JPH02229733A - Method and device for producing low-scattering fluoride fiber - Google Patents

Abstract

PURPOSE:To produce a low-scattering fluoride fiber and to eliminate the glass defect due to internal stress by providing a specified cooling stage or external force impressing stage and thermally drawing the fiber from the molten fluoride glass through its preform. CONSTITUTION:The molten fluoride glass is cooled and solidified to produce the preform for a fluoride fiber. The preform is then heated and drawn to produce the fluoride fiber. In this case, the molten fluoride glass is cooled to control the generation of tensile stress in the preform, or external force is impressed to reduce the tensile stress produced in the preform or the glass defect resulting from the tensile stress. Although a fluoride fiber having a lower loss than the quartz fiber can be theoretically made, the low-loss fiber is not produced by this time. However, the low-loss fluoride fiber is produced by this method.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to a fluoride fiber for long-distance optical communication using light in the wavelength range of 2 to 3 μm, and in particular to manufacturing a fluoride fiber with low scattering. The present invention relates to a method for manufacturing a low scattering fluoride fiber and an apparatus therefor.

(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 you cut out a very high quality part of about 0 to 20 meters and measure it, it will be a little less than 1 dB/Km, and if you average it over a length of Loom or more, it will be 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. Forucm vol. 19-20 (1987)
pp. 381, microcrystals, oxide particles, metal particles, air bubbles, phase separation, etc. present in the fiber are cited.

Therefore, until now, in order to manufacture fluoride fibers with low scattering, it has been necessary to remove microcrystals, oxide particles, which cause excessive scattering,
Various manufacturing methods and apparatuses for removing metal particles, air bubbles, phase separation, etc. have been proposed. However, in these conventional manufacturing methods and their equipment, the main causes of excessive scattering are not these, and appropriate measures are not taken to eliminate the real causes, so low scattering fluoride fibers are used. It was difficult to make.

Therefore, there has been a strong desire to find out the true cause of excessive scattering, and to provide an appropriate method and apparatus for manufacturing a low scattering fluoride fiber to reduce excessive scattering, but nothing has been disclosed to date.

The present invention was devised in view of the problems of the prior art described above, and it clarifies the true main cause of excessive scattering in fluoride fibers, and provides a low-destroying fluoride fiber that can produce low-scattering fluoride fibers. An object of the present invention is to provide a method for manufacturing a compound fiber and an apparatus therefor.

(Means for Solving the Problems) The first feature of the present invention is that after cooling and solidifying a fluoride glass melt to produce a fluoride fiber preform,
In the method for producing a fluoride fiber, the fluoride fiber is drawn while heating the fluoride fiber preform to produce a fluoride fiber, in which the fluoride glass is heated to suppress generation of tensile stress in the fluoride fiber preform. Perform at least one of a cooling step of cooling the melt and an external force application step of applying an external force to reduce the tensile stress inherent in the fluoride fiber preform or defects in the glass caused by the tensile stress. There is a particular thing.

A second feature of the present invention is to reduce tensile stress inherent in the fluoride fiber preform in a fluoride fiber manufacturing apparatus for manufacturing a fluoride fiber from a fluoride fiber preform that has been prepared in advance. The object of the present invention is to have such an external force applying means.

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

Until now, it has been thought that the main causes of excessive scattering in fluoride fibers are microcrystals, oxides, fine particles such as metals, air bubbles, phase separation, etc. 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
544) and the above-mentioned method for manufacturing a preform without scattering centers (Japanese Patent Application No. 63-17109), excessive scattering also existed. Moreover, even when such a fluoride fiber (hereinafter simply referred to as "Fiber J") was actually carefully examined using an optical microscope and an electron microscope, no scattering center 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) Second, as shown in Figure 1, when a 2-3 cm fiber piece is observed under a transmission microscope, "unevenness" of brightness and darkness, which cannot be seen in quartz fibers, is seen in the core. This is related to the shape and strength of the "unevenness" and 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 this application believed that macroscopic "unevenness" in the refractive index corresponding to brightness and darkness was distributed within the core, causing scattering. In addition, when the fiber length to be observed becomes several meters, it becomes impossible to observe brightness and darkness, so the refractive index "
It is presumed that the "unevenness" has such randomness that it is averaged out while the light propagates through the fiber for several meters.

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 drawing a preform having a core with such strong tensile stress and defects caused by it, it is natural that the core glass heated above its softening point will shrink in volume and try to relieve 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, we will discuss two experimental examples that support the above model. (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 is greater in a preform with more vacuum bubbles. 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, even though it was clearly a vacuum bubble when it was formed, there was an air component inside. This means that N. ,O. This indicates that there are defects in the glass that allow easy penetration of gases such as . 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, the present invention has been made based on the above-mentioned principle, and is a manufacturing method for realizing a low scattering fiber by eliminating or reducing tensile stress in a preform, which is a main cause of excessive scattering in the fiber. This paper also proposes a device for the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of an apparatus and method for manufacturing a low scattering fluoride fiber according to the present invention will be explained in detail below with reference to the drawings.

