JPS61178434A - Production of base material for optical fiber - Google Patents

Abstract

PURPOSE:To eliminate the occurrence of cracking of a base material in and after production thereof and improve the yield, by removing a dummy rod at a specific time of a collapsing step in jacketing a glass pipe around the outer periphery of a glass rod of core. CONSTITUTION:A dummy rod 12 at one end of a glass rod 11 is mounted on a chuck of a glass lathe, and a glass pipe 13 at the other end is mounted and supported on the glass lathe respectively. The left and right ends of the pipe 13 are set respectively at the starting and completing ends of the heat shrinkage. The rod 11 and pipe 13 are synchronously rotated in the direction of (R), and the pipe 13 is heated by a heater 15 moving in the direction (X). The interior of the pipe 13 is decompressed by a vacuum suction means to start the heat shrinkage of the pipe 13 at the same time to heat fuse the pipe 13 to the rods 12 and 11 one after another. When the heat fusion region goes over the boundary part of the rods 11 and 12, the rod 12 is cut by fusion and removed from the mutual heat fusion part of the rods 11 and pipe 13. A supporting rod 16 is then heat fused to the removed end part to support the end part. The rod 11 and pipe 13 are then integrated by movement of the heater 15 to cut the right end part of the pipe 13 by fusion, and the aimed base material 17 is obtained by removing strain.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing an optical fiber preform by jacketing a glass pipe around the outer periphery of a core glass rod.

``Prior Art'' As a means of manufacturing a large-sized optical fiber preform, a glass rod with a large diameter (e.g., 25 mm in diameter and 100,100 mm in length) is combined with a glass rod with a large diameter and thick wall (e.g., with an outer diameter of 50 mm).
The method used is to jacket and integrate a glass pipe with a thickness of 14 m + * and a length of 100100 O.

FIGS. 2(a) to 2(d) show the main steps in the above conventional example, and these will be explained below.

The core glass rod l shown in FIG. 2(a) has a dummy rod 2 at one end thereof.

Generally, the glass rod 1 for the core is made of doped quartz, but the dummy rod 2 is made of normal quartz.

Therefore, the core glass rod 1 and the dummy rod 2 have different coefficients of thermal expansion.

The outer periphery of the core glass rod 1 is covered with a jacket glass pipe 3 made of pure quartz or doped quartz, as shown in FIG. 2(b).
The glass pipe 3 is set into a glass lathe. At this time, the glass rod 1 is supported by attaching the dummy rod 2 at one end to the chuck 4a of the glass lathe, and the other end of the glass pipe 3 is attached to the chuck 4b of the lathe. It is supported by attaching it to.

Next, as shown in the same figure (b), the glass rods 1. The glass pipes 3 are synchronously rotated in the direction of the arrow R, heated by a heater 5 such as an oxyhydrogen flame burner moving in the direction of the arrow x1, and depressurized by a vacuum suction means.

Through this collapse process, the glass pipe 3 is thermally shrunk, and the glass rod 1 and the glass pipe 3 are integrated by thermal fusion.

When the glass rod 1 and the glass pipe 3 are fused and integrated in this way, a residual strain is generated in the base material 8 produced by the integration due to the difference in the thermal expansion coefficients of the glass rod 1 and the glass pipe 3.

Therefore, during the collapse process, the base material B is heated by a separate heater running after the heater 5, or after the collapse process, as shown in FIG.
Heating of the strain relief is performed by appropriate means such as reheating with the heater 5 that reciprocates in two directions,
After the heating, the base material B is naturally cooled.

After the strain relief process, the dummy rod 2 is cut off from one end of the base material 6 by fusing, as shown in FIG. is obtained.

"Problems to be solved by the invention" In the case of the above-mentioned example, the collapse process, distortion removal process,
Cracks often occur in the base material during subsequent processes, and cracks may also occur one day to one month after the base material is manufactured. ■The incidence of cracking is high in larger base materials.

One of the causes of these cracks is that the core glass rod 1 and the dummy rod 2 have different coefficients of thermal expansion.

That is, while the glass rod 1 contains a dopant for increasing the refractive index, the dummy rod 2 does not contain this, and the difference in thermal expansion coefficient between the two is large. A large thermal strain occurs near the boundary of
Therefore, the above-mentioned cracks occur during the collapse process and the like.

