JPH0450130A - Production of preform for optical fiber - Google Patents

Abstract

PURPOSE:To make it possible to stably produce a preform for optical fiber over long period by introducing a mixed gas into a furnace core tube made of carbon, subjecting a porous glass preform to dehydration and high purification treatment and then vitrification. CONSTITUTION:A mixed gas of inert gas, halogenated gas and carbon monoxide is introduced into a carbon furnace core tube having SiC or thermally decomposed carbon film and a porous glass preform charged into the furnace core tube is subjected to dehydration and high-purity treatment thereby. Then a fluorine compound is introduced and the treated porous glass preform is vitrified in a mixed atmosphere of fluorine compound and inert gas. Thereby the aimed preform for optical fiber free from contamination with impurities and water can be stably produced and deformation, crystallization and breakage produced in a conventional quartz furnace and oxidation consumption of coating film in SiC-coated carbon material are suppressed and the furnace core tube can be used over long period.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing an optical fiber preform by heat-treating a porous glass preform for an optical fiber.

<Conventional technology> Typical method for producing optical fiber base material 1 This method involves producing a cylindrical or cylindrical porous optical fiber (base material) using methods such as the vapor phase shaft attachment method and the external attachment method. Furthermore, this porous optical fiber base material is dehydrated and purified in a sintering furnace in an atmosphere containing an inert gas such as argon helium, chlorine gas, or fluorine gas, and then fluorine is added. There is a method that uses transparent treatment to obtain a highly pure optical fiber preform.

The operating conditions of the furnace during this vitrification vary depending on the type and content of the dopant, but are approximately 1200 to 1600°C.
is within the range of In addition, since impurities easily enter the base material at high temperatures, a furnace core tube made of high-purity quartz is generally used to reduce the contamination of impurities as much as possible. has the problem that it rapidly becomes brittle and has poor durability due to a phenomenon called "devitrification" in which it changes from a glassy state to a crystalline state at high temperatures.

Therefore, in order to solve the problem of durability of the quartz furnace, we decided to
CVD method,
A SiC film or a pyrolytic carbon film is coated using the PCVD method.

<Object to be solved by the invention *a> The conventional technology introduced above has the following drawbacks.

(1) When using a quartz furnace core tube, the temperature is below the crystal transition temperature (below 300°C) due to softening deformation at temperatures above 1400°C and formation of crystals (cristobalite) above 1200°C.
There is a problem in that the furnace temperature cannot be lowered to a certain degree, and that once the furnace temperature has been heated to -1000 degrees, it must be used continuously.

(2) When using a SiC film or a SiC-coated carbon furnace core tube, there is a problem that the following reaction occurs at high temperatures and low O2 concentrations, causing the coating to be oxidized and consumed.

SiC (g) + Oω → SiO axis) ↑ 10 Co (g) ・
・(1)C(s) +170.2 billion) → CO(g)
(2) Therefore, if furnace core tubes are used for a long period of time, the coating will be consumed by the decomposition reaction of equation (1), and the carbon material will be exposed.

In this case, carbon will be consumed by the reaction of formula (2).

The present invention has been made in view of the above-mentioned circumstances, and the S
An optical fiber base material that does not cause deterioration of the iC film or pyrolytic carbon film, allows stable production of the optical fiber base material over a long period of time, and produces an optical fiber base material that provides an optical fiber with low transmission loss. The purpose is to provide a method for manufacturing materials.

Means for Solving the Problems> The structure of the method for manufacturing an optical fiber base material of the present invention to achieve the above-mentioned object is to include an inert material in a carbon furnace core tube having a SiC film or a pyrolytic carbon film. The method is characterized in that a mixed gas of gas, halogen-based gas, and carbon monoxide is introduced to dehydrate and purify the porous glass base material, and then to make it transparent.

Hereinafter, the configuration of the present invention will be explained in detail with reference to the drawings.

