JPH11322356A - Dehydration and making transparent glass of optical fiber porous preform and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for dehydrating and making transparent glass of an optical fiber porous preform by which the fluctuation of characteristics in the longitudinal direction of the preform after the dehydration and making transparent glass can be reduced. SOLUTION: An optical fiber porous preform 2 is placed in a furnace core tube 1 and heated with a heater 7 of a heating furnace 8 arranged outside the furnace core tube 1 to carry out the dehydration and transparentization treatment. In the process, the temperature of the heater 7 is controlled with a controller so as to provide the temperature of the preform 2 in a site for measuring the temperature within a required range while measuring the temperature of the preform 2 in the furnace core tube 1 with measuring window means 31 and 32 and preform temperature measuring means 33 and 34 installed in the heating furnace 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for dehydrating and vitrifying a porous optical fiber preform.

[0002]

2. Description of the Related Art A core optical fiber preform synthesized by a VAD method or a core optical fiber preform obtained by dehydrating and clearing the core optical fiber preform is externally attached as a starting material. As shown in FIG. 8, an apparatus for dehydrating and vitrifying an optical fiber porous preform for dehydrating and vitrifying an optical fiber porous preform composed of a drawing optical fiber porous preform synthesized by a method as shown in FIG. Furnace tube 1 made of ceramics such as quartz glass or alumina or carbon and arranged vertically
It has. In the furnace core tube 1, an optical fiber porous preform 2 is held by a chuck 4 at the lower end of a preform support rod 3 and suspended therefrom. A base material insertion opening 1a at the upper end of the furnace core tube 1 is closed by an upper lid 5, and a gas supply port 6 for supplying a required gas into the furnace core tube 1 is provided at a lower portion of the furnace core tube 1. . A heating furnace 8 is provided outside the furnace core tube 1 to heat the optical fiber porous preform 2 with a temperature distribution as shown in FIG. The reason why the furnace core tube 1 is used is that a halogen-based gas is used for heat treatment of the optical fiber porous preform 2 so that the gas does not enter the furnace body 9 of the heating furnace 8. The furnace body 9 is also provided with a gas supply port 10 independently. An exhaust pipe 1 for exhausting gas supplied into the furnace core tube 1 is provided on an upper side surface of the furnace core tube 1.
The exhaust pipe 11 is connected to an exhaust system (not shown) to process exhaust gas.

The optical fiber porous preform 2 in the furnace core tube 1 is
The elevating mechanism 12 moves up and down. The elevating mechanism 12 is driven by a drive motor 13, a ball screw 14 which is driven to rotate in a vertical direction by the drive motor 13, is fitted to the ball screw 14 by screw connection, and is prevented from rotating by detent means (not shown). It comprises a nut-like member 15 that is raised and lowered by the rotation of the ball screw 14, and a support rod holder 16 that is integrated with the nut-like member 15 and that raises and lowers the base material support rod 3.

Further, this apparatus receives a rotational position signal S1 from the drive motor 13 and a heater temperature signal S10 from the heating furnace 8 and outputs a speed signal S2 to the drive motor 13 to input an optical fiber porous substrate. A controller 17 that controls the raising and lowering of the material 2 and outputs a control signal to the heater 7 to control the heater temperature to a required range is provided.

[0005] Dehydration of such a porous optical fiber preform
In the transparent vitrification apparatus, the optical fiber porous preform 2 is inserted into the furnace core tube 1 from the upper part of the furnace core tube 1 by a descending operation by an elevating mechanism 12, and the optical fiber is placed at a predetermined position in the furnace core tube 1. The porous base material 2 is stopped, and heat treatment for dehydration and transparent vitrification is started. In this case, the base material insertion opening 1a at the upper end of the furnace core tube 1 is closed with an upper lid 5, and nitrogen gas or the like is supplied from a sealing gas supply unit (not shown) so that air does not enter through a gap between the base material support rod 3 and the upper lid 5. Gas sealing is performed by supplying an inert gas such as an argon gas.

