JPH01145346A - Production of optical fiber preform - Google Patents

Abstract

PURPOSE:To obtain the title preform having low transmission loss at a low cost, by dehydrating a porous glass composed mainly of SiO2 under specific condition and converting to transparent glass with heat. CONSTITUTION:A porous glass composed mainly of SiO2, containing GeO2, TiO2 and/or Al2O3, having a bulk density of 0.2-0.8g/cm<3> and produced by VAD process, OVD process, sol-gel process or pressing process is dehydrated at 500-950 deg.C in an inert gas atmosphere (He, Ar or N2 atmosphere) containing Cl or Cl compound (e.g., CCl4) and NO at a Cl/NO ratio of 2/1 and having a Cl or Cl compound concentration of 1-10vol.% in terms of Cl2. The supply of Cl or Cl compound and NO is stopped and the heat-treated product is converted into a transparent material by heating.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a method of manufacturing a transparent glass base material for optical fiber by subjecting a porous glass body whose main component is silicon dioxide to dehydration treatment and transparency treatment. The present invention relates to a method for producing a low-loss optical fiber base material at a reduced cost through a novel and advantageous dehydration treatment.

[Conventional technology]

As a method for producing a transparent glass base material for optical fibers, a method is known in which a porous glass body containing silicon dioxide as a main component, created by a soot method, a sol-gel method, or other methods, is sintered and made transparent. Prior to or at the same time as this sintering and transparency, a dehydration process is generally performed to remove water (OH) that has been incorporated into the porous glass body during the manufacturing process. This is to prevent the hydroxyl groups of the fiber from adversely affecting the characteristics, such as increased transmission loss, when the obtained optical fiber preform is t-drawn to form an optical fiber.

As such conventional technology, for example, Japanese Patent Publication No. 58-155
No. 03 proposes that the porous glass body be exposed to an atmosphere containing helium and an effective amount of chlorine up to 5 t% by volume while heating the porous glass body for the time required to make it transparent. chlorine in the atmosphere permeates into the porous glass body and replaces the hydrophilic acid group ions contained therein, resulting in dehydration, resulting in substantially no water content and attenuation at 950 nm. is 10 (LB/10
There is a method characterized by forming a glass of 1 or less.

Further, as proposed in Japanese Patent Publication No. 57-40096, a porous glass body can be heated at 800°C to 99°C in a gas atmosphere containing halogen or halides and having a dehydrating effect.
Heating at a constant temperature increase rate from 50°C to 1250°C, then holding for a certain period of time in the temperature range of 950°C to 1250°C, and further reducing the concentration of the gas having the dehydration effect until reaching the vitrification temperature. A method characterized by heating at an elevated temperature to obtain a transparent glass body.

[Problem that the invention seeks to solve]

By the way, in the former of the above conventional methods, chlorine is flowed while making it transparent and a dense glass is created in an atmosphere containing chlorine, so it is difficult to maintain sufficient airtightness in equipment and equipment that is easily contaminated with air. Even when manufactured, it has the advantage of being able to reduce the moisture content in the glass, but at 1400℃
Since chlorine is flowed at a vitrification temperature of ~1600°C, it has the disadvantage of inevitably causing defects in the glass.

In addition, in the latter, most of the dehydration treatment is carried out at temperatures below t-1250c, and at higher temperatures the concentration of gases that have a dehydrating effect is reduced, so that defects are generated by reacting with the dehydrating agent at high temperatures. You can hold back your friend. However, in recent years, the reliability required for fibers has become even more stringent, and it has become necessary to further reduce glass defects that cause increased loss in hydrogen atmospheres and radiation exposure. A suitable method is desired.

The present invention has been made in view of the current situation, and is a novel method that solves the above-mentioned problems and can produce a high-quality base material for optical fiber through effective dehydration and transparency treatment without producing glass defects. I am here to suggest a method.

[Means and effects for solving the problems] The present invention provides a porous glass body whose main component is silicon dioxide in an inert gas atmosphere containing chlorine or a chlorine compound and nitrogen monoxide.
Dehydrated within a temperature range of 00°C or higher and 950°C or lower,
The present invention relates to a method for manufacturing an optical fiber base material, which comprises thereafter ceasing the supply of chlorine or a chlorine compound and nitrogen monoxide to the atmosphere and heating to make the base material transparent.

Porous glass body in the present invention 1) G 60* tT
ie! , M40m, and preferably has a bulk density of α2 to Q and Bf/α1. The concentration of chlorine or chlorine compounds in the atmosphere is 1 to 1o by volume in terms of Cle, and the ratio of chlorine or chlorine compounds to nitrogen monoxide is 2 to 2 atoms of aZ.
In a particularly preferred embodiment, the chlorine compound is carbon tetrachloride and the inert gas is helium, argon, or nitrogen.

