JPH03183632A - Production of glass preform for optical fiber - Google Patents

Abstract

PURPOSE:To minimize the contamination from a furnace core tube and to obtain the glass preform for optical fibers added uniformly with fluorine by heat treating an SiO2 glass particle body in specific two stages in a gaseous fluorine- contg. atmosphere, then vitrifying the body to transparent glass in an inert gaseous atmosphere. CONSTITUTION:The SiO2 glass particle body synthesized by a flame hydrolysis method is put into the furnace core tube and is dehydrated by a heating treatment in the gaseous chlorine system. The body is then subjected to the heating treatment in the atmosphere contg. the fluorine-based gas (e.g.; SiF4) to add fluorine to the SiO2 glass particle body. The furnace temp. is then set at the temp. from the same temp. as the fluorine addition temp. up to 1400 deg.C and the SiO2 glass particle body is subjected to the 2nd heating treatment in the fluorine-based gas-contg. atmosphere to shrink the SiO2 glass particle body to the extent of preventing the evaporation of fluorine in the outer peripheral part of the SiO2 glass particle body. The particle body is then vitrified to the transparent glass in the inert gaseous atmosphere, by which the glass preform for the optical fibers added with fluorine is obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a glass base material for optical fibers, in particular, a core made of pure quartz (SjOt) and a cladding made of fluorinated quartz (F5h).
The present invention relates to a method for manufacturing a glass preform for optical fibers.

[Conventional technology]

So-called pure silica core optical fiber, in which the core is made of pure silica (Sj Ot) glass and the cladding is made of fluorine-doped silica glass doped with F to lower the refractive index, has low transmission loss and is suitable for long-distance communication lines. Attention has been paid.

The fluorine-doped silica glass used as the cladding material for this pure silica core optical fiber is normally doped with fluorine in a sintering furnace.

To list 2.3 of the methods, the method described in JP-A No. 60-90842 dehydrates a silica glass particle aggregate and firstly evaporates it in an α compound-containing atmosphere to remove impurities.
This is a method in which a heat treatment is performed, then a second heat treatment is performed in a gas atmosphere containing fluorine or a fluorine compound, and finally a third heat treatment is performed for transparent vitrification.
The method described in Japanese Patent No. 275035 involves impregnating fluorine at a temperature at which the silica glass particle laminate is in a porous state;
Thereafter, the material is made transparent in a high-temperature furnace in an atmosphere containing fluorine compound gas. Further, as the fluorine compound used at this time, it is desirable to use 5iF4, which does not cause a reaction with quartz, SIC, etc. used to maintain airtightness.

5FI-CF4 and CCetFw decompose at 1000° C. or higher, generate F and gas, and etch 5iOt, SiC, etc.

[Problem to be solved by the invention]

However, it was confirmed that when slF4 and 5ift coexisted at 1400°C or higher or 5IFl and SjC coexisted at 1380°C or higher, consumption of Sing and stc was observed, albeit in a small amount. This means that under high temperatures, especially 1400°
When processing the base material in the coexistence of SiF4 at a temperature of C or higher,
Sing or SiC is the material of the reactor core tube that maintains airtightness.
This means that the carbon, etc. coated with this material is etched, and impurities contained in the reactor core tube are mixed into the base material.

Then, it would be better not to use a fluorine compound at the time of transparency, but in the commonly performed fluorine addition treatment at 1150°C to 1300°C, the soot shrinks sufficiently (bulk density 1.7
If the fluorine compound is not flowed during transparentization, the fluorine at the outer periphery of the base material will volatilize, resulting in a poor refractive index. Also, during fluorine addition treatment, force → density 1
．． If the material is sufficiently shrunk to about 7 g/a/ or more, fluorine volatilization can be prevented, but the processing time for the base material becomes longer, causing problems in productivity.

The present invention has been made in order to solve the above-mentioned problems, and its object is to obtain a glass base material for optical fibers uniformly doped with fluorine by improving the heat treatment process.

