JPS6321232A - Production of perform for optical fiber - Google Patents

Abstract

PURPOSE:To produce the title preform for fluoride optical fiber for a low-loss optical transmission line by successively injecting the melt of fluoride clad glass and the melt of core glass into a bottomed cylindrical casting mold, and opening the bottom of the casting mold before solidification. CONSTITUTION:The melt 4 of the clad fluoride glass obtained by adding NH4 F4.HF to a mixture of fluorides such as ZrF4, BaF2, GdF3, and AlF3, and heating and melting the mixture is charged into a bottomed cylindrical casting mold A made of brass and plated with gold. The melt 5 of the core fluoride glass obtained by adding NH4F.HF to a mixture of fluorides such as ZrF4, BaF3, AlF3, and PbF3, and heating and melting the mixture is then charged. The bottom lid is opened before the materials are solidified, and the melt in the central part of the casting mold is discharged. A double-structure preform 6, wherein a core glass layer is formed on the inner surface of a clad glass tube, is formed in the casting mold, the preform is cooled to room temp. in an inert gas atmosphere having <=0<= dew point, then the upper and lower openings are closed with a resin, and a preform for fluorine optical fiber having extremely low transmission loss is produced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is expected to be used as an ultra-low loss optical transmission line.
The present invention relates to a method for manufacturing a preform for a tsuride optical fiber.

[Prior Art/Problems to be Solved by the Invention Aphthalate glass can emit light at longer wavelengths than silica glass, and has a smaller Rayleigh scattering loss than silica glass. Therefore, if an optical fiber is made of fluoride glass, it can be expected to realize an optical communication line with lower transmission loss than that of a silica-based optical fiber.

For silica-based optical fibers, there are many methods for manufacturing preforms, such as the VAD method (vapor phase shafting is the method) and the MCVD method.
A preform synthesis method using a gas phase method is adopted. However, unlike oxide glasses, fluoride glasses have the characteristic that the temperature change in p1 viscosity decreases extremely rapidly, and they tend to crystallize in the glass softening temperature range. Therefore, the VAD method and VCD method used to form silica-based optical fiber preforms cannot be applied.
In addition, it is difficult to apply the scouring method to double overlaps.

As a preform manufacturing method for fluoride optical fiber,
Build-in casting method [Patent application No. 56-76684
(% 1977-191240))-'P rotational casting method [Patent application 1987-7409
8-125630): 1 has been proposed, but in both of these methods, the molten core glass is injected while the cladding glass is solidified in the mold.
There was a drawback that the cladding glass was easily reheated4 and crystallized at the cladding interface.

To improve this drawback, the raw material for clad glass is poured into the mold as a melt, the raw material for core glass is poured onto this melt, and the unsolidified glass melt is poured out from the bottom of the @ mold at a constant rate to form the core. - A method of forming a cladding structure has been proposed (Japanese Patent Application No. 59-213074).

However, even if these methods are used, it is difficult to produce a preform for a vertical mode optical fiber having a small core diameter and a core/cladding diameter ratio of 1:6 or more.

The ultra-low loss light transmission custom made with fluoride glass is available at Dai J.
i;1 & It is effective to be used for distance transmission method),
A method for producing a single mode optical fiber preform for this purpose has been sought.7 The object of the present invention is to produce a fiber optic fiber having a small diameter core diameter and a sufficiently large core/cladding diameter ratio. An object of the present invention is to provide a preform for an optical fiber obtained from a compound single mode optical fiber.

[Means for solving the labor problem] In the present invention, after the clad glass melt is cast into a bottomed cylindrical mold, the core glass melt is deposited on the glass melt.
fXf Chiasting Grease, before the entire chiastained glass melt solidifies, leave the bottom of the mold for one week, pour out the glass melt in the center of the casting, and add a core glass layer to the circle side.
The most important feature is to form a cylindrical glass having a clad glass layer on the outside.

In the conventional technology, a small amount of glass melt is poured out and the back of the clad glass tube is filled with core glass melt as a base material, but in the present invention, the core glass melt is not filled in the inside of the clad glass tube. This method differs from conventional methods in that a core glass layer is formed on the wall.

〔Example〕

Figure 1) to (d) are process diagrams of embodiments of the present invention.
In this figure, A is a bottomed cylindrical mold made of gold-plated brass, 1 is the side wall of mold A, 2 is the bottom of casting ff1A, 3 is clad glass melt, 4 is a gold crucible, and 5 is core glass. 6 is a preform.

