JPS5920612B2 - Optical fiber manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an optical fiber in which a glass tube having a glass film deposited on its inner wall is heat-welded while applying external pressure.

Research on optical fibers has developed rapidly over the past few years, and we have reached the stage where successful prototypes with ultra-low losses of several dB/km are being announced one after another.

These low-loss optical fibers are mainly composed of quartz glass synthesized by a gas phase chemical reaction and a quartz glass material doped with a small amount of at least one metal oxide for controlling the refractive index. FIG. 1 shows an example of a process for manufacturing an optical fiber using a conventional gas phase chemical reaction method.

In the same figure, a glass tube 3 (usually made of quartz tube, Pycor, etc.) is attached to the glass processing plate 1 with a chuck, and while rotating it in the direction of arrow 5 (or the opposite direction), a source for forming a glass film is applied from the direction of arrow 2. is introduced, and the heating source 4 (an oxyhydrogen burner is used in the figure) is moved in the direction of arrow 7 (or T') to deposit a glass film that will become the core material of the optical fiber on the inner wall surface of the glass tube 3. It's a process.

Figure b shows a circular bar made of a two-layer (or multi-layer) glass structure made by welding a glass tube 3 on which a glass film 6 has been deposited while heating it with a heating source 4 so that the inner surfaces of the glass tube 3 are in close contact with each other. This is the process of converting the optical fiber into a parallel ohm (hereinafter abbreviated as preform) 8.

Further, FIG. 3C shows a process in which the obtained preform 8 is fed into a heating furnace 10 at a constant speed, heated and melted while being drawn, thereby forming an optical fiber 9 with a small diameter. However, as a result of theoretical and experimental studies conducted by the present inventor on the manufacturing process of the preform shown in FIG.

First, in Figure b, a model of the glass tube 3 is shown in which the flame of the oxyhydrogen burner is uniformly blown from the outer surface of the glass tube 3, which is rotating around its axis at a constant rotational angular velocity ω. The state is as shown in Fig. 2a.

That is, the glass tube 3 is heated from the outside surface to a uniform flow temperature by the blowing from the oxyhydrogen burner, while being exposed to the external pressure P.
can be uniformly applied and radially compression welded to form the preform shown in FIG. 2b. In addition, since the external pressure P acts uniformly due to the flame spray of the oxyhydrogen burner, the deformation of the glass tube is symmetrical about its axis, and
It is assumed that there is no change in the axial direction. Furthermore, it is assumed that the composition of the glass film deposited on the inner wall surface of the glass tube is almost the same as the composition of the glass tube, and the thickness of the film is considered to be included in the wall thickness of the glass tube. Assuming this, the radial stress σ, which acts on the Bd surface in Figure 2 C (7) during welding, and A
The circumferential stress σ1 acting on the d-plane and the Cd-plane is expressed by the following equation.

In equation (1), the first term on the right side of σ and σ is
If the external pressure P is sufficiently larger than P, compressive stress will act on the glass tube 3. Therefore, by increasing the external pressure P, the glass tube 3
are compression welded to form a preform 8.

In this case, the speed of forming the glass tube 3 into the preform 8 is determined by the viscosity and elastic constant of the glass, and is expressed by the following equation. Here, S: amount of deformation However, from equation (1), the circumferential stress σ, which acts on the outer diameter portion and inner diameter portion of the glass tube 3 during welding, and the radius applied σ, are expressed by the following equation.

As can be understood from equation (3), in order to make the core diameter and outer diameter of the preform into an inertia circle, it is necessary and sufficient to configure the glass tube 3 so that symmetrical external pressure P and centrifugal force act on its axis. be.

However, in such a welding method using external pressure, (
3) As shown in the formula, only the circumferential stress σ1 due to centrifugal force acts on the inner diameter of the glass tube, so if the circumferential stress σ1 due to centrifugal force becomes slightly asymmetrical, the core diameter becomes circular. There is a drawback.

