JP2006327896A - Method for heat-treating optical fiber preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for heat-treating an optical fiber preform where a glass rod connected to the preform can be reutilized in its sintering or drawing step and thus a production cost can be reduced furthermore. <P>SOLUTION: When the optical fiber preform is connected to a glass rod 12 and hung in a heat-treating device with a hanging jig in its sintering or drawing step, the glass rod 12 whose outside diameter is D2 is inserted into a pipe 13 whose inside diameter is D1 and which constitutes the hanging jig. The clearance (D1-D2) between the pipe 13 and the glass rod 12 satisfies the equation: 1.0&le;[(D1-D2)/D2]&times;100&le;5.0. A flexible shock absorbing material 20 whose main raw material is carbon and whose carbon content is 97% or more is favorably inserted into at least a part of the clearance 19 between the pipe 13 and the glass rod 12. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to a heat treatment method for an optical fiber preform that further reduces production costs in the step of sintering the optical fiber porous preform and the step of stretching the optical fiber glass preform.

In recent years, with the decrease in demand, in the manufacturing process of optical fiber preforms, reduction of production cost has been demanded. Various methods can be considered to reduce the production cost, and one of them is to reduce the amount of quartz glass rods used.
More specifically, quartz glass rods are used for various purposes in the optical fiber preform manufacturing process. For example, seed rods used at the beginning of deposition in the VAD process, glass rods connected as dummy bars (ineffective portions) at both ends of the target material having a core in the OVD process, and gripping at both ends of the base material in the flame polishing process -There are glass rods connected as support parts.

The glass rods used for the above applications were previously disposed of each time. However, due to the recent demand for reduction in production costs, the glass rods used in the production costs can be reduced by repeatedly using them as much as possible. There is a need to reduce the rate.
However, at present, glass rods connected to both ends of a target material having a core used in the OVD process are difficult to use multiple times for the following reasons. That is, the glass rod provided with the recessed part or the convex part used in order to hang it to a sintering apparatus in the next sintering process is connected to one end of the target material used in the OVD process (Patent Publication 1). reference).

  For example, an apparatus as shown in FIG. 1 is used for sintering of a porous base material manufactured by the OVD process. The porous base material 1 is suspended in a furnace core tube 2 by a rotation mechanism 3 so as to be freely rotatable, and is heated and sintered by a heater 4. He gas or chlorine-based gas is supplied from the gas inlet 5 and exhausted from the exhaust port 6. The porous base material 1 is connected to a rotating shaft 8 via a hanging tool 7 that is a hanging jig. A support rod 10 is engaged with the screw rod 9, and the porous base material 1 is moved up and down by the elevating mechanism 11 through the support rod 10 and the rotating shaft 8 as the sintering progresses.

  FIG. 2 is an enlarged view of the hanging tool 7 of FIG. 1, (a) is a front view thereof, and (b) is a side view thereof. The glass rod 12 connected to the porous base material 1 is inserted into a quartz glass tube 13 having a slight margin, and is provided in the tube 13 in a cut recess 14 provided on one side of the glass rod 12. By inserting the pin 16 from the hole 15 into the gap between the inner wall surface of the tube 13 and the cut recess 14, the glass rod 12, that is, the porous base material 1 is suspended from the rotating shaft 8 via the hanging tool 7. . The pin 16 has a flat surface 17 on the side surface of the cylinder, and the flat surface 17 contacts the inclined surface of the cut recess 14.

When the porous preform 1 is suspended using such a hanging tool 7, it is possible to prevent the eccentricity of the core portion that causes connection loss in the optical fiber. However, when sintering is performed using such a hanging tool 7, a dent-like scratch of about 0.5 to 2.0 mm is generated on the surface of the glass rod 12 inserted into the tube 13.
In addition, the following is estimated as a cause of generation | occurrence | production of a damage | wound. That is, the sintering of the porous base material 1 is performed by a series of temperature cycles in which the porous base material 1 is exposed to a high temperature in a state where it is suspended by the hanger 7 and then cooled to room temperature. For this reason, at the time of cooling, a damage | wound generate | occur | produces on a glass rod by the shrink-fitting phenomenon produced by the difference of the thermal expansion coefficient between the pipe | tube of a hanger, and a glass rod.

  When such a glass rod having a cut recess and having scratches is reused, cracks originating from scratches occur in the glass rod during the deposition of glass particles in the OVD process, and the base material is in the middle of the deposition. An accident of falling occurs. Even if no accident occurs in the deposition process, there is a high possibility that a fall accident will occur in the sintering process due to the heat cycle in the sintering process. If a base material fall accident occurs, in addition to discarding the base material, the equipment will be seriously damaged. For this reason, the damaged glass rod is not reused but discarded.

