JPH0471017B2 - - Google Patents

Description

Detailed Description of the Invention <Technical Field to Which the Invention Pertains> The present invention relates to a method for making a porous optical fiber base material transparent and an improvement in an apparatus for carrying out the method, and more specifically relates to a method for making a glass raw material transparent by adding fire and water to When a porous optical fiber base material obtained by decomposing and laminating the resulting medium glass particles is heated and sintered in a high-temperature furnace to dehydrate and/or add a refractive index increasing or decreasing substance and make it transparent. The present invention relates to a method and apparatus for transparentizing a porous optical fiber base material, which can be made transparent evenly and reliably up to the center more quickly than conventional methods.

<Prior art> There is already a method of fire-hydrolyzing a glass raw material, depositing the resulting medium glass particles, and heating and sintering the resulting porous optical fiber base material in a high-temperature furnace to make it transparent. It is publicly known. For example, JP-A-55
Publication No. -67533 discloses the following two methods.

○B The first method is to heat a laminate of glass particles to a temperature of 1000°C or lower in an atmosphere containing fluorine compound gas, and then heat the laminate of glass particles to a temperature of 1400°C or higher in an inert gas atmosphere. This is a method of making the fiber transparent by holding it in a soaking furnace 31 as shown in FIG. 3, and 2 in FIG. 3 is a porous optical fiber base material.

○B The second method is to heat the porous optical fiber base material 2 to 1400°C or higher in a mixed atmosphere of fluorine compound gas and inert gas to form a glass body containing fluorine, as shown in FIG. This is carried out by passing through a zone furnace 41 equipped with a heating element 40 so that the temperature is high.

However, according to the results of studies conducted by the present inventors regarding the above-mentioned two types of conventional methods, the following was found. That is, when the glass fine particle laminate is heated at 1000° C. or lower in an atmosphere containing a fluorine compound gas with the configuration shown in FIG. 3 according to the first method, the glass fine particle laminate remains porous. Subsequently, when the glass fine particle laminate is heated in an inert gas atmosphere, it becomes transparent. When we measured the refractive index distribution of the optical fiber base material obtained in this way, we found that the refractive index in the peripheral part was higher than that in the central part, as shown in Figure 4. In other words, the fluorine content in the peripheral part was proportional to the amount of fluorine added in the central part. There was less compared to that. However, R shown in the figure is the outer diameter of the base material.

This is 1.00℃ in an atmosphere containing fluorine compound gas.
This is considered to be because the glass fine particle laminate is still in a porous state at the time when the fluorine addition treatment is completed and the fluorine once added is volatilized again by the high temperature treatment.

By the second method, the glass fine particle laminate could be made transparent by passing it through a zone furnace (Fig. 5) with an atmosphere consisting of fluorine compound gas and inert gas while heating it to 1400°C or higher. .

As shown in FIG. 6, the refractive index distribution of the obtained optical fiber base material had a low fluorine content in the center.
This is thought to be because the time in the zone furnace during which fluorine is added is substantially short because the glass particle laminate becomes transparent at the same time, and a sufficient amount of fluorine does not reach the center of the glass particle laminate. It will be done.
As an experiment, we reduced the moving speed of the glass particle stack in the furnace to less than half of the normal transparency.
Although fluorine was added uniformly to the center, it was found that the processing time increased significantly.

The present inventors proposed a method for eliminating the disadvantages of the conventional method for making a porous optical fiber base material transparent as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 55-67533. In No. 61-78379, as a method that can uniformly add fluorine to an optical fiber base material and make it transparent in a short time, after adding fluorine to the glass particle laminate at a temperature where the glass particle laminate is in a porous state, proposed that the laminate be made transparent by holding or passing it through a high-temperature furnace in a fluorine compound gas atmosphere.

<Problems to be Solved by the Invention> However, the method for manufacturing an optical fiber base material proposed in Japanese Patent Application No. 61-78379 involves the inclusion of a refractive index controlling substance in a transparent optical fiber base material. The analysis results focus on the quantity, and do not analyze the relationship between high temperature heating and the transparency of the porous laminate.