According to various experiments conducted to confirm this, for slight changes in parameters such as the rotational angular velocity ω and the moving speed of the external pressure P1 oxyhydrogen burner, ×200%) becomes a perfect circle within a few percent. Despite being close to , the inertia of the core diameter is 4~
The result was that it fluctuated sensitively within a range of 30%.

The reason for this result is that in the glass film forming process shown in Figure 1a, the glass tube 3 is heated at a temperature of around 1500°C for several hours, which inevitably causes deformation and misalignment of the glass tube 3. It is.

For this reason, imperfections have already occurred in the glass tube 3 before the welding process, so even if the rotational angular velocity ω is slowed down and the circumferential stress σt is reduced, the outer diameter will approach a perfect circle. It was extremely difficult to make the core diameter close to a perfect circle. As described above, the conventional welding method using external pressure has the disadvantage of being unstable and having poor reproducibility, and furthermore, it has been impossible to obtain a core inertia of 2% or less over the entire length of the preform.

An object of the present invention is to eliminate the above-mentioned drawbacks, to manufacture a brifohm having a perfectly circular core diameter and an outer diameter, and in particular, even if the glass tube before welding is rounded, the present invention has a perfectly circular core diameter and an outer diameter. An object of the present invention is to provide a method for manufacturing an optical fiber that can manufacture a preform having a diameter.

In the present invention, after a glass film is deposited on the inner wall surface of a glass tube, one end is sealed, and a gas is introduced into the tube from the other end.
When heating and welding the optical fiber hub rib ohm while applying external pressure from outside the tube, the external pressure and internal pressure are selected so that compressive stress is applied to the molten glass tube.

Hereinafter, the present invention will be explained in detail based on Examples. In this example, the structure is such that not only external pressure from a heat source is applied, but also internal pressure is applied by supplying gas into the glass tube on which the glass film is deposited to perform heat welding. Figure 3 shows the pressure P inside the glass tube with the glass film deposited.
FIG. 3 is a model diagram of a cross section of a glass tube when heat welding is performed using an oxyhydrogen burner at a uniform pressure P while adding gas of i.
In this case, the circumferential stress σt acting on the molten glass tube
As a result of various analyzes regarding the radial stress σr, the present inventor found that it is expressed by the following equation. As can be understood from equation (4), even under conditions where both external pressure P and internal pressure Pi are applied to the glass tube, σt and σr become negative values, and compressive stress can be applied to the glass tube during welding. As a result of various analyses, the inventor found that the conditions for applying compressive stress are expressed by the following equation. Here, equation (5) means that the stress based on the external pressure and internal pressure caused by the gas is made larger than the centrifugal stress caused by the rotation of the glass tube, and equation (6) means that the stress based on the external pressure and internal pressure caused by the gas applied to the glass tube is increased. This is a condition that ensures that the stress based on is a positive value.

That is, under conditions where the stress based on the external pressure and internal pressure caused by the gas applied to the glass tube is greater than the centrifugal stress accompanying the rotation of the glass tube, the external pressure P is adjusted to satisfy equation (6).
By appropriately selecting the internal pressure P1 and the internal pressure P1, compressive stress acts on the molten glass tube, making it possible to manufacture a preform having a perfectly circular core diameter and outer diameter. Examples of the method for manufacturing an optical fiber according to the present invention will be described below.

FIG. 4 shows the relationship between the core diameter ellipticity of the BRIFORM and the spindle rotation speed based on actual measurement results.

The figure shows that the main shaft rotational speed of the glass lathe, that is, the rotational angular velocity, decreases in a direction that satisfies equations (5) and (6), and that a preform with a perfectly circular core diameter is thereby obtained. There is.