This can be understood from the fact that the locations where cracks occur during the collapse process etc. are concentrated on one end side of the base material 6 (dummy load 2 side).

Another reason why cracks occur is due to insufficient strain relief.

That is, when the base material B is heated for a predetermined period of time (approximately 1 hour) in the strain relief step, and then the base material 8 is naturally cooled, the base material 6 cools and hardens before the strain is sufficiently removed. Therefore, cracks occur in the optical fiber preform 6 during stocking after the preform is manufactured.

In view of the above-mentioned problems, the present invention aims to provide a method for manufacturing an optical fiber preform that does not cause cracking of the preform during or after the preform is produced, that is, has a high yield. .

The present invention involves fitting together a glass rod for a core having a dummy rod at one end and a glass pipe for a jacket, and shrinking the glass pipe while heating it. By: In a method of manufacturing an optical fiber base material in which a glass rod and a glass pipe are heat-sealed to each other, after the glass rod and glass pipe are fitted together, one longitudinal end where the dummy rod is located is Heat shrinkage of the glass pipe is started with the heat shrinkage start end and the opposite end as the heat shrinkage end end, and the heat fused area between the glass rod and the glass pipe due to the heat shrinkage crosses the boundary between the dummy rod and the glass rod. When.

The dummy rod is removed from the heat-sealed ends of these glass rods and glass pipes, and a support rod is welded to the removed end of the dummy rod, and then, while vacuum suction is applied, the welded part is moved from the above-mentioned heat-shrinkable end. After heat-shrinking the glass pipe over a period of time, thereby completing the heat-sealing of the glass rod and the glass pipe, remove the heat-sealed end of the glass rod and the glass pipe at the end of the heat shrinkage, and then, It is characterized in that the base material made of these glass rods and glass pipes is placed in a heated atmosphere below the softening temperature of the base material and slowly cooled, thereby removing the distortion of the base material.

r Effect 1 The method of the present invention includes a collapse process when jacketing a glass pipe around the outer periphery of a glass rod, a process of removing both ends of the base material, a process of removing distortion of the base metal, etc. During the collapse process, which is the start point of heat shrinkage, when the heat-sealed area between the glass rod and glass pipe crosses the boundary between the dummy rod and the glass rod, the heat-sealed end of the glass rod and glass pipe crosses the boundary between the dummy rod and the glass rod. Since the dummy rod is removed from the end, the cause of cracking due to the difference in thermal expansion coefficient between the glass rod and the dummy rod is eliminated, and therefore the support rod is welded to the dummy rod removed end, and the welded part is heat-shrinked to the end. The subsequent steps, such as heat shrinking the glass pipe over the entire length of the pipe, removing the glass rod at the end of the heat shrinkage, and the heat-sealed ends of the glass pipes, can be carried out without any problems, and the yield of the base material can be improved by eliminating the causes of cracking mentioned above. will improve.

Furthermore, in the method of the present invention, instead of removing the distortion of the base material by heating the base material and then naturally cooling it, the base material is placed in a predetermined heating atmosphere and slowly cooled, so the cooling rate of the base material can be appropriately controlled. Strain relief can be performed without the residual strain seen in natural cooling.

Therefore, the problem that is the main cause of cracks in the base material is eliminated, and the yield of the base material is significantly improved.

In the strain relief process of the method of the present invention, the optical fiber base material is placed in an atmosphere below the softening temperature of the base material and is slowly cooled to remove the strain. There is no undesirable elongation of the base material that occurs when using it as a starting point, and there are no problems when removing distortion.

``Example J'' Specific examples of the method of the present invention will be described below with reference to the drawings.

In FIGS. 1A to 1G, 11 is a glass rod for the core, 12 is a dummy rod at one end of the core glass rod, and 13 is a glass pipe for the jacket.

Glass rod for core! is made of doped quartz containing a dopant for a high refractive index such as germanium, the dummy rod 12 is made of ordinary quartz, and the jacket glass pipe 12 is made of pure quartz or doped quartz containing a dopant for a low refractive index. .

First, the outer periphery of the glass rod 11 is covered with a glass pipe 13 for a jacket as shown in FIG. 1(B), and the glass rod 11 and the glass pipe 13 are set in a glass lathe.

