JP2751176B2 - Manufacturing method of base material for optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application field] The present invention uses a multi-tube burner to perform VAD
The present invention relates to a method for producing a porous preform for an optical fiber by producing a granular glass (soot) such as a VPO method (external vapor phase oxidation method), which can improve the deposition efficiency of the granular glass.

[Conventional technology]

In the soot generation method, a glass raw material is supplied into a flame of a combustion burner, and the glass raw material is subjected to a flame hydrolysis or oxidation reaction to interpose granular glass, which is then rotated around a rotating starting material or mandrel. Deposit to produce a porous glass preform. At this time, a burner for combustion having a concentric multiple pipe structure and having a plurality of ejection ports for combustion gas, auxiliary gas, and inert gas for glass raw material and flame formation is usually used.

As a combustion burner of this kind, a set of glass raw materials having a concentric multi-layer structure is provided outside the inner flame forming nozzle comprising a pair of glass raw materials and a port for forming a flame in the above concentric multi-layer structure. And / or an outer flame forming nozzle comprising a flame forming port, wherein the outer flame forming nozzle is higher than the inner flame forming nozzle by a distance sufficient to complete the synthesis of the granular glass. A burner having a step has been proposed (Japanese Utility Model Publication No. 60-4979). With such a double structure, the synthesis of the granular glass and the temperature control of the deposition surface can be performed separately.

In Japanese Patent Publication No. Sho 62-50418, as shown in FIG. 4, a double-structure multi-tube burner (double flame burner) is used to retreat the inner flame forming nozzle 41 with respect to the outer flame forming nozzle 42. There has been proposed a burner capable of adjusting the step L ′ between the tip portions of both nozzles. This burner can increase the diameter of granular glass by increasing the burner flame length, and can increase the diameter in the radial direction. It is described that doubling the flame increases the amount of granular glass deposited, thereby realizing high-speed synthesis of a porous preform for optical fiber.

[Problems to be solved by the invention]

In the synthesis of a porous preform for an optical fiber, it is an important issue how to efficiently deposit the particulate glass synthesized by the burner on the preform. The ratio of the granular glass to be synthesized attached to the base material is called the yield. If the yield is low, not only the rate of synthesis of the porous base material cannot be increased, but also the inside of the reaction vessel (referred to as Matsufuru) for the synthesis of the base material for the optical fiber, the granular glass that has not adhered to the base material ( Since the floating soot floats, various adverse effects occur as described below.

If the floating soot adheres to the deposition surface, when the porous base material is turned into a transparent glass, bubbles are generated in the glass, so that a high-quality base material cannot be obtained.

If soot adheres to the inner wall of Matsufuru, particularly to a monitor window or the like, it becomes impossible to monitor the production status, and the base material cannot be produced stably.

The adverse effect of increasing the flow rate of the raw material in order to increase the synthesis rate is described in JP-B-62-50418. One of the disadvantages is that the growth (particle size) of the granular glass is insufficient. It is considered as a factor. Thus, the present inventors tried to increase the step L 'between the inner layer 41 and the outer layer 42 in order to increase the grain size in the conventional burner shown in FIG. 4, but tried to make L' too long. On the contrary, it has been found that the yield is deteriorated, and furthermore, there is an adverse effect that the granular glass adheres to the tip of the inner wall 43 which is the boundary between the inner and outer layers. FIG. 5 shows the change in yield when the step L ′ (mm) is changed. Further, even if the length L 'is short, the burner was separated from the base material, and the production was carried out. Remained changed. Also, the particle size is
It was found that it grew only to about 0.3 μm and did not grow any more.

The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a method capable of synthesizing a base material for an optical fiber by improving the deposition efficiency of granular glass using such a double flame forming type burner. It is intended for.

[Means for solving the problem]

The present inventors have examined the relationship between the glass material flow rate and the double flame forming type burner structure in detail, and as a result, the relation between the glass material flow rate, the step L of the burner, and the material ejection port size satisfy a certain relational expression. It has been found that the above-mentioned object can be achieved by making the selection so as to achieve the present invention.

That is, the present invention relates to a method for producing a porous preform for an optical fiber by depositing particulate glass synthesized by supplying a gaseous glass material into a flame of a multi-tube burner and subjecting it to flame hydrolysis. An outer flame comprising an inner flame forming portion provided with a multiple ejection port for flame formation on the outer periphery of a central port for material ejection, and a multiple ejection port for flame formation arranged on the outer periphery of the flame forming portion. Using a double flame forming type burner having a forming part,
In addition, when the step between the inner flame forming part and the outer flame forming part is L (mm), the inner diameter d of the raw material ejection port (mm), and the glass raw material flow rate Q (cc / min), 0.4Q ≦ L × d ² ≦ 0.
The present invention relates to a method for producing a base material for an optical fiber, wherein granular glass is deposited while satisfying the relationship of 6Q.

The method of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic explanatory view of a specific example of a double flame forming type burner used in the method of the present invention. In this example, from the center port 1 to the fourth port 4 is an inner flame forming part, the center port 1 is for the raw material gas, the second port 2 is for the combustion gas, the third port 3 is for the auxiliary gas,
The fourth port 4 is for an inert gas. The fifth to eighth ports are outer flame forming parts, the fifth port 5 is for inert gas, the sixth port 6 is for combustion gas, and the seventh port 7
Is for inert gas, and the eighth port 8 is for auxiliary gas. 9 is an inner wall. In the present invention, when the inner diameter of the center port 1 for the glass raw material gas is d (mm) and the level difference between the tip of the inner flame forming portion and the tip of the outer flame forming portion is L (mm), the flow rate Q (cc) / min), the deposition is performed so as to satisfy the relationship of 0.4Q ≦ L × d ² ≦ 0.6Q. FIG. 1 is merely an example, and the number of ejection ports is not limited to this.

