JPS596820B2 - Manufacturing method of optical fiber base material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing an optical fiber preform by a VAD method.

Conventionally, optical fibers used for optical communications have been manufactured by various methods, but the VAD method has recently been attracting attention.

In the VAD method (vapor phase attachment method), soot-like glass particles are attached and deposited on the lower end of a rod-shaped base material that moves upward while rotating.
This is a method in which a rod-shaped porous glass preform is formed by growing soot-like glass particles in the axial direction while pulling up a rod-shaped base material, and then a predetermined process is performed to form an optical fiber preform. Then, this optical fiber preform is spun to form an optical fiber. By the way, when manufacturing optical fibers by the VAD method, refractive index profiles of various shapes are formed.

The refractive index profile is determined by the reaction state within the flame, the flow of sooty glass particles, temperature distribution, etc., but in the case of the VAD method, the refraction at the outer vicinity a is shown by the solid line in Figure 1. Profiles with relatively high rates tend to be obtained. An optical fiber having a profile in which the refractive index near the outside a is relatively high does not have good band characteristics. Therefore, some attempts have been made to improve the refractive index distribution by adjusting the reaction state within the flame, the flow of soot-like glass particles, temperature distribution, etc., but no satisfactory results have been obtained. There is a need for a method to reduce the rate. The inventors of the present invention conducted extensive research in order to meet the above-mentioned needs, and found that when a porous glass preform formed by the VAD method is heat-treated in the presence of a boron compound and oxygen, and then subjected to a predetermined treatment, As shown by the dotted line in Figure 1, the refractive index in the vicinity of the outer side a decreases, and it has been found that an optical fiber having a square distribution type and good band characteristics can be obtained.

The present invention was completed based on this knowledge, and its feature lies in adjusting the refractive index distribution by heat treating a porous glass preform formed by the VAD method in the presence of a boron compound and oxygen. The present invention will be explained in detail below.

First, soot-like glass particles are attached and deposited on the lower end of a rod-shaped substrate that moves upward while rotating by the VAD method, and the soot-like glass particles are grown in the axial direction while pulling up the rod-shaped substrate to form a rod-shaped porous glass. Form a preform.

Next, the porous glass preform is heat treated in the presence of a boron compound and oxygen.

Specifically, the porous glass prehiome 2 is housed in a reaction vessel 1 formed of, for example, a quartz pipe or an alumina pipe as shown in FIG. A gaseous boron compound is supplied from a boron compound gas introduction pipe 3, and oxygen gas is simultaneously introduced from an oxygen gas introduction pipe 4 provided at the bottom of the reaction vessel 1. As the boron compound, BBr3, B2H6, etc. are used. Since B2H6 is a gas at room temperature, it is sent as it is to the reaction vessel 1, and since BBr3 is a liquid at room temperature,
It is vaporized by bubbling with an inert gas such as Ar gas, and then supplied into the reaction vessel 1 . The amount of boron compound gas supplied depends on the size of the porous glass preform 2,
Although it depends on the treatment temperature, treatment time, and type of boron compound, a desirable value is usually in the range of 10 to 30 CC/min. In addition, the amount of oxygen gas supplied is determined by the amount of boron compound gas supplied, and the boron compound is sufficiently oxidized.
An amount that can be used as an oxide is required, and the amount is set at 200 to 600 CC/min depending on the amount of the boron compound gas supplied. A heater 5 surrounding the reaction vessel 1 is provided on the outside of the reaction vessel 1, and this heater 5 heats the porous glass preform 2, boron compound, and oxygen in the reaction vessel 1. , heat treatment is performed.

The treatment temperature depends on the type of boron compound, treatment time, etc., but in the case of a normal SiO2 glass preform, it is in the range of about 1000 to 1300°C. Also,
The processing time varies depending on the processing temperature, the size of the porous glass preform 2, the type of boron compound, etc.
The range is 0 hours. Furthermore, during this heat treatment, the porous glass preform 2 is rotated at a rotation speed of 10 to 30 rpm and transferred downward at a feed rate of 2 to 10 mm/min to ensure uniform heat treatment. Through such heat treatment in the presence of the boron compound gas and oxygen, the boron compound gas and oxygen gas are first diffused into the porous glass preform 2, and the reaction is expressed by the following reaction formula. A thermal oxidation reaction results in boron oxide (B, O.).
This B2O3 is deposited and attached to the porous portion near the outside of the porous glass preform 2. Since the refractive index of B2O is smaller than that of SiO2, the refractive index near the outside of the porous glass preform 2 decreases, resulting in a square distribution type refractive index distribution (profile) as shown by the dotted line in Figure 1. ) is obtained. At this time, the degree of decrease in the refractive index of the outer portion of the porous glass preform 2 can be controlled by appropriately changing the supply amount of boron compound gas and oxygen gas, processing temperature, processing time, etc. within the above ranges. Can be done.
Next, the porous glass preform 2 that has undergone this heat treatment is heated to become transparent vitrification, and becomes an optical fiber base material. Note that the exhaust gas generated during the heat treatment is discharged to the outside of the reaction container 1 from an exhaust pipe 6 provided at the upper part of the reaction container 1.

