JP2006056755A - Method for producing optical fiber preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an optical fiber preform where the deformation of a support rod or bar supporting a porous preform for an optical fiber is reduced when dehydrated and vitrified to be transparent, where the effective length of the optical fiber preform can be lengthened and where the optical fiber with small eccentricity of a core and small non-circularity of clad can be obtained. <P>SOLUTION: The optical fiber preform is produced by the method that the porous preform for the optical fiber whose upper edge is supported with a supporting bar 6 is inserted in a core tube 5 at a heating furnace, dehydrated and vitrified to be transparent. When the maximum temperature in the furnace is set to 1,500°C at the longitudinal center axis of the core tube 5, the temperature difference in a same face at a circumferential direction in the position of the core tube 5 where the temperature of the longitudinal center axis of the core tube 5 is 1,000-1,300°C is 10°C or lower. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber used for optical communication or the like, and more particularly to a method for manufacturing an optical fiber preform for obtaining an optical fiber having a small core eccentricity and a low cladding non-circularity.

Conventionally, when manufacturing an optical fiber preform made of transparent glass, a porous preform for optical fiber synthesized by the VAD method or the OVD method is gradually inserted into a heating furnace having a furnace core tube and dehydrated and transparent. A vitrification method is employed.
Further, in recent years, the optical fiber preform is becoming larger in order to reduce the cost.

  However, as the size of the optical fiber preform increases, the quartz glass support rod supporting the upper end of the optical fiber porous preform and the lower end of the quartz glass support rod further supporting the support rod However, there is a problem that it is thermally deformed such as being thinned by the weight of the suspended optical fiber preform.

Hereinafter, a heating furnace, that is, an optical fiber preform manufacturing apparatus will be specifically shown.
As shown in FIG. 12, a porous base material for optical fiber in which a soot 2 for cladding is further deposited around a glass rod 1 including a core (hereinafter referred to as a core rod 1) is used as a support rod made of quartz glass. 4a is supported at the lower end of a support rod 6 made of quartz glass, and is rotated around the axis of the support rod 6 into the core tube 5 provided with the heater 7 and heated by the heater 7 Dehydration and transparent vitrification yield a transparent optical fiber preform 8.
In FIG. 12, reference numeral 3 denotes a vitrified portion of the soot 2, reference numeral 4b denotes a support rod connected to the other end of the optical fiber porous preform, and reference numeral 9 denotes a heater 7 or a core tube 5. A heating furnace protector made of, for example, a heat insulating material is shown.
As shown in this figure, when the entire furnace core tube 5 is covered with a heating furnace protector 9 made of a heat insulating material at the upper part of the heater 7, the temperature distribution is generally on the right side of the heater 7 in FIG. As shown in FIG.

When the optical fiber porous preform is exposed to such a wide temperature range, that is, a broad temperature distribution, a quartz glass support rod 4a connecting the optical fiber porous preform to the support rod 6 is provided. However, there are problems such as being exposed to high temperature for a long time and being thinned by the weight of the optical fiber preform 8 as described above.
Normally, the support bar 4a is used repeatedly, but if it is deformed as described above, it cannot be used thereafter.

Further, when the porous preform for optical fiber is exposed to the broad temperature distribution as described above, the difference in the progress rate of transparent vitrification between the surface and the inside becomes large, and as shown in FIG. as such, where the wish for shorter length L ₁ of the upper portion of tapered optical fiber preform 8 after the transparent vitrification, and ended up with longer L ₂ than L ₁ as shown in FIG. 13 (b), the There is a problem that the effective length of the optical fiber preform 8, that is, the length that can be actually used is shortened, and it is difficult to reduce the cost.
Incidentally, the effective length of the optical fiber preform 8 is a portion where the outer diameter D of the preform is stable within a predetermined range, that is, a portion where the outer diameter D excluding both end portions of the optical fiber preform 8 is constant. The length of the portion where the usable optical fiber can be drawn.

Therefore, for example, in order to solve the problem of the former support rod, Patent Document 1 proposes a heating furnace capable of adjusting the length of the heater during the transparent vitrification process of the optical fiber porous preform. .
Specifically, as described in Patent Document 1, when the transparent vitrification of the optical fiber porous preform progresses, and the connecting portion with the quartz glass support rod on the upper part approaches the heater, The upper part of the heater is removed so that the support rod is not overheated.

