JPH05339024A - Heating device for glass preform - Google Patents

Abstract

PURPOSE:To obtain a transparent glass body with the characteristic stabilized in the longitudinal direction by minimizing the furnace temp. fluctuation even when the upper part of a porous vitrifying the glass body. CONSTITUTION:A porous glass body 5 is inserted into a furnace core tube 2 surrounded by a heater 3 and moved, the part of the tube not to be heated is covered with a heat reflecting material 82 and a heat insulating material 81, and a dummy porous glass body 7 formed with a heat reflecting material 72 and a heat insulating material 71 is fixed to the upper end of the body 5 in the tube 2 and moved in the tube 2 along with the body 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dehydration treatment or a sintering (transparent vitrification) treatment by heating a porous glass body in a process of manufacturing an optical fiber preform by a VAD method or an external attachment method. The present invention relates to a heat treatment device for a glass base material, which performs the step of.

[0002]

2. Description of the Related Art When manufacturing an optical fiber preform by a VAD method or an external attachment method, an oxyhydrogen flame is generated from a burner and a raw material gas of glass such as silicon tetrachloride and a gas of a dopant material are sent into the flame. Glass particles (silicon dioxide) are generated by a hydrolysis reaction and a thermal oxidation reaction, and the glass particles are deposited on a target to form a cylindrical porous glass body. Then, by heating this porous glass body, the OH groups are removed (dehydrated), and a transparent vitrification (sintering) process is performed. The dehydration treatment and the sintering treatment may be performed at the same time, or only the dehydration treatment is performed by first heating at a temperature lower than the melting temperature of glass, and then the temperature is raised to the glass melting temperature to clear glass. In some cases, the processing is performed in two steps, such as performing the processing. In this way, a transparent glass preform is produced, and this is melted and drawn and spun to form a thin optical fiber.

Conventionally, a heat treatment apparatus for performing the dehydration and sintering treatments is constructed as shown in FIG. In FIG.
The heating furnace 1 is provided with a quartz glass core tube 2 in the center thereof,
A heater 3 is provided around it. The porous glass body 5 formed on the starting member 4 is moved into the furnace core tube 2. In other words, suspend the starting member 4 from above,
The porous glass body 5 is moved downward (traverse) while being rotated so that the porous glass body 5 passes from the top to the bottom through the heat zone 31 in which a constant vitrification temperature is maintained by the heater 3. Process gas of a dehydrating agent such as helium and chlorine gas is fed from the lower part of the core tube 2, and the porous glass body 5 is processed at a high temperature of 1,000 to several hundreds of degrees when passing through the heat zone 31, The OH group of is removed and transparent vitrification is performed. As a result, the porous glass body 5 passing through the heat zone 31 becomes the transparent glass body 6.

At this time, it is considered that the heat conduction in the core tube 2 is governed by the radiation and the conduction by the process gas, and the peak wavelength of the heat radiation in the temperature range of 1,000 to a few hundreds of degrees is between 1 μm and 3 μm. It is in. This wavelength region is almost transmitted by the transparent quartz glass, but is scattered by the porous glass body 5 in which the glass fine particles are aggregated and deposited. Therefore, when the central part of the porous glass body 5 is located in the heat zone 31, the radiation in the above wavelength region is scattered by the porous glass body 5 which is not yet vitrified, and is emitted to the outside of the furnace. The amount of heat radiation is small. Further, since glass basically has low thermal conductivity, the amount of heat released to the outside of the furnace through the lower transparent glass body 6 is small. However, when both ends (upper and lower ends) of the porous glass body 5, especially the upper end, move to the heat zone 31, there are few porous glass bodies 5 that scatter thermal radiation, so much heat is radiated to the outside of the furnace, A decrease in temperature is brought about.

In recent years, in order to reduce the manufacturing cost of optical fibers, the transparent glass base material tends to increase in size, and accordingly, the heating furnace for the glass base material also increases in size. When the heating furnace becomes larger as the base material becomes larger in this way, the cross-sectional area for releasing heat further increases, so that the temperature fluctuation in the furnace becomes larger.

Therefore, the temperature inside the furnace is usually controlled. Regarding the temperature detection, if impurities are mixed in the base material, the transmission loss of the optical fiber is adversely affected. Therefore, these temperature measuring parts are placed in the core tube 2 in order to avoid mixing of impurities from the thermocouple or the protection tube. I can't help but core tube 2
The temperature near the outer heater 3 is measured, or the temperature inside the furnace is measured from outside the furnace using a radiation thermometer or the like.

[0007]

However, just by measuring the temperature from the outside of the furnace and controlling the temperature as in the conventional case, the temperature distribution in the furnace due to the position of the porous glass body accompanying the enlargement of the base material and the heating furnace will be described. However, there is a problem in that the characteristics of the base material change in the length direction of the base material. That is, even if the temperature measured from the outside is constant, when the upper part of the porous glass body reaches the heat zone 31, the amount of heat released to the outside of the furnace increases, so the actual temperature inside the core tube decreases. In some cases, the dehydration may be insufficient, or the transparent vitrification treatment may be insufficient to increase the number of opaque portions.

