FI94747B - Process for producing a porous preform for an optical fibre - Google Patents

Description

94747

A method for producing a porous preform of an optical fiber

The present invention, which is separated from the application FI-904390, relates to a method for producing a glass preform made of optical fiber. More particularly, the invention relates to a method for heating a porous glass preform consisting of a fine quartz glass particle and thermally treated. The method of the present invention can prevent contamination of the glass preform from impure elements and has good durability.

The term "thermal treatment" is understood to mean that the glass preform is heated in a sleeve tube located inside the heating element to separate the heating atmosphere from the heating element, thereby removing water from the preform, adding fluorine and / or vitrifying it.

One common method for mass-producing a glass preform used in the manufacture of optical fiber 20 is known as Vapor Phase Axial Deposition (VAD). The VAD method involves depositing fine glass particles generated in an explosive gas flame on a rotating starting member such as a glass plate or rod. 25 in which a cylindrical porous preform (soot preform) is formed and wherein said porous preform is sintered to obtain a transparent glass preform used in the manufacture of an optical fiber.

In the VAD process, to sinter a porous preform to make it: transparent glass, the preform must be heated in a noble gas (e.g., helium or argon) to a temperature of 1600 ° C, or higher. As the heating furnace for sintering the preform, a heating furnace with 35 carbon heaters is usually used. What must be taken into account when sintering a preform in such a heating furnace is whether the preform contains transition metals such as copper, iron or 2 94747 water. If the glass preform contains one ppm or more of transition metals, the transmission wavelength transmission losses of the manufactured optical fiber are for the most part degraded over the entire wavelength range. If the glass preform contains 0.1 ppm or 5 more water, the properties of the manufactured optical fiber deteriorate over a longer wavelength range.

Therefore, the porous preform is usually dewatered before or during glazing. The dewatering method uses heating the porous preform to a high temperature in an atmosphere containing noble gases, a gas containing chlorine. When the porous precursor is heated to a high temperature in an atmosphere containing noble gases, a fluorine-containing gas, the fluorine is transferred to the porous precursor. When the fluorine is added to the porous preform, the refractive index important for the optical fiber is advantageously adjusted. In this connection, reference is made to Japanese Patent Publication 15682/1980 and Japanese Patent Kokai Publication 67533/1980. We will return to these publications later.

The treatment with the fluorine-containing gas is carried out in a heating furnace before or simultaneously with the glazing.

In order to avoid losses of the carbon heater due to the moisture generated during the heating of the preform. 25 or oxygen, a sleeve tube is installed to separate the carbon heater and the sintering atmosphere. The sleeve tube used is normally made of quartz glass.

Japanese Patent Publications 58299/1983 and 42136/1983 and Japanese Patent Kokai Publication 86049/1985 explain: the use of a quartz tube made of quartz glass in detail.

The fluorine-containing gas decomposes or reacts at high heat-35 to form F2 gas or HF gas. These gases react with quartz glass to form SiF4 gas according to the following formula and in this reaction the quartz glass corrodes: li 3 94747 S1O2 + 2F2 = SiF4 + O2 SiO2 + 4HF = SiF4 + 2H2O This corrosion results in the transfer of copper and iron inside the quartz glass to the surface of the porous preform 5 and contaminate it. In addition, during corrosion, needle holes are formed in a sleeve tube made of quartz glass due to the ingress of ambient air or leakage of atmosphere into the sleeve tube. These are not advantageous to the production method.

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In addition, the quartz glass tube has the bad property that it tends to easily deform at high temperature.

When the quartz glass is at a temperature of about 1300 ° C for a long time, it starts to flow sticky. In addition, when used at a temperature of 1150 ° C or higher for a longer period of time, it crystallizes, and as the furnace temperature decreases, stresses are generated due to the difference in thermal expansion coefficient between the glass phase and the crystalline phase, and eventually the tube breaks.

20 As a material that hardly reacts with a gas containing fluorine or chlorine, carbon is a possible alternative. Carbon does not react with substances SF6, C2Fg, CF4, nor with quartz. And of course carbon does not react with SiF4.

Japanese Patent Publication No. 28852/1981 discloses the use of a carbon sleeve tube in an atmosphere containing a fluorinated gas such as F2, although no working example has been described.

30 However, coal has the following disadvantages: 1. Although coal is less porous, gases can penetrate through it. Nitrogen permeability through carbon is 106 times greater than through quartz glass.

35 2. Carbon is easily oxidized and at temperatures below 400 ° C it. readily reacts with oxygen to form CO2 or CO.

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To avoid oxidation, attempts have been made to form a ceramic layer on the inner surface of the carbon sleeve tube, such as SiC, Al 2 O 3 and BN. Although the ceramic layer prevents oxidation, it reacts unfavorably with either chlorine or fluorine-containing gas 5. Impurities generated in such a reaction crystallize the soot precursor and create bubbles in the soot precursor.

Because carbon is a material with wide gas permeability, the gas passes in and out through the walls of the sleeve tube, allowing moisture in the air to penetrate the sleeve tube through the walls. Therefore, the glass preform has a relatively large amount of water and hydroxyl groups on the other hand. In addition, gases such as Cl2 and SiF4 remain outside the furnace wall and may contaminate the working environment, and contaminants such as copper and iron may enter the furnace from the outside. These weaknesses can be greatly reduced by increasing the thickness of the carbon, but not completely eliminated.

However, as can be seen from the above, adding fluorine to the quartz glass of the coating part in a conventional method presents various difficulties.

It is an object of the present invention to solve in such conditions the problems of a conventional sleeve tube used for dewatering and glazing an optical fiber preform and adding fluorine to the preform, and to provide a sleeve tube for producing an optical fiber glass preform having good durability and long life and preventing air penetration. muffle tube.

