JP2022127161A - Method for manufacturing optical fiber preform and method for manufacturing optical fiber - Google Patents

Abstract

To reduce the concentration of OH group in a portion near a seed rod in a transparent glass preform to make it possible to manufacture an optical fiber having a small transmission loss derived from the OH group even when using the portion near the seed rod in the transparent glass preform.SOLUTION: A method for manufacturing an optical fiber preform comprises: a seed rod manufacturing step of manufacturing a seed rod having an average OH group concentration of 1 ppm or less in a region having a distance of 0.05 mm or more and 0.5 mm or less from the surface in a tip; a glass fine particle deposit manufacturing step of depositing glass fine particles formed by supplying a glass raw material to a burner on the rotating seed rod to manufacture a glass fine particle deposit; a dehydration step of heating and dehydrating the glass fine particle deposit by a heater in dehydration gas; and a transparent vitrification step of heating the glass fine particle deposit by the heater after the dehydration step to obtain a transparent glass preform.SELECTED DRAWING: Figure 2

Description

The present disclosure relates to a method for manufacturing an optical fiber preform and a method for manufacturing an optical fiber.

Patent Literature 1 discloses a method of depositing glass particles on a seed rod to produce a glass particle deposit using the VAD method (Vapor Phase Axial Deposition). The thus-obtained glass fine particle deposited body, with a seed rod attached to one end thereof, is subjected to a dehydration process and a transparent vitrification process by sintering to form a transparent glass base material. An optical fiber is manufactured by drawing the obtained transparent glass base material.

JP 2017-178630 A

When a transparent glass base material is produced from the glass particle deposit obtained by the method disclosed in Patent Document 1, the transparent glass base material is produced even though the OH groups are removed from the glass particle deposit in the dehydration step. The concentration of OH groups was high in the portion near the seed rod in . If an optical fiber is manufactured using such a portion with a high concentration of OH groups, the transmission loss will increase. Therefore, conventionally, the portion of the transparent glass base material near the seed rod was discarded and was not used for the production of optical fibers. Discarding part of the transparent glass preform has resulted in an increase in the manufacturing cost of the optical fiber.

An object of the present disclosure is to reduce the concentration of OH groups in the portion near the seed rod in the transparent glass base material, and even when using the portion near the seed rod in the transparent glass base material, the transmission loss derived from the OH group is small. It is to enable the production of optical fibers.

A method for manufacturing an optical fiber preform according to an aspect of the present disclosure includes:
a seed rod manufacturing step of manufacturing a seed rod having an average OH group concentration of 1 ppm or less in a region at a distance of 0.05 mm or more and 0.5 mm or less from the surface at the tip;
a glass particle deposit manufacturing step of depositing glass particles generated by supplying a glass raw material to a burner on the rotating seed rod to manufacture a glass particle deposit;
a dehydration step of dehydrating the glass particulate deposit by heating it with a heater in dehydration gas;
a transparent vitrification step of obtaining a transparent glass base material by heating the glass particle deposit with the heater after the dehydration step;
Prepare.

A method for manufacturing an optical fiber according to an aspect of the present disclosure includes:
An area within 10 mm in the axial direction from the tip of the seed rod of the optical fiber preform obtained by the method for producing an optical fiber preform is used, and the transmission loss at a wavelength of 1380 nm is Producing an optical fiber having an increase of less than 40 mdB/km relative to an optical fiber produced using a portion axially located 100 mm from the seed rod tip.

According to the configuration disclosed above, it is possible to manufacture an optical fiber with little transmission loss due to OH groups even when using a portion of the transparent glass base material near the seed rod, which has conventionally been discarded. As a result, the manufacturing cost of the optical fiber can be reduced.

FIG. 1 is a schematic diagram showing diffusion of OH groups from a seed stick in a transparent vitrification process. FIG. 2 is a schematic diagram showing a method of manufacturing an optical fiber preform according to an embodiment of the present disclosure. FIG. 3 is a graph showing an example of OH group concentration at the tip of a seed rod according to the present disclosure. FIG. 4 is a graph showing how OH groups diffuse when a seed rod according to the present disclosure is heated. FIG. 5 is an example of an infrared absorption spectrum of a seed stick. FIG. 6 is a graph showing the transmission loss of the optical fiber manufactured from the optical fiber preform manufactured in each manufacturing example. FIG. 7 is a graph showing the relationship between the length of the tip of the seed rod and the rate of increase in OH loss.

