JPS6311534A - Production of jacket pipe for drawing optical fiber - Google Patents

Abstract

PURPOSE:To produce a jacket pipe consisting of fluoride glass having a smooth inside wall surface by casting a fluoride glass melt into a cylindrical casting mold and allowing the glass melt in the central part to flow to the outside, then cooling the melt slowly down to a room temp. CONSTITUTION:The fluoride glass melt 3 obtd. by heating and melting the fluoride glass in a gold crucible 2 is cast into the cylindrical casting mold 1 made of gold-plated brass. The part of a low viscosity in the central part of the casting mold is then discharged to the outside before the entire part of the above-mentioned glass melt 3 in the mold 1 vitrifies. The above-mentioned cylindrical casting mold 1 is then rotated around the central axis thereof at need; thereafter, the glass melt remaining in the mold 1 is slowly cooled down to the room temp. to yield the fluoride glass tube 4. The respective stages mentioned above are preferably executed in an inert gaseous atmosphere. The fluoride glass jacket pipe having the smooth inside wall surface to obviate the generation of irregular structure at the boundary between the preformed clad during drawing of an optical fiber is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a Jackert tube for drawing optical fibers, which has uniform outer and inner diameters and a smooth inner wall surface.

[Prior art] Fluoride glass can transmit light with a longer wavelength than silica glass, and its Rayleigh scattering loss is smaller than that of silica glass. It is expected that a waveguide for optical communication having lower transmission loss than fiber can be obtained.

For silica-based optical fibers, methods for producing preforms include the VAD method (vapor phase axis attachment method) and the MCVD method, and a preform synthesis method using the vapor phase method is adopted.

However, unlike oxide glass, fluoride glass has the characteristic that its viscosity changes extremely rapidly with temperature and is easily crystallized in the glass softening temperature range. Therefore, the VAD method and CVD method used to form preforms for silica-based optical fibers cannot be applied to fluoride glass, and the build-in casting method is used as a preform manufacturing method for fluoride optical fibers. and rotational casting methods have been proposed.

By the way, fluoride optical fiber has the potential to be used as a so-called ultra-low-loss optical transmission medium, which has lower transmission loss than silica-based optical fiber. It is advantageous to use optical fibers as long-distance, large-capacity optical transmission lines in order to bring out their characteristics.

One way to draw a car-mode optical fiber is to insert a preform into a glass tube (called a jacket tube).
There is a jacketing method in which wire is drawn together with the glass tube, and it is commonly used. In the case of a silica-based optical fiber, a pure quartz tube is generally used as the jacket tube.

When drawing wire using the jacketering method, the inner wall of the jacket tube is coated with hydrofluoric acid in order to suppress the occurrence of interface irregularities such as microcrystals and bubbles at the interface between the preform and the jacket tube during wire drawing. After etching, the glass surface is smoothed by flame polishing using an oxyhydrogen flame.

In the case of fluoride glass, weather resistance against acids is generally very poor, especially Zr used in fluoride optical fibers.
In the case of ZrF-based glass containing F as its main component, when it comes into contact with an acidic solution, the glass surface is not etched uniformly and becomes uneven.

Furthermore, the melting point of such ZrF4 glass ranges from 600°C to 7.
The temperature is as low as 00°C, so flame polishing cannot be applied. Therefore, even if it is possible to perforate a fluoride glass rod by ultrasonic processing, it is difficult to make the inner wall surface smooth, and it is difficult to obtain a jacket tube that can be subjected to the jacketering method.

[Problems to be Solved by the Invention] Therefore, as described above, an object of the present invention is to have a smooth inner wall surface that does not cause structural irregularities at the preform-clad interface during wire drawing, which has been difficult to produce in the past. An object of the present invention is to provide a method for manufacturing fluoride glass jacketed tubes.

[Means for Solving the Problems] To achieve these objects, the present invention involves casting a fluoride glass solution into a cylindrical mold and then pouring out the central portion. Alternatively, after casting the glass melt, the mold is rotated around its central axis of rotation to form a glass tube, and then slowly cooled to room temperature. In the present invention, these steps are performed in a low-humidity inert gas atmosphere.

That is, the present invention includes a step of casting a fluoride glass melt into a cylindrical mold, a step of pouring out the glass melt at the center of the cast glass melt to the outside of the cylindrical mold, and a step of casting a fluoride glass melt into a cylindrical mold. The method is characterized by comprising a step of slowly cooling the remaining glass melt to room temperature.

Another aspect of the present invention includes the steps of: casting a fluoride glass melt into a cylindrical mold; pouring out the glass melt at the center of the cast glass melt to the outside of the cylindrical mold; It is characterized by comprising the steps of rotating the shape mold around its central axis, and slowly cooling the glass melt remaining in the cylindrical mold to room temperature.

