JPS6220139B2 - - Google Patents

Description

Detailed Description of the Invention (Technical Field) The present invention relates to a method for manufacturing an optical fiber base material, and in particular, in the vapor phase axial deposition method (VAD method), when producing a porous base material, uniformity in the longitudinal direction is obtained. This paper proposes a manufacturing method for optical fiber base material that improves properties.

(Background technology) As one method of creating a porous base material using the VAD method, a method is known in which the porous base material is synthesized by flowing raw materials with different compositions in the radial direction using multiple burners. This method is used as a method for manufacturing a base material for a single mode fiber, and also as a method for simultaneously synthesizing a core portion and a jacket portion for a multimode fiber.

This will be explained below with reference to the drawings.

FIG. 1 schematically shows a method for producing a porous base material for a single mode fiber. In Figure 1, the burner for synthesizing the porous base material for the core is a burner for synthesizing the porous base material for the cladding, and usually, for example, the burner for the core is
After synthesizing SiO ₂ - GeO ₂ glass and fine particles of SiO ₂ glass in the cladding burner,
It is sintered into transparent glass and used as a glass base material for optical fibers.

At this time, the position of the growth point at the bottom of the porous base material (point G indicated by the arrow in the figure), which is an important parameter during the deposition of the porous base material, is usually controlled to keep the distance from the burner constant. be. In this method, the growth point position of the porous base material is measured optically without contact, and in order to keep the distance from the burner constant, it is common to change the pulling speed in the direction of the arrow in the figure. It is.

However, when a porous base material is synthesized by flowing raw materials with different compositions in the radial direction using a plurality of burners, the above method alone causes the following problems. When only the pulling position is controlled, the pulling speed is changed as a control variable, but a change in the pulling speed significantly changes the deposition state in areas other than the tip of the porous base material.

In particular, this will be specifically explained using the example shown in FIG. A constant flow rate of SiCl ₄ , H ₂ , O ₂ , and other raw materials are fed into the core burner and clad burner shown in Figure 1 to synthesize glass, and the core burner and the tip of the porous base material are connected to each other. When the base material is produced by pulling the base material while controlling the distance to be constant, the pulling speed is not constant but is varied as shown in FIG. In this case, it goes without saying that the position of the tip of the porous base material is maintained at a constant distance from the core burner. However, the deposited state of the synthesized porous base material becomes extremely non-uniform as shown in FIG.

Both Figures 2 and 3 show two typical cases where the problem occurred. In each figure, A shows a case where the pulling speed gradually increases, and B shows a case where the pulling speed gradually increases. The figure shows the velocity fluctuations and the changes in the deposition shape of the porous base material when the velocity is slowed down. As can be easily deduced from these examples, there is generally a negative correlation between the pulling speed and the cladding diameter of the synthesized porous base material.

Such variations in the outer diameter of the porous base material are preserved in a similar manner even through the subsequent sintering and transparent vitrification process, so that the fiber core diameter and other structural parameters are uniform in the longitudinal direction. It has a negative impact on sexuality.

(Object of the Invention) An object of the present invention is to present a method for solving such problems.

(Structure of the Invention) In order to produce a uniform base material in the longitudinal direction (axial direction) using multiple burners, we believe that it is necessary to keep the pulling speed constant, as can be inferred from the above example. It will be done. Through various experiments, the present inventors discovered a method of intentionally changing the pulling speed by changing the conditions of the core burner, and arrived at the present invention.

In other words, the gist of the present invention is that when glass fine particles with different compositions are deposited in the radial direction using a plurality of burners in manufacturing a porous base material by the VAD method, the distance between the burners and the tip of the growth surface is kept constant. , the light is irradiated from the side of the growing surface, grazing the tip of the growth surface, and the pulling speed is adjusted while keeping the amount of transmitted light constant, and the gas flow conditions of the burner that synthesizes the growing tip among multiple burners are changed. The present invention provides a method for manufacturing an optical fiber base material, characterized in that the pulling speed is controlled to be as constant as possible.

The method of the present invention will be explained in detail below.
Figure 4 is a graph showing the influence of the gas flow rate of the core burner (tip synthesis burner) on the pulling speed, and the horizontal axis represents the flow rate of Ar, H ₂ and O ₂ in each core burner. The vertical axis indicates the pulling speed to maintain a constant distance between the tip of the porous base material and the core burner. As shown in Figure 4, increasing the H ₂ content of the combustion gas reduces the pulling speed;
It was also found that there is a similar tendency for the pulling speed to decrease as the inert gas Ar is increased. The reason for this phenomenon is that in the former case of _H2 , as the amount of combustion gas increases, the temperature at the tip of the porous body increases and the density of the porous body increases, which reduces the effective growth rate. Yes, and also the latter Ar
In this case, the growth rate may be slowed down because the efficiency of the glass synthesis reaction decreases as the amount of inert gas increases. Furthermore, the influence of the O ₂ flow rate is thought to be due to the existence of both of the above effects.

