CA1323193C

CA1323193C - Furnace for heating glass preform for optical fiber and method for producing glass preform

Info

Publication number: CA1323193C
Application number: CA000572413A
Authority: CA
Inventors: Ichiro Tsuchiya; Michihisa Kyoto; Yoichi Ishiguro
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1987-06-15
Filing date: 1988-07-19
Publication date: 1993-10-19
Anticipated expiration: 2010-10-19

Abstract

Abstract:
A furnace for heating a porous preform of fine particles of quartz base glass for making an optical fiber in an atmosphere containing fluorine to add fluorine to the preform and to vitrify the preform to produce the glass preform. The furnace has a heater and a muffle tube positioned inside the heater to separate the heating atmosphere from the heater. The invention is characterized in that at least an inner layer of the muffle tube consists of highly pure carbon. As a result, the life of the muffle tube is significantly lengthened.

Description

~3231~3 Furnace for heating glass preform for optical fiber and method for producing glass preform The present invention relates to a furnace for heating a glass preform for an optical fiber, and to a method for producing such a glass preform. More particularly, it relates to a furnace and to a method for heating a porous glass preform consisting of fine particles of quartz glass, adding fluorine to the preform and vitrify it. The furnace of the present invention can prevent contamination of the glass preform with impuri~y elements and has good durability.
As one of the general methods for mass producing a glass preform for use in the fabrication of an optical fiber, the VAD (Vapor Phase Axial Deposition) method is known. The VAD method comprises depositing fine particles of glass generated in an oxyhydrogen flame on to a rotating starting member, such as a glass plate or rod, to form a cylindrical porous preform (soot preform), and sintering said porous preform to obtain a transparent glass preform for use in the fabrication of an optical fiber.
In the VAD method, for sintering the porous preform to covert it into transparent glass, the preform should be heated in an atmosphere of inert gas (e.g. helium and :

` ~ ' ~ ., ' ' ' ' -.

132~93 argon) to a temperature of 1,6000C or higher. As the furnace for sintering the preform, a furnace having a carbon heater is usually used. What should be avoided when sintering the preform in such a furnace is the inclusion of transition metals, such as copper or iron, and water. When 1 (one) ppb or larger of a transition metal is included in the glass preform, the transmission loss wavelength characteristics of the fabricated optical fiber are greatly deteriorated over the entire wavelength range. When 0.1 ppm or larger of water is included in the preform, the characteristics of the fabricated optical fiber are impaired in the longer wavelength range.
Therefore, the porous preform is usually dehydrated before or during vitrification. As a dehydration method, it is known to heat the porous preform to a high temperature in an atmosphere of an inert gas containing a chlorine-containing gas, a fluorine-containing gas, etc.
When a fluorine-containing gas is used, not only is the porous preform dehydrated but also fluorine is added to the porous preform. When fluorine is added to the porous preform, the refractive index profile which is essential to the optical fiber is improved. In this connection, Japanese Patent Publication No. 15682/1980 and Japanese Patent Rokai Publication No. 67533/1980 can be referred to. These publications will be discussed below.
The treatment with a fluorine-containing gas is carried out in the furnace before or simultaneously with vitrification. To prevent wastage of the carbon heater due to moisture or oxygen that is generated during heating of the preform, a muffle tube is installed for separating the carbon heater and the sintering atmosphere. As the muffle tube, an alumina one is conventionally used (cf.
Japanese Patent Publication No. 40096/1982 and U.S. Patent No. 4,338,111). However, when an alumina muffle tube is ~ .

~323193 used, alkali components contained in the alumina float into the heating atmosphere at the high temperature and adhere to the surface of the porous preform to form a cristobalite layer.
A muffle tube of quartz has been used. In comparison with the alumina muffle tube, the use of a quartz tube gives following advantages:
1. The quartz has better mechanical processing accuracy and therefore the airtightness of the atmosphere is maintained so that the soot preform is effectively dehydrated.

2. The quartz tube contains few impurities, such as iron and alkali, and is much purer than an alumina muffle tube.

3. The glass preform produced by means of a quartz muffle tube does not suffer from surface devitrification caused by alkali.

4. The quartz tube hardly suffers from thermal breakage (breakage due to thermal shock).

5. When a fluorine~containing gas is used, no contaminating gas such as AlF3 or the like is generated. Although gaseous SiF4 is generated, it does not act as an impurity that has an adverse influence on the glass preform.
Methods utilizing a quartz muffle tube are described in detail in Japanese Patent Publication Nos. 58299/1983 and 42136/1983 and Japanese Patent Kokai Publication No.
86049/1985.
If copper and iron are contained in the quartz glass, they react easily with a chlorine-containing gas contained in the dehydration atmosphere according to the following reaction formulae, to form volatile chlorides which penetrate into the porous preform and severely deteriorate the transmission loss characteristics of the finally ~' .

~3231~3 fabricated optical fiber. This is a new problem associated with the quartz muffle tube.

CuO ~ CU2C12 Fe23 > FeC13 Another problem is that, since copper tends to diffuse easily in the quartz glass at a high temperature, copper which is liberated from the furnace itself or the heater penetrates through the muffle tube and contaminates the glass preform.
Further, a fluorine-containing gas is decomposed or reacts to form F2 gas or HF gas. These gases react with the quartz glass according to the following reaction formulae, to generate SiF4 gas, and the quartz glass is etched by these reactions:

SiO2 + 2F2 ~ SiF4 ~ 2 SiO2 + 4HF ~ SiF4 + 2h2O

Because of this etching, copper and iron present inside the quartz glass appear on its surface and contaminate the porous preform. In addition, pin holes are formed in the quartz made muffle tube by etching, which is a cause of intake of environmental air or leakage of the dehydration atmosphere, effects that are disadvantageous in the production method.
Furthermore, a quartz glass tube has the problem that it tends to deform easily at a high temperature. That is, if the quartz glass is kept at about 1,3000C for a long time, it deforms due to viscous flow. In addition, if it . ~
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is used at a temperature of l,150C or higher for a long time, it is devitrified, and, once the furnace temperature is lowered, strain is generated due to a difference of thermal expansion coefficients between the glass phase and the devitrified phase, and finally the tube breaks.
A glass preform for an optical fiber comprises a core part and a cladding partr the core part occupying a centee portion of the glass preform and having a higher refractive index than the cladding part, so as to transmit light to form this refractive index difference between the core and the cladding, the refractive index of the core is increased and/or that of the cladding is decreased. The term "Refractive index difference~ as used herein is intended to mean a difference of refractive index between a certain glass and pure silica.
To increase the refractive index of the core part, a refractive index increasing dopant, such as GeO2, A12O3 or TiO2 is added to a glass-forming raw material during synthesis of the quartz glass, so that an element such as Ge, Al and Ti is added to the glass.
However, when such a metal oxide is used, the following defects will arise:
In proportion to the increase of the amount of the added dopant, the light scattering (Rayleigh scattering) due to the dopant increases, which is not preferable. If a large amount of the dopant is added, bubbles or crystalline phases are generated in the glass preform.
For example, when GeO2 is used, bubbles of GeO gas tend to form, and when A12O3 is used, clusters of A12O3 tend to form. ~uch bubbles or crystalline phases have an undesired influence on the light transmission characteristics and also on the strength of the optical fiber.
Therefore, the core part preferably consists of pure ~323~3 quartz glass or a quartz base glass containing dopant in an as small an amount as possible.
One proposed measure for achieving the desired refractive index difference between the core part and the cladding part while overcoming the various above described drawbacks associated with the addition of a dopant to the core part, is to provide a glass preform for an optical fiber comprising a cladding part to which fluorine (which decreases the refractive index) has been adde~. One of the advantages achieved by the use of fluorine as a dopant is that the core part can be made of pure quartz or a quartz base glass containing as small as possible an amount of the dopant, since the refractive index of the cladding can be made lower than that of the pure quartz.
To enable the prior art to be described with the aid of diagrams, the figures of the drawings will first be listed.
Figs. lA and lB show general structures of a single mode optical fiber and a multi-mode optical fiber, respectively, Fig. 2 shows the structure of a low dispersion type of optical fiber comprising a cladding to which fluorine has been added, Fig. 3 schematically shows a cross section of a first embodiment of a furnace according to the present invention, Fig. 4 schematically shows a cross section of a modification of this embodiment, Fig. 5 schematically shows a cross section of a second embodiment according to the present invention, Fig. 6 schematically shows a cross section of a third embodiment according to the present invention, Fig. 7 schematically shows a cross section of a pressurized heating furnace, , -, . , : ' .............. . . . . : , . .