The apparatus for carrying out the method of the present invention is as shown in FIG. 1
A furnace body 15 is provided surrounding the outer periphery of the porous glass base material 2, which is equipped with a heater 14 that heats the porous glass base material 11 and a heat insulating material that prevents heat radiation from the heater 14 from reaching the base material side. ing. This furnace body 15 is provided with an inert gas inlet @16 for introducing inert gas. In addition, a gas introduction pipe 17 is connected to the lower part of the furnace core tube 12 in the figure to introduce dehydrated and high-purity gas required for dehydration and sintering into the furnace core tube 12. A mixed gas of an inert gas, a halogen gas, and CO is introduced into the introduction pipe 17 via a gas mixer (not shown). The furnace core tube 12 uses a carbon furnace core tube base material 18, and its surface is coated with an SiC single film made of high-purity silicon carbide and a SiO2 coated with SiO2 coated on the surface of the SiC film.
A SiC film 19 such as a film is formed by a known method described later.

Here, the dehydrated/high purity gas according to the present invention is an inert gas such as helium (He), argon (Ar), nitrogen (N2), SiCj4. SiF,.

CCl4. A mixed gas containing a halogen gas selected from CF4 and carbon monoxide (CO).

Next, an experimental example that led to the present invention will be explained.

Experiment 1 A coating material coated with 50 μm of SiC coating by CVD was heated at 1600°C under 0.2100 ppm of He.
Heat treatment was performed at 0 hours.

The weight reduction of the SiC coated part after this treatment is 10%.
The coating pores were observed in some areas.

Experiment 2 The coating material manufactured using the same method as in Experiment 1 was heated to 1600°C.

Heat treatment was performed for 10 hours under He containing 0.0100 ppm and 500 ppm of CO.

The weight reduction of the SiC coated part after this treatment is 0.5
%, and there was no difference at all from the initial stage of SIC coating.

Experiment 3 The coating material manufactured using the same method as in Experiment 1 was heated to 1600°C.

0500ppm, under He containing 0500ppm CC, 10
Heat treated for a period of time.

The weight loss of the SiC coated part after this treatment was 3%, indicating that it was worn out.

Experiment 4 The surface of a graphite base material was coated with 30 μm of pyrolytic graphite by CVD using CH4. This was heat-treated at 1600° C. in 0.100100 pp for 5 hours.

After this treatment, the coating disappeared and the graphite substrate was exposed. Moreover, the weight reduction rate including the base material was 2%.

Experiment 5 The same coating material as in Experiment 4 was used at 1600°C.

0, 100 ppm, Co 1000 ppm containing H
The mixture was heat treated for 5 hours under e.g.

The coating thickness after this treatment was reduced to 25 μm, but the coating remained.

From these experimental results, CO? The addition of I improves the high temperature oxidation resistance of SiC or pyrolytic carbon coatings. It was also shown that when COlo and specific force were low by 9, it could not be completely suppressed.

As described above, in a furnace core tube that uses a carbon material that does not cause deformation of the furnace core tube even at 2000°C as the base material of the furnace core tube and coats it with SiC or pyrolytic carbon as a gas-impermeable coating, , carbon monoxide (C
It has been found that by incorporating O) the durability of the coating is significantly improved.

This can be explained by examining the reaction shown in reaction formula (3) below (the case using SiC will be explained)
.

The equilibrium constant of the reaction SiC+O→SiO+CO (3) is expressed as K=PS・O” Co (4).

P.S. is one of the decomposition reaction products of SiC, and (4
) formula is P, . =K-PO2/P,. ...
(4)' is transformed. Equation (4)' shows that in order to lower the partial pressure of SiO, which is a decomposition product of SiC, mixing CO or reducing O is effective (mixing SiO can also be considered at high temperatures). ).

Therefore, the effectiveness of CO addition is demonstrated.

At this time, the partial pressure of co gas is 10" atm ~ 10-'
The partial pressure of CO is preferably three times the partial pressure of 02.

In addition, the halogen resistance of SiC is 8 for halogen gas alone.
Although the temperature limit is 00 to 1000°C, corrosion of SiC can be prevented by adding a Sl-based halogen compound (SiCj).

This fact can be explained by the equilibrium equation of equation (5).

S i C+2CI = S i C1+C...(5)
It is sufficient that the reaction (1) above increases the corrosion reaction of SiC due to CI2.

Next, an example of manufacturing an optical fiber preform based on the above results will be shown.

