step). At this time, the concentration of the alkali metal starts from the second initial concentration C2 and decreases with time to reach the second final concentration C3. Further, the concentration decreasing 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, as in the concentration increasing step (second traverse).

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

Next, the alkali metal so that the first final concentration C4 of the alkali metal becomes higher than the first initial concentration (that is, the second final concentration of the concentration reducing step performed immediately before) C3 in the vicinity of the inner surface of the glass tube 21. Increase the concentration (concentration increase step). At this time, the concentration of the alkali metal starts from the first initial concentration C3, reaches the maximum, and then reaches the 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 reciprocated once (third traverse).

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

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

Next, the outer diameter of the glass tube 21 to which the alkali metal is added is reduced (diameter reduction step). The diameter reduction step is usually carried out under the condition that the heating of the alkali metal compound 30 by the first heat source 41 is stopped to stop 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, and 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 be. The added alkali metal diffuses from the inner surface side to the outer surface side of the glass tube 21 even in the diameter reduction step.

Subsequently, the reduced diameter glass tube 21 is finally solidified to obtain the core material 11 (solidification step). The added alkali metal also diffuses from the inner surface side to the outer surface side of the glass tube 21 or from the center of the core material 11 to the outer peripheral side even in the solidification step.

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

Then, the core material 11 is stretched and externally ground as necessary to form the core portion 1 (core portion forming step).

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

Since the clad portion 2 usually has a larger volume than the core portion 1, it may be formed in layers by dividing it into a plurality of times. The difference in refractive index and the radius ratio between the core portion 1 and the clad portion 2 may be appropriately selected according to the required characteristics of the optical fiber. Further, the clad portion 2 may be formed of a plurality of layers having different refractive indexes. The optical fiber base material 10 is manufactured as described above.

According to the above manufacturing method, a second heat source that traverses along the longitudinal direction of the glass tube 21 while supplying the vapor of the alkali metal compound 30 to the hollow portion inside the glass tube 21 _{together with the carrier gas containing the 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. Then, in the alkali metal addition step, two concentration increasing steps are performed in which the concentration of the alkali metal is increased so that the first final concentration of the alkali metal in the vicinity of the inner surface of the glass tube 21 is higher than the first initial concentration. At the point between the two concentration increasing steps, there is a concentration lowering step of lowering the alkali metal concentration so that the second final concentration of the alkali metal in the vicinity of the inner surface of the glass tube 21 is lower than the second initial concentration. Then, the concentration increasing step is performed after the concentration decreasing step. That is, the step consisting of the concentration increasing step and the concentration decreasing step is repeated a plurality of times. In this way, the core material 11 is prepared, and the core portion 1 is formed using the core material 11. Therefore, when the total amount of alkali metals added to the bulk of the glass tube 21 is the same, the alkali metals are formed by performing the above concentration decreasing steps at two of the locations between the concentration increasing steps. The final concentration of the alkali metal in the vicinity of the inner surface of the glass tube 21 in the concentration increasing step finally performed in the addition step can be reduced as compared with the case where the concentration decreasing step is performed after all the plurality of concentration increasing steps are performed. can.

For example, in FIG. 4, the concentration decreasing step in the second traverse of the second heat source 42 is changed to the concentration increasing step as shown by the broken line, and the concentration increasing step in the third traverse is changed to the concentration decreasing step as shown by the broken line. When changed to, after performing all the two concentration increasing steps, the concentration decreasing step is performed twice in succession. In this case, the first final concentration of the alkali metal in the vicinity of 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 the present 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 is lower than C6 by ΔC.

Since the concentration of the alkali metal can be further reduced in the vicinity of 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, when the optical fiber base material 10 is manufactured by surrounding the core portion 1 with the clad portion 2, an optical fiber in which an increase in transmission loss is suppressed is formed when the optical fiber is manufactured by drawing. It is possible to manufacture an optical fiber base material 10 capable of producing the same.

Further, in the above manufacturing method, a diameter reduction step is performed after the alkali metal addition step. Therefore, since the wall thickness of the glass tube 21 is relatively thin as compared with the case where the alkali metal addition step is performed after the diameter reduction step, the alkali metal can be diffused to the outer peripheral side in the glass tube 21. Therefore, the alkali metal can be more widely distributed in the core portion 1 of the obtained optical fiber base material 10.

Further, in the above production method, when switching from the concentration decreasing step to the concentration increasing step, the condition for reducing the concentration of the alkali metal in the concentration decreasing step is set in advance, and the concentration of the alkali metal is set in advance in the concentration increasing step immediately before the concentration decreasing step. When returning to the increasing condition and switching from the concentration increasing step to the concentration decreasing step, the condition for increasing the alkali metal concentration in the concentration increasing step is set in advance to reduce the alkali metal concentration in the concentration decreasing step immediately before the concentration increasing step. It is returned to the condition to make it. Therefore, even when the total time of the concentration increasing step and the concentration decreasing step is the same, that is, even when the amount of the alkali metal added is the same, the concentration of the alkali metal near the inner surface of the glass tube 21 is the same. The glass tube 21 is less likely to crystallize because the temporary increase in the amount of the glass tube 21 is further suppressed.

