JPH02229732A - Production of fluoride glass fiber preform - Google Patents

Abstract

PURPOSE:To obtain a preform capable of producing a low-scattering fluoride fiber by hollowing the molten core glass and clad glass, successively laminating the glasses, heating the laminate close to the softening point and extruding the laminate under pressure. CONSTITUTION:The molten core glass and clad glass are cooled and solidified to produce the fluoride glass fiber preform. In this case, the molten glasses are hollowed and successively laminated in the first stage. The hollow glasses are heated close to the softening point, extruded under pressure and made solid in the second stage. In this process, the pipe can be made solid even at the temp. which does not cause crystallization by heating the fluoride glass pipe close to the softening point while exerting a pressure on the periphery of pipe, the tensile stress in the glass is eliminated, and a low-scattering fiber can be obtained.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to a method for manufacturing a fluoride fiber preform for manufacturing a fluoride fiber for long-distance optical communications using light with a wavelength in the 2 to 3 μm band. It is related to.

(Prior art) Fluoride fiber is attracting attention as a long-distance communication fiber that uses light in the 2-3 μm wavelength band, and is theoretically expected to have an ultra-low loss of 0.01 dB/Km or less. ．． However, the loss of current fluoride fibers is 1
Even if a very high-quality part of about 0 to 20 m is cut out and measured, it is a little less than 1 dB/Km, and when averaged over a length of 100 m or more, it becomes several dB/Km. Scattering accounts for most of this loss, and reducing scattering is the most important issue in reducing the loss of fluoride fibers.

(Problems to be Solved by the Invention) The cause of this excessive scattering caused by fluoride fibers is described, for example, by France et al. Mat. Sc
i. Forum vol. 19-20 (1987)
pp. 381, microcrystals, oxide particles, metal particles, air bubbles, phase separation, etc. present in the fiber are cited.

Therefore, in order to manufacture preforms for fluoride fibers with low collapse, various manufacturing methods have been used to remove the causes of excessive scattering, such as microcrystals, oxide particles, metal particles, air bubbles, and phase separation. It has been proposed. However, in these conventional manufacturing methods, these are not the main causes of excessive scattering, and appropriate measures are not taken to eliminate the true main causes, resulting in low scattering fluoride fiber fibers. It was difficult to make renovations. Therefore, there has been a strong desire to elucidate the true cause of excessive scattering and to find a method for manufacturing an appropriate low-scattering fluoride fiber preform for reducing excessive scattering, but nothing has been disclosed so far.

The present invention was devised in view of the problems of the prior art described above, and it has been made to clarify the true main cause of excessive scattering in fluoride fibers, and to make it possible to produce low scattering fluoride fibers. The purpose of the present invention is to provide a method for manufacturing a preform.

(Means for Solving the Problems) The present invention is characterized in that a preform for a fluoride glass fiber is produced by cooling and solidifying a glass melt of at least core glass and Talad glass. The manufacturing method includes a first step of making the glass melt hollow and sequentially stacking the glass, and a second step of making the hollow glass solid by pressure extrusion while heating it to a temperature near its softening point. There is a particular thing.

(Principle of the Invention) First, we will explain what is the true main cause of excessive scattering in fluoride fibers.

Until now, the main causes of excessive scattering in fluoride fibers were thought to be microcrystals, oxides, metal particles, etc., bubbles, and phase separation present in the fiber. However, a patent application has already been filed by the same applicant for a core-Tallad glass composition that does not produce crystals (Japanese Unexamined Patent Publication No. 61-63
Excess scattering also existed in fluoride fibers manufactured using the above-mentioned method for manufacturing preforms without scattering centers (Japanese Patent Application No. 17109/1983). Furthermore, even when such fluoride fibers (hereinafter simply referred to as "fibers") were carefully examined using an optical microscope and an electron microscope, no scattering centers as described above could be found.

Therefore, the inventors of the present application believed that there was another main cause of excessive scattering, and in order to find the true main cause, they conducted an investigation to find a phenomenon that is correlated with the scattering intensity of the fiber. As a result, we were able to discover two phenomena.