[Action]

The present inventors have studied in detail the mechanism of the deposition of the granular glass generated in the flame of the combustion burner on the base material for the optical fiber. As shown in FIG. Due to the diffusion or thermophoresis effect (a phenomenon in which fine particles move from a high temperature to a low temperature under a force) while flowing along the streamline 13 and flowing on the surface of the base material 10, the deposition surface of the base material 10 It was found to reach the surface. That is, in order for the granular glass 11 to be deposited on the base material 10, it is necessary to flow through a portion as close to the center of the flame 12 as possible.

Based on such findings, using the burner shown in FIG. 1, the flow rate of the glass raw material was fixed at Q = 5500 cc / min, and L (m
When the yield was examined by changing the parameters of m) and d (mm), the results shown in the graph of FIG. 3 were obtained. L × d ² / Q is a parameter corresponding to the time when the raw material gas passes through the step L of the burner (this parameter can be converted to time by multiplying by π / 4). Equivalent to. FIG. 3 shows that a high yield of about 70% can be obtained when 0.4 ≦ Ld ² /Q≦0.6. If L is short,
After exiting the outer flame forming portion, the glass particles are diffused to the outside and the deposition efficiency is deteriorated. If L is too long, the glass particles are mixed in the inner flame forming portion, so that the glass fine particles adhere to the inner wall 9. And the diffusion efficiency also degrades the deposition efficiency.

Although the above description has been made with reference to a double flame forming type burner composed of concentric multi-tubes, the cross-sectional area s of a multi-tube burner having a cross section other than circular is also considered 0.4 ≦ L for the assumed inner diameter d assuming a circle
Similar effects can be expected if the deposition is performed so as to satisfy the relationship of d ² /Q≦0.6.

〔Example〕

Example 1 Using a concentric octuple tube double flame forming type burner shown in FIG. 1, a granular glass was synthesized and deposited according to the present invention to produce a porous preform for an optical fiber. The conditions are as follows.

Burner parameters: L = 80 mm, d = 6 mm Inner flame forming layer (center port 1 to port 4): Glass material flows only to center port 1. SiCl ₄ 5.5 l / min, H ₂
12 l / min, O ₂ 25 l / min, Ar 3 l / min. Outer flame forming layer (5th to 8th ports): 5th and 7th
Flow Ar gas through the port. Ar 8l / min, H ₂ 40l / min, O ₂ 26l /
min.

At this time, L × d ² /Q=80×36÷5500=0.52. The deposition rate of the porous glass preform synthesized above was 10.6 g / mi.
n, The yield was as good as about 72%.

Comparative Example 1 A porous glass base material was produced under the same conditions as in Example 1 except that only L was 120 mm. Then L × d ²
/Q=0.78, out of the range of the present invention. As a result, the deposition rate was as low as 7 g / min, and the yield was only about 48%. In addition, soot was deposited on the inner wall 9 of the burner and was fixed. Comparing the deposition rate and yield of Example 1 and Comparative Example 1 above,
The effect of the present invention can be clearly understood.

〔The invention's effect〕

As described above, the method of the present invention adjusts the glass raw material flow rate, the step size of the double flame forming type burner, and the inner diameter of the raw material ejection port to synthesize and deposit granular glass, thereby improving the deposition rate. In addition, a base material for optical fiber can be manufactured with a high yield. Therefore, the present invention can be widely applied to a method of manufacturing a base material for optical fiber by so-called sooting, thereby improving the productivity and contributing to cost reduction.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a gas ejection portion of a specific example of a double flame forming type burner for synthesizing granular glass used in the present invention.
FIG. 2 is a schematic diagram for explaining how the granular glass flows in a flame, FIG. 3 is a table showing the relationship between L × d ² / Q (horizontal axis) and yield (%, vertical axis) in the present invention, FIG. 4 is a schematic explanatory view of a gas ejection portion of a burner used in the conventional method, and FIG. 5 is L ′ (mm, mm) obtained when synthesis and deposition of granular glass are performed by the conventional method using the burner of FIG. 4 is a chart showing a change in yield (%, vertical axis) when the horizontal axis is changed.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masahiro Takagi 1 Tayacho, Sakae-ku, Yokohama City, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. Yokohama Works (56) References JP-A-61-186238 (JP, A) JP-A Sho 61-270226 (JP, A) JP-A-62-187428 (JP, A) JP-A-61-186239 (JP, A)

Claims (1)

(57) [Claims] 1. A method for producing a porous preform for an optical fiber by depositing particulate glass synthesized by supplying a gaseous glass raw material into a flame of a multi-tube burner and subjecting it to flame hydrolysis. As a burner, an inner flame forming part in which multiple flame-forming ports are provided on the outer periphery of a raw material discharging center port, and an outer flame comprising multiple flame-forming ports in the outer periphery of the flame forming part A double flame forming type burner having a flame forming portion is used, and a step at the tip of the inner flame forming portion and the outer flame forming portion is L (mm). ), A method of manufacturing a base material for an optical fiber, comprising depositing granular glass satisfying a relationship of 0.4 Q ≦ L × d ² ≦ 0.6 Q when a glass raw material flow rate is Q (cc / min).