Japanese Patent Application Laid-Open No. 06-048760

  However, in the thing of patent document 1, when a support rod comes to a predetermined position, a part of heater must be removed each time, and work becomes complicated. Also, if this is done automatically, there is a problem that the apparatus becomes complicated and expensive, which is in conflict with the cost reduction.

Therefore, a part of the heating furnace protector 9 in FIG. 12, more specifically, a part immediately above the heater 7 is removed over the entire circumferential direction, the furnace core tube 5 is cooled with the outside air, and a high temperature region is obtained. Attempts have been made to sharpen the rising edge.
According to this method, since the rising portion of the furnace temperature becomes steep, the heating time to the support rod 4a is shortened, and the thermal deformation problem of the support rod 4a is almost solved. Also, the difference in vitrification speed inside and outside the porous preform for optical fiber is reduced, and the taper of the upper part of the optical fiber preform 8 is surely shaped as shown in FIG. 13 (a) after vitrification. Can now get. That is, the optical fiber preform 8 having a long effective length can be obtained more reliably.
Moreover, this method is considered to be an extremely inexpensive method because only a part of the heating furnace protector 9 needs to be removed, and it is considered that this method is also excellent in terms of cost reduction.

  However, when the optical fiber is drawn from the obtained optical fiber preform 8, the optical fiber preform 8 obtained before removing a part of the heating furnace protector 9 is obtained when the core eccentricity and the cladding non-circularity of the optical fiber are removed. It turned out to be larger than that of the optical fiber obtained by drawing.

  In view of the above problems, the object of the present invention is to reduce the deformation of the support rod and the support rod that support the optical fiber porous preform when the porous preform for optical fiber is dehydrated and made into a transparent glass, and It is an object of the present invention to provide a method for manufacturing an optical fiber preform that can obtain an optical fiber that can take a long effective length of the optical fiber preform and that has a small core eccentricity and a small cladding non-circularity.

  In order to achieve the above-mentioned object, the method for manufacturing an optical fiber preform according to claim 1, wherein a porous preform for an optical fiber having an upper end supported by a support rod is inserted into the furnace core tube of a heating furnace having a furnace core tube. In the method of manufacturing an optical fiber preform to be dehydrated and made into transparent glass, when the temperature of the heating furnace is set so that the maximum temperature at the longitudinal central axis of the furnace core tube is 1500 ° C., the temperature of the longitudinal central axis of the core tube Is 1300 ° C., the temperature of the longitudinal central axis of the core tube is within 300 mm above the axial direction of the core tube from the position where the temperature is 1400 ° C., and the temperature of the longitudinal central axis of the core tube is 1000-1300 ° C. The temperature difference in the same circumferential direction in the core tube at the position is 10 ° C. or less.

According to the method of manufacturing an optical fiber preform according to claim 1, the furnace tube is set when the temperature of the heating furnace is set so that the maximum temperature at the longitudinal central axis of the furnace tube is 1500 ° C. The position where the temperature of the longitudinal central axis of the core tube is 1300 ° C. is within 300 mm above the axial direction of the core tube from the position where the temperature of the longitudinal central axis of the core tube is 1400 ° C. Since the temperature distribution is steep, the deformation of the support rod and the support rod for supporting the optical fiber porous preform can be reduced more reliably, and the effective length of the optical fiber preform can be increased. It is also possible to reduce the cost of the optical fiber.
At the same time, the temperature difference in the same plane in the circumferential direction in the core tube at a position where the temperature of the longitudinal central axis of the core tube is 1000 to 1300 ° C. is set to 10 ° C. or less. As a result, it is possible to easily manufacture an optical fiber preform having a small core eccentricity and a low cladding non-circularity. Therefore, if this optical fiber preform is drawn, an optical fiber having a small core eccentricity and a low cladding non-circularity can be obtained.

  A method for manufacturing an optical fiber preform according to claim 2 is the method for manufacturing an optical fiber preform according to claim 1, wherein the temperature of the core tube in the longitudinal center axis of the reactor core tube is in the region of 1000 to 1300 ° C. A heat-radiating shield is provided on the outer periphery.

According to the method for manufacturing an optical fiber preform according to claim 2, the heat shield is provided on the outer periphery of the core tube in the region where the temperature at the longitudinal central axis of the core tube is 1000 to 1300 ° C. Since it is provided, the temperature distribution in the 1000-1300 ° C. region of the reactor core tube is more steep and more reliably excluded, and the temperature distribution in the circumferential direction of the reactor core tube is more uniform, specifically 10 ° C. The following temperature difference can be maintained.
Therefore, the deformation of the support rod and the support rod can be reduced, the effective length of the optical fiber preform can be increased, and if the manufactured optical fiber preform is drawn, the core eccentricity and the cladding non-circularity are extremely small. An optical fiber can also be obtained.

