JPS6374932A - Production of preform for optical fiber - Google Patents

Abstract

PURPOSE:To produce a preform optical fibers, having a low loss and capable of controlling refractive index, by feeding a raw material gas into a burner repeating the movement in the oblique direction and stop in a plane including the rotation axis of a deposited body while changing the flow rate of the raw material gas according to the position of the burner. CONSTITUTION:Fine glass particles are produced from one or more burners 2 and 3 to deposit a soot preform 4 on a seed rod 1 while rotating and moving the seed rod 1 in the direction of rotation axis and the resultant deposited body is then transparentized and vitrified to afford the aimed preform for optical fibers. In the process for producing the preform for the optical fibers, at least one burner 3 is fixed on a guide 61 and moved in the oblique direction to the rotation axis in a plane including the above-mentioned rotation axis while repeating the high-speed movement for a given distance and stop for a given time. The flow rate of the raw material gas to the above-mentioned burner 3 is simultaneously changed as a function of the position of the burner 3. Thereby the aimed preform for optical fibers capable of readily providing a low loss is obtained without causing dip, etc., in a refractive index distribution.

Description

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

The present invention relates to a method for manufacturing a communication optical fiber with low loss and a controlled refractive index distribution, and in particular, it involves depositing fine glass powder (hereinafter referred to as soot) into a predetermined shape and then forming the preform. The present invention relates to a method of manufacturing an optical fiber preform by heating and converting it into transparent vitrification.

[Conventional technology]

Conventionally, the so-called external deposition method and the VAD method have been known as methods for manufacturing optical fiber preforms by once depositing soot in a predetermined shape and then heating this preform to make it transparent. There is. In the external method, as shown in FIG. 9, a frit gas such as a metal halide is supplied into the flame of a burner 92, soot is generated by a flame hydrolysis reaction, and this soot is transferred to the outside of a long and narrow mandrel 91. The spray builds up to form a soot layer 93. Usually burner 92 is applied to mandrel 9 more than 10 times.
The soot layer 93 having a desired dopant concentration distribution in the radial direction is formed by moving the soot layer 93 back and forth in the longitudinal direction of the glass and changing the composition of the glass deposited during that time. This soot layer 93 is finally turned into transparent glass in a high-temperature heating furnace to form an optical fiber preform having a predetermined refractive index distribution in the radial direction.

[Problems to be solved by the invention]

However, in the external method, the central mandrel 91, which is the target of soot deposition, needs to be removed either before or after the soot layer 93 is clarified, resulting in a hole in the center of the preform. This hole is, in many ways, an obstacle to manufacturing high-quality optical fiber. In other words, first of all, if this hole becomes contaminated with metal impurities or dirt, since this hole is located near the center, the part that should become the core will be contaminated, and the hole near the center will become contaminated. This will only serve to create opportunities that will hinder loss reduction. Second, the dopant evaporates from the part facing the hole in the center of the preform during heating, which tends to result in a refractive index distribution with a dip in the center as shown in Figure 10. . On the other hand, when producing, for example, a graded optical fiber using the VAD method, it is necessary to control the refractive index distribution into the desired shape while adjusting the temperature of the lower end surface of the soot-deposited preform. tried many times,
The current situation is that you have to repeat AND and errors. Furthermore, there are too many factors that affect the refractive index distribution, such as not only the surface temperature, but also the flow rate of the raw material gas and the surface shape of the lower end of the soot preform, making it extremely complicated. This is described in detail in Japanese Patent Publication No. 75943. Furthermore, as a modification of the VAD method, a method using a plurality of burners 94 as shown in FIG. 11 has been proposed (Japanese Patent Publication No. 56-33327). In this case, the refractive index distribution is formed by depositing soot by changing the concentration of the raw material gas supplied to each of the plurality of burners 94.However, in reality, a plurality of burners are arranged adjacently. Then, the burners interfere with each other, making the soot deposition process by each burner unstable, making it impossible to control the refractive index distribution.In other words, in the so-called VAD method, as is well known, the refractive index cannot be controlled. Germanium tetrachloride (GeC1) of Dovan I, which is commonly used for
The oxidation reaction (4) takes place partly in the flame, but mostly on the surface of the deposited soot preform. In other words, the amount of Ge doped largely depends on the surface temperature of the soot preform being heated by the burner. For this reason, when a plurality of burners are close to each other, the surface temperature tends to rise more than when there is only one burner, and the doping becomes unstable. The present invention provides an optical fiber base material that can easily reduce loss without causing dips in the refractive index distribution, and that can perform substantially stable refractive index control without requiring complicated adjustments. The purpose is to provide a manufacturing method.