The dehydration treatment and the vitrification treatment are usually performed while feeding the porous optical fiber preform 2 into a heating zone formed by one heater 7. In this case, the amount of heat received by the optical fiber porous preform 2 is
In the longitudinal direction. This is the optical fiber porous preform 2
Is different from the middle parallel portion of the optical fiber porous preform 2, a difference occurs in the amount of heat received from the heating zone. Also, in the portion where the density distribution of the optical fiber porous preform 2 fluctuates in the longitudinal direction, the state of heat conduction in the radial direction and the longitudinal direction of the received heat changes, so that the temperature distribution in the same heating zone is changed. Even if formed, the state of dehydration and transparent vitrification will change. Further, the processed gas is cooled by the supplied gas, and the unprocessed gas is heated. In the case of dehydration, the dehydration state differs between the treated part and the untreated part. Therefore, it can be easily imagined that the dehydration history and the heating history greatly differ depending on the longitudinal direction of the optical fiber porous preform 2.

In order to make the amount of heat received by the porous optical fiber preform 2 from the heating zone substantially the same, it is possible to adjust the moving speed of the porous optical fiber preform 2 down in the longitudinal direction. It has been proposed to adjust the temperature of the heating zone (JP-A-8-225337). In Japanese Patent Application Laid-Open No. Hei 8-225337, an optical fiber porous preform 2 is disclosed.
The amount of change in the temperature in the longitudinal direction and the amount of change in the feed rate applied to the optical fiber porous preform 2 during dehydration and transparent vitrification of
The control pattern is determined in advance by experiments so that the refractive index distribution of the base material after the heat treatment becomes substantially constant in the longitudinal direction.

[0008]

However, VAD
The optical fiber porous preform 2 synthesized by a method or an external method is not necessarily the same. That is, the outer diameter and length of the optical fiber porous preform 2 may be different for each production. Also, these differences due to different devices are even greater. The density of the porous optical fiber preform 2 has a distribution in the radial direction, and the distribution also varies in the longitudinal direction of the porous optical fiber preform 2. It is considered that the density fluctuation of the optical fiber porous preform 2 greatly affects the dehydration state and the fluctuation of the refractive index distribution in the longitudinal direction of the optical fiber porous preform 2 during the dehydration treatment. In addition, there is a density variation for each production of the optical fiber porous preform 2 and a density variation between apparatuses. Therefore, it is considered that such a change can be statistically processed to set the optimal temperature of the heating zone and the setting of the feed rate. However, when the density change of the individual optical fiber porous preform 2 is considered, In addition, even if the dehydration / transparent vitrification treatment is controlled at a predetermined temperature or a lowering speed, there is a problem that the refractive index distribution of the optical fiber preform after the treatment is not always substantially constant in the longitudinal direction.

An object of the present invention is to provide a method and an apparatus for dehydrating / clearing a porous optical fiber preform, which can reduce fluctuations in the longitudinal properties of the preform after dehydration / transparent vitrification.

[0010]

According to the present invention, an optical fiber porous preform is inserted into a furnace core tube, and the optical fiber porous preform is heated by a heater of a heating furnace disposed outside the furnace core tube. It is an object of the present invention to improve a method of dehydrating / transparent vitrifying an optical fiber porous preform in which heating is performed to effect dehydration / transparent vitrification.

In the method of dehydrating and vitrifying a porous optical fiber preform according to the present invention, while measuring the temperature of the preform in a furnace tube, the temperature of the preform at the temperature measurement point is kept within a required range. In addition, one or both of the temperature of the heater and the pull-down speed of the base material are controlled.

As described above, while measuring the temperature of the base material in the core tube, one or both of the heater temperature and the base material lowering speed are adjusted so that the temperature of the base material at the temperature measurement point falls within a required range. When controlled, even if the outer diameter of the optical fiber porous preform changes in the longitudinal direction or the optical fiber porous preform having a different outer diameter, the preform itself is subjected to a temperature in a required range. Since the heat treatment can be performed, fluctuations in the properties in the longitudinal direction of the base material after dehydration and transparent vitrification can be reduced.

Further, according to the present invention, a porous optical fiber preform is inserted into a furnace core tube, and the optical fiber porous preform is heated and dehydrated by a heater of a heating furnace arranged outside the furnace core tube. An object of the present invention is to improve an apparatus for dehydrating and transparently vitrifying an optical fiber porous preform that performs a transparent vitrification treatment.