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~323193 Fig. 8 schematically shows apparatus used in an experiment for measuring an amount of inflow air, Fig. 9 is a graph showing the amount of inflow air, Figs. 10 and 11 schematically show cross sections of a fourth embodiment according to the present invention, Figs. 12A and 12B illustrate methods for producing a soot preform by flame hydrolysis, Figs. 13A to 13C show the structures of the soot preforms produced in Examples 9 to 11 or Examples 13 to 15, respectively, Figs. 14A to 14C show the structures of the glass preforms that were produced by adding fluorine to the soot preforms produced in Examples 9 to 11 or Examples 13 to 15, respectively, Fig. 15 is a graph showing weight loss of a carbon muffle tube, Fig. 16 is a graph showing the results to a tensile test of an optical fiber, and Fig. 17 is a graph showing the relationship between the heating temperature and the specific refractive index difference ~n(F) of the optical fiber in Example 16.
In the refractive index structures of a single mode optical fiber and a multi-mode fiber, shown in Fig. lA and lB, respectively, the "A" part and the "B" part respectively correspond to the core part and the cladding part.
Fig. 2 shows the refractive index structure of a quartz base glass optical fiber having a cladding to which fluorine has been added. By such a structure, the light scattering (Rayleigh scattering) due to the dopant contained in the core through which the light propagates is reduced, and the core has preferable properties as a _, - ~' ' , " ' ' ' .
, . .

~323~93 light transmitting guide.
Further, the resource for fluorine is richer than that for other dopants, such as GeO2, and purification of the raw material is easy, which is economically advanta~eous.
In addition, the fluorine gas not only acts as the dopant for adjusting the refractive index of the glass, but also acts as an excellent dehydrant for removing moisture contained in the soot preform. This is also one of the characteristics of fluorine.
For adding (or doping) fluorine to the quartz glass, several methods have been proposed.
Firstly, Japanese Patent Publication No. 15682/1980 describes a method comprising supplying fluorine-containing gas in a gaseous phase synthesis of glass, so as to add fluorine to the glass. Although this method can add fluorine to the glass, it has the drawbacks that the glass deposition efficiency and the fluorine addition efficiency (doping yield) are both low. The reason for this may be that in the flame hydrolysis which utilizes an oxyhydrogen flame, the moisture in the flame and the fluorine-containing gas, such as SF6, react according to formula (1) to generate hydrogen fluoride (HF) gas:

SF6 ~ 3H20 ~ SO3 ~ 6HF (1) Since the generated HF gas is stable, almost all the fluorine-containing gas is converted to HF gas at a high temperature as long as moisture is present, and only a slight amount of the remaining fluorine-containing gas is utilised as the dopant.
The HF gas etches the glass, particularly quartz, and reacts with the fine particles of the glass synthesized in the flame according to formulae (2) and (3)-6~
: ' ' ' ' ' ' ' . ' '.

132319'~
g SiO2(s) + 2HF(g) ~ SiOF2(g) + H2O(g) (2) siO2(s) + 4HF(g)-~ SiF4(g) + 2H2O(g) (3) wherein (s) and (g) stand for a gas and a solid, respectively. Thereby, the synthesized fine particles of the glass are consumed so that the deposition efficiency is decreased.
Accordingly, an increase of the addition of the fluorine-containing gas results in a decrease of the deposition rate of the soot particles.
Secondly, Japanese Patent Kokai Publication No.
67533/1980 discloses a method comprising synthesizing fine particles of the glass by flame hydrolysis, depositing them to form a soot preform, heating the formed soot preform in an atmosphere comprising a fluorine~containing gas to dope fluorine into the soot, whereby a glass preform containing fluorine is produced.
However, this method also has several drawbacks. In one embodiment of the method described in said Japanese Patent Kokai Publication, the soot preform is heated in the atmosphere comprising the fluorine-containing gas at a temperature of not higher that l,OOOoC. However, the addition rate of the fluorine is low and sometimes copper and ixon are present in the finally fabricated optical fiber. Copper and iron are known to cause absorption loss, which is a cause of an increase of transmission loss.
It is also suggested to treat the soot preform in the gaseous atmosphere comprising the fluorine-containing gas at a temperature of not lower than l,400C. However, the surface of the produced glass preform is etched, and also a muffle tube, such as a ~uartz muffle tube, for maintaining the atmosphere is sometimes severely damaged by etching. Such etching of the muffle tube is one of the ~ .

.

-~' ' .' - : .
.

~ 32~93 causes for increased contamination oE the soot preform with the impurities in the muffle tube.
In addition, the fabricated optical fiber in said Japanese Patent Kokai Publication suffers from a change of absorption loss with time due to hydroxyl groups, and the absorption loss greatly increases at high temperatures.
To overcome such problems, Japanese Patent Kokai Publication No. 239337/1985 discloses a method in which SiF4 is used as the fluorine-containing gas.
SiF4 is a fluorine-containing gas that does not etch the soot preform or the quartz glass muffle tube, so that it does not induce breakage of the muffle tube due to etching.
However, in addition to the above described drawbacks, a quartz glass muffle tube has the following drawbacks.
Through the quartz, impurities such as alkali and copper penetrate. If a slight amount of water is present, it reacts with SiF4 to form HF which etches the quartz glass muffle tube so that the impurities contained in the muffle tube material can contaminate the soot preform.
Penetration of the impurities can be prevented by lining the whole muffle tube with a highly pure material. But such lining increases the production cost of the muffle tube and is uneconomical. To prevent etching of the muffle tube, the soot preform and the muffle tube are throughly dried to remove moisture before the supply of SiF4 into the muffle tube, which requires airtight equipment or careful operation.
As a material that hardly reacts with a fluorine-containing gas or a chlorine~containing gas, carbon is contemplated. Carbon does not react with SF6, C2F6, CF4 or the like, which easily react with the quartz. Of course, carbon does not react with Si~4.
Japanese Patent Publication No. 28852/1981 suggests ~ 323193 the use of a carbon muffle tube in an atmosphere comprising a fluoeine-containing gas such as F2, although no working example is described.
However, the use of carbon has the following drawbacks:
1. Since the use of carbon has minute pores, gases can penetrate therethrough. The permeability of nitrogen through the carbon is 106 times larger than through quartz glass.
2. The carbon is easily oxidized and, at a temperature not lower than 4000C, it reacts easily with oxygen to form CO2 or CO.
To prevent oxidation, it has been proposed to form a layer of ceramic, such as SiC, A12o3 or BN, on an inner wall of the carbon muffle tube. Although the ceramic layer prevents oxidation, it reacts disadvantageously with at least one of the chlorine-containing gas and the fluorine-containing gas.
Impurities generated by such reaction devitrify the soot preform and generate bubbles in the soot preform.
Although F2 gas has no possibility of liberating carbon or sulfur, it reacts explosively with water.
Therefore, F2 gas is not suitable as a fluorine-doping gas.
Since carbon is a material having a large gas permeability, as described above, the gas goes in and out through the wall of the muffle tube, so that the moisture in the air penetrates into the muffle tube through its wall. Therefore, the glass preform contains a comparatively large amount of water and in turn hydroxyl groups. In addition, gasses such as C12 and SiF4 are released outside the furnace and may pollute the work environment, and impurities (e.g. copper and iron) can penetrate into the furnace from the outside. These defects can be partly overcome by increasing the thickness - :

.
.