[Means to solve the problem]

According to this invention, at least one of the burners for soot generation is moved at high speed by a predetermined distance in a plane including the preform rotation axis in a diagonal direction with respect to the axis, and after the movement The soot is deposited to form a soot preform by moving the soot while repeatedly stopping it for a predetermined period of time, and by changing the flow rate of at least the source gas among the gases fed to the burner as a function of the position of the burner. That's what I do.

[For production]

At least one burner for soot generation,
The preform is moved in a plane that includes the rotation axis at high speed by a predetermined distance diagonally with respect to the axis, and then stopped for a predetermined period of time. This is the same as fixing one burner at each stop position. Moreover, since there is actually only one burner, it is possible to avoid the inconvenience of multiple burners interfering with each other. Since the flow rate of the raw material gas fed into the burner is changed as a function of the position of the burner, the refractive index distribution can be easily controlled.

【Example】

As shown in FIG. 1, the burner 2 is fixed to the seed rod 1, and the burner 3 is movable. These burners 2.3 generate soot (fine glass powder), which deposits the soot on the tip of the seed rod 1 and pulls it upward while rotating the seed rod 1. , a soot preform 4 is formed by soot deposition. When this soot preform 4 is heated and made into transparent glass, it should become a glass preform for optical fiber. An exhaust device 5 is provided to exhaust exhaust gas from these burners 2.3. The burner 3 is moved by the drive mechanism 6 in a plane including the rotational axis of the preform 4 (in a diagonal direction with respect to this axis (in this embodiment, in a linear direction at an angle of approximately 40° with respect to the rotational axis). has been done. That is, the drive mechanism 6
It consists of a guide 61 that guides the burner 3 in the above-mentioned diagonal direction, a high-speed pulse motor 63 as a driving source, and a gear box 62 that converts rotational motion into linear motion. This pulse motor 63 is controlled by a computer 65 via a pulse motor drive 64 to effect movement between each step position so that the burner 3 takes up, for example, 16 step positions. The time required for this movement, that is, the time in the movement mode, is made sufficiently shorter than the time when the burner 3 is stopped, that is, the time in the stop mode. In this example, the burner 3 was moved at a speed of 500 mm for 7 seconds. Since the moving range of the burner 3 is 200 mm, this distance is moved in 16 steps (that is, the number of step intervals is 15, and the burner moves 15 times), and the time required for moving between steps is 0. In this example, a graded multimode optical fiber is manufactured, and the stop time of the burner 3 at each step from the lowest step number O to the highest step number 15 is very short, just over 0.028 seconds. Define as shown in Table 1 below. Furthermore, the flow rates of the raw material 5iC14 and GeC14 gases and the fuel H2,0□ gas to be supplied to the burner 3 are changed by a mass flow controller 67 controlled by a computer 65 via a D/A converter group 66. I made it as shown in Table 1. Here, the reason for increasing the flow rate of H2,02 on the upper end side is as follows.
This is to avoid the heating time per unit area becoming shorter as a result of the diameter of the preform 4 becoming larger, and the temperature becoming lower than that at the lower end. That is, by changing the flow rate of the fuel gas in this way, the surface temperature of the preform 4 is controlled and soot is deposited favorably. As a result, the time required for the 11-rubber was approximately 53 seconds. Incidentally, after the burner 3 which started accumulating soot from the lower end reaches the upper end, it returns to the lower end again and repeats the operation from the lower end to the upper end. Of course, you can also reverse the steps in Table 1. In this way, the soot preform 4 was grown, and finally a preform 4 having a diameter of about 120 mm was obtained. Note that gas was supplied to the burner 2 fixed near the center under the conditions shown in Table 1 and Table 2, and as a result, a thin soot