Therefore, the present inventors repeatedly investigated the relationship between the above-mentioned high-temperature heating of the glass particle laminate in a furnace and its transparency, and found that the glass particle laminate was heated in a zone where the heating zone was shorter than the effective length of the glass particle laminate. When passing through a heating furnace, it takes a long time to dehydrate and/or fluoride the entire glass particle laminate to make it transparent.

In addition, when heating is carried out in a uniform heating furnace where the heating zone is longer than the effective length of the glass particle laminate, the outer surface becomes highly transparent, but gas components remain inside the laminate and are not discharged outside the laminate. Air bubbles remained, making it difficult to obtain a uniformly transparent optical fiber base material.

The present invention was made in order to eliminate the drawbacks of the above-mentioned conventional method for making a porous optical fiber base material transparent, and it uses a medium glass fine particle laminate obtained by fire hydrolysis of a glass raw material. The present invention aims to provide a method for transparentizing a porous optical fiber base material, which can be quickly and uniformly transparentized to the inside in a high-temperature furnace.

Further, the present invention aims to provide an apparatus suitable for implementing such a method of making a porous optical fiber base material transparent.

<Means for Solving the Problems> In order to achieve the above object, the method for transparentizing a porous optical fiber base material of the present invention involves laminating medium glass fine particles produced by fire hydrolysis of a glass raw material. In a method for transparentizing a porous optical fiber preform, the porous optical fiber preform obtained by placing the obtained porous optical fiber preform into a high-temperature furnace and dehydrating it and/or adding a refractive index controlling substance to make it transparent. A plurality of zone heating bodies each having a heating zone shorter than the effective length of the base material are arranged in series from upstream to downstream on the insertion side of the porous optical fiber base material at a constant interval from each other, and these plurality A zone heating member is used which forms a uniform heating zone in which the total length of the heating region is at least approximately equal to the length of the porous optical fiber base material, and the porous optical fiber base material is inserted and held in the high temperature furnace. After uniform heating treatment at a temperature below the transparentization temperature for a certain period of time, the temperature of the zone heating element in the high-temperature furnace is individually controlled to transparentize the porous optical fiber base material from the top to the bottom or from the bottom to the top. It is characterized by moving a heating zone that is heated to a temperature higher than that temperature for a certain period of time.

In addition, in the apparatus for transparentizing a porous optical fiber base material of the present invention, a furnace core tube is inserted into the furnace body in the direction of the central axis, and the furnace core tube is circulated to create a heating zone shorter than the effective length of the porous optical fiber base material. A plurality of zone heating bodies having a plurality of zone heating bodies are sequentially arranged in series along the furnace tube from the upper end to the lower end of the insertion side of the porous optical fiber, and the heating area formed by the entire plurality of zone heating bodies is small. a high-temperature furnace that forms a uniform heating region with a length substantially equal to the effective length of the porous optical fiber base material; a support device that holds or moves in and out the porous optical fiber base material inserted into the furnace core tube; Connect to the reactor core tube and create a gas atmosphere containing a dehydrating agent or a gas atmosphere that increases or decreases the refractive index within the core tube.
A gas supply device that creates a gas atmosphere containing He as a main component; an exhaust device that is connected to the core tube and exhausts the gas in the core tube; and a heating temperature control device that freely controls the heating temperature of the zone heating body. It is characterized by:

<Function> As described above, the method for making a porous optical fiber base material transparent according to the present invention involves using a plurality of zone heating bodies as a high-temperature furnace whose heating zone is shorter than the effective length of the porous optical fiber base material to be made transparent. The zone heating elements are arranged in series from the upstream side to the downstream side of the porous optical fiber base material at a constant interval from each other, and the total length of the heating area formed by these multiple zone heating elements is small. In this high-temperature furnace, the plurality of porous optical fiber base materials placed in the furnace are After uniformly heating the whole body to a temperature below the transparentization temperature for a certain period of time using the zone heating element, the temperature of the zone heating element is individually controlled to move the porous optical fiber base material from the upper end to the lower end. As a result of sequential zone heating to a temperature equal to or higher than the transparentization temperature for a certain period of time, bubbles based on gas components remaining inside are sequentially discharged to the outside of the porous optical fiber base material.