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To solve the above problem, it has been found that if the inner and outer walls of the sleeve tube are coated with pyrolytic graphite or solid carbonized vitreous carbon, the sleeve tube does not deteriorate even when in contact with high temperature corrosive gases such as fluorine and chlorine. It's because

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5,94747 that the sleeve tube does not react with a gas containing fluorine or chlorine, since the inner wall is coated with a carbon coating. And such a sleeve pipe has a much longer service life than a conventional pipe. In addition, the carbon is so dense that it does not suffer from problems such as gas penetration that arise when using conventional carbon sleeve tubes.

The present invention relates to a method for producing a porous preform of optical fiber made of fine particles of quartz glass according to independent claims 10 to 6, characterized in that the method is carried out in a heating furnace comprising a heater and a sleeve tube located inside the heater. -15 to separate the heating atmosphere from the heater, and having a sleeve tube body made of high purity carbon, wherein both the inner and outer walls of the body are coated with a carbon material selected from the group consisting of pyrolytic graphite and solid carbonized glassy carbon.

The invention will be explained in more detail with reference to the accompanying figures, in which:. Fig. 1 schematically shows a cross-section of an example of an embodiment of an optical fiber preform heating furnace, Fig. 2 schematically shows an example of another embodiment of an optical fiber preform heating furnace: Fig. 3 schematically shows a third embodiment of an optical fiber preform heating furnace Figures 4A and 4B show methods for preparing a carbon black preform by flame hydrolysis, Fig. 5 shows the refractive index profile of a preform prepared by the method of the invention, Fig. 6 , each 10 separately and Fig. 9 schematically shows a cross section of a heating furnace used in Examples 5 and 6 to be described later.

In the present invention, the porous glass preform consists of fine particles of quartz base glass (hereinafter referred to as "soot preform"), typically comprising soot preforms, and having the following structures: 1. A solid or hollow preform assembly consisting of fine glass preforms. particles. In the case of shaping, after the glazing of the soot preform, a channel is formed in the middle part, after which a glass rod is placed in it, whereby the final glass preform is obtained.

. 2. The soot preform includes a glass interior, and a fine glass particle deposited around the interior.

3. The carbon black preform comprises a glass core around which a portion of the shell is formed and a fine glass particle is deposited around the shell.

30 Pyrolytic graphite (also called "pyrolytic carbon") and a solid carbonized glass-like carbon coating on the inner and outer walls of the sleeve tube body are the following types of carbon.

Pyrolytic graphite means graphite 35 which is deposited on the surface of a body by carbon deposition,

It 7 94747 which is formed by decomposing a raw material at high temperature (700 - 1200 ° C), mainly comprising hydrocarbon. The sleeve tube body can be coated with pyrolytic graphite in many different ways. For example, the body of a 5-sleeve tube is heated directly by an electric current or indirectly heated from outside it, and the carbon material is then obtained by thermally decomposing hydrocarbon gas (e.g. propane, methane, etc.), which is then deposited on the body.

10 Solid carbonized vitreous carbon means carbon produced by very slow curing and carbonization of the resin. These resins include, for example, thermosetting resins such as phenolic resins and furan resins and polyvinyl chloride resins. Glazed carbon can also be made by carbonizing sugar, cellulose, polyvinylidene chloride, etc. When coating the sleeve tube body with glazed graphite, a thermosetting resin is applied to the body surface by suitable shaping or multiple spraying and carbon-high resin.

The carbon material used for the sleeve tube body is preferably a very pure carbon material, the purity of which is explained below. The coating thickness can be selected depending on the conditions under which the preform is thermally treated.

In general, the purity of the carbon used for the sleeve pipe bodies is preferably such that the total ash content is less than 50 ppm, preferably less than 20 ppm. If the total ash content of the carbon is 30 to 1000 ppm, it cannot be used to silence the sleeve tube body because impurities such as iron and copper will enter. The impurities and their amounts in the carbon at a total carbon ash content of 20 ppm or less are shown in the table below.

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Table 1 B <0.1 ppm Ca <0.1 ppm

Mg <0.1 ppm Ti <0.1 ppm 5 Al <0.1 ppm V <0.1 ppm

Si <0.1 ppm Cr <0.1 ppm P <0.1 ppm Fe <0.1 ppm S <0.1 ppm Cu <0.1 ppm N i <0.1 ppm 10

The preform with fine glass particles is dewatered with a dehydrating agent, most commonly a chlorine-containing gas. When using the heating furnace according to the invention, a chlorine-containing gas must be selected which does not contain any oxygen-containing atoms, e.g. Cl2, SiCl2 or CCl4. If a chlorine-containing gas containing an oxygen atom is used, e.g. SO 2 Cl 2, it decomposes upon removal of water at high temperature, releasing oxygen, whereby oxygen unfavorably destroys the carbon material. CCl 4 is thermally decomposed to form carbon dust. Cl2 reacts with water in the porous preform to form a small amount of oxygen according to the following formula:

Cl 2 + H 2 O = 2HCl + (1/2) 02 25 In addition, SiCl 2 reacts with water in the porous preform as follows:

SiCl. + 2H-O = SiO-, + 4HC1 4 2 2 However, no oxygen or carbon dust is generated. Therefore, SiO 2 is the most preferred dewatering agent when using the sleeve tube of this invention. The temperature is normally 800 to 1200 ° C when removing water. The dewatering treatment is carried out in an atmosphere containing chlorine-containing gas mixed with a noble gas such as argon or helium in an amount of 0.1 to 10% of the molar amount of chlorine-containing gas.

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A fluorine-containing gas is generally used as the fluorine addition gas. When a sleeve tube made of quartz is used, the fluorine-containing gas decomposes at a high temperature, producing fluorine gas, which corrodes the sleeve tube. Therefore, a fluorine-containing gas that does not corrode the sleeve pipe should be selected.

Since the present furnace sleeve tube-ki does not react with fluorine gas, a wider range of different fluorine addition gases can be used, and then silicon fluoride and carbon fluoride can be selected. Silicon fluorides include SiF 4, Si 2 Fg, etc., and carbon fluorides include CF 4, C 2 F 6, C 3 F 8, CC 12 F 2, C 2 Cl 3 F 3, etc.

