JPH033617B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to manufacture an arbitrary refractive index distribution with good reproducibility and improve lot-to-lot variations in optical properties in a method for manufacturing an optical fiber base material using a vapor phase axising method. The present invention relates to a method for manufacturing the target optical fiber base material.

The vapor phase axial method (hereinafter referred to as the VAD method) is
Glass-forming raw materials such as SiCl ₄ , GeCl ₄ , and POCl ₃ are fed into a fine particle synthesis torch (hereinafter referred to as the torch), and a flame hydrolysis reaction is caused by an oxyhydrogen flame formed in front of the torch's outlet. _SiO2 ,
Glass particles such as GeO ₂ and P ₂ O ₅ are generated, and these are sprayed onto one end surface of a rotating rod-shaped starting material to deposit and grow in the direction of the rotational axis of the rod-shaped starting material to form a porous glass body. In this method, the fiber is inserted into a high-temperature furnace either continuously or discontinuously to remove bubbles and turn it into transparent glass to obtain a preform for optical fiber. This method uses long
Large-sized optical fiber preforms can be manufactured continuously, and optical fibers with low loss and broadband optical properties can be obtained. However, in order to manufacture a broadband optical fiber by the VAD method, the refractive index distribution must be optimized during manufacturing of the porous base material. According to Figure 1, which shows the relationship between distribution coefficient α and band, distribution coefficient α=1.99±
When the α value is 0.02, the optimum refractive index distribution is obtained, but if there is a slight deviation from the optimum α value, the band characteristics deteriorate significantly. Therefore, in order to manufacture a broadband optical fiber in particular, it is necessary to search for manufacturing conditions for the porous base material so that the distribution coefficient α=2. Furthermore, in order to manufacture a broadband optical fiber preform with good reproducibility (improve the yield), the porous preform must be manufactured while keeping the optimum manufacturing conditions constant.

The manufacturing conditions for controlling the refractive index distribution include (1) raw material gases (SiCl ₄ , GeCl ₄ ) introduced into the torch;
Controls _H2 gas, _O2 gas, Ar gas.

(2) Control the temperature distribution on the growth surface of the porous base material.

(3) Optimize the installation position and angle of the torch to be used.

However, by adjusting the above three points, a base material having an optimal refractive index distribution (α=2) can be manufactured. Therefore, in order to manufacture broadband base materials with good reproducibility, it is sufficient to keep the optimal conditions obtained in (1), (2), and (3) constant. Although the conditions can be precisely controlled and kept constant using a mass flow meter/controller, optimal conditions can be found for conditions (2) and (3), especially condition (3), but these conditions cannot be kept constant. It's difficult. In the current VAD technology, it is difficult to keep the optimal condition (3) constant, which results in large variations in band characteristics.

The reason why it is difficult to keep the optimum condition (3) constant is as follows. In other words, (a) Since the commonly used chiru torches are made of quartz glass, it is difficult to obtain a torch with a perfectly round appearance.
Therefore, when the torch is removed from the torch holder when cleaning the torch, etc., and when the torch is reattached, the torch will be slightly shifted from its original position (optimal position). This deviation also occurs due to the accuracy of the torch holder.

(b) Due to fluctuations in the amount of exhaust from the exhaust port of the reaction vessel, the central axis of the flame flow formed in front of the torch shifts slightly, causing deviation from the optimal conditions. This phenomenon occurs even if the torch is installed at the optimal position (even if the phenomenon described in (b) does not occur). This is because the amount of discharged air fluctuates little by little over a long period of time because excess glass particles adhere to the inner surface of the exhaust port.

(c) As the porous base material grows, the pressure inside the reaction vessel changes, which causes the central axis of the flame flow to shift slightly and deviate from the optimal conditions. This variation occurs over a short period of time.

Due to reasons (a), (b), and (c) above, the optimal position and angle of the torch, that is, the optimal central axis of the flame flow formed in front of the torch, is slightly shifted, so we reproduced the broadband base material. It is difficult to manufacture with good performance. In addition, in the case of VAD method, whether it is the optimal position or not,
Because the refractive index distribution is formed spatially, there is no established technology for measuring it each time, and the reality is that it is impossible to know unless the manufactured base material is made into a fiber and the band characteristics are measured.

The present invention has been proposed in view of the above-mentioned circumstances. Focusing on the fact that the growth rate of the porous glass changes, we use a porous glass growth rate setting section, a porous glass growth rate detection section, and a torch holder that has the ability to move back and forth and left and right to control the growth rate of the preset pores. The torch holder is held in position so that the growth rate detected from the porous glass body is equal to the growth rate of the porous glass body, and the central axis of the flame flow and the central axis of the porous base material are aligned. It is characterized by controlling the intersection point and angle to be constant,
The present invention aims to solve the drawbacks of the VAD method and provide a method for manufacturing an optical fiber base material with extremely low lot-to-lot variation and reproducibility.

Hereinafter, a method for manufacturing an optical fiber base material according to the present invention will be explained based on the embodiments shown in the drawings.