  As described above, according to the present invention, when the optical fiber porous preform is dehydrated and made into a transparent glass, the support rod and the support rod that support the optical fiber porous preform are reduced in deformation, and It is possible to provide a method for manufacturing an optical fiber preform that can increase the effective length of the optical fiber preform and obtain an optical fiber having a small core eccentricity and a low cladding non-circularity.

Hereinafter, a method for producing an optical fiber preform of the present invention will be described in detail with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same part as the manufacturing apparatus of the conventional optical fiber preform shown in FIG. 12, and detailed description is abbreviate | omitted.
FIG. 1 is a schematic view of an optical fiber preform manufacturing apparatus used in an embodiment of the optical fiber preform manufacturing method of the present invention, and FIG. 5 is a schematic view of a manufacturing apparatus used in still another embodiment. .
The production types in the examples and comparative examples in this specification are the most common single mode optical fibers.

  As shown in FIG. 1, quartz glass support rods 4a and 4b are respectively connected to both ends of a porous optical fiber preform formed by depositing a soot 2 for cladding around a core rod 1. The fiber preform is connected to the support rod 6 made of quartz glass through the support rod 4a. By the way, the support bar 4a and the support bar 6 are connected by being pinned, for example.

  When the porous optical fiber preform is attached to the tip of the support rod 6 via the support rod 4a as described above, the core tube 5 provided with the heater 7 is rotated while rotating the support base 6 around the axis of the support rod 6. The glass fiber is fed at a predetermined speed, dehydrated by heating, and further formed into a transparent glass to obtain a transparent optical fiber preform 8. Note that the axial center of the support rod 6 coincides with the longitudinal central axis of the core tube 5.

The feature of the optical fiber preform manufacturing apparatus, that is, the heating furnace used in the first embodiment is that, as shown in FIG. 1, the heating furnace protector 9 covering the furnace core tube 5 and the heater 7 is removed and the heating furnace is heated. The heat-dissipating shield 10 is provided coaxially with the longitudinal central axis of the core tube 5 at a portion immediately above the heater 7.
This heat radiating shield 10 is an annular one in which an aluminum plate is mounted on the inside as a heat radiating plate and a heat insulating material is provided on the outside in consideration of the safety of the operator. It is installed coaxially with the tube 5 and in this example with a space of 150 mm from the core tube 5.

Here, the reason why the heat-radiating shield 10 is provided immediately above the heater 7 as described above is that the core tube 5 immediately above the heater 7 is covered by the heat-dissipating shield 10 so as to cover the external environment. The temperature of the core tube 5 is partially dissipated to the outside due to the heat dissipation of the heat-shielding shield 10, and the temperature distribution created by the heater 7 steeply rises to the high-temperature part. It is to make it.
As shown in the temperature distribution curve on the right side in FIG. 1, when compared with the temperature distribution along the longitudinal central axis of the furnace core tube 5 in the heating furnace of FIG. 12 indicated by the dotted line in the drawing, in the heating furnace of FIG. The temperature distribution (shown by the solid line) has a steep rise to the high temperature part.

  That is, as described above, the inventor of the present invention removes the heating furnace protector 9 immediately above the heater 7 in FIG. 12 over the entire circumferential direction, cools the furnace core tube 5 with the outside air, and moves to the high temperature region. As a result of making an attempt to sharpen the rising portion of the optical fiber, the core eccentricity and the cladding non-circularity of the optical fiber were larger than that of the optical fiber obtained before removing the heating furnace protector 9. It was speculated that the core tube 5 exposed to the outside air by removing the protective body 9 was cooled unevenly in the circumferential direction due to the influence of the outside air. Therefore, the heat-shielding shield 10 is provided for the two purposes of shielding the outside air and cooling the core tube 5.