Among these, SiF 2 is preferred because it is readily available, although it is toxic and expensive. In addition, carbon fluoride is even more advantageous because it is not only cheap and safe, but also easy to handle. In particular, CF 2 is most preferred when using the sleeve pipe of the present invention, since CF 2 does not generate any carbon dust.

The fluorine addition temperature is normally between 1100 and 1600 ° C. The fluorine addition treatment is performed in an atmosphere in which the fluorine-containing gas is diluted with a noble gas such as argon or helium-25a, which is 0.1 to 10% of the molar amount of fluorine-containing gas.

When the present heating furnace sleeve tube is used to vitrify a glass preform, a noble gas such as helium, nitrogen or argon is fed to the sleeve tube at a temperature of 1400 to 1600 ° C.

The experiments and concepts on which this invention is based are explained below. The concepts explained below have been obtained through the hard work of the inventors and have not been easy to achieve.

35 Experiment 1. A sleeve tube made of quartz glass with an inner diameter of 10,94747 of 100 mm, a length of 300 mm and a wall thickness of 2 mm was heated to 1500 ° C and maintained at that temperature for one day. The sleeve tube was extended to a length of 400 mm.

5 Experiment 2. A sleeve tube made of high-purity carbon having the same dimensions as in Experiment 1 was used. The inner and outer surface of the sleeve tube had a pyrolytic graphite coating with a thickness of 30 μπι. The sleeve tube was heated as in Experiment 1, but the tube 10 did not expand. Also, no expansion occurred when the coating was a solid carbonized glass-like carbon having a thickness of 10 μm.

In this experiment, a pyrolytic graphite drop coating was applied to the surface of the sleeve tube body by introducing carbon-15 hydrogen gas at a temperature of 1000 ° C. The glazed carbon coating was formed on the surface of the body by curing and carbonizing the polyvinyl chloride resin.

In the following examples, the coatings were shaped as described above, unless otherwise stated.

Experiment 3. The same sleeve tube as in Example 1 was heated from room temperature to 1500 ° C for more than three hours and kept at that temperature for one day and then cooled from 1500 ° C to room temperature for one day. By repeating heating and cooling for 20 days, the sleeve tube broke due to crystallization.

Experiment 4. The same sleeve tube as in Experiment 2 was subjected to the same heating test as in Experiment 3.

30 After 20 days, no problems appeared.

Oxidation resistance is explained below.

Experiment 5. Moss sleeve tube as in Experiment 2 in an atmospheric atmosphere at 500 ° C for one hour. No oxidation was observed.

35 Experiment 6. The same oxidation test was repeated as

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94747 11 in 5 using a sleeve tube having neither pyrolytic graphite coating nor solid carbonized vitreous carbon coating on the surface. The surface of the sleeve tube made of carbon was oxidized to a thickness of 50.

The corrosion resistance is explained below.

Experiment 7. A pyrolytic graphite coating having a thickness of 30 μm was applied to the inner and outer surfaces of the sleeve tube having the same dimensions as the sleeve-10 tube of Experiment 1. The sleeve tube was heated to 1050 ° C in a helium atmosphere containing 5% mole by weight of Cl 2. No corrosion was observed in the sleeve tube. Nor had it leaked through the wall of the pipe.

In a similar manner, a sleeve tube made of high-purity carbon with a solid carbonized glassy carbon coating having a thickness of 10 μm was heated. Again, no corrosion or CI2 leakage was observed.

It is due to the pyrolytic graphite coating 20 or solid carbonized vitreous carbon preventing CI2 from leaking.

Experiment 8. The same test as in Experiment 7 was repeated, using a very pure sleeve tube made of pure carbon, but no coating was used. Through the wall, ha-25 prevented CI2 leakage.

Experiment 9. The same test as in Experiment 7 was repeated using a sleeve tube with a silicon carbide coating on the inner and outer surface instead of a pyrolytic graphite coating or a solid carbonized glassy carbon coating. Silicon carbide coatings reacted and evaporated and leakage occurred.

SiC reacts according to the following formula:

SiC + 2Cl-, = SiCl. + C 2 4

Experiment 10. The same sleeve tube as used in Experiment 35 2 was heated at 1400 ° C in a helium atmosphere containing 3% of the molar amount of 12,94747 SiF. SiF was not found to leak and no corrosion was observed.

Experiment 11. The same test as in Experiment 10 was repeated using the same sleeve tube as used in Experiment 9, 5 with a SiC coating on the inside and outside. The interior and exterior coatings reacted and evaporated.

Such a sleeve tube had a problem with long-term use.

The following conclusions can be drawn from the results of Experiments 1-11: 1. A sleeve tube made of very pure carbon with coatings made of pyrolytic graphite or solid carbonated glassy carbon on the inner and outer surface withstands very high temperatures better than a sleeve tube made of comparable pure quartz glass.

2. A sleeve tube having on its inner and outer surfaces Pyrolytic graphite coatings or solid coatings made of carbonated glassy carbon 20 are significantly more resistant to oxidation than a comparable sleeve tube made of very pure carbon.

3. When used or with a fluorine-containing gas, the sleeve tube of the heating furnace according to the present invention is corrosion-resistant. A sleeve pipe made of quartz cannot withstand gases containing 25 fluorines. For example, SiF 2 corrodes a sleeve tube made of quartz at 1.6 μm / h at 1100 ° C. A sleeve pipe made of high-purity carbon with a SiC coating is not resistant to chlorine- or fluorine-containing gases.

: 30 Preferred embodiments of the present heating furnace are explained below with reference to the accompanying drawings.

One embodiment is shown schematically in a cross-sectional view.

