JP2003146686A - Method for manufacturing optical fiber preform and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To promote deposition of soot in MCVD method. SOLUTION: In a method for manufacturing an optical fiber preform in which a quartz tube 1 for the optical fiber preform is heated by moving a burner 3 back and forth along the axial direction of the quartz tube 1 while feeding gaseous starting materials into the quartz tube 1 to generate and deposit soot 2 in the quartz tube 1, the outer periphery of the quartz tube is covered with a cooling jacket 5 in a zone in front and/or in the rear of the forward direction of the burner 3 and the quartz tube in the zone is cooled by flow contact of an appropriate inert gas containing fine mist of water generated by a mist generator 6.

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 producing an optical fiber preform, more specifically, a method for producing an optical fiber preform by the MCVD method, and particularly to an optical fiber preform adapted to promote soot deposition. The present invention relates to a manufacturing method and device.

[0002]

2. Description of the Related Art First, a method of manufacturing an optical fiber preform by the MCVD method will be briefly described with reference to FIG. 4 showing a standard example thereof. The quartz tube 1 which should be the outermost clad portion in the optical fiber preform of the final product is heated by reciprocally moving the burner 3 over its entire length while rotating while holding both ends by chucks of a lathe (not shown). To do.

The quartz tube 1 has an O₂gas
Of the quartz material vaporized by SiCl_FourAnd dopant (refractive index
What raises) Raw material GeCl_FourCarrier gas O₂
Pour with. The inside of the quartz tube 1 is heated by the burner 3.
Therefore, some of the following oxidation reactions that have already occurred
Further promoted, SiO₂, GeO₂Is suit
Soot 2) is deposited on the inner surface of the quartz tube 1. SiCl_Four+ O₂= SiO₂+ 2Cl₂ GeCl_Four+ O₂= GeO₂+ 2Cl₂

After the soot 2 is sufficiently deposited, a collapse process is performed. This is used in English as it is in Japan because there is no suitable translation, but in short, it is a process of completely expelling the gas part remaining inside and leaving only a solid solid part, and the temperature of the burner 3 from the outside. And the reciprocating speed is usually lowered to sufficiently heat it. In this way, a rod-shaped optical fiber preform (preform) having a circular cross section is completed.

Next, a conventional technique will be described. First of all, it should be strongly pointed out that the following techniques do not correspond to the prior art in the exact sense of the invention. So you don't have to take it up,
Although it can be said that there is no corresponding prior art, some of the steps disclosed therein have a step operation similar to that used in the present invention, and therefore will be explained here.

This is the technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-290036 "Optical Fiber Manufacturing Method". The outline of the technology is outlined with reference to FIG. 5. This is because the dopant GeO ₂ which is the main cause of bubbles is contained in the collapse process in order to prevent bubbles from being contained in the optical fiber preform of the product. This is a countermeasure technique in which GeO derived from is ejected by causing the cooling nozzle 103 to precede the burner 102 as illustrated.

In a nutshell, the theory is that the cooling nozzle 10
The cooling reaction step of blowing nitrogen or air, which serves as a refrigerant by 3 to the optical fiber preform 101 on which the soot deposition is completed, discharges the generation reaction rate of GeO from GeO ₂ to the outside of the collapse step. It is to keep GeO out of the base metal at a rate lower than the transfer rate.

As explained in the above item [0006],
It may not be necessary to further comment on this publication technology because the use process shown in (1) and the purpose shown in (2) are different, but further, the cooling nozzle is arranged only in front of the heating burner. It can be pointed out that its drawbacks are that the refrigerants are nitrogen and air, which have a small specific heat (heat capacity) and a low cooling efficiency.

[0009]

Despite the prior art, the present invention generally provides a way to promote soot deposition to increase the efficiency of optical fiber preform manufacturing.

[0010]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and the solution according to the invention of claim 1 is provided inside a quartz tube to be an optical fiber preform. In the method of manufacturing an optical fiber preform, feeding a source gas, heating the quartz tube by reciprocating a burner in the axial direction of the quartz tube, thereby generating and depositing soot inside the quartz tube, In the method of manufacturing an optical fiber preform, a part of the burner in the forward direction, the rear direction, or both axial directions of the burner is cooled.

According to a second aspect of the present invention, in the cooling step, a relatively inert gas flow containing fine water droplets (H ₂ O) is brought into contact with the outer circumference of the quartz tube by using a cooling jacket. The method for producing an optical fiber preform according to claim 1, wherein the optical fiber preform is produced by

Further, the solution means according to the invention of claim 3 is to control the cooling step so that a temperature difference between a heating portion of the quartz tube by the burner and a cooling portion of the cooling jacket becomes constant. The method for producing an optical fiber preform according to claim 1 or 2, characterized in that.

