JPH0459630A - Production of optical fiber - Google Patents

Abstract

PURPOSE:To improve long-term stability by impregnating a preform consisting essentially of quartz or an intermediate preform to become a preform with H2, before melt spinning. CONSTITUTION:A perform for single mode fiber comprising a quartz core and a fluorine-containing quartz clad or an intermediate preform to become a pre form is heated in a furnace in an atmosphere of 100% H2 under 1-3atm. at 200-1,200 deg.C for 1-100 hours and H2 is sufficiently diffused into the core part. Then, the preform is subjected to melt spinning to give optical fiber having improved long-term stability. The preform is heated by using an oxyhydrogen burner in a ratio of H2/O2 of >=3 at 1,500-1,700 deg.C, impregnated with H2 along the whole length and then subjected to melt spinning to also give an optical fiber having the same characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing an optical fiber, and more particularly to a method for heat treating a base material for an optical fiber.

[Conventional technology]

The core is pure quartz (Si) containing no additives (dopants).
Ox ) glass, the cladding is quartz glass doped with fluorine (F) to lower the refractive index (fluorine-doped S1
0. Optical fibers made of glass (glass) have the following characteristics: ■The core does not contain dopants that cause scattering loss (Rayleigh scattering), so the transmission loss is theoretically low; ■It is stable against hydrogen; ■Pure silica glass has no structural defects. Since there are few defects induced by radiation, it has excellent radiation resistance characteristics.
For these reasons, it is attracting attention as a highly reliable, high-performance fiber.

As a manufacturing method for this type of fiber, a patent application was filed in 1983.
The following methods are proposed in the specifications of No. 0-83243 and No. 61-72433.

That is, Toppan 1-
A pure silica glass sintered body containing no moisture is produced, and this is heated and stretched in an electric furnace to form a sintered body with a diameter of 2~
A glass rod for a core of 4 amφ is prepared. Separately, V
A sintered silica glass body containing 1% by weight of fluorine is produced by the AD method, and a hole is bored in this body using an ultrasonic drilling machine to produce a pipe to serve as a cladding (a clad pipe). The glass rod containing the core obtained above is inserted into the hollow part of the clad pipe, and in this state, the clad pipe is heated from the outside to reduce its diameter, thereby integrating the two (collapsing). A porous glass body is deposited on the outer periphery of the obtained glass body by a VAD method, and this is sintered in an atmosphere containing a fluorine compound, thereby forming a jacket portion of fluorine-doped quartz glass. The thus obtained optical fiber preform is heated and melted in an electric furnace and immediately spun to produce an optical fiber in which the core is made of pure quartz and the crat is made of fluorine-doped quartz.

[Problem to be solved by the invention]

In the method of manufacturing an optical fiber in which the core is made of pure quartz and the crat is made of fluorinated quartz, as proposed in the above specifications, the viscosity of the core is greater than that of the crat at high temperatures during melt spinning of the preform. Therefore, most of the tension applied to the fiber by wire drawing and spinning is borne by the core, which has a higher viscosity. In particular, in the case of a ngle mode fiber whose core diameter is small, about 8.0 to 10.5 μm, while the cladding diameter is about 125 μm, the cross-sectional area of the core is about 1/250 of that of the cladding. The tensile stress on the core of the seed fiber is very large;
The weak part of the -Si OSi network structure that constitutes the core is cut as shown in equation (1) below.

=SL OS+=→=iS+ O・10・5l=(1
) Such bond defects (existence of broken bonds are caused by λ−0
．． This results in an increase in transmission loss near 63 μm.

In addition, in particular, =S+-0・ easily reacts with hydrogen, and easily combines at room temperature to become =S+'OH, and λ ÷ 1.38 μm
This causes an increase in absorption, which poses a problem in terms of long-term reliability of optical fibers.

Defects exist even in the best preforms.

In the conventional method, no means was taken to eliminate defects in the drawn fiber and preform.

