JPS6311539A - Production of base material for optical fiber - Google Patents

Abstract

PURPOSE:To produce a base material for an optical fiber at a high yield and high speed by projecting laser light into the flame of a combustion burner for flame hydrolysis of a gaseous glass raw material, detecting the scattered light of fine glass particles and controlling the flow rate of a combustion gas. CONSTITUTION:The gaseous glass raw material is ejected from the combustion burner and is subjected to flame hydrolysis. The fine glass particles formed in such a manner is deposited on a rotating starting material, etc., and is grown in the rotating axis direction to form the porous glass base material. The laser light from an He-Ne laser is projected through a chopper into the flame where said fine glass particles are formed. The incident light is scattered by the fine glass particles and the scattered light is received by a photodetector. Said light is then detected by a lock-in amplifier. The flow rate of the combustion gas is so controlled as to maximize the intensity of the scattered light detected in such a manner.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a novel method for producing a porous glass preform for optical fibers using a flame hydrolysis reaction.

[Conventional technology]

Generally, in the production of porous glass base material using flame hydrolysis reaction, combustion gas and glass raw materials are mixed and ejected from a burner, and glass fine particles generated by the hydrolysis reaction of the glass raw materials in an oxyhydrogen flame are produced. is deposited on a rotating starting material or mandrel. In this method, the flow rate conditions of the combustion gas govern the vitrification reaction within the flame, and do they have a decisive influence on the size and number density of the glass particles produced? have It has been found that the larger the glass particles produced, the better the deposition efficiency [Reference: Suda et al., Electronics Letters the Third, January, 1985.21, vol.-1, p.
29-30].

[Problem that the invention seeks to solve]

However, conventionally, in the above manufacturing method, in the flame,
There was no method to control the flow rate of combustion gas while observing the size of the glass particles produced, and the method was determined by trial and error based on experience.

h is the size of the glass particles generated within the flame.
Even though it is known that the larger the size, the better the deposition efficiency,
This is because it was not possible to directly measure the size of particles within the flame.

After thinning and depositing glass fine particles, SEM.

Since the BET method is used for measurement, it is impractical to determine the gas flow conditions based on the size of the glass particles because of the large size and lack of continuous workability.

In view of the current situation, the present invention provides a method for manufacturing an optical fiber base material that can control the combustion gas flow rate in direct response to the size of glass particles being generated in a flame, which has not been realized in the past. It is something to do.

[Means for solving problems]

The present invention produces a porous glass base material by ejecting gaseous glass raw materials from a combustion burner and subjecting them to flame hydrolysis, depositing the resulting glass fine particles on a rotating starting material or mandrel and growing them in the direction of the rotation axis. In this method, the laser beam is totally incident on the flame flow where glass particles are generated
A method for manufacturing an optical fiber base material, characterized in that the incident light is scattered by glass particles and the scattered light is detected, and the flow rate of combustion gas is controlled so that the intensity of the scattered light is maximized. be.

First, the basic idea of the present invention will be explained.

The present inventors have discovered that the glass particles produced in the flame are directly
Is the use of light scattering the best way to control the gas flow rate while observing? I got an idea. This light scattering method utilizes the fact that the intensity of light scattered by fine particles is a function of the particle diameter.According to Rayleigh scattering theory, a spherical particle with a particle size dp and a refractive index m has a wavelength The scattered light intensity when irradiated with light with λ and intensity is given by equation (1).

However, L: Distance θ from the particle to the light receiving part: Scattering angle Therefore, as the particle diameter dp within the flame increases, the intensity of the scattered light increases.

The present invention utilizes this principle, and in the WAD method, a laser beam is incident into a flame flow where glass particles are being generated, and the intensity of the scattered light when the incident light is scattered by the glass particles is measured, and the scattered light intensity is the maximum. By controlling the flow rate of combustion gas so that the following is achieved, large glass particles can be deposited and the deposition efficiency of glass particles can be greatly improved.

The reason why laser light is used as the incident light in the present invention is that laser light is superior to other light sources in terms of monochromaticity, polarization, directivity, high energy density, etc., and is suitable for measuring light scattered by microparticles. This is because it is suitable.

The laser beam used in the present invention is as follows.

For example, a HeNe laser or an Ar laser can be used, but the invention is not limited to these, and as described below, a laser beam in a wavelength range suitable for the particle size of the glass particles can be used. Bye.