60ZrF-32Bc-F-4GaFs-
6mob4 In the composition of AlF3, ZrF4# BaF2
゜GdF and AlF3 were weighed, 60 g of a mixture consisting of the fluoride (20.9 g of diNH4F-HF was added, and the mixture was first heated at 400°C for 2 hours using a metal crucible 4 in an electric furnace maintained in a nitrogen atmosphere. Fluorination treatment with Nf4F-1' is carried out, and then the temperature is raised to 850°C and heated and melted for 2 hours, after which the clad glass melt 3 is cast ah (see Figure 1(a)). Baso, ga4i
The li part 1 is preheated to 260°C, and the bottom part 2 is soo
'c+ = preheated 0th order C2, 60ZrF -3
0BaF-4GdF-4AtFa-2 mob
A fluoride mixture weighed to the composition of P bF 2 30
A core glass melt 5 obtained by heating and melting 10Ji' of g(2NH4F-HF) in the same process as the clad glass is applied to the clad glass melt in history A (1st (see Figure (1))), before the entire glass melt inside WA solidifies, the mold bottom i2 is
The glass melt with low viscosity in the center of the mold is poured out by removing it from the um part 1 and opening the bottom of the mold (81) (see Figure 1 <c>). After the melt flows out, the core glass melt flows down from the top, and a part of the core glass melt adheres to the inner wall surface of the clad glass tube in a concentric circle with the clad glass tube and solidifies. Cylindrical glass body 6 having a two-layer structure with a glass layer
(preform) was obtained (see Fig. 1(d)), the shape of the preform was 10uφ in outer diameter, 7.5uφ in inner diameter,
The mother was 150u.

Thereafter, the glass tube was slowly cooled to room temperature, and the two preform openings were covered with epoxy resin to prevent the inner wall surface of the preform from coming into contact with the outside air.

The process from the above-mentioned Kiastain ink to sealing with epoxy resin was carried out in a glove box purged with a low-humidity nitrogen gas atmosphere with a water dew point of -80°C.

After optically polishing the outer periphery of the preform, the noepoxy resin was removed and Teflon Fip? I jiaketed Yup and drew a line. In addition, during the wire drawing, the inside of the preform was sufficiently replaced with dry nitrogen gas to remove moisture remaining on the glass surface, and then the inside was evacuated.
The preform is smoothly solidified into a fiber.
When solidified, no irregularities such as crystallization were observed on the inner surface. As a result, the core diameter is 13 μm, the cladding is 140 μm, and the refractive index difference between the core and the cladding is 0.491i.
, cutoff wavelength is 2.3 μm, minimum loss is 2.6
We were able to obtain a step-index fluoride single mode fiber with a performance of 1 dB/Rm in μm (2501).

The preform is placed in a phonetic atmosphere with a moisture dew point of 0°C.
When the preform was placed in a replaced glove box using the above method and pulled for a while, some structural irregularities due to crystallization were observed on the inner surface of the preform during solidification during the first pull.

This means that the moisture in the atmosphere may contact the inner surface of the preform, for example "-ZrF4-) H2O-Zr(OH)F3+')
This is because the IP causes a water splitting reaction and a hydroxide layer is generated on the inner wall surface, so crystallization occurs more easily during gland extraction.

Since the above reaction is likely to occur at high temperatures, it is particularly desirable to keep the humidity of the atmosphere in the space where the chiasting is performed and the preform is cooled to its normal temperature as low as possible.

The above example "I: is Z>-F4- BaF2-
Preform 8 using GdF3-AtF3 glass
However, ZrF -BaF -LaF3-A
tF3-NaF glass, Zrl;'-nap”
2- L'F3-ALF -LiF series glass, Z
l-F-l3aF2-LaF-YF3-L
iF-NaF glass, ZrF-HfF
-BaF -G (IF -A7F3 glass or ZrF -HfF -BaF2 -LaF3-
Even when YF -AAF -LiF -NaF glass was used, it was possible to obtain a suitable reform with a smooth interior 11)1i where irregularities would not occur during line drawing by the method shown in the example. [Effects of the Invention] As explained above, even if argon gas or helium gas with a moisture dew point of -80°C was used as the inert gas, a preform without structural irregularities during wire drawing could be produced. According to the method for creating an optical fiber preform according to the present invention, a heptadide optical fiber preform with a small core diameter and a sufficiently thick core/cladding diameter can be produced, and by drawing it, a fluoride fiber can be produced. Since a single-mode optical fiber can be easily obtained, it has the advantage of contributing to the realization of ultra-low loss, high-capacity optical communication systems using fluoride optical fibers.

[Brief explanation of drawings]

FIGS. 1(a) to 1(d) are process diagrams showing one embodiment of the present invention. A... Mold, 2... Bottom, 3...
... Clad glass melt, 5 ... Core glass melt, 6 ... Cylindrical glass mold (preform).

Claims (2)

[Claims] (1) After the clad glass melt is chiastained into a bottomed cylindrical mold, the core glass melt is chiastained onto the glass melt, and the bottomed mold is opened before the entire glass melt solidifies. A method for manufacturing an optical fiber preform, characterized in that a glass melt at the center of a mold is poured out to form a cylindrical glass body with a core glass inside, and this is slowly cooled to room temperature. (2) The preform for an optical fiber according to claim 1, wherein the step of slowly cooling the cylindrical glass body to room temperature is performed in an inert gas atmosphere having a humidity of 0° C. or less at a water dew point. Production method.