(a) First, there is a correlation with fiber strength; fibers with high mechanical strength have low scattering, and conversely, fibers with low mechanical strength have high scattering. Based on this phenomenon, the inventors of the present application believed that it was because some mechanical defect existed in the fiber and was strongly related to both the intensity and scattering.

(b) The second one is 2-3 cm as shown in Figure 1.
When a piece of fiber is observed under a transmission microscope, light and dark ``unevenness'' that is not seen in quartz fibers can be seen in the core, and the shape and strength of this bright and dark ``unevenness'' are related to the scattering intensity. It was also found that this contrast becomes less visible as the length of the fiber being observed becomes longer. From this phenomenon,
The inventors of the present application considered that macroscopic "unevenness" in the refractive index corresponding to brightness and darkness is distributed within the core, causing scattering. Furthermore, since it becomes impossible to observe brightness and darkness when the length of the fiber being observed becomes several meters, this refractive index "unevenness" has such randomness that it is averaged out while the light propagates through the fiber for several meters. It is assumed that

The inventors of the present invention considered the model that causes the mechanical defects and refractive index "unevenness" in the fiber as follows.

Fluoride fiber preforms, unlike quartz fiber preforms, are produced by cooling and solidifying glass melt. Since cooling proceeds from the periphery of the glass toward the inside, tensile stress naturally exists in the cooled and solidified glass. The most problematic point in this conventional manufacturing method (cooling and solidification process) for preforms for fluoride fibers is that the core glass melt is injected into the TALLAD glass tube once produced and is then cooled and solidified. This means that the tensile stress acting on the material becomes extremely large.

The volumetric shrinkage of fluoride glass when it solidifies from a melt is much larger than that of silica glass, reaching about 10%. Therefore, during cooling and solidification, stress so strong as to exceed the elastic limit of the glass may be generated, and it is thought that there are strong tensile stresses and defects sheared by the stress inside the core glass. This can be easily inferred from the fact that vacuum bubbles are generated in the glass unless special measures are taken in the cooling process of the core glass.

When a preform with such strong tensile stress and resulting defects in its core is drawn, it is natural that the core glass, heated above its softening point, will shrink in volume and try to alleviate the stress. There would be no problem if this stress relaxation occurred completely uniformly in both the longitudinal and radial directions of the fiber, but in short-term dynamic processes such as during drawing, it is incomplete and uneven. It is considered more natural for uniform relaxation to occur. As a result, non-uniform stress distribution and density distribution occur in the drawn fiber, which causes refractive index "unevenness" and causes scattering. Furthermore, it is natural that the mechanical strength of the fiber decreases as the fiber has such uneven stress distribution and defects.

Next, let's look at two experimental examples that support the above model. I will explain it.

(a) The magnitude of the tensile stress inherent in a preform can be estimated using the number of vacuum bubbles present in the preform as a guide, and it is thought that the tensile stress of a preform with more vacuum bubbles is greater. Therefore, scattering was measured for fibers drawn from several preforms with distinct differences in the number of vacuum bubbles. As a result, even though there were no scattering centers such as bubbles or crystals in the measurement area, the scattering was stronger in fibers drawn from preforms with a larger number of vacuum bubbles, that is, preforms with larger tensile stress.

(b) Next, the preform containing a considerable amount of vacuum bubbles was left in the air for 7 days after production, and the presence or absence of gas within the vacuum bubbles was examined by laser Raman spectroscopy.

As a result, although it was clearly a vacuum bubble at the time of formation, air components were present inside. This means that
This indicates that there are defects in the glass that allow gases such as N2, 0■ to easily penetrate.

As explained in detail above, the true main cause of excessive scattering in fluoride fibers is the conventional theory that microcrystals, oxides, metal particles, etc., air bubbles, etc. that exist in the fiber,
It was possible to infer that this was caused by tensile stress within the preform rather than phase separation.

Therefore, in order to produce a low scattering fiber, it is considered necessary to produce a preform free of tensile stress.