preform with a diameter of about 12 mm was produced at the center. Therefore, the moving burner 3 deposits soot on the outside of this narrow central preform. This preform 4 was treated in a chlorine-containing helium atmosphere in a heating furnace at a maximum temperature of 800°C. Further, the maximum temperature in the furnace was increased to 1630'C, and the atmosphere in the furnace was replaced with 100% helium gas, and the preform 4 was heat-treated to become transparent vitrification. At this time, the shrinkage rate of the soot deposited preform 4 was about 8 times. When the refractive index distribution of the preform after the transparentization was confirmed using a preform analyzer, an almost perfect graded refractive index distribution as shown in FIG. 3 was obtained. The preform obtained by this is the part that should become the core of the optical fiber, so this transparent vitrified preform was stretched in a heating furnace to a diameter of 13 mm, and then a separately prepared quartz glass was used. The tube was inserted into a tube, and a collapse operation was performed using an oxyhydrogen burner that produced a temperature of about 2000° C. in Table 2, Table 3, and Table 4 so that the gap between them was collapsed. Furthermore, the preform after the collapse was drawn in a heating furnace at 2100° C. to form an optical fiber. When the transmission characteristics of this optical fiber were measured, the transmission loss was 0.5 at a wavelength of 1°3 μm.
It was confirmed that a high-quality optical fiber with a transmission bandwidth of 850 MHz (at an optical fiber length of 1 km) and a transmission bandwidth of 5 dB/kn+ was obtained. As far as we can see from Figure 3, there is no effect of the step movement on the refractive index distribution, but this is because the time required for the 11-rubber of burner 3 is less than 1 minute, and the change in glass deposition conditions during this time is The width of the soot band deposited by the burner 3 during one stop mode is effectively 12
This is probably due to the fact that the difference in refractive index of each step was eliminated. In addition,
When the number of steps is 10 or less, a decrease in transmission bandwidth was observed in the case of graded optical fibers. In the second embodiment shown in FIG. 4, the drive mechanism 7 has an arcuate guide 71 and uses a pulse motor 73 to drive the burner 3 via a flexible wire 72. In this case, the burner 3 is moved stepwise while changing its direction so that the upper end of the movement range is oriented substantially horizontally, and the lower end thereof is oriented at an angle of about 40° with respect to the vertical axis of rotation. By changing the direction of the burner 3 in this manner, soot can be deposited in a cylindrical shape. In all of the above embodiments, combustion of the fuel gas supplied to the burner is used as the heat source, but the fifth embodiment
A high frequency plasma torch 31 having an induction coil 32 as shown in the figure and generating high frequency plasma 33 can also be used. Alternatively, a burner using resistance heating or induction heating may be used. If such a heat source is used, no OH groups are generated, which is advantageous in terms of reducing loss. In the several embodiments described above, a separate fixed burner 2 was used to form a thin preform at the center, but such a fixed burner 2 is not necessarily necessary. In many cases, there is no need to form a renovation.Also, as shown in FIG. Soot may be blown onto the surface by the moving burner 3 and deposited thereon. Furthermore, it can easily accommodate refractive index distributions other than graded type. For example, as shown in FIG. 7, the number of steps of the burner 3 was set to 3, and deposition was performed under the conditions shown in Table 3 above. Gas was supplied to the fixed burner 2 for the narrow preform in the center under the conditions shown in Table 4 above. As a result, a special refractive index distribution as shown in FIG. 8 could be formed. The optical fiber having this refractive index distribution is a so-called dispersion shifted I optical fiber, and the diameter of the ring-shaped high refractive index portion outside the refractive index distribution in Fig. 8 is approximately 14 mm.
By doing so, the wavelength dispersion could be made almost zero at a wavelength of 1.55 μm.