1 323~1~3 of the carbon, but not completely.
As explained above, the addition of fluorine to the quartz glass of the cladding part by the conventional methods encounters various difficulties.
In view of these circumstances, the present invention aims to solve the problems of the conventional muffle tube that is used in the dehydration of a preform for an optical fiber and the addltion of fluorine to the preform, and to provide a muffle tube that has improved durability and long life and can prevent the penetration of air into the muffle tube.
As a result of an extensive study to solve the above described problems, it has been found that, when an inner wall of a muffle tube consists of a carbon layer, the muffle tube is not deteriorated even if a corrosive gas, such as a fluorine-containing gas or a chlorine-containing gas, is supplied at high temperature. This is because the muffle tube does not react with the fluorine-containing gas or the chlorine-containing gas, since the inner wall is coated with the carbon layer. As a result, such the muffle tube has a much longer life than conventional ones~
Accordingly, the present invention provides a furnace for heating a porous preform of fine particles of quartz base glass for making an optical fiber in an atmosphere containing fluorine to add fluorine to the preform and to vitrify the preform to produce the glass preform, said furnace comprising a heater and a muffle tube positioned inside the heater to separate the heating atmosphere from the heater, wherein at least an inner layer of the muffle tube consists of highly pure carbon.
In the present invention, a porous glass preform - consisting of fine particles of quartz base glass (herein occasionally referred to as a "soot preform") typically includes soot preforms having the following structures:

~ .

~2319~

1. A solid or hollow soot preform that consists entirely of Eine particles of glass. After vitrifying the soot preform, a bore is formed at a center part, and then a glass rod is inserted into this bore to produce the final glass preform.
2. A soot preform comprising a glass core and fine particles of the glass deposited around the core.
3. A soot preform comprising a glass core around which cladding has been formed, with fine particles of glass deposited around the cladding.
In the first embodiment of the present invention, the muffle tube has an inner layer made of carbon and an outer layer of silicon carbide. Preferred examples of such muffle tube are one made of carbon, an outer wall of which is coated with silicon carbide, or a silicon carbide muffle tube, an inner wall of which is coated with carbon.
Generally, the purity of the carbon is such that the total ash content is not larger than 50 ppm, preferably not larger than 20 ppm. Carbon having a total ash content of 1,000 ppm cannot be used for making a muffle tube in view of the impurities such as iron and copper. The impurities and their amounts contained in carbon having a total ash content of 20 or less are as shown in following Table.

Table 1 B <O.l ppm Ca <0.1 ppm Mg <0.1 ppm Ti <0.1 ppm Al C0.1 ppm V <0.1 ppm Si 0.8 ppm Cr <0.1 ppm P <0.2 ppm Fe ~0.1 ppm S <0 1 ppm Cu <0.1 ppm ~i <0.1 ppm ~, ,...... ~

- ~323~g~

Silicon carbide containing iron in an amount of several ppm or less and copper in an amount of 1 ppm oe less is preferably used.
When a muffle tube according to the present invention is used, as the fluorine-containing gas, silicon fluorides (e.g. SiF4, Si2F6, etc.) and carbon fluorides (e.g.
4, C2F6, C3F8, CC12F2, etc.) are preferred. Among them, SiF4 is particularly preferred.
A fluorine-containing compound containing oxygen is not preferred.
To coat the silicon carbide or carbon film, a film-forming method by a gas phase reaction, such as plasma CVD coating, chemical CVD coating or the like, is preferred, since a highly pure and dense film can be formed in this way.
Experiments and concepts on which the present invention is based will now be explained. The concepts explained below were established from the findings of the experiments.
Analysis of heat resistance Experiment 1 A quartz glass muffle tube having an inner diameter of 100 mm, a length of 300 mm and a wall thickness of 2 mm was heated to 1,500C and kept at the same temperature one day. The muffle tube was expanded to a length of 400 mm.

Experiment 2 A silicon carbide muffle tube having the same dimensions as the muffle tube used in Experiment 1, but having a dense carbon layer of 0.5 ~m in thickness on its inner surface, was subjected to the same test as in Experiment 1, and no expansion of the muffle tube was ~323~93 observed.

Experiment 3 The same muffle tube as used in Experiment 1 was heated from room temperature to 1,5000C over 3 hours in one day and cooled from 1,5000C to room temperature throughout the next day. After repeated heating and cooling for 20 days, the muffle tube was broken due to devitrification.

Experiment 4 The same muffle tube as used in Experiment 2 was subjected to the same heating test as in Experiment 3.
After 20 days, no problem arose.

Experiment 5 The same heating test as in Experiment 1 was carried out on a carbon muffle tube having an inner diameter of 110 mm, a length of 300 mm and a wall thickness of 6 mm, and a silicon carbide layer of 200 ~m in thickness on the outer wall. No expansion of the muffle tube was observed.

Analysis of oxidation resistance Experiment 6 A carbon muffle tube having an inner diameter of 110 mm, a length of 300 mm and a wall thickness of 5 mm, and a silicon carbide layer of 200 ~m in thickness on the outer wall was used and its interior space was filled with a helium atmosphere and its outer wall was exposed to the air. After keeping the muffle tube at 1,500C for 3 hours, no oxidation was observed.

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.

~23193 Experiment 7 The same oxidation test as in Experiment 6 was repeated while changing the thickness of the silicon carbide layer to about 5 ~m. Some parts of the outer wall were oxidized.

Analysis of corrosion resistance Experiment 8 The same heating test as in Experiment 6 was repeated while filling the inner space o the muffle tube with an atmosphere of helium containing 10 ~ by mole of C12 and 10 ~ by mole of SF6. No corrosion of the outer and inner walls of the muffle tube was observed. In addition, no leakage of the C12 and SF6 gasses through the tube wall was observed. This is because the dense silicon carbide layer prevented leakage oE the gasses.

Experiment 9 The same test as in Experiment 8 was repeated using a carbon muffle tube having no silicon carbide layer. The outer wall was severely oxidized and leakage of ~12 and SF4 gasses through the tube wall was observed.

Experiment 10 The same test as in Experiment 8 was repeated using a muffle tube having a silicon carbide layer on the inner wall instead of the outer wall. The silicon carbide layer on the inner wall reacted with the gasses to dissipate, and the ou~er wall was oxidized.
. ..
Experiment 11 The same corrosion test as in Experiment 8 was .

..