The main reason why strong tensile stress occurs in the core of preforms manufactured using conventional manufacturing methods is that the TALLAD glass tube is completely filled with core glass melt before cooling and solidifying. The point is that it is extremely unlikely to cause volumetric contraction associated with this. In other words, since the core glass is already surrounded by the solidified inner wall of the cladding tube, radial contraction cannot occur. I have no choice but to shrink. However, since the upper end naturally cools faster than the center of the glass, only a small amount of such shrinkage due to the side of the machine occurs, resulting in a large tensile stress being generated in the core glass.

As described above, the cause of the stress generation is that the glass melt is cooled and solidified in a confined state with almost no open surface, and therefore volumetric contraction is not possible. In other words, when glass is created by cooling and solidifying a melt, strong tensile stress is generated inside glass with a shape that has almost no open surface, such as a cylinder, whereas glass with a shape with a wide open surface, e.g. , it is thought that the generation of tensile stress is extremely small in pipe-shaped glass that has an open surface in the center. Therefore, in order to manufacture a preform with low internal stress, it is considered necessary to first cool and solidify the glass melt in a shape with a wide open surface, such as a vibrator. However, even if a vibrator-shaped glass is created, cooling of the glass always proceeds from the outer circumference toward the inside, making it difficult to completely suppress the generation of internal stress. In addition, since a pipe-like shape with a hollow portion inside cannot be used as a preform, the hollow portion must be made solid.

In the case of preforms for quartz fibers, preforms have already been manufactured by solidifying pipes. For example, in manufacturing a preform by the MCVD method, a quartz tube with a layer of synthetic quartz deposited on its inner wall is solidified by heating it at a high temperature with an oxyhydrogen flame.

However, it is difficult to apply the solidification method for quartz fiber preforms to fluoride fiber preforms. The reason for this is that fluoride glass crystallizes extremely easily and cannot be heated to the high temperatures required to solidify it. Therefore, no method has been proposed to date for solidifying fluoride glass pipes.
This has not yet been achieved. Therefore, the inventors of the present application conducted studies to solve the problem of realizing a low scattering fiber and a solid preform, and as a result, they heated the fluoride glass pipe to near its softening point temperature while applying pressure to the outer periphery of the pipe. It was discovered that by doing so, it is possible to make the vibrator solid even at low temperatures that do not cause crystallization, and the tensile stress inside the glass is also eliminated, making it possible to create a low-scattering fiber.

Accordingly, the present invention has been made based on the above-mentioned principles and is directed to a fluoride fiber for realizing a low scattering fiber by eliminating or reducing the tensile stress in the preform, which is the main cause of excessive scattering in the fiber. This paper proposes a method for manufacturing preforms for

Below, a method for manufacturing a fluoride fiber preform according to the present invention will be explained using the drawings. (Example 1) First, a method for manufacturing a preform for a step type single mode fiber with low scattering will be explained as an example. FIG. 2 is a cross-sectional view of a manufacturing apparatus for manufacturing a hollow glass pipe according to the present invention (hereinafter referred to as "glass pipe manufacturing apparatus"). In the figure, l is a cylindrical mold with one end narrowed so that it is thinner than the other, 2 is a plug for the small diameter part, 3 is the other plug of the mold, and 4 holds the mold. 5 is an electric furnace for keeping the mold at a predetermined temperature, and 6 is a rotation drive unit.

Moreover, FIG. 3 is a cross-sectional view of the medium practical extrusion equipment if (hereinafter referred to as "solidification equipment") according to the present invention, which is an apparatus for solidifying a glass vibrator produced by a glass pipe manufacturing equipment. It is. In the figure, 7 is a clad glass layer, 8 is a core glass layer, 9 is a pressure-resistant container for reinforcing the strength of the mold 1, IO is a piston for pressurized extrusion, 11
is an electric furnace. The manufacturing process when manufacturing a preform using this glass pipe manufacturing apparatus will be explained.