That is, as in the conventional example, the glass rod 11 is supported by attaching the dummy rod 12 at one end to the chuck 14a of the glass lathe, and the glass pipe 13 is supported by attaching the other end to the chuck 14b of the lathe. Ru.

The glass pipe 13 in the set state is set so that the left end (on the side of the dummy rod 12) in the figure is the start end of heat shrinkage during the collapse process, and the opposite end is the end end of the heat shrinkage during the collapse process.

Next, as shown in FIG. 2(c), the glass rod 11 and the glass pipe 13 that are fitted to each other are synchronously rotated in the direction of the arrow R, and the glass pipe 13 is moved in the direction of the arrow X in a heater such as an oxyhydrogen flame burner. 15, and the pressure is reduced by a vacuum suction means.

The glass pipe 13 starts to shrink due to the collapse process, and the glass pipe 13 and the dummy rod, and the glass pipe 13 and the glass rod 11 are sequentially heat-sealed, but these heat-sealed regions are the glass rod 11. When the boundary of the dummy rod 12 is crossed, the glass rod 11. The dummy rod 12 is removed from the heat-sealed ends of the glass pipes 13.

At this time, the dummy rod 12 is torn off by heating and softening the heat-sealed end portion and applying a tensile force in the axial direction to the dummy rod 12 and the glass pipe 13.

Thereafter, as shown in FIG. 1(e), a support rod 1B made of a quartz rod is welded by heat welding to the removed end of the dummy rod, and the support rod 16 is supported by the chuck 14a.

After these steps are completed, a full-scale collapse process is performed from the welded part of the support rod 1B to the other end of the glass pipe 13. At this time, the heater 15 is moved to the position shown in FIG. The glass rod 11 and the glass pipe 13 are fused and integrated by a collapse process using the heater 15.

After the process shown in FIG. 1(e), the other ends of the glass rod 11 and the glass pipe 13 (the right end in the figure) which are fused and integrated as shown in FIG. 1(f) are removed by the same tearing means as described above. ,
In this way, the optical fiber preform 17 before strain relief is obtained.

Thereafter, as shown in FIG. 1(g), the optical fiber preform 17 is placed in a predetermined heating atmosphere using an electric furnace 1B, and the preform 17 is strained.

In the strain relief process shown in FIG. 1(d), the heating atmosphere by the electric furnace 1B is set to a temperature lower than the softening temperature of the optical fiber preform 17, and the optical fiber preform 17 is placed in the heated atmosphere, and further The base material 1 is heated at a slow temperature drop rate.
7. Cool slowly.

At this time, the heating atmosphere temperature is determined by the material of the optical fiber base material 17 (
The annealing rate is also set depending on the size (size) of the optical fiber preform 17. For example, the outer diameter (diameter) is 40 mm or more.

When creating a large base material with a diameter of 50 m layers and a length of 1000 mm using jacket glass pipes with a wall thickness of 9 mm or more, and removing the distortion of the base material, the temperature is approximately 1000°C.
It takes about 2 hours for the base material to reach room temperature due to natural cooling.

Therefore, when placing a large base material as described above in a heated atmosphere to remove its distortion, it is recommended to set the slow cooling rate of the base material to less than about 500" O/hr, which is slower than natural cooling. good.

In order to confirm the effectiveness of the method of the present invention, the following various experiments were conducted.

When producing the optical fiber preform in each experimental example, the glass rod for the core had a diameter of 25 mm and a length of 900 mm.
, Δ-1% Gl type core base materials (with quartz dummy rod) were used, and natural quartz tubes with an outer diameter (diameter) of 50 mm, a thickness of 12 m, and a set length of 1000 m were used as jacket glass pipes.

Experimental Example 1 When producing an optical fiber preform by the method shown in Fig. 2, two oxyhydrogen flame burners (total hydrogen flow rate 1931/i
When the collapse process was carried out using a total oxygen flow rate of n% (89/■ in) and a feed rate of approximately 10 m/cm for each heater, the collapse progressed to a point approximately 2,001 mm long from the end of the dummy rod. , cracks occurred in the base metal at the end of the dummy rod.