Specifically, in order to prove this, in the core tube 5 immediately above the heater 7, the outer environment in the circumferential direction of the core tube 5 is changed non-uniformly by blowing wind, etc. The relationship between the maximum temperature difference in the circumferential direction and the average value of the core eccentricity of the optical fiber was examined by changing the circumferential temperature of the core tube 5 at a position where the temperature at the longitudinal central axis of the tube was about 1250 ° C.
The result is shown in FIG. As shown in FIG. 2, the average value of the core eccentricity is also increased in proportion to the temperature difference in the circumferential direction of the core tube 5, and it is understood that the inventor's guess is correct. In particular, when the maximum temperature difference in the circumferential direction exceeds 10 ° C., the average value of the core eccentricity tends to suddenly increase, and it is understood that the maximum temperature difference in the circumferential direction needs to be 10 ° C. or less.
The position where the maximum temperature difference in the circumferential direction needs to be 10 ° C. or less is a region where the temperature at the longitudinal central axis of the core tube is 1000 to 1300 ° C. It is generally known that the temperature at which soot contraction begins is 1000-1300 ° C. This is because if the maximum temperature difference in the circumferential direction of the core tube is large in this temperature region, the soot contracts asymmetrically, and as a result, an optical fiber preform having a large core eccentricity and a large cladding non-circularity is produced.
Here, the definition of the core eccentricity and the cladding non-circularity of the optical fiber shall conform to IEC 60793-1-20.

  By the way, among the reasons for installing the heat-radiating shield 10 at a position immediately above the heater 7, the reason from the viewpoint of the temperature distribution of this portion is that the maximum temperature at the longitudinal central axis of the core tube 5 is 1500 ° C. When the temperature of the heating furnace is set as described above, the position where the center temperature of the core tube 5 is 1300 ° C. is higher than the position where the temperature of the longitudinal center axis of the core tube 5 is 1400 ° C. This is because it is desired to be within 300 mm. That is, in the temperature distribution created by the heater 7, the temperature range from 1300 ° C. to 1400 ° C. corresponding to the rising portion to the high temperature portion is made steeper, and the support rod 4a and the support rod 6 connected to the support rod 4a are used. The soot 2 is vitrified almost simultaneously with the inside and outside so that the lower end of the optical fiber is not heated for a long time, and the upper portion of the optical fiber preform 8 is finished as shown in FIG. This is because the effective length can be increased.

With the heat-dissipating shield 10 installed in this manner, the temperature of the heating furnace is set so that the maximum temperature at the center of the furnace core tube 5 is 1500 ° C. required for the transparent vitrification of the soot 2, and the soot 2 He was flowed 10 liters / minute into the core tube 5 in accordance with the conditions for transparent vitrification. In this state, a thermocouple was inserted from above along the longitudinal central axis of the furnace core tube 5, and the temperature in the furnace was measured.
In the measurement, first, a thermocouple is inserted along the longitudinal central axis of the core tube 5 and moved by 50 mm in the longitudinal direction of the core tube 5 to find a region where the measured temperature is about 1000 ° C. to 1300 ° C. Thereafter, in this region, the temperature difference in the circumferential direction in the core tube 5 was measured. Specifically, the measurement position was set at a position 20 mm away from the inner wall of the core tube 5, and the thermocouple was shifted 45 degrees in the circumferential direction at each position, that is, eight points were measured on the same plane in the circumferential direction. As a result, in the core tube 5 of Example 1, the maximum temperature difference was 8 ° C.

  Using the manufacturing apparatus thus formed, the porous preform for optical fiber was actually dehydrated and made into a transparent glass. As the porous base material for optical fiber, a core rod 1 that has been dehydrated and transparently vitrified and externally provided with a soot 2 having an outer diameter of 280 mm for cladding is used by a known OVD method. The optical fiber preform 8 was manufactured by feeding the porous preform for optical fiber at a constant speed while rotating it into the core tube 5 having an inner diameter of 320 mm, and dehydrating and forming transparent glass.

The upper end of the obtained optical fiber preform 8 is as shown in FIG. 13A, and the optical fiber preform has a long effective length.
When the core eccentricity and the cladding non-circularity of the optical fiber obtained by drawing the optical fiber preform 8 were measured, the average value of the core eccentricity was 0.11 μm, and the average value of the cladding non-circularity was 0.00. The standard deviation of 09% was 0.01 μm and 0.01%, respectively.
Further, no deformation or damage was observed at the lower ends of the support bar 4a and the support bar 6.

