JPS6311542A - Method and apparatus for producing preform for optical fiber - Google Patents

Abstract

PURPOSE:To produce a preform for an optical fiber which is long-size and is low in a scattering loss by casting core glass from the inside pipe of concentrical nozzles and clad glass from the outside pipe thereof respectively simultaneously into a casting mold. CONSTITUTION:The concentrical double nozzles 2 which are respectively discretely connected to a core glass melting part 1A and a clad glass melting part 1B are put onto the upper part of a platinum crucible 1 consisting of said two melting parts. Respective raw materials are then melted in the above-mentioned two melting parts 1A, 1B to form a core glass melt 7 and a clad glass melt 8. The front end 2C of the double nozzles 2 is inserted into the casting mold 3. The crucible 1, the double nozzles 2 and the casting mold 3 are thereafter inverted by means of a frame 9 for casting. The crucible 1 supported by a crucible base 5 via the nozzles 2 and the casting mold 3 supported by a supporting base 10 via a casting mold base 4 are isolated from each other in the above-mentioned inverted state, by which the core glass melt 7 and clad glass melt 8 in the crucible 1 are simultaneously cast into the casting mold 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method and apparatus for manufacturing a preform for a fluoride optical fiber, which is expected to be used as an ultra-low-loss optical transmission line.

[Prior art] Fluoride glass can transmit light with a longer wavelength than silica glass, and its Ley scattering loss is smaller than that of silica glass. For example, it is expected that optical communication waveguides with lower transmission loss than quartz fibers will be obtained.

However, for silica-based optical fibers, methods for manufacturing preforms include the VAD method (vapor phase shafting is the method) and the V[l] method for M.
There is a method of pre-forming using a gas phase method, which has the characteristics that the temperature change in viscosity is extremely rapid and it is easy to crystallize in the glass softening temperature range. Therefore, the V used for forming the preform of silica-based optical fiber
The AD method and CVD method cannot be applied to fluoride glass, and it is also difficult to apply the double crucible drawing method.

The build-in casting method and the rotational casting method have been proposed as preform manufacturing methods for fluoride optical fibers, but in these methods, a TALLAD glass tube is first created and then the hollow part is filled with core glass melt. Therefore, after the clad glass has been cooled, it is fused with the core glass melt and heated, and at this time,
Crystallization occurs at the core-clad interface, making it difficult to provide an optical fiber preform with low scattering loss.

Furthermore, it is not possible to manufacture a preform with a small diameter core and a sufficiently large core-to-cladding diameter ratio, and only preforms for multimode optical fibers can be manufactured. It is difficult to fabricate single-mode optical fibers for use in large-capacity, long-distance transmission systems that can exhibit these characteristics.

[Problems to be Solved by the Invention] Therefore, an object of the present invention is to provide a method and apparatus for manufacturing an optical fiber preform that can manufacture a long fluoride optical fiber with low scattering loss.

[Means for Solving the Problems] In order to achieve such an object, the method of the present invention includes a container consisting of a core glass melting section and a clad glass melting section surrounding the core melting section, and a container disposed in the upper part of the container. A double nozzle section having a concentric section that connects the core glass melting section and the clad glass melting section individually is used to melt the core glass, and then cover the container with the double nozzle section, and the core glass raw material is melted. and a step of simultaneously melting the clad glass raw materials in a core glass melting section and a clad glass melting section, respectively; a step of covering the double nozzle section with a mold, and then simultaneously inverting the melting container and the mold;
In the inverted state, the container and mold are relatively separated,
It is characterized by comprising the step of simultaneously casting the core glass melt and the clad glass melt in the container into a mold.

The manufacturing apparatus of the present invention includes a container consisting of a core glass melting section and a clad glass melting section surrounding the core melting section, and a concentric circle disposed on the top of the container and connected individually to the core glass melting section and the clad glass melting section. a double nozzle part having a part, a mold having a hollow part into which the double nozzle part can be fitted; 8
The present invention is characterized by comprising a moving member, a member capable of simultaneously inverting the container and the mold, and a member heating the container and the mold.