In manufacturing a preform for a graded optical fiber, the present inventors first studied the inter-lot variation in band characteristics with respect to the relative positional relationship between the torch (flame flow) and the growth surface of the porous glass body. As a result, it was revealed that the band characteristics were significantly deteriorated due to a misalignment of approximately 100 μm between the initially set torch (flame flow) and the porous glass growth surface. This deviation occurs due to changes in the flow rate of the exhaust port for exhausting excess particles, pressure fluctuations in the reaction vessel, etc., but the most significant one is when the torch is removed from the torch holder to clean it.
It's time to reinstall it. Ordinary torches are made of quartz glass, so the external dimensional accuracy is not very good, so when reinstalling, the
Or even more misalignment can easily occur. However, because the flame stream and the porous glass body formed in front of the torch or the torch outlet are spatially arranged, it is very difficult to measure deviations from the initial position setting, and this makes it difficult to measure the deviation between lots. It has become clear that this is a major hindrance to eliminating deterioration in band characteristics. As a result of further studies to solve the above problem, the present inventors found that if the position of the torch or the flame stream formed in front of the torch is slightly shifted relative to the growth surface of the porous glass body, the porous glass body We found that the growth rate also changed depending on the shift. This phenomenon is the same in the production of a single-mode optical fiber base material in which a porous glass body is formed into a thin (10 mmφ to 15 mmφ), and in this case, the above-mentioned phenomenon was even more pronounced.

This phenomenon will be explained with reference to FIG. 2. FIG. 2a is a schematic diagram of the production of a porous glass body to be used as a base material for graded optical fiber, in which 1 is a torch;
2 is a flame flow, 3 is a glass particle flow formed in the center of the flame flow, and 4 is a porous glass body (diameter 45 mmφ
~55mmφ), A is the rotation center axis of the porous glass body,
B is the central axis of the torch. Figure 2b shows the movement distance and the growth of the porous glass body when the torch center axis B is moved in the x and y directions while keeping the angle θ at which the porous glass body central axis A intersects with the torch central axis B constant. It is a diagram examining the relationship between speeds. Now Figure 2a
, the angle θ is 20°, and H ₂ forms flame flow 2.
Gas was bubbled at 3/min, O ₂ gas at 6/min, and carrier Ar gas was bubbled at 100 c.c./min while SiCl ₄ forming glass particle flow 3 was maintained at 40°C.
Also, while keeping GeCl ₄ at 32℃, carrier Ar gas
Each gas was sent into the torch 1 by bubbling at a rate of 90 c.c./min to produce a porous glass body 4 having a diameter of about 50 min. When this was converted into an optical fiber, the band characteristics were
It was 1.2GHz・Km. Next, the central axis B of the torch 1 was moved in the x direction while keeping each gas condition and angle θ constant. As a result, the band characteristics deteriorate depending on the moving distance, and with a movement of 100 μm, 650 MHz・Km,
420MHz Km with 200μm movement, 300μm movement
It was 360MHz/Km. Similar results were also obtained in the y direction. The relationship between these moving distances and the growth rate of the porous glass body was as shown by the solid line shown in FIG. 2b. The growth rate when 1.2GHz/Km was obtained was 42mm/hr (point 0 in the figure), but when the torch center axis was moved in the x direction, the growth rate increased, and with a moving distance of 300μm. The speed was 64mm/hr. Furthermore, when moving in the y direction, the growth rate decreased to 20 mm/hr at a moving distance of 300 μm. The dotted line in the figure shows the change in growth rate when a porous glass body for a single-mode optical fiber was manufactured, which was much larger than the change in the growth rate for a graded porous glass body. The absolute value of these speed changes varies depending on the shape and dimensions of the torch, the flow rate conditions of each gas sent to the torch, etc., but the positioning of the torch relative to the growth surface of the porous glass body changes the growth rate of the porous glass body and It has become clear that there is a close relationship with the band characteristics of the resulting optical fiber.

The present invention was carried out based on the above study results, and involves moving the torch so that the growth rate from lot to lot is constant during the production of porous glass bodies. That is, the present invention provides a growth rate setter for inputting an arbitrary growth rate of the porous glass body, a growth rate detector for detecting the growth rate of the porous glass body to be manufactured, and a growth rate setter for inputting an arbitrary growth rate of the porous glass body; It is equipped with a growth rate controller that processes electric signals and electric signals output from a growth rate detector, and a torch moving device that moves left and right according to the electric signals output from the controller, and which controls the growth rate of the porous glass body. can be controlled to any speed. Therefore, according to the present invention, the growth rate can always be kept constant under the manufacturing conditions of a porous glass body that provides good band characteristics, so it is possible to extremely reduce lot-to-lot variation in band characteristics. .