In Example 1, when the maximum temperature at the center of the core tube 5 is 1500 ° C., the position at which the center temperature of the core tube 5 is 1300 ° C. is a position 190 mm above the position at 1400 ° C. .
Moreover, each fluctuation | variation of the core eccentricity of a longitudinal direction of this optical fiber and a cladding non-circularity is shown in FIG.3 and FIG.4. It can be seen from these figures that the optical fiber obtained has no problem in quality. Here, in both figures, the horizontal axis indicates the optical fiber length, the left is the support bar 4a side, and the right is the support bar 4b side.

  5 shows another embodiment of the present invention. As in FIG. 1, the heat shield 10 has a core in the region where the temperature of the longitudinal center axis of the core tube 5 is 1000 ° C. to 1300 ° C. when the maximum temperature of the longitudinal center axis of the core tube 5 is 1500 ° C. The tube 5 is installed so as to surround the tube 5, and the shape thereof is a Baumkuchen type box shape in which the core tube 5 side is opened as shown in FIG. 5. This heat-radiating shield 10 is obtained by attaching an aluminum material excellent in heat-dissipating properties to the inside of a box made of heat insulating material.

Specifically, the heat-dissipating shield 10 has an outer diameter that is approximately three times the outer diameter of the core tube 5, and its inner diameter is slightly larger than the outer diameter of the core tube 5. There is very little gap.
As described above, by covering the outside of the core tube 5 just above the heater 7 and forming a space between the core tube 5 and the core tube 5, moderate heat dissipation can be achieved in the region of 1000 ° C. to 1300 ° C., and The temperature in the circumferential direction of the core tube 5 can be made uniform.
In addition, the area | region where the furnace temperature above the heat-radiating shielding object 10 becomes 1000 degrees C or less may remain open, and you may give the heating furnace protector 9 conventionally.

Also in Example 2, when the temperature in the circumferential direction in the region of 1000 ° C. to 1300 ° C. was measured as in Example 1, it was confirmed that the maximum temperature difference was 6 ° C.
In addition, as shown in the temperature distribution curve on the right side of FIG. 5, the temperature distribution in the heating furnace in FIG. 5 (shown by the solid line) is compared with the temperature distribution in the heating furnace in FIG. Has a steep rise to the high temperature part. Further, the position where the temperature of the longitudinal central axis in the furnace core tube 1 was 1300 ° C. was a position 210 mm above the point where it became 1400 ° C.

With the optical fiber preform manufacturing apparatus shown in FIG. 5, the optical fiber porous preform having the same size as in Example 1 was dehydrated and made into transparent glass to obtain an optical fiber preform 8. The upper part of the optical fiber preform 8 was finished as shown in FIG.
When the core eccentricity and the cladding non-circularity of the optical fiber obtained by drawing this optical fiber preform 8 were measured, the average value of the core eccentricity was 0.08 μm, and the average value of the cladding non-circularity was The standard deviation was 0.07% and 0.01 μm and 0.01%, respectively.
Also in Example 2, no deformation or the like was found at the lower ends of the support bar 4a and the support bar 6.
6 and 7 show the fluctuation of the core eccentricity and the cladding non-circularity in the longitudinal direction of the optical fiber. As can be seen from these figures, there is no problem with respect to the quality of the optical fiber in terms of the core eccentricity and the cladding non-circularity. In these figures, the horizontal axis indicates the optical fiber length, the left is the support bar 4a side, and the right is the support bar 4b side.

Comparative Example 1

Comparative Example 1 is shown below. The optical fiber preform manufacturing apparatus shown in FIG. 8 is the same as that of the manufacturing apparatus shown in FIG. The same optical fiber porous preform as that used in the first embodiment is fed into the core tube 5 of the optical fiber preform manufacturing apparatus under the same conditions as in the first embodiment, dehydrated and made into a transparent glass, and its upper end is An optical fiber preform 8 as shown in FIG.
The optical fiber preform 8 was drawn and the core eccentricity and the cladding noncircularity of the obtained optical fiber were measured. The average value of the core eccentricity was 0.20 μm, and the average value of the cladding noncircularity was 0. The standard deviation of 13% was 0.03 μm and 0.02%, respectively.

Incidentally, the fluctuation of the core eccentricity and the cladding non-circularity in the longitudinal direction of the optical fiber are shown in FIGS.
In both figures, the horizontal axis indicates the optical fiber length, the left is the support bar 4a side, and the right is the support bar 4b side. As shown in FIGS. 9 and 10, the values of the core eccentricity and the cladding non-circularity increase as they go to the support rod 4 b side in the longitudinal direction of the optical fiber.