Here, the container, the double nozzle part, and the mold are housed in the core tube, and the core tube is provided with a partition wall that can divide the inside of the furnace core tube into a glass melting part and a mold preheating part. It is preferable to arrange a nozzle section, a mold in the mold preheating section, and a moving member attached to the furnace core tube.

[Function] The present invention has a container portion such as a double-structured platinum crucible consisting of a core glass melting portion and a clad glass melting portion surrounding the core glass melting portion, and the core glass melting portion and the cladding glass melting portion are provided in the upper part of the container. After arranging a double nozzle part connected to the glass melting part and melting the core glass and clad glass in the above-mentioned container, covering the double nozzle with a mold for forming an optical fiber preform, With the mold placed over the double nozzle, invert the glass melting vessel and pull the mold downward, or lift the glass melting vessel upward to remove the core glass solution and cladding glass melt in the glass melting vessel. Cast into the mold at the same time.

Conventionally, after forming a clad glass tube, the core glass melt was cast into the tube, whereas the present invention differs in that the clad glass melt and the core glass melt are simultaneously cast into the mold. .

According to the present invention, a long preform for a fluoride optical fiber without crystallization at the core-cladding interface can be produced, and a low-loss fluoride fiber can be easily obtained. This can contribute to the realization of ultra-low loss optical communication systems.

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

Example 1 FIG. 1 shows an embodiment of the manufacturing apparatus of the present invention.

Here, 1 is the core glass melting part IA and this part IA
This is a platinum crucible having a clad glass melting section IB surrounding the cladding glass melting section IB. 2 is a platinum double nozzle whose central part 2A is removably connected to the core melting part IA of the platinum crucible 1, and its peripheral part 2B to the clad glass melting part IB. Reference numeral 3 denotes a gold-plated cylindrical brass mold, which has a hollow portion 3A for accommodating the tip portion 2C of the double nozzle 2.

Reference numeral 4 denotes a mold stand for raising and lowering the mold 3, and 5 and 6 denote crucible stands for supporting a crucible consisting of a platinum crucible 1 and a double nozzle 2. 7 is the core glass melting part IA of platinum crucible 1
8 is the core glass melt in the clad glass melting section IB. 9 is a casting frame, and this frame 9 includes a crucible stand 5. A support stand lO that supports the crucible stand 6 and the mold stand 4 so as to be able to rise and fall freely
Attach.

Next, an embodiment of the method of the present invention for manufacturing an optical fiber preform using the manufacturing apparatus of the present invention shown in FIG. 1 will be described with reference to FIGS. 1 and 2.

First, ZrF, BaF2. GdF3. Δj2 F3.
PbF2 to 80ZrF4-30BaFz-4GdFs-
4AI F3-2mol! A mixture obtained by adding 5 g of NH4F.HF to 12 g of a fluoride mixture for core glass weighed to have a composition of kPbF2 was placed in the core glass melting zone IA at the center of the platinum crucible I. Next, ZrF.

BaF, GdF,, Aj! F3 to 60ZrF4-3
0BaF2-4GdF, -6molXA4 To 60g of the mixture weighed to the composition of F3, add 20g of N)
The added material is transferred to the clad glass melting section IB of the platinum crucible I.
I put it in.

This platinum crucible 1 was covered with a double nozzle 2. This was placed in an electric furnace maintained in a nitrogen atmosphere, and first heated at 400°C for 2 hours to perform fluorination treatment with NH4F.IIF. Next, the temperature was raised to 850° C. in the same atmosphere, and the mixture was heated and melted for 2 hours.

After that, the platinum crucible 1 is taken out from the electric furnace, the double nozzle 2 is covered with the mold 3 preheated to 260°C, and the platinum crucible 1 and the mold 3 are assembled for casting as shown in FIG. A mold stand 4 and crucible stands 5 and 6 were installed on a frame 9.

Next, as shown in FIG. 2, the casting frame 9 is inverted, the core glass melt 7 is placed in the center 2A of the double nozzle 2, and the clad glass melt 8 is placed in the outer area 2B of the double nozzle 2. It poured out.

