JP2004210598A - Apparatus and method for producing optical fiber preform, and optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for producing an optical fiber preform which apparatus can prevent an inert gas from remaining in the inside during the vitrification of an optical fiber core preform, thus preventing the increase in transmission loss of an optical fiber; to provide a production method using the apparatus; and to provide an optical fiber. <P>SOLUTION: In order to prevent an inert gas from coming into an optical fiber preform during its transparent vitrification, when a core preform is produced by dehydrating/sintering a porous preform for an optical fiber core, the inert gas concentration in the area above the location position of a heating means 11 in a furnace tube 2 for dehydrating/sintering the porous preform is controlled to 10% or lower. The inert gas comes into the furnace tube 2 through minute gaps between a penetration hole 4a of an inside seal lid 4 and a supporting rod 8. Thus, the inert gas is prevented from coming into the inside of an optical fiber, reducing the transmission loss of the optical fiber. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a manufacturing apparatus and a manufacturing method for a silica-based optical fiber preform mainly used for optical communication, and an optical fiber manufactured by the manufacturing apparatus.
[0002]
[Background]
The optical fiber is formed by forming a clad on the outer peripheral surface of the core, and can be manufactured as follows. For example, first, a core preform serving as a core of an optical fiber is manufactured using a VAD method (Vapor-phase Axial Deposition). In the VAD method, for example, oxygen gas and hydrogen gas are supplied from a burner to create an oxyhydrogen flame, and GeCl is contained in the oxyhydrogen flame. ₄ And SiCl ₄ To produce glass particles. The glass particles are deposited and grown on a seed rod, and GeO ₂ And SiO ₂ A base material made of a rod-like porous body made of And the base material which consists of this porous body is spin-dry | dehydrated and sintered, and the core base material made into transparent glass is produced.
[0003]
After that, for example, an OVD method (Outer Vapor Deposition (external method)) is used to form a porous cladding original layer that becomes a cladding of the optical fiber on the outer peripheral surface of the core preform. Thereafter, the clad original layer is dehydrated and sintered to form a transparent glass. In this way, an optical fiber preform that is the base of the optical fiber is manufactured. An optical fiber can be manufactured by drawing this optical fiber preform.
In the specification of the present application, a base material formed of a porous base material that is a precursor of a core base material and a base material on which a porous cladding base layer that is a precursor of an optical fiber base material is formed is collectively referred to as porous. I will call it the base material.
[0004]
FIG. 3 is a schematic sectional view showing an example of the structure of an apparatus for producing an optical fiber preform by dehydrating and sintering a porous preform. The apparatus 1 for sintering the porous base material has a furnace core tube 2. The upper end portion of the core tube 2 is doubly closed by an outside air seal lid 3 and an inner seal lid 4. The outside air seal lid 3 and the inside seal lid 4 are arranged with a space therebetween, and a gap between the outside air seal lid 3 and the inside seal lid 4 is a seal chamber 5. A gas supply port 6 and an exhaust port 7 are formed in the wall portion of the seal chamber 5. Argon, for example, enters the seal chamber 5 from the gas supply unit (not shown in FIG. 3) through the supply port 6. An inert gas such as helium or nitrogen gas is supplied, and the gas in the seal chamber 5 is exhausted from the exhaust port 7 by, for example, an exhaust fan 9. The sealing gas used here is often nitrogen gas in terms of cost.
[0005]
The outside air seal lid 3 and the inner seal lid 4 are formed with through holes 3a and 4a through which the support rods 8 are inserted, respectively. The support bar 8 supports the porous base material 10 which is an optical fiber base material. The support bar 8 is connected to a rotating means (not shown). The rotating means has a configuration for rotating the porous base material 10 around the central axis of the porous base material 10 as a rotation axis.
[0006]
A heating means 11 for heating the porous base material 10 is provided on the side of the core tube 2. A gas supply port 12 is formed at the lower end of the core tube 2, and chlorine gas and helium gas are supplied from the gas supply port 12 to the inside of the core tube 2 as atmospheric gases. Further, a gas exhaust port 13 is formed at a position slightly below the inner seal lid 4 on the upper side of the core tube 2, and the atmosphere gas in the core tube 2 is caused to flow by an exhaust fan (not shown), for example. The exhaust air is exhausted from the exhaust port 13.
[0007]
In such an apparatus 1, when the porous base material 10 is dehydrated / sintered, for example, first, the porous base material 10 supported by the support rod 8 is placed more than the heating means 11 inside the core tube 2. Place in the upper position. Thereafter, the porous base material 10 is gradually lowered while rotating the porous base material 10. Thereby, the porous base material 10 is sequentially heated by the heating means 11 from the lower end side toward the upper end side. At this time, chlorine gas and helium gas are supplied into the core tube 2 from the gas supply port 12, and the porous base material 10 is moved from the lower end side by the heating of the chlorine gas, helium gas and heating means. It dehydrates and sinters.