The characteristics of the present invention are to control the pulling speed to keep the position of the growth point constant by utilizing the above-mentioned phenomenon, and to feed back the pulling speed itself to the gas flow rate conditions of the core burner. The goal is to keep it constant.

Next, the present invention will be specifically explained with reference to Examples.

FIG. 5 is a schematic explanatory diagram of a method for manufacturing a porous base material according to the method of the present invention, and the core burner and the cladding burner indicate the porous base material.

A concentric multi-tube burner with an outer diameter of 10 mmφ is used as the core burner, and two concentric multi-tube burners with an outer diameter of 20 mmφ are used as the burner for the cladding.The arrangement shown in Figure 5 creates a two-layer structure of the core and cladding. We prepared a porous base material with The raw materials were supplied so that each composition would have a core of SiO ₂ -GeO ₂ and a cladding of SiO ₂ . For the core burner, H ₂
(2.0/min), O ₂ (5.0/min) Ar2.0/
min, SiCl ₄ [100c.c./min (35℃)] _{GeCl 4} [30
cc/min (35℃)] into each layer of the concentric multi-tube,
The burner for the cladding is equipped with H ₂ (6/
min) O ₂ (6/min), Ar3.0/min, SiCl ₄
[150 c.c./min (40°C)] was flowed respectively.

The porous base material pulling speed was adjusted so that the distance between the core burner and the tip position of the porous base material was kept constant. Specifically, the core burner is fixed,
The position of the tip of the porous base material is determined by irradiating a He-Ne laser beam that passes over the tip of the base material, monitoring the amount of transmitted light, and extracting the amount of transmitted light as a voltage signal with a photoelectric conversion element, which connects the core burner and the porous base material. It was used as a monitor for relative positional relationships. At this time, in addition to the above control, the H ₂ flow rate was changed while monitoring the pulling speed to keep it constant.

The amount of change in the H ₂ flow rate was within the range of 2.0/min±0.2/min, and the change in the pulling speed was within the range of ±2 mm/h with respect to the central value of 50 mm/h. Approximately during this time
In 10 hours, the outer diameter of a porous base material of 450 mm
The uniformity was 80mmφ±3mmφ.

Although the above example shows a method using the H ₂ gas flow rate as the control variable, the O ₂ or Ar flow rate may also be used as the control variable. However, it is necessary to adjust the proportional gain of each control as appropriate based on the data shown in FIG. Regarding control, it is preferable to include some integral elements in order to improve control accuracy.

(Effects of the Invention) The present invention is not limited to the embodiment shown in FIG. 5, but generally uses VAD using a plurality of burners.
It is applied to keep the tip position constant when synthesizing a porous base material by the method, and is extremely effective in maintaining uniformity in the growth direction.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a method for producing a porous base material for single mode fiber. FIG. 2 is a diagram showing an example of variation in pulling speed when producing a porous base material. FIG. 3 is a diagram showing non-uniformity in the growth direction of the porous base material when the speed fluctuation shown in FIG. 2 occurs. FIG. 4 is a graph showing the influence of various gas flow conditions of the core burner on the pulling speed. FIG. 5 is an explanatory diagram of a method for producing a porous base material in a control example.

Claims (1)

[Claims] 1. In the production of a porous base material by the VAD method,
When depositing glass fine particles with different compositions in the radial direction using multiple burners, in order to keep the distance between the burners and the tip of the growth surface constant, light is irradiated from the side of the growth surface so as to brush over the tip of the growth surface. An optical fiber characterized in that the pulling speed is adjusted while keeping the amount of transmitted light constant, and the pulling speed is controlled to be as constant as possible by changing the gas flow conditions of a burner that synthesizes the growth tip among a plurality of burners. Manufacturing method of base material. 2. The method for manufacturing an optical fiber base material according to claim 1, wherein the gas that changes the gas flow rate condition is hydrogen. 3. The method for manufacturing an optical fiber base material according to claim 1, wherein the gas that changes the gas flow rate condition is an inert gas. 4. The method for manufacturing an optical fiber base material according to claim 1, wherein the gas that changes the gas flow rate condition is oxygen.