The present inventors conducted various investigations to find out the cause of the problem that the concentration of OH groups increases in the portion near the seed rod in the transparent glass base material. First, the present inventors focused on the dehydration process and conducted experiments by changing conditions such as the temperature and time in the dehydration process and the concentration of chlorine gas introduced during dehydration. did not resolve. In addition, the possibility of outside air entering from the outside of the core tube during the transparent vitrification process was investigated, but it was found that this was not related to the above problem.

As a result of various investigations as described above, the present inventors have found that the above problem is caused by the release of OH groups from the heated seed rod. FIG. 1 is a schematic diagram showing diffusion of OH groups from a seed rod 10' in the transparent vitrification process. In the transparent vitrification step, a glass particle deposit M1 with a seed rod 10' attached to one end is placed in a furnace core tube 31 and heated by a heater 32 to obtain a transparent glass preform. At this time, the OH groups contained in the seed rod 10' are diffused to the outside by the heating, and are diffused to the core portion M11 and the clad portion M12 of the glass fine particle deposit M1. Then, transmission loss occurs due to the OH groups that finally remain in the core portion M11. The present disclosure is made based on such findings.

[Description of Embodiments of the Present Disclosure]
Embodiments of the present disclosure are listed and described.
A method for manufacturing an optical fiber preform according to an aspect of the present disclosure includes:
a seed rod manufacturing step of manufacturing a seed rod having an average OH group concentration of 1 ppm or less in a region at a distance of 0.05 mm or more and 0.5 mm or less from the surface at the tip;
a glass particle deposit manufacturing step of depositing glass particles generated by supplying a glass raw material to a burner on the rotating seed rod to manufacture a glass particle deposit;
a dehydration step of dehydrating the glass particulate deposit by heating it with a heater in dehydration gas;
a transparent vitrification step of obtaining a transparent glass base material by heating the glass particle deposit with the heater after the dehydration step;
Prepare.
According to this configuration, the concentration of OH groups in the portion near the seed rod in the transparent glass base material is reduced, and even when the portion near the seed rod in the transparent glass base material is used, the transmission loss due to the OH groups is small. It becomes possible to manufacture optical fibers. As a result, the manufacturing cost of the optical fiber can be reduced.

In the method for manufacturing the optical fiber preform,
It is preferable that the seed rod manufacturing step is a step of manufacturing seed rods having an average OH group concentration of 3 ppm or less in a region of the tip portion having a distance of 0.05 mm or more and 2.0 mm or less from the surface.
According to this configuration, it is possible to further reduce the concentration of OH groups in the portion near the seed rod in the transparent glass base material. As a result, even when an optical fiber is manufactured using a portion of the transparent glass preform close to the seed rod, it is possible to stably manufacture an optical fiber with little transmission loss due to OH groups.

In the method for manufacturing the optical fiber preform,
It is preferable that the length of the tip portion is at least half the length of the heater.
According to this configuration, it is possible to further reduce the concentration of OH groups in the portion near the seed rod in the transparent glass base material. As a result, even when an optical fiber is manufactured using a portion of the transparent glass preform close to the seed rod, it is possible to stably manufacture an optical fiber with little transmission loss due to OH groups.

A method for manufacturing an optical fiber according to an aspect of the present disclosure includes:
An area within 10 mm in the axial direction from the tip of the seed rod of the optical fiber preform obtained by the method for producing an optical fiber preform is used, and the transmission loss at a wavelength of 1380 nm is Producing an optical fiber having an increase of less than 40 mdB/km relative to an optical fiber produced using a portion axially located 100 mm from the seed rod tip.
According to this configuration, it is possible to manufacture an optical fiber with little transmission loss due to OH groups even when using a portion of the transparent glass base material near the seed rod, which has conventionally been discarded. As a result, the manufacturing cost of the optical fiber can be reduced.

[Details of the embodiment of the present disclosure]
Hereinafter, examples of embodiments according to the present disclosure will be described with reference to the drawings. In the following description, the same or corresponding elements are denoted by the same reference numerals even in different drawings, and overlapping descriptions are omitted as appropriate. Also, in each drawing used for the following explanation, the scale is appropriately changed so that each member has a recognizable size.

