JPS63206325A - Production of optical fiber - Google Patents

Abstract

PURPOSE:To contrive to reduce loss of optical fiber in heat-treating a perform having glass fine particle deposited layer on a transparent glass rod dividedly at two stages, by specifying the heat treatment temperature. CONSTITUTION:For example, SiCl4 is sent to O2/H2 burner 2, glass fine particles 4 are produced in oxyhydrogen flame 3 and deposited on a transparent glass rod 1 to form a glass fine particle deposited layer 5. A composite preform consisting of the glass rod 1 and the glass fine particle deposited layer 5 is inserted into a heating furnace 7 having a heat source 8. In the operation, the preform is heat-treated in a fluorine-containing atmosphere at a temperature >=1,000 deg.C and not to transparentize glass as the first stage. Then the perform is heat-treated in a fluorine-containing atmosphere at >=1,600 deg.C to give a transparent glass preform. By this method, optical fiber having sufficiently low loss can be readily produced.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for manufacturing a silica-based glass optical fiber, and more particularly to an improvement in a method for manufacturing a low-loss optical fiber in which fluorine is added to the cladding.

[Conventional technology]

In an optical fiber, in order to form a core and a cladding, it is necessary to provide an effective difference in refractive index between the two. Therefore, in optical fibers whose main component is glass, a component called a dopant is added to the core or cladding to change the composition of the glass and create a difference in refractive index. Currently, the most used optical fiber is silica-based optical fiber.
Usually, the core portion is doped with an oxide such as germanium or phosphorus to make the refractive index higher than that of the cladding. In addition, boron or fluorine is introduced into the cladding part to make the refractive index of the cladding part lower than that of pure silica glass. Controlling the refractive index of the core and the refractive index of the cladding are often performed simultaneously. An optical fiber whose cladding includes glass containing fluorine is known as one of the optical fibers that have been frequently used in recent years. Several methods are known for producing fluorine-doped glass, but one method that can produce fluorine-doped glass in large quantities is to deposit a glass particle preform made by depositing glass particles in a high-temperature fluorine gas-containing atmosphere. A method is known in which fluorine is doped into glass by processing it inside the glass. This manufacturing method will be explained in a little more detail, for example, as follows. First, as shown in FIG. 1, silicon tetrachloride is fed into an oxygen/hydrogen burner 2 to generate glass particles 4 in a flame 3. The glass particles 4 are made of a transparent glass rod (
Here, for example, it is assumed that the glass is made of glass which contains germanium in advance and whose refractive index is about 0.1% higher than that of pure silica glass) 1 to form the glass fine particle deposition layer 5. The composite preform consisting of the glass rod 1 and the glass fine particle deposition layer 5 thereon is then inserted into a heating furnace 7 having a heat source 8, as shown in FIG. At this time, as a first step, the maximum temperature inside the furnace is set to approximately 8
Keep at around 50℃. In this example, the dimensions of each part are as follows. Center transparent glass rod: diameter 12mm, length 500mm Composite preform: diameter 120mm, effective length 42
0 mm Inner diameter of heating furnace 150 mm Heat zone length of heating furnace 180 mm In this first stage, the following gases are introduced into the heating furnace 7. Helium 90% Carbon tetrafluoride 10% And the insertion speed of the composite preform into the furnace is about 400
mm, and this composite preform is moved back and forth twice in the furnace. Next, as a second step, the temperature inside the heating furnace 7 is set to about 1500
℃ and feed the composite preform into the high temperature area of the furnace sequentially from the lower end. In this way, in this second step, the glass fine particle deposited layer 5 is made into transparent vitrification, and a transparent vitrified portion 6 is obtained. The reason why the heat treatment of the composite preform is divided into two stages is that it is necessary to sufficiently reduce the amount of residual OH groups in the optical fiber base material. That is, in the first stage, the temperature is set at 850° C. so that the glass fine particle deposited layer 5 remains porous, and the fluorine generated from the thermally decomposed carbon tetrafluoride is passed through the gaps between the glass fine particles 4 into the deposited layer. The remaining OH groups are discharged to the outside of the deposited layer 5 in the form of HF (hydrogen fluoride). Furthermore, the reason why carbon tetrafluoride is flowed during the second stage of transparent vitrification is that if even a small amount of moisture enters the furnace from outside the furnace, OH groups will re-enter into the transparent glass. This is to prevent this. Fluorine doping is carried out in the first step of removing OH groups (dehydration) by heat treatment and the second step of making transparent vitrification by heat treatment.