First, we will begin by explaining how the present invention was achieved.

In order to dehydrate a porous glass body using a dehydrating agent such as chlorine without increasing defects in the glass, dehydration should be performed at as low a temperature as possible. However, in reality, as the dehydration temperature is lowered, the amount of water remaining in the porous glass body increases.

As a result of various studies conducted by the present inventors, it has been found that the cause of this is that the number of active Cl atoms decreases as the dehydration temperature decreases. Therefore, we investigated a method that would allow sufficient dehydration even at low temperatures by increasing the number of Cl atoms that are active at low temperatures, and discovered that it is effective to coexist chlorine or chlorine compounds with nitrogen monoxide, leading to the present invention. That's what happened.

-The reason why active Cl atoms do not decrease even at low temperatures due to the coexistence of nitrogen oxide is explained in detail by giving a chemical reaction formula.
First, in the conventional method, the dehydration reaction is thought to proceed according to the following equations (1) and (2): C2l → 2Cl
... (1) 81-OH+ 01
4 B i + HQL↑ ・−・(2) Here, the activation energy of the reaction in which chlorine decomposes in formula (1) is u 57 Kcal [References: W, L,
Jolly 1 The Prinkles Op Inorganic Chemistry, published by McGraw-Hill, New York, 1976].

On the other hand, in the method of the present invention, reactions of the following formulas (3) and (4) are considered to occur.

C2t+NO→noaz+Cl...(
3) Sl-oH+Cl→st+HCl↑・
...(4) Here, the activation energy of the reaction of formula (3) is 22 Kcal [Ibid. in the document]. Therefore, the activation energy required to generate active Cl atoms in the method of the present invention is significantly reduced to less than A of that in the conventional method, so if carried out at the same temperature, the OL concentration will increase. It can be considered that sufficient dehydration can now be performed even at low temperatures. Furthermore, the present invention achieves dehydration at a lower temperature than conventional methods, thereby preventing the formation of defects in the glass, reducing loss due to residual OH and increased loss under hydrogen atmosphere or radiation. A base material for optical fiber can be obtained.

The porous glass body in the present invention is silicon dioxide (SiO
m) t” is the main component, and other components include, for example, germanium dioxide (aeo@), titanium dioxide (T
metal oxides such as aluminum oxide (At10), poron oxide (Jog), phosphorus pentoxide (Pros), magnesium oxide (MgO), zinc oxide (ZnO), nitrogen X (8), fluorine (F), etc. May contain.

It is preferable that the bulk density of the porous glass body is within the range of 1 (L2 to Q, 81/tsuba3. If the bulk density is less than α2f/cm", the porous glass body is easily broken and difficult to handle.
This is because if the length exceeds 817 cm, it is difficult to obtain transparent glass without bubbles, and it is also difficult to dehydrate and add fluorine to the center of the porous body.

In such a porous glass body, for example, a glass raw material gas is subjected to a flame hydrolysis reaction to generate glass fine particles, which are then deposited in the axial direction to obtain a porous glass body using the VAD method (
(vapor phase axis attachment method), OvD method (vapor phase axial attachment method), in which the above-mentioned glass particles are sequentially deposited on the surface of the starting iron and the diameter is increased to obtain a porous glass body (
Sol-gel method, in which a porous glass body is obtained by hydrolyzing metal alcolade 1, gelling it in a designated container, and then drying it; a porous glass body is obtained by press-molding glass fine particles using hydrostatic pressure; It may be manufactured by various known techniques such as a pressing method to form a glass body.

In the present invention, the porous glass body is first heated to a temperature of 500
Dehydration treatment is carried out by exposing the sample to an inert gas atmosphere containing chlorine or chlorine compound gas and nitrogen monoxide (MO) at a temperature of -950°C. If the temperature is less than 500°C, the reaction of formula (3) will only occur insufficiently, while if it exceeds 950°C, defects will occur in the glass, which is not preferable.

The chlorine or chlorine compound gas is, for example, chlorine (C2
carbon tetrachloride (CO4,), thionyl chloride (EOCl)
1), silicon tetrachloride (51ct4), germanium tetrachloride (Ge04), etc., with chlorine or carbon tetrachloride being particularly preferred. The concentration of the chlorine or chlorine compound gas is preferably 1 volume i% or more and 10 volume i% or less in terms of chlorine,
If it is less than 1% by volume, the resulting Cl 2 atom concentration is low and dehydration tends to be insufficient, and if it exceeds 10% by volume, bubbles tend to remain in the glass, which is not preferable. The ratio of chlorine or chlorine compound to nitrogen monoxide is 2 O atoms to 2 N atoms.
It is preferable that o be 1, since this creates an equivalence relationship in the above formula (3).