Further, in the apparatus for making a porous optical fiber preform transparent according to the present invention, the furnace core tube is inserted into the furnace body in the central axis direction, and the outer periphery of the furnace core tube has a length shorter than the effective length of the porous optical fiber preform. A plurality of zone heating bodies each having a hollow heating zone are disposed sequentially from upstream to downstream at regular intervals along the furnace core tube, and the plurality of zone heating bodies collectively heat at least a porous optical fiber. A high temperature furnace that forms a uniform heating zone with a length approximately equal to the effective length of the base material; and a heating temperature control device that controls the temperature of each zone heating element is provided, so that the porous optical fiber base material can be heated. Even without moving, the porous optical fiber base material is heated from the upper end to the lower end in the same high-temperature furnace at a temperature below the clearing temperature for a certain period of time, and then at a temperature above the clearing temperature for a certain period of time. Alternatively, the heating zones can be sequentially moved in the opposite direction to carry out zone heating and transparentization treatment.

<Example> Next, a method for making a porous optical fiber transparent will be specifically described according to a typical example of the apparatus for making a porous optical fiber transparent according to the present invention.

FIG. 1 is a vertical cross-sectional view of the main part showing a schematic configuration of a porous optical fiber transparentizing device, and 16 is a furnace core tube that penetrates the center of a vertical furnace body 17. Heaters 18, 19, and 20 are arranged around the outer circumference of the furnace core tube 16 in series from upstream to downstream along the furnace core tube to form a heating zone within the furnace core tube. Heater 1
8, 19, 20 are provided with temperature sensors 21, 22, 23 near their respective installation positions, and the temperatures indicated by these sensors are transmitted to heating temperature control devices 24, 25,
26, the temperature is controlled to a preset temperature.

The core tube 16 has a porous optical fiber base material 2 produced and laminated by, for example, a vapor phase axial attachment method (hereinafter referred to as "VAD method") on a seed rod 15 that is rotatably supported within the core tube by a support device 14. is installed.

Furthermore, a gas supply device 9 is provided in the core tube 16.
It is configured so that the type and flow rate of the gas to be fed can be adjusted and fed from the furnace core tube 1.
An exhaust device 10 is connected to exhaust the gas inside 6.

The heating temperature control devices 24, 25, and 26 are controlled so that the temperature values detected by the temperature sensors 21, 22, and 23 are all the same, and the temperature of the furnace 17 when the heaters 18, 19, and 20 are operated is controlled. The vertical length of the uniform heating region (for example, a temperature range of about ±25° C.) needs to be longer than the longest effective length of the porous optical fiber in which this device is used.

The dehydration, fluoridation, and transparency treatment of a porous optical fiber base material using the apparatus of this example is carried out as follows.

First, the furnace core tube 16 is preheated as shown in FIG. 2 so that the temperatures indicated by the temperature sensors 21, 22, and 23 are all T ₀ , and the porous optical fiber 2 to be processed is heated by the support device. Insert the attached porous optical fiber into the reactor core tube and rotate it.

Next, the temperature is raised so that the temperatures indicated by the temperature sensors 21, 22, and 23 are all the dehydration temperature T ₁ (lower than the transparentization temperature), and the gas atmosphere in the core tube is changed to a mixture of He and chlorine gas as a dehydration agent. Create a mixed gas atmosphere. When the dehydration is completed under these conditions, the temperatures indicated by the temperature sensors 21, 22, and 23 all rise to the fluorine addition temperature _T2 , and the gas atmosphere in the core tube is changed to a He/fluorine mixed gas atmosphere.