The present invention has been made with the object of overcoming the problem of the presence of defects, which affects the radiation resistance characteristics, long-term reliability, etc. of optical fibers described above. An object of the present invention is to provide a method for manufacturing an optical fiber with excellent radiation resistance and long-term reliability (no change over time).

[Means to solve the problem]

As a means for solving the above problems, the present invention provides a method for manufacturing an optical fiber in which a preform containing quartz as a main component is melt-spun. It is characterized by

It is particularly preferable that the hydrogen impregnation treatment is a heat treatment in an atmosphere containing hydrogen, and it is further preferable that the preform or the intermediate base material that will become the preform is held in a heating furnace that maintains an atmosphere containing hydrogen. .

Hydrogen impregnation treatment can be carried out at atmospheric pressure or under pressure in a heating furnace that maintains an atmosphere containing hydrogen.

Another particularly preferred embodiment of the present invention is a method in which the hydrogen impregnation treatment is performed by heating with a flame using hydrogen as the combustion gas, and in this case, the Hz/02 ratio is adjusted using an oxyhydrogen burner. It is particularly preferable to use 3 or more.

In the present invention, the above-mentioned preform is a single-mode fiber preform consisting of a pure quartz core and a fluorine-doped quartz cladding, or the intermediate base material that becomes the above-mentioned preform is a pure quartz core used in the collapse method or a pure quartz core used in the collapse method. It is particularly preferable to use fluorine-doped quartz glass.

[Effect]

In order to overcome problems such as radiation resistance and long-term reliability that are affected by defects existing in optical fibers, the inventors of the present invention have proposed that if hydrogen is sufficiently diffused into the preform at the preform stage, Even if defects are generated during wire drawing, hydrogen diffused into the preform ensures that −=Si・
,=S10・=S4 H,=Si
We came up with the idea that by changing to OH, defects could be eliminated, that is, defect generation could be suppressed, and we were able to arrive at the present invention.

It was confirmed that by the hydrogen impregnation treatment of the present invention, although the loss at 1.38 μm slightly increased, the loss at 0.63 μm could be significantly reduced.

After creating the preform or the intermediate base material that will become the preform, we perform a hydrogen impregnation treatment in hydrogen to sufficiently diffuse hydrogen into the glass base material, so that defects that occur after drawing are removed from the hydrogen in the glass. It is possible to manufacture optical fibers with excellent radiation resistance and long-term reliability (no change over time).

In particular, hydrogenating a single mode fiber having a smaller core diameter than the cladding, especially a single mode fiber preform consisting of a pure quartz core and a fluoridated quartz cladding, before melt spinning, is advantageous. It is very effective in reducing defects resulting from large tensions applied to the surface.

The temperature of the heat treatment may be at least room temperature and at most 1600° C. (melting point of glass fiber). Diffusion coefficient of hydrogen (D,
) is represented by the following formula (2).

D u = 6.63X 10-”exp(-1),
5 (kcal/mol) / RT)...(21 Diffusion length L = 2 (DHL) l" R: Gas constant (1,987 cal/mol. K) T:
Absolute temperature (k) [From Lee, Glass Data Handbook p. 161-162] Above. , L. When calculating the hydrogen diffusion distance from the equation, the results shown in Table 1 below are generally obtained with respect to temperature and time.

Therefore, the heat treatment temperature in a hydrogen atmosphere is 200~
The temperature is preferably 1200°C, and the hydrogen treatment may be carried out for an arbitrary period of time depending on the temperature.

Furthermore, although the relationship between metal impurities and radiation resistance properties is not clear, hydrogen impregnation treatment of the preform or the intermediate base material that will become the preform can change the valence of the transition metals present in small amounts in the glass. It may also be possible to change the absorption and radiation resistance properties.