35 Figure 1 shows a list of parts: number 1 is a porous preform, 2 is a support rod, 3 is a sleeve tube, 4 is a heater, 5 is a furnace body, 6 is a noble gas import, 7 is an atmospheric gas (e.g. SiF 2 and helium ) si-import. 31 is the body of a carbon sleeve tube and 5 32 is a pyrolytic graphite coating or a solid carbonized vitreous carbon coating. In the embodiment shown in Figure 1, the sleeve tube has a coating on both its inner and outer surface (incl. Its bottom part).

When using a sleeve tube as described above, a 10 carbon black preform passes through the sleeve and is heat treated.

In another embodiment, the sleeve tube consists of upper, middle and lower portions removably attached and at least the central portion is made of high purity carbon coated with pyrolytic graphite or solid carbonized glassy carbon and the upper and lower portions are made of heat and corrosion.

The second embodiment will be explained with reference to Fig. 20.

Figure 2 schematically shows a cross-section of this embodiment of a heating furnace. Inside the oven body 5 there is a heater 4 and the sleeve tube 3 is located in the middle of the oven body.

The sleeve tube consists of an upper part 34, a central part 35 and a lower part 36, and the adjacent parts are releasably fastened to each other by suitable means, e.g. with threads. The central part 35 of the sleeve tube is made of a very pure carbon material 31, the surfaces of which have a coating 32 made of pyro-, lytic graphite or solid carbonized glassy carbon.

Since the upper and lower parts (34 and 36) are not heated to as high a temperature as the central part 35, they can be made of a high-purity carbon material 35 coated with SiC or SiO 2 material which is resistant to oxidation, corrosion, chlorine-containing and and / or fluorine-containing gas. SiC and Si 2 N 2 do not react with C ± 2 or F substances at low temperature. For simplicity, the top and bottom coatings 5 are not shown.

When applying a SiC or Si 2 N 2 coating to the surface of the body, for example, the CVD method (Chemical Vapor Deposition) can be used.

A sleeve tube made of high purity carbon is preferred because it does not react with halogen-containing compounds unless there is oxygen and water in the atmosphere and has excellent heat resistance.

When treating the porous preform, the carbon temperature of the central portion 35 is raised high and is consumed as the moisture and moisture in the preform 15 and oxygen migrate out of the outer surface after a long period of use. The carbon inner wall wears out due to special reasons for processing the porous preform, which are explained in more detail below.

SiO 2 dust is released from the porous preform adhering to the carbon inner wall and reacts with carbon to form SiC and oxygen is evolved in said reaction by further reacting with carbon to form CO. The resulting SiC, on the other hand, reacts with chlorine-containing gas, * 25 which is used for dewatering. SiO 2 dust consumes the carbon inner wall in this reaction.

These reactions can be represented by the following formulas:

SiO 2 + C = SiC + O 2 O 2 + 2C = 2CO

30 SiC + Cl., = SiCl. + C

: 2 4

Therefore, the middle carbon part must be renewed after a longer service life.

On the other hand, since the upper and lower parts of the sleeve tube do not reach such a high temperature, in which case they also do not wear particularly, only the middle part preferably needs to be replaced when the sleeve tube consists of three parts.

Because the carbon is porous, it is necessary to completely remove the absorbed moisture at a high temperature. Therefore, in order to remove the absorbed moisture, it is preferable to replace the carbon sleeve pipe as infrequently as possible. When the middle part of the sleeve tube of this embodiment is worn out, it is not necessary to remove the absorbed moisture from the upper and lower parts, but they can still be used. From an economic point of view, the three-part sleeve tube of this embodiment is advantageous in all respects.

One measure to prevent oxidation of this type of sleeve tube is to lower the temperature to 500 ° C or below, where carbon is not oxidized during weathering and removal of the glass preform si-15. But at such a low temperature, the operating ratio of the furnace decreases poorly. Contamination of the inside of the sleeve with airborne dust cannot be avoided. The inflow of air into the sleeve tube can be prevented by the third embodiment of the present invention. In addition, the heater and the sleeve tube in this heating furnace of the third embodiment have a front chamber through which the porous preform is inserted and removed from the sleeve tube. The front chamber .. can preferably be heated to 800 ° C and the pressure can be reduced to 10 torr or less.

The front chamber is preferably made of a heat-resistant material which does not emit any impurities such as quartz glass, SiC, Si 2 N 2 or the like. The front chamber may be made of the same or different material than; 30 sleeve pipe.

A third embodiment of the heating furnace will be explained with reference to the accompanying figures.

Fig. 3 is a schematic cross-sectional view of an example of a third embodiment of a heating furnace. This heating furnace is the same as in Fig. 2, on which a front chamber 11 is placed 16 94747. In addition to all parts of the heating furnace of Fig. 2, this heating furnace comprises a front chamber 11, a front chamber gas outlet 14, a front chamber gas inlet 15 to clean the front chamber gas 5, and a partition wall. 3 also shows the top and bottom coatings.

The introduction of the porous preform into the heating furnace according to Fig. 3 takes place as follows: 1. The partition wall 16 separating the heating atmosphere of the front chamber 11 and 10 is closed. (In the starting position and during processing of the soot preform, the partition is open).

2. For rotary vertical movement, a porous preform 1 is attached to a support rod 2.

3. The upper part of the front chamber 11 is open and the porous preform 1 is lowered into the front chamber 11.

4. Close the top and clean the inside of the front chamber with a noble gas (eg nitrogen or helium).

5. The septum 16 is opened and the porous preform 20 1 is introduced into a heating atmosphere having the same temperature as where the preform is thermally treated.

The preform is removed from the heating furnace of the present invention as follows: 1. The preform is drawn from the heating atmosphere up to the front chamber 11 after heat treatment. It is not necessary to lower the temperature of the heating atmosphere at this stage.