In the apparatus for dehydrating and transparently vitrifying an optical fiber porous preform according to the present invention, a temperature measuring means for measuring the temperature of the preform is disposed outside the furnace core tube made of quartz.
Based on the measurement result of the temperature of the base material from the temperature measuring means, one or both of the heater temperature and the base material lowering speed are adjusted so that the temperature of the base material at the temperature measurement point falls within a required range. Control means for controlling is provided. As the temperature measuring means, for example, a radiation thermometer or a thermal image measuring device can be used.

As described above, the temperature of the base material is measured by the temperature measuring means, and the temperature of the heater or the base material is set so that the temperature of the base material at the temperature measurement point falls within a required range based on the measurement result of the temperature. When either or both of the pull-down speeds are controlled by the control means, the outer diameter of the optical fiber porous preform changes in the longitudinal direction, or the optical fiber porous preform has a different outer diameter. Also, since the base material itself can be heat-treated under a temperature condition in a required range, fluctuations in the longitudinal properties of the base material after dehydration and transparent vitrification can be reduced.

[0016]

1 and 2 show a first embodiment of an apparatus for dehydrating and transparently vitrifying an optical fiber porous preform according to the present invention, and FIG. 1 shows the apparatus. FIG. 2 is an enlarged vertical sectional view of a portion of the apparatus shown in FIG. 1 where a heating furnace is present. This example is 1
1 shows a case where the present invention is applied to an apparatus for dehydrating and vitrifying a porous optical fiber preform in which a heating zone is formed by two heaters. Note that parts corresponding to those in FIG. 8 described above are denoted by the same reference numerals.

In this apparatus for dehydrating and vitrifying a porous optical fiber preform, the furnace core tube 1 is made of translucent quartz. The furnace body 9 of the heating furnace 8 is made of metal such as stainless steel, and has a tubular portion 9a, an annular upper wall portion 9b that covers an upper portion of the tubular portion 9a, and an annular lower portion that covers a lower portion of the tubular portion 9a. And a wall 9c. Furnace body 9
Are water-cooled by well-known water-cooling means (not shown). Such a furnace body 9 has a sealing material presser 1 provided on an upper wall portion 9b and a lower wall portion 9c.
At 8, the sealing material 19 is pressed against the outer periphery of the furnace core tube 1 so as to be hermetically sealed and attached to the furnace core tube 1, and is supported by a load (not shown) under a load. In the furnace body 9, a heat insulating material 20 made of a material such as carbon fiber is arranged along the inner wall of the furnace body 9 so as to surround the heater 7 made of a material such as carbon. Power is supplied to the heater 7 by power supply means (not shown). The control of the power supply to the heater 7 is performed by the controller 17.

In the heating furnace 8, through holes 21, 22 which penetrate the heat insulating material 20 and the cylindrical portion 9 a of the furnace body 9 horizontally on one side of the furnace core tube 1 at positions above and below the heater 7. Is provided. Measuring windows 23 and 24 are provided on the outer surface of the cylindrical portion 9a corresponding to the through holes 21 and 22, respectively. The measurement windows 23 and 24 are provided with through holes 21 and 22 on the outer surface of the cylindrical portion 9a.
Window frame parts 25 and 26 provided integrally around the
The window frames 25 and 26 are applied to the through holes 21 and
It is composed of transparent plates 27 and 28 such as quartz plates closing the cover 2, and fixed frames 29 and 30 for fixing the transparent plates 27 and 28 to the window frame portions 25 and 26. These through holes 21 and 2
2 and the measurement windows 23 and 24 constitute measurement window means 31 and 32 for measuring the temperature of the optical fiber porous preform 2 in the furnace core tube 1.

Outside the measuring windows 23 and 24, the temperature of the porous optical fiber preform 2 in the furnace core tube 1 is measured through the light-transmitting furnace core tube 1 through the through holes 21 and 22. Base material temperature measuring means 33 and 34 such as a radiation thermometer and a thermal image measuring device are arranged. The base material temperature signals S4 and S5 obtained by the measurement by the base material temperature measuring means 33 and 34 are transmitted to the controller 1
7 is input.