[0008]
In this way, the porous preform 10 can be sintered by the apparatus 1 to produce a vitrified optical fiber preform.
[0009]
On the other hand, it is necessary to prevent the chlorine gas inside the furnace tube 2 from flowing out to the outside because it is very dangerous because it leads to contamination in the factory due to leakage of highly toxic gas and deterioration of the working environment. In addition, when the air flows into the core tube, the atmosphere inside the core tube is contaminated, and the optical fiber preform being dehydrated and sintered is contaminated by impurities such as transition metals in the atmosphere, resulting in fiber transmission loss. This also needs to be prevented because it gets worse. However, in this apparatus 1, since the support bar 8 moves up and down while rotating, the gap between each support hole 8 and the through holes 3a, 4a of the outside air seal lid 3 and the inner seal lid 4 is completely closed. I can't. For this reason, it is common to set the pressure of an inert gas such as nitrogen gas flowing in the seal chamber 5 to be higher than the atmospheric pressure to prevent the above-described gas leakage and mixing. is there.
[0010]
In many cases, the core tube 2 is made of quartz glass. In this case, the core tube 2 is softened by a high temperature when the porous base material 10 is sintered. Therefore, the internal pressure P of the core tube 2 ₂ Outside atmospheric pressure P ₀ Higher than (P ₂ > P ₀ ), The softened core tube 2 has an external atmospheric pressure P ₀ To prevent it from being crushed. Therefore, the gas pressure in the seal chamber is P ₁ If so, the relationship between these three pressures is P ₂ > P ₁ > P ₀ It becomes.
[0011]
Further, when the heating means 11 is a heater made of, for example, carbon, the heater is housed inside the housing and is cut off from the atmosphere to prevent combustion. For this reason, an inert gas is always supplied into the housing of the heating means 11 from the pipe 11a. In order to prevent the pressure in the housing from becoming higher than the internal pressure of the core tube 2, the pressure P in the housing of the heating means 11 is used. ₃ Is the internal pressure P of the core tube 2 ₂ Pressure P in the housing in order to prevent the outside air from entering the housing of the heating means 11. ₃ To be higher than atmospheric pressure (P ₂ > P ₃ > P ₀ Controlled).
[0012]
2. Description of the Related Art Conventionally, as an optical fiber preform manufacturing apparatus and manufacturing method that satisfies such demands, an invention that has examined an inert gas supply method has been proposed (see, for example, Patent Document 1).
[0013]
[Patent Document 1]
JP 2001-64032 A
[0014]
[Problems to be solved by the invention]
By the way, in silica-based optical fibers, reduction of transmission loss has been one of the most important development issues so far, but in recent silica-based optical fibers, particularly single-mode optical fibers, transmission loss is close to the theoretical limit. There is a feeling of reaching the level. Typical values for the wavelength band and transmission loss used for transmission are 0.34 dB / km at a wavelength of 1.31 μm and 0 at a wavelength of 1.55 μm for a 1.3 μm zero-dispersion single mode fiber (hereinafter abbreviated as SMF). In the case of a dispersion shifted optical fiber (hereinafter abbreviated as DSF), it is about 0.37 dB / km at a wavelength of 1.31 μm and about 0.21 dB / km at a wavelength of 1.55 μm.
[0015]
However, the problem of further reducing transmission loss still remains at the present time. One of the reasons is that, by performing WDM (Wavelength Division Multiplexing) transmission, the power of the total laser light incident on the optical fiber increases, a nonlinear phenomenon occurs, and the transmission is inhibited. This is because it is advantageous to reduce the power of incident light from each laser as much as possible. Obviously, if the transmission loss is small, transmission can be performed with a smaller incident optical power.
[0016]
Another reason is the spread of the wavelength band used for transmission. The conventional system mainly uses the 1.3 μm band and the 1.55 μm band. However, in the transmission system using the WDM transmission, the 1.55 μm band is divided into 1530 to 1565 nm (1.530 to 1.30 nm). 565 μm) is called the C-Band band, and 1570 to 1625 nm (1.570 to 1.625 μm) is called the L-Band band, and transmission is carried out at the respective wavelengths. Furthermore, a system that performs transmission also in the vicinity of 1450 nm (1.450 μm) in the S-Band band has become realistic due to the practical application of the Raman amplifier system.
[0017]
In this way, it is now meaningful to further reduce the transmission loss of optical fibers, with an eye toward broadband transmission bands and the practical application of Raman amplifier systems, but improvements have been made to near theoretical limits. A method for further improving the transmission loss of an advanced optical fiber has not been easily found.