As a specific means for the hydrogen impregnation treatment, heat treatment is performed in a hydrogen atmosphere at normal pressure or under increased pressure in the above temperature range. More specifically, treatment in a heating furnace or treatment using a flame using hydrogen as a combustion gas can be mentioned. In the former method, the atmosphere in the heating furnace is hydrogen (100%) at 1 to 3 atm and 20
Heat at 0-1200°C for 1-100 hours. Also,
In the latter method, for example, an oxyhydrogen burner is used, and the Hz
/Ox ratio is preferably 3 or more when heating. The flame temperature at this time is 1500 to 1700°C. Heating with an oxyhydrogen flame is advantageous in that both the means and equipment are simple, low cost, and can be completed in a short time.

The glass rod containing the core used in the present invention (generally refers to those consisting only of the core and those having the inner part of the crat on the outer periphery of the core) and the glass pipe serving as the crat (also collectively referred to as the jacket part) are conventionally used. known method,
For example, it is created using the VAD method. More specifically, the methods described in the above-mentioned Japanese Patent Applications No. 60-83243 and No. 61-72433 are preferred, but the method is not limited thereto.

〔Example〕

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited thereto.

Example 1 Pure silica core with fluoridated cladding, outer diameter 25
A glass preform (preform) for a single mode optical fiber of IIIInφ was heat-treated in a hydrogen atmosphere in a heating furnace. The processing conditions were a temperature of 800°C for 124 hours, and H
l 100% atmosphere (1 atm). Expected H1
The diffusion distance is approximately 10 degrees, and if the distance is 10, hydrogen (H2) can be sufficiently diffused to the core by heat treatment during drawing.

The treated preform of the present invention is melt-spun to obtain an outer diameter of 125 mm.
It was made into a μm optical fiber. The transmission loss characteristics of the obtained optical fiber are shown by solid lines in FIG. The absorption at λ-0.63 μm was 5.5 dB/km, and no absorption peak was observed. From this fact, it is presumed that =si O. is not produced.

Example 2 A pure silica core rod was made of pure silica glass prepared by the VAD method, had a diameter of 4IIlfflφ, and had an OH group content of 0. A glass rod was prepared that had been sufficiently dehydrated to have an O of 5 ppm or less. In addition, the glass pipe used as the clasp is made of quartz glass containing 1.3% by weight of fluorine, created by the VAD method, and drilled with an ultrasonic drilling machine to have an outer diameter of 60 m + oφ and an inner diameter of 4.5 mm. I prepared something like this. The clad/guive is placed in a 100% hydrogen atmosphere,
After heat treatment at 1000° C. for 24 hours, the core rod and the clamp rod were heated and melted to form a preform. After melt-spinning this preform into an optical fiber with an outer diameter of 125 μm, the absorption at λ = 0.63 μm was measured, and the loss was 5.5 dB, which was the same as in Example 1 shown by the solid line in Figure 1. /km, and there was no absorption peak at this wavelength.

Example 3 A glass preform for an optical core injector having a pure silica core lot was subjected to pressure and heat treatment in a hydrogen atmosphere.

The treatment conditions were: temperature 800°C 1) 0 hour treatment, hydrogen 10
0% and 3 atm. When this preform was melt-spun into an optical fiber with an outer diameter of 125 μm, the absorption at λ = 0.63 μm was measured, and it was 5.5 dB/km, which was the same as in Example 1.2 shown by the solid line in Figure 1. There was no absorption peak.

Example 4 A glass preform for an optical fiber having an outer diameter of 25 mm and having a pure silica core was impregnated with hydrogen using oxyhydrogen sulfur. Oxyhydrogen gas contains H as a combustion gas.
3 was supplied at 12 Of/min, and 02 was supplied as an auxiliary gas at 301/min, and the burner was moved back and forth 5 times at a speed of 5/min to impregnate hydrogen over the entire length of the preform. The preform was cut into 50 sections and the surface was etched with HF, and the OH peak amount at 3670 cm-' was measured each time using an IR (infrared spectrometer). The peak has disappeared. Therefore, it is estimated that hydrogen was diffused from the preform surface to a depth of 2 am t by the hydrogen impregnation treatment of this example. depth 2
If it is diffused up to mm, it can be diffused to the core part in the wire drawing process.