(Manufacturing method of preform for optical fiber)
FIG. 2 is a schematic diagram showing a method for manufacturing an optical fiber preform (hereinafter also referred to as a “transparent glass preform”) M2 according to an embodiment of the present disclosure. As shown in FIG. 2, the manufacturing method of the optical fiber preform M2 according to the present embodiment includes steps 1 to 4. As shown in FIG.

Step 1 is a seed rod manufacturing step for manufacturing seed rods 10 . The seed rod 10 satisfies Condition 1 described later regarding the average OH group concentration at least at the tip portion 11 (the portion between the tip P2 and the position P1). The axial length a of the tip portion 11 is preferably, for example, half or more of the axial length b of the heater 32 . This is because it is preferable that the tip P2 of the seed rod 10 descends to the vicinity of the center of the heater 32 in step 4 (transparent vitrification step) described later. In this case, the portion from the tip P2 to the position b/2 in the axial direction enters the heat zone of the heater 32 in the transparent vitrification step. Since the portion that has entered the heat zone is heated more strongly than the other portion of the seed rod 10, the OH groups are likely to diffuse into the transparent glass base material M2. Such diffusion can be prevented by being half or more of b.

FIG. 3 is a graph showing an example of the OH group concentration at the tip portion 11 of the seed rod 10. As shown in FIG. The seed rod 10 satisfies Condition 1 that the average OH group concentration in the region R1 at a distance from the surface of 0.05 mm or more and 0.5 mm or less is 1 ppm or less. The OH groups that have conventionally caused transmission loss are mainly due to the OH groups present in the region R1. Therefore, by setting the average OH group concentration of the region R1 to 1 ppm or less, it is possible to suppress the OH groups derived from the seed rod 10 from diffusing and remaining in the transparent glass base material M2. In addition to condition 1, seed rod 10 preferably satisfies condition 2 that the average OH group concentration in region R2 at a distance from the surface of 0.05 mm or more and 2.0 mm or less is 3 ppm or less.

In addition, the region R3 at a distance of 0 mm or more and less than 0.05 mm from the surface has no condition regarding the average OH group concentration, and the average OH group concentration may reach several thousand ppm, for example. This is because the OH groups contained in the region R3 diffuse from the seed rod 10 to the glass particle deposit M1 and are further removed from the glass particle deposit M1 in step 2 (dehydration step).

FIG. 4 is a graph showing how OH groups diffuse when the seed rod 10 is heated, and is a theoretical graph calculated from a diffusion equation. As shown in FIG. 4, the OH groups existing in the region where the distance from the surface exceeds 0.5 mm remain inside the seed rod 10 even after heating for 4 hours, and there is a risk of diffusing to the outside of the seed rod 10. Few. From this, it can be seen that the average OH group concentration in the region R1 at a distance of 0.05 mm or more and 0.5 mm or less from the surface is the most important.

The average OH group concentration of the seed rod 10 can be measured using a conventionally known method. For example, by observing the OH stretching vibration of Si—OH in the infrared absorption spectrum, the average OH group concentration of the seed rod 10 can be calculated.

FIG. 5 is an example of an infrared absorption spectrum of a seed stick. The average OH group concentration of the seed _rod is calculated by the ^Lambert - can be calculated by Beer's formula.

C _OH =α/εt (1)

In the above formula (1), the absorbance α is determined by "α=Log10 (I0/I)". The molar extinction coefficient ε is the value “ε = 77.5 L / mol-cm" can be used. The sample thickness t can be measured with a vernier caliper or the like.

The OH group concentration distribution in the radial direction of the seed rod 10 can be calculated, for example, by the following method. First, the transmittance in the radial direction of the seed rod 10 is measured by the above formula (1), and the average OH group concentration in the radial direction of the seed rod 10 is obtained. Next, the surface of the seed rod 10 is etched by immersing the seed rod 10 in a hydrofluoric acid solution or the like. After that, the average OH group concentration of the seed rod 10 is determined again by the above formula (1). From the outer diameter of the seed rod 10 before and after etching and the average OH group concentration, the OH group concentration of the etched portion is calculated by the following formula (2). By sequentially repeating etching and measurement in the same procedure, the OH group concentration distribution in the radial direction of the seed rod 10 can be obtained.

(DC-D'C')/(D-D') (2)

In addition, in the above formula (2), "D" is the outer diameter of the seed rod 10 before etching. "D'" is the outer diameter of the seed rod 10 after etching. "C" is the average OH group concentration of the seed rod 10 before etching. "C'" is the average OH group concentration of the seed rod 10 after etching.

