JPH01219034A - Production of optical fiber glass preform - Google Patents

Abstract

PURPOSE:To obtain the subject optical fiber glass preform having a refractive index distribution by heating the fine glass particles consisting essentially of quartz at specified temp. and pressure in an atmosphere contg. fluorine or a fluorine compd. to vitrify the particles and to simultaneously vary the amt. of fluorine added in the radial direction. CONSTITUTION:The fine glass particles consisting essentially of quartz are heated in an atmosphere contg. fluorine or a fluorine compd. to add fluorine into the particles, and vitrified. In this case, the heating temp. is controlled to >=1,000 deg.C, and the pressure of the atmosphere is kept at 400-0.1Torr. The fine glass particles having >=0.5 porosity is preferably used. As the fluorine or fluorine compd., F2, SiF4, CF4, CCl2F2, NF3, etc., can be exemplified.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a method for producing a glass base material for optical fibers having a refractive index distribution by changing the amount of fluorine added in the radial direction. .

[Conventional technology]

Optical fibers made of quartz glass for optical communication technology must have a suitable refractive index profile decreasing from the fiber core towards the fiber cladding. This is a material that increases the refractive index in the fiber core, such as GaO2, A/2.
03. doping with P2O3, etc., or glass additives to lower the refractive index of the fiber clad;
For example, it can be obtained by doping with 2o5 or fluorine (F), in which case fluorine is bonded to Sl instead of 1/202-. B2 for use with five optical wavelengths
O3 and P2O3 pose little problem. This is because these doping substances increase the absorption loss (see ``Influence of doping substances on the transmission loss of optical fibers with low OH content'').
ants on transmission loss
low-OH-content optical fi
bers) Jx Lectronics Letters (Erec
tr, Lett, ), Vol. 12 (1976), pp. 549-550). Moreover, the doping substances Goo2°AIO and P2O3 increase the Rayleigh scattering losses of the fiber (Propagation in glass optical waveguides).
ptical waves) J, Review
Op Modern Physics (Remy Wor Mo
d, Phyg, ), Volume 51 (1979), No. 34
(See pages 1 to 367). Another important drawback of oxide doping materials is their tendency to react with hydrogen molecules that can diffuse into the fiber from the environment, forming OH groups (l); .4μ This results in a permanent reduction in optical transmission in the wavelength range near the grave. In contrast, only fluorine-doped fused silica glass achieves the lower losses of pure fused silica glass, and still has 51
02 has low reactivity with J H2 (“Effect of residual halogen in optical fiber on hydrogen loss increase characteristics (I
Cffectg of residual haloge
n in optical flbers on hy
4rogen 1ossincrease chara
oteristicss) J Conference Organic Fiber Committee (co n r *Op
t, Fiber Oomm, ), San Diego, 19
February 11-13, 1985, Technical Digest (Techn, Digcs+s, pages 46-47, Opt, Boo, of America)
a), 1985). Fluoride is required in addition to the oxide doping material when fibers with multiple claddings are to be manufactured, and in such fibers at least one cladding should have a refractive index less than that of fused silica. (2 pg/kJ over the wavelength range 1.28 μm to 1.65 μm
Low-loss quadruple-clad single-mode optical fiber with scattering less than
uple-clad singla-moda lig
ht-gu14es with dispersion
below2 pg/FAm nu over th
e 1.28- L65 pw+ wave length
hrange) J, x Lectonics Letters (
]E1ectr.

Lett, Volume 18 (1982), pages 1023 to 1024).

810□ Fine glass particles are produced by flame hydrolysis and deposited on the outer surface of a cylindrical substrate. A highly efficient and inexpensive outer surface deposition process such as the VAD method (vapor deposition method) is a fluorine-doped method. Unfortunately, it is not suitable for the production of optical fiber glass.
ng in theMAD method andit
applications to optica
lfiber fabrication) J, 9th Eurobiancon 7 Elens Optical Committee (]Europ, Conf, Opt, Ooa+
m,) il proceedings, Geneva, October 23, 1983.
(See pages 13-16, Amsterdam, Netherlands, 1985). The basis for this is 2 H2O+ SiF4# 4 HF 15in2 ・
... (1) The chemical equilibrium given by the above equation (11) is in an unfavorable state, and the equilibrium is on the 7) hydrogen chloride side at high temperatures. Therefore, in a high temperature flame, for example, OF4.
The mixed fluoride, which may be present in the leon form such as 02F6 or in the form OF, C1 etc. or in the form SiF4, is eventually converted to hydrogen fluoride (HF), which is converted into hydrogen heptadide in 5102. Since it has only a small solubility in the gas, most of it escapes in the form of a gas.