It has been found that the occurrence of the flaws also occurs in the same phenomenon in the glass base material stretching step performed after the porous base material sintering step.
The porous base material and the glass base material are hereinafter referred to as an optical fiber base material or simply as a base material.
Japanese Patent Laid-Open No. 2001-287916

  The present invention provides a heat treatment of an optical fiber preform that enables reuse of a glass rod connected to the preform in the sintering process or the stretching process of the optical fiber preform and further reduces the production cost. It aims to provide a method.

The method for heat-treating an optical fiber preform according to the present invention comprises connecting the preform to a glass rod and suspending it from a heat treatment apparatus via a suspension jig in the optical fiber preform sintering or stretching process. At this time, the glass rod having the outer diameter D2 is inserted into the tube having the inner diameter D1 constituting the suspension jig, and the gap (D1-D2) between the tube and the glass rod is 1.0 ≦ [(D1-D2) / D2] × 100 ≦ 5.0 is satisfied.
In addition, it is preferable to insert a flexible cushioning material mainly made of carbon into at least a part of the gap between the tube and the glass rod, and the carbon content of the flexible cushioning material is 97% or more.

  According to the present invention, in the optical fiber preform sintering process or drawing process, the glass rod inserted into the hanger is prevented from being scratched on the surface, and the glass rod can be reused. Contributes to reducing the manufacturing cost of the base material.

As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge and achieved the present invention.
In order to prevent the occurrence of scratches due to shrink fitting between the hanger tube and the inserted glass rod in the heat treatment of the optical fiber preform, that is, in the sintering process of the porous preform or the stretching process of the glass preform. As one of the effective means, it is conceivable to provide a clearance of a certain width or more between the glass rod and the inserted tube.

However, in this case, the following problem occurs.
Simply increasing the clearance between the tube to be inserted and the glass rod only, when the rotation is started in the sintering process, the base material will run out and will not be heated evenly. Causes eccentricity. Similarly, in the stretching process, when the clearance is increased, the suspended base material is inclined with respect to the vertical direction, and the preform obtained by stretching is curved.
As described above, the size of the gap between the tube into which the glass rod is inserted and the scratch on the surface of the glass rod after the treatment are in a trade-off relationship, and a condition where both are compatible is required. .

  As a result of intensive research, the inventors have found that the relationship between the inner diameter D1 of the tube into which the glass rod is inserted and the outer diameter D2 of the glass rod, and the gap (D1-D2) between the inserted tube and the glass rod is 1.0. ≦ [(D1−D2) / D2] × 100 ≦ 5.0 is satisfied, and a flexible cushioning material mainly composed of carbon is inserted into a part of the gap during the heat treatment. The present invention has been achieved by finding that damages caused by shrink fitting between a tube and a glass rod can be prevented without causing bending in the eccentric after stretching and the preform obtained by stretching.

  Here, when [(D1−D2) / D2] × 100 <1.0, the gap is too small and it becomes very difficult to insert the flexible cushioning material. Even if a buffer material with a small thickness is to be inserted, the buffer material is partially wrinkled or cracked during the insertion, and as a result, a part of the surface of the glass rod after processing is damaged.

  Further, in the case of 5.0 <[(D1-D2) / D2] × 100, whirling due to rotation occurs in the sintering process even when the flexible cushioning material is inserted. Due to the occurrence of this swaying, the base material is not heated evenly, resulting in uneven thickness and eccentricity. Further, in the stretching process, the suspended base material is inclined, and the preform after stretching is curved.

Considering that the flexible buffer material is heated to about 1,500 ° C. in the sintering process, a material having carbon as a main raw material is used. An example is a Nika film (trade name, manufactured by Nippon Carbon Co., Ltd.). However, this example is merely an example, and does not limit the present invention.
In addition, it is preferable that the carbon used as the main raw material of the flexible buffer material has a purity of 97% or more. If the purity of the carbon is less than 97%, devitrification occurs on the surface of the glass rod due to impurities contained in the flexible buffer material under high temperature treatment conditions.

Examples of the present invention are shown below, but the present invention is not limited to these, and various modes are possible.
Example 1
The optical fiber preform (porous preform) having a diameter of 300 mm and a weight of 80 kg was sintered using the vertical sintering apparatus shown in FIG.
The base material 1 was attached to a suspension jig (suspender) 7 attached to the tip of a ceramic shaft 8 and suspended from the apparatus. The base material 1 is rotated at 10 rpm by the motor 3 and the heater 4 is heated to 1500 ° C. in this state, and then the base material 1 is lowered at a speed of 2.0 mm / min by the lifting mechanism 11 using the screw rod 9. Sintering was performed sequentially from the tip. During sintering, He gas and chlorine-based gas were introduced into the core tube 2 from the gas inlet 5 at flow rates of 10 L / min and 3 L / min, respectively, and exhausted from the exhaust port 6.