[0018]
Therefore, in order to achieve the above-described problem of further reducing the transmission loss of the optical fiber, the present inventors, as a factor that deteriorates the transmission loss of the optical fiber, have made the glass constituting the optical fiber, particularly the MFD in which light propagates. Focusing on the inert gas contained in the glass in (Mode Field Diameter; mode field diameter), the present invention has been made.
[0019]
That is, the present invention relates to a manufacturing apparatus, a manufacturing method, and an optical fiber obtained as a result, in a quartz-based single mode optical fiber, so that at least a portion in the MFD through which light propagates does not substantially contain an inert gas. The purpose is to provide.
[0020]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a method in which the inert gas is converted into a core tube when the optical fiber preform is vitrified by setting the pressure of the inert gas in the seal chamber lower than the pressure in the core tube. It is characterized by not entering inside.
This will be specifically described below.
[0021]
1st invention WHEREIN: After forming the porous preform | base_material which is a precursor of an optical fiber preform | base_material, the porous preform | base_material is internalized in the apparatus which sinters the porous preform | base_material and produces an optical fiber preform | base_material. A core tube for heating the porous base material, the upper portion of the core tube being an outside air seal lid, and an inside seal lid arranged inside the outside air seal lid with a space therebetween In addition, a gas ventilation means for flowing an inert gas is provided in the seal chamber formed between the outside air seal lid and the inner seal lid so that the inside of the seal chamber is more than the atmospheric pressure outside. Controlled to a low pressure to prevent the atmospheric gas inside the reactor core tube from leaking to the outside due to the pressure difference with the outside, and the outside air seal lid and the inside seal lid have coaxial through holes, respectively. Formed and supported by support rods inserted through these through-holes The porous base material is disposed inside the core tube, and the porous base material is provided on the side of the core tube while rotating the porous base material around the central axis of the porous base material. The heating means is relatively moved in the vertical direction to heat and sinter the porous base material, and the inert gas flowing in the seal chamber or the atmosphere entering the seal chamber from the outside From the upper position of the heating means in the core tube by preventing the inert gas from entering the inside of the core tube through the gap between the through hole of the inner seal lid and the support rod inserted through the through hole. Is also provided with an inert gas concentration suppressing means for suppressing the upper nitrogen gas concentration to 10% or less.
[0022]
The second invention is characterized in that, in the above invention, the inert gas concentration suppressing means is constituted by a core tube internal pressure control means for controlling the inside of the core tube to a pressure set higher than the atmospheric pressure. .
[0023]
The third invention is characterized in that, in the above invention, the pressure in the furnace core tube is 50 to 150 Pa, the pressure fluctuation range is 50 Pa or less, and the pressure in the seal chamber is lower than the atmospheric pressure.
[0024]
In a fourth aspect of the present invention, in the above invention, the pressure in the core tube is set to 100 to 150 Pa, the pressure fluctuation range is set to 50 Pa or less, and the pressure in the seal chamber is set to 40 Pa or less so that the gas in the seal chamber does not flow into the core tube. It is characterized by that.
[0025]
According to a fifth invention, in the above invention, oxygen gas is flowed into the seal chamber in place of the inert gas to measure the oxygen gas concentration above the position of the heating means in the core tube, and based on the measured value The nitrogen gas concentration is estimated, and the estimated pressure is used to determine the pressure inside the core tube to suppress the inert gas concentration below 10% from the position of the heating means in the core tube. The reactor core tube internal pressure control means stabilizes the pressure inside the reactor core tube by controlling the interior pressure of the reactor core tube to the set pressure.
[0026]
A sixth invention is a method of manufacturing an optical fiber preform by forming a porous preform serving as a precursor of an optical fiber and then sintering the porous preform. Dosing double and flowing an inert gas into the seal chamber formed between the double plugs to control the inside of the seal chamber to a pressure lower than the external atmospheric pressure. Arranged by a support rod inserted through the through-beam formed coaxially in the vertical direction of the seal chamber in the reactor core tube that prevents atmospheric gas from leaking to the outside, and rotates the central axis of the porous base material When the porous preform and the heating means provided on the side of the core tube move relatively in the vertical direction while rotating the porous preform as an axis, the porous preform is heated and sintered. , Inert gas flowing in the seal chamber, or entering the seal chamber from the outside The pressure control inside the core tube allows inert gas in the atmosphere to enter the inside of the core tube through the gap between the through hole inside the core tube of the seal chamber and the support rod inserted into the through hole. Therefore, the inert gas concentration above the upper position of the heating means in the core tube is suppressed to 10% or less.
[0027]
A seventh invention is characterized in that, in the above-mentioned invention, the inert gas concentration suppressing means is constituted by a core tube internal pressure control means for controlling the interior of the core tube to a pressure set higher than the atmospheric pressure. .