The doped quartz glass can be produced by deposition on the inner wall of a tubular basic body from a hydrogen-free atmosphere, for example by the well-known method of mounting-on-the-wall (MOVD). However, this method has a low deposition rate.

The problem of fluorine doping during deposition on the external surface of the substrate is '! -j'5102 An undoped stack of glass particles is produced, which is later exposed to a gas containing fluorine or fluorine compounds during a hydrogen-free drying and sintering process. This solution has been previously solved by doping by diffusion using

Efforts have been made to take advantage of the fact that the rate of diffusion of fluorine into a porous body is inversely proportional to the density of the material in order to create a doping distribution. To this end, the 8102 glass particles must have a higher density in the core and a lower density in the cladding. N
ewiy aevelopell fluorine
doping techniquefox VAD p
rocas) J, 10th European Conference Optical Committee (ICurop, G
onf, Opt, Gown, ), Stuttgart, September 3-6, 1984, pages 294-295, VDiC Verlag, Berlin, 1
984). By using a hollow rod made of SiO
ars by the outside vapor
deposition process) J, Technical Digest (Teohn.

Digest), Conference Optical Fiber Committee (Oonf, Opt, Fiber
r Oomm, ), New Orleans, 1984 1
25th or 25th of the month, pages 22 or 23
This can be achieved by precipitation of powders of different densities consisting of glass particles. However, sintering of the glass that begins simultaneously at the temperature at which fluorine diffuses has drawbacks. This is because when the temperature is low for thick glass particles, a very long diffusion time is required.

[Problem to be solved by the invention]

As mentioned above, in the conventional technology, fluorine or fluorine compound gas is diffused into the glass particulate body in the so-called sintering process in which the glass particulate body is heated in an atmosphere containing fluorine or fluorine compound gas to sinter and become transparent. There is a problem in that the time required for the process reduces the manufacturing efficiency of optical fibers. Furthermore, when it is necessary to locally reduce the pores 4 of the glass fine particles in order to suppress the diffusion of fluorine or fluorine compound gas, for example, when trying to change the refractive index distribution of the fluorine-doped portion in the radial direction, There is also the problem that gas replacement in that area is not performed well and bubbles remain in the resulting transparent glass body.

The present invention aims to solve these problems and provide a new method for manufacturing an optical fiber base material in which the amount of fluorine added is varied in the radial direction to form a refractive index distribution.

[Means to solve the problem]

The present inventors have repeatedly researched the mechanism of fluorine addition to glass particles, and found that the reaction rate with fluorine on the surface of glass particles has a strong temperature dependence. It has been found that by forming a temperature distribution, it is possible to form a distribution of weight. Based on this knowledge, in a completely different way from the conventional technology, by reducing the atmospheric gas pressure during the sintering process, the heat conduction by the atmospheric gas is inhibited, and the temperature rises rapidly in the radial direction of the glass particles. We have arrived at the method of the present invention, which changes the amount of fluorine added by forming a distribution and changing the reaction rate with the fluorine.

That is, in the sintering process 11 of the present invention, glass fine particles containing quartz as a main component are heated in an atmosphere containing fluorine or a sulfuric compound to form transparent vitrification while adding fluorine t-a into the glass fine particles. , the pressure t of the atmosphere containing the two or fluorine compound gas at a heating temperature of 1000°C or higher
The present invention relates to a method for producing a glass preform for an optical fiber that exhibits a t% characteristic when maintained at a reduced pressure within a range of -400 TOrr or less and 0.1 Torr or more.

In the present invention, the glass fine particles have a porosity of 0.5.
It is particularly preferable that it is above.

Hereinafter, the present invention will be explained in detail with reference to the drawings. Figure 1 is an explanatory diagram of a specific example of the present invention. is inserted into the furnace core tube 2 of a heating furnace (resistance heating furnace shown) and heated by the heater 3. Fluorine or 7 into the furnace
An atmospheric gas containing a compound gas is supplied. When the temperature inside the furnace reaches 1000° C., atmospheric gas is guided to the piping 6 side by the internal pressure regulating valve 5. In a region where the furnace temperature is 1000°C or higher, the furnace atmosphere gas pressure is set to 400 Torr or less.
Vacuum pump 4 so that the pressure is reduced to more than I Torr.
The pressure is regulated using the needle valve 5. 7 is an exhaust port.