(Dehydration/high purity treatment process) In the present invention, when using a carbon furnace tube having a SiC or pyrolytic carbon film, a mixed gas of an inert gas and a silicon halide is used as a dehydration/high purity gas. The ratio of this mixed gas is 0.3 to 10% by volume, preferably 2% silicon halide to 100% by volume of inert gas.
Contains ~5% by volume, with a partial pressure of CO of 1O-4a tm~l
O-'atm, 02f) minute pressure 100-2at
It is preferable that the partial pressure of CO is at least three times the partial pressure of 02. This contains 0.3% by volume of silicon halide.
If it is below, the dehydration ability will not be sufficient, and if it exceeds 10% by volume, the effect of the addition will be diminished.

Further, the heating temperature for efficiently performing dehydration and high purification treatment is 900 to 1200°C.

This is because if the temperature is below 900°C, the dehydration/purification reaction will be insufficient; on the other hand, if the temperature is above 1200°C,
This is because the porous glass body shrinks, and the diffusion of dehydrated and high-purity gas into the porous glass base material and the volatilization of impurities to be removed from inside the porous glass base material are suppressed.

By the way, when halogen-based gas is mixed with dehydrated and high-purity gas, SiC! As another means of preventing the deterioration of l,
A possible method is to dehydrate and purify the porous glass base material in advance in a heating furnace with a quartz core tube, and then heat treat the porous glass base material with a SiC coated furnace core tube. . Quartz furnace core tubes are not corroded by halogen gases such as CI2, but as mentioned above, 120
If the temperature is set to a high temperature of 0° C. or higher, there will be a problem that the temperature cannot be lowered due to crystallization, so to avoid this problem, it is desirable to carry out the dehydration and purification treatment in a quartz furnace tube at a temperature of 1100° C. or lower.

Pyrolytic carbon is not corroded by chlorine-based gas alone, but the 0□ produced by the soot dehydration reaction in equation (6) below causes the oxidative depletion reaction in equation (7) below. Is required.

S 1-OH+Cj, -81-Cj+HCj+v20.
- (6) O2+c→CO (7) It is also desirable that the dehydration treatment be carried out at a temperature of 900° C. or higher, at which the dehydration reaction is efficient, and at a temperature of 1200° C. or lower, at which soot shrinkage does not occur.

(Fluorine Addition Step) Next, adding fluorine to a porous glass base material using a furnace core tube having a SiC film will be described.

The furnace core tube 12 having the SiC film 19 is kept at a high temperature, the inside of the furnace core tube is made into a mixed gas atmosphere of fluorine compound gas and inert gas, and the treated porous glass base material is held or passed through it to heat it. Fluorine is added to the porous glass base material by setting the temperature to 1400° C. or lower.

Here, as the fluorine compound gas, silicon fluoride such as S i F4, carbon fluoride such as CF4, SF6, etc. are used. In addition, if the porous glass base material does not become transparent vitrified by the fluorine addition treatment at 1400°C or less,
Transparent vitrification of the porous glass base material is performed again at 1400°C or higher in an atmosphere of only inert gas, or
SiF,
In addition, at least 812F6 or Si3F is contained and fluorine is added thereto to form transparent glass.

By doing this, the deterioration of the SiC film will be reduced.
Fluorine addition treatment can be performed using a carbon furnace core tube with an SIC film.

Although it is possible to add F to a carbon furnace core tube coated with pyrolytic carbon using the same process as SiC coating, carbon coating has better F compound gas resistance than SiC and requires 2F addition conditions. The range of is wide.

F compound gas 11t, rc! , S i F,, CF4
°C2F, SF6 are used. - For boat fishing conditions, 2F addition treatment is carried out at 1200 to 14oooC in a mixed gas atmosphere of inert gas and F compound gas, and 145
It is made into transparent glass at a temperature above 0°C fi in a mixed atmosphere of F compound gas and inert gas.

Note that the method for manufacturing the optical fiber preform illustrated here is just one example, and the optical fiber preform may be manufactured as appropriate using a desired manufacturing method such as the low-turn rad-in tube method.

Example of implementation A preferred embodiment of the present invention will be described below.

Using the H2102 flame hydrolysis method as shown in Figure 4, the outer diameter is 120φ, the length is 450cm, and the density is 0.35g/cl.
r glass particle deposit body 11 (hereinafter referred to as "porous glass base material")
) was manufactured.

The inner layer of this porous glass base material is shown in FIG.