2. The partition 16 is closed.

3. The upper part of the front chamber 11 is opened and the preform 1 is removed from the chamber 11.

The present invention relates to a process for producing a glass preform made of optical fiber, comprising heat treating a porous preform consisting of fine quartz glass particles in a heating furnace comprising a 94747 17 sleeve tube made of high-purity carbon and having an inner and outer wall a carbon material selected from the group consisting of pyrolytic graphite and solid carbonated vitreous carbon containing a noble gas atmosphere as a medium for adding fluorine to glass, at least one fluoride selected from the group consisting of silicon fluoride and carbon fluoride for adding fluorine to glass, preferably after dehydration and simultaneously or subsequently vitrifying fine glass particles to give a glass preform.

In order to completely remove impurities generated during the process from the sleeve tube, or absorbed dust and moisture, the sleeve tube is heated for several hours in an atmosphere containing chlorine-containing gases, in particular Cl 2, at a temperature above 1500 μg. If the optical fiber is made from a glass preform and without the sleeve tube being heated in the above manner, it may have considerable absorption due to moisture or impurities.

The gases and conditions described in connection with the present embodiments of the heating furnaces may be used in the process of this invention.

Among the fluorine alloying agents used in the process of the invention, SiF. is the most advantageous. SiF. is preferably a fairly pure product, 3N or higher.

Although SiF 2 does not react with carbon at all, if the carbon black precursor is used without thorough dewatering, it may generate steam in the sleeve tube made of carbon at the rate of fluorine addition. Such steam can be generated by the reaction between the moisture of the soot precursor and SiF4 or carbon. The result is deposits, which may be carbon particles, accumulated on the top of the soot preform. To prevent this, the soot preform 35 is preferably dewatered prior to heat treatment 18,94747 in a sleeve tube having SiF 2 in the atmosphere.

Although dewatering of the soot preform is possible simultaneously with the addition of fluorine, dewatering of water-5 is performed before the addition of fluorine, due to the reasons mentioned above as well as the efficiency of dewatering.

Addition of fluorine to the soot preform SiF. by means of 4 is carried out efficiently at a temperature of 1000 ° C or higher, preferably 1100 to 1400 ° C. A sufficient amount of fluorine should be added to the soot preform before the shrinkage of the soot preform is complete. If the soot preform shrinks before a sufficient amount of fluorine has been added, no fluorine is added to the uniform preform 15 and the addition of added fluorine is unevenly distributed in the preform.

The carbon black preform is normally prepared by a flame hydrolysis method and has a fine glass particle size of 0.1 to 0.2 μm.

The present invention will be explained in detail below.

To produce fine quartz glass particles by mass flame hydrolysis method using a coaxial multi-tube burner made of quartz glass according to Fig. 4A, oxygen, hydrogen and as a raw material gas, SiCl 4 or SiCl 4 and an additive (e.g. GeCl 4) and with helium or argon, which act as catalysts for the reaction.

. The shielding noble gas is introduced so that the raw material gas reacts a few mm from the front of the burner 41. To produce a rod-like soot preform, glass particles deposit at the lower end of the rotating seed rod 46 in the axial direction of the seed rod 46. To make the tubular preform 35, the glass particles are deposited

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19 94747 about a rotating quartz or carbon rod 46 passing over the burner 41 according to Fig. 4B, and then the rod 46 is removed. The rod 46 may be an inner glass rod.

In that case, there is no need to remove the rod.

5 There can be several burners 41.

In a sleeve tube (a cylindrical sleeve tube with top and bottom flanges) made of high-purity carbon, the inner and outer walls of which are coated with a material with low gas permeability, 10 e.g., as shown in Figure 2, a soot preform hangs at the top of the heater, and the inside of the sleeve tube is filled with a helium atmosphere, including gas. After the heater atmosphere has been heated to 1050 ° C, the soot preform is lowered at a rate of no-15 at a rate of 2-10 mm / min. After the entire soot preform has passed through the heater, the soot preform counting ends and Clj gas import is stopped. The atmosphere is then converted to a helium atmosphere containing SiF. When the temperature of the heater has risen to 1400 ° C, the soot preform 20 is raised at a rate of 4 mm / min. while adding fluorine to the precursor.

When the entire preform has passed through the heater again, the soot preform lifting is stopped. SiF ^ imports will cease and helium imports will continue. When the temperature of the heater 25 is 1600 ° C, the soot preform is lowered at a rate of 3 mm / min to vitrify the preform.

The present invention is illustrated by the following examples.

Example 1. Water was removed from a porous glass preform, fluorine was added and vitrified using the heating furnace of Figure 2. The inner diameter of the sleeve tube was 200 mm, the wall thickness 10 mm and the length 1000 mm. The sleeve tube consisted of three parts. The middle part was made of very pure carbon and had a pyrolytic graphite coating on its inner and outer walls with a thickness of 30 .mu.m. The top and bottom were made of very pure carbon and had a SiC coating on their inner and outer walls to a thickness of 50.

The heat treatment conditions were as follows: 5 Dewatering

heater temperature: 1050 ° C

atmospheric gas: C ^ / He = 5 mole% / 95 mole% preform descent rate: 5 mm / rain

Addition of fluorine

10 heater temperature: 1370 ° C

atmospheric gas SiF 2 / He = 3 mole% / 97 mole% preform lift rate: 3 mm / min Glazing heater temperature: 1600 ° C 15 atmospheric gas: He 100% preform lift rate: 5 mm / min.

The difference Λ n of the characteristic refractive index of the obtained transparent preform was 0.34%. The difference in the specific refractive index used here is defined as follows: 20 (nSiO “nF} Δη = 2_ x 100 (%) nsio2 (where ^ 3 ^ 02 3a np denote the refractive indices of pure quartz and fluorine-added precursor).

The pure quartz interior of the monofilament optical fiber was prepared using the prepared preform. The Opti-30 fiber had a transmission loss of 0.17 dB / km at a wavelength of 1.55 μτα and had no absorption by Cu and Fe.

The preform described above was thermally treated with the sleeve tube described above, and no deterioration was observed in the sleeve tube.