Further, as viewed in the axial direction of the furnace core tube 1,
The heat insulating material 20 and the cylindrical portion 9a of the furnace body 9 are positioned between the positions
Is provided on one side of the furnace core tube 1 in a horizontal direction. A measurement window 36 is provided on the outer surface of the cylindrical portion 9a corresponding to the through hole 35. The measurement window 36 includes a window frame portion 37 provided integrally with the outer surface of the cylindrical portion 9a so as to surround the through hole 35, and a transparent plate such as a quartz plate applied to the window frame portion 37 to close the through hole 35. The light plate 38 and the light transmitting plate 38
Is fixed to the window frame portion 37. The through-hole 35 and the measurement window 36 constitute a measurement window means 40 for measuring the temperature of the heater 7 in the furnace core tube 1.

Outside the measuring window 36, a heater temperature measuring means 41 such as a radiation thermometer or a thermal image measuring device for measuring the temperature of the heater 7 in the furnace core tube 1 through the through-hole 35 is arranged. The heater temperature signal S3 obtained by the measurement by the heater temperature measuring means 41 is input to the controller 17.

The controller 17 controls the porous optical fiber preform 2 heated by the heater 7 based on the input signal given thereto.
The heater power is controlled so that the temperature of the portion is within a required range.

In the method for dehydrating and vitrifying an optical fiber porous preform using such an apparatus, in the case of dehydration, the state of the preheating portion of the optical fiber porous preform 2 is compared with that of transparent vitrification. Since a large influence is exerted, a control method is adopted in consideration of the influence of the temperature of the portion of the optical fiber porous preform 2 above the portion facing the heater 7 (measured temperature of the preform temperature measuring means 33). On the other hand, in the case of transparent vitrification, since the temperature of the glass is important, the temperature of the portion of the optical fiber porous preform 2 below the portion facing the heater 7 (the temperature measured by the preform temperature measuring means 34) is taken into consideration. The adopted control system. Thus, it is preferable to change the control method between dehydration and transparent vitrification.

First, the dehydration treatment will be specifically described. In this case, after the optical fiber porous preform 2 is lowered to a predetermined position in the furnace core tube 1 by the elevating mechanism 12,
The gas is switched from the standby gas (Ar gas) to the He gas, and when the pressure difference from the atmosphere in the furnace core tube 1 becomes stable, the temperature of the heating furnace 8 is raised to the temperature of the dehydration treatment. Hee Yu 7
After the temperature is stabilized, oxygen and chlorine are further supplied into the furnace core tube 1, and the optical fiber porous preform 2 is pulled down at a predetermined speed by the elevating mechanism 12. The dehydrated optical fiber porous preform 2 is subjected to a dehydration process while passing through the heating zone HZ formed in the furnace core tube 1 by the heater 7. At this time, in this apparatus, measurement windows 23 and 24 are provided above and below the heating furnace 8 with the heater 7 interposed therebetween.
The temperatures of the portions of the optical fiber porous preform 2 facing the base materials 3 and 24 are measured by preform temperature measuring means 33 and 34. The base material temperature signals S4 and S5 measured by the base material temperature measuring means 33 and 34 are input to the controller 17. The controller 17 controls the heater 7 so that the measured temperature of the optical fiber preform 2 becomes a temperature suitable for the dehydration treatment (in this case, 1300 ° C.).
Control the temperature of the At the start of dehydration, the temperature of the portion of the optical fiber porous preform 2 above the portion facing the heater 7 can be measured stably (the portion below the heating portion of the heater 7 is affected by the contraction of the preform). Therefore, it was difficult to make measurements.)

Next, the transparent vitrification treatment will be specifically described. After the completion of the dehydration, the optical fiber porous preform 2 is pulled up to the dehydration start position, the gas supplied to the furnace core tube 1 is changed to a gas for the transparent vitrification treatment, and the heater 7 is heated to the vitrification temperature. When the temperature is stabilized, the optical fiber porous preform 2 is lowered at a predetermined speed. At this time,
In this apparatus, measurement windows 23 and 24 are provided above and below the heating furnace 8 with the heater 7 interposed therebetween.
The temperature of the portion of the optical fiber porous preform 2 facing 24 is measured by preform temperature measuring means 33, 34. The base material temperature signal S4 measured by these base material temperature measuring means 33 and 34.
, S5 are input to the controller 17. The controller 17 controls the temperature of the heater 7 such that the measured temperature of the optical fiber porous preform 2 falls within a required range suitable for transparent vitrification (in this case, 1650 ° C.). In this case, the control was performed with emphasis on the temperature of the glass portion, that is, the temperature of the portion of the optical fiber porous preform 2 below the portion facing the heater 7.