[Problems to be solved by the invention]

However, it has been found that such conventional manufacturing methods do not sufficiently reduce the loss of the finally obtained optical fiber. An object of the present invention is to provide an optical fiber manufacturing method that can easily manufacture an optical fiber with sufficiently low loss, based on the above-mentioned study of the conventional manufacturing method.

[Means to solve the problem]

The method for producing an optical fiber according to the present invention includes the steps of depositing glass particles on a transparent glass rod, and heating a composite preform of the glass rod and glass particle stack obtained in the step at 1000°C or higher and making it transparent. A first heat treatment step in which heat treatment is performed in a fluorine-containing atmosphere having a temperature at which no vitrification occurs, and a second heat treatment step in which a transparent glass preform is obtained by heat treatment in a fluorine-containing atmosphere at a temperature of 1600° C. or higher.
A special feature characterized by comprising a heat treatment step of

[For production]

By performing the first heat treatment step in an atmosphere containing fluorine and fluorine at a temperature of 1000°C or higher and at a temperature that does not cause transparent vitrification, residual OH groups can be sufficiently discharged, and the second step Since the heat treatment step is performed in a fluorine-containing atmosphere at a temperature of 1600° C. or higher to make the glass preform transparent, the loss of the optical fiber can be reduced to the minimum. Regarding this, we first investigated the reasons why conventional manufacturing methods cannot sufficiently reduce the loss of optical fibers, and from the results of that study, we found that the above manufacturing method can significantly reduce the loss of optical fibers. Let me clarify. According to numerous studies by the inventors, the reasons why conventional manufacturing methods cannot sufficiently reduce losses can be summarized as the following two points. First, conventionally, the temperature of the heat treatment step for dehydration (hereinafter simply referred to as dehydration and dehydration temperature) is inappropriate. In the conventional example described above, the setting of the dehydration temperature simply followed the previous optical fiber manufacturing conditions. That is, conventionally, glass particles are deposited to create a sintered body of glass particles (so-called soot preform).
There is a known method for producing a glass base material for optical fibers by converting this into transparent vitrification, but at that time, if necessary, the soot preform is dehydrated at a temperature that prevents complete transparent vitrification. things were being done. The dehydrating agent used therein is a chlorine-containing gas such as chlorine gas or thionyl chloride, but these gases are generally highly reactive even at room temperature, for example, to the extent that they produce a very strong pungent odor even at room temperature. Therefore, these dehydrating agents are 85
Since the glass particles are sufficiently activated at a temperature of 0°C, the factors to consider when removing the OH groups remaining in the glass particles are setting a temperature that is sufficient for the diffusion of the OH groups in the glass particles. It was found that the decomposition and reaction of dehydrating agents did not require much care. However, fluorine-containing gas materials that are currently commonly used to add fluorine to glass particles include CF4, C2F, SF6, etc., and these materials are extremely stable and harmless to humans and animals at room temperature. Therefore, when using such materials, the question is how much these materials are decomposed and activated at the so-called dehydration and transparent vitrification temperatures. In our measurements, CF was released within about 5 seconds of heating time.
It was found that a temperature of about 1000° C. or higher is required for almost 100% decomposition of 4. Of course, the decomposition of CF4 starts at about 700°C, so dehydration is possible even at relatively low temperatures, but in that case, the decomposed fluorine will not be sufficiently distributed throughout the soot preform, so the final result will be The refractive index distribution of transparent glass preforms is often not flat. On the other hand, if the temperature is too high, the transparent vitrification will be completed before the time required for the HF gas generated by dehydration to pass through the gaps in the porous soot preform and be discharged outside the soot preform. As a result, sufficient dehydration will not occur. According to our study results, the upper limit depends on the average particle size of the glass particles, but the bulk density of the deposited glass particles is 0.
The temperature was 1100°C under conditions of about 12 to 0.28°C. Normally, there is a
If the OH group of p p tn remains, about 60 d −
The absorption loss of B/km is 1.38. It is known that this occurs in um. As a result, when drawing the glass preform produced in this way to make an optical fiber, if the optical communication is 1.3 μm to 1.55 μm