'' repeated using a silicon carbide muffle tube having an lnner diameter of 100 mm, a length oE 300 mm and a wall thickness of 5 mm, and a carbon layer of about 1 ~m in thickness. The same results were achieved.
From the results of Experiments :L to 11, the following can be concluded:
1) A carbon muffle tube and a silicon carbide muffle tube can be resistant to very high temperatures in comparison with a pure quartz glass tube.
2) When a fluorine-containing gas is used, a silicon carbide muffle tube having a carbon layer on the inner wall is not etched. A carbon muffle tube having a silicon carbide layer on the outer wall has the same effect.
Based on the above experiments, it has been found that, as a muffle tube for heating a porous preform at a temperature not lower than 1,500OC, a heat resistant muffle tube having a carbon inner layer is suitable, particularly when a fluorine-containing gas is used. Such findings can be explained as follows.
A muffle tube made of quartz glass (SiO2) is etched by the reaction of the SiO2 of the muffle tube or the porous preform with SF6 according to the following formula (I):

SiO2(s~ + SF6~g) ~ SiF4(g) ~ SF2(g) + O2(9)... (I) wherein (s) and (g) stand for a solid and a gas.
On the other hand, since carbon does not react with SF6, the SF6 does not etch the carbon.
In the case of a muffle tube made of silicon carbide, the thickness of the carbon layer formed on the inner wall is about 0.01 to 500 ~m to achieve the objects of the :

.

~323193 present invention. There is no limitation on the method to be used for forming the carbon layer on the inner wall, and any of the conventional methods can be used. For example, a method comprising heating a muffle tube to be coated to a temperture of 1,200 to 1,500C and flowing a mixture of argon and a vapor of CH4 or CCl~ through the inner space of the muffle tube to deposit the carbon on the inner wall of the muffle tube (the CVD method) is known. In this method, the thickness of the deposited carbon per run is preferably about 0.2 ~m to prevent surface cracking or peeling off. Therefore, the deposition procedure is repeated 500 times to form a carbon layer of 100 ~m.
The thickness of the silicon carbide layer is generally from 10 to 300 ~m, preferably from 50 to 250 ~m.
Each of Figs. 3 and 4 illustrates the first embodiment of furnace according to the present invention.
In Fig. 3, numeral 1 is a porous preform, 2 a supporting rod, 3 a muffle tube, 4 a heater, 5 a furnace body, 6 an inlet for introducing an inert gas, and 7 an inlet for introducing an atmosphere gas (e.g. SF6 and helium). 31 is the body of a carbon muffle tube and 32 is a silicon carbide coating layer.
In Fig. 4, numeral 1 is a soot preform, 2 a supporting rod, 3 a muffle tube, 4 a heater, 5 a furnace body, 6 an inlet for introducing an inert gas, and 7 an inlet for introducing an atmosphere gas (e.g. SF6 and hçlium).
31' is the body of a silicon carbide muffle tube and 32' is a carbon coating layer.
The second embodiment of the present invention is illustrated in Fig. 5. A heater 4 is installed inside the body of a furnace 5, and a muffle tube 3 is installed at the center of the furnace.
A body of the muffle tube 3 consists of a quartz glass d~ , `3.' ~32319~

tube having a coating of a carbon layer 33 on the inner wall thereof.
The carbon layer is coated by one of the same methods as in the above first embodiment.
The thickness of the carbon layer 33 is preferable from 0.01 to 500 ~m. If the thickness of the carbon layer is larger than 500 ~m, the layer tends to peel off, and if it is less than 0.01 ~m, the desired effect is not achieved.
Instead of the carbon layer, a ceramic film that has a higher meiting point and is corrosion resistant to the fluorine-containing gas can be formed on an undercoat of silicon nitride having a thickness of 2 to 20 ~m. The following compounds are suitable as the ceramic:
Carbides: SiC, WC, TaC
Nitrides: AlN, ThN, ZrN, BN, TaN
Oxides: A12O3, CaO, ZrO2, Th2 Borides: SiB, TaB2, ZrB
At a side of the furnace body 5, an inlet 6 for supplying a blanketing gas (e.g. argon and nitrogen) is provided. At a lower end of the muffle tube 3, an inlet 7 for supplying a treating gas (e.g. helium, argon, chlorine, the fluorine-containing compound, etc.) is provided. In the upper part of the muffle tube 3, the porous preform 1 is suspended by means of the supporting rod 2.
In this construction, a quartz glass muffle tube having an inner lining of a carbon layer is more dense and has a smaller coefficient of thermal expansion than an aluminum or carbon tube, so that it is less likely to be broken by thermal stress and has good durability.
To prevent contamination of the preform due to diffusion of impurities in the quartz glass, it is preferred to make the muffle tube body from a quartz glass ~ , ...

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~32~1~3 that is as pure and transparent as possible. Preferably, the purity of the quartz glass is 0.5 ppm or less of copper in terms of a CuO content and 1 ppm or less of iron in terms of a Fe2O3 content. Particularly suitable is a transparent quartz glass containing no copper components.
Since impurities such as copper, iron and water, which are diffused from the outer heater body 5 and the heater 4, cannot penetrate through the carbon layer 33, they are shielded by the carbon layer 33 and cannot migrate into the inside of the muffle tube 3. Contamination of the optical fiber preform with such impurities is thus prevented.
Further, since the inner wall of the quartz glass tube is lined with the carbon layer 33, corrosion of the muffle tube is prevented, even when the porous preform is sintered in an atmosphere comprising a fluorine-containing compound. The etching effect of a HF solution on the quartz glass and the carbon are shown in the table below.

Table _ Quartz glass Carbon Etched amount -0.1 0 (g/cm3.day) Note: After immersing a sample in a 46 % HF
solution at room temperature for 7 days, the change of sample weight was measured.

As is clear from the above table, the carbon has a notable corrosion resistance. Therefore, copper, iron and water contained in the quartz glass are not exposed on the surface and do not contaminate the preform, so that the .
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l 323193 purity of the glass preform is further improved.
In the third embodiment of the present invention (Fig.
6), the muffle tube consists of upper, middle and lower parts that are detachably connected to each other, at least the middle part being made of highly pure carbon, while the upper and lower parts are made of a heat and corrosion resistant material. A heat:er 4 is installed inside a body of a furnace, and the Muffle tube 3 is installed at the center of the furnace. The muffle tube 3 consists of an upper part 34, a middle part 35 and a lower part 36, the adjacent parts being detachably connected by suitable means such as screw threads. The middle part 35 is made of highly pure carbon. The purity of the carbon is the same as in the first embodiment.
Since the upper and lower parts are not heated to as high a temperature as the middle part, they are not necessarily made of a material so highly pure as the middle part, insofar as said material is heat and corrosion resistant. The upper and lower parts are preferably made of a conventional carbon for economy.
Since the upper and lower parts are heated to a temperature not higher than l,OOOoC, they can be made of a quartz material that is less corrosion reisistant to a fluorine-containing gas. However, in such a case, the iron and copper content, particularly the copper, should be taken care of and should preferably be less than 0.1 ppm-A muffle tube, the middle part of which is made ofhighly pure carbon, is preferable, since it does not react with the halogen-containing compound unless the atmosphere contains oxygen, and has excellent heat resistance.
During the treatment of the porous preform, the carbon of the middle part 35 is exposed to a high temperature and is worn by moisture occluded in the preform and moisture ~.