Optical fiber porous preform 2 during dehydration described above
2 is depicted in FIG. In the heating zone HZ in the furnace core tube 1 by the heater 7, the optical fiber porous preform 2 is deformed, and the optical fiber porous preform 2 slightly shrinks. The portion of the porous optical fiber preform 2 facing the heating zone HZ is a dewatering section 2B, and the portion of the optical fiber porous preform 2 below the heating zone HZ is a dewatered section 2A. The portion of the optical fiber porous preform 2 above the heating zone HZ is a non-dewatered portion 2C. The degree of shrinkage of the dewatering section 2B of the porous optical fiber preform 2 facing the heating zone HZ depends on the temperature distribution, outer diameter distribution, density distribution and the like of the optical fiber porous preform 2. It is considered that this is largely due to the temperature history of the porous optical fiber preform 2. Therefore, it can be easily imagined that the temperature of the optical fiber porous preform 2 and the shape (fluctuation in outer diameter) after dehydration have a close relationship. The concept of the present invention is to make the shape of the optical fiber porous preform 2 in the heating zone HZ the same. For that purpose, it is effective to set the temperature of the optical fiber porous preform 2 to a required range. Therefore, in the present invention, the temperature of the optical fiber porous preform 2 is directly measured.

The same can be said for the vitrification treatment described above. However, in this case, below the portion facing the heater 7, the base material is not a porous body but a transparent glass state.

The gas conditions at the time of dehydration are as follows: He gas is 100
l / min, oxygen at 10 1 / min and chlorine at 11 / min. The gas conditions during the vitrification were 100 He / min for He gas.

In this embodiment, the temperature of the heater 7 is controlled, but the pulling speed of the porous optical fiber preform 2 is controlled so that the temperature of the temperature measuring section of the porous optical fiber preform 2 falls within a target temperature range. You can also.

Also, the temperature of the heater 7 and the rate of lowering the porous optical fiber preform 2 can be controlled together so that the temperature of the temperature measuring section of the porous optical fiber preform 2 is within the target temperature range.

The fluctuation of the refractive index difference at the center of the preform of the optical fiber preform obtained by performing the dehydration / transparent vitrification treatment as described above is reduced to 3% or less, and the conventional optical fiber preform is not changed. It was confirmed that the dehydration / transparent vitrification treatment of this example was effective for the change (5%) in the refractive index difference at the center of the base material.

FIGS. 3 to 5 show a second embodiment of an apparatus for dehydrating / clearing a porous optical fiber preform according to the present invention, and FIG. 3 shows a schematic configuration of the apparatus. FIG. 4 is an enlarged vertical sectional view of a portion where the heating furnace of the apparatus shown in FIG. 3 exists, and FIG. 5 is a temperature distribution diagram of the heating furnace in the apparatus shown in FIG. This example shows a case in which the present invention is applied to an apparatus for dehydrating and vitrifying an optical fiber porous preform in which a heating zone is formed by two heaters. 1 and 2 are denoted by the same reference numerals.

In this apparatus for dehydrating and vitrifying a porous optical fiber preform, a heating furnace 8 includes a main heater 7A for heat treatment and a preliminary heater 7B for preheating.
And the preliminary heater 7B is arranged so as to be located upstream with respect to the direction in which the porous optical fiber preform 2 is moved downward.

Such a main heater 7A and a preliminary heater 7B
With the presence of the through hole 21 of the measurement window means 31 on the upstream side.
Is provided above the main heater 7A, in other words, between the main heater 7A and the preliminary heater 7B, so that the temperature of the optical fiber porous preform 2 can be measured. It is provided so that the temperature of the optical fiber porous preform 2 can be measured below the main heater 7A.