The impregnated preform is melt-spun, and λ=0
．． When the loss at 83 μm was measured, it was as shown by the solid line in FIG. 1, and it was confirmed that there was no absorption peak at a wavelength of 5.5 dB and 7 km.

Example 5 In Example 1, the hydrogen heat treatment conditions were changed to a temperature of 200°C;
After 2 hours and drawn into fiber, λ=0.6
No absorption peak at 3 μm was observed.

Example 6 In Example 1, the hydrogen heat treatment conditions were changed to a temperature of 200°C;
After 1 hour, a slight absorption peak (1.5 dB/km) was observed at λ=0.63 μm.

Comparative Example 1 A preform similar to Example 1 was melt-spun without heat treatment in a hydrogen atmosphere. λ of this optical fiber = 0.63
When the absorption in μm was measured, it was 8.5 dB/km, and there was an absorption peak of 3 dB/km.

Comparative Example 2 A pure silica core lot and a fluorine-doped clad pipe similar to those in Example 2 were prepared. The clad vibrator was heat-fused and integrated with the core without heat treatment in a hydrogen atmosphere, the obtained preform was melt-spun into an optical fiber with an outer diameter of 125 μm, and the loss at λ = 0.63 μm was measured. , as shown by the dashed line in Figure 1, I! lz/B
/ka+.

That is, an absorption peak of 9.54 B/ktm exists.

〔Effect of the invention〕

The present invention provides an optical fiber having a pure quartz core.
After melt spinning, the glass network structure is cut = S
The defects i−0・,=S1・ are filled with hydrogen molecules that have been diffused into the preform in advance, resulting in the formation of MiSi−OH,=S
Since t H is used, the absorption peak at 0.63 μm can be eliminated, and the long-term stability of the fiber after spinning can be improved.

[Brief explanation of drawings]

Figure 1 is a diagram comparing and showing the transmission loss characteristics of each optical fiber manufactured in Examples 1 to 4 of the present invention and Comparative Example 2, where the solid line is for the fiber of Examples 1 to 4, and the broken line is for the fiber of Examples 1 to 4. The case of the fiber of Comparative Example 2 is shown. Ina 1 diagram

Claims (10)

[Claims] (1) A method for producing an optical fiber in which a preform containing quartz as a main component is melt-spun, which is characterized in that the preform or an intermediate base material serving as the preform is subjected to hydrogen impregnation treatment before melt-spinning. Method. (2) The method for manufacturing an optical fiber according to claim 1, wherein the hydrogen impregnation treatment is a heat treatment in an atmosphere containing hydrogen. (3) The method for manufacturing an optical fiber according to claim (2), wherein the heat treatment involves holding the preform or an intermediate base material to become the preform in a heating furnace in which an atmosphere containing hydrogen is maintained. (4) The method for manufacturing an optical fiber according to claim (3), wherein the heat treatment is a hydrogen impregnation treatment under atmospheric pressure or pressurization in a heating furnace that maintains an atmosphere containing hydrogen. (5) The method for manufacturing an optical fiber according to claim (1), wherein the hydrogen impregnation treatment is a heat treatment using a flame using hydrogen as a combustion gas. (6) The method for manufacturing an optical fiber according to claim (5), wherein the heat treatment is performed using an oxyhydrogen burner. (7) The method for manufacturing an optical fiber according to claim (6), wherein the oxyhydrogen burner has a H_2/O_2 ratio of 3 or more. (8) The method for manufacturing an optical fiber according to claim (1), wherein the preform is a single mode fiber preform consisting of a pure quartz core and a fluorine-doped quartz cladding. (9) Claim characterized in that the intermediate base material that becomes the preform is a pure quartz core used in the collapse method (
1) The method for manufacturing the optical fiber described above. (10) The method for manufacturing an optical fiber according to claim (1), wherein the intermediate base material serving as the preform is fluorine-doped silica glass used in the collapse method.