Here, the above-mentioned casting process and pouring [function] Conventionally, in order to obtain a fluoride glass tube, a hole was drilled in a fluoride glass rod using an ultrasonic processing machine, and the inner wall surface of the hole was polished. According to the present invention, a jacket tube having a smooth inner wall surface can be obtained without polishing.

According to the present invention, a fluoride glass tube with an extremely smooth inner wall surface can be obtained, so even if this is used as a jacket tube and a fluoride optical fiber preform is inserted and drawn, the preform and jacket Can be smoothly fused to pipes. However, with =), such a glass tube can be used for drawing a low-loss fluoride single-mode optical fiber, thereby contributing to the realization of an ultra-low-loss, high-capacity optical communication system.

[Example code] The present invention will be described in detail below with reference to the drawings.

Example 1 FIG. 1 is a process diagram explaining the first example of the present invention, where l is a gold-plated brass mold, 2 is a gold crucible, and 3 is a fluoride glass mold. It is a melt. 4 is the obtained fluoride glass tube.

ZrF, , [1aF2. GdF3. 8nF3 to 60Zr
F4-30BaF2-4GdFa-6mo42%8λF
, and the fluoride mixture 6
0g and 20g of NH4F-HF were added and heated to 400°C in an electric furnace maintained in a dry nitrogen atmosphere using metal crucible 2.
The mixture was heated for 2 hours to undergo fluorination treatment with N)I4F-HF, then heated and melted at 850°C for 2 hours, and cast into mold 1 preheated to 260°C.

After that, before the entire cast glass melt 3 is vitrified, the low-viscosity part at the center of the mold is poured out and slowly cooled to room temperature, and the length is 150101° and the outer diameter is 8.5°.
A fluoride glass tube 4 having an inner diameter of 5aIIIlφ and a diameter of 5aIIIlmm was obtained.

The above steps from casting to slow cooling of the fluoride glass tube to room temperature were carried out in a glove box purged with 7 atmospheres of low-humidity nitrogen gas with a moisture dew point of -80°C.

Next, the opening of the fluoride glass tube 4 obtained in this way is closed with an epoxy resin in the glove box, and the outer periphery is optically polished.The epoxy resin is removed and the fluoride glass The tube is used as a jacket tube, and the cladding composition is 60ZrFa-308aF2.
-4GdFi-6moj2%8fLh, core composition 60Z
rF4-30BaFz-4GdF3-4811 F2-
2molL%PbF2, core diameter 0.7+nmφ,
Cladding diameter 5. OLI1mφ core/clad refractive index difference 0
．． The outer circumference of a 4% fluoride optical fiber preform was optically polished so that it could be inserted into the jacket tube. Furthermore, this fluoride jacket tube was coated with Teflon FEP and drawn to form a fiber.

Note that the drawing of the optical fiber was carried out while the inside of the jacket tube was sufficiently replaced with dry nitrogen gas to sufficiently remove moisture, and then evacuated with a vacuum pump.

As a result, the core diameter was 13 μmφ, the cladding diameter was 90 μmφ,
A step index type single mode optical fiber having a fiber diameter of 148 μm, a cutoff wavelength of 2.3 μm, a minimum loss of 2.6 μm, and a speed of 16 B/km was obtained with a length of 250° C.

During wire drawing, the preform and jacket tube were smoothly fused together, and no structural defects such as crystallization occurred.

On the other hand, when the humidity inside the glove box where the fluoride glass tube was manufactured was maintained at a water dew point of 0°C, the fluoride glass tube obtained in the above process was subjected to drawing as a jacket tube to manufacture a fiber. Crystal grains were observed at the interface with the tube, and the transmission loss characteristics of the fiber deteriorated.

This is because moisture is present on the inner wall surface of the glass tube, for example ZrF4+
This is because the river water decomposition reaction t+2O-1r (OH)F3+ %l+F occurs and a layer of hydroxide is generated on the inner wall surface, making crystallization more likely to occur during wire drawing. be.

Therefore, in order to suppress crystallization during wire drawing, considering that the above reaction tends to occur at high temperatures, it is necessary to reduce the humidity of the atmosphere as much as possible during the process of casting, forming a glass tube, and cooling it to room temperature. It is desirable to prevent reactions between the compound glass tube and moisture.

Example 2 FIG. 2 is a process diagram illustrating a second example of the present invention, in which 11 is the side wall of a gold-plated brass mold, 5 is the bottom of the mold, and 14 is a side wall of a gold-plated brass mold. The resulting fluoride glass tube.