(1) First step■ First, keep the temperature of the mold 1 near the glass transition temperature,
Tilt the entire device and pour the molten Talad glass into the mold.
After injecting, put on the stopper 3.

■After that, return the device to the horizontal position and rotate the mold l at high speed around the axial direction. The injected Talad glass melt spreads uniformly on the inner wall of the mold 1 due to centrifugal force, and the glass melt is cooled and solidified from the wall surface of the mold 1.

- After tilting the entire device again and injecting molten core glass with a different refractive index from TALLAD glass, the device is leveled again and mold 1 is rotated at high speed. The injected core glass melt spreads uniformly on the inner wall of the clad glass tube due to centrifugal force and is cooled and solidified to form a core glass layer. This is the first step, and in the present invention, when forming a glass tube by uniformly cooling a cylindrical glass melt from the outer circumference, the glass melt is cooled so that the glass easily contracts in the radial direction. These are hollow glass tubes that are stacked one after another to minimize the residual stress (internal stress) in each tube. (2) Second step ■ The mold 1 in which the glass tube consisting of the two layers of the Tallard glass layer 7 and the core glass layer 8 was formed in the first step is moved to the pressure-resistant container 9 in the solidifying device. ■Heat the glass tube consisting of two layers, the TALLAD glass layer 7 and the core glass layer 8, in a 1L electric furnace until it reaches a temperature close to its softening point. ■Pressure is applied by piston 10. The two-layer glass tube pressurized by the piston 10 is subjected to compressive stress toward the center when passing through the narrow diameter portion of the mold l. Therefore, the hollow part of the glass tube gradually becomes thinner and becomes solid. At this time, the glass tube, whose temperature is close to its softening point, is subjected to compressive force, so the tensile stress inside the glass can also be relieved. In addition, when applying pressure to make a glass tube solid, the glass easily deforms even at relatively low temperatures near its softening point, so it is possible to make the glass tube solid without inducing crystallization. It is possible to As mentioned above, the present invention involves the first step of producing a hollow glass pipe, and the second step of making the glass pipe heated to a temperature close to its softening point solid by extruding it by applying an external force.
A preform for fluoride fiber that does not crystallize and has low scattering can be manufactured by simply performing the following steps. Furthermore, by applying this manufacturing method, it is easy to manufacture preforms for single mode fibers or low dispersion fibers. For example, if the core glass composition is 53Zr-20Ba-20N
a-4La-3AI, Talad glass composition 40Zr-
13Hf-18Ba-22Na-4La-3AI,
A mold 1 with an inner diameter of 40 mm was used, and the thickness of the core glass was 0.
By making a glass tube with a diameter of 4 mm and an inner diameter of 8 mm and making a preform, a preform with a core-clad ratio of 1:10.7 can be made. The relative refractive index of these glasses is 0.4
4%, the outer diameter is 150 μm from this preform.
When the fiber is spun, it exhibits single mode characteristics at a wavelength of around 2.5 μm.

As is clear from the above manufacturing method, it is easy to further deposit a layer with a different refractive index on the inner wall of the glass tube manufactured in the first step of the present invention. Therefore, Talad glass has a composition of 10Zr-43Hf-18Ba-22Na-
A glass tube with an inner diameter of 8 mm was made using 4La-3AI, and the second layer had a composition of 53Hf-18Ba-22Na-4La.
-3Al glass with a thickness of 0.4 mm and a composition of 53 as the third layer
Each glass of Zr-20Ba-20Na-4La-3AI is successively injected to a thickness of 0.4 mm to produce a glass tube consisting of three layers each having a different refractive index.
After that, a preform is produced by extruding under pressure in the same manner. The relative refractive index of these glasses with respect to Talad glass is -0.15% and +0.85%, respectively, and the refractive index distribution at the center of the preform has characteristics as shown in FIG. It is known that the dispersion characteristics of a fiber with such a refractive index distribution exhibit a low value over a wide range of wavelengths, and it is possible to fabricate a fiber with low dispersion characteristics even in the case of a fluoride glass fiber.