The seed rod 10 is manufactured by heat-treating a glass rod of fused silica glass manufactured by the electric melting method or synthetic silica glass manufactured by the soot method in a dry atmosphere to reduce the average OH group concentration in the region R1 to 1 ppm or less. can. As the seed rod 10, quartz glass produced by the flame fusion method or the direct method can also be used, but since the OH group concentration contained in the glass is high, it takes time to reduce the average OH concentration. Synthetic quartz glass sintered in vacuum in the soot method is preferable because it is easy to obtain a low average OH concentration in the region R1. Furthermore, the synthetic quartz glass sintered after dehydration treatment in the soot method is preferable because it provides a low average OH group concentration not only in the region R1 but also in the vicinity of the center (for example, the region R2).

The seed rod manufacturing process may include a step of flame polishing the surface of the seed rod 10 . By performing flame polishing, the surface of the seed rod 10 is cleaned and smoothed, and it is possible to prevent the interface with the deposited glass particles from crystallizing and remaining air bubbles. Since crystallization and residual air bubbles cause cracking, it is preferable to prevent these occurrences.

Since the flame polishing involves applying a burner flame to the surface of the seed rod 10, the OH group concentration on the surface of the seed rod 10 is increased by the flame polishing. However, this increase in OH group concentration is within a range of 0.05 mm from the surface of seed rod 10 . As described above, within this range, it is released from the surface of the seed rod 10 by heating during the dehydration process, and the OH group concentration is reduced during the transparent vitrification process, so there is no effect on the transmission loss of the optical fiber. .

Hereinafter, referring back to FIG. 2, steps 2 to 4 will be described. Step 2 is a step of manufacturing a glass particle deposit body M1. In the glass particle deposit manufacturing process, for example, the VAD method is used to manufacture the glass particle deposit M1. Specifically, the glass raw material gas G1 is supplied to the core burner 21, and the core glass microparticles produced by the flame hydrolysis reaction are deposited on the rotating seed rod 10. As shown in FIG. Further, the glass raw material gas G2 is supplied to the clad burner 22 to deposit the clad core glass microparticles generated by the flame hydrolysis reaction on the core glass microparticles. Thus, the glass fine particle deposit M1 is manufactured.

Further, the glass fine particle deposit M1 may be manufactured using an OVD (Outside vapor deposition) method. In the OVD method, a mandrel is inserted into a pipe-shaped seed rod, glass microparticles are deposited on the outside of the seed rod and the mandrel, and after the deposition of the glass microparticles is completed, the mandrel is pulled out, and the seed rod is attached to one end. , a transparent glass preform is manufactured through a dehydration step and a transparent vitrification step by sintering. Therefore, in the case of using the OVD method, the average OH group concentration in the region at a distance of 0.05 mm or more and 0.5 mm or less from the surface is set to 1 ppm or less on both the inner surface and the outer peripheral surface of the pipe-shaped seed rod.

The glass raw material gas G1 is, for example, a core gas containing dopants such as silicon tetrachloride (SiCl ₄ ) or germanium tetrachloride (GeCl ₄ ) in addition to siloxane. Further, the glass raw material gas G2 is, for example, a clad gas containing silicon tetrachloride or siloxane and containing no dopant. In the step of manufacturing the glass fine particle deposit, a conventionally known configuration can be appropriately employed.

Step 3 is a dehydration step for dehydrating the glass particulate deposited body M1. In the dehydration step, the glass particle deposit M1 is put into the furnace core tube 31 . Then, a dehydration gas G3 (for example, a mixed gas of an inert gas such as nitrogen and chlorine gas) is supplied to the furnace core tube 31 to lower the glass particle deposit M1 within the furnace core tube 31 . The glass particle deposit M1 is heated by the heater 32 arranged around the furnace core tube 31 in an atmosphere containing the dehydration gas G3 to be dehydrated. The dehydration treatment removes OH groups contained in the glass fine particle deposit M1 and the like. Conditions such as temperature and time in the dehydration step can be appropriately set based on conventionally known conditions.