ZrF, , BaF2. GdF3. Al1 F3 60Z
rF4-30BaF2-4GdF3-61!1of1%
Weighed the composition of ^uF, added 20 g of Ni14F.IIF to the fluoride mixture Bog, heated and melted it using the process shown in Example 1 using gold crucible 2, and then
It was cast into a mold consisting of sections 11 and 5 which were preheated to .degree. After that, before the entire cast glass melt 3 is vitrified, the bottom part 5 of the mold is removed and the low viscosity part at the center of the mold is poured out, and it is slowly cooled to room temperature to form a mold with a length of 150 mm and an outer diameter of 8. .5 mmφ, inner diameter 5m
A fluoride glass tube 14 of mφ was obtained.

The above steps from casting to slow cooling of the fluoride glass tube to room temperature were performed in a glove box replaced with a low humidity argon gas atmosphere with a moisture dew point of -80°C.

When the fluoride glass tube 14 thus obtained was used as a jacket tube to draw a fluoride single mode optical fiber, no irregularities due to crystal formation occurred at the interface between the preform and the jacket tube. Both were able to be smoothly fused together.

Embodiment 3 FIG. 3 is a process diagram illustrating a third embodiment of the present invention, where 6 is a small electric furnace for heating the mold 1.

ZrF4. BaF2. GdF, , Al1 F, 60Z
rF4-30BaF2-4GdF3-6moj2%Au
f in the composition of F3! Then, 20 g of NH,F-HF was added to 60 g of the fluoride mixture, heated and melted using the process shown in Example 1 using a metal crucible 2, and then cast into a mold 1 preheated to 260°C. Thereafter, this mold 1 was held in an electric furnace 6 and rotated around the center I sleeve of the electric furnace 6 at a rotation speed of 300 rpm while being heated, and then slowly cooled to room temperature to form a mold with a length of 150 + nm. A fluoride glass tube 4 having an outer diameter of 8.5 mlφ and an inner diameter of 51IIlnφ was obtained.

The above steps from casting to slow cooling of the fluoride glass tube 4 to room temperature were carried out in a glove box purged with a low-humidity helium gas τ atmosphere with a moisture dew point of -80°C.

When the thus obtained fluoride glass tube 4 was used as a jacket tube to draw a fluoride single moat optical fiber, no irregularities due to crystal formation occurred at the interface between the preform and the jacket tube, and both were able to be fused smoothly.

In the above Examples 1 or 63, ZrF4-BaF2-Gd
An example was shown in which a fluoride glass tube was made using F, -Ax F3i glass, but ZrF, -BaF2-LaF3
-AIIF3-NaF glass, ZrF4-BaF2
-LaF3-8fl F3-LiF glass 5ZrF,
-BaF2-LaF3-YF3-AIt F3-LiF
-NaF glass.

ZrF4-1IfFa-BaF2-GdF3- AJZ
F3 glass or ZrF4-tlfF4-BaF
2-LaF3-YF3- AJZ F3-LiF-Na
Even when F-based glass was used, a fluoride glass tube having a smooth inner wall surface that could be used for drawing fluoride optical fibers could be obtained by the methods shown in Examples 1 to 3.

[Effects of the Invention] As explained above, according to the method for manufacturing a fluoride glass tube according to the present invention, a fluoride glass tube with an extremely smooth inner wall surface can be obtained, and this can be used as a jacket tube.
Even if a fluoride optical fiber preform is inserted into this and drawn, the brief form and the jacket tube can be smoothly fused together. Therefore, such a glass tube can be used for drawing a low-loss fluoride single-mode optical fiber, which has the advantage of contributing to the realization of an ultra-low-loss, high-capacity optical communication system.

[Brief explanation of drawings]

FIG. 1 is a process diagram of Example 1 of the present invention, FIG. 2 is a process diagram of Example 2 of the present invention, FIG. 3 is a process diagram of Example 3 of the present invention. l...brass mold, 2...Gold crucible, 3...Fluoride glass melt, 4...Fluoride glass tube, 5...Mold bottom, 6...Small electric furnace, 11... Mold side wall, 14...Fluoride glass tube. One step ＼　　　　Cwa

Claims (1)

[Scope of Claims] 1) A step of casting a fluoride glass melt into a cylindrical mold, and a step of pouring out the glass melt at the center of the cast glass melt to the outside of the cylindrical mold; 1. A method for manufacturing a jacket tube for optical fiber drawing, comprising the step of slowly cooling a glass melt remaining in a cylindrical mold to room temperature. 2) The method for manufacturing a jacket tube for drawing optical fiber according to claim 1, wherein the casting step, the pouring step, and the slow cooling step are performed in a low-humidity inert gas atmosphere. 3) a step of casting a fluoride glass melt into a cylindrical mold; a step of pouring out the glass melt at the center of the cast glass melt to the outside of the cylindrical mold; A method for producing a jacket tube for drawing an optical fiber, comprising the steps of rotating the glass melt remaining in the cylindrical mold to room temperature. 4) The method for manufacturing a jacket tube for optical fiber drawing according to claim 3, wherein the casting step, the pouring step, and the slow cooling step are performed in a low-humidity inert gas atmosphere.