[Means to solve the problem]

The present invention provides a method for producing a glass preform for optical fibers, which includes a fluorine addition step in which 5 loz glass particles synthesized by flame hydrolysis or the like are heat-treated in an atmosphere containing a fluorine-based gas. After the treatment, the furnace temperature was set from the same as the fluorine addition temperature to 1400°C, a second heat treatment was performed in an atmosphere containing fluorine gas, and then transparent vitrification was performed in an inert gas atmosphere. This is a method of manufacturing a glass preform for optical fiber, characterized by carrying out the following steps.

The furnace temperature in the first heat treatment (fluorine addition treatment) is preferably from 1150°C to 1300°C,
The furnace temperature in the second heat treatment is preferably 1300°C
The temperature is above 1400°C.

The 5-int glass particles in the present invention are glass raw materials such as si CI! 5iCf+ in 02 atmosphere under heating
+0t-4SIO*+ 2 A method of reacting like Get, or 5iCJ'i+ 4 HtO4SiOt+ 4 HCI in heated steam
It can be synthesized by a reaction method as shown in '.

As the inert gas for transparency after the second heat treatment of the present invention, 1K is desirably used.

[Effect]

The present inventors first 810! A thermodynamic study was conducted on the reaction between SIC and 5IFt. As a result, Si
It was recognized that Fl can cause the reaction of the following formula (1) with 5ift.

st F4 + 310s → 2 sl OF*↑
...+11 Furthermore, in the case of the reaction between SIC and st F4, etching of SiC occurs due to the reaction of formula (2) below.

From the above, it can be seen that etching of St Of or SiC by St Fl progresses at temperatures above 1400°C.

When the core tube is etched, impurities present in the core tube are scattered into the atmospheric gas, causing an increase in transmission loss caused by the impurities. Therefore, it is preferable not to use Si Fl under transparent conditions of 1450° C. or higher.

However, the conventional fluorine addition process (1150℃~13℃
When the soot is made transparent in an atmosphere without Si Fl, the fluorine at the outer periphery of the soot is volatilized, resulting in a refractive index distribution as shown in FIG.

When the fluorine addition temperature is lowered, volatilization at the outer periphery disappears, but due to shrinkage of the soot due to fluorine addition, sufficient fluorine is not added to the center, resulting in the profile shown in FIG. 3.

In the present invention, the first heat treatment (temperature 1150°C to 1
After adding fluorine to the entire glass particle body at a temperature of 300°C, a second heat treatment is performed at a temperature of 1400°C or lower to shrink the soot enough to prevent volatilization of the fluorine in the outer periphery. By performing transparency at a temperature of 1,450°C or higher in an inert gas, it is possible to prevent core tube etching caused by SiF4 at high temperatures, achieve homogeneous fluorine addition to the base material, and produce a high-purity glass body. Obtainable.

SIO created using flame hydrolysis method etc. The soot usually contains about 1000-2000 ppm IIsO. 11. Since the quartz article is etched by 1-IF produced by the reaction between O and StF4, and because litO decomposes at temperatures above 1000°C to produce O3, the moisture in the soot is It is desirable to remove it using an α-based gas such as CL, Sii'4, Cα4, etc. Dehydration conditions are 850℃ or higher and 1100℃
α concentration 0.5-8%<ce*- in the temperature range below °C
c is 0.25-4%, 5IC11, Cα4 is 0, 12
It is preferable to carry out in the range of 5-2%).

〔Example〕

Example 1 Synthesized by VAD method, outer diameter 120 mm, length 500 mm
mm Stow glass particles were heated at 1050°C in a heating furnace using a ring-shaped resistance heating heater with a 5-lot furnace tube.)Ie15J! /min, C4', 300cc
Dehydration treatment was carried out at a traverse speed of 10 an/min in an atmosphere of 10 an/min. After dehydration, 1220℃, 151℃! /min, SiF* Switch gas to 600 cc/min, 10+
Fluorine addition treatment was performed at +ua/min. Next, increase the furnace temperature to 1
The temperature was raised to 375° C., and shrinkage treatment was performed at a traverse speed of 12 an/min using the same amount of gas.

