JPS621333B2 - - 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) It is sensitive to fluctuations in the pumping speed of the pumping device 17.

(2) Since the exhaust gas contains corrosive gases such as chlorine gas and is high temperature, the valve opening/closing device 1
6 (usually a butterfly valve is used) is easily corroded, resulting in deterioration of equipment performance.

(3) Since this method controls the pressure by changing the cross-sectional area of the exhaust pipe, glass particles contained in the exhaust gas tend to adhere to the valve opening/closing device and cause the pressure to fluctuate.

(4) When using a butterfly valve, it is difficult to vary the pressure linearly over a wide range.

In order to solve the above problems, the present inventor developed a method of controlling the pressure inside the reaction vessel by first flowing a bias gas along the flow direction of exhaust gas and adjusting the gas flow rate. We made a proposal and filed a patent application. (See Figures 2 and 3). Since this method does not include a valve opening/closing device for controlling the flow rate of exhaust gas on the exhaust pipe side, the problems of the conventional method do not occur. In addition, by operating the gas supply adjustment device 19,
The pressure inside the reaction vessel 6 can be varied linearly over a wide range, resulting in good controllability. An example is shown in FIG. This is an internal pressure control gas F _C (in this case, N ₂ was used, but
Ar, He, O ₂ , air, etc. may also be used. ) and the pressure ΔP in the reaction vessel 6 (this is the differential pressure value from atmospheric pressure). This result makes F _C 20
/min, and set ΔP to -0.5, -1, -, respectively.
∆P was measured while changing F _C with the exhaust device 17 set to 1.5 and -2 mmAq (marked with a cross in the figure). However, in this method, the input supply gas flow rate F _I into the reaction vessel 6
(This includes the gas flow rate injected from the burner 1, the gas flow rate flowing in the direction of the arrow 11', and the flame protection gas flowing along the outer circumference of the burner, which is not shown in FIG. 2). Since ΔP changes sensitively, it is necessary to control _FI with high precision in order to prevent ΔP from fluctuating.
FIG. 5 shows an example of the relationship between _FI and ΔP. This keeps F _C at 20/min, initial values of F _I are 10/min and 30/min, respectively, and ΔP is -0.5.
ΔP was measured by changing F _I from a state in which the exhaust device 17 was set to -1 mmAq (≦ mark in the figure). In this way, ΔP tends to change with a slight change in _FI . Therefore, it is highly desirable to have a method in which ΔP does not respond sensitively to even slight changes in F _I .

The present invention proposes a method in which ΔP is less likely to respond sensitively to changes in the input supply gas flow rate into a reaction vessel, and a method in which pressure is controlled with good linearity with respect to the operation of a pressure regulating device. be. In other words, an air intake amount adjustment mechanism that sucks air from the atmosphere into the exhaust pipe on the exhaust side and applies a bias to the exhaust gas is provided to suck in the desired air, and the flow direction of the exhaust gas or the inside of the reaction vessel is adjusted. In this method, a gas supply regulating device is provided that has the function of supplying and regulating gas along the flow direction of the gas supplied to the reactor, and the pressure inside the reaction vessel is controlled by operating the regulating device.

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

FIG. 6 shows a schematic diagram of the internal pressure control method of the present invention.
It shows two internal pressure control methods. One method is to detect the internal pressure in the reaction vessel 6 with the pressure detector 15 and feed back this detection signal to the gas supply adjustment device 19 through the control section 18. In this case, arrows 21″, 21, 23, 2
The gas flow rate sent in the directions 3' and 23'' was constant. 24 is a hole for sucking air from the atmosphere into the exhaust pipe 13, and arrows 25, 25', 2
5'' indicates the flow direction of air sucked into the exhaust pipe 13 through the hole. Arrow 14
indicates the flow direction of exhaust gas, and arrow 26 indicates the flow direction of gas in which the intake air is biased against the exhaust gas. The larger the size and number of the air suction holes 24, and the larger the number, the larger the amount of air suction, and the easier it is to alleviate pressure fluctuations in response to fluctuations in the input supply gas flow rate _FI . The shape of the hole may be circular, square, rectangular, etc., or it may be slit-shaped or may have a number of branched tubes. Further, this hole may be located closer to the exhaust device, or may be provided in the gas supply pipe 20. Furthermore, if it is provided throughout the exhaust pipe 13, it is effective to suppress the adhesion of glass particles to the inner wall of the exhaust pipe. Next, a specific example of this method will be described.

