JPH0352418B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an optical fiber preform manufacturing apparatus.

The so-called internal CVD method is known as a typical manufacturing method for optical fiber base materials.

Although the above method can produce a low-loss optical fiber, it also has the following drawbacks.

In other words, when forming a glass layer on the inner peripheral surface of a reaction tube made of quartz tube or the like, the tube is heated to a high temperature of, for example, 1,400 to 1,600 degrees Celsius, so it softens and gradually contracts due to its own surface tension. Eventually, it became difficult to form a homogeneous glass layer on the inner surface, and the thickness of the glass layer that could be formed was limited, and the length of the optical fiber obtained from one preform was about 1 km. Yes, we cannot hope for a longer optical fiber.

This tendency is especially noticeable when forming pure silica glass (SiO ₂ ), which requires high temperatures. Also, since the quartz tube is heated to high temperatures during glass formation, deformations such as ovalization are likely to occur in the cross section of the quartz tube, and the resulting preform As a result, the cross-sectional dimensional accuracy at the optical fiber stage is reduced.

In order to deal with the above problem, a proposal has already been made to control the diameter of the quartz tube at a constant level by maintaining the internal pressure of the quartz tube higher than its external pressure during glass formation. In order to generate an internal pressure that is balanced with the contraction force, an internal pressure adjusting section is provided at one end of the tube, and the internal pressure of the quartz tube is adjusted according to the measured value of the outside diameter of the quartz tube.

However, in this case, since the internal pressure of the quartz tube is controlled based on an indirect measured value of the outer diameter of the quartz tube, there is a problem in that the control is delayed, that is, the responsiveness is poor. In addition, if the variation in the outer diameter with respect to the variation in the internal pressure is small, the adjustment range of the internal pressure becomes large, resulting in repeated internal pressure adjustments, resulting in the outer diameter value being repeatedly set too large or too small. There was even a fear that things would go out of control.

The present invention has been made to address the above-mentioned problems, and the configuration thereof will be explained below with reference to the illustrated embodiment. One end of the reaction tube 1 is connected to a glass raw material supply section (not shown) via a pipe 3, and from this supply section, a glass raw material in a gas phase, such as silicon tetrachloride (main raw material), along with oxygen, is supplied into the reaction tube 1. Germanium tetrachloride (dope raw material) etc. are being introduced.

A heating section 4 consisting of a burner for heating the reaction tube is movable along the longitudinal direction of the reaction tube 1.
This heating section 4 is supplied with oxygen and hydrogen.

A control chamber 6 is connected to the other end of the reaction tube 1 via a rotary seal 5.
There is a pressure measuring device 7, a high pressure gas inlet 8 and an exhaust port 9.
A supply source (not shown) is connected to the high-pressure gas inlet 8 via a high-pressure gas introducer 10, and high-pressure gas is introduced into the inlet 8 from the supply source.

On the other hand, an electric valve 11 is connected to the exhaust port 9, and the orifice of this electric valve 11 is controlled to open and close by a motor 12. Therefore, the internal pressure of the control chamber 6 is made higher than the external pressure. The motor 12 makes the orifice smaller so that the temperature can be maintained, but at this time, if only the gas phase glass raw material is supplied to the reaction tube 1, the orifice diameter must be made extremely small, which is difficult to control. The orifice diameter can be increased by introducing high-pressure gas through the high-pressure gas inlet 8, since it will be immediately blocked by a large amount of glass fine powder such as SiO ₂ GiO ₂ present in the exhaust gas. It's becoming like that.

A calculation section 14 is connected to the motor 12 of the electric valve 11 via a drive section 13, and an external setting device 15 and the pressure measuring device 7 are connected to the input side of the calculation section 14.

Note that the control room 6 also serves as a collector for the glass fine powder.

In the present invention, as is known, the reaction tube 1 is connected to the rotating section 2.
While rotating the reaction tube 1, a gaseous glass raw material and oxygen were passed through the pipe 3 and ignited from the outside in the heating section 4, so that the glass raw material and oxygen reacted and generated. It is applied to the CVD method in which fine glass powder is attached to the reaction tube 1.

Here, the operation of the above device will be described. During glass deposition in the reaction tube 1, the pressure measured by the pressure measuring device 7 and the set pressure of the external setting device 15 are compared in the calculation unit 14, and if there is a difference between them, At this time, a control signal is sent to the drive unit 13 of the electric valve 11, and the motor 12 of the electric valve 11 is rotationally controlled so that the internal pressure of the reaction tube 1 is maintained at the set pressure.

That is, when the set pressure is higher than the measured pressure, the motor 12 is operated to close the electric valve 11 by the output of the drive unit 13, and conversely, when the measured pressure is higher than the set pressure, the motor 12 is operated to close the electric valve 11. Motor 12 to open
is operated to control the internal pressure of the reaction tube 1 to be constant.

Example A quartz tube with an outer diameter of 24 mm and an inner diameter of 21 mm was used as the reaction tube 1, and an optical fiber base material having 90 glass deposited layers was manufactured at a surface temperature of 1800°C.

When the internal pressure was the same as the external pressure, the outer diameter contracted to 20 mm, whereas when the internal pressure was a positive pressure of 20±1 mmH ₂ O, the outer diameter remained within ±0.2 mm.

As explained above, the present invention includes a rotating part that rotatably holds a reaction tube, a raw material supply part that supplies a vapor phase glass raw material into the inside of the reaction tube from one end of the reaction tube,
In an optical fiber preform manufacturing apparatus comprising a heating section for heating the reaction tube, a control chamber having a pressure measuring device, a high-pressure gas inlet, and an exhaust port is provided at the other end of the reaction tube; is provided with a control valve, and a control system for controlling the control valve based on the measured value from the pressure measuring device is connected to the control valve to maintain the internal pressure of the reaction tube at a substantially constant positive pressure. It constitutes a means for doing so.

Therefore, in the case of the present invention, the internal pressure of the reaction tube can be maintained at a substantially constant positive pressure through the above-mentioned means, and an optical fiber preform can be produced without fluctuations in the tube diameter.

Moreover, in the case of the present invention, unlike the conventional indirect feedback control in which the tube diameter is held constant by combining an outer diameter measuring means and a means for adjusting the inner pressure of the reaction tube, the inner pressure of the reaction tube is directly measured. Since the internal pressure is controlled to a positive pressure based on the value, the control response time can be shortened and control can be performed with high precision. As a result, it has become possible to obtain a high quality optical fiber base material.

[Brief explanation of drawings]

The drawing is an explanatory diagram showing an embodiment of the optical fiber preform manufacturing apparatus according to the present invention. 1... Reaction tube, 2... Rotating section, 4... Heating section,
6...Control room, 7...Pressure measuring device, 8...High pressure gas inlet, 9...Exhaust port, 11...Electric valve for control valve.

Claims (1)

[Claims] 1. An optical fiber motherboard comprising a rotating part that rotatably supports a reaction tube, a raw material supply part that supplies a vapor phase glass raw material from one end of the reaction tube to the inside of the reaction tube, and a heating part that heats the reaction tube. In the material manufacturing apparatus, a control chamber having a pressure measuring device, a high-pressure gas inlet, and an exhaust port is provided at the other end of the reaction tube, and the exhaust port is equipped with a control valve, and the measured value from the pressure measuring device is provided. manufacturing an optical fiber base material comprising means for maintaining the internal pressure of the reaction tube at a substantially constant positive pressure by connecting a control system to the control valve to control the control valve based on Device.