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Example 2. A sleeve pipe according to Figure 6 with a double pipe structure was used. The structure of the sleeve tube was such that the sleeve tube 3 used in Example 1 was placed in an outer tube 10 made of high purity carbon 5. The outer tube 10 had a pyrolytic graphite coating on both its outer and inner walls, 30 μm thick. For simplicity, the outer tube coating is not shown.

The heat treatment was performed under the same conditions as in Example 1. In this example, He gas was introduced into the space between the outer tube and the sleeve tube at 10 l / min from the inlet 8, so that the working pressure in this state was lower than in the sleeve tube.

The pure quartz core 15 of the monofilament optical fiber was prepared using the prepared preform as a coating part. The transmission loss of the optical fiber was 0.17 dB / km at a wavelength of 1.55 μm and the residual water in the optical fiber was less than 0.01 ppm. The heating furnace of this example had the advantage that the working environment does not directly li-20 because atmospheric gas, resulting in Example 1, quietly leaks from the joints of the sleeve pipe parts, which gas mixes with the He gas between the sleeve pipe and the outer pipe.

A sleeve tube having a solid carbonized glass-like carbon coating (thickness 10 μm) instead of a pyrolytic graphite coating was used under the same conditions as the sleeve tube used with a pyrolytic coating. Here, the same results were used as in the case of the sleeve tube coated. 30 with pyrolytic graphite.

Example 3. A heating furnace according to Figure 3 was used. the sleeve tube placed in the heating furnace had a pyrolytic graphite coating (thickness 30 μm) or a solid carbonized glassy coating (thickness 35 10 μm) throughout the inner and outer walls.

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The porous preform was placed in a front chamber 11 heated to 200 ° C and the top closed. Nitrogen gas was introduced at 10 1 / min to replace the nitrogen in the inner atmosphere of the anterior chamber. The septum 16 was then opened and the porous preform 1 was placed in the sleeve tube from the front chamber and dewatered, fluorine was added and vitrified to obtain a transparent optical fiber glass preform. To remove the preform from the heating furnace, the preform was transferred to the anterior chamber, the septum was closed, and the top was opened as a result of the removal of the preform.

The heat treatment conditions were as follows: Dewatering

heater temperature: 1050 ° C

3 atmospheric gas: SiCl 2 200 cm / min 15 He 20 1 / min preform transfer rate: 5 mm / min

Addition of fluorine

heater temperature: 1270 ° C

3 atmospheric gas: Sif ^ 800 cm / min 20 He 20 1 / min preform transfer rate: 4 mm / min

Glazing

heater temperature: 1500 ° C

atmospheric gas: He 20 1 / min 25 SiF ^ 800 cm ^ / min precursor transfer rate: 10 mm / min.

The optical fiber was prepared using the produced glass preform as a coating part and had a low transmission loss of 0.17 dB / km with a wavelength of 1.55 μm.

; In this example, when the heating furnace comprises a front chamber, the temperature of the sleeve tube does not have to be lowered below 800 ° C, this embodiment being advantageous in view of the productivity of the thermally treated preforms.

35 Comparative example 1. Same procedure as

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94747 in Example 1, but now a sleeve tube made of quartz glass with 1 ppm copper but no carbon coating was used and an optical fiber was produced. The optical fiber contained residual water of 0.01 to 5 ppm and had an absorption due to copper near the wavelength of 1.30 μm. This was sufficiently low compared to the absorption of conventional optical fibers and the absorption number was 2-3 dB / km at a wavelength of 0.8. But the inside wall of the sleeve tube was extremely eat-10. This means that the corrosion resistance of this sleeve tube is not sufficient.

Comparative Example 2. (Heating resistance of a sleeve tube made of quartz glass). The same procedure as in Example 1, but now using a quartz glass sleeve tube instead of a carbon sleeve tube, and a carbon black preform was prepared. The sleeve tube made of quartz glass expanded during the glazing of the soot preform and could not be reused.

Comparative Example 3. (Corrosion of 20 sleeve tubes made of quartz glass). In the process of Comparative Example 2, SiF4 was used instead of SF4> In this case, needle holes were etched into the walls of the sleeve tube made of quartz glass near the heater. The glass preform produced contained several ppm of water. And, of course, the sleeve tube had expanded considerably, and it could no longer be used.

Comparative Example 4. The same heating furnace as in Example 1 was used, except that the sleeve tube made of high-purity carbon had a SiC coating, 50 μm thick, and only on its outer wall; ‘30 instead of pyrolytic graphite coating.

The glass preform was thermally treated under the same conditions as in Example 1. The monofilament optical glass fiber was prepared from the produced preform. The optical fiber had a low transmission loss of 0.18 dB / km at a wavelength of 1.55 Jim.

"4747 24 '^

But when the heating furnace was used repeatedly, the gas coming through the carbon body peeled off the SiC coating and penetrated the furnace body and then the heating furnace could no longer be used. And in addition, there were 5 serious problems in the work environment.

Example 4. The same heating furnace as in Example 1 was used, except that the sleeve tube made of high purity carbon had a coating of solid carbonated glassy carbon, 10 to 10 μm thick, on its inner and outer walls instead of a pyrolytic graphite coating.

The glass preform was thermally treated under the same conditions as in Example 1.

Made with optical fibers, the first to fifth preforms had a higher transmission loss of 0.02 to 0.03 dB / km at a wavelength of 1.55 μm than those made from the preforms treated in a sleeve tube with a pyrolytic graphite coating.

This is due to the distribution of the element concentration according to the thickness of the coating 20, as can be seen from the graphs in Figures 7 and 8.