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

Example 1 SiCι4200CC/min, GeCt45OCC/min,
POct35CC/min, H23OOCC/min, 025
500 CC/min was supplied to the quadruple tube burner, and the rod-shaped base material was rotated at a rotational speed of 20 rpm while pulling at a pulling speed of 50 mm.
/ hour, and the outer diameter was 50 mm using the VAD method. length 35
A zero-pore porous glass preform was formed.

Next, this porous glass preform was placed in a reaction vessel 1 shown in FIG. 2, and heat-treated in the presence of B2H6 gas and oxygen gas.

The conditions for heat treatment are as follows. B2H6 supply amount 20
CC/min02 Supply rate 600CC/min Heat treatment temperature 1200℃ Heat treatment time 3 hours Porous glass brifform rotation speed and 25 rpm
Feed rate: 5 mTfL/min In this way, the heat-treated porous glass preform was vitrified transparent at 1450° C. to obtain an optical fiber preform, and an optical fiber was manufactured from this optical fiber preform.

The obtained optical fiber had a refractive index distribution as shown in FIG. On the other hand, for comparison, when an optical fiber was manufactured by the same method from a porous glass preform that was not treated with B2H6 gas or 02 gas, an optical fiber having a refractive index distribution as shown in Fig. 4 was obtained. Ta.

As is clear from comparing the refractive index distributions in FIG. 3 and FIG. 4, the profile of the one processed by the method of the present invention is of a square distribution type and the refractive index near the outside is low, whereas
The profile of the conventional one without treatment is not a perfect square distribution type, and the refractive index near the outside is relatively high.

From this, it can be seen that the optical fiber formed from the optical fiber base material treated by the method of the present invention has better band characteristics than the conventional fiber. Example 2 A porous glass preform obtained in the same manner as in Example 1 was heat treated in the presence of BBr3 gas and 02 gas.

The heat treatment conditions were a BBr3 supply rate of 10 CC/min,
The procedure was the same as in Example 1 except that the supply amount of 02 was 200 cc/min. The refractive index distribution of the optical fiber obtained from the porous glass preform after heat treatment was a square distribution type almost similar to that shown in FIG. As explained above, the method for producing an optical fiber preform of the present invention involves heat-treating a porous glass preform obtained by the VAD method in the presence of a boron compound and oxygen.

Therefore, according to the present invention, the refractive index in the vicinity of the outer side of the porous glass preform can be lowered, and a square distribution type refractive index distribution can be reliably formed. Therefore, the optical fiber obtained from the porous glass preform treated by the method of the present invention has a square distribution type refractive index distribution (profile curve) and has good band characteristics.

[Brief explanation of drawings]

FIG. 1 is a graph showing the refractive index distribution of optical fibers obtained by the method of the present invention and the conventional method. FIG. 2 is a schematic diagram showing an example of an apparatus suitable for carrying out the method of the present invention. FIG. 3 is a graph showing the refractive index distribution of the optical fiber formed from the optical fiber base material obtained in Example 1, and FIG. 4 is a graph showing the refractive index distribution of the optical fiber obtained by the conventional method. DESCRIPTION OF SYMBOLS 1... Reaction container, 2... Porous glass preform, 3... Boron compound gas introduction pipe, 4... Oxygen gas introduction pipe, 5.・・・・・・
・Heater, 6...Exhaust pipe.

Claims (1)

[Claims] 1. A porous glass preform obtained by the VAD method is heat-treated in the presence of a boron compound and oxygen, and boron oxide is deposited on the outer part of the porous glass preform to lower the refractive index. A method for manufacturing an optical fiber preform, characterized in that the refractive index distribution of the glass preform is a square distribution type.