[0028]
The eighth invention is characterized in that, in the above invention, the pressure in the reactor core tube is 50 to 150 Pa, the pressure fluctuation range is 50 Pa or less, and the pressure in the seal chamber is lower than the atmospheric pressure.
[0029]
According to a ninth aspect of the present invention, in the above invention, the pressure in the core tube is set to 100 to 150 Pa, the pressure fluctuation range is set to 50 Pa or less, and the pressure in the seal chamber is set to 40 Pa or less so that the gas in the seal chamber does not flow into the core tube. It is characterized by that.
[0030]
According to a tenth aspect of the present invention, in the above invention, oxygen gas is allowed to flow in the seal chamber instead of the inert gas to measure the oxygen gas concentration above the position of the heating means in the core tube, and based on the measured value The nitrogen gas concentration is estimated, and the estimated pressure is used to determine the pressure inside the core tube to suppress the inert gas concentration below 10% from the position of the heating means in the core tube. The reactor core tube internal pressure control means stabilizes the pressure inside the reactor core tube by controlling the interior pressure of the reactor core tube to the set pressure.
[0031]
An eleventh invention is an optical fiber manufactured by the above manufacturing method.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments according to the present invention will be described below with reference to the drawings. In the description of this embodiment, the same components as those of the apparatus 1 shown in FIG. 3 are denoted by the same reference numerals, and duplicate descriptions of the common portions are omitted.
[0033]
The inventor has paid attention to nitrogen gas which is often used as an inert gas flowing into the seal chamber and also exists in the atmosphere. Therefore, first, a base material composed of a porous body including a portion in the MFD synthesized using the VAD method is dehydrated and sintered in an atmosphere containing nitrogen gas to form a transparent glass, and nitrogen molecules or An experiment was conducted in which a core base material in which nitrogen atoms were dissolved was manufactured, and then an optical fiber base material obtained by forming a clad base layer, dehydrating and sintering, and vitrifying it into a fiber, and comparing the transmission loss. . Here, experimental results are shown for a general SMF (Single Mode Fiber) having a zero dispersion wavelength in the 1.3 μm wavelength band and adding germanium for improving the refractive index of glass to the MFD portion.
[0034]
Specifically, the amount of nitrogen gas contained in the atmosphere in which a base material made of a porous body manufactured by the VAD method is dehydrated and sintered to form a transparent glass is changed to seven levels, The resulting preform was made into a transparent glass, and then a cladding layer was further provided to obtain an optical fiber preform. Next, this optical fiber preform was drawn to obtain seven types of optical fibers for evaluation.
[0035]
Next, the transmission loss of these seven types of optical fibers was measured, and the relationship with the concentration of nitrogen gas contained in the atmosphere during transparent vitrification was investigated. The result is shown by ■ in FIG. In this way, there is a correlation between the transmission loss and the amount of nitrogen gas contained in the atmosphere in which the porous base material is made into a transparent glass, and the case where the nitrogen gas concentration is manufactured under low atmospheric conditions. It was found that the transmission loss was low. Further, it was also found that the transmission loss at a wavelength of 1310 nm (1.310 μm) can provide good characteristics of 0.34 dB / km or less when the nitrogen gas concentration in the atmosphere is 10% or less.
[0036]
By the way, according to the previous manufacturing method, when manufacturing a base material made of a porous body that is a precursor of a core base material, a base material made of a porous body or a base material made of a porous body is used. An inert gas is not used as the atmospheric gas in the furnace to be manufactured. For this reason, normally, the inert gas should not be contained in the core base material obtained by forming a porous base material into a transparent glass. However, in reality, nitrogen gas in the atmosphere enters the base material made of the porous body while being left in the atmosphere for a long time, and the nitrogen gas remains in the base material made of the porous body as it is. Can be considered. Alternatively, when dehydration / sintering of a base material made of a porous body is performed using the apparatus 1 shown in FIG. 3 described above, the atmospheric gas rapidly increases at a high temperature during the dehydration / sintering process. When the internal pressure of the core tube 2 fluctuates greatly and falls to the lowest limit, for example, as shown in FIG. ₁ Is the internal pressure P of the core tube 2 ₂ It is considered that nitrogen gas, which is the atmospheric gas in the seal chamber 5, enters into the core tube 2 and is contained in the core base material.
[0037]
In the apparatus 1 shown in FIG. 3, as described above, the internal pressure P of the core tube 2 ₂ Is the atmospheric pressure P ₀ (In other words, the internal pressure P of the seal chamber 5) ₁ The operating conditions of the apparatus 1 (for example, the pressure of the gas supplied into the core tube 2 from the gas supply port 12, the rotational speed of the exhaust fan for exhausting the core tube 2) The pressure of the inert gas supplied to the seal chamber 5 and the rotational speed of the exhaust fan 9 for exhausting the seal chamber 5 are set.