According to a fourth aspect of the present invention, there is provided a means for solving the problem, which is mounted on a burner stand for carrying the burner so as to be movable and adjustable in the axial direction of the quartz tube, and surrounds the quartz tube, and the burner stand in the axial direction. At least one hollow cooling jacket that can move with, and fine water droplets (H
₂ O), a mist generator, a mist feed pipe for feeding the minute water droplets from the mist generator into the cooling jacket in a flow of a relatively inert gas, and the cooling jacket. And an exhaust pipe for exhausting the flow of the gas described above.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An apparatus according to an embodiment of the present invention will be described with reference to FIGS. The quartz tube 1 shown in FIG. 1, the soot 2 that is being deposited in the quartz tube 1, and the burner 3 that reciprocates in the axial direction of the quartz tube 1 are exactly the same as those described in the section of the prior art. is there.

The burner 3 is carried on a burner table 4, and the burner table 4 is reciprocated in parallel with the axial direction of the quartz tube 1 by a traverse shaft 42. In the present invention, a part of the burner table 4 has a quartz tube. 1 is provided with a mounting base plate 41 that can be moved in parallel with the axial direction of the quartz tube 1, and its position, that is, the distance along the axial direction of the quartz tube 1 with respect to the burner 3 can be adjusted by the adjusting screw 43.

However, the mounting base plate 41 has a support column 51.
The cooling jacket 5 is mounted around the quartz tube 1 with a slight clearance 5A therebetween. The cooling jacket 5 has a hollow annular body. To be more precise,
As shown in the drawing, a hollow tube having a trapezoidal shape, a semicircular shape, or any other shape is wound around the quartz tube 1 with a gap 5A with the relatively large area of the tube wall inside.
It can be described as a shape in which the tube wall is cut off with respect to the outer circumference of. Quartz, which has high heat resistance, or ceramics is preferable.

A mist feed pipe 52 extending from the mist generator 6 is connected and connected above the cooling jacket 5, and a mist discharge pipe 53 is similarly arranged below.

The mist generator 6 is a device for forming fine water droplets (H ₂ O) used as a refrigerant. This may simply be a device that heats water to boil to generate water vapor and condenses it, but in order to facilitate various quantitative controls, in the present invention, the principle of the ultrasonic humidifier is used. Use the equipment used.

H that is inactive as a carrier gas for sending the water droplets produced by the mist generator 6 to the mist inlet pipe 52
Using e, N ₂ or air, it is appropriately pressurized and sent into the mist generator 6 via the flow rate controller 7.

For the above-mentioned control, the temperature is measured at three points of A (heating section by the burner), B (end point of the cooling zone) and C (start point of the same) shown in the figure. All of these are points on the outer peripheral surface of the quartz tube 1 and are measured by a radiation thermometer in a non-contact manner.

The purpose of the present invention is to promote the deposition of soot 2 as described at the beginning, and the mechanism is to quickly cool the high temperature soot that has just been generated, and to have its potential energy (partly). Heat energy, and partly in the form of kinetic energy) to achieve this by a thermophoresis effect.

As for the case where the cooling jacket 5 is provided in front of the burner 3, the sooted source gas has a kinetic energy when supplied from the left end of FIG. It enters the cooling zone of the cooling jacket.

When only the rear cooling jacket 9 is operated as shown in a modified embodiment described later, the soot 2 produced by the burner 3 is deposited on the inner wall of the quartz tube 1 and is further vitrified by the burner 3. It The vitrified quartz tube is cooled by the jacket 9 and creates a temperature difference with the burner 3 at the next traverse.

Specifically, the heating section A by the burner 3
The larger the temperature difference at the point B where the cooling process is most advanced, the better the deposition efficiency of the soot 2, and the more stable the temperature difference, the more stable the characteristics of the soot. Then, as a result of the experiment, this temperature difference (T _A −T _B ) is equal to the temperature difference (T
_B - _TC ).

Further, the temperature difference (T _A -T _B ) also varies depending on the distance AB between the points A and B, and generally, the less the influence of heating by the burner 3 is, that is, the distance A.
It is obvious that the larger B is, the larger it is. Of course, it depends on the traverse speed of the burner 3.

Based on the above findings, the cooling jacket 5
A program for controlling the cooling efficiency by means of will be described with reference to the block diagram in the lower part of FIG. First of all, regarding the size of water droplets for cooling, it can be said that the larger the water droplets, the higher the heat capacity and therefore the higher the cooling effect. On the other hand, considering the transport efficiency by the carrier gas, the smaller the water droplets, the better as a refrigerant. The overall distribution contact amount cannot be said either. This should be determined experimentally. Then, the set frequency (frequency) of the mist generator 6 may be determined based on the experimental result.

Next, the temperature difference (T _A -T _B ) between the heating section A and the point B where the cooling process is most advanced is a function of the temperature difference (T _B -T _C ) at both ends of the cooling zone, as described above. Therefore, the flow rate controller 7 controls the flow rate of the carrier gas and the adjustment screw 43 adjusts the distance AB so that the temperature difference (T _B −T _C ) that achieves the best soot deposition can be achieved. .