FIGS. 7 and 8 show characteristics of specific embodiments.
First of all, in Figure 7, O ₂ gas is pumped in the direction of arrow 21 at 10%
internal pressure control gas _F
( _N2 gas) is kept at 20/min and ΔP is -
The characteristics are obtained by setting the exhaust device 17 to 0.5 mmAq (marked with a cross in the figure), and then changing _FC and measuring ΔP. The hole provided in the exhaust pipe 13 (in this case, the diameter was 5 mm) was used as a parameter. As the number of holes increases, F _C −
The slope of the ΔP curve is gradual. The variation of ΔP during deposition of actual porous glass matrix is up to 2
Since it is about 30%, it is possible to control with good linearity even when the number of holes is 24 in the case shown in FIG. 7 above. Next, the results shown in Figure 8 show that the gas F _I (in this case O ₂ gas was used) flowing in the direction of arrow 21 is 10
/min, and maintain a constant flow rate (number/min to 10 number/min) in the directions of arrows 23, 23', and 23''.
Set the exhaust device 17 so that ΔP is -0.5 mmAq (marked with – in the figure) while flowing FC (minimum min) and keeping F _C at 20/min, then change _F This is a measured characteristic. and exhaust pipe 13
The hole (in this case, the diameter was 5 mm) was used as a parameter. This is a desirable result in that as the number of holes increases, the amount of change in ΔP becomes smaller with respect to the change in _FI . That is, F _I
The effect of the present invention is that ΔP becomes less sensitive to small changes in ΔP.

The results shown in Figure 9 are for the case where a hole (in this case, the diameter was 3.8 mm) for inhaling air from the atmosphere is not provided on the side of the gas supply pipe 20, and when it is provided (the number of holes is 9 mm).
f) shows the F _I -ΔP characteristic curve.
In this case as well, similar to the results shown in FIG. 8, it is shown that the amount of change in ΔP with respect to the change in _FI can be reduced by providing the holes.

Next, another internal pressure control method shown in FIG. 6 will be described. This is a method in which the pressure detector 15 detects the internal pressure in the reaction vessel 6, and this detection signal is fed back through the control section 18 to the input supply gas regulating device 19'. However, in this case, arrows 23, 2
The flow rate of gas sent in the directions 3' and 23'' is constant.A constant flow rate of gas may be sent in the direction of the arrow 21, or air from the atmosphere may be sucked in as in the hole 24. Therefore, 19 has a function as a supply gas flow rate adjustment device or an air suction amount adjustment device.By operating the input supply gas adjustment device 19' to adjust the gas flow rate in the direction of arrow 21, Δ
It is clear from the results shown in FIGS. 5, 8, and 9 that P can be controlled. A feature of this method is that ΔP does not vary greatly with variations in the exhaust speed of the exhaust device 17. FIG. 10 is an example. This is the result of providing a butterfly valve in the exhaust pipe on the exhaust device 17 side to disturb the exhaust volume, and measuring how much ΔP changes by rotating the valve. It can be seen that ΔP does not change much. That is, it is stable against disturbances and has good controllability.