[0038]
However, in the conventional driving method of the apparatus 1, when the gas flows in the high temperature core tube 2, the internal pressure P of the core tube 2 ₂ Greatly fluctuates (the pressure fluctuation range is 300 to 500 Pa), and the internal pressure P of the core tube 2 ₂ Is the internal pressure P of the seal chamber 5 ₁ It might be considerably lower than. For this reason, the inert gas flowing through the seal chamber 5 and the nitrogen gas in the atmosphere that has entered the seal chamber 5 from the outside may enter the core tube 2 from the seal chamber 5. It is considered that this inert gas such as nitrogen gas enters the inside of the porous base material 10 during dehydration and sintering and is contained in the core base material.
[0039]
The inventor examined the relationship between the nitrogen gas concentration in the atmosphere gas above the position of the heating means 11 in the furnace core tube 2 and the increase in the transmission loss of the optical fiber by experiments. The experiment was conducted as follows. First, the apparatus 1 is driven in a state as shown in FIG. That is, instead of the support rod 8, a hollow glass rod 14 having the same outer diameter as that of the support rod 8 is inserted into the through holes 3 a and 4 a of the outside air seal lid 3 and the inner seal lid 4. And the front-end | tip part of the glass rod 14 is arrange | positioned in the position a little above the arrangement | positioning area | region of the heating means 11. FIG. The reason why the tip of the glass rod 14 is arranged in this way is that the heating means 11 heats the porous base material 10 to such a high temperature that it becomes transparent glass. This is because it is not preferable in terms of heat resistance of the glass rod 14 if it is arranged in the arrangement region.
[0040]
Inside the glass rod 14, a gas introduction pipe 16 for guiding the atmospheric gas inside the furnace core tube 2 to the oxygen measuring device 15 is arranged.
[0041]
In this experiment, it is necessary to detect the concentration of the atmospheric gas in the core tube 2, here, the nitrogen gas concentration, but the nitrogen gas concentration in the core tube 2 cannot be measured directly. For this reason, in this experiment, the oxygen gas concentration above the position where the heating means 11 in the furnace core tube 2 was positioned was measured by the oxygen measuring device 15, and the nitrogen gas concentration was estimated and detected based on this measured value. When measuring the oxygen concentration in the furnace core tube 2, oxygen gas was allowed to flow into the seal chamber 5 instead of nitrogen gas.
[0042]
In this way, the operating conditions of the apparatus 1 are changed variously while estimating and detecting the nitrogen gas concentration in the furnace core tube 2. And the operating condition of the apparatus 1 for the nitrogen gas density | concentration above the heating means 11 in the furnace core tube 2 to become a predetermined density | concentration is calculated | required. Thereafter, the glass rod 14 is removed, and the base material 10 made of a porous body is disposed inside the core tube 2 as described above, and the apparatus 1 is driven under the obtained operating conditions to form the porous body. The base material 10 is sintered. Note that nitrogen gas was allowed to flow into the seal chamber 5 when the base material 10 made of a porous body was dehydrated and sintered.
[0043]
In this way, a plurality of types of core preforms produced in the core tube 2 with different nitrogen gas concentrations are obtained, and an optical fiber preform is produced using these core preforms to produce a plurality of optical fibers. did. Thereafter, a test for examining the transmission loss of the optical fiber at a wavelength of 1.31 μm was performed on each of the manufactured optical fibers.
[0044]
The result is also shown by a circle in FIG. This result is in good agreement with the experimental results in which the amount of nitrogen gas was changed to 7 levels in an atmosphere in which the porous base material was dehydrated and sintered in the furnace core tube 2 to form a transparent glass.
[0045]
As a result of this experiment, the core base material 10 is obtained by dehydrating and sintering the base material 10 made of a porous body in a state where the nitrogen gas concentration in the atmosphere gas above the heating means 11 in the core tube 2 is 10% or less. It has been found that an optical fiber satisfying optical fiber characteristics such as transmission loss can be manufactured by manufacturing the optical fiber. Since the nitrogen gas (nitrogen) concentration in the MFD of the optical fiber should be as low as possible, the nitrogen gas concentration above the heating means 11 in the core tube 2 is more preferably 1% or less.
[0046]
From the above, in this embodiment, the apparatus 1 is provided with nitrogen gas concentration suppression means for suppressing the nitrogen gas concentration above the heating means 11 in the core tube 2 to 10% or less. . Various configurations of the nitrogen gas concentration suppressing means are conceivable.
[0047]
For example, as shown in FIG. 1, a gas supply unit 18 constituting a gas ventilation unit is connected to the seal chamber 5 via a gas supply pipe 17. The gas supply unit 18 is configured to supply nitrogen gas to the seal chamber 5.