Step 4 is a transparent vitrification step in which the glass particle deposit M1 is heated by the heater 32 to obtain a transparent glass base material M2. In the transparent vitrification step, the glass particle deposits M1 that have moved below the furnace core tube 31 in the dehydration step are raised to the upper part of the furnace core tube 31 . Then, an inert gas G4 (for example, nitrogen, helium, etc.) is supplied to the furnace core tube 31, and the glass particulate deposited body M1 is lowered within the furnace core tube 31. As shown in FIG. The glass fine particle deposit M1 is heated by the heater 32 in an atmosphere containing the inert gas G4 to be sintered to become the transparent glass base material M2. In the transparent vitrification step, it is preferable to lower the tip P2 of the seed rod 10 to near the center of the heater 32 . Conditions such as temperature and time in the transparent vitrification step can be appropriately set based on conventionally known conditions.

The transparent glass base material M2 manufactured through the above steps 1 to 4 has a lower OH group concentration in the region M21 near the seed rod 10 than when manufactured by the conventional manufacturing method. Therefore, the area M21, which has conventionally been discarded, can also be used as the preform for the optical fiber. Note that the area M21 is an area between the positions P5 and P6. Position P5 is a position corresponding to tip P2 of seed rod 10 . The position P6 is, for example, a position at a distance of 100 mm downward in the axial direction from the position P5.

(Method for manufacturing optical fiber)
A method for manufacturing an optical fiber according to this embodiment will be described below. The method for manufacturing an optical fiber according to the present embodiment uses an area within 10 mm in the axial direction from the tip P2 of the seed rod 10 of the optical fiber preform M2 obtained by the above-described method for manufacturing an optical fiber preform. , The increase in transmission loss at a wavelength of 1380 nm is less than 40 mdB/km based on the optical fiber manufactured using the portion of the optical fiber preform M2 located 100 mm in the axial direction from the tip P2 of the seed rod 10 It is a manufacturing method for manufacturing an optical fiber. Note that an increase of less than 40 mdB/km from the reference includes cases where the transmission loss is reduced from the reference.

The optical fiber preform M2 becomes an optical fiber through a conventionally known process for manufacturing an optical fiber from the transparent glass preform M2 such as a drawing process. Since the region M21 of the optical fiber preform M2 obtained by the above-described optical fiber preform manufacturing method has a low average OH group concentration, the region within 10 mm in the axial direction from the tip P2 of the seed rod 10 is used. The transmission loss at a wavelength of 1380 nm (hereinafter also referred to as transmission loss T10) of the optical fiber obtained by using the portion located 100 mm in the axial direction from the tip P2 of the seed rod ₁₀ The increase is less than 40 _mdB /km compared to the loss (hereinafter also referred to as transmission loss T100). That is, “T ₁₀ −T ₁₀₀ <40 mdB/km”. In the method for manufacturing an optical fiber according to the present embodiment, an optical fiber is manufactured using a region within 10 to 100 mm in the axial direction from the tip P2 of the seed rod 10 or a region below the region M21 shown in FIG. including.

[Example]
Hereinafter, the present disclosure will be described in more detail by showing Examples according to the present disclosure. It should be noted that the present disclosure is not limited to the following examples.

(Production Examples 1 to 5)
Synthetic quartz glass produced by the soot method was heat-treated in a dry atmosphere to produce three low-OH seed rods. Also, synthetic quartz glass produced by the soot method was heat-treated in a dry atmosphere to dehydrate to produce three low-OH species rods, one of which was confirmed to satisfy the above conditions 1 and 2. In addition, two ordinary seed rods, which are commercially available fused quartz glass manufactured by an electric melting method, were prepared. From one of them, it was confirmed that the average OH group concentration was 130 ppm in the region of the tip of the normal seed rod at a distance of 0.05 mm or more and 0.5 mm or less from the surface.

For each of the four low-OH seed rods and one normal seed rod, the above-described glass fine particle deposition body manufacturing process, dehydration process, and transparent vitrification process were performed under the same conditions to manufacture five transparent glass base materials M2. . Next, all the obtained transparent glass preforms M2 were drawn under the same conditions to manufacture optical fibers. The transmission loss at 1380 nm was measured for each optical fiber obtained. The results are shown in FIG. Production Examples 1 and 2 are production examples using low OH species rods that satisfy Condition 1 but do not satisfy Condition 2. Production Examples 3 and 4 are production examples using low OH species rods that satisfy Conditions 1 and 2. Production Example 5 is an example of production using a normal seed stick.

FIG. 6 is a graph showing the transmission loss of the optical fiber manufactured from the optical fiber preform M2 manufactured in each manufacturing example. Each plotted point in the graph of FIG. 6 has a corresponding relationship with the transparent glass base material M2 shown at the top of the graph. For example, the transmission loss of the optical fiber manufactured using the position P6 as the base material is the value on the line indicating the position P6.

