JPS60122738A - Manufacture of preform for optical fiber - Google Patents

Abstract

PURPOSE:To prevent the expanding and bursting of a reaction tube at the position by keeping the internal pressure of the reaction tube in the vicinity of the position on the supply sie of a glass material where the forward trip of a burner is started higher than the external air pressure and lower than the internal pressure of the other part in an internal CVD method. CONSTITUTION:A reaction tube 2 is rotated, and a glass material is supplied from one end. A burner 5 is travelled rightward, and high-pressure air is supplied into a control chamber 3 by a fan 13 to keep the internal pressure of the chamber 3 at P1 which is necessary for keeping the equilibrium with the shrinking force of the reaction tube 2. when the burner 5 reaches the terminal of the forward trip and strikes against a limit switch 6b, the opening degree of a motor valve 14 is decreased and the flow rate of the high-pressure air is reduced to keep the internal pressure of the control chamber 3 at P2 which is lower than the pressure P1 and slightly higher than the external air pressure. The pressure P2 is kept until the burner 5 collides against a limit switch 6a on the other side and changes its direction and a specified time elapses. The pressure P1 is increased to P2 after a specified time.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for manufacturing an optical fiber preform by an internal CVD method.

(Prior art) In the internally sealed CVD method (MCVD method), the inner surface of the quartz tube is heated by reciprocating a burner along the longitudinal direction of the quartz tube while supplying a vapor phase glass raw material from one end of the quartz tube. A preform is obtained by collapsing the hollow quartz pipe obtained in this way.

By the way, in the above MCVD method, since the quartz tube normally contracts when heated by a burner, thermal plastic deformation of the quartz tube is prevented by maintaining the internal pressure of the quartz tube higher than the external pressure.

However, in such a case, the following problems arise.

In other words, the temperature of the gaseous glass raw material supplied from one end of the quartz tube increases as it moves towards the other end due to the heat of the burner. The temperature inside the quartz tube does not rise rapidly at any point.

However, at one end of the quartz tube, which is the supply side of the glass raw material, the temperature of the glass raw material is still low, so when the burner returns and starts heating to deposit the next layer, the glass raw material inside the quartz tube is It is rapidly heated and expands, and as a result, the internal pressure locally increases near the start point of the burner's reciprocal movement, and the equilibrium between the contraction force of the quartz tube and the internal pressure shifts in the direction of the internal pressure. As a result, the quartz tube swells locally, and as this happens repeatedly, the quartz tube eventually breaks.

(Objective of the Invention) An object of the present invention is to prevent expansion and rupture of the quartz tube at the starting point of forward movement of the burner while suppressing thermal plastic deformation of the quartz tube.

(Structure of the Invention) The present invention provides heating by reciprocating a burner along the longitudinal direction of the reaction tube while supplying glass raw material in one gas phase into the reaction tube, thereby forming a glass layer on the inner circumferential wall of the reaction tube. In a method for producing an optical fiber preform by a depositing CVD method, the internal pressure of the reaction tube is slightly lower than the external pressure near the starting point of the reciprocation of the core on the side where the glass raw material is supplied. By setting the internal pressure high and the internal pressure at other locations low, the increase in internal pressure caused by the expansion of the glass raw material in the gas phase is offset, and the expansion and rupture of the reaction tube at the relevant locations are prevented. be.

(Example) The present invention will be explained below with reference to the examples shown in the drawings. Figure 1 (tag, tb) compares the setting conditions of the internal pressure of the reaction tube between the conventional example and the method according to the present invention. , the vertical axis shows the internal pressure Pl of the reaction tube, and the horizontal axis shows the time required for the reciprocation of the burner.

As shown in the figure (aJ), in the conventional example, the entire length of the reaction tube was set to a value higher than the external pressure (normally equal to atmospheric pressure, and in the figure, the external pressure is set to zero) by Pl.

Therefore, for the above-mentioned reason, as shown by the solid line in Fig. 2, near the forward movement start point A of one burner, the outer diameter of the reaction tube is d,
The outer diameter d showed a tendency to gradually increase as the number of layers of glass accumulated, n, until it finally burst.

Therefore, as shown in FIG. 1b3K, in the present invention, the internal pressure of the reaction tube is changed in two stages.

That is, in the area where the gas phase glass raw material reaches an appropriate temperature and the hydrolysis reaction is occurring steadily, the internal pressure is set to P1 necessary to maintain equilibrium with the contraction force of the reaction tube, and the forward movement of the burner is Near the starting point, P2 is set to be smaller than P1 and slightly larger than the outside pressure.

The result of this setting is shown by the dotted line in FIG. 2, where d2 indicates the outer diameter of the reaction tube.

Usually, the point where the outer diameter expands the most is the point where the burner returns and starts heating again, and the temperature begins to rise due to heating and reaches a predetermined reaction temperature, and the time to reach that point is: It depends on the heating conditions such as the dimensions of the reaction tube, the burner speed and the firepower of the burner, and the value of P2 and the transition time from P2 to P also depend on these conditions.

