JPS621332B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A conventional method for manufacturing an optical fiber preform is shown in FIG. This is a flame hydrolysis burner 1 that generates a flame 5 containing fine glass particles, and a target 4 that moves in the direction of arrow 7 while rotating in the direction of arrow 8.
to grow a rod-shaped porous glass base material 2. This method is followed by sintering with a heat source 3 to form a transparent optical fiber base material. In order to suppress fluctuations in the refractive index distribution and outer diameter of the optical fiber base material, the pressure inside the reaction vessel 6 is detected by the pressure detection device 15, and the output signal is fed back to the valve opening/closing device 16 through the control section 18. The structure is such that the displacement is controlled by However, as a result of examining this method, the following problems were found.

(1) A butterfly valve is used as the valve opening/closing device 16, but since the exhaust gas 14 contains many glass particles, they tend to adhere to the valve and change the opening/closing amount.

(2) The butterfly valve is made of stainless steel because the exhaust gas contains corrosive gases such as chlorine gas and is high temperature (over 100℃). However, even those made of stainless steel are susceptible to corrosion if used for a long time. Also, since it is made of stainless steel, the weight of the valve is heavy, the effect of inertia occurs, and the response of the valve to pressure fluctuations is poor.

(3) The operation of the valve opening/closing device is likely to deteriorate due to the high temperature of the exhaust gas. In particular, when a motor is used to drive the opening and closing of a valve, a cooling device must be used in the motor drive section, making the device extremely complex.

The object of the present invention is to provide a method that solves the problems of the above-mentioned conventional methods. That is, a valve opening/closing device 1 that adjusts and controls the pressure inside the reaction container.
In this method, instead of 6, a gas supply pipe is provided through which a bias gas flows along the flow direction of the exhaust gas, and the pressure inside the reaction vessel is controlled by adjusting the gas flow rate.

The method of the present invention will be explained below using the drawings.

FIG. 2 shows an embodiment of the method for manufacturing an optical fiber base material of the present invention. The gas supply pipe 20 is provided so that the gas flows along the exhaust gas 14 flowing in the exhaust pipe 13, and the gas flows from the direction of the arrow 21 through the gas supply adjustment device 19 in the direction of the arrow 21' and merges with the exhaust gas 14. be. arrow 2
The gas flow rate that is supplied or sucked from one direction is set to be enough to sufficiently control pressure fluctuations within the reaction vessel 6. This gas flow rate is determined by the flow rate of the exhaust gas 14, the cross-sectional area of the exhaust pipe 13, and the exhaust device 1.
Although it depends on the pumping speed of No. 7 and the reference pressure setting value predetermined by the control unit 18, it is usually selected from a range of several tens of minutes per minute to several tens of minutes per minute.
The type of gas may be an inert gas (N ₂ , Ar, He, Ne, etc.), an oxidizing gas, air, or a mixture of the above gases. Then, the pressure detection device 15 detects the pressure in the reaction vessel 6.
The internal pressure P _i is detected and compared with the reference pressure set value P _r set in advance by the control unit 18. If P _i ≠ P _r , an output signal is generated at the output end of the control unit. This is fed back to the gas supply adjustment device 19. As a result, the gas flow rate flowing in the direction of arrow 21' is controlled so that P _i =P _r .
Here, when P _i >P _r , the flow rate of gas flowing in the direction of arrow 21' is increased; conversely, when P _i <P _r
In this case, the control unit 18 and the gas supply adjustment device 1 are operated so as to reduce the flow rate of gas flowing in the direction of the arrow 21'.
9 is configured. As is clear from the above explanation, the method of the present invention is not a method in which a valve opening/closing device is provided in the passage through which the exhaust gas 14 flows and the amount of opening/closing thereof is controlled in accordance with pressure fluctuations.
Problems like (2) and (3) do not occur at all. Since the gas supply regulator 19 is neither corroded by the exhaust gas 14 nor heated to high temperatures, a valve opening/closing device with a quick response can be used, and a motor (such as a DC motor or a pulse motor) can be used. The valve can be opened and closed using a drive device such as Furthermore, a butterfly valve may be used to control the flow rate of air taken in from the direction of arrow 21 to the direction of arrow 21'.