[0048]
Further, the internal pressure P of the core tube 2 can be controlled by accurately controlling the pressure in the core tube 2. ₂ The internal pressure P of the seal chamber 5 ₁ Always higher. This pressure difference may reliably prevent nitrogen gas from entering the core tube 2 from the seal chamber 5.
[0049]
For example, as shown in FIG. 1, the pressure in the exhaust passage 19 is set in the passage 19 for exhausting the atmospheric gas in the core tube 2. Pressure detecting means 22 for detecting the pressure is provided. In addition, a reactor core internal pressure control unit 21 is provided in the control device 20 of the device 1. The core tube internal pressure control unit 21 takes in, for example, the detected value of the pressure in the core tube 2 from the pressure detection means 22 every moment. The reactor core tube internal pressure control unit 21 then determines the reactor core tube 2 internal pressure P based on the detected pressure value. ₂ Is the atmospheric pressure P ₀ For example, the flow rate variable solenoid valve 23 in the exhaust passage 19 is controlled to maintain a higher pressure. As another means, the pressure in the core tube 2 may be controlled by controlling the rotation amount of an exhaust fan (not shown) in the exhaust passage 19 and varying the flow rate of the exhaust gas. As another means, as shown in FIG. 5, the pressure in the core tube 2 can be controlled by inserting atmospheric air into the exhaust pipe from the outside by means of the insertion blower 30. In this case, the rotation speed control of the blower 30 is performed by the pressure detection means 22, the controller 21, and the blower driver 21a. In this way, the pressure in the core tube is changed to the atmospheric pressure P. ₀ The pressure fluctuation width in the furnace core tube can be stabilized within 50 Pa at a pressure of +50 Pa to 150 Pa.
[0050]
Further, in the same manner as the pressure control in the furnace core tube 2, as shown in FIG. ₁ Is the atmospheric pressure P ₀ It is preferable to control or adjust the pressure to a lower setting. Also in this case, for example, a pressure detection means 25 for detecting the pressure in the seal chamber 5 is provided, and a seal chamber pressure control unit 26 is provided in the control device 20. This seal chamber pressure control unit 26 is based on the internal pressure of the seal chamber 5 detected by the pressure detection means 25 and maintains the internal pressure of the seal chamber 5 at a set pressure. The variable solenoid valve 27 and an exhaust fan (not shown) are driven and controlled. When continuous control is not performed, measures such as adjusting the opening degree of the electromagnetic valve 27 for variable flow in advance are taken. In this way, the internal pressure P of the seal chamber 5 ₁ Is controlled or adjusted to about −50 Pa below atmospheric pressure. An example of the execution data of this control is shown in FIG. Atmospheric pressure P in FIG. ₀ Is 0 Pa and the pressure in the reactor core P ₂ And the internal pressure P of the seal chamber 5 ₁ Is shown. Furnace core pressure P ₂ Pressure fluctuation is 40 Pa, and the pressure in the reactor core P ₂ Is the pressure in the seal chamber P ₁ Therefore, nitrogen in the seal chamber no longer enters the core tube 2.
[0051]
In addition, an appropriate internal pressure P of the core tube 2 for making the nitrogen gas concentration above the heating means 11 in the core tube 2 10% or less. ₂ And the internal pressure P of the seal chamber 5 ₁ For example, as described above, while detecting the oxygen concentration in the core tube 2 as the nitrogen gas concentration, the operating condition of the apparatus 1 is changed to change the internal pressure P of the core tube 2. ₂ And the internal pressure P of the seal chamber 5 ₁ An appropriate value can be obtained by variously changing. Of course, an appropriate internal pressure P of the core tube 2 for setting the nitrogen gas concentration above the heating means 11 in the core tube 2 to 10% or less. ₂ And the internal pressure P of the seal chamber 5 ₁ May be obtained by simulation.
[0052]
As described above, the internal pressure P of the core tube 2 ₂ And the internal pressure P of the seal chamber 5 ₁ A means for accurately controlling the pressure, a means for controlling the pressure by providing a nitrogen gas concentration suppressing means, and a means for suppressing the nitrogen gas concentration comprising a means for flowing the nitrogen gas into the seal chamber 5 are provided. Thus, the nitrogen gas concentration above the heating means 11 in the core tube 2 can be suppressed to 10% or less.
[0053]
In addition, if the clearance of each through-hole 3a, 4a of the outside air seal lid 3 and the inner seal lid 4 is 0.5 mm or less, preferably 0.3 mm or less, the inert gas can enter the core tube 2 more reliably. Can be prevented.