In addition, when carbon tetrachloride is used as a dehydrating agent, additional oxygen (Os) is added to the frame and atmospheric gas to remove free carbon (
0) will be prevented from occurring. The reactions at this time include the following (5) to (7).
) is as follows.

CC26+ otomi → 2 Ctl + 2
CO・ ・ ・ (5)C! 4+No →moCl
+ ct...(6) st-OH+ CL→
Day 1+ HCl (7) As the inert gas in the present invention, for example, helium (H
θ), argon (Ar), nitrogen (Nu), etc. can be used.

Although the above dehydration treatment may vary depending on the density, size, etc. of the porous glass body, it is carried out for about 15 to 2 hours, and then chlorine or chlorine-based compound gas and -nitrogen oxide are supplied to the atmosphere. is stopped and heated to the vitrification temperature in an inert gas atmosphere to form transparent vitrification.

[Implementation row]

Examples 1 and 2 and Comparative Examples 1 to 3 Fabricated by VAD method, average bulk density is 1217 cm
”, the composition of the center is Sin, 94% ii%-Go
O16 weight t%, peripheral composition is 8101)00 weight 1i%
A porous glass body (120 mφ x 600 mm) was dehydrated for 30 minutes under the conditions -1 to -5 in the table. -1~
-4, after dehydration, it was further kept in a Hθ atmosphere at 1600℃ for 50 minutes to make it transparent, and the outer diameter was 125μ.
9. Fabricate it into a fiber and evaluate its characteristics. Although Na1 (Comparative Example 1) was dehydrated with the addition of No gas, residual O
The H concentration is 5.5 in terms of loss at a wavelength of 1.38 μm. O
It was as high as dE/km. This is thought to be because the dehydration temperature was as low as 400°C.

-2 and N&3 (Examples 1 and 2) have a residual OH concentration (10-,: 20 ppb) at a level that poses no practical problem, and there is no absorption peak at wavelength L52 μm that indicates the presence of glass defects. , there was no visible evaporation of germanium at all.

-4 and Na5 (comparison rows 2 and 3), the residual OH concentration has a value that does not cause any practical problems, but a peak at a wavelength of 1.52 μm appears, germanium evaporates, and the refractive index profile It's chaotic.

Note that the residual OR concentration that causes no practical problems is the wavelength of 1.38.
It means 20 ppb or less, which corresponds to an OH absorption peak value of 1 dB/km in μm.

Practical row 3 Average bulk density 14F/an'' produced by OVD method
, and the outermost periphery contains SiO containing 50% of Ti1l.
Porous body (100mφ×500 mml)
, in a quartz furnace tube at a temperature of 600°C, with a volume of C1) of 10,
It was dehydrated by being exposed to an atmosphere of No. 1 G volume t% and NR 80 volume for 30 minutes, and then kept in a 1550° C. ■θ atmosphere for 30 minutes to make it transparent. The obtained base material for nine optical fibers did not contain air bubbles. From this base material, the outer diameter is 125μ
When an optical fiber of m was prepared, the absorption at a wavelength of 1.3 μm due to residual OH was 15 aB/km, a level that poses no problem for practical use, and no absorption peak at a wavelength of 1.52 μm appeared. Also, Tie.

No evaporation was observed.

Practical row 4 Produced by pressing method, average bulk density is α7 f/α
3, a porous glass body consisting of 810,100 yen (
Dehydration treatment, fluorine addition treatment, and transparentization treatment t-II were then carried out in a 80WφX 300 mml) 'ks810 furnace tube to obtain an optical fiber preform. For dehydration conditions, temperature 6
50℃, ccj4s capacity section, Ol 5 capacity section, No 5
Expose the sample to an atmosphere consisting of 885% N8 by volume for 2 hours. Next, the inside of the furnace was heated to 1) 50℃, 81F, 3
Exposure to an atmosphere containing 97 volume t% He for 3 hours, add fluorine, and then raise the furnace temperature to 1550°C in the same atmosphere.
The mixture was heated for 1 hour to become transparent. The optical fiber base material obtained above had no pores and contained fluorine in the amount of t2 weight.

Is it possible to drill a hole in the center of this optical fiber base material and use high-purity quartz rond that does not contain dopants? They were inserted and integrated using flaps to create a waveguide structure. Draw this line outside 9
Using a 125 μm fiber, we measured the transmission loss and found that at a wavelength of 1.3 μm, (L 32 dB/kll s and α18 dB/) at a wavelength of 1.55 μm, 1>, 1, 381) m T absorption a due to the OB group. CL 2 aB/
km, and does not occur at the absorption peak of 1.52 μm, which indicates the presence of defects.