FIG. 3 shows another embodiment of the invention. The pressure detection device 15 detects the pressure inside the reaction vessel 6, and the detection signal is fed back to the gas supply adjustment device 19 through the control unit 18. In this method, the pressure is controlled by adjusting the gas flow in the direction indicated by 21'. 22,2
2', 22'', 22 are cylindrical protective tubes (made of glass) for protecting the flame of the burner, and gas flows inside each protective tube as shown by arrows 21', 23, 23', 23'', 23. be. In this embodiment, in order to keep the flame as undisturbed as possible, arrow 21'
Pressure control is performed by feedback to the gas flow rate in the direction of arrows 23, 23', 2.
Control is possible by feeding back to either 3'' or 23.

Next, specific examples of the present invention will be described.

In FIG. 3, a pneumatically operated micro differential pressure detector (manufactured by Brandt, USA, product name: Pi Valve, differential pressure measurement range 0 to 2.5 mmAq) was used as the pressure detector 15 in FIG. This is the supply air (1.4Kg/cm ² ±0.14Kg/cm ² )
It was set to operate at 2.5 mmAq and generate an output of 1 V as an air-electrical conversion output. A circuit shown in FIG. 4 was used as the control section 18. In this circuit, V _i is the output signal of the pressure detection device, _{V r} is the voltage corresponding to the reference setting differential pressure, and V _p is the output voltage of _the control section. It is configured to produce positive and negative outputs. For No. 19, a valve opening/closing device in which a motor was directly connected to the valve opening/closing shaft was used. Then, due to the positive and negative voltages of the output signal V _p of the control section, the arrow 2
The flow rate of gas flowing in the 1' direction is increased or decreased. The exhaust system 17 has an exhaust speed of 140/min.
It was evacuated using a pump that Protection tube 2
2' to 22 are set to flow from several/min to 10 several/min (gas is O ₂ ), and in the direction of arrow 21', gas (N ₂ gas) is initially flowed at 15/min, and V _r =
Glass porous base material 2 was deposited so that the voltage was 0.8V. However, the reaction vessel used was a Pyrex glass tube with an outer diameter of 178 mmφ and a length of 600 mm. Then, when V _i > V _r , the flow rate of gas flowing in the direction of arrow 21' is increased, and conversely, when V _i < V _r , the flow rate of gas flowing in the direction of arrow 21' is reduced. Deposition was carried out. As a result, it has become clear that the problems of the conventional method can be solved and pressure fluctuations can be controlled with extremely fast response (2 to 3 seconds). For a deposition time of about 5 hours, a porous base material having an outer diameter of 75 mmφ, a length of 450 mm, and a deposited weight of 142 g was obtained. The outer diameter variation of the porous base material is less than 1%, and the differential pressure can be controlled to less than ±2%, making it possible to reduce the outer diameter variation and differential pressure variation to less than 1/2 compared to conventional methods. Ta.

Next, another embodiment of the present invention will be described.