FIG. 3 shows a modified embodiment of the present invention in which two cooling jackets 5 and 9 are provided before and after the burner 3. Although the description with reference numerals is omitted, the pipes connected to the jacket are the same as those already described. Although not shown, a plurality of cooling jackets may be provided on the front side, the rear side, or both sides of the burner.

The apparatus of the embodiment shown in FIG. 3 is the same as the apparatus of the embodiment shown in FIG. 1 if only the cooling jacket 5 is operated. Even if only the rear cooling jacket 9 is operated, the soot deposition is promoted, and if both cooling jackets 5 and 9 are operated at the same time, the effect is greater than in the case of one front cooling jacket.

The method of the present invention has been developed to promote soot deposition as described, but in the case of the apparatus as well as in the case of the collapse shown in the above-mentioned prior art. It can be used for the purpose of eliminating bubbles.

As a reminder, the difference between the device of the present invention and the prior art is that, unlike the cooled gas flow,
An inert gas containing fine water droplets with a large heat capacity is used as a refrigerant.Unlike just blowing a refrigerant gas, it is contacted and circulated by being surrounded by a cooling jacket so that it can stay in contact with a cooling portion for a long time. It is possible to perform precise control for maximum efficiency, and it is possible to selectively cool not only the front of the burner but also one or both of the front and rear.

[0032]

According to the first aspect of the present invention, since the partial area in front of and behind the heating section by the burner is selectively cooled, the kinetic energy of the generated soot is rapidly removed. It has the effect of promoting the deposition on the inner wall of the quartz tube.

According to the invention of claim 2, the flow of the inert gas containing fine water droplets having a large heat capacity as the refrigerant is
Since the cooling jacket that can be retained in contact with the outer circumference of the quartz tube for a relatively long time is used, the soot deposition promoting effect described above is further enhanced.

According to the third aspect of the invention, the cooling efficiency can be controlled so that the cooling process having the highest efficiency and the soot quality can be selected depending on the temperatures of the heating portion and the cooling area of the burner, so that the manufacturing efficiency is simply increased. However, there is an advantage that an optical fiber preform having good quality can be manufactured as a whole.

According to the invention apparatus of claim 4, there is an advantage that all the effects described in the above [0032] to [0034] can be obtained.

[Brief description of drawings]

FIG. 1 is a side sectional view showing an apparatus according to an embodiment of the present invention together with its control circuit.

FIG. 2 is a sectional view taken along line II-II of FIG.

FIG. 3 is a side sectional view showing a modified example of the present invention.

FIG. 4 is a side sectional view for outlining a typical MCVD method.

FIG. 5 is a schematic view for explaining an example of a conventional collapse process for removing bubbles.

[Explanation of symbols]

1 quartz tube 2 suit 3 burners 4 burner stand 41 Mounting base plate 42 traverse axis 43 Adjustment screw 5,9 cooling jacket 51 props 52 Mist injection pipe 53 Mist discharge pipe 6 Mist generator 7 Flow controller 8 thermocouple 101 Optical fiber base material 102 burners 103 cooling nozzle

Claims (4)

[Claims] 1. A quartz tube (1) to be an optical fiber preform.
While supplying the raw material gas to the inside of the quartz tube (1), the burner (3) is reciprocated in the axial direction of the quartz tube (1).
In the method for manufacturing an optical fiber preform, in which the soot (2) is generated and deposited inside the quartz tube (1) by heating the same, a front side or a rear side of the traveling direction of the burner (3), or both of them. A method for manufacturing an optical fiber preform, characterized by cooling a part of the area in the axial direction. 2. The cooling step, fine water droplets (H ₂ O)
2. The optical fiber preform according to claim 1, wherein the flow of a relatively inert gas containing is brought into contact flow with the outer circumference of the quartz tube (1) using a cooling jacket (5). Production method. 3. The cooling step is controlled so that a temperature difference between a heated portion of the burner (3) of the quartz tube (1) and a cooled portion of the cooling jacket (5) becomes constant. The method for producing an optical fiber preform according to claim 1 or 2, which is characterized in that. 4. A burner stand (4) carrying said burner (3) is mounted so as to be movable in the axial direction of said quartz tube (1), and surrounds said quartz tube (1) in the axial direction thereof. At least one hollow cooling jacket (5) that can move with the burner stand (4) and fine water droplets (H ₂ O)
(6) capable of generating a mist, and a mist feed for feeding the fine water droplets from the mist generator (6) into the cooling jacket (5) in a flow of a relatively inert gas. An apparatus for producing an optical fiber preform, comprising a pipe (52) and a discharge pipe (53) for discharging the gas flow from the cooling jacket (5).