[0054]
Therefore, by providing such a nitrogen gas concentration suppressing means, the nitrogen gas concentration above the heating means 11 in the furnace core tube 2 can be suppressed to 10% or less.
[0055]
A core preform is manufactured using the apparatus 1 having the above-described configuration, and an optical fiber based on the core preform is produced, thereby providing a high-performance optical fiber with reduced transmission loss. be able to.
[0056]
In addition, this invention is not limited to this example of embodiment, Various embodiment can be taken. For example, in addition to the configuration of this embodiment, at least one of the outside air seal lid 3 and the inner seal lid 4 is provided with a labyrinth seal in the gap between the through holes 3a, 4a and the support rod 8. This can prevent nitrogen from entering the core tube 2 more reliably.
[0057]
Further, in the present embodiment example, the manufacture of the core base material that the present invention is considered to function more effectively is taken up. However, the optical fiber base is formed by dehydrating and sintering the clad base layer formed around the core base material. Obviously, it can also be applied to the production of materials.
[0058]
【The invention's effect】
In the invention of the optical fiber core preform manufacturing apparatus, nitrogen gas concentration suppression means is provided, and by this nitrogen gas concentration suppression means, a porous preform serving as a core of the optical fiber is sintered. The upper side of the arrangement position of the heating means in the furnace core tube can be an atmosphere having a nitrogen gas concentration of 10% or less. In other words, the core base material is produced by sintering the porous base material in an atmosphere with a very low nitrogen gas concentration such that the nitrogen gas concentration in the upper part of the furnace core tube is 10% or less above the position where the heating means is disposed. Can do. Therefore, an optical fiber with a small transmission loss can be obtained reliably by manufacturing an optical fiber using the core preform.
[0059]
In addition, since the nitrogen gas in the upper part from the position where the heating means is disposed in the core tube enters from the seal chamber provided in the upper portion of the core tube, the nitrogen gas does not flow into the seal chamber. The nitrogen gas concentration in the upper part of the heating tube in the furnace core tube can be easily kept low.
[0060]
Furthermore, in the configuration in which the reactor core tube internal pressure control means for controlling the interior of the reactor core tube to a pressure set higher than the atmospheric pressure is provided as the nitrogen gas concentration suppression means, the fluctuation of the pressure inside the reactor core tube is suppressed. The internal pressure of the core tube is always kept higher than the pressure in the seal chamber. As a result, the nitrogen gas concentration in the upper part of the heating means in the furnace tube can be suppressed to 10% or less.
[0061]
Although it is very difficult to measure the nitrogen gas concentration in the heated furnace core tube, it is easy to measure the oxygen concentration. Therefore, the oxygen concentration is measured instead of the nitrogen gas concentration, and the nitrogen gas concentration is estimated and detected based on the measured value. Based on this estimated detection value, the pressure in the furnace core tube for suppressing the nitrogen gas concentration in the upper part of the heating means in the furnace core tube to 10% or less is determined, so that it can be easily above the heating means in the furnace core tube. The operating condition of the apparatus for suppressing the nitrogen gas concentration in the portion to 10% or less can be set.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining an embodiment of an optical fiber preform manufacturing apparatus according to the present invention.
FIG. 2 is a diagram for explaining a method for measuring an oxygen concentration in a furnace core tube.
FIG. 3 is a diagram for explaining a structural example of an apparatus for producing a core base material by sintering a porous base material.
FIG. 4 shows the relationship between the nitrogen loss and the transmission loss at a wavelength of 1310 nm of a single mode fiber obtained from a core produced by mixing and flowing nitrogen gas from the lower part of the core tube to a desired concentration together with helium and chlorine. FIG. 5 is a diagram showing the same relationship when the amount of nitrogen mixed from the seal chamber is measured and nitrogen gas is mixed from the seal chamber under those conditions.
FIG. 5 is a view for explaining another embodiment of the optical fiber preform manufacturing apparatus according to the present invention.
FIG. 6 shows the internal pressure P of the seal chamber in the optical fiber preform manufacturing apparatus according to the present invention. ₁ It is a figure which shows an example of the implementation data which controlled to about -50 Pa below atmospheric pressure.
7 shows an internal pressure of a furnace core tube and an internal pressure P of a seal chamber in a conventional optical fiber manufacturing apparatus. ₁ It is a figure which shows the relationship data.