Finally, change the gas to l1elO1/min only and reduce the furnace temperature to 1
Transparent vitrification was carried out at 580° C. and a traverse speed of 12 III/min. The processing time at this time was about 6 hours.

The refractive index distribution of the base material obtained above was as shown in FIG. 4, and the difference in refractive index between the inside and the outer periphery was less than 0.01%. Base material 10 subjected to the same fluorine addition vitrification treatment as the above treatment
Regarding the book, pure 5lot core SM fiber (Δn=0.
34%, core diameter 9.6 ts). The average fiber loss is 0.1735 d at a wavelength of 1.554
It was B/km.

Example 2 SIO with the same shape as Example 1! Glass particle body, Sa
The amount of impurities 5p coated with c by CVD method to 80-
The inside of a heating furnace using a resistance heating heater having a carbon core tube of less than pm was subjected to dehydration treatment at a temperature of 1050° C1) & 151/min and a traverse speed of 1 ON/min in an atmosphere of 5iC1'4150 cc/min.

The subsequent fluorine addition treatment was carried out in the same manner as in Example 1 to produce a glass body.

The refractive index distribution of the obtained base material was as shown in FIG. In addition, pure 5ins core SM fiber (Δn
= 0.34%, core diameter 9.5 ppm).

The average loss value of the fiber is 0.17 at a wavelength of 1.55p.
It was 40 dB/km.

Comparative Example 1 Using the same heating furnace and furnace tube as in Example 1, Sj Ot glass particles were first heated at 1050° C. in an atmosphere of Cf*300cc/min and Ib15Z/min at a traverse speed of 10an.
/min. After dehydration, 1220℃, l1e
The gas and temperature conditions were changed to 15 f/min and 5iF4600 cc/min, and fluorine addition treatment was performed at a traverse speed of 10 an/min. Next, with the same gas, the furnace temperature was set to 1600.
℃ and passed through a heater at a speed of 8 mm/min to achieve transparent vitrification.

The refractive index distribution of the base material obtained above was as shown in FIG. Based on the base material, pure SIO! When core SM fiber was manufactured, the average loss value of 10 fibers was 0.1792.
It was dB/km.

Comparative Example 2 Using the same heating furnace and core tube as in Example 1, SI O! Dehydrate the glass particles (1050°C, Clt/1-1e
= 0.3/15, traverse speed 1 ON/min), then fluorine addition (1220°C, SiF4/l = 0.6
/15, traverse speed 5/min) After the treatment, the gas type was changed to 1K and transparentization was performed at 1600°C. As shown in FIG. 2, the profile of the obtained base material showed that fluorine volatilization was large in the outer periphery.

〔Effect of the invention〕

As explained above, in the present invention, element 16 can be uniformly added to the preliminary Sl 2 O* glass grains, and contamination from the furnace tube can be minimized.

Therefore, the transmission loss of a pure 5ift core optical fiber made from a preform produced by the method of the present invention can be stabilized.

[Brief explanation of drawings]

FIG. 1 is a flow diagram showing the heat treatment process of the present invention, FIGS. 2 to 4 are refractive index distribution diagrams of a non-additive glass base material, and FIG. FIG. 3 shows an example of fluorine addition at high temperature (1), and FIG. 4 shows examples of Example 1, Example 2, and Comparative Example 1.

Claims (1)

[Claims] (1) SiO_2TEL synthesized by flame hydrolysis method etc.
In a method for manufacturing a glass base material for an optical fiber, which includes a fluorine addition step in which a glass particle body is heat-treated in an atmosphere containing a fluorine-based gas, after the heat treatment, the furnace temperature is adjusted from the same as the fluorine addition temperature to 1400°C. 1. A method for producing a glass preform for an optical fiber, comprising: performing a second heat treatment in an atmosphere containing a fluorine-based gas, and then performing transparent vitrification in an inert gas atmosphere.