(Example 1) Figure 2 shows a first example according to the present invention, which is a low scattering fluoride fiber manufacturing apparatus (hereinafter referred to as a "drawing apparatus") for producing a fiber by drawing a preform. ). Note that FIG. 3A shows a configuration diagram of the entire wire drawing device, and FIG. 1B shows a cross-sectional view of the heater portion.

In the figure, 1 is an extrusion port, 2 is a tapered extrusion container, 3 is a preform, 4 is a ring heater, 5 is a temperature control heater that controls the temperature of the extrusion port 1, 6 is a preheating heater, and 7 is a 8 is a pressure extrusion section, 8 is a wire diameter control section, 9 is a wire guide roller, and 100 is a winding drum. Next, the operation will be explained.

(1) First, the preform 3 is loaded into an extrusion container 2 having an extrusion port 1 having a diameter smaller than the outer diameter of the preform 3. At this time, the preform 3 may be uncoated, but if it is coated in advance with a resin such as Teflon heat-shrinkable tube, it has the effect of improving the slippage with the extrusion container 2. (2) Adjust each part of the preform to a predetermined temperature using the ring heater 4, the temperature control heater 5 of the extrusion port 1, and the preheating heater 6. In addition, the approximate temperature of each part is 10'-106cp inside the ring heater 4, 10'cp inside the temperature control heater 5 of the extrusion port 1, and 10'' inside the preheating heater 6 based on the viscosity of glass. The temperature is such that it becomes 10"cp.

(2) Next, the preform 3 is extruded into the ring heater 4 by the pressure extrusion section 7. At this time, in the present invention, since the structure is such that when the preform 3 passes through the extrusion port 1, it receives stress in the opposite direction to the tensile stress inside the preform 3, the tensile stress inside the preform 3 and Glass defects caused by tensile stress can be eliminated. Therefore, even if the extruded preform 3 is drawn using the ring heater 4, the scattering caused by the internal stress of the preform 3 has already been eliminated, so it becomes a fiber with low scattering.

In addition, in order to continuously draw a fiber with a uniform diameter,
The wire diameter control section 8 is used to adjust the wire drawing roller 9 and the extrusion speed, as is usually done, and the wire is wound onto the winding drum 100.

As described above, in the present invention, when drawing the preform 3 by heating it to a temperature above the softening point and below the crystallization temperature,
By pressurizing and ejecting the preform from the tapered extruder 2, scattering due to internal stress of the preform 3 is reduced.

FIG. 3 is a scattering characteristic diagram for comparing the scattering of the fluoride fiber according to the present invention and the scattering of a conventional fiber.

In the figure, characteristic curve A shows the scattering intensity of a conventional fluoride fiber, characteristic curve B shows the scattering intensity of the fluoride fiber according to the present invention, and characteristic curve C shows the scattering intensity of a conventional quartz fiber. As is clear from the figure, the scattering intensity of the fluoride fiber according to the present invention is significantly reduced compared to the scattering intensity of the conventional fluoride fiber, and can be improved to almost the same level as the scattering intensity of the quartz fiber.

(Example 2) FIG. 4 shows a second example according to the present invention, in which the configuration of a device (hereinafter referred to as "stress relief device") for eliminating tensile stress in the preform 3 by pressure stretching is shown. It is a diagram. In addition. FIG. 3(a) shows a longitudinal sectional view of the preform 3 set in the apparatus, and FIG. 3(b) shows a longitudinal sectional view of the preform 3 being stretched under pressure.
In the figure, 10 is a bell jar, l1 is a gas inlet, and 1
2 is a gas discharge port, 13 is a drawer rod, 14 is a drawer control unit, 15 is an electric furnace, and the others are the same as in Example 1.

Next, the operation will be explained.

(1) First, the extrusion container 2 placed inside the bell jar 10 is filled with the preform 3.

(2) An inert gas atmosphere is created in the bell jar 10 by introducing an inert gas through the gas inlet 11 and exhausting it through the gas outlet l2.

(2) Next, the preform 3 is heated to a temperature above its softening point using the electric furnace 15, and the preform is extruded from the extrusion port 1 using the pressure extrusion section 7. At this time, preform 3
The external pressure in the opposite direction to the tensile stress inherent in the extrusion port 1
Since the tensile stress is applied in the vicinity of the tensile stress, tensile stress and defects in the glass caused by the tensile stress are eliminated.