Next, FIG. 11 shows an example of the control section used in the example. Vi is the detection signal of the pressure detector, Vr is the voltage corresponding to the reference setting value of ΔP (in this case, -1 mmAq
It was set to 0.1V. ), Vo is the output signal. This Vo
is the DC of the gas flow regulator at 19 or 19'.
This is the motor drive voltage. 19 or 19' is a flow meter with a needle valve with a DC motor directly connected to the valve shaft, or a rotating disk shaft.
A configuration in which a DC motor was directly connected was used. The pressure detector used is a strong gauge type (with amplifier) that can measure up to ±10 mmAq, and is designed to generate a maximum output of ±1 V. The reaction vessel used was one with an outer diameter of 175 mm and a length of 500 mm, and the exhaust pipe used was one with an inner diameter of 48 mm.

While creating a porous glass base material with the above configuration,
Internal pressure control was carried out for about 8 hours. the result,
The internal pressure can be controlled to ±1% or less, and the outer diameter of the porous glass base material and the refractive index distribution of the optical fiber base material are extremely uniform in the axial direction (several percent or less).
It was confirmed that

The invention is not limited to the above embodiments. For example, the pressure detector may be located within the reaction vessel or within the exhaust pipe. The shape of the reaction container may be cylindrical or spherical, and its size may be arbitrary.
The base material 2 may be directly made into transparent glass. Alternatively, the porous glass base material and the vitrification may be performed in separate steps. The protection tubes 22', 22'', 22 may further be concentric multiple tubes or may not be provided.The burner 1 may have one or more burners.
There is no need for gas to flow from the arrows 23, 23', 23''.The position of the burner 1 may be such that the gas is blown diagonally other than in the case shown in Fig. 6.Exhaust pipe 1
3 may be not only one but multiple. Base material 2
is not only for optical fiber but also for simple glass (e.g.
(SiO ₂ glass) may also be used. Arrow 21″, 23, 2
The gases sent in the 3′ and 23″ directions are N ₂ , He, Ar,
O ₂ , air, or a mixture thereof may be used. Furthermore, in the above embodiment, atmospheric air is sucked in from the hole 24 as the gas to bias the exhaust gas, but air, O ₂ ,
Gases such as N ₂ , Ar, and He may also be introduced.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view explaining the conventional method for manufacturing an optical fiber base material, FIGS. 2 and 3 are schematic cross-sectional views explaining the method for manufacturing the optical fiber base material of the prior application, and FIG. Figure 5 is a graph showing the relationship between the internal pressure control gas flow rate and the internal pressure of the reaction vessel when the manufacturing method of the optical fiber base material of the earlier application is used. A graph showing the relationship between the input supply gas flow rate and the internal pressure of the reaction vessel, FIG. 6 is a schematic cross-sectional view illustrating the method for manufacturing the optical fiber base material of the present invention, and FIG. 7 is the internal pressure control in one embodiment of the present invention. FIGS. 8 and 9 are graphs showing the relationship between input supply gas flow rate and reaction vessel internal pressure in other embodiments of the present invention, and FIG. 10 is a graph showing the relationship between input supply gas flow rate and reaction vessel internal pressure. 11 is a graph showing the relationship between the valve opening degree of the exhaust pipe and the pressure inside the reaction vessel in another embodiment of the present invention, and FIG. 11 is a circuit diagram of the control unit used in the embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Flame hydrolysis burner, 2... Porous glass base material, 3... Heat source, 5... Flame, 15... Pressure detector, 17... Exhaust device, 18... Control unit,
19... Gas supply regulator, 19'... Input supply gas regulator, 24... Hole.

Claims (1)

[Claims] 1. A target 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 target, and porous holes are formed in the axial direction of the target. In the method for growing a quality 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 confluence point of the bias gas and the above-mentioned A method for manufacturing an optical fiber base material, characterized in that an exhaust pipe between the exhaust port and the exhaust port is provided with at least one hole for sucking air from the atmosphere. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein the flow rate of the bias gas is adjusted according to pressure fluctuations within the reaction vessel. 3. The method for manufacturing an optical fiber preform according to claim 2, wherein the pressure fluctuation is detected by a pressure detector provided inside the reaction vessel.