In the comparative example 1, as shown by the temperature distribution curve described on the right side of FIG. 8, the temperature distribution in the heating furnace of FIG. (Shown) has a steep rise to the high temperature portion, and when the maximum temperature of the longitudinal central axis of the core tube 5 is 1500 ° C., the position where the temperature of the longitudinal central axis of the core tube 5 is 1300 ° C. is 1400 The position was 150 mm above the position at which the temperature was changed.
In the same manner as in the above embodiments, the temperature of the longitudinal central axis of the core tube 5 is in the temperature range of 1000 ° C. to 1300 ° C., and the thermocouple is arranged in the circumferential direction on the same plane at a position 20 mm away from the inner wall of the core tube 5. When the temperature was measured at 8 locations by moving 45 ° at a time, the maximum temperature difference was observed at a position where the temperature of the longitudinal central axis of the core tube was about 1250 ° C. and was 15 ° C.
After all, in this comparative example 1, since the heat-radiating shield 10 does not exist, the temperature difference in the circumferential direction of the core tube 5 in the temperature region where the temperature of the longitudinal central axis of the core tube 5 is 1000 ° C. to 1300 ° C. becomes large. Therefore, it is considered that the core eccentricity and the cladding non-circularity have increased.

Comparative Example 2

Next, another comparative example is shown. FIG. 11 shows an apparatus for manufacturing an optical fiber preform used in Comparative Example 2, that is, a heating furnace. This heating furnace is characterized in that when the maximum temperature of the longitudinal central axis of the core tube 5 is 1500 ° C., the core tube In the region where the temperature of the longitudinal central axis of 5 is in the range of 1000 ° C. to 1300 ° C., a shield 11 made only of a ceramic fiber heat insulating material is installed immediately above the heater 7 so as to surround the core tube 5. is there.
Incidentally, when the temperature of the longitudinal central axis in the core tube 5 of this manufacturing apparatus is in the region of 1000 ° C. to 1300 ° C., the temperature in the circumferential direction of the core tube 5 is measured in the same manner as in Examples 1, 2 and Comparative Example 1. A temperature difference of 2 ° C. was confirmed.
Further, as shown by the temperature distribution curve described on the right side of FIG. 11, the temperature distribution in the heating furnace of FIG. 11 (shown by the solid line) is substantially the same as the temperature distribution in the heating furnace of FIG. The position where the temperature of the longitudinal central axis in the furnace core tube was 1300 ° C. was 330 mm above the point where it became 1400 ° C.

Using this heating furnace, the same optical fiber porous preform as in Examples 1 and 2 and Comparative Example 1 was fed into the furnace core tube 5 in the same manner to obtain an optical fiber preform 8. The top of the taper of the optical fiber preform 8 had become one long, as shown in L ₂ in FIG. 13 (b).
Further, the average value of the core eccentricity of the optical fiber obtained by drawing the obtained optical fiber preform 8 is 0.08 μm, the average value of the cladding non-circularity is 0.07%, and the standard deviation is 0.01 μm, respectively. 0.01%.
Incidentally, the fluctuation of the core eccentricity and the cladding non-circularity in the longitudinal direction of this optical fiber were measured. The results are almost the same as those in FIGS. 6 and 7, and both the core eccentricity and the non-circularity of the cladding are optical fibers. There was no problem in quality.

However, in Comparative Example 2, the support rod 4a was elongated, and the lower end of the support rod 6 was also partially deformed. This is a broad temperature distribution in the high temperature portion in the longitudinal central axis in the core tube 5, specifically, 330 mm from the point where the temperature of the longitudinal central axis in the core tube 5 becomes 1400 ° C. This is probably because the lower end of the support bar 4a and the support bar 6 has been heated too much. Further, the upper part of the optical fiber preform 8 is also as shown in FIG. 13B, and the effective length is short, which is not suitable for cost reduction.
Thus, in Comparative Example 2, the average value of the core eccentricity of the optical fiber and the average value of the cladding non-circularity were satisfactory in terms of the quality of the optical fiber, but the effective length of the optical fiber preform 8 Is not suitable for cost reduction, and the support rod 4a and the support rod 6 are thermally deformed. In particular, the support rod 4a cannot be reused.