This actual measurement result is shown in Figure 5 (main shaft rotation speed is 10 to 60 rpm).
1) and the apparatus shown in FIG. 6 (when the spindle rotational speed is O). In addition, 1a and 1b in Fig. 5
Reference numeral 18, 18a and 18b in FIG. 6 are supporting devices for holding the glass tube vertically. Reference numeral 18, 18a and 18b in FIG. In addition, in FIG. 6, since the glass tube is not rotated, a ring-shaped oxyhydrogen burner is used as the heating source 4, and the glass tube 3 is
ensure that it is heated evenly. The conditions during actual measurement are described below.

First, in the apparatus shown in FIG. 1a, a quartz glass tube with an outer diameter of 141" and a wall thickness of 111" is used as the glass tube 3, and oxygen gas and oxygen gas mixed with the vapors of each component through liquid SiCl4 and POCl3, respectively, are used. 120cc/Mi each! t and 80cc/Mi! t and 400CC/Mi! Feed in the direction of arrow 2 at a flow rate of L. Next, the oxyhydrogen burner 4 is used to heat the
(Actual value measured using an optical pyrometer)
A glass film 6 made of phosphoosilicate glass with a thickness of 0 μm is formed. In this way, the glass tube 3 with the glass film forming the core material formed on the inner surface is shown in FIG. 5 or 6.
It is attached to the apparatus shown in the figure, and while oxygen gas is pressurized into the tube, it is heated with an oxyhydrogen burner 4 to weld.

In this case, oxygen gas is supplied from a cylinder 11 through a pressure reducing valve 12, its pressure is monitored with a pressure gauge 13, and its flow rate is 550 CC/7! U! The t1 pressure was 0.2 kg/d. The welding operation begins with the tip 15 of the glass tube 3 before welding.
is sealed and oxygen gas is passed through the glass tube 3 through the blow tube 14.
to be introduced within. Here, the oxyhydrogen burner 4 is moved from the vicinity of the portion 16 on the side of the sealed end 15 of the glass tube 3 slightly inside the support bases 1b, 18b by 0.61! 7' at a speed of Tm/Sec
When the oxyhydrogen burner 4 reaches the vicinity of the portion 17 of the support bases 1a, 18a on the open end side of the glass tube,
It is moved in the opposite direction of arrow 7 at a high speed of 5 m1/SeC. Next, the oxyhydrogen burner 4 was moved again in the 7' direction at a speed of 0.2 m11/Sec, and when it reached around 17, the welded part of the glass tube became a completely circular bar shape, and preform 8 was obtained. . The temperature of the portion heated by the oxyhydrogen burner 4 was 1,600 to 1,650°C. FIG. 7 shows the results of actual measurement of the relationship between the core diameter ellipticity of the preform and the amount of oxygen gas introduced into the glass tube. 4 shows the case where the internal pressure Pi is kept constant and the rotational angular velocity ω is changed, but in FIG. 7, the rotational angular velocity ω is kept constant and the amount of oxygen gas introduced into the glass tube is changed. The internal pressure Pi is changed. FIG. 7 shows the increase in internal pressure Pi, that is,
The stress based on the external pressure and internal pressure due to the gas increases, (5)
It is shown that a preform having a perfectly circular core diameter can be obtained by satisfying formula (6), and the effects of the present invention are clearly demonstrated. In the same figure, the dangerous area is where the amount of oxygen gas introduced is excessive and there is a risk that the glass tube will break. FIG. 8 shows a case where the rotational angular velocity ω is kept constant as in the case of FIG. 7, and the internal pressure Pi is varied depending on the pressure of oxygen gas introduced into the glass tube.

In this case as well, the internal pressure P1 increases, that is, equations (5) and (
It is shown that by satisfying 6), a preform having a perfectly circular core diameter can be obtained. However, in the same figure, when the oxygen gas pressure exceeds 1.5k9/d, the core diameter becomes elliptical, but this is because the values of internal pressure P1 and external pressure P are approaching each other, and internal pressure Pi expands the glass tube. This is due to the fact that I came to Nazutau. For this development, it is not enough to simply increase the internal pressure; it also means that the external pressure should be increased in accordance with the increase in internal pressure, that is, the internal pressure and external pressure should be selected so as to satisfy equation (6). be. FIG. 9 is an example showing the position of the glass tube and preform for each outer diameter ellipticity, and the relationship between the core diameter ellipticity and the preform position.