【Effect of the invention】

According to this invention, refractive index distribution control can be performed stably. Moreover, no complicated adjustment is required, and loss can be easily reduced. Furthermore, no dip or the like occurs in the center of the refractive index distribution.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the first embodiment of the present invention, FIG. 2 is a block diagram of the control system, FIG. 3 is a graph showing the refractive index distribution of the preform obtained in the first embodiment, and FIG. 4=z A schematic diagram of the second embodiment, FIG. 5 is a schematic diagram of another embodiment, FIG. 6 is a schematic diagram of another embodiment, FIG. 7 is a schematic diagram of yet another embodiment, and FIG. 8 is a schematic diagram of another embodiment. The figure is a graph showing the refractive index distribution of the optical fiber obtained in the example of Fig. 7, Fig. 9 is a schematic diagram of the conventional example, and Fig. 10 is a graph showing the refractive index distribution of the optical fiber obtained in Fig. 9. The graph in FIG. 11 is a schematic diagram of another conventional example. 1... Seed rod, 2.3.21.92.94... Burner, 4... Soot preform, 5... Exhaust device, 6.
7... Drive mechanism, 61.71... Guide, 62...
・Gear hox, 63.73...Pulse motor-16
4... Pulse motor drive device, 65... Computer, 66... D/A converter group, 67... Mass flow controller, 72... Flexible wire, 31
...High frequency plasma torch, 32...Induction coil,
33...High frequency plasma, 11...Transparent glass rod,
91...Mandrel, 93...Soot layer

Claims (9)

[Claims] (1) A deposited body that has one or more burners that generate fine glass powder that becomes glass for optical fibers by turning it into transparent vitrification, and that the fine glass powder generated from the burner is sprayed and deposited. In the method of manufacturing an optical fiber preform by moving the preform in the direction of its rotation axis while rotating it to form a cylindrical fine glass powder preform, at least one of the burners is moved in the direction of its rotation axis. At least the raw material of the gas sent to the burner is moved at high speed by a predetermined distance in a diagonal direction with respect to the axis within a plane that includes the axis, and is repeatedly stopped for a predetermined period of time after the movement. A method for manufacturing an optical fiber preform, characterized in that the flow rate of gas is varied as a function of the position of the burner. (2) The method for manufacturing an optical fiber preform according to claim 1, wherein the burner is moved on a non-linear trajectory. (3) The burner is oriented in a direction substantially perpendicular to the axis at an end of its movement range away from the axis of rotation, and relative to the axis at the other end near the axis. 4
Claim 1, characterized in that the angle changes with movement so as to point in a direction at an angle smaller than 5°.
A method for manufacturing an optical fiber preform as described in Section 1. (4) The amount of heating by the burner is increased at the end of the moving range of the burner on the side away from the rotation axis, and is decreased at the other end on the side closer to the axis, so that the amount of heating by the burner is increased. The method for manufacturing an optical fiber preform according to claim 1, characterized in that the surface temperature of the glass fine powder preform is controlled. (5) The method for manufacturing an optical fiber preform according to claim 1, wherein the heat source of the burner is the combustion of fuel gas supplied to the burner. (6) The method for manufacturing an optical fiber preform according to claim 1, wherein the heat source of the burner does not generate OH groups. (7) The flow rate of the fuel gas supplied to the burner is increased at an end of the movement range of the burner on the side away from the rotation axis, and decreased at the other end on the side closer to the axis. A method for manufacturing an optical fiber preform according to claim 5. (8) Another burner for generating no glass fine powder is fixed near the rotation center axis of the fine glass powder preform, and while forming a fine glass powder preform with the burner, the outside of the fine glass powder preform is formed. 2. The method of manufacturing an optical fiber preform according to claim 1, wherein the moving burner deposits the glass fine powder. (9) A transparent glass rod prepared in advance is moved in the axial direction while being rotated around its axis, and fine glass powder is deposited by the moving burner on the outside of the transparent glass rod. 2. A method for manufacturing an optical fiber preform according to item 1.