With the presence of the main heater 7A and the auxiliary heater 7B, measurement window means 40A and 40B for measuring the temperatures thereof and heater temperature measuring means 41A and 41B are provided, respectively. These measurement window means 40A, 4
0B and the structure of the heater temperature measuring means 41A and 41B are the same as those of the example of the measuring window means 40 and the heater temperature measuring means 41 shown in FIG. 2, so that the same numbers as those constituent elements correspond to the main heater 7A. Indicates that the symbol A is added, and the symbol corresponding to the preliminary heater 7B is indicated by the symbol B.
In this case, heater temperature signals S10 and S11 are output to the controller 17 from the heater temperature measuring means 41A and 41B, respectively.

Dehydration of such a porous optical fiber preform
The way of the dehydration / transparent vitrification processing by the transparent vitrification apparatus is the same as in the first example of the above-described embodiment.

In the second example of this embodiment, the main heater 7A for heat treatment and the preliminary heater 7B for preliminary heating are provided with an optical fiber porous heater as described above so that more detailed temperature control can be performed. The temperature control of the upper part (upstream side) of the heating zone HZ in the furnace core tube 1 is performed by the preliminary heater 7 </ b> B. The temperature control of the lower portion (downstream side) of the heating zone HZ is performed by the main heater 7A. FIG. 5 shows an example of the temperature distribution in the heating zone HZ in the furnace tube 1 by the main heater 7A and the preliminary heater 7B. Since the control systems interfere with each other, the design of the control system is complicated. However, since the control system is more thermally stable, the outer diameter and the density of the optical fiber porous preform 2 are smaller than those of the first example of the embodiment. Even with large fluctuations,
Fluctuations in the longitudinal outer diameter and density of the vitrified optical fiber preform can be reduced. Also, the optical fiber porous preform 2
In the case where the fluctuations of the outer diameter and the density are the same, there is an advantage that the fluctuations of the outer diameter and the density in the longitudinal direction can be made smaller.

Specifically, the optical fiber porous preform 2 is lowered to a predetermined position in the furnace core tube 1 by the elevating mechanism 12. The gas in the furnace tube 1 is switched from the standby gas (Ar gas) to He gas, and when the pressure difference from the atmosphere in the furnace tube 1 becomes stable, the temperature of the heating furnace 8 is raised to the temperature of the dehydration treatment. I do. After the temperature is stabilized, oxygen and chlorine are further supplied into the furnace core tube 1 and the optical fiber porous preform 2 is pulled down at a predetermined speed. The dehydrated optical fiber porous preform 2 is dehydrated while passing through the preheating zone PZ and the heating zone HZ. In this example, since the heating furnace 8 is provided with measuring window means 31 and 32 above and below the main heater 7A, the optical fiber porous preform 2 is provided at these positions by the preform temperature measuring means 33 and 34. Measure the temperature of. The measured temperatures of the porous optical fiber preform 2 at positions above and below the heating zone HZ are within a required range suitable for dehydration treatment (in this case, the preform temperature above the heating zone HZ is 1300 ° C., Heating zone H
The temperature of each of the heaters 7B and 7A was controlled by the controller 17 so that the base material temperature below Z was 1250 ° C.).

The fluctuation of the refractive index difference at the center of the preform of the optical fiber preform obtained by performing the dehydration / transparent vitrification treatment as described above is reduced to 2% or less, and the conventional optical fiber preform is not changed. It was confirmed that the dehydration / transparent vitrification treatment of this example was effective for the change (5%) in the refractive index difference at the center of the base material.

In this embodiment, the temperature of the heater 7 is controlled, but the pull-down speed of the porous optical fiber preform 2 is controlled so that the temperature of the temperature measuring section of the porous optical fiber preform 2 falls within a target temperature range. You can also.

Further, by controlling both the temperature of the heater 7 and the lowering speed of the porous optical fiber preform 2, the temperature of the temperature measuring section of the porous optical fiber preform 2 can be kept within a target temperature range.

FIG. 6 is an enlarged vertical sectional view of a portion where a heating furnace according to a third embodiment of the present invention is present in an apparatus for dehydrating and vitrifying an optical fiber porous preform according to the present invention. This example shows another example in which the present invention is applied to an apparatus for dehydrating and vitrifying an optical fiber porous preform in which a heating zone is formed by one heater. Parts corresponding to those in FIG. 2 described above are denoted by the same reference numerals.