- . . ~ .

and oxygen migrated from outside after long time use. The carbon inner wall tends to wear due to causes associated with the treatment of the porous preform, which will now be explained.
SiO2 powder liberated from the porous preform adheres to the carbon inner wall and reacts with the carbon to form SiC, and oxygen generated by said reaction further reacts with the carbon to form CO. The SiC reacts readily with the chlorine-containing gas that is used for dehydration. The carbon inner wall is worn by such reaction with the SiO2 powder.
These reactions can be expressed by the following formulae:

SiO2 + C ~ SiC + 2 O + 2C ~ 2CO
SiC + C12 ~ SiC14 + C

Therefore, the middle carbon part should be replaced by a new one after prolonged use.
On the other hand, since the upper and lower parts of the muffle tube are not so severely worn, only the middle part need be replaced.
Since the carbon is porous, it is necessary to thoroughly remGve absorbed moisture at a high temperature. Therefore, in view of this need to remove absorbed moisture, it is preferred to replace the carbon muffle tube as infrequently as possible. When the middle part of the muffle tube is worn out, it is not necessary to remove the absorbed moisture from the upper and lower parts, since they can still continue to be used. Apart - 30 from economy, a three part muffle tube has various advantages.
Ac described above, the upper part 34 and the lower ~323~ 93 paet 36 can be made of the quartz glass instead of a porous material such as carbon. Particularly, highly pure quartz containing no impurities such as copper and iron is preferred. Copper and the like tend to generate oxide vapors of CuO at a temperature higher than 600OC and to contaminate the porous preform.
At the side of the furnace body 5, an inlet 6 for supplying the blanketing gas (e.g. argon, helium and nitrogen) is provided, and at the lower end of the muffle tube 3, an inlet 7 for supplying the treating gas (e.g.
helium, argon, the chlorine-containing gas and the fluorine-containing gas) is provided. In the upper part of the muffle tube 3, the porous preform 1 is suspended by means of the supporting rod 2. Generally, the furnace is constructed as illustrated in Figs. 3 to 6, or as illustrated in Fig. 7 which will now be explained.
Fig. 7 shows an example of a furnace for carrying out thermal treatment under elevated or reduced pressure. The furnace body 5 consists of a pressure vessel. The furnace comprises a carbon heater 4, a muffle tube 3, an insulator 4', an inlet 6 for supplying the gas constituting the muffle tube atmosphere, and an outlet 8 for the gas and a pump 9.
Since the furnace can be designed as shown in Fig. 6 or 7, air (environmental atmosphere) flows into the muffle tube interior space when the preform is inserted into or removed from the muffle tube.
Fig. 8 schematically shows equipment that is used in measurement of the amount of air inflow into the muffle tube. This equipment comprises a muffle tube 101, an inlet for purging gas 102, a gas sampling tube 103, a -- device 104 for measuring oxygen concentration and a pump 105. The inner diameter of the muffle tube 101 is 150 mm, and the front end of the gas sampling tube 103 is fixed at - ~ ' ~3231~

a point which is 1 mm below the upper edge of the muffle tube. The results are shown in Fig. 9. These results suggest that air flows into the muffle tube, and that such air inflow cannot be prevented by an increase of the purging nitrogen gas.
Inflow of the air will cause various problems.
Firstly, the interior space of the muffle tube is contaminated by dust in the air. Such dust comprise SiO2, A1203, Fe203 and the like. Among them, A1203 will cause devitrification of the preform, and Fe203 will cause an increase of transmission loss of the optical fiber. Secondly, the inner surface of the carbon muffle tube is oxidized. During oxidation of the sintered body of carbon, it is known that tar or pitch which is used as a binder is firstly oxidized. Therefore, the remaining graphite particles are dropped or splashed out and float in the furnace. Since these particles adhere to the surface of the sintered glass preform, an optical fiber fabricated from such a glass preform has many parts with low strength. As a natural consequence, the lifetime of the carbon muffle tube is shortened. Fig.
15 shows results of measured weight loss of a carbon muffle tube. After repeating the removal of the glass preforms 40 times, a surface of 0.4 mm in thickness of the carbon muffle tube was oxidized and worn. The lifetime of the carbon muffle tube having a wall thickness of 1 cm is estimated to be about 2.5 months.
One of the measures to prevent such oxidation of the muffle tube is to reduce the temperature to 4000C or lower, at which the carbon is not oxidized, during the insertion and removal of the glass preform. However, at - such a low temperature, the operating rate of the furnace is greatly decreased. The contamination of the interior space of the muff:Le tube with dust in the air cannot be .
`~ ` : :
:

.

~323193 prevented. The inflow of the air into the muffle tube can be prevented by the fourth embodiment (Fig. 10) of furnace according to the present invention. In addition to the heater and the muffle tube, the furnace of the fourth embodiment comprises a front chamber through which~ the porous preform is inserted into and removed from the muffle tube.
Preferably, the front chamber can be heated up to 8000C and evacuated down to a pressure of 10 2 Torr. or less.
The front chamber is preferably made of a heat resistant material which does not liberate impurities, such as quartz glass, SiC, Si3N4, BN and the like.
The front chamber may be made of the same material as or lS different from that of the muffle tube. The front chamber can be evacuated by a rotary pump. To prevent a back flow of pump oil, a liquid nitrogen-cooled trap can be connected between the pump and the front chamber. At the upper wall of the front chamber, a rotary installing mechanism having a magnetic seal is provided.
This embodiment is particularly useful when the muf1e tube is made of highly pure carbon, although it can be used for a muffle tube made of other materials such as quartz glass.
Fig. 10 schematically shows this fourth embodiment which is the same as that of Fig. 6 to which a front chamber 11 is attached. In other words, in addition to all the parts of the furnace of Fig. 6, this furnace comprises the front chamber 11, an outlet 14 for front chamber gas, an inlet 15 for a gas for purging the front chamber and a partition 16.
Fig. 11 shows an example of the fourth embodiment for carrying out thermal treatment under elevated or reduced pressure. This furnace is the same as that of Fig. 7 to ~. .

.

.
' 2319~

which the front chamber 11 is attached. In other words, in addition to all the parts of the furnace of Fig. 7, this furnace comprises the front chamber 11, a heater 12, a pump 13, the outlet 14 for front chamber gas, the inlet 15 for a gas for purging the front chamber and a partition 16.
The insertion of the porous preform into the heating furnace of Fig. 10 is carried out as follows:
1. To a rotatable, vertically movable chuck, the porous preform 1 is attached by the supporting rod 2.
2. An upper cover of the front chamber 11 is opened, and the porous preform 1 is lowered into the front chamber 11.
3. The upper cover is closed, and the interior space of the front chamber is purged with an inert gas (e.g. nitrogen or helium).
4. The partition 16 which separates the front chamber 11 and the heating atmosphere is opened, and the porous preform 1 is introduced in the heating atmosphere which has been kept at a temperature at which the preform is thermally treated.
5. The partition 16 is closed.
The preform is removed from the heating furnace of the present invention as follows:
1. The partition 16 is opened.
2. The preform 1 which has been thermally treated is pulled up from the heating atmosphere to the front chamber 11. In this step, the ~emperature of the heating atmosphere is not necessarily lowered.
3. The partition 6 is closed.
4. The upper cover of the front chamber 11 is opened, and the preform is removed from the chamber 11.
According to another aspect of the present invention, there is provided a method for producing a glass preform ~ , ' ..~ -~i .~.,..
:
. : ~ ' . .

-.
.