It was inserted into a heating furnace having a furnace core tube 12 having a SiC film 19 on the outer layer.

This SiC film 19 is coated by CVD method 6
The film thickness was 5 μm.

The heater 14 was heated to bring the furnace temperature to 1070°C. The 02 concentration in the furnace at this time was 50 to 70 ppm (N2
(in gas).

Gas: He 101/min, SiCj, 200 c
c/min.

GO 10%-He 90% gas was switched to 100 cc/min, and the porous glass base material 11 was passed through the heater section at a descending speed of 10 cc/min to perform dehydration. Next, the gas types were changed to He1Oj/min, SiF300cc/min, and GOIO.
%-He90% gas at 100 cc/min, and the descending speed of the porous glass base material 11 was set to 3.5 μ/min.
The sample was passed through a heater section at 0°C. Next, the heater temperature was set to 1600°C, and the gas was He 101/min, C0I
The flow rate was changed to 100 cc/min of O%-He90% gas, and the gas was passed through the heater part at a descending speed of 10 ml/min to obtain transparent vitrification.

The obtained base material has a relative refractive index difference of 0.014% with respect to quartz glass, and an OH concentration of 0.014%. It was less than lppm (
It was determined from Si-OH vibration absorption at 3670 am1 using an infrared spectrophotometer. ).

A hole with a diameter of 5 m was made in the center of the obtained transparent glass body using an ultrasonic wide opening machine, and then heated and stretched using an H2102 burner to an outer diameter of 25 φ, and the pipe and the quartz rod were heated and integrated.

The obtained base material had a relative refractive index difference of 0.34% and a core-cladding ratio of 5.1. This rod is stretched to 10φ and has an inner diameter of 1
It is inserted into an F-added quartz pipe with a diameter of 2φ and an outer diameter of 30φ, heated and integrated to form a pipe with an outer diameter of 28φ and a core-clad ratio of 12.5. An optical fiber preform with a relative refractive index difference of 0.34% was manufactured.

This preform was drawn to obtain the single mode optical fiber 20 shown in Fig. 6, and its loss was measured to be 0.173 dB/km at a wavelength of 1.55 μm (second
(see figure).

A similar transparent vitrification process was performed for up to 90 tubes, but no damage to the furnace core tubes was observed. In addition, 1.55μ for 50 preforms manufactured using the obtained base material
The average loss of m was 0.1722 dB/, which was good.

Comparative Example 1 The porous base material 19 was heated with CO under the same temperature conditions as in Example 1.
A single mode optical fiber was obtained by processing in the same manner as in Example 1 under gas conditions without flowing.

The average loss for the 1st to 40th sintered pieces is 0 or 1?
The loss was 31 dB/, but the loss increased thereafter, and by the 55th run, it was 0.189 dB/.

When the furnace core tube was examined, small cracks had formed near the heater and some of the coating had disappeared.

Example 2 A porous glass preform 11 having an outer diameter of 100φ, a length of 450 m, and a density of 0.33 g/car was manufactured by the H,/02 flame thermal decomposition method shown in FIG.

This porous glass base material was set in a furnace core tube 12 shown in FIG. 1 whose carbon surface was coated with 65 .mu.m of SiC, and the temperature of the furnace 14 was raised to 1060.degree.

Gas: He 101/min, SiCj, 300cc/min.

The flow rate was changed to Co 10%-He 90% 200 ce/min, and the base material was passed through the converter section at a speed of 5 ce/min.

The gas was then heated to He 101/min, C0IO%-1 (e
The speed was changed to 200 ec/min at 90%, and the speed was set to 4 sm/min at a heater temperature of 1670° C., and the porous glass base material 11 was passed through the heater to form transparent glass.

The obtained base material 11 (pure SiO8) has an OH concentration of 0 and lp
It was less than pm. After stretching in a resistance heating furnace,
By the rod-in-tube method shown in Fig. 3, it was integrated with the F-doped quartz tube 21, which has a refractive index lower than that of 0.4% quartz, and was then drawn, resulting in a loss of 0.175 at 1.55 μm.
dB/kmt.

The OH absorption at 1.38 μm was 0.6 dB/km.