In the apparatus for dehydrating and vitrifying the optical fiber porous preform, the temperature of the optical fiber porous preform 2 at the center of the heating zone HZ, in other words, at the center of the heater 7 in the vertical direction is controlled. The space 4 is provided in the heater 7 so that measurement can be performed.
2, a through hole 4 that penetrates horizontally through the heat insulating material 20 and the cylindrical portion 9 a of the furnace body 9 on one side of the furnace core tube 1.
3 are provided. A measurement window 44 is provided on the outer surface of the cylindrical portion 9a corresponding to the through hole 43. Measurement window 44
Is a window frame 45 integrally provided on the outer surface of the cylindrical portion 9a so as to surround the through hole 43, and a light transmitting plate 46 such as a quartz plate applied to the window frame 45 to block the through hole 43. And a fixing frame 47 for fixing the light transmitting plate 46 to the window frame portion 45. The space 42, the through hole 43, and the measurement window 44 constitute a measurement window means 48 for measuring the temperature of the optical fiber preform 2 in the furnace core tube 1.

Outside the measurement window 44, the radiation temperature at which the temperature of the optical fiber porous preform 2 in the furnace core tube 1 is measured through the light-transmitting furnace core tube 1 through the through holes 42 and 43 is measured. A base material temperature measuring means 49 such as a meter or a thermal image measuring device is provided.
A base material temperature signal obtained by the measurement by the base material temperature measuring means 49 is input to the controller 17.

With this configuration, since the temperature of the optical fiber porous preform 2 corresponding to the center of the heater 7 can be measured, the temperature of the optical fiber porous preform 2 can be more stably evaluated, and control becomes easy. . As shown in FIGS. 7A and 7B, the shape of the heater 7 used in this example is such that there is no space between the portion where the temperature is measured and the opposite portion so that there is no heat generating portion.
Are provided, respectively, so that errors in temperature measurement hardly occur. In this heater, the heat generating portion is formed in a meandering shape by alternately providing slits 7a in upper and lower portions of a tubular tropic.
b, 7c are provided.

The procedure of dehydration and transparent vitrification is the same as that of the first embodiment, and the only difference is that the temperature measurement point and the control method are increased because the number of measurement points is increased.

The fluctuation of the refractive index difference at the center of the preform of the optical fiber preform obtained by performing the dehydration / transparent vitrification treatment as described above is reduced to 2% or less, and the change of the conventional optical fiber preform is reduced. It was confirmed that the dehydration / transparent vitrification treatment of this example was effective for the change (5%) in the refractive index difference at the center of the base material.

The above description has been given of the core optical fiber porous preform synthesized by the VAD method. However, a drawing method in which glass fine particles are deposited by an external method on a starting rod formed by extending the core optical fiber porous preform. In the case of an optical fiber porous preform, if dehydration unevenness occurs in the longitudinal direction of the preform, it affects transmission loss. Therefore, in the past, the transmission loss of an optical fiber obtained by drawing from a transparent / vitrified optical fiber preform sometimes fluctuated in the longitudinal direction.
In the present invention, the transmission loss of an optical fiber obtained by drawing from a transparent and vitrified optical fiber preform could be reduced to 0.002 dB / km or less.

When the outer diameter fluctuation after vitrification is large, the fluctuation of the diameter of the optical fiber becomes large, and the fluctuation of the clearance between the drawing furnace and the optical fiber preform increases the consumption of the heating furnace. Or the number of times increases. In the present invention, the temperature in the longitudinal direction was controlled so that the elongation of the optical fiber preform hardly occurred (specifically, the temperature at the end of vitrification was reduced by about 50 ° C. with respect to the temperature at the start of vitrification). However, there was almost no change in outer diameter in the longitudinal direction. Therefore, the molding variation was significantly reduced as compared with the conventional case, and the maintenance interval of the furnace tube and the furnace body could be extended to about twice that of the conventional case.