~l32319~

for an optical fiber which comprises thermally treating a porous preform comprising fine particles of quartz glass in a heating furnace comprising a muffle tube, at least an inner layer of which is made of carbon, in an inert gas atmosphere containing at least one fluoride selected from the group consisting of silicon fluorides and carbon fluorides so as to add fluorine to the glass, and simultaneously or thereafter, vitrifying the fine particles of the glass to produce a glass preform.
As the muffle tube, one of the above described muffle tubes of various embodiments of the present invention can be used.
To completely remove the contaminations during processing of the muffle tube or to absorbed dust and moisture, the carbon muffle tube is preferably baked for several hours in an atmosphere comprising a chlorine-containing gas, particularly C12 at a temperature not lower than 1500C. When the optical fiber is fabricated from a glass preform that was produced by means of an unbaked muffle tube, it may have considerable absorption due to moisture or impurities.
Further, to prevent penetration of impurities from outside, the outer wall of the muffle tube is preferably covered with a heat resistant material. As the covering material, ceramics or metals that have a nitrogen permeability of-the order of 10 6cm2/sec. or less are preferred. As ceramics, in addition to the above exemplified silicon carbide, A1203, BN and the like can be used. Particu]arly, ~-SiC which is formed by the CVD method is preferred. Since silicon carbide has good affinity with the carbon and no pin holes or microcracks, it can maintain high denseness. This is because the coefficient of thermal expansion of silicon carbide is close to that of carbon. Further, silicon carbide has ,~

~3231 ~3 excellent heat resistance and moisture resistance.
A1203 is less preferred than other ceramics, since it may generate AlC at high temperatures.
As the metals, those that do not react with carbon, such as platform and tantalum are preEerably used. The metal is coated on the carbon surface by flame spray coating. When a metal that is highly reactive with carbon, such as titanium and nickel is used, the carbon surface is precoated with a ceramic and then the metal is flame spray coated.
A larger thickness of the outer wall coating is better. However, too thick a coating may suffer from peeling off due to thermal stress. Therefore, the thickness of the outer wall coating is generally from 10 to 300 ~m, and preferably from 50 to 250 ~m, although it can vary with the kind of the material.
Among the fluorine-dopants to be used in the method of the present invention, SiF4 is most preferred. SiF4 is preferably a highly pure product of 3N or higher.
Although SiF4 does not react with carbon at all, if the soot preform is used without thorough dehydration, it may generate fumes in the carbon muffle tube during the step of the addition of fluorine. Such fumes can be generated by the reaction of the moisture in the soot preform with SiF4 or the carbon. As a result, deposits which may be carbon particles are accumulated on the upper portion of the soot preform. To prevent this the soot preform is preferably dehydrated before thermally treating it in the muffle tube having an atmosphere containing SiF4. The dehydration of the soot preform is carried out at à temperature at which the soot preform does not shrink, in an atmosphere of an inert gas (e.g. argon or helium) containing no more than 10 ~ by mole of a chlorine-containing gas having no oxygen, such as C12, ~ f~

.

~3231 9~

CC14 and S2C12, particularly C12 and CC14. The dehydration temperature is usually from 800 to 1,2000C.
Although it is possible to dehy~rate the soot preform simultaneously with the addition of fluorine, the 5 dehydration is preferably carried out before the addition of fluorine for the reasons described above and the dehydration effect.
The addition of fluorine to the soot preform with SiF4 is effectively performed at a temperature of 1,000C or higher, preferably from 1,100 to 1,400C. A
sufficient amount of fluorine should be added to the preform before the shrinkage of the soot preform is completed. If the soot preform shrinks before a sufficient amount of fluorine has been added, fluorine is not added to the entire preform and is ununiformly added, so that a distribution of the amount of added fluorine is present in the preform.
The soot preform is generally produced by the flame hydrolysis method and consists of fine particles of glass having a particle size of 0.1 to 0.2 ~m.

Production of soot preform To produce a mass of fine particles of quartz glass by flame hydrolysis, using a quartz glass coaxial multi tube burner 41 as shown in Fig. 12A, oxygen 42, hydrogen 43 and, as a raw material gas, SiC14 or a mixture of SiC14 and a doping compound (e.g. GeC14) are supplied to and reacted in the center of the oxyhydrogen flame from an inlet 45 together with a carrier gas.
An inert gas for shielding is supplied from an inlet 44 so that the raw material gas reacts in a space several millimeters away from the front end of the burner 41. To produce a rod form soot preform, the particles of the glass are deposited on the lower tip of a rotating seed 132~1~3 rod 46 in the direction of the axis of the seed rod 46.
To produce a pipe form soot preform, the particles of the glass are deposited around the periphery of a rotating quartz or carbon rod 46 while traversing the burner 47, and then the rod 46 is removed. The rod 46 can be a glass rod for the core. In such case, it :is not necessary to remove the rod. A plurality of rods can be used.
The soot preforms produced as above have refractive index structures as shown in Figs. 13A, 13B and 13C, in which "A" and "B" correspond to the core part and the cladding part, respectively.

Fluorine addition to soot preform and vitrification (sintering) of preform In a muffle tube (cylindrical muffle tube with upper and lower flanges) made of highly pure carbon the outer peripheral surface of which is coated with a material having small gas permeability, for example, as shown in Fig. 6, a soot preform produced in the above manner is suspended at a position above the heater, and the interior of the muffle tube is filled with an atmosphere of helium containing C12 gas. After heating the atmosphere to l,0500C by the heater, the soot preform is lowered at a rate of 2 to 10 mm/min. After the whole soot preform has passed the heater, the lowering of the soot preform is 25 stopped and the supply of the C12 gas is terminated.
The atmosphere is then changed to a helium atmosphere containing SiF4. After the heater temperature reaches l,6500C, the soot preform is pulled up at a rate of 4 mm/min. so as to add fluorine to the preform and simultaneously to make the preform transparent. In the refractive index structure of the glass preform, since fluorine is added, the refractive indices of the core and cladding parts are decreased as shown in Figs. 14A, 14B

~, , ~, .

~323~93 and 14C.
The present invention will be i]lustrated by foilowing Examples.

Example 1 A quartz made muffle tube having a carbon inner lining of 0.5 ~m was heated to 1,6000C by the heater, and SF6 and helium were flowed therein at rates of 50 ml/min. and 5 l/min., respectively. A porous preform was then inserted into the muffle tube at a lowering rate of 2 mm/min. The transparent glass preform thus obtained was drawn, to fabricate an optical fiber. The optical fiber contained 0.01 ppm of residual water, and had no light absorption due to copper or iron.
By using the same muffle tube, 100 transparent glass preforms were produced. No deterioration of the muffle tube body or the carbon coating was observed.

Comparative Example 1 In the same manner as in Example 1 but using a quartz glass muffle tube containing 1 ppm of copper, but having no carbon inner lining, an optical fiber was fabricated.
The optical fiber contained 0.01 ppm of residual water, and had absorption due to copper near to a wavelength of 1.30 ~m. This was sufficiently low in comparison with absorption by a conventional optical fiber, and the absorption value was 2 to 3 dB/km at a wavelength of 0.8 ~m. However, the inner wall of the muffle tube was severely etched. This means that this muffle tube had insufficient corrosion resistance.

Example ~
By using the same muffle tube as in Example 1 and filling the interior of the muffle tube with 100 % SiF4 ~, ,.~

~L323~93 atmosphere, the porous preform was doped with fluorine and vitrified simultaneously in the muffle tube. The transparent glass preform contained fluorine in an amount corresponding to ~~ of 0.7 %. The produced glass preform was bored along its axis to form a cladding member. By using such cladding member, a single mode optical fiber was fabricated. The optical fiber had no absorption due to impurities, and its transmission loss at a wavelength band of 1.5 ~m was as low as 0.25 dB/km.