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

Specifically, the concentration lowering step can be realized by performing the following steps (1) to (4).
(1) A step of reducing the supply amount of the alkali metal compound 30 from the supply amount of the alkali metal compound 30 in the concentration increasing step.
(2) the supply amount of O ₂ gas, the step of reducing than the supply amount of O ₂ gas in the concentration increasing step.
(3) A step of raising the temperature of the second heat source 42 to be 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, in the step (1), the heating temperature of the alkali metal compound 30 by the first heat source 41 is set to a temperature lower than the melting point of the alkali metal compound 30, the heating by the first heat source 41 is stopped, or the composite glass tube. This can be done by providing a shutter (not shown) in the hollow portion inside the 20 and preventing the alkali metal compound 30 from being supplied from the first dummy tube 22 to the hollow portion inside the glass tube 21. ..

Specifically, in the step (2), the supply amount of the carrier gas is reduced from the supply amount of the carrier gas in the concentration increasing step, or _{the ratio of the O 2} gas in the carrier gas is reduced in the concentration increasing step. O ₂ ratio of gas to lower than (to, for example, 0% to 100%) that can be carried out by such.

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 that the amount of alkali metal oxide deposited on the inner surface of the glass tube 21 is increased. It can be reduced, and the concentration of alkali metal on the inner surface of the glass tube 21 can be efficiently reduced.

According to the step (3), when the temperature on the outer surface of the glass tube 21 by the second heat source 42 in the concentration lowering step is lower than the temperature on the outer surface of the glass tube 21 in the concentration increasing step, the glass tube 21 The amount of alkali metal oxide produced on the inner surface can be reduced. On the other hand, the temperature on the outer surface of the glass tube 21 by the second heat source 42 is made higher than the temperature on the outer surface of the glass tube 21 in the concentration increasing step, so that the glass of the alkali metal oxide deposited on the inner surface of the glass tube 21 is formed. It is possible to promote diffusion into the tube 21. Therefore, the concentration of the alkali metal in the vicinity of 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, conversely when the concentration decreasing step is completed and the next concentration increasing step is performed, when an acid hydrogen flame is used as the second heat source 42, the second 2. The temperature on the outer surface of the glass tube 21 by the heat source 42 can be changed extremely easily, the concentration decreasing step after the concentration increasing step can be immediately performed, and the concentration increasing step after the concentration decreasing step can be performed. It can also be done immediately.

According to the step (4), when the traverse speed of the second heat source 42 is smaller than the traverse speed of the second heat source 42 in the concentration increasing step in the concentration decreasing step, the heating time per unit time of the glass tube 21 is long. Therefore, the alkali metal oxide tends to diffuse from the inner surface of the glass tube 21 toward the outer diameter before the amount of the alkali metal oxide deposited is small or the glass tube 21 does not crystallize. On the other hand, when the traverse rate of the second heat source 42 is higher than the traverse rate of the second heat source 42 in the concentration increasing step in the concentration decreasing 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 the alkali metal on the inner surface of the glass tube 21 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 second heat source 42 can be changed extremely easily, and the concentration decreasing step after the concentration increasing step can be performed immediately. Alternatively, the concentration increasing step after the concentration decreasing step can be performed immediately.

The above steps (1) to (4) can be performed independently, but it is also possible to perform the steps (1) to (4) in combination.

<Manufacturing method of optical fiber>
Next, the method for manufacturing the optical fiber of the present invention will be described. The optical fiber manufacturing method of the present invention includes a base material preparation step for preparing the optical fiber base material 10 manufactured by the above-described optical fiber base material manufacturing method and an optical fiber base material 10 prepared in the base material preparation step. Includes a drawing step of drawing an optical fiber to obtain an optical fiber.

According to this method for manufacturing an optical fiber, it is possible to make it difficult for crystallization to occur in the glass tube 21 and reduce the number of crystals in the core portion 1 by the above-mentioned method for manufacturing an optical fiber base material. Therefore, when the optical fiber base material 10 manufactured by surrounding the core portion 1 with the clad portion 2 is drawn to manufacture the optical fiber, the optical fiber in which the increase in transmission loss is suppressed is manufactured. Can be done.

<Line drawing process>
In the drawing step, the optical fiber base material 10 is set in the drawing device, heated in a heating furnace, and drawn. As a result, an optical fiber having a core to which an alkali metal is added and a clad is manufactured.