The limitation of the wavelength range is determined by the particle size according to the theory of Mie scattering. Let the wavelength of the incident light be λ, the particle size be Dp,
If the particle size parameter is α==to constant fiλ, then if α×2, the scattered light intensity increases monotonically in proportion to α6. He-Nθ laser λ=α6328μm
Therefore, the particle size that satisfies the condition is Dp (a) 4 μm, which satisfies the purpose of this patent.

Glass particles produced by the WAD method are generally (LO1~
(It is said that L is about 2 μm, and the wavelength λ that satisfies the condition is C1016μ1 or more.

A detailed explanation will be given below with reference to the drawings.

FIG. 1 is a diagram illustrating a method for measuring the distribution of scattered light in a flame in one embodiment of the present invention. The light source was a He-We laser chopped to 225 Hz, and the light scattered by glass particles in the flame was received by an SL photodiode at 90° to the side, and synchronously detected by a lock-in amplifier. .

A quadruple tube burner is used as a burner, and the center layer has a 51 cta
, vg2 layer H! , 3rd layer Ar % 4th layer 0! Flow away,
Glass particles were generated in the flame.

At this time, the flow rate of the raw material 5iC24 was kept constant, and H8°Ar
,O! As a result of changing the flow rate of Hl and injecting a laser beam into the flame as shown in Figure 1, and measuring the intensity of the scattered light, Hl
, Ar, 0! Relationship between flow rate change and scattered light intensity Fi
It was as shown in FIG. 2, FIG. 3, and FIG. 4, respectively.

From these results, it can be seen that when the raw material S i Ct4 flow rate is constant, there is an appropriate value for the sH1 flow rate (40 t/min in this case), and for Ar, 02, the smaller the flow rate, the greater the scattered light intensity. Ru.

Next, investigate the relationship between scattered light intensity and raw material yield (porous base material/raw material input amount). Select some appropriate gas flow conditions, set the scattered light intensity, and actually The results are shown in Fig. 7. As predicted earlier, it was found that the yield was higher under gas flow conditions where the scattered light intensity was higher.

From the above experiments and studies, it has been confirmed that the method of the present invention, which controls the gas flow rate conditions so that the intensity of scattered light is maximized, is very effective in improving yield.

[Implementation row]

EXAMPLE Conventionally, a porous base material was produced at a raw material supply rate of 2000 cc/min, a deposition rate of 2.5 y/min, and a yield of about 50 y/min. The combustion gas it at this time is H, :54
t/min, Ar: 14t/min, 0! =52t/min. The intensity of scattered light by the 8102 particles in the flame under these manufacturing conditions was measured and was approximately 200 (arbitrary scale).

On the other hand, if the raw material input amount is fixed, the combustion gas flow rate (H
When l, Ar, and 0 were varied, the scattered light intensity reached a maximum of 310 (arbitrary scale) at a H2 flow of 41 t/min. O@
, The more the amount of Ar was reduced, the more the scattered light intensity increased, but due to the structure of the burner, o, s2t/min, Ar11t
/ It was not possible to reduce the number of minutes. These gas flow conditions: H: a 1t 7 minutes, Ar: 1117 minutes,
0! : The scattered light intensity at 52 t/min was 300 (arbitrary scale). When a porous base material was produced under these conditions, the yield was improved by 15% compared to the conventional method, and the deposition rate was also improved to 5,252/min.

〔Effect of the invention〕

As explained above, the present invention utilizes a flame hydrolysis reaction to generate glass fine particles and deposit them to form a porous matrix. Since the combustion gas flow rate is determined by directly observing the light intensity to maximize the scattered light intensity, the optimum flow rate conditions can be easily determined in a short time, resulting in a significant improvement in yield and deposition rate. play.

Furthermore, the size control and flow rate control of glass particles through direct observation has been realized for the first time by the present invention.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating an embodiment of the present invention. Figures 2 to 4 are diagrams showing the relationship between the scattered light intensity and the change in each gas flow rate for the MAD method (Ar, 01). Figure 5 shows the relationship between the scattered light intensity and the deposition yield of glass particles. FIG.

Claims (1)

[Claims] (1) A gaseous glass raw material is ejected from a combustion burner and subjected to flame hydrolysis, and the resulting glass fine particles are deposited on a rotating starting material or mandrel and grown in the direction of the rotation axis to produce a porous glass base material. In the method of 1. A method for manufacturing an optical fiber base material, the method comprising controlling the flow rate of the fiber.