Example 3 The furnace of Fig. lO was used. The porous preform was inserted in the front chamber and the upper cover was closed. Nitrogen gas was supplied at a rate of lO Q/min.
for 10 minutes to replace the interior ~as in the front chamber with nitrogen. Then, the partition was opened, and the porous preform was inserted into the muffle tube from the front chamber. After closing the partition, the preform was thermally treated to produce a transparent glass preform. To remove the preform from the furnace, the partition was opened, the preform was moved to the front chamber, the partition was closed, and then the upper cover was opened, followed by removal of the preform.
An optical fiber fabricated from this gIass preform had a low transmission loss of ~.18 dB/km at a wavelength of 1.55 ~m.

Example 4 In the same manner as in Example 3, 40 transparent glass preforms were produced. The weight loss of the carbon muffle tube was 20 g, which corresponds to an oxidation wear of 50 ~m from the surface~ This worn amount suggests that the carbon muffle tube could be used for about 1.5 years.

~ .
.
.

Example 5 The apparatus of Fig. 11 was used. A porous preform was inserted into the front chamber, the upper cover was closed, and nitrogen gas was supplied at a rate of 10 Q/min. for 10 minutes to the front chamber to replace the interior of the front chamber with the nitrogen gas.
Then, the partition was opened, the porous preform was moved into the muffle tube which was kept at l,OOOoC and then the partition was closed. Thereafter, the furnace was evacuated to 10 2 Torr. and heated to 1,600C to vitrify the porous preform so as to produce a glass preform. The produced glass preform was used as a jacketing member and an optical fiber was fabricated therefrom and subjected to a tensile test. The results (Weibull plot) is shown in Fig. 16. The low strength parts were only 5 %.

Example 6 The apparatus of Fig. 11 was used. A porous preform was inserted into the front chamber, and the interior of the front chamber was replaced with nitrogen. Then, the partition was opened, the porous preform was moved into the muffle tube which was kept at l,000C and the partition was closed. Thereafter, the furnace was pressurized to 2 kg/cm2 while introducing SiF4, and the porous preform was vitrified under such conditions as to produce a transparent glass preform. The produced glass preform contained 3 % by weight of fIuorine. By using this glass preform as a cladding material, a single mode optical fiber was fabricated. It had a transmission loss of 0.22 dB/km at a wavelength of 1.55 ~m, and its low strength parts according to the tensile test were 5 %.
Example 7 The apparatus of Fig. 11 was used. After the porous ,~

13~3~3 preform was inserted into the front chamber, the front chamber was kept under a pressure of 10 2 Torr. at 8000C. Then, the partition was opened, the porous preform was moved into the muffle tube and the partition was closed. Thereafter, the preform was subjected to the thermal treatment to produce a transparent glass preform.
An optical fiber fabricated from the produced preform and used as a core material had a very low transmission loss of 17 dB/km at a wavelength of 1.55 ~m.
Example 8 In the same manner as in Example 7, 40 porous preforms were thermally treated. The weight loss of the carbon muffle tube was 15 g, which corresponds to an oxidation wear of 40 ~m from the surface. This wear amount suggests that the carbon muffle tube could be used for about 2 years.
In subsequent Examples, the muffle tube of Fig. 6 was used. The muffle tube was made of carbon and had a silicon carbide layer of 150 ~m in thickness on the outer wall. It had an inner diameter of 150 mm, an outer diameter of 175 mm and a length of 1,500 mm.

Example 9 -On a peripheral surface of a starting member consisting of a quartz glass rod containing 17 % by weight of Ge02, which constituted the core part, soot of pure quartz ~Si02) was deposited by flame hydrolysis to produce a soot preform having the refractive index profile of Fig. 13A.
The soot preform was suspended at a position about 5 cm above the heater 3 in an atmosphere of helium containing 1 ~ by mole of C12. When the heater temperature reached 1,050C, the soot preform was lowered at a rate of 3 mm/min. After the whole soot preform had `

. . .

~ 32319~

passed the heater 3, it was pulled up at a rate of 20 mm/min. until the lower end of the preform reached a position about 5 cm above the heater.
Then, the heater temperature was raised to 1,750C and the supply of C12 was terminated. Instead, helium containing 20 ~ by mole of SiF4 was supplied to the muffle tube, and the soot preform was lowered at a rate of 2 mm/min. to make it transparent.
The produced glass preform had the refractive index profile of Fig. 14A.
The glass preform was drawn to fabricate an optical fiber having an outer diameter of 125 ~m by means of a drawing furnace. The content of the OH groups in the optical fiber was 0.01 ppm and its transmission loss at a wavelength of 1.30 ~m was as low as 0.45 dB/km. No absorption peak due to impurities such as copper and iron was observed.

Example 10 In the same manner as in example 9 but using a pure quartz rod having a diameter of about 8 mm as a starting member, soot of pure Si02 was deposited to produce a soot preform having the refractive index profile of Fig.
13B.
In the same manner as in Example 9 but supplying SiF4 in a concentration of 10 % by mole, the soot preform was thermally treated (dehydration, fluorine addition and vitrification). The produced glass preform had the refractive index profile of Fig. 14B.
The composition of the part of the preform to which fluorine was added was analyzed by an IR spectrometer to find that the content of the OH groups was less than 0.1 ppm.

~323~9~

Example ll On a peripheral surface of a starting member consisting of a quartz glass rod containing 0 to 17 % by weight of Ge02 and having ~ refractive index profile of Fig. 13C, soot of pure Si02 was deposited by flame hydrolysis. Then, in the same manner as in Example 9, the soot preform was thermally treated. The produced glass preform had the refractive index profile of Fig. 14C.
Comparative Example 2 (Heat resistance of a quartz glass muffle tube) In the same manner as in Example 9 but using a quartz glass muffle tube in place of the carbon muffle tube, a soot preform was produced. The quartz glass muffle tube was expanded during vitrification of the soot preform and could not be reused.

Comparative Example 3 (Etching of a quartz glass muffle tube) In the procedures of Comparative Example 2, SF6 was used in place of SiF4. Then, the quartz glass muffle tube was heavily etched to form pin holes in the wall near the heater. The produced glass preform contained several ppm of water. Of course, the muffle tube was considerably expanded and could not be reused.

Example 12 (Repeated use of the carbon muffle tube) In the same manner as in Example 10, ten glass preforms were produced. All the glass preforms had substantially the same quality.

Examples 13-15 A glass preform was produced in the same manner as in each of Examples 9 to ll but supplying no Cl2 gas.

. .

13~3~9~

The soot preforms and the glass preforms had substantially the same refractive index profiles as those produced in Examples 9 to 11, respectively.

Characteristics of an optical fiber fabricated .
from a glass preform The characteristics of the optical fibers fabricated from the glass preform produced in Examples 9 to 11 were measured. The optical fibers showed no absorption increase due to impurities and had a sufficiently low transmission loss, for example 0.4 dB/km at a wavelength of 1.3 ~m. Further, the absorption peak due to the OH
groups did not change with time.
On the contrary, the optical Eibers fabricated from the glass preforms produced in Examples 13 to 15 contained a comparatively large amount of OH groups so that the absorption loss at a wavelength of 1.30 ~m was slightly larger but still acceptable. From this fact, it is understood that it is better to dehydrate the preform in the presence of a chlorine-containing gas for the purpose of decreasing the transmission loss of the optical fiber.
In the present method, the addition of fluorine and the vitrification of glass can be performed separately from each other by using different furnaces or the same furnace. In either case, the same amount of fluorine is added and the optical fiber has the same characteristics.