Here, the drawing temperature (heating 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, in the alkali metal addition step, the concentration increasing step is performed twice, but the concentration increasing step may be performed a plurality of times and may be performed three times or more. Further, in this case, there are two or more locations between the concentration increasing steps, but in this case, the concentration decreasing step may be performed at at least one location. Therefore, the concentration reducing step may be performed at all points. For example, in the alkali metal addition step, when the concentration increasing step is performed four times, there are three places between the concentration increasing steps, but the concentration decreasing step may be performed at all three places, one place or two. A concentration reducing step may be performed at the site.

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

Further, in the above embodiment, when switching from the concentration decreasing step to the concentration increasing step, the condition for reducing the concentration of the alkali metal in the concentration decreasing step is set in advance, and the concentration of the alkali metal is set in advance in the concentration increasing step immediately before the concentration decreasing step. It is returned to the condition to increase it, but it is not always necessary to return it.

Further, in the above embodiment, when switching from the concentration increasing step to the concentration decreasing step, the condition for increasing the concentration of the alkali metal in the concentration increasing step is set in advance, and the concentration of the alkali metal is set in advance in the concentration decreasing step immediately before the concentration increasing step. It has been returned to the conditions for lowering it, but it is not always necessary to return it.

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 base material, which manufactures an optical fiber base material having a core portion and a clad portion surrounding the core portion.
The core material preparation process and the core material preparation process
A core portion forming step of forming the core portion using the core material, and
Including a clad portion forming step of forming the clad portion so as to surround the core portion.
The core material preparation process
While supplying vapor of an alkali metal compound to a hollow portion inside the glass tube together with a carrier gas containing oxygen gas, the glass tube is heated by a heat source traversing along the longitudinal direction of the glass tube, and the glass tube is heated. It includes an alkali metal addition step of depositing an alkali metal oxide on the inner surface and adding the alkali metal into the bulk of the glass tube.
The alkali metal addition step
A plurality of concentration increasing steps of increasing the concentration of the alkali metal so that the first final concentration of the alkali metal in the vicinity of the inner surface of the glass tube is higher than the first initial concentration.
The concentration of the alkali metal is carried out at least one of the locations between the concentration increasing steps so that the second final concentration of the alkali metal in the vicinity of the inner surface of the glass tube is lower than the second initial concentration. A method for producing an optical fiber base material, which comprises a concentration reducing step of reducing the concentration. The method for producing an optical fiber base material 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 temperature on the outer surface of the glass tube by the heat source is made lower or higher than the temperature on the outer surface of the glass tube in the concentration increasing step, so that the alkali in the vicinity of the inner surface of the glass tube. The method for producing an optical fiber base material according to claim 1 or 2, wherein the concentration of the alkali metal is reduced so that the second final concentration of the metal is lower than the second initial concentration. By making the traverse rate of the heat source smaller or larger than the traverse rate of the heat source in the concentration increasing step in the concentration decreasing step, the second final concentration of the alkali metal in the vicinity of the inner surface of the glass tube is second. The method for producing an optical fiber base material according to any one of claims 1 to 3, wherein the concentration of the alkali metal is reduced so as to be lower than the initial concentration. When switching from the concentration lowering step to the concentration increasing step in the alkali metal addition step, the condition for lowering the alkali metal concentration in the concentration lowering step is set in advance by setting the alkali metal concentration in the concentration increasing step. The method for producing an optical fiber base material according to any one of claims 1 to 4, which returns to the conditions for increasing the number. When switching from the concentration lowering step to the concentration increasing step in the alkali metal addition step, the condition for lowering the alkali metal concentration in the concentration lowering step is set in advance in the concentration increasing step immediately before the concentration lowering step. The method for producing an optical fiber base material according to claim 5, returning to the condition of increasing the concentration of the alkali metal. When switching from the concentration increasing step to the concentration decreasing step in the alkali metal addition step, the condition for increasing the alkali metal concentration in the concentration increasing step is set in advance by setting the alkali metal concentration in the concentration decreasing step. The method for producing an optical fiber base material according to any one of claims 1 to 6, which returns to a lowering condition. When switching from the concentration increasing step to the concentration decreasing step in the alkali metal addition step, the condition for increasing the alkali metal concentration in the concentration increasing step is set in advance in the concentration decreasing step immediately before the concentration increasing step. The method for producing an optical fiber base material according to claim 7, wherein the conditions for reducing the concentration of the alkali metal are returned to. A base material preparation step for preparing an optical fiber base material produced by the method for producing an optical fiber base material according to any one of claims 1 to 8.
A method for manufacturing an optical fiber, which comprises a drawing step of drawing the optical fiber base material prepared in the base material preparation step to obtain an optical fiber.

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