In order to avoid such temperature fluctuation, empirically, the set temperature is raised at the position where the upper part of the porous glass body has moved to the heat zone 31, or the moving speed of the porous glass body is changed. Attempts have also been made to suppress characteristic fluctuations, but such favorable results have not been obtained. Moreover, when heat treatment is performed focusing only on the stabilization of the characteristics of the base material, the upper end of the base material and the starting member are stretched by the weight of the base material itself, and the outer diameter becomes small. There may be problems such as

In view of the above, the present invention reduces empirical corrections to the heating temperature to minimize temperature fluctuations in the furnace even when the upper part of the porous glass body moves to the heat zone. Stable processes such as dehydration and transparent vitrification are performed to make the heat history of the base material uniform in the length direction,
An object of the present invention is to provide a heat treatment apparatus capable of obtaining an optical fiber preform of transparent glass, the characteristics of which are stable in the length direction and the dimensions such as the outer diameter are stable.

[0010]

In order to achieve the above object, in the heat treatment apparatus for a glass base material according to the present invention, the periphery of the non-heated portion of the furnace tube is covered with a heat reflecting material and a heat insulating material, and Attaching a pseudo-porous glass body composed of a heat reflecting material and a heat insulating material to the upper end of the glass body,
This pseudo porous glass body is characterized in that it is moved together with the porous glass body in the core tube.

[0011]

[Function] The porous glass body is moved from the top to the bottom in the core tube, successively passes through the heat zone, and is subjected to treatments such as dehydration and transparent vitrification, but the upper part of the porous glass body is heated. Even when fewer porous glass bodies move to the zone to dissipate heat, the pseudo-porous glass body takes its place. That is, this pseudo-porous glass body is composed of a heat-reflecting material and a heat-insulating material, and can be almost equivalent to the porous glass body in terms of heat scattering and heat conduction. This pseudo-porous glass body is attached so as to cover the periphery of the upper end of the porous glass body, and moves together with the porous glass body in the furnace tube. Can be considered as extending, and the heat treatment should be performed up to the upper end of the original porous glass body in a state similar to the central part (the same degree of radiation of heat to the outside of the furnace). You can
As a result, even if the base material and the heat treatment apparatus are increased in size, temperature fluctuations can be reduced, and dehydration, transparent vitrification and the like can be stably performed in the length direction of the base material. As a result, the heat history of the glass base material becomes uniform in the lengthwise direction, the characteristics are stabilized in the lengthwise direction, and the correction amount of the heating temperature is small, so fluctuations in the lengthwise direction such as the outer diameter dimension. Can be minimized.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. In FIG. 1, a heating furnace 1 has a quartz glass core tube 2 in the center thereof, and a heater 3 is provided around the furnace core tube 2. A porous glass body 5 is formed at the lower end of the starting member 4, and the heating member 31 formed by the heater 3 is made porous by suspending the starting member 4 from above and traversing downward while rotating. The glass body 5 is passed through. The pseudo-porous glass body 7 is attached to the starting member 4 above the porous glass body 5. Process gas of a dehydrating agent such as helium and chlorine gas is fed into the core tube 2, and the porous glass body 5 is processed at a high temperature of 1,000 to several hundreds of degrees when passing through the heat zone 31, As the OH group is removed, transparent vitrification is performed. As a result, the porous glass body 5 passing through the heat zone 31 becomes the transparent glass body 6.

The pseudo-porous glass body 7 is mainly composed of a heat-reflecting material 72, which is a material that reflects the visible to infrared wavelength region, and a material 71 having a low thermal conductivity (herein, called a heat insulating material). By being attached to the member 4, when the starting member 4 traverses downward, it moves downward with it. The heat insulating material 71 and the reflecting material 72 of the pseudo-porous glass body 7 are selected from materials that are excellent in heat resistance and do not react with atmospheric gas such as glass and dehydrating agent, and their size is almost the same as the inner diameter of the furnace tube 2. It is desirable to be equal. Even a material that is reactive with a dehydrating agent or the like can be used if it is protected by a film made of another material such as transparent quartz glass. Also,
Although not shown here, when the upper part of the porous glass body 5 reaches the heat zone 31, the pseudo-porous glass body 7 is also exposed to high temperature, so the pseudo-porous glass body 7 is cooled. It is also possible to provide a mechanism so as to extend the life of the pseudo porous glass body 7.

Further, the outer peripheral portions of the furnace core tube 2 in the upper portion and the lower portion of the heating furnace 1 are covered with a reflecting material 82, and the periphery thereof is further covered with a heat insulating material 81. This reflective material 8
The reference numeral 2 mainly comprises a heat reflecting material which is a material that reflects the visible to infrared wavelength range.

As described above, the periphery of the non-heated portion of the core tube 2 is covered with the reflector 82 and the heat insulator 81 to prevent heat radiation to the outside at that portion, and the temperature required for this dehydration / transparent vitrification treatment. A pseudo-porous glass body 7 similar to the porous glass body 5 in terms of heat scattering and conduction in the above is attached to the upper part of the porous glass body 5 and moved downward together therewith. Porous glass body 5 in which the upper part of the body 5 moves to the heat zone 31 to scatter heat
Even when the amount of heat is reduced, heat can be reflected by the pseudo-porous glass body 7, heat loss due to radiation of heat is reduced, temperature is stabilized, and stable dehydration, transparent vitrification, etc. can be performed. It will be possible.