Figure 7 shows the element concentration distribution in the pyrolytic graphite coating and Figure 8 shows the same in the solid carbonized glassy carbon-25 coating. These graphs were obtained by measuring the distribution of impurity concentrations with a secondary ion mass spectrometer along the thickness directions of the coating. The vertical axis indicates the concentration of the element and the horizontal axis indicates the thickness of the coating and the left end of the horizontal axis corresponds to the surface of the coating. As can be seen qualitatively from the element concentrations, the concentrations of impure elements (e.g. alkali metal or transition metal) in the glazed carbon coating are much higher than those in the pyrolytic graphite coating. 94747 25 which can be seen as surface concentrations plotted and as curves. The above results of this example apparently indicate differences in element concentrations.

5 But also when using a sleeve tube with a glazed carbon coating, optical fibers made from the sixth and later preforms had a low transmission loss of 0.17 dB / km. This was because the contaminants had been removed from the coating.

Example 5. In this example, the heating furnace of Figure 9 was used. In this heating furnace, a heater 4 was mounted on a cylindrical furnace body 5 through which a sleeve tube 3 fit. A preform placed in the sleeve tube was thermally treated with a heater 11a.

The sleeve tube 3 in the heating furnace according to Fig. 9 has three parts (34, 35 and 36) and further has a front chamber 11 to prevent oxidation of the sleeve tube. The inner and outer walls of the central portion 35 of the sleeve tube had a gas-impermeable pyrolytic graphite coating 20 having a thickness of 30 to 40 μm. Because the top 34 and bottom 36 did not heat to such a high temperature, they had SiC coatings 37 that withstood oxidation and chlorine- and fluorine-containing gases. The preform to be processed was 500 mm in length and 140 mm in diameter and was prepared by the VAD method.

The processes by which the heating furnace of Figure 9 was used were essentially the same as those used with the heating furnace of Figure 3. Initially, the preform 1 was placed in the front chamber 11, where the partition 16 was closed and then the upper part 8 was closed. Nitrogen gas was introduced at 10 1 / min into the front chamber inlet to clean the interior of the 15 front chambers. The septum 16 was then opened and the porous preform was placed in a sleeve tube and thermally treated to obtain a transparent preform. To remove the preform, the preform was introduced into the front chamber and then the septum 16 was closed after the preform was removed after opening the top.

After the preform was transferred from the front chamber to the sleeve-5 tube, the preform was thermally treated by raising the temperature of the sleeve tube to 1050 ° C, in an atmosphere of 20 l / min 3

They gas and 200 cm / min SiCl 2 gas at a rate of descent of 5 mm / min to remove preformed water and impurities. After the preform was completely removed from the heater, the SiCl 4 import was stopped and the He gas import was continued at 20 L / min and the sleeve tube was heated to 1550 ° C. The preform was then raised at 4 mm / min to vitrify the preform.

15 As explained above in the manufacture of the inner part-. . 3 h, the preform was dewatered in an atmosphere of 200 cm / min SiCl 2 gas and 20 l / min He gas. The sleeve tube temperature was then raised to 1200 ° C and gases were introduced at 900 cm 3 / min CF 2 and 20 l / min He. The preform was raised at a rate of 3 mm / min to which fluorine was added. Thereafter, the temperature of the sleeve tube was raised to 1500 ° C and the preform was lowered at a rate of 4 mm / min, while introducing 20 l / min He gas to vitrify the preform.

The prepared preform contained 1.3% by weight of fluorine and had a specific refractive index An of 0.35%.

The pure quartz preform obtained in connection with the production of the above-mentioned inner part was stretched in an electric resistance furnace so that its diameter became 5 mm.

A 5 mm diameter hole was drilled with fluorine. 30 to a glass preform made in the production of the aforementioned coating section, and the ends of the tubes were connected to both ends of the preform. The preform was placed in an electric furnace and the inner surface of the preform was leveled SF, by etching, keeping the inner surface temperature high.

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35 The stretched clean preform was then placed in tubes

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27,94747 and were heated and made together to complete CI2 in an atmosphere to obtain an optical fiber precursor.

The finished preform was stretched to a diameter of 35 mm, and then the pure quartz core of the single-mode optical fiber, 10.5 μm in diameter, and the coating having a diameter of 125 μm were withdrawn from the preform. The optical fiber had a low transmission loss of 0.18 dB / km at a wavelength of 1.55 μm.

Example 6. A glass rod consisting of a quartz glass center portion 10 containing 6% by weight of GeO 2 and a pure quartz glass core portion was prepared by the VAD method. The diameter of the rod was 18 mm. The porous glass layer was further coated on the surface of the glass rod. The diameter of the rod was 140 mm.

The above-mentioned and prepared preform was dewatered and vitrified under the same conditions as in Example 5. The preform was placed in the anterior chamber, and the interior of the anterior chamber was purged with nitrogen gas. The preform was then transferred to a sleeve tube. The temperature of the sleeve tube 20 was 1050 ° C and the preform was lowered through the sleeve tube at a rate of 5 mm / min in an atmosphere of 20 l / min He gas 3 and 200 cm / min SiCl 4 <whereby water was removed from the preform.

After the preform had passed through the entire heater, the sleeve. the heat of the tube was raised to 1500 ° C. SiCl 2 import was stopped and He import 201 / min continued. The preform was raised at a rate of 4 mm / min to vitrify the preform.

The outer diameter of the glazed preform was 65 mm, and the ratio of the outer diameter of the preform to the inner part containing GeO 2 was 15.0. When the preform was stretched to diameter. 30 35 mm, an optical fiber, 125 μm in diameter, was drawn.

The fabricated optical fiber had a low transmission loss of 0.35 dB / km at a wavelength of 1.3 μm and 0.20 dB / km at a wavelength of 1.55 μm. The initial tensile strength of the optical fiber was sufficient at 5.5 kg.

35 After the 200 preforms had been heat-treated, the inner wall of the muffler was inspected. The inner part of the middle part of the sleeve tube was slightly oxidized and no deformation or deterioration was observed.

According to the present invention, an optical fiber glass preform which is not contaminated with impurities such as iron or copper can be produced by reducing the wear of the sleeve tube, and further, an optical fiber having a low transmission loss can be produced from the produced glass preform.