In order to carry out the process at a wavelength of , it is necessary to reduce the residual OH group concentration to several tens of ppb or less, preferably to several ppb or less. In addition, when using a saturated silicon fluoride material such as CF4, by sufficiently permeating fluorine into the glass particles and the gaps between the particles, it is possible to uniformly coat the glass in the cladding part in the subsequent transparent vitrification process. This ensures a high level of fluorine addition and increases the reproducibility of the final optical fiber's transmission characteristics. Second, the transparent vitrification temperature is conventionally inappropriate. That is, the optical fiber loss depends on the transparent vitrification temperature. Although it has been described above that the absorption of residual OH groups has an adverse effect on optical fiber transmission, it is presumed that scattering loss is the cause of the loss that depends on the temperature of the transparent glass. This loss is thought to be caused by fluctuations at the core-cladding interface, but according to transmission theory, the amplitude of these fluctuations is smaller than μm and is distributed over the entire length of the optical fiber. Please note that this has not been completely confirmed. In other words, even very fine fluctuations will cause a loss of about 0.1 dB/km if distributed over the entire length. Using the conventional manufacturing method, a composite preform with a transparent glass rod in the center and glass particles deposited around it is made into a transparent material in a high-temperature heating furnace after undergoing a sufficient dehydration process as described above. When the transmission loss was measured with respect to the transparent vitrification temperature during the vitrification treatment, the results shown in FIG. 3 were obtained. Here, helium gas and CF4 gas are introduced into the heating furnace,
The relative refractive index difference between the core and cladding of the finally obtained optical fiber is set to be approximately 0.3%. As can be seen from Fig. 3, the lowest loss wavelength band of 1.55μ is the optical fiber whose main component is silica glass.
The loss in
The loss was about 0.18 dB/km, but at a transparent vitrification temperature of 1600°C or higher, the loss decreased to 0.18 dB/km, which is almost the ultimate loss.
It has decreased to km. The following hypothesis can be considered to positively explain this second reason. When fluorine penetrates glass, under normal conditions the diffusion distance is believed to be on the order of 0.1-0.2 μm. On the other hand, it is said that the average particle size of normal glass particles is about 0.1 μm, so if the particle size is like this, the composite preform can be heated in a fluorine-containing atmosphere to form the cladding of an optical fiber. It is thought that fluorine can be added almost uniformly into the glass that is to be formed. However, in reality, as shown in FIG. 4A, the particle size of glass fine particles 4 produced from silicon tetrachloride, trichlorosilane, etc. by flame hydrolysis or thermal oxidation reaction is not uniform but varies. , there is a distribution in the particle size. Therefore, probabilistically speaking, in the glass fine particle deposition layer 5, glass fine particles 4 having a considerably large particle size will be deposited on the transparent glass rod 1 at a certain rate. If the particle size of the glass particles 4 is larger than the diffusion distance of fluorine at high temperatures, the fluorine will not penetrate sufficiently into the inside of the large particles 4, so after becoming transparent vitrified, as shown in Figure 4B. As such, in the cladding portion 10 around the core portion 9, portions with a low concentration of fluorine doped occur unevenly (in FIG. 4B, the shaded portion indicates a portion where fluorine is sufficiently doped). The above discussion concerns single-mode optical fibers, which are most susceptible to the effects of refractive index fluctuations near the core-cladding boundary, but the same theory applies to multimode optical fibers, which have a large number of propagation modes. be. Such microscopic causes of scattering loss are difficult to identify by so-called general measurements of transmission characteristics, and macroscopically cause a uniform increase in the transmission loss coefficient in the length direction of the optical fiber. The cause of such loss should also depend on the concentration of fluorine added. That is, by adopting a transparent vitrification temperature higher than conventional conditions, it becomes possible to promote the diffusion of fluorine within the glass particles. In addition, by raising the transparent vitrification temperature in this way, fluorine is sufficiently diffused into the central transparent glass rod, which is the core glass, so that the actual core-cladding boundary is inside the optical fiber. As a result, the region with the refractive index fluctuation at the core-cladding boundary that once occurred will be located at a distance from the core-cladding boundary by that amount, and even if it is not sufficiently penetrated by fluorine. It is thought that the degree of scattering loss will be reduced even if there are glass particles present.