In practice, a heating furnace 1 having a core tube 2 with an inner diameter of 250 mm was used, and the outer diameter was 200 mm and the length was 16 mm.
The 00 mm porous glass body 5 was suspended by the starting member 4 for dehydration and transparent vitrification treatment. At this time, a cylindrical pseudo-porous glass body 7 protected by a transparent quartz glass cylinder having an outer diameter of 210 mm and a length of 200 mm was attached to the starting member 4. The pseudo porous glass body 7 uses a platinum foil as the reflecting material 72 and an alumina heat insulating material as the heat insulating material 71. Further, the non-heated portion of the furnace core tube 2, that is, the periphery of the upper portion and the lower portion of the heating furnace 1, is an alumina heat insulating material 81 having a platinum foil (reflecting material 82) attached to its inner surface.
Covered with. At this time, helium was introduced into the core tube 2 from the lower part of the core tube 2 at a flow rate of 15 liters / minute and chlorine at a flow rate of 150 cc / minute. Further, helium was poured from the upper portion through the starting member 4 at a flow rate of 3 liter / min to cool the pseudo porous glass body 7.

The transparent glass body 6 thus obtained was spun into an optical fiber, and its wavelength was 1.3 μm.
The loss in the 1.38 μm band is as shown by the solid lines in FIGS. 2 and 3, respectively, and is 0.34 ± 3
The loss was as low as 0.01 dB / km and 0.37 ± 0.01 dB / km, and it was confirmed that the product was sufficiently dehydrated.

On the other hand, for reference, the conventional pseudo-porous glass body 7 and the heat treatment apparatus not provided with the heat insulating material 81 and the reflecting material 82 for covering the furnace tube 2 are used to prepare the porous glass body 5 I tried dehydration and transparent vitrification. In this case, the unsintered part is about 150 m above the porous glass body.
When the loss was measured for the optical fiber produced by spinning the remaining transparent glass body, the data shown by the dotted lines in FIGS. 2 and 3 were obtained. 2 and 3
Indicates the loss in the wavelength band of 1.3 μm and the loss in the wavelength band of 1.38 μm, respectively. According to this, when the conventional heat treatment device is used, the loss gradually increases at the upper part of the transparent glass body, and the loss becomes 0.42 dB / km (wavelength 1.3 μm at the uppermost part).
Band), 1.2 dB / km (wavelength 1.38 μm band), which indicates that the dehydration treatment is not performed sufficiently compared to the central part of the base material, and it is understood that the transmission characteristics are poor.

Further, the amount of electric power for heating by the heater 3 for dehydration and transparent vitrification treatment is the same as that of the conventional heat treatment apparatus without the heat insulating material 81 covering the pseudo porous glass body 7 and the furnace tube 2 and the reflecting material 82. It was necessary to increase about 25% in the upper part of the base material compared to the central part of the base material, but in the above example, the base material is covered by the heat insulating material 81 and the reflecting material 82 that cover the pseudo porous glass body 7 and the furnace core tube 2. Since we were able to suppress the heat loss in the upper part, we only needed to increase it by about 5%. Furthermore, the amount of the porous glass body 5 that remains without being made into transparent glass is also about 20%.
Therefore, the dehydration and transparent vitrification treatment could be performed more efficiently.

[0020]

As described above with reference to the embodiments,
According to the glass base material heat treatment apparatus of the present invention, even when the upper part of the porous glass body has moved to the heat zone, the empirical correction amount for the heating temperature is made small while the temperature fluctuation in the furnace is suppressed. It is possible to stably carry out treatments such as dehydration and transparent vitrification while minimizing the amount. This makes it possible to manufacture a transparent glass optical fiber preform having a uniform heat history and stable characteristics over the entire length of the preform. In addition, dimensions such as the outer diameter of the glass base material can be stabilized in the length direction.

[Brief description of drawings]

FIG. 1 is a schematic view of an embodiment of the present invention.

FIG. 2 is a graph showing a loss in a length direction of a base material in a wavelength band of 1.3 μm.

FIG. 3 is a graph showing a loss in a length direction of a base material in a wavelength band of 1.38 μm.

FIG. 4 is a schematic diagram of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Core tube 3 Heater 31 Heat zone 4 Starting member 5 Porous glass body 6 Transparent glass body 7 Pseudo porous glass body 71, 81 Insulation material 72, 82 Reflective material

Claims (1)

[Claims] 1. A furnace core tube into which a porous glass body is inserted,
A heater disposed around the core tube to heat the core tube; a heat reflecting material and a heat insulating material provided so as to cover the periphery of the core tube in a portion other than a portion heated by the heater; A heat treatment device for a glass preform, comprising: a pseudo-porous glass body composed of a heat reflecting material and a heat insulating material, which is arranged in the furnace core tube so as to move together with the porous glass body.