By providing the inner and outer walls of the sleeve tube with a pyrolytic graphite coating or a solid carbonized glassy carbon coating, the sleeve tube hardly wears under the influence of heat or corrosive gases even at high temperatures, in which case it has good durability. Therefore, the sleeve-15 pipe according to the present invention is also economically advantageous.

Further, by making the sleeve tube body from high purity carbon to prevent fouling of the porous preform from impurities, the sleeve tube does not react with fluorine-containing gases (e.g., CF 2, SF 2, SiF 4, etc.) and the sleeve tube does not decompose at very high temperatures such as 1800 ° C or above. Therefore, the durability of the sleeve pipe has increased.

If the heating furnace has a front chamber, it prevents the flow of air (office atmosphere) into the heating air ring, thus preventing the inside of the sleeve tube from becoming dirty. In addition, crystallization of the preform is prevented and the transparency of the preform is increased. Since the temperature does not drop when the preform is inserted and removed, the efficiency of the heating furnace is good. If the sleeve tube is made by coating with 30 carbons using pyrolytic graphite or • solid carbonated glassy carbon, and since the carbon hardly oxidizes, the life of the sleeve tube increases and the graphite particles do not float in the sleeve tube.

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29 9 4 7 4 7 The use of a carbon sleeve tube avoids deformation and crystallization due to heat, as well as fractures due to crystallization over time, which can occur unexpectedly when using a quartz sleeve tube, in addition to which the sleeve tube can be used for a longer period of time. If a quartz material is used, it is difficult to produce a large-diameter sleeve pipe due to the machinability. But if a carbon material is used according to the invention, a sleeve pipe with a larger diameter can be manufactured. In addition, it is preferable to be able to process a preform having a larger diameter.

In the examples described above, a zone furnace was used, but the same power achieved with zone 15 furnaces is also achieved with a well furnace.

Claims (6)

A method of manufacturing a porous preform (1) for an optical fiber made of fine glass portion + quartz glass clar, which method comprises dehydrating the porous preform in an atmosphere comprising at least one chlorine-containing compound selected from the group consisting of of CI2, CC14 and SiCl4 as water eliminating agents, then adding fluorine in the preform in an atmosphere containing at least one fluorine-containing compound selected from the group consisting of silicon fluorides and carbon fluorides, and then vitrifying the preform, characterized in that it is realized in a heating furnace comprising a heater (4) and a sleeve tube (3) arranged inside the heater (4) to distinguish the heating atmosphere from the heater (4), and the sleeve tube body (31) is made of extremely pure koi , the inner and outer walls of the body (31) being coated with carbon material (32) selected from the group consisting of pyrolytic graphite and solid carbonated glassy koi. 20 A method of manufacturing a porous preform (1) for an optical fiber made from fine quartz glass particles, which method comprises dehydrating the porous preform in an atmosphere comprising S1 Cl4 as a water. And then glazing the preform, characterized in that the process is realized in a heating furnace comprising a heater (4) and a sleeve tube (3) arranged inside the heater (4) to separate the heating atmosphere from the heater (4). and which socket tube body (31) is made of extremely pure koi, wherein: the inner and outer walls of the body (31) are lined with carbon material (32) selected from the group consisting of pyrolytic graphite and solid carbonated glassy koi. 35 A method of manufacturing a porous preform (1) for an optical fiber made of fine quartz glass glass particles, comprising a dehydration of the porous 94747 preform in an atmosphere comprising CF4 as a fluorine additive, and then vitrification of the preform , characterized in that the process is realized in a heating furnace comprising a heater (4) and a sleeve tube 5 (3) arranged inside the heater (4) to separate the heating atmosphere from the heater (4), and the sleeve tube body (31) is made of extremely pure koi, the inner and outer walls of the body (31) being coated with carbon material (32) selected from the group consisting of pyrolytic graphite and solid carbonated glassy koi. A method of manufacturing a porous preform (1) for an optical fiber made of fine quartz glass particles, which method comprises dehydrating the porous preform in an atmosphere comprising at least one chlorine-containing compound selected from the group consisting of Cl 2 , CC14 and SiCl4 as water eliminating agents, then adding fluorine in the preform in an atmosphere containing at least one fluorine-containing compound selected from the group consisting of silicon fluorides and carbon fluorides, and simultaneously glazing the preform, characterized in that process a heating furnace comprising a heater (4) and a sleeve tube (3) arranged within the heater (4) to separate the heating atmosphere from the heater (4), and the sleeve tube body (31) being made of particularly pure koi, (31) the inner and outer walls are lined with carbon material (32) selected from the pyrolytic graphite resin group and solid carbonated glassy koi. 30 A method of manufacturing a porous preform (1) for: an optical fiber made of fine quartz glass particles, which method comprises dehydrating the porous preform in an atmosphere comprising SiCl 4 as a water eliminating agent, and simultaneously glazing the preform Characterized in that the process is realized in a heating furnace comprising a heater (4) and a sleeve tube (3) arranged inside the heater (4) to separate the heating atmosphere from the heater (4), and the socket of the sleeve tube. (31) is made of extremely pure koi, the inner and outer walls of the body (31) being coated with carbon material (32) selected from the group consisting of pyrolytic graphite and solid carbonated glassy koi. A process for manufacturing a porous preform (1) for an optical fiber made of fine quartz glass particles, which process comprises dehydrating the porous preform in an atmosphere comprising CF4 as a fluorine additive, and then glazing the preform. , characterized in that the process is realized in a heating furnace comprising a heater (4) and a sleeve tube (3) arranged inside the heater (4) to separate the heating atmosphere from the heater (4), and which socket tube body (31) is made of extremely pure koi, the inner and outer walls of the body (31) being coated with carbon material (32) selected from the group consisting of pyrolytic graphite and solid carbonated glassy koi. 20