In the present invention, glass particles mainly composed of quartz can be produced by a soot-applied method such as a WAD method or an OVD method, as well as a sol-gel method or a press molding method (glass particles are collected and molded using a hydrostatic press, etc.). This can be done by various known techniques such as method).

Examples of the fluorine or fluorine compound gas that can be used in the present invention include F2. SiF4. SF6. For example, fluorocarbon gas such as OF4, for example CG12F2. , C
Chlorofluorocarbons such as C/3F, e.g. L td N
Examples include fluorinating agents that do not contain metal elements such as F3*''25.

In the present invention, 400 Torr% 0. I Tor
The temperature range for reducing the pressure of r is 1000°C or higher, and the upper limit is usually 1600°C. Below 1000°C, the reaction rate of fluorine is extremely slow and production efficiency is poor.

Furthermore, even when a temperature difference is created between the surface and the center of the glass particle body by reducing the pressure as in the present invention, the surface of the glass particle body follows the furnace temperature.
When the temperature exceeds 00° C., at least the surface of the fine particles has completely become transparent and vitrified, and there is virtually no point in reducing the pressure.

Note that, for example, a known technique such as a vacuum pump may be used as the pressure reducing means.

[Effect]

The mechanism of adding fluorine into the glass particles during the sintering process consists of three stages.

That is, in the first stage, fluorine or fluorine compound gas diffuses and permeates into the glass fine particles, in the second stage, fluorine atoms and glass@particles react on the surface of the glass fine particles, and in the third stage, fluorine diffuses into the glass fine particles. be. As described above, methods for controlling the diffusion rate of fluorine or fluorine compound gas in the first stage have been devised in the prior art, but there are various problems.

On the other hand, studies conducted by the present inventors have revealed that the rate of reaction occurring on the surface of the glass particles in the second stage exhibits strong temperature dependence.

Finally, it has been found that by forming a temperature distribution in the radial direction of the glass particles, it is possible to provide a distribution in the amount of fluorine added. The second case is an example of data showing this temperature dependence. A glass particulate body is inserted into a resistance furnace in which the concentration of fluorine raw material gas is kept constant, and the amount of fluorine added to the particulate body ( Two levels (
The figures are shown for furnace temperatures of T=1200°C and T=1000°C. As you can see from the figure, the furnace temperature is 1200
When the temperature is 1000°C, the reaction rate is about twice as high as that at 1000°C.

Therefore, for example, if the furnace internal pressure is kept at atmospheric pressure and the furnace temperature is raised at an extremely fast heating rate, the surface temperature of the glass particles will follow the rise due to radiation from the heater, but inside there will be a delay due to heat conduction. This creates a temperature difference with the surface. However, in order to provide a distribution in the amount of fluorine added in this method, an extremely fast heating rate of K is required (1), and there is a problem in that bubbles remain in the resulting glass body.

In response to this, the inventors of the present invention have conducted further intensive studies and have found a means that is effective (for forming the distribution of the amount of fluorine added) and does not cause the problem of bubbles remaining. The present inventors first studied the thermal conductivity mechanism of the glass fine particles, and found that the thermal conductivity of the glass fine particles themselves is as follows (its porosity (Pa) expressed by equation 21: Porosity (Pc) = 1- ρ/ρ... It was found that the thermal conductivity of the glass particles is 1/1000 or less of that of the atmospheric gas when the porosity is 0.5 or more. Under atmospheric pressure, the temperature distribution of glass particles can be formed by controlling the thermal conductivity of the atmospheric gas.In fact, the pressure of the atmospheric gas can be adjusted to 40
When the temperature was set to 0 Torr, the conductivity of the hot plate was approximately %), making it possible to form an effective temperature distribution. Moreover, according to this means, there is an advantage that even if the heating rate is increased, bubbles are less likely to remain than in the case of atmospheric pressure. Therefore, the temperature near the surface and near the center of the glass particle body can be made extremely high. Note that when the atmospheric gas is reduced in pressure to about Q, I Torr, its thermal conductivity is equal to that of the glass particles, y, and even if the pressure is reduced below this level, it is difficult to expect an increase in the effect.

As described above, according to the present invention, not only is there no restriction due to the time required for the diffusion process of atmospheric gas in the sintering process, but also a good glass base material without many bubbles can be produced by sintering under reduced pressure. can do.

The above explanation is based on a heating method that raises the temperature inside the furnace (soaking furnace). This method is also effective for inserting glass particles into a furnace, and the same effects as described above can be obtained.

〔Example〕

Example 1 #! by VAD method! The remaining pure silica glass particles were inserted into the core tube of the resistance heating furnace shown in FIG. 1, and fluorinated and sintered according to the present invention. The porosity of the glass fine particles was 0.7.