(2) Once a small amount of preform 3 has been extruded, the tip of the preform 3 is fixed to the pull-out rod l3, and the preform 3 is stretched straight while adjusting the pull-out and extrusion speed by the pull-out section 14 and the pressurized extrusion section 7.

After the tensile stress is eliminated by the above-described stress relief device, a low scattering fiber can be obtained by performing a normal drawing process. In addition, if the tensile stress of the preform cannot be sufficiently relieved in one stress relief process (processes from ■ to ■), the above-mentioned stretching process is necessary using an extrusion container 2 whose extrusion port has a gradually smaller diameter. You may repeat it as many times as you like.

In Example 1, a method and device for resolving the tensile stress within the preform 3 during the wire drawing process were explained, but in Example 2, a method for resolving the tensile stress within the preform 3 between the preform manufacturing process and the wire drawing process was explained. A new stress relief process and stress relief device have been added for this purpose.

(Embodiment 3) FIG. 5 shows a third embodiment of the present invention, and is a configuration diagram of another stress relief device.

The difference from Example 2 is that the tensile stress inside the preform is eliminated by hot pressing.

In the figure, 3 is a preform, 15 is an electric furnace for heating, 16 is a cylindrical cylinder, 17 is a pressure medium that includes the preform 3 and equalizes the pressure applied to the preform 3, and 18 is a pressure medium for uniformizing the pressure applied to the preform 3. a pressurizing section 19 for applying pressure to the preform 3 via a piston 19 to be described later;
is a piston, and 20 is a pedestal.

Next, the operation will be explained.

(2) Filling the cylinder 16 with the preform 3 with a pressure medium 17 such as Teflon resin interposed therebetween.

(2) Next, while heating the preform 3 to a temperature above its softening point using the electric furnace 15, the piston 19 is gradually pushed into the cylinder 16 under the control of the pressurizing section 18.

When the stress relief process is performed using such an external force applying means, a substantially uniform pressure is applied to the outer periphery of the preform 3 via the pressure medium 17. Glass defects can be eliminated.

If the preform 3 is drawn after passing through this stress relief step, the tensile stress has been eliminated, so that a low scattering fiber can be obtained.

In the above explanation, the stress relief process using a hot press was explained by taking as an example a piston-cylinder type uniaxial pressurizing means that can use extremely strong pressure. A stress relief device can be similarly realized using hydraulic means or the like.

Further, in the present invention, a stress relief process is newly provided between the preform manufacturing process and the wire drawing process in order to eliminate the internal stress of the preform, but it is possible to perform the stress relief process as in Example 2. This makes it possible not only to realize a low scattering fiber but also to realize a long preform necessary for manufacturing a long fiber.

(Example 4) FIG. 6 is a fourth example according to the present invention, and is a configuration diagram of a preform manufacturing apparatus. FIG. 5A shows a longitudinal sectional view of the apparatus when pouring the core glass has started, and FIG.

In the figure, 21 is a moving rod, 22 is a Talad glass tube,
23 is a core glass melt, 24 is a cooling gas blower serving as a cooling means, 25 is a core glass, and 26 is a concave portion at the upper end of the core glass 25.

Next, the operation will be explained.

(2) As shown in FIG. 6(a), a core glass melt 23 is poured into a TALLAD glass tube 22 fixed to a moving rod 21.

(2) Next, as shown in FIG. 6(b), the moving rod 21 is gradually lowered so that the preform passes through the cooling gas flow produced by the cooling gas blower 24.

In this way, the core glass 25 is heated by the cooling gas.
As the core glass 25 is forcedly cooled and solidified sequentially from the bottom to the top, a recess 26 is formed at the upper end of the core glass 25.
Body contractions due to cooling occur uniformly. That is, when the core glass 25 solidifies, it undergoes volumetric contraction, but at that time, a cooling process is performed to cause the volumetric contraction so that no force is applied to the core glass 25, thereby reducing the tensile stress generated inside the core glass 25. It was suppressed.

As mentioned above, in Example 4, special glass cooling was performed during preform production to reduce tensile stress inside the glass. Therefore, if the preform produced in Example 4 is used to remove stress as in Examples 2 or 3, or to draw while applying an external force as in Example 1, the tensile stress can be easily eliminated. ,
The production of low scattering fiber becomes more feasible.

As described above, the present invention realizes a low scattering fiber by reducing tensile stress during preform production or by reducing or eliminating tensile stress existing inside the produced preform. It is.

(Effects of the Invention) As described above, the present invention provides at least one of cooling to suppress the tensile stress generated during production of the preform and external force application to reduce the tensile stress inherent in the preform. By doing this, it is possible to realize a fluoride fiber with low scattering, and it is also possible to eliminate defects in glass caused by internal stress.