Example 5 Produced by MAD method with t1 average bulk density of 12 f/c
rm" and the composition is 0eO1) 5% by weight - At!Os
A porous glass body (60-φ x 400 mm) with 15 i amount %-8 iO content 70 weight and 3996 was heated in a quartz furnace tube at a temperature of 600 °C, Cl, to volume %-NO10
Case i# - Dehydration treatment was carried out by exposing the sample to an atmosphere of 80 volumes of He for 30 minutes, and then it was kept in a He atmosphere at 1600° C. for 30 minutes to make it transparent.

The obtained optical fiber base material did not contain any air bubbles and had a relative refractive index difference of Δn (relative refractive index difference) of 2%. After jacketating this base material with a commercially available high-purity quartz tube, it was made into an optical fiber with a diameter of 125 μm.
．． The absorption HQ at 3 μm is 56 B/kflI, which is a level that poses no practical problem, and the absorption peak at the wavelength of 1.52 μm does not appear.

Example 6 Fabricated using the sol-gel method, the average bulk density was α5 fi
! / ct* "A porous glass body (30wφx1
ooom-) in a quartz furnace tube at a temperature of 600℃, 01)
10 volume - 1a 010 volume 1% - 1) e 80 volume Exposed in an atmosphere for 51)] minutes to dehydrate, then 1300
Keep it in He atmosphere at ℃ for 30 minutes to make it transparent. The obtained optical fiber base material did not contain air bubbles. A hole is made in the center of this base material, a high-purity quartz rond is inserted, and the material is integrated by collapsing to create a waveguide structure. This was drawn to have an outer diameter of 125 μm and made into a 7-eyeper wire, and the transmission loss was measured to be 132 dB/km at a wavelength of 1.5 μm.
s wavelength L 55 pm (L 1 s IIE/-, and absorption at 1.38 μm by OH groups [L2 dB/
The absorption peak at 1.52 μm, which indicates the presence of defects, was not observed.

〔Effect of the invention〕

From the above explanation and the results of the practical and comparative columns, by dehydrating in the temperature range and atmospheric gas conditions of the present invention, the residual 01) concentration was converted to light absorption at 1.3 μm.
Oan/km or less, a base material for optical fiber can be stably obtained, and there are few glass defects in the base material, and the diameter is 1.52 μm.
It can be seen that no absorption peak appears.

Therefore, it is very effective to use the optical fiber produced from the optical fiber base material according to the present invention as an optical fiber for submarine cables with low loss and little change over time.

In addition, since the method of the present invention does not flow chlorine or chlorine compound gas at high temperatures exceeding 950°C, there is almost no evaporation of the dopant, so there is an advantage that the desired composition (refractive index distribution) can be obtained. On the other hand, this sophisticated method contributes to improving production yields and is expected to reduce costs, and also has the advantage that materials such as 81C, which react with chlorine or chlorine compounds at high temperatures, can be used as the core tube material. It is.

Claims (8)

[Claims] (1) A porous glass body whose main component is silicon dioxide is dehydrated in an inert gas atmosphere containing chlorine or a chlorine compound and nitrogen monoxide within a temperature range of 500°C or more and 950°C or less, and then 1. A method for producing an optical fiber base material, which comprises heating and making the base material transparent by stopping the supply of chlorine or a chlorine compound and nitrogen monoxide to the post-atmosphere. (2) Porous glass body is made of GeO_2, TiO_2, Al
The method for manufacturing an optical fiber preform according to claim (1), characterized in that it contains one or more of _2O_3. (3) The bulk density of the porous glass body is 0.2 g/cm^3
The method for manufacturing an optical fiber preform according to claim 1 or 2, wherein the preform is 0.8 g/cm^3 to 0.8 g/cm^3. (4) For the optical fiber according to any one of claims (1) to (3), wherein the porous glass body is produced by a VAD method, an OVD method, a sol-gel method, or a press method. Method of manufacturing base material. (5) The method for manufacturing an optical fiber preform according to claim (1), wherein the concentration of chlorine or a chlorine compound is 1% by volume or more and 10% by volume or less in terms of Cl_2. (6) The light according to claim (1) or (5), characterized in that the ratio of chlorine or chlorine compound to nitrogen monoxide NO^7 is 1 NO to 2 Cl atoms. Method for manufacturing fiber base material. (7) The method for manufacturing an optical fiber preform according to any one of claims (1), (5), and (6), wherein the chlorine compound is carbon tetrachloride. (8) The method for manufacturing an optical fiber preform according to claim (1), wherein the inert gas is helium, argon, or nitrogen.