[0050]

According to the method for dehydrating and vitrifying a porous optical fiber preform according to the present invention, the temperature of the preform is measured by a temperature measuring means, and the temperature is measured based on the measurement result of the temperature of the preform. Either or both of the heater temperature and the base material lowering speed are controlled by the control means so that the temperature of the base material at the measurement point falls within a required range, so that the outer diameter of the optical fiber porous base material is in the longitudinal direction. Even if the preform changes to a predetermined value or the optical fiber porous preform has a different outer diameter, the preform itself can be heat-treated under a required temperature range. Variations in the properties in the longitudinal direction of the material can be reduced. Thereby, it is possible to reduce the variation in the longitudinal direction of the refractive index distribution of the base material due to the dehydration, and the variation in the transmission loss due to insufficient or excessive dehydration as compared with the conventional case. In addition, by optimizing the processing temperature of the base material in the longitudinal direction during vitrification, fluctuations in the outer diameter in the longitudinal direction due to elongation of the base material can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a schematic configuration of a first example of an embodiment of a dehydrating / transparent vitrification apparatus for an optical fiber porous preform according to the present invention.

FIG. 2 is an enlarged vertical sectional view of a portion where a heating furnace of the apparatus shown in FIG. 1 is present.

FIG. 3 is a longitudinal sectional view showing a schematic configuration of a second example of the embodiment of the dehydration / transparent vitrification apparatus for an optical fiber porous preform according to the present invention.

4 is an enlarged vertical sectional view of a portion where a heating furnace of the apparatus shown in FIG. 3 exists.

FIG. 5 is a temperature distribution diagram of a heating furnace in the apparatus shown in FIG.

FIG. 6 is an enlarged longitudinal sectional view of a portion where a heating furnace according to a third example of the embodiment of the present invention is present in the apparatus for dehydrating / transparent vitrifying an optical fiber porous preform according to the present invention.

7A is a plan view of a heater used in the heating furnace of the third example, and FIG. 7B is a cross-sectional view taken along line XX of FIG. 7A.

FIG. 8 is a longitudinal sectional view showing a schematic configuration of a conventional optical fiber porous preform dehydration / transparent vitrification apparatus.

9 is a temperature distribution diagram of a heating furnace in the apparatus shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Furnace core tube 1a Base material insertion port 2 Optical fiber porous base material 3 Base material support rod 4 Chuck 5 Upper lid 6 Gas supply port 7 Heater 7a Slit 7b, 7c Power supply terminal 8 Heating furnace 9 Furnace body 9a Tubular part 9b Upper wall Part 9c Lower wall part 10 Gas supply port 11 Exhaust pipe 12 Elevating mechanism 13 Drive motor 14 Ball screw 15 Nut-like member 16 Support rod holder 17 Controller 18 Sealing material holding 19 Sealing material 20 Heat insulating material 21, 22 Through hole 23, 24 Measurement Windows 25, 26 Window frame portions 27, 28 Light transmitting plates 29, 30 Fixed frames 31, 32 Measurement window means 33, 34 Base material temperature measurement means 35, 35A, 35B Through holes 36, 36A, 36B Measurement windows 37, 37A, 37B Window frame part 38, 38A, 38B Light transmitting plate 39, 39A, 39B Fixed frame 40, 40A, 40B Measurement window means 41, 41A 41B heater temperature measuring means 42 space portion 43 through hole 44 measuring window 45 the window frame portion 46 transparent plate 47 fixed frame 48 measurement window means 49 preform temperature measuring means

Claims (2)

[Claims] 1. An optical fiber porous preform is inserted into a furnace core tube, and the optical fiber porous preform is heated by a heater of a heating furnace arranged outside the furnace core tube to dehydrate / clear the glass. In the method of dehydrating and vitrifying an optical fiber porous preform for performing a vitrification treatment, while measuring the temperature of the preform in the furnace tube, the temperature of the preform at the temperature measurement point is in a required range. A method for dehydrating and vitrifying a porous optical fiber preform, wherein one or both of the temperature of the heater and the pull-down speed of the preform are controlled. 2. An optical fiber porous preform is inserted into a furnace core tube, and the optical fiber porous preform is heated by a heater of a heating furnace arranged outside the furnace core tube to dehydrate / clear glass. An optical fiber porous preform for dehydration and transparent vitrification, wherein the temperature measuring means is disposed outside the quartz core tube and measures the temperature of the preform; and Based on the measurement result of the temperature of the base material, one or both of the temperature of the heater and the lowering speed of the base material are controlled so that the temperature of the base material at the temperature measurement point is within a required range. And a vitrifying / transparent vitrifying apparatus for an optical fiber porous preform.