By performing an external force application process after the cooling process, a fluoride fiber with lower scattering can be manufactured. By performing the external force application process after producing the fluoride fiber preform and during drawing, it becomes possible to eliminate internal stress and to produce long preforms necessary for producing long fibers. By performing the external force application process while drawing the fluoride fiber preform while heating it, the process of eliminating internal stress and the drawing process can be performed in succession, which simplifies the manufacturing process.

By having an external force applying means that reduces the tensile stress inherent in the fluoride fiber preform,
Fluoride fibers with low scattering can be manufactured with a simple equipment configuration. The external force applying means includes a container in which the extrusion port for the fluoride fiber preform is smaller than the insertion port, and a pressure extrusion device disposed on the insertion port side of the container.
The apparatus has a simple configuration, and a low scattering fluoride fiber can be manufactured while drawing is performed at the same time.

The external force applying means has a pressure means for applying pressure to the fluoride fiber preform and a pressure medium that equalizes the pressure applied to the fluoride fiber preform by the pressure means. It becomes possible to eliminate internal stress.

Therefore, the present invention makes a significant step forward toward the realization of fluoride fibers, which have hitherto been difficult to reduce loss, although it is theoretically possible to create fibers with lower loss than silica fibers. , the effect is extremely large.

[Brief explanation of the drawing]

Figures 1 (a) and (b) are enlarged views of fluoride fiber and quartz fiber pieces observing "unevenness" to explain the principle of the present invention, and Figure 2 is a first embodiment of the present invention. A block diagram of a drawing device for producing a fiber by drawing a preform, FIG. 3 is a scattering characteristic diagram for comparing scattering between a fluoride fiber according to the present invention and a conventional fiber, and FIG. 4 is a diagram showing the present invention. FIG. 5 is a vertical sectional view of another stress relief device as a third embodiment of the present invention, and FIG.
) and (b) are configuration diagrams of a preform manufacturing apparatus as a fourth embodiment of the present invention. 1... Extrusion port, 2... Extrusion container, 3...
Preform, 4... Ring heating, 5...
・Temperature control heater, 6... Preheating heater,
7... Pressure extrusion section, 8... Wire diameter control section,
9... Line drawing roller, lO... Bell jar
11... Gas inlet, 12... Gas outlet,
l3... Pull-out rod, 14... Pull-out control section, 15... Electric furnace, 16... Cylindrical cylinder, 17... Pressure medium, 18... Pressurizing section, 19... Piston, 20... Pedestal, 21
...Moving rod, 22...Crund glass tube, 2
3... Core glass melt, 24... Cooling gas blower,
25... Core glass, 26... Concave portion, 100
... Winding drum. Figure 1 Figure 2 (b) Figure 3 Figure 7 High! i (cm) Younger brother figure 5 (b)

Claims (7)

[Claims] (1) Production of fluoride fiber by cooling and solidifying a fluoride glass melt to produce a fluoride fiber preform, and then drawing the fluoride fiber preform while heating it to produce a fluoride fiber. The method includes a cooling step of cooling the fluoride glass melt so as to suppress generation of tensile stress in the fluoride fiber preform, and a tensile stress inherent in the fluoride fiber preform or the tensile stress. 1. A method for producing a low scattering fluoride fiber, the method comprising performing at least one of the steps of applying an external force to reduce defects in the glass caused by the process. (2) The method for manufacturing a low scattering fluoride fiber according to claim 1, wherein the external force application step is performed after the cooling step. (3) The method for manufacturing a low scattering fluoride fiber according to claim 1, wherein the external force application step is performed between the production of the fluoride fiber preform and the drawing. (4) The method for manufacturing a low scattering fluoride fiber according to claim 1, wherein the external force application step is performed while heating the fluoride fiber preform while drawing it. (5) In a fluoride fiber manufacturing apparatus for manufacturing fluoride fiber from a fluoride fiber preform prepared in advance, an external force applying means is provided to reduce the tensile stress inherent in the fluoride fiber preform. An apparatus for producing a low scattering fluoride fiber, comprising: (6) The external force applying means includes a container whose extrusion port for the fluoride fiber preform is smaller than the insertion port, and a pressure extrusion device disposed on the insertion port side of the container. An apparatus for producing a low scattering fluoride fiber according to claim 5. (7) The external force applying means includes a pressure means for applying pressure to the fluoride fiber preform, and a pressure medium that equalizes the pressure applied to the fluoride fiber preform by the pressure means. An apparatus for manufacturing a low scattering fluoride fiber according to claim 5, characterized in that it has the following.