【Example】

First, a glass fine particle deposition layer 5 was formed around a transparent glass rod 1 as shown in FIG. This glass rod 1 is
It was a transparent glass rod with a diameter of 10 mm and a flat surface. Its composition does not contain any impurities that are harmful to its optical properties, and it is made of quartz glass doped with germanium. Silicon tetrachloride was introduced into the oxygen/hydrogen flame 3 of the burner 2 to generate fine particles 4 of quartz glass. The flow rate of each gas was 10 liters/minute for hydrogen, 12 liters for 7 minutes for oxygen, 0.5 liters/minute for argon, and 0.25 liters/minute for silicon tetrachloride. Note that argon is a carrier gas for silicon tetrachloride. The burner 2 was traversed in the length direction of the glass rod 1 about 20 times while the glass particles 4 thus produced were sprayed onto the transparent glass rod 1 and deposited thereon. As a result, the thickness of the glass fine particle deposit layer 5 was approximately 60 mm, and the diameter of the composite preform consisting of the glass rod 1 and the glass fine particle deposit layer 5 thereon was approximately 130 mm. Note that the length of the deposited layer 5 is approximately 500 mm. Next, as shown in FIG. 2, this composite preform was inserted into a heating furnace 7 maintained at a temperature of approximately 1020° C. to dehydrate it, and at the same time, fluorine was permeated into the voids of the deposited layer 5. . In this way, the first stage of heat treatment was performed. This causes some shrinkage in the diameter of the composite preform, approximately 1'
It became 20rnm. However, at this stage, the deposited layer 5 is still
The void inside is not closed. Gases containing 90 parts of helium and 10 parts of carbon tetrafluoride were introduced into the heating furnace 7. The composite preform that had been subjected to the first heat treatment in this manner was turned into transparent vitrification in the same heating furnace 7. In this second stage heat treatment process, the temperature inside the heating furnace 7 is approximately 16
The temperature was 30°C. Further, the gas flow rate into the furnace 7 was set to 86 parts of helium and 14 parts of carbon tetrafluoride. The above steps were repeated to obtain a final transparent glass preform to obtain a sufficient rad thickness for a single mode optical fiber. The refractive index distribution of this transparent glass preform was as shown in FIG. As can be seen from FIG. 5, the refractive index distribution is approximately flat in the cladding portion, demonstrating one of the effects of the manufacturing method of the present invention. When this transparent glass preform was drawn and spun to produce a single mode optical fiber, the wavelength characteristic of transmission loss was as shown in FIG. It can be seen from FIG. 6 that the loss (indicated by the dotted line) caused by the fluctuation of the refractive index in the vicinity of the conventional core-cladding interface has been improved. In addition, this single mode optical fiber has a core diameter of approximately 12μTn, a cladding diameter of approximately 125μm, a relative refractive index difference between the core and cladding of 0.3% LPl, a mode cutoff wavelength of 1.28μm, and other transmission characteristics. I found it to be satisfactory.

Effects of the Invention According to the optical fiber manufacturing method of the present invention, a fluorine-doped optical fiber with very low loss can be easily manufactured.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing a glass particle deposition process according to an embodiment of the present invention and a conventional example, Fig. 2 is a schematic diagram showing a heat treatment process according to an embodiment and a conventional example, and Fig. 3 is a transparent view of transmission loss. Graphs showing the vitrification temperature dependence, Figures 4A and 4B are enlarged cross-sectional views of the glass fine particle deposited layer; Figure A shows the state before transparent vitrification, and Figure B shows the state after transparent vitrification.
The figure is a graph showing the refractive index distribution of the transparent glass preform obtained in one example, and FIG. 6 is a graph showing the wavelength characteristics of the transmission loss of the optical fiber produced from the same side. DESCRIPTION OF SYMBOLS 1... Transparent glass rod, 2... Burner, 3... Oxygen/hydrogen flame, 4... Glass fine particles, 5... Glass fine particle accumulation layer, 6... Transparent vitrified part, 7 ...
Heating furnace, 8... Heat source, 9... Core part, lO... Clad part.

Claims (3)

[Claims] (1) A step of depositing glass fine particles on a transparent glass rod, and heating a composite preform of the glass rod and glass fine particle laminated layer obtained in this step to a temperature of 1000°C or higher and at a temperature at which transparent vitrification does not occur. A first heat treatment step of heat treating in a fluorine-containing atmosphere having a temperature of 1,600° C. or higher, and a second heat treatment step of obtaining a transparent glass preform by heat-treating in a fluorine-containing atmosphere at a temperature of 1600° C. or higher. A method for manufacturing optical fiber. (2) The transparent glass rod is 0% compared to pure quartz glass.
．． 2. The method of manufacturing an optical fiber according to claim 1, wherein the optical fiber is made of silica-based glass or pure silica glass containing an additive at a concentration less than or equal to an additive concentration that provides a refractive index difference of 1%. (3) The optical fiber according to claim 1 or 2, wherein in the first heat treatment step, the main component of the fluorine-containing atmosphere is helium, and the remaining component is saturated fluorocarbon. manufacturing method.