When fluorine addition is completed under these conditions, the temperatures indicated by the upstream and central temperature sensors 21 and 22 are set to _T4 , and the heater is turned on so that the temperature of the most downstream temperature sensor 24 becomes the transparent temperature _T3 . The heating temperature of step 20 is gradually raised.

In this example, as shown in Figure 2, the temperature
Although T ₄ is lower than the temperature at which fluorine is added, T ₂ , this is not necessarily a requirement. When the temperature of the sensor 23 reaches _T3 , the heating temperature of the heater 9 is gradually increased so that the temperature of the sensor 22 reaches _T2 .
When the temperature of the sensor 22 also reaches _T2 , the heating temperature of the heater 18 is gradually increased so that the temperature of the sensor 21 reaches _T3 . Further, the heater 20 is turned off and cooled down until the temperature sensor 23 reaches the standby temperature T ₀ . Then, the temperature indicated by the sensor 21 is
When T ₃ is reached, turn off the power to the heater 19,
The temperature is lowered until the temperature indicated by the sensor 22 reaches the standby temperature T ₀ .

In this state, when the porous optical fiber 6 is completely sintered and made transparent, the power to the heater 18 is turned off and the temperature of the temperature sensor 21 is also lowered to the standby temperature T ₀ . In addition, the gas atmosphere inside the reactor core tube was changed to N ₂
Create an inert gas atmosphere such as

When the core tube gas atmosphere replacement is completed, the transparent optical fiber base material can be removed from the support device even if the values of the temperature sensors 21, 22, 23 near all the heaters have not fallen to _T0 . .
The above time versus heat treatment process results in a time table as shown in FIG.

The above embodiments have described the steps of dehydrating, adding fluorine, and sintering the porous optical fiber base material to make it transparent. In some cases, addition of an increasing refractive index controlling substance or dehydration treatment may not be necessary. In that case, the process is performed excluding the refractive index addition or dehydration process in the process described in this example.

Medium silica fine particles produced by fire hydrolysis of SiO ₂ are stacked and deposited on a seed rod with a length of 750 mm.
The porous optical fiber base material was preheated to a temperature of 800° (=
T ₀ ), dehydration temperature (=T ₁ ) 1070℃, fluorine addition temperature (=T ₂ ) 1290℃, sintering transparency temperature (=T ₃ ) 1550
℃, standby temperature before sintering/clarification (= T ₄ ) under heating conditions of 1200℃, dehydration treatment for 1 hour, fluorine addition time
Dehydration and fluorine addition were completed in a total of 6.2 hours, including 2.4 hours, 1.2 hours for sintering/clarification, temperature raising/cooling, and gas replacement.

Moreover, no air bubbles were observed in the optical fiber base material obtained.

On the other hand, a porous optical fiber base material of the same size, in which medium silica obtained by fire hydrolysis of SiO ₂ was deposited in layers using the same method, was dehydrated, fluoridated, sintered, and Dehydration and fluorine addition must be completed for a total of 10.0 hours, including 1.6 hours of dehydration, 5.3 hours of fluorine addition, 1.6 hours of sintering/clarification treatment, temperature raising/lowering, and gas replacement operations under the same temperature conditions. However, the optical fiber base material could not be obtained.

In addition, when a porous optical fiber base material of the same size prepared by the same method was treated with dehydration, fluoridation, and sintering to make it transparent using a conventional homogeneous furnace,
A certain amount of air bubbles inevitably remained, making it impossible to produce a good optical fiber base material at a good yield.