Example 3 Using a carbon furnace core tube having a pyrolytic carbon film coated with pyrolytic carbon inside and outside as a furnace core tube, F# added silica glass was manufactured under the same conditions as in Example 1, and the relative refractive index difference was 0. When a single-mode fiber preform with a core-cladding ratio of .34% and a core-cladding ratio of 12.5 was fabricated using the shoulder-in-tube method and drawn, the loss at 1.55 μm was 0.1715 dB on average for 40 fibers. there were.

Comparative Example 2 A heating process similar to that in Example 3 was performed using a pyrolyzed graphite furnace core tube having the same structure as in Example 3. No abnormality was observed in the sintered star bodies with 1 to 20 pieces, but
On the 20th starburst, bubbles (dots) could be seen on the glass surface.

This occurred because the carbon coat disappeared and carbon powder generated due to consumption of the graphite base material adhered to the quartz.

Example 4 A heating furnace 18 (
A silicon carbide inner layer with a thickness of 50 μm and an outer silicon carbide inner layer with a thickness of 50 μm, formed by CVD method, purity 100%) were used. Heat the furnace core tube 1 to 1050°C using the heater 14.
2 at a rate of 300 cc/min, He 101/min, Co 10%-He 90% 200 oe/min, and the porous glass base material 11 was lowered into it at a descending speed of 10-min.
/minute inserted. After the porous glass base material 11 passes through the heater 14, the gas is heated at 160 cc/min of SiF and He1.
01/min, C010%-He 90% 200cc/min, set the temperature of the reactor 14 to 1400°C, and
The porous glass base material 21 was moved at a rate of 1/min. Then, the gas was switched to He 101/min and the heater temperature was 170
The porous glass base material was processed at 20 m/min at 0° C. to make it transparent.

The obtained transparent glass base material had a relative refractive index difference that was 0.34% lower than that of quartz.

Using this obtained glass base material, a 125 μm diameter l
When a li S i O core type car-mode fiber was fabricated, the transmission loss value at a wavelength of 1.55 μm was 0.1.
It was 73 dB/km. Further, even when the heat treatments were repeated for a long period of time, no damage was observed to the SiC film 19.

<Effects of the Invention> As described above in detail along with experimental examples and examples, according to the present invention, it is possible to stably manufacture an optical fiber base material free of impurities and moisture over a long period of time, and Since the deformation, crystallization, and destruction that occur in conventional quartz furnaces and the oxidative wear and tear of the coating film in SiC-coated carbon materials are suppressed, the furnace core tubes can be used interchangeably for a long period of time, making it economical.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of the heating furnace used in the method of the present invention, Fig. 2 is a graph showing the loss characteristics of optical fiber, Fig. 3 is a preform manufacturing diagram by the loft-in-tube method, and Figs. A conceptual diagram of the base material manufacturing method using the hydrolysis method, and FIG. 6 is a refractive index distribution diagram of the fiber. In the drawing, 11 is a porous glass base material, 12 is a furnace core tube, 13 is a support rod, 14 is a heater, 15 is a furnace body, 16 is an inert gas introduction path, 17 is an atmospheric gas introduction path, and 18 is carbon. The furnace core tube base material 19 is a SiC film or a pyrolytic carbon film.

Claims (1)

[Claims] 1) A mixed gas of an inert gas, a halogen gas, and carbon monoxide is introduced into a carbon furnace core tube having a SiC film or a pyrolytic carbon film to form a porous glass base material. A method for producing an optical fiber base material, which is characterized by dehydration, high purity treatment, and transparent vitrification treatment. 2) In the method for manufacturing an optical fiber preform according to claim 1, after the porous glass preform is dehydrated and purified, Si_2F_6 is added to SiF_4 in the furnace core tube.
and/or a fluorine compound mixed with Si_3F_8 is introduced into the porous glass base material after dehydration and high purity treatment, and the porous glass base material is subjected to fluorine addition treatment, and then subjected to transparent vitrification treatment. manufacturing method. 3) In the method for manufacturing an optical fiber base material according to claim 1 or 2, the mixed gas to be dehydrated and purified is He, Ar, N_
one or more inert gases selected from 2, SiCl_4,
A method for manufacturing an optical fiber base material, characterized in that the mixed gas is a halogen gas selected from SiF_4, CCl_4, and CF_4 and CO.