[Explanation of symbols]
2 Core tube
3 Outside air seal lid
4 Inner seal lid
5 Sealing room
8 Support rod
10 Porous base material
11 Heating means

Claims (11)

In an apparatus for producing an optical fiber preform by forming a porous preform that is a precursor of an optical fiber preform and then sintering the porous preform, the porous preform is disposed inside the porous preform. A core tube for heating the base metal, and the upper portion of the core tube is double-blocked by an outside air sealing lid and an inside sealing lid arranged inside the outside air sealing lid with a space therebetween. In addition, a gas ventilation means for flowing an inert gas is provided in the seal chamber formed between the outside air seal lid and the inner seal lid, and the inside of the seal chamber is controlled to a pressure lower than the external atmospheric pressure to The atmospheric gas inside the reactor core tube is prevented from leaking to the outside due to the pressure difference between the outer air seal lid and the inner seal lid, and through holes are formed on the same axis. The porous base material is supported by the inserted support rod. Arranged inside the core tube, while rotating the porous base material around the central axis of the porous base material, the porous base material and the heating means provided on the side of the core tube are relatively The porous base material is heated and sintered in the vertical direction, and the inert gas flowing in the seal chamber or the inert gas in the atmosphere that has entered the seal chamber from the outside is inside. The upper position of the heating means in the core tube is prevented by controlling the pressure inside the core tube through the gap between the through hole of the seal lid and the support rod inserted through the through hole. An apparatus for producing an optical fiber preform, comprising inert gas concentration suppressing means for suppressing the inert gas concentration on the upper side to 10% or less. 2. The optical fiber preform according to claim 1, wherein the inert gas concentration suppression means is constituted by a reactor core tube internal pressure control unit that controls the interior of the reactor core tube to a pressure set higher than the atmospheric pressure. manufacturing device. 3. The pressure according to claim 1, wherein the pressure inside the core tube is 50 to 150 Pa, the pressure fluctuation range is 50 Pa or less, and the pressure in the seal chamber is lower than atmospheric pressure. 4. Fiber base material manufacturing equipment. The apparatus for producing an optical fiber preform according to claim 3, wherein the pressure inside the furnace core tube is 100 to 150 Pa, the pressure fluctuation range is 50 Pa or less, and the pressure in the seal chamber is 40 Pa or less. Instead of the inert gas, oxygen gas is allowed to flow into the seal chamber to measure the oxygen gas concentration above the position of the heating means in the furnace core tube, and the inert gas concentration is estimated based on the measured value. Then, using this estimated value, the pressure inside the core tube for suppressing the inert gas concentration on the upper side from the arrangement position of the heating means in the core tube to 10% or less is obtained. The apparatus for manufacturing an optical fiber preform according to any one of claims 1 to 4, wherein the control means controls the pressure inside the furnace core tube to the set pressure. In a method for producing an optical fiber preform by forming a porous preform serving as a precursor of an optical fiber and then sintering the porous preform, the upper portion of the porous preform is closed twice, and An inert gas is allowed to flow through the sealing chamber formed between the two times, and the inside of the sealing chamber is controlled to a pressure lower than the atmospheric pressure outside, and the internal atmospheric gas leaks to the outside due to the pressure difference with the outside. In the core tube which is prevented from being disposed by a support rod inserted through the penetrating light formed coaxially in the vertical direction of the seal chamber, the porous base material with the central axis of the porous base material as the rotation axis When the porous base material and the heating means provided on the side of the core tube move relatively in the vertical direction to heat and sinter the porous base material, Inactive gas or inactive in the atmosphere entering the seal chamber from the outside Preventing gas from entering the inside of the core tube through the gap between the through hole on the inside of the core tube of the seal chamber and the support rod inserted through the through hole, by controlling the pressure inside the core tube, A method for producing an optical fiber preform, wherein the inert gas concentration above the upper position of the heating means in the furnace tube is suppressed to 10% or less. 7. The optical fiber preform according to claim 6, wherein the inert gas concentration suppressing means is configured by a core pressure control means for controlling the inside of the core tube to a pressure set higher than the atmospheric pressure. Production method. 8. The pressure according to claim 6, wherein the pressure inside the core tube is 50 to 150 Pa, the pressure fluctuation range is 50 Pa or less, and the pressure in the seal chamber is lower than atmospheric pressure. 9. Manufacturing method of fiber preform. 9. The method of manufacturing an optical fiber preform according to claim 8, wherein the pressure inside the furnace core tube is 100 to 150 Pa, the pressure fluctuation range is 50 Pa or less, and the pressure in the seal chamber is 40 Pa or less. Instead of the inert gas, oxygen gas is allowed to flow into the seal chamber to measure the oxygen gas concentration above the position of the heating means in the furnace core tube, and the inert gas concentration is estimated based on the measured value. Then, using this estimated value, the pressure inside the core tube for suppressing the inert gas concentration on the upper side from the arrangement position of the heating means in the core tube to 10% or less is obtained, and the pressure inside the core tube is obtained. The method of manufacturing an optical fiber preform according to any one of claims 6 to 9, wherein the pressure is controlled to the set pressure. An optical fiber manufactured by the method for manufacturing an optical fiber according to any one of claims 6 to 10.

Images