Example 16 Treating temperature in an atmosphere comprising a fluorine-containing gas and the relationship between the amount of added fluorine and refractive index difference Fig. 17 shows refractive index differences (~n %) achieved by keeping a porous preform at the temperature .,. .~, ~0 ~32319~

indicated on the abscissa in an atmosphere of an inert gas containing 1 % by mole of chlorine gas and 2 % by mole of SiF4 for 3 hours. From these results, it was understood that the fluorine could be effectively added to the soot preform in a temperature range of 1,100 to 1,4000C.

Examples 17(1~ to 17(3) Three soot preforms substantially the same as those of Examples 9 to 11 were produced (in Examples 17(1), 17(2) and 17(3)). Each of them was heated and dehydrated in an atmosphere of argon containing 1 % by mole of C12 in a temperature range of 800 to l,100C, and heated from l,100C to 1,400C in an atmosphere of helium containing 20 % by weight of highly pure SiF4 to make it transparent.
From each of the produced glass preforms, an optical fiber was fabricated and its characteristics were measured. All the optical fibers showed no increase of absorption due to impurities and had a sufficiently low absorption loss, for example, less than 0.5 dB/km at a wavelength of 1.30 ~m. Further, the absorption peak due to the OH groups did not change with time.

Example l8 In the same manner as in Example 9 but using, as the starting member, a glass rod of 10 mm in diameter consisting of a center part of pure quartz and a quartz layer which was formed on the periphery of the center part and contained l % by weight of fluorine, a soot preform was produced.
The soot preform was inserted from one end to the other at a rate of 4mm/min. into a zone furnace kept at 1,200~C and having an atmosphere of helium gas containing 2 % by mole of C12. After the furnace had been heated ~
.

~323193 in an atmosphere of helium gas containing 20 % by mole of SiF4 at 1,6500C, the soot preform was inserted from one end at a rate of 4 mrn/min. into the furnace to make it transparent. From the glass preform, an optical fiber was fabricated.
By measurement of the characteristics of the fabricated optical fiber, it was found that the fiber showed no absorption due to impurities and had a sufficiently low transmission loss, for example, less than 0.4 dB/km at a wavelength of 1.30 ~m.

Effects of the present invention According to the present invention, a glass preform for an optical fiber that is not contaminated with iron or copper can be produced while decreasing the wear on the muffle tube. An optical fiber having a small transmission loss can be fabricated from the glass preform.
By forming the inner wall of the muffle tube from the carbon layer and the outer wall from silicon carbide, the muffle tube is hardly worn by heat or the corrosive gases even at high temperatures, so that it has good durability.
By providing a carbon coating on its inner wall, the corrosive wear of a quartz glass muffle tube by fluorins can be prevented, so that the durability of the muffle tube is improved.
Further, by making a middle part of the muffle tube from highly pure carbon, the contamination of the porous preform with an impurity is prevented, said part does not react with the fluorine-containing gas (e.g. CF4, SF6, SiF4-etc.), and the muffle tube is not broken at an extremely high temperature, such as 1,800C or higher.
Therefore, the durability of the muffle tube is further increased.
When a front chamber is provided on the furnace, the ~3 2 '~ ~ .'33 inflow of air (atmosphere of the work room) into the heating atmosphere is prevented, and contamination of the preform with impurities in the muffle tube material is prevented. Therefore, devitrification of the preform is 5 prevented and the transparency of the preform is increased. Since the temperature is not decreased during the insertion and removal of the preform, the operational rate of the furnace is high. When the muffle tube is made of carbon, since the carbon is not oxidized, the life time of the muffle tube is increased, and the graphite particles do not float in the muffle tube, so that the ratio of the low strength part in the optical fiber fabricated from the glass preform is decreased. If the front chamber is heated up to 800OC and evacuated down to 10 2 Torr., the impurities (e.g. metals and moisture) adhered to the porous preform are removed in advance in the front chamber. Therefore, the purity of the glass preform is much increased, and oxygen is not generated by the thermal decomposition of water, so that the life time of the carbon muffle tube is further improved.

' . . ~ . . .' : ' ~ '" ', , ' " ` ' ' ' ` ` ' ~

Claims

1. A heating furnace for heating a porous preform made of fine particles of quartz base glass for an optical fiber in an atmosphere comprising a fluorine-containing atmosphere to add fluorine to the preform and to vitrify the preform to produce a glass preform for an optical fiber, which comprises a heater and muffle tube positioned inside the heater to separate an atmosphere heated by the heater from the heater, wherein the muffle tube comprises an inner layer made of highly pure carbon and an outer layer made of silicon carbide.

2. The heating furnace according to claim 1, wherein the muffle tube comprises a tube body made of silicon carbide and a layer of highly pure carbon coated on an inner wall of the tube body.

3. The heating furnace according to claim 1, wherein the muffle tube comprises a tube body made of highly pure carbon and a layer of silicon carbide coated on an outer wall of the tube body.

4. A heating furnace for heating a porous preform made of fine particles of quartz base glass for an optical fiber in an atmosphere comprising a fluorine-containing atmosphere to add fluorine to the preform and to vitrify the preform to produce a glass preform for an optical fiber, which comprises a heater and a muffle tube positioned inside the heater to separate an atmosphere heated by the heater from the heater, wherein the muffle tube comprises a tube body made of quartz glass and a layer of carbon coated on an inner wall of the tube body, the carbon layer being formed by a CVD method or a plasma CVD method and the carbon layer having a thickness of from 0.01 to 500 µm.

5. A heating furnace for heating a porous preform made of fine particles of quartz base glass for an optical fiber in an atmosphere comprising a fluorine-containing atmosphere to add fluorine to the preform and to vitrify the preform to produce a glass preform for an optical fiber, which comprises a heater and a muffle to be positioned inside the heater to separate an atmosphere heated by the heater from the heater, wherein the muffle tube consists of upper, middle and lower parts, the adjacent parts thereof being detachably connected, the middle part being made of highly pure carbon and the upper and lower parts being made of a heat and corrosion resistant material.

6. The heating furnace according to claim 5, wherein purity of the carbon constituting the middle part is such that a total ash content is not larger than 20 ppm.

7. A heating furnace for heating a porous preform made of fine particles of quartz base glass for an optical fiber in an atmosphere comprising a fluorine-containing atmosphere to add fluorine to the preform and to vitrify the preform to produce a glass preform for an optical fiber, which comprises a heater and a muffle tube positioned inside the heater to separate an atmosphere heated by the heater from the heater, wherein at least the inner layer of the muffle tube consists of highly pure carbon, and which further comprises a front chamber in which the porous preform is kept and inserted in or removed from the muffle tube.

8. A method for producing a glass preform for an optical fiber which comprises thermally treating a porous preform comprising fine particles of quartz glass in a heating furnace comprising a muffle tube, at least an inner layer of which is made of carbon and an outer wall of which is coated with silicone carbide, in an inert gas atmosphere containing, as an agent for adding fluorine to the glass, at least one fluoride selected from the group consisting of silicon fluorides and carbon fluorides so as to add fluorine to the glass, and simultaneously or thereafter, vitrifying the fine particles of the glass to give a glass preform.

9. The method according to claim 8, wherein the heating furnace further comprises a front chamber in which the porous preform is kept and inserted in or removed from the muffle tube and the front chamber is heated up to 800°C and evacuated down to 10-2 Torr.

10. The method according to claim 8, wherein the nitrogen permeability of the silicon carbide is 10-6 cm2/sec. or less.