An outline of the hanging jig used in this example is shown in FIG. 3A is a longitudinal sectional view, and FIG. 3B is a transverse sectional view.
The inner diameter D1 of the insertion tube 13 constituting the hanger 7 is 41.0 mm, and the outer diameter D2 of the inserted glass rod 12 is 40.0 mm. The length L of the glass rod 12 at the portion inserted into the insertion tube 13 is 80 mm. Furthermore, in the gap (D1-D2) between the insertion tube 13 and the glass rod 12, a carbon flexible cushioning material 20 having a height of 80 mm, a width of 30 mm, and a thickness of 0.2 mm is inserted from the direction A in FIG. Was inserted to cover.
In this state, the base material 1 was sintered. After the sintering, the base material 1 was removed from the apparatus and the surface of the glass rod 12 was observed. As a result, no scratch was observed on the surface. The eccentricity of the transparent vitrified base material was 0.1% or less.

(Examples 2 to 4)
Three base materials 1 having the same outer diameter and weight as the optical fiber base material (porous base material) used in Example 1 were prepared. The structure shown in Fig. 3 is used. Insertion tubes (40.5mm, 41.5mm, 42.0mm) 13 with different inner diameters D1 are used, and the gap between the insertion tube 13 and the glass rod 12 is 80mm high and wide. Sintering was performed in the same manner as in Example 1 except that carbon flexible buffer material 20 having a thickness of 30 mm and a thickness of 0.4 mm was inserted.
After completion of sintering, the glass rod surface of the three transparent vitrified base materials was removed from the apparatus, and no damage was observed on the glass rod surface using any of the insertion tubes 13 . In addition, the eccentricity of each base material was 0.1% or less.

(Comparative Examples 1 and 2)
Two base materials 1 having the same outer diameter and weight as the optical fiber base material (porous base material) used in Example 1 were prepared. The insertion tube (40.4mm, 42.5mm) 13 has a structure as shown in Fig. 3, each with a different inner diameter D1, and the gap between the insertion tube 13 and the glass rod 12 is 80mm high, 30mm wide, and thick. Sintering was performed in the same manner as in Example 1 except that a carbon flexible buffer material 20 having a thickness of 0.4 mm was inserted.
When the insertion tube 13 having an inner diameter D1 of 40.4 mm is used, the gap with the glass rod 12 is very narrow. occured.

  After the sintering was completed, the glass rod surface of the two transparent vitrified base materials was observed. When an insertion tube having an inner diameter D1 of 40.5 mm was used, the surface of the glass rod 12 had a diameter. One 0.2mm large scratch was confirmed. In addition, in the case of using an insertion tube having an inner diameter D1 of 42.5 mm, no scratches were confirmed on the surface of the glass rod, but the contact area of the base material was visually confirmed during sintering, and as a result The eccentricity was 0.2%, which was worse than that of Examples 1 to 4.

  The glass rod connected to the optical fiber preform can be reused, contributing to a reduction in manufacturing costs.

It is a schematic longitudinal cross-sectional view which shows an example of a vertical sintering apparatus. It is the schematic which shows an example of the hanging tool of the preform | base_material for optical fibers, (a) is the front view, (b) is a side view. BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows an example of the hanging tool of the optical fiber preform | base_material of this invention, (a) is the longitudinal cross-sectional view, (b) is a cross-sectional view.

Explanation of symbols

1 ... Porous base material (base material),
2 ... Core tube,
3 …… Motor (rotation mechanism),
4 ... Heater,
5 …… Gas inlet,
6 …… Exhaust port,
7 …… Hanging jig (hanging tool),
8 …… Shaft (rotating shaft),
9 …… Screw rod,
10 …… Support bar,
11 …… Elevating mechanism,
12 …… Glass rod,
13 …… Insertion tube (tube),
14 …… Incision recess,
15 …… hole,
16 …… Pin,
17 …… Flat surface,
19 …… Gap,
20 …… Flexible cushioning material.

Claims (3)

In the sintering process or drawing process of the optical fiber preform, when the preform is connected to the glass rod and suspended in the heat treatment apparatus via the suspension jig, the inner diameter D1 constituting the suspension jig The glass rod having the outer diameter D2 is inserted into the tube, and the gap (D1−D2) between the tube and the glass rod satisfies the relationship of 1.0 ≦ [(D1−D2) / D2] × 100 ≦ 5.0. Heat treatment method for optical fiber preform. The method for heat-treating an optical fiber preform according to claim 1, wherein a flexible cushioning material mainly made of carbon is inserted into at least a part of a gap between the tube and the glass rod. The method for heat-treating an optical fiber preform according to claim 2, wherein the flexible buffer material has a carbon content of 97% or more.

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