Experimental example 2 Collapse conditions using two oxyhydrogen flame burners are experimental example 1
However, the method for preparing the base material is shown in Figure 1 (a) ~
The process in (f) was adopted.

When the optical fiber preform thus obtained was left to naturally stand indoors, cracks occurred in the preform after about 12 hours.

Experimental Example 3 An optical fiber preform was prepared in the same manner as in Experimental Example 2, and then two heaters (
oxyhydrogen flame burner) at a total hydrogen flow rate of 20% at 10 minute intervals.
The heating power was gradually weakened while throttling so that fL/sin decreased and the total oxygen flow rate decreased by 5 layers, thereby removing the strain in the base metal.

When the base material that had been strain-removed was left to naturally stand indoors, cracks occurred in the base material after about 15 hours.

Experimental Example 4 As in Experimental Example 2, an optical fiber base material was made through the steps shown in Figure 1 (A) to (F), and the base material was heated to a hydrogen flow rate of 7541/si.
n, oxygen flow rate 30/win, feed speed 100mm/++
Flame polishing was performed using a heater set to +in, and after the flame polishing, 20
Within minutes, the base material was placed in an electric furnace (set temperature: 1570°C) as shown in Figure 1 (G), and the temperature drop rate was 150°C.
Strain of the base material was removed by slow cooling at ℃/hr.

The optical fiber preform thus obtained has a diameter of 50111 mm.
1. The length was 850 mm, and when the base material was left to naturally stand indoors, no cracking occurred in the base material even after about 3 months.

As is clear from the above-mentioned experimental results, in Experimental Example 4, which corresponds to the method of the present invention ( Right-handedness can be seen.

r Effects of the invention j As explained above, when using the method of the present invention, the cause of base material cracking is due to the different coefficients of thermal expansion of the core glass rod and the dummy rod, the cause of base material cracking due to residual strain in the base material, etc. Since the optical fiber preform is manufactured by eliminating both of these problems, the yield during and after manufacturing the preform is significantly improved.

[Brief explanation of drawings]

FIGS. 1(a) to (g) are process explanatory diagrams schematically showing one embodiment of the method of the present invention, and FIGS. 2(a) to (d) are process explanatory diagrams schematically showing a conventional method. 11...Glass rod for core 12φ...Dummy rod 13...Glass pipe for jacket 15...Heater (for glass pipe heat contraction) 1B fist...
Support rod 17...Optical fiber base material 18...Electric furnace (for heating atmosphere for strain relief) agent
Patent Attorney Yoshio Saito Figure 1

Claims (5)

[Claims] (1) A core glass rod with a dummy rod at one end and a jacket glass pipe are fitted together,
In a method for manufacturing an optical fiber base material in which the glass rod and the glass pipe are thermally fused to each other by shrinking the glass pipe while heating, the dummy Heat shrinkage of the glass pipe is started with one longitudinal end where the rod is located as the heat shrinkage start end and the opposite end as the heat shrinkage end end, and the heat fused area between the glass rod and glass pipe due to the heat shrinkage becomes a dummy rod. When the boundary between the glass rod and the glass rod is crossed, the dummy rod is removed from the heat-sealed ends of the glass rod and the glass pipe, and a support rod is welded to the end of the dummy rod removed, and then vacuum suction is applied. However, the glass pipe is heat-shrinked from the welded part to the heat-shrinkable end, and after the glass rod and glass pipe are thermally fused together, the glass rod and glass pipe are bonded to each other at the heat-shrinkable end. The heat-sealed ends are removed, and then the base material consisting of these glass rods and glass pipes is placed in a heated atmosphere below the softening temperature of the base material and slowly cooled to remove the distortion of the base material. Characteristic method for manufacturing optical fiber preform. (2) The method for manufacturing an optical fiber preform according to claim 1, wherein the slow cooling rate is less than 500° C./hr. (3) A method for manufacturing an optical fiber preform according to claim 1, wherein the slow cooling is performed in an electric furnace. (4) The method for manufacturing an optical fiber preform according to claim 1, wherein the outer diameter of the glass pipe for the jacket is 40 mm or more. (5) The method for manufacturing an optical fiber preform according to claim 1, wherein the glass pipe for the jacket has a wall thickness of 9 mm or more.