Thereafter, the mold stand 4 was lowered and the mold 3 was pulled out from the double nozzle 2.

As a result, the core glass melt 7 flowed out into the center of the mold 3, and the clad glass melt 8 flowed out around it in a concentric solid state and solidified.

This was slowly cooled to room temperature to obtain a step index type fluoride optical fiber preform having a core diameter of 3.3 m1Ilφ, a cladding diameter of 10 mmφ, and a length of 150 mm.

The above casting operation was carried out in a glove box purged with a nitrogen gas atmosphere whose humidity was -80° C. at the water dew point.

After optically polishing the outer periphery of the preform obtained in this way, it was coated with a Teflon FEP tube and drawn.
An optical fiber having a cladding diameter of 140 μm and a length of 600 m was obtained. The minimum transmission loss value of this optical fiber is wavelength 2.6μ
It was 1 dB/km at m. Also, the wavelength is 2.8μ
The absorption loss due to OH group impurities appearing in m is 10 dB/
It was km.

In the conventional manufacturing method for fluoride optical fiber preforms, after forming a clad glass tube, the hollow part of the tube is filled with core glass melt kept at high temperature, which tends to cause crystallization at the core-clad interface. However, according to the manufacturing method of the present invention, the length of the fiber with low scattering loss could be increased by more than 10 times.

Example 2 FIG. 3 shows another embodiment of the manufacturing apparatus of the present invention.

Here, 11 and 12 are electric furnaces, and 13 is a furnace tube disposed in the core of the electric furnaces 11 and 12. This furnace tube 1
3 is divided into a mold preheating section 13A and a glass melting section 13B by a partition wall 14 disposed in the middle thereof so as to be openable and closable. Reference numerals 15 and 16 are an inlet and an outlet, respectively, for atmospheric gas to the mold preheating section 13A. 1
Reference numerals 7 and 18 are an inlet and an outlet, respectively, for atmospheric gas to the glass melting section 13B. 19 is a crucible stand on which the platinum crucible 1 is mounted. 20 is a mold stand on which the mold 3 is placed. These stands 19 and 20 are supported on support stands 21 and 22 so as to be able to rise and fall freely, and further,
These supports 21 and 22 are fixed to the glass melting section 13B and mold preheating section 13A of the furnace tube 13, respectively.

Next, an embodiment of the method of the present invention for manufacturing an optical fiber preform using the manufacturing apparatus of the present invention shown in FIG. 3 will be described.

First, ZrF, BaF2. GdF3. Al1 F...
PbF, 60ZrF4-308aF2-4GdF3-
4AA F3-2mol! Nll, F-HF to 12 g of a fluoride mixture for core glass weighed to a composition of kPbFz
5 g was added to the core glass melting zone IA at the center of the platinum crucible 1. ZrF4. BaF2°GdF3.
AλF3 60ZrF4-30BaFz-4GdFs
Mixture a weighed to a composition of -6 molX 8 flFs
og and 20 g of N114F-HF were added to the clad glass melting section IB of the platinum crucible 1.

This platinum crucible 1 was covered with a double nozzle 2 and placed in a furnace core tube 13. Further, the mold 3 is installed on the mold table 20, and the furnace core tube 13 is connected to the mold preheating section 1 by the partition wall 14.
It was separated into glass melting part 3A and glass melting part 13B.

First, in an electric furnace 11, a platinum crucible l was heated to 400°C for 2 hours.
The fluoride glass raw material was heated for a period of time to undergo fluorination treatment with N)14F-HF, and then the temperature was raised to 850° C. and the core glass was melted by heating for 2 hours.

At this time, dry nitrogen gas was introduced through the air gas inlet 17, and the glass melting section 13B of the furnace tube 13 was filled with nitrogen gas air.

Further, the mold 3 was heated to 260° C. using the electric furnace 12. At this time, dry nitrogen gas was introduced into the mold preheating section 13B of the furnace tube 13 through the τ ambient gas inlet 15 to maintain a nitrogen gas atmosphere.