  By the way, in each Example mentioned above, although the thing which mounted | wore aluminum inside the heat insulating material is used as the heat-radiating shielding object 10, other than aluminum as a heat-radiating material, for example, copper plate, copper foil, etc. Any nonflammable material that is excellent, in other words, excellent in thermal conductivity can be used. Carbon can be used as the heat insulating material. However, in view of the purpose of the heat-dissipating shield 10 is not heat insulation but heat radiation, the thickness of the heat insulating material is necessary for the handling and safety of the operator. It is important to make it thicker and not thicker than that.

  Moreover, although the heat-radiating shield 10 is installed immediately above the heater 7, the portion further above the heat-shielding shield 10, that is, the portion where the temperature of the longitudinal central axis of the core tube 5 is 1000 ° C. or less, The furnace core tube 5 may be left exposed or may be covered with the heating furnace protector 9 as in the prior art. What is required is to cover the outside of the core tube 5 where the temperature of the longitudinal central axis of the core tube 5 is 1000 ° C. to 1300 ° C. with the heat-radiating shield 10 as shown in the first and second embodiments. The periphery of the core tube 5 in the part where the temperature of the longitudinal central axis is within 300 mm upward from the position where the temperature of the longitudinal central axis is 1400 ° C. is within 300 mm. The maximum temperature difference in the direction is within 10 ° C.

  As described above, according to the method for manufacturing an optical fiber preform of the present invention, when the optical fiber porous preform is dehydrated and made into transparent glass, the support rod for supporting the optical fiber porous preform, Provided is a method of manufacturing an optical fiber preform that can reduce the deformation of the support rod and increase the effective length of the optical fiber preform, and that can obtain an optical fiber with a small amount of core eccentricity and cladding non-circularity. can do.

It is the schematic which shows the manufacturing apparatus of the optical fiber preform | base_material used for Example 1 of this invention. It is a graph which shows the relationship between the maximum temperature difference of a core tube circumferential direction, and the average value of the core eccentricity of an optical fiber. 6 is a graph showing fluctuations in the amount of core eccentricity in the longitudinal direction of the optical fiber obtained in Example 1. 6 is a graph showing fluctuations in the cladding non-circularity in the longitudinal direction of the optical fiber obtained in Example 1. It is the schematic which shows the manufacturing apparatus of the optical fiber preform used for Example 2 of this invention. It is a graph which shows the fluctuation | variation of the amount of core eccentricity of the optical fiber longitudinal direction obtained in Example 2. FIG. It is a graph which shows the fluctuation | variation of the clad non-circularity of the optical fiber longitudinal direction obtained in Example 2. FIG. 3 is a schematic view showing an optical fiber preform manufacturing apparatus used in Comparative Example 1. FIG. 6 is a graph showing fluctuations in the amount of core eccentricity in the optical fiber longitudinal direction obtained in Comparative Example 1; 6 is a graph showing the fluctuation of the cladding non-circularity in the longitudinal direction of the optical fiber obtained in Comparative Example 1. 6 is a schematic view showing an optical fiber preform manufacturing apparatus used in Comparative Example 2. FIG. It is the schematic of the manufacturing apparatus of the conventional optical fiber preform. It is an enlarged vertical sectional view showing the shape of the tip part of an optical fiber preform, (a) is an enlarged sectional view of an optical fiber preform with a long effective length, and (b) is an enlarged sectional view of an optical fiber preform with a short effective length. It is.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Core rod 2 Soot 4a, 4b Support rod 5 Reactor core tube 6 Support rod 7 Heater 9 Heating furnace protector 10 Heat radiation shield 11 Shield

Claims (2)

  In a manufacturing method of an optical fiber preform in which a porous preform for an optical fiber, the upper end of which is supported by a support rod, is inserted into the furnace core tube of a heating furnace having a furnace core tube to be dehydrated and made into a transparent glass, the length of the furnace core tube is When the temperature of the heating furnace is set so that the maximum temperature at the central axis is 1500 ° C., the position where the temperature of the longitudinal central axis of the core tube is 1300 ° C. is the temperature of the longitudinal central axis of the core tube is 1400 ° C. The temperature difference in the same plane in the circumferential direction in the core tube at a position within 300 mm above the axial direction of the core tube from a certain position and the temperature of the longitudinal central axis of the core tube is 1000 to 1300 ° C. is 10 ° C. or less. A method for manufacturing an optical fiber preform, characterized in that:   2. The optical fiber preform according to claim 1, wherein a heat-radiating shield is provided on an outer periphery of the core tube in a region where the temperature at the longitudinal central axis of the core tube is 1000 to 1300 ° C. 3. Method.

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