FIG. 3 is a schematic diagram of an apparatus for carrying out the method of the present invention, in which 5 is a starting material, 6 is a reaction vessel, 7 is an exhaust port,
8 is a rotation device, 9 is a ball screw for pulling, 10 is a growth rate (pulling rate) detector, 11 is a growth rate setting device, 12 is a growth rate controller, 13 is a left-right moving device, and 14 is a torch holder. Manufacturing of the porous glass body 4 is started by inputting the growth rate of the porous glass body at which the broadband characteristics have been obtained into the speed setter 11, and the growth rate at this time is inputted to the speed detector 10.
Detected by. The detected speed signal and the speed signal output from the speed setter 11 are compared by the growth speed controller 12, and the signal from the speed detector 10 is smaller than the signal from the speed setter 11 (the speed is slow). In this case, the controller 12 sends a signal to the moving device 14 so that the torch holder 14 attached to the left-right moving device 13 moves in the x direction. In addition, the signal from the speed detector 12 is higher than the signal from the speed setter 11.
When the signal is large (speed is fast), a signal is sent to the moving device 14 so that the torch holder 14 moves in the y direction. In this way, by moving the torch holder so that the signal output from the speed setter 11 and the signal output from the speed detector 10 are equal, the relationship between the growth surface of the porous glass body 4 and the torch 1 is increased. The positional relationship, that is, the point and angle at which the central axis of the flame flow intersects with the central axis of the porous base material, can be kept constant at all times. Therefore, the variation between lots in band characteristics, which was a problem in the past, can be extremely reduced. It is possible to manufacture an optical fiber preform with good reproducibility.

Next, a specific example of the present invention will be described.
Manufacturing conditions (H ₂ flow rate, O ₂ flow rate, SiCl ₄ flow rate, GeCl ₄ flow rate) under which a band characteristic of 1.2 GHz/Km was obtained in the experiment shown in the figure.
Each flow rate was set to , and a speed signal of 42 mm/hr was input to the speed setting device 11 to manufacture 10 porous glass bodies. At this time, the torch 1 was removed from the torch holder 14 four times and washed. As a result, the growth rate of all 10 porous glass bodies was 42 mm/hr, and when the 10 base materials were made into fibers and the band characteristics were measured, they were within the range of 1.05 GHz・Km to 1.2 GHz・Km. It was hot. For comparison, the band characteristics of 10 lines without implementing the present invention are 460MHz・Km ~ 1.2G
It fluctuated in the range of Hz/Km.

According to the present invention, it is possible to reduce not only the variation between lots but also the variation in the properties of the long porous glass body in the longitudinal direction. In the conventional method, when producing a long porous glass body, the pressure inside the reaction vessel decreases over time, and the relative position of the porous glass body growth surface and the flame flow (the position of the flame flow shifts). This is the same phenomenon as when the center axis of a torch shifts.) This shift causes the growth rate to change and the band characteristics to fluctuate in the longitudinal direction. However, according to the embodiments of the present invention, even a long porous glass body could be manufactured at a constant speed, and fluctuations in the band characteristics in the longitudinal direction could be reduced.

As explained above in detail based on the embodiment shown in the drawings, the growth rate is the same as the growth rate arbitrarily set by the growth rate setting device, the growth rate detector, and the torch holder equipped with the ability to move forward, backward, left, and right. According to the present invention, a porous glass body is manufactured by moving a torch holder so that the speed of the torch holder becomes constant, and controlling the point and angle at which the central axis of the flame flow intersects with the central axis of the porous base material to be constant. For example, there are advantages in that it is possible to extremely reduce inter-lot variation in band characteristics, which has been a problem in the past, and also in that variation in longitudinal characteristics of the base material for a long optical fiber can be reduced.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the refractive index distribution coefficient and the band, Figure 2a is a schematic diagram of the production of a porous glass body that will be the base material for graded optical fibers, Figure 2b
3 is a graph showing the relationship between the position of the torch with respect to the growth surface of the porous glass body and the growth rate of the porous glass body, and FIG. 3 is a schematic diagram of an apparatus for carrying out the method of the present invention. In the drawing, 1 is a glass particle synthesis torch, 2 is a flame stream, 3 is a glass particle stream, 4 is a porous glass body, 5 is a starting material, 6 is a reaction vessel, 7 is an exhaust port, 8
10 is a rotation device, 9 is a pulling ball screw, 10 is a growth rate detector, 11 is a growth rate setter, 12 is a controller, 13 is a moving device, and 14 is a torch holder.

Claims (1)

[Claims] 1. Glass forming raw materials are subjected to an oxidation reaction using a glass particle synthesis torch, and the synthesized glass particles are sprayed onto the tip of a rotating starting material to grow and form a round bar-shaped porous glass base material, which is then turned into transparent glass and exposed to light. In the vapor phase axis method for obtaining a fiber base material, a holder for a glass fine particle synthesis torch is equipped with a growth rate setting section for the porous glass base material, a growth rate detection section for the porous glass body, and a function to move back and forth and left and right. The position of the glass particle synthesis torch holder is held so that the signal input to the growth rate setting section and the signal output from the growth rate detection section match, and the central axis of the flame flow and the center of the porous base material are maintained. 1. A method for manufacturing an optical fiber base material, which comprises controlling the point and angle at which the axes intersect to be constant.