<Effects of the Invention> As is clear from the above explanation, according to the method for making a porous optical fiber base material transparent according to the present invention,
Compared to conventional methods for making porous optical fiber base materials transparent, it is possible to reliably obtain an optical fiber that is uniformly transparent throughout the interior in an extremely short time.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the main parts showing the schematic structure of the apparatus for transparentizing a porous optical fiber base material of the present invention, and FIG. A timetable showing the relationship between time and treatment temperature for dehydration, fluoridation, and sintering to make it transparent; Figure 3 is a perspective view showing a conventional method for making a porous optical fiber base material transparent using a soaking furnace; Figure 4 The figure is the third
Fig. 5 is a radial refractive index distribution diagram of an optical fiber preform made transparent by a soaking furnace; Fig. 5 is a perspective view showing a conventional method for making a porous optical fiber preform transparent using a zone furnace; FIG. 6 is a radial refractive index distribution diagram of the optical fiber base material made transparent by the zone furnace shown in FIG. In the figure, 2... porous optical fiber base material, 14...
Supporting device for the porous optical fiber base material transparentization device of the present invention, 15... Seed rod, 16... Furnace tube, 1
7... Furnace body, 18, 19, 20... Heater,
21, 22, 23...Temperature sensor, 24, 25,
26...Heating temperature control device for controlling the heating temperature of the heater, 31...Soaking furnace used for making the conventional porous optical fiber base material transparent, 40...Soaking furnace used for making the conventional porous optical fiber base material transparent Heater of soaking furnace used for transparentization, 41...Zone furnace used for conventional transparentization of porous optical fiber base material.

Claims (1)

[Claims] 1. A porous optical fiber base material obtained by laminating medium glass fine particles produced by fire hydrolysis of a glass raw material is placed in a high temperature furnace, and dehydrated and/or a refractive index controlling substance is added. In a method for transparentizing a porous optical fiber base material, a plurality of zone heating bodies each having a heating zone shorter than the effective length of the porous optical fiber base material are used as a high temperature furnace. A device which is arranged in series from upstream to downstream on the insertion side and forms a uniform heating zone in which the total length of the heating area formed by the plurality of zone heating bodies is at least approximately equal to the length of the porous optical fiber base material. At the same time, the porous optical fiber base material is inserted and held in this high-temperature furnace, and after being uniformly heated to a temperature below the transparentization temperature for a certain period of time, the zone heating body in the high-temperature furnace is operated to remove the porous optical fiber base material. 1. A method for transparentizing a porous optical fiber base material, which comprises moving a heating zone that heats the material above the transparentization temperature for a certain period of time from the upper end to the lower end or from the lower end to the upper end. 2 Uniform heat treatment of the above porous optical fiber base material at a temperature below the transparentization temperature in a high temperature furnace in a gas atmosphere containing a dehydrating agent; The porous optical fiber base material according to claim 1, wherein the zone heating treatment in which the heating zone is moved for a certain period of time at a temperature higher than the oxidation temperature is carried out in an atmosphere containing He as a main component. Transparency method. 3. The uniform heat treatment of the porous optical fiber base material in a high-temperature furnace at a temperature below the transparentization temperature is performed in a high-temperature furnace in a gas atmosphere containing a refractive index controlling substance that increases or decreases the refractive index of the optical fiber base material. The zone heating treatment, in which the porous optical fiber base material is moved from the upper end to the lower end or from the lower end to the upper end through a heating zone that is heated for a certain period of time at a temperature higher than the transparentization temperature, must be performed in a gas atmosphere containing He as the main component. A method for making a porous optical fiber base material transparent according to claim 1, characterized in that: 4 A furnace core tube penetrates the furnace body in the direction of the central axis, and a plurality of zone heating bodies that go around the furnace core tube and have a heating zone shorter than the effective length of the porous optical fiber base material are spaced at regular intervals along the furnace core tube. The porous optical fibers are sequentially arranged in series from the upper end to the lower end on the insertion side, and the heating area formed by the entire plurality of zone heating bodies is at least as long as the effective length of the porous optical fiber base material. A high-temperature furnace that forms a uniform heating area of approximately equal length; a support device that holds a porous optical fiber base material inserted into the reactor core tube or supports it so that it can be freely moved in and out; a gas supply device that creates a material-containing gas atmosphere, a refractive index increasing or decreasing gas atmosphere, or a gas atmosphere containing He as a main component; an exhaust device that is connected to the reactor core tube and exhausts the gas in the reactor core tube; and a heating temperature of the zone heating element. 1. A device for transparentizing a porous optical fiber base material, comprising a heating temperature control device for freely controlling the heating temperature.