After inserting the particulate material into a nine furnace that maintains the furnace temperature at 1,000°C, the furnace temperature was raised from 1,000°C to 1,500°C at a rate of 10°C/min, held at t500°C for 1 hour, and then lowered to 1,000°C. ℃
The base material T-removed was taken out. Within the temperature range of 1000°C and 1500°C, the temperature inside the furnace was maintained at f 20 Torr. 81F4t-200CC was continuously supplied into the furnace for 7 minutes.

At 1300° C. or higher, the atmosphere was 1He (atmospheric pressure). The transparent glass base material obtained as a result had a refractive index distribution as shown in FIG. 3), which was satisfactory. In addition, Example 2, in which no air bubbles remained in the base material, was prepared by adding fluorine and sintering according to the present invention, using the same pure silica glass Wk particle body with a porosity of 0.7 as in Example 1 and having the configuration shown in Figure 1. did. The fine particles were inserted into a furnace kept at 1000°C, and
The temperature was raised from 000°C to 1500'ct at a rate of 5°C/min, and the atmosphere at this time was 81F47G up to 1300°C.
Torr vacuum, over 1300℃ Ha 517
minutes (atmospheric pressure), and held at 1500℃ for 30 minutes.
The temperature was lowered to ℃ and the transparent glass body was taken out. The refractive index distribution of the obtained transparent glass body, as shown in FIG. 4, was satisfactory and no bubbles were observed.

Example 3 A pure quartz glass fine particle body having a porosity of 0.7 was added with fluorine according to the present invention and sintered using the same structure as in Example 1, and the structure shown in FIG. 1000℃ at initial temperature of 1000℃ in the furnace
The temperature was raised at a rate of 15°C/min from
It was set as r.

The transparent glass body taken out after cooling to 1000°C had no bubbles and had the refractive index distribution shown in FIG.

Example 4 In Example 3, 5iF450 cc/min, furnace pressure was 0
．． When the same procedure was carried out except that the pressure was changed to 1 Torr, a transparent glass base material without bubbles and having the refractive index distribution shown in FIG. 6 could be obtained.

Example 5 A transparent glass body was obtained in the same manner as in Example 1 except that 8F6'i was used as the fluorine compound gas. The refractive index distribution of this glass body was r! Same as Figure 3, Atsushi. This shows that the same effect can be obtained regardless of the type of gas.

Comparative example Furnace pressure was atmospheric pressure, 5iF4200α/min, Ha
The same procedure as in Example 1 was carried out except that the setting was 7.4J3/min.
A transparent glass body (comparative product) t- was obtained. Note that in this example as well, Si
The partial pressure of F4 is the same as in Example 1. There were no air bubbles in the glass body.The amount of fluorine inlaid was uniform in the radial direction from the outer periphery to the center.

From the results of Examples 1 to 5 and Comparative Examples above, it is clear that within the heating temperature range of 1000°C and 51600°C, the atmospheric pressure containing fluorine compound gas is 4005°C to 4005°C.
It can be seen that by setting the amount within the range of 1, a fluorine doping amount distribution is formed in the radial direction of the glass, and thereby a glass base material having a refractive index distribution in the radial direction can be obtained without residual air bubbles.

〔Effect of the invention〕

As described in detail above, in the sintering process of glass microparticles, the present invention is performed at a temperature of 1000°C or higher and an atmospheric gas pressure f:
By controlling the temperature within the range of 400 Torr or less and 0.1 Torr or more, a temperature distribution is formed in the radial direction of the glass fine particles, and thereby the refractive index distribution is controlled by controlling the reaction rate with fluorine. can. For this purpose, an optical fiber base material doped with fluorine and having a refractive index distribution in the radial direction can be manufactured efficiently and with good controllability without problems such as restrictions due to diffusion speed or bubbles remaining in the glass base material. This is an advantageous method that can be used.

[Brief explanation of the drawing]

Claims (2)

[Claims] (1) In the sintering process in which a glass particulate body mainly composed of quartz is heated in an atmosphere containing fluorine or a fluorine compound to make it transparent while adding fluorine to the glass particulate body, the heating temperature is 1000°C. A method for manufacturing a glass preform for an optical fiber, characterized in that the pressure of the atmosphere containing the fluorine or fluorine compound gas is maintained at a reduced pressure within a range of 400 Torr or less and 0.1 Torr or more. (2) The method for manufacturing a glass preform for an optical fiber according to claim 1, wherein the glass fine particles have a porosity of 0.5 or more.