That is, Figure a shows the relationship between the outer diameter ellipticity of the glass tube before welding and the outer diameter ellipticity of the preform after welding, and the respective positions, and Figure b shows the relationship between the preform core diameter ellipticity and position. show. However, in this case, the moving speed of the oxyhydrogen burner, the amount of oxygen gas introduced into the glass tube, and the gas pressure are the same as in the case of Fig. 4, and the main shaft rotation speed is 10 r.
An example of p1 is shown. In each of the above examples, the core diameter and outer diameter ellipticity of the preform are both improved than the outer diameter ellipticity of the glass tube before welding.

This is the case when formulas (5) and (6) are satisfied,
The effectiveness of the present invention is clearly proven. On the other hand,
Figure 10a shows that under the conditions of Figure 9, only the oxygen gas pressure introduced into the glass tube is changed from 0.2k9/i to 1.9k9/i.
(This is the result when the internal pressure P1 and external pressure P are made closer to each other by changing to V7l.

In this case, there is no improvement effect as shown in FIG. 9, and on the contrary, it can be seen that both the core diameter and the outer diameter ellipticity of the preform after welding are worse than the outer diameter ellipticity of the glass tube before welding. From this point on, it is clear that the internal pressure Pl and the external pressure P should be selected not only to simply increase the internal pressure Pi but also to satisfy equation (6). The results described above are merely examples, and the present invention is not limited to these examples. As an example, the glass film formed on the inner wall surface of a quartz glass tube contains B2O3, GeO2, TiO
A quartz glass film doped with a metal oxide such as 2 may also be used. Further, the number of metal oxides may be not only one, but many types. Furthermore, the doping amount is O~25 mol%
The effects of the present invention can still be obtained even if the following changes are made. As can be seen from the above description, the present invention is to select the pressure applied inside the glass tube so as to satisfy the above equations (5) and (6) when heating and welding the glass tube with an oxyhydrogen burner. As a result, a perfectly circular preform can be obtained with good reproducibility, and in particular, an optical fur hub that has a perfectly circular core diameter and outer diameter even when using a glass tube that is oval before welding. It becomes possible to create rib ohms.

As described above, the present invention is suitable for industrial mass production and is extremely advantageous in constructing a high-quality optical fiber transmission line.

[Brief explanation of the drawing]

Figures 1 a-c are explanatory diagrams showing the process of the conventional optical fiber manufacturing method, Figures 2 a-c are welding model diagrams for analyzing the conventional welding process, and Figure 3 is the welding process of the present invention. Fig. 4, Fig. 7, Fig. 8, Fig. 9, and Fig. 10 are characteristic diagrams experimentally showing the effects of the present invention. FIG. 3 is an explanatory diagram showing the configuration of each example of a welding device used to experimentally demonstrate the effects of the present invention. 3: glass tube, 4: heating source, 6: glass membrane, 8: preform, 9: optical fiber, 10: heating furnace.

Claims (1)

[Scope of Claims] 1. After depositing a glass film on the inner wall surface of a glass tube, one end is sealed, gas is introduced into the tube from the other end, and external pressure is applied from outside the tube while heat welding is performed to form an optical fiber preform. Under the condition that the compressive stress based on the external pressure and internal pressure applied to the glass tube is larger than the centrifugal stress accompanying the rotation of the glass tube,
)/(r_2^2-r_1^2)r_1^2(1-[2
_2r^2/r^2]) Here, P: External pressure applied to the glass tube Pi: Internal pressure of the glass tube 2r_1: Inner diameter of the glass tube 2r_2: Outer diameter of the glass tube r: Distance in the radial direction from the central axis of the glass tube A method for manufacturing an optical fiber, characterized in that an external pressure P and an internal pressure Pi are selected so as to satisfy the following.