After heating and melting for 2 hours, the partition wall 14 is removed, the crucible stand 19 is raised upward, and the double nozzle 2 is inserted into the hollow part of the mold 3. Next, the entire furnace core tube 13 is reversed, and the glass melting container consisting of the platinum crucible 1 and the double nozzle 2 is moved upward by the crucible stand 19, and the core glass melt 7 and the clad glass melt 8 are respectively , mold 3 from the central part 2A of the double crucible 2 and its peripheral part 2B.
It poured out into the hollow part 3A of. As a result, the core glass melt flowed concentrically into the mold center 2A and the clad glass melt flowed into the surrounding area 2B and solidified.

This was slowly cooled to room temperature to obtain a step index type fluoride optical fiber preform having a core diameter of 3.3 mm, a cladding diameter of 10 mm, and a length of 150 mm.

In the casting method of this embodiment, since there is no need to take the crucible 1 containing the glass melt out of the furnace tube 13, the contamination of moisture into the glass melts 7 and 8 can be suppressed to the utmost. Therefore, the wavelength of the optical fiber obtained by drawing such a preform is 2.8 μ.
The OH absorption loss appearing in m could be reduced to 10 dB/km or less.

[Effects of the Invention] As explained above, according to the present invention, a long fluoride optical fiber preform without crystallization at the core-cladding interface can be manufactured, and a low-loss fluoride optical fiber can be easily produced. This has the advantage that it can contribute to the realization of ultra-low loss optical communication systems using fluoride optical fibers.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing one embodiment of the manufacturing apparatus of the present invention, FIG. 2 is a cross-sectional view illustrating the steps of an example of carrying out the method of the present invention using the manufacturing apparatus shown in FIG. 1, and FIG. FIG. 3 is a sectional view showing another embodiment of the manufacturing apparatus of the present invention. DESCRIPTION OF SYMBOLS 1... Platinum crucible, IA... Core glass melting part, 1B... Clad glass melting part, 2... Double nozzle, 2A... Center part, 2B... Surrounding part, 2C...・Tip part, 3... Mold, 4... Mold stand, 5.6... Crucible stand, 7... Core glass melt, 8... Clad glass melt, 9... For casting Frame, lO... Support stand, 11, 12... Electric furnace, 13... Furnace tube, 14... Partition wall, 15, 17... Atmosphere gas inlet, 16, 18... Atmosphere gas Exhaust port, 19... Crucible stand, 20... Mold stand, 21, 22... Support stand.

Claims (1)

[Scope of Claims] 1) A container comprising a core glass melting section and a clad glass melting section surrounding the core glass melting section; A core glass raw material and a clad glass raw material are introduced into the core glass melting zone and the clad glass melting zone, respectively, using a double nozzle section having concentric circular portions that are individually connected. a step of simultaneously melting the core glass raw material and the clad glass raw material in the core glass melting section and the clad glass melting section, respectively; and covering the double nozzle section with the mold. a step of simultaneously inverting the melting container and the mold; in the inverted state, the container and the mold are relatively separated, and the core glass melt and the clad glass melt in the container are 1. A method for manufacturing an optical fiber preform, comprising the step of simultaneously casting into a mold. 2) A container consisting of a core glass melting section and a clad glass melting section surrounding the core glass melting section, and a concentric circle disposed above the container and individually connected to the core glass melting section and the clad glass melting section, respectively. a mold having a hollow part into which the double nozzle part can be fitted; a mold having a hollow part into which the container and the double nozzle part can be fitted; An optical fiber device comprising: a member for relatively moving a nozzle and the mold; a member for simultaneously reversing the container and the mold; and a member for heating the container and the mold. Preform manufacturing equipment. 3) In the manufacturing apparatus according to claim 2, the container, the double nozzle section, and the mold are housed in a furnace core tube, and the inside of the furnace core tube can be divided into a glass melting section and a mold preheating section. A partition wall is provided in the furnace core tube, the container and the double nozzle part are arranged in the glass melting part, the mold is arranged in the mold preheating part, and the moving member is attached to the furnace core tube. An optical fiber preform manufacturing device characterized by: