JPH05221676A - Production of preform of optical fiber - Google Patents

Abstract

PURPOSE:To provide the subject method for producing a preform of an optical fiber so that the amount of residual dehydrating gas in the preform may be uniformly distributed in the longitudinal direction, in clarification of a porous preform. CONSTITUTION:When clarifying a glass by heating and sintering a porous preform (M), a dehydrating gas is supplied from a dehydrating-gas supply unit 5 into a muffle 2 while controlling a flow control device 4 based on a concentration signal sent from a concentration sensor 7 till completion of clarification of the glass by connecting, at least, the dehydrating-gas supply unit 5 filled with the dehydrating gas through the flow control device 4 to the muffle 2, equipping the concentration sensor 7 for detecting the concentration of the dehydrating gas in the muffle 2 and watching the concentration of the dehydrating gas in the muffle 2. Thereby, the concentration of the dehydrating gas is gradually increased and the amount of the residual dehydrating gas in the preform (M) is uniformly distributed in the longitudinal direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for gradually vitrifying a porous base material obtained by the VAD method or the OVD method into a transparent vitrification in a muffle in a heating furnace from the start to the end of sintering. The present invention relates to a method for producing an optical fiber preform in which the concentration is increased to obtain a transparent vitrification preform in which the amount of dehydrated gas remaining in the preform is uniformly distributed in the longitudinal direction.

[0002]

2. Description of the Related Art Conventionally, in order to dehydrate a porous base material obtained by a VAD method or an OVD method into a transparent vitreous material, for example, a vertical muffle of a heating furnace (such as a gradient furnace in which the temperature is distributed in a gradient) In addition to the porous base material from above into the muffle, a mixed gas of He gas and dehydration gas (chlorine-based gas, etc.) is poured into the muffle from above or below, and the porous base material is inserted into the furnace. It is sent to the heat zone of the heating furnace heated to about 1100 ° C. and first dehydrated.

Next, this dehydrated porous base material is sent as it is to the same heating furnace or another heating furnace, for example, while flowing He gas in a muffle, and is sent to a heat zone heated to about 1500 ° C. , Treated with transparent vitrification.

[0004]

However, in the sintering in such a transparent vitrification treatment, the porous base material has a long rod shape and the start of the sintering corresponds to the feeding of the base material, for example. to be done toward increasingly preform upper end from the base material lower end, the time lag over the ends from the start of the sintering occurs. Therefore, at the start of sintering, a relatively large amount of dehydration gas used in the dehydration process, which is the previous process, remains in the porous matrix, but it decreases as the sintering progresses. When viewed from the length direction of the base material, the residual dehydrated gas is unevenly contained. That is, at the start of sintering, the glass is turned into transparent glass before the dehydration gas is exhausted. Since the presence of such dehydrating gas affects the refractive index and hardness of the obtained transparent glass base material, the dehydrating gas remaining non-uniformly in the length direction of the base material is There is a drawback that the characteristics change in the length direction.

The present invention has been made in view of the above conventional problems, and the amount of dehydrated gas remaining in the base material is uniformly distributed in the longitudinal direction in the muffle in the heating furnace. The present invention also provides a method for producing an optical fiber preform in which the dehydration gas concentration is controlled to gradually increase from the start to the end of sintering.

[0006]

A more specific feature of the present invention is a method for producing an optical fiber preform, in which a porous preform is put into a heated muffle, and the mixture is heat-sintered for transparent vitrification. In, in the muffle through a flow rate control device to connect a dehydration gas supply unit filled with at least dehydration gas and a concentration sensor for detecting the concentration of dehydration gas in the muffle is installed, the concentration sensor in the muffle While monitoring the dehydration gas concentration, start the sintering of the porous base material, and toward the end thereof, control the flow rate control device by the concentration signal from the concentration sensor to control the flow rate from the dehydration gas supply unit into the muffle. It is a method of manufacturing an optical fiber preform in which the dehydration gas concentration in the muffle is gradually increased by supplying the dehydration gas.

[0007]

As described above, since the dehydration gas concentration in the muffle is gradually increased from the start of sintering of the porous base material to the end thereof, as a result, the amount of residual dehydration gas in the obtained transparent glass base material becomes long. Since the particles are evenly distributed in the direction, an optical fiber preform having excellent characteristics can be obtained.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of a manufacturing apparatus system for carrying out the method for manufacturing an optical fiber preform according to the present invention. In this manufacturing apparatus system, 1 is a heating furnace (tilt furnace), 2 is a vertical muffle standing upright in the center of the heating furnace 1,
3 is a gas supply path connected to the lower end of the vertical muffle 2, 4 is a flow rate control device incorporating a gas flow rate control valve,
Reference numeral 5 denotes a dehydration gas supply unit including a gas tank or the like connected to the flow rate control device 4 and filled with dehydration gas such as chlorine gas, and 6 denotes a gas such as He gas connected to the flow rate control device 4 and the like. An inert gas supply unit composed of a gas tank or the like, and the sensing units 7a and 7b of the vertical muffle 2
Concentration sensors mounted on the upper and lower sides of the sensor for detecting the concentration of dehydrated gas inside, and 8 receives a concentration signal from the concentration sensor 7 and supplies a gas supply control signal by a preset program or the like. The flow control device 4
, M is a porous base material to be processed, which is put in the muffle 2, 9 is a seed rod extending from the upper end of the porous base material M, and 10 is a rotary grip for gripping the seed rod portion. Reference numeral 11 denotes a traverse mechanism for moving the porous base material M up and down through the rotary grip portion 10, reference numeral 12 denotes a cap at the upper end of the muffle 2 having a seed rod 9 penetrating its center hole 12a, and reference numeral 13 denotes a cap 12 portion It is an exhaust unit including a pipe for sucking and exhausting gas.

Therefore, in order to carry out the method for manufacturing an optical fiber preform according to the present invention with this manufacturing apparatus system, the following process may be carried out.

First, the dehydrated porous base material M is put in the muffle 2, and the seed rod 9 is traversed by the traverse mechanism 11.
Then, He gas is supplied into the muffle 2 from the inert gas supply unit 6 and the heating furnace 1 is driven to raise the temperature of the heat zone to about 1500 ° C. In this state, for example, sintering is started from the lower end Ma of the porous base material M,
The porous base material M is gradually lowered by the traverse mechanism 11 and the sintering is advanced to the upper end Mb side. Then, after the sintering of the entire length of the porous base material M is completed, the porous base material M is taken out of the muffle 2. Thereby, a transparent glass base material is obtained.

At the start of sintering, the concentration of dehydrated gas in the muffle 2 is monitored by a concentration sensor 7. Usually, at the start of this sintering, a relatively large amount of the dehydrating gas used in the dehydrating step remains in the porous base material M, so that the transparent vitrification is completed before the dehydrating gas is sufficiently exhausted. Therefore, for example, in the lower end Ma portion of the porous base material M, a relatively large amount of dehydrated gas is taken into the transparent glass base material. However, since it takes time until the sintering progresses and reaches the upper end Mb portion of the porous base material M, the dehydration gas escapes and the residual amount of the dehydration gas in the transparent glass base material reaches the upper end Mb portion. It gradually decreases, which is preferable in view of the characteristics of the base material, but the residual amount of dehydrated gas becomes nonuniform when viewed from the entire length of the base material.

Therefore, according to the present invention, at the start of sintering, the dehydration gas supply unit 5 hardly supplies the dehydration gas and the concentration of the dehydration gas is reduced, while the concentration signal from the concentration sensor 7 is changed over time. The flow rate control device 4 is controlled by a program preset by the controller 8 so that the dehydration gas is supplied to the muffle 2 while gradually increasing the dehydration gas amount from the dehydration gas supply unit 5. Since the rise of the dehydration gas concentration in the muffle 2 suppresses the escape of the residual dehydration gas in the porous base material M, the transparent glass base material obtained by this transparent vitrification treatment has its entire length. The amount of residual dehydrated gas is evenly distributed over the entire area. That is, an optical fiber preform having excellent characteristics can be obtained.

That is, in the transparent vitrification treatment of the porous base material M, it is ideal that the dehydration gas should not remain in the base material, but the retention is avoided at the start of sintering. If not, it is the position of the present invention that it is better to remain uniformly rather than unevenly over the entire length of the base material.

Incidentally, an example of the relationship between the sintering time and the dehydration gas concentration in He gas in the present invention is shown in FIG.
Is like. For example, when the dehydration gas concentration during the dehydration treatment is about 3%, in the porous base material M thus obtained, the residual dehydration gas escapes at the start of sintering and is small at the end of sintering. Therefore, as shown in the figure, the supply amount of the dehydration gas may be gradually increased from the start of the sintering (for example, the dehydration gas concentration in the muffle 2 is 1.5 at the end).
Supply so that it will be about%). As a result, an excellent transparent glass preform in which the residual dehydrated gas amount is uniformly distributed over the entire length can be obtained.

[0015]

As described above, according to the method of manufacturing an optical fiber preform according to the present invention, the following excellent effects can be obtained. (1) First, since the concentration of dehydrated gas in the muffle gradually increases with time, an excellent transparent glass base material in which the amount of residual dehydrated gas is evenly distributed over its entire length can be obtained. That is, an excellent transparent glass base material having no change in refractive index or hardness over the entire length can be obtained. (2) Since uniform characteristics can be obtained over the entire length of the base material, there is an advantage that it is sufficient to measure only one point in the measurement of the base material characteristic estimation.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing an example of a manufacturing apparatus system for carrying out a method for manufacturing an optical fiber preform according to the present invention.

FIG. 2 is a graph showing an example of the relationship between the sintering time and the dehydration gas concentration in He gas in the present invention.

[Explanation of symbols]

 1 heating furnace, 2 muffle, 4 flow rate control device, 5 dehydration gas supply part, 6 inert gas supply part, 7 concentration sensor, 8 controller, 9 type rod, 11 traverse mechanism, 13 exhaust part, M porous base material,

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koichi Takahashi 1440 Rokuzaki, Sakura City, Chiba Prefecture Fujikura Electric Wire Co., Ltd. Sakura Factory

Claims (1)

[Claims] 1. A method for producing an optical fiber preform in which a porous preform is put into a heated muffle and heat-sintered to obtain a transparent vitrification, wherein at least the dehydrated gas is supplied to the muffle through a flow rate controller. A concentration sensor for detecting the concentration of dehydrated gas in the muffle is connected to the filled dehydrated gas supply unit, and the concentration of the dehydrated gas in the muffle is monitored by the concentration sensor, and the porous base material is baked. The concentration of dehydrated gas in the muffle is controlled by controlling the flow rate control device by the concentration signal from the concentration sensor to supply dehydrated gas into the muffle from the dehydrated gas supply unit. A method for manufacturing an optical fiber preform, characterized by gradually increasing the temperature.