FIG. 5 shows a case where the pressure detector 15 is provided within the reaction vessel 6 or in the immediate vicinity of the reaction vessel. The diameter of the reaction vessel 6 is 178 mm, the length is 500 mm, and the exhaust pipe 1
The diameter of 3 is 50mm. A butterfly valve 28 roughly adjusts the pressure inside the reaction vessel 6, and is set by a DC motor 27 and a potentiometer 26. 2
Reference numeral 9 denotes a buffer tank that suppresses fluctuations in the exhaust speed of the exhaust device 17. 24 is an amplifier that amplifies the output voltage of the pressure transducer to a desired value, and 25 is a preset reaction vessel internal pressure setting circuit. Next, the embodiment shown in FIG. 5 will be described. FIG. 6 shows the relationship between the flow rate of the internal pressure control gas flowing in the direction of the arrow 21 and the internal pressure inside the reaction vessel 6. Here, the internal pressure is a pressure difference between the atmospheric pressure and the atmospheric pressure, and is measured using an ordinary strain gauge type pressure transducer 15. In this embodiment, _N2 gas was used as the internal pressure control gas flowing in the direction of arrow 21 (air, Ar, _O2 , etc. may also be used). The results shown in Figure 6 show that the internal pressure in the reaction vessel was adjusted to 0.5 mm when the gas flow rate for internal pressure control was N ₂ = 15/min.
Aq, 1.0 mmAq, and 1.5 mmAq were set using the butterfly valve 28, and the internal pressure measurement results were obtained when the internal pressure control gas flow rate was changed from 0 to 30/min under these conditions. This shows that the internal pressure can be controlled with good linearity by controlling the internal pressure control gas flow rate. Next, the internal pressure of 0.5 is applied to the output of the amplifier 24.
Make sure that the output is 1V for mmAq, and V _r = -
The control circuit shown in Fig. 4 was used with an internal pressure setting of 1V (ie, an internal pressure setting value of 0.5 mmAq). A DC motor was connected to the output of the control circuit. The shaft of this DC motor is directly connected to the valve opening/closing part of the gas flow rate regulator 19.
The flow rate of gas flowing in the direction of arrow 21' is increased or decreased by positive or negative voltages applied to the DC motor. Porous glass base material 2 was deposited for about 7 hours using this equipment configuration, but the fluctuations in internal pressure were
It was below 0.5mmAq±0.01mmAq.

The invention is not limited to the above embodiments. For example, an internal pressure control gas may be forcibly introduced into the gas pipe 30 and the gas flow rate may be controlled at 19, or air in the atmosphere may be forced into the gas pipe 30 by the exhaust device 17 and the gas flow rate may be controlled at 19. Let it be sucked in from
The sucked flow rate may be controlled by 19. The regulating gas flowing along the exhaust gas is 1
By flowing gas from multiple locations instead of from one location, using only one location for adjustment and feedback, and using the other as bias gas, the ratio between the supply gas flow rate and exhaust speed can be varied over a wide range, resulting in a disturbance suppression effect. can be raised. Alternatively, a method combining the method shown in FIG. 2 and the method shown in FIG. 3 may be used.

[Brief explanation of the drawing]

FIG. 1 is a schematic sectional view illustrating an apparatus used in a conventional optical fiber preform manufacturing method, and FIGS. 2 and 3 are schematic sectional views illustrating an optical fiber preform manufacturing apparatus used in an embodiment of the present invention. 4 is a circuit diagram showing the control circuit of the optical fiber preform manufacturing apparatus used in an embodiment of the present invention, and FIG. FIG. 6, which is a schematic cross-sectional view for explaining the manufacturing apparatus, is a graph showing the relationship between the internal pressure of the reaction vessel and the flow rate of the internal pressure control gas. DESCRIPTION OF SYMBOLS 1... Flame hydrolysis burner, 2... Porous glass base material, 3... Heat source, 4... Target, 5...
... Flame, 13 ... Exhaust pipe, 15 ... Pressure detection device, 16 ... Valve opening/closing device, 17 ... Exhaust device, 18 ... Control section, 19 ... Gas supply adjustment device, 20 ... Gas supply pipe.

Claims (1)

[Claims] 1. A starting member is placed in a reaction vessel having an exhaust port connected to an exhaust device, glass fine particles synthesized by a flame hydrolysis burner are sprayed onto the starting member, and the axial direction of the starting member is In the method for growing a porous glass base material, a bias gas is forcibly supplied between the exhaust port and the exhaust device along the flow direction of the exhaust gas flowing in the exhaust pipe, and the pressure in the reaction vessel is increased. A method for manufacturing an optical fiber base material, characterized in that the flow rate of the bias gas is adjusted according to fluctuations. 2. The method of manufacturing an optical fiber preform according to claim 1, wherein the pressure fluctuation is detected by a pressure detector provided between the exhaust port and the exhaust device. 3. The method of manufacturing an optical fiber preform according to claim 1, wherein the pressure fluctuation is detected by a pressure detector provided inside the reaction vessel.