As can be seen from FIG. 6, when a normal seed rod that does not satisfy Condition 1 is used, the optical fiber manufactured using the region M21 shows a significant increase in transmission loss. On the other hand, when a low OH seed rod that satisfies Condition 1 is used, no significant increase in transmission loss is observed even in the optical fiber manufactured using the region M21. In particular, Production Examples 3 and 4 using low OH seed rods satisfying Conditions 1 and 2 show low values of transmission loss. Specifically, the values of T ₁₀ −T ₁₀₀ are 3 mdB/km for Production Example 1, 3 mdB/km for Production Example 2, 3 mdB/km for Production Example 3, and 3 mdB/km for Production Example 4. 5 is 46 mdB/km. Further, in Manufacturing Examples 1 to 4, the transmission loss of the optical fiber manufactured using the region M21 is 0.31 dB/km or less.

(Production Examples 6 to 10)
The same is true except that the length a of the tip 11 of the seed rod 10 is changed to 0.1, 0.2, 0.5, 0.8, and 1 times the length b of the heater 32, respectively. A transparent glass base material M2 was manufactured under the conditions of . In addition, in the transparent vitrification step, the tip P2 of the seed rod 10 was lowered to near the center of the heater 32 .

Next, all the obtained transparent glass preforms M2 were drawn under the same conditions to manufacture optical fibers. Whether or not an increase in OH loss (transmission loss due to OH groups) occurred was determined for an optical fiber manufactured using a portion of each transparent glass preform M2 located 10 mm below the position P5 in the axial direction. The increase in OH loss is obtained when the transmission loss at 1380 nm increases by 40 mdB/km or more based on the optical fiber manufactured using the portion located 100 mm below the position P5 in the axial direction. I judged yes.

The production of the transparent glass base material M2 under each of the above conditions regarding the length a of the tip portion 11 was performed multiple times, and the rate of increase in OH loss under each condition (of the transparent glass base material M2 manufactured under the same conditions, The number of occurrences of increased OH loss/the number of transparent glass preforms M2 manufactured under the same conditions) was calculated. The results are shown in FIG.

As can be seen from FIG. 7, the occurrence rate of OH loss increase can be reduced by setting the length a of the tip portion 11 to be 0.5 times or more the length b of the heater 32 .

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. Further, the number, positions, shapes, etc., of the constituent members described above are not limited to those of the above-described embodiment, and can be changed to suitable numbers, positions, shapes, etc. in carrying out the present invention.

10', 10: seed rod 11: tip 21: core burner 22: cladding burner 31: core tube 32: heater

G1, G2: glass raw material gas G3: dehydration gas G4: inert gas M1: glass fine particle deposit M11: core part M12: clad part M2: transparent glass base material (optical fiber base material)
M21: area R1, R2, R3: area

Claims (4)

a seed rod manufacturing step of manufacturing a seed rod having an average OH group concentration of 1 ppm or less in a region at a distance of 0.05 mm or more and 0.5 mm or less from the surface at the tip;
a glass particle deposit manufacturing step of depositing glass particles generated by supplying a glass raw material to a burner on the rotating seed rod to manufacture a glass particle deposit;
a dehydration step of dehydrating the glass particulate deposit by heating it with a heater in dehydration gas;
a transparent vitrification step of obtaining a transparent glass base material by heating the glass particle deposit with the heater after the dehydration step;
A method for manufacturing an optical fiber preform, comprising: 2. The method according to claim 1, wherein the seed rod manufacturing step is a step of manufacturing a seed rod having an average OH group concentration of 3 ppm or less in a region at a distance of 0.05 mm or more and 2.0 mm or less from the surface in the tip portion. A method for producing the optical fiber preform described above. 3. The method of manufacturing an optical fiber preform according to claim 1, wherein the length of said tip portion is at least half the length of said heater. Using an area within 10 mm in the axial direction from the tip of the seed rod of the optical fiber preform obtained by the method for producing an optical fiber preform according to any one of claims 1 to 3,
The light whose transmission loss at a wavelength of 1380 nm increases by less than 40 mdB/km based on an optical fiber manufactured using a portion of the optical fiber preform located 100 mm in the axial direction from the tip of the seed rod. A method of making an optical fiber comprising manufacturing a fiber.

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