(Embodiment 2) FIG. 5 is for explaining a second embodiment of the present invention, which is the same as FIG. 2 except for the structure of the lid and the nozzle for injecting molten glass. The lid in this embodiment has a small opening in the center, and a molten glass injection nozzle 12 is inserted through the small opening.

In Example 2, a two-layer pipe structure was once created in a molten state by using glass with a composition in which the specific gravity of TALLAD glass was larger than that of the core glass, and then the glass was cooled. This makes it possible to produce a homogeneous double-layered glass pipe with extremely low internal stress.

■In this manufacturing method, first, the temperature of mold 1 is set above the melting temperature of the fluoride glass, and the mold is rotated at high speed. ■ After that, the melted composition 40Zr-13Hf-18B
A Talad glass having a high specific gravity such as a-22Na-4La-3AL is injected through the nozzle 12. The injected glass remains liquid and adheres uniformly to the inner wall of the mold due to centrifugal force.

■Next, the composition 53Zr-208a-20Na-4La-3
Inject core glass melt with low specific gravity like AI.
The injected core glass also remains liquid due to centrifugal force and spreads uniformly over the inner surface of the TALLAD glass, forming a melt layer with a different composition. In this case, the two melts do not mix with each other because the specific gravity of the crund glass is greater than the specific gravity of the core glass.

■After that, it is cooled and solidified while rotating at high speed.

■Next, if it is solidified using a medium practical extrusion device,
It is possible to produce a preform with a step-shaped refractive index distribution with extremely low internal stress.

(Effects of the Invention) As explained in detail above, the present invention includes the first step of laminating glass melts having different refractive indexes in the shape of a hollow glass pipe, and the process of heating the hollow glass to near its softening point temperature. By including the second step of solidifying by pressure extrusion, it is possible to produce a glass pipe with low residual stress, which is less susceptible to stress due to volumetric shrinkage of the glass as it cools. By forming a hollow space in the glass melt using the centrifugal force caused by rotation, the thickness of the laminated glass can be controlled only by the amount of glass injected, and the refractive index can be appropriately selected depending on the glass composition. In addition to the three-layer refractive index distribution shown in Example 2, pipes with even more complex refractive index distributions can be easily fabricated. A preform for a fluoride glass fiber with low dispersion can be manufactured by forming a stack of glass melts with at least three glasses having mutually different refractive indexes.

Therefore, it is possible to produce a fluoride glass fiber for long distance communication in the 2 to 3 μm band, which is expected to have low loss and low breakage characteristics in the future, and its effects are extremely large.

FIG. 3 is a schematic diagram of a solidifying device for solidifying a fluoride glass vibe according to the present invention, and FIG. 4 is a schematic diagram showing the refractive index of a preform manufactured by the manufacturing method according to the present invention. The distribution characteristic diagram, FIG. 5, is a schematic diagram of another glass pipe apparatus for producing a fluoride glass pipe according to the present invention.

DESCRIPTION OF SYMBOLS 1... Mold, 2... Plug (bottom cover), 3... Plug (top cover), 4... Jig, 5... Electric furnace, 6... Rotation drive unit, 7...・Tallad glass layer, 8...Core glass layer, 9...Pressure vessel, 10...Piston, 1l...
・Electric furnace, l2... nozzle.

[Brief explanation of the drawing]

Claims (3)

[Claims] (1) In a method for producing a preform for a fluoride glass fiber in which a preform for a fluoride glass fiber is produced by cooling and solidifying a glass melt of at least a core glass and a clad glass, the glass melt is hollowed. A fluoride glass fiber fiber plug comprising: a first step of sequentially laminating the glass; and a second step of making the hollow glass solid by pressure extrusion while heating it to a temperature close to its softening point. Renovation manufacturing method. (2) The method for manufacturing a preform for a fluoride glass fiber according to claim 1, wherein the hollow of the glass melt is formed using centrifugal force due to rotation. (3) The stacked layers include at least three layers having mutually different refractive indexes.
2. The method of manufacturing a preform for a fluoride glass fiber according to claim 1, wherein the preform is formed from a glass melt.