In FIG. 2, the vertical axis indicates the outer diameter of the reaction tube, the horizontal axis indicates the longitudinal direction of the reaction tube, and B is the start point of the backward movement (end point of the forward movement) of the burner.

Next, the apparatus used in the above method will be explained.

As shown in FIG. 3, both ends of a quartz reaction tube 2 are rotatably mounted on the glass lathe 1, and glass raw material in a vapor phase is supplied from one end of the reaction tube 2. The other end is attached to the control chamber 3 via a rotary lever 4 to keep the control chamber 3 and the reaction tube 2 in communication.

Further, a heating burner 6 is installed in the reaction tube 2 so as to be freely movable along the hand direction, and limit switches 6 a and 6 b are installed at both ends of the travel range of the burner 6 , and the switches are connected to the arithmetic circuit 7 . do.

A measuring device 8 for measuring the internal pressure is connected to the control room 3, and the measured value is input to an operator 9.

An external setting device 10 is also connected to the operator 9, and a signal from the operator 9 is sent to a drive circuit 11 connected to the external setting device 10.
motor M1 according to the signal from the same circuit 11.
is activated to adjust the opening degree of the electric valve 112.

The control room 3 further includes a fan 13 for supplying high-pressure air.
are connected via the electric valve 14, and the electric valve 1
114 is driven by a motor M2 that operates in response to a signal from the arithmetic circuit 7.

Further, the signal from the arithmetic circuit 7 is also supplied to the drive circuit 11.

Next, the operation of the device having the above configuration will be described.

While the reaction tube 2 is rotated and glass raw material is supplied from one end, the burner 6 is run rightward in the drawing, and the fan 13 is operated to supply high pressure air into the control chamber 3.

The pressure inside the control room 3 is measured by a measuring device 8, and the pressure inside the control room 3 is measured by a measuring device 8.
The opening degree of the electric valve 112 is controlled so that the pressure inside is maintained at the above-mentioned P, and the gas in the control chamber 3 is exhausted from the exhaust port 15 according to this opening degree.

Note that the value of P is specified by the external setting device 10.

Further, the flow rate from the fan 13 is controlled by an electric valve [14].

When the burner 6 reaches the end of its forward path and collides with one of the limit switches 6b, the switch 6b operates and issues a signal. This signal is input to the arithmetic circuit 7, and the electric valve
14 becomes smaller in opening and the flow rate decreases, while the electric valve 112 is fixed at a predetermined opening, and the control chamber 3
The pressure inside is maintained at %P2.

This pressure P2 is applied to the burner 6 by the other limit switch 6.
After colliding with point a and changing direction, it is held until a predetermined time has elapsed, and after the predetermined time, the pressure is increased to P1.

In other words, the opening degree of the electric valve U14 is increased to increase the flow rate, and at the same time, the internal pressure control of the control chamber 3 is activated.

The switching from P and P to P is performed by a timer.

Here are some more specific examples: OD 24,
Using a reaction tube with an inner diameter of 21 mm, Pl = 20 m + H20,
P2=5■H20, burner running speed 200 Mn/M
When a glass layer was deposited on the inner circumferential wall of the reaction tube at a heating temperature of 1800°C, the outer diameter changed only slightly by 24±0.
．． It was 2 tan.

(Effects) As is clear from the above, the present invention reduces the internal pressure at one end of the reaction tube to which the gaseous glass raw material is supplied, that is, near the starting point of the forward movement of the burner, to that at the point where the reaction occurs steadily. , so even if the gas-phase raw material near the forward movement start point expands rapidly and the internal pressure increases, this increase will be offset by the decrease in the set internal pressure. This prevents the tube from expanding and bursting.

[Brief explanation of the drawing]

Figure 1 La) is a graph showing the relationship between the conventional burner position and the internal pressure of the reaction tube, Figure tbl is a graph showing the relationship between the burner position and internal pressure in the present invention, and Figure 2 is a graph showing the relationship between the burner position and the internal pressure in the conventional example. A graph comparing the variation of the outer diameter of the reaction tube with that of the present invention, FIG. 3 is a schematic diagram of the apparatus according to the present invention. 2...Reaction tube 5 ＠ll+l＠＠Barna Patent applicant representative Patent attorney Makoto Ifuji

Claims (1)

[Claims] An inner-enclosed CV in which glass JZ is deposited on the inner circumferential wall of the reaction tube by heating it by reciprocating a burner along the longitudinal direction of the reaction tube while supplying glass raw materials in a gas phase into the reaction tube.
In the method for manufacturing an optical fiber preform by method D, the internal pressure of the reaction tube is set to be slightly higher than the external pressure near the reciprocating start point of the burner on the side where the glass raw material is supplied, and at a point other than that point. A method for using a preform for optical fiber, which is characterized by setting the internal pressure to a low level.