JP2000281376A - Production of preform for optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To sufficiently heat upper part of a soot preform so as to dehydrate by adjusting and controlling traverse speed or adjusting and controlling the temp. of a heater of a heating furnace when at least upper end part of the soot preform suspended in the heating furnace approaches the heating area of the heating furnace. SOLUTION: The traverse of a soot preform suspended in a heating furnace in the dehydration process is carried out by three steps of the upward traversing, waiting for chance and the downward traversing. The speeds of upward and downward traverses are set to be nearly constant. The speed of the soot preform at the time of downward traversing is allowed to be reduced by providing a speed reduction control process slightly before a conical taper part of the upper end of the soot preform approaches the heating area of the heating furnace and enters the waiting process. Thereby, the upper end part of the soot preform is sufficiently heated in the speed reduction control process and the waiting process. In the case of the transition from the waiting process to the upward traversing process, the speed of the soot preform is gradually accelerated by providing an acceleration control process. It is preferable that the conical taper part of the lower end of the soot preform is heated by the same process as the process mentioned above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a preform for an optical fiber in which dewatering of a soot preform is improved.

[0002]

2. Description of the Related Art Conventionally, in an external soot preform manufactured by a vapor phase axial method (VAD method), prior to the vitrification by sintering, the moisture (primarily) contained in the inside is reduced. In order to remove the OH group, the solution is suspended in a heating furnace (dehydration furnace) and is heated and dehydrated while traversing.

[0003] In this dewatering step, the soot preform is usually inserted into a heating furnace and performed in one reciprocation of a lower traverse which is gradually lowered and an upper traverse which is gradually raised from the end position (lowest lower end position) of the lower traverse. ing.

In practice, as shown in FIG. 4, for example, a soot preform 20 suspended from a dummy rod 21 is inserted into a heating section (electric heater) 10 such as a cylindrical shape of a heating furnace, and The upper and lower traverses are reciprocated once.

[0005]

However, in the case of the external soot preform 20 by the VAD method, the soot deposition shape is downward at the upper end portion 20a at the base, which is the connection portion with the dummy rod 21. Because of the gradually expanding conical shape (tapered shape), the conventional heating tends to be insufficient particularly at the upper end portion 20a.

In the upper end portion 20a, as shown in FIG. 5, the distance between the soot surface and the heating section 10 is
For the effective diameter portion 20b of the soot preform 20,
This is considered to be due to the fact that heat becomes easy to escape upward due to the increase in size and the formation of a large gap on the upper side between the heating units 10. The conical tapered portion is also at the lower end side of the soot preform 20, but at the lower end side, the portion above the lower end side is the effective diameter portion 20b, so the escape path (gap) for heat becomes narrower, Since the heating is sufficiently performed, there is no problem as much as the upper end portion 20a.

Therefore, although the traverse speed is kept almost constant even in the prior art, the upper end portion 20a of the soot preform 20 reaches the heating section 10 and waits for a while after entering the heating area. After that, the upper traverse is started.

[0008] However, even if such standby heating is performed, it has been found that dehydration is still insufficient from the transmission loss characteristics of the obtained optical fiber.

SUMMARY OF THE INVENTION The present invention has been made in view of the above. When at least the upper end portion of the soot preform approaches the heating area of the heating furnace, the traverse speed is adjusted and controlled, or the heater of the heating furnace is controlled. It is an object of the present invention to provide a method of manufacturing a preform for optical fiber in which sufficient heating is performed by adjusting and controlling the temperature.

[0010]

According to a first aspect of the present invention, in the step of traversing a soot preform suspended in a heating furnace and dewatering the soot preform, at least an upper end portion of the soot preform is formed of the heating furnace. A method of manufacturing a preform for optical fiber, comprising adjusting and controlling a traverse speed of the soot preform when approaching a heating area.

According to a second aspect of the present invention, in the step of traversing and dewatering a soot preform suspended in a heating furnace, at least an upper end portion of the soot preform approaches a heating area of the heating furnace. And a method for producing a preform for an optical fiber, wherein a heater temperature of the heating furnace is adjusted and controlled.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, in the present invention, as shown in FIG. 1, each of a lower traverse step T1, a standby step T2, and an upper traverse T3 in dehydrating a soot preform. Steps shall be performed.

Although the upper and lower traverse speeds are set to be substantially constant (V1, for example, about 600 mm / h), the deceleration adjustment process T1a is performed at least slightly before the standby process T2 of the lower traverse process T1. Provide and decelerate. Ideally, this deceleration is performed gradually as shown in the figure.
The speed may be decelerated at a constant speed, or may be decelerated a plurality of times stepwise.

In the deceleration adjusting step T1a before the standby step T2, the upper end portion of the soot preform receives a large amount of heat and enters the standby step T2. For this reason,
As a result, sufficient heating is performed, and conventional insufficient heating is eliminated.

On the other hand, in the present invention, as a preferable case, the upper traverse step T3 started from the standby step T2
Also in this case, an acceleration adjustment step T3a is provided to gradually accelerate. In principle, the deceleration adjustment step T
The use of 1a enables sufficient heating. However, if this acceleration adjustment step T3a is used in combination, the combination of the acceleration adjustment step T3a, the deceleration adjustment step T1a, and the standby step T2 allows the heating in each step. Adjustment of the heating allotment becomes easy, and balanced process control becomes possible.

As described above, since the conical tapered portion is also at the lower end of the soot preform,
If necessary, even when the lower end portion approaches the heating area of the heating furnace, the same deceleration adjustment step and acceleration adjustment step as described above can be provided. As a result, the same effect as above can be obtained even at the lower end portion side.

Incidentally, the base material obtained by the method of the present invention (total length = 1400 mm, base material diameter = 160 mm, dummy rod diameter = 25 mm, length of tapered portion = 200 m)
Comparison between the transmission loss characteristics of the optical fiber using m) and the optical fiber using the preform obtained by the conventional method (each data such as the overall length is substantially the same as the preform of the present invention). FIG.

That is, according to the transmission characteristic curve II of the optical fiber according to the conventional method, a large loss increasing portion IIa is observed at the wavelength of 1380 nm, whereas the transmission characteristic curve I of the optical fiber according to the method of the present invention is: It can be seen that the loss increasing portion Ia during this period is small and greatly improved.

Next, in the present invention according to the first aspect, the temperature control on the soot preform side is performed by controlling the traverse speed.
Even if the traverse speed is constant, the heating temperature can be controlled by controlling the heating temperature on the heating furnace side when the upper end portion of the soot preform approaches the heating area of the heating furnace.

In view of the above, according to the present invention, as shown in FIG. 3, the dehydration of the soot preform is performed in the same manner as described above in the lower traverse step T.
1, a standby step T2 and an upper traverse step T3, while at least a standby step T2 of the lower traverse step T1
Slightly before reaching, a temperature raising adjustment step T1b for raising the temperature is provided. If necessary, the upper traverse step T3 started from the standby step T2 is also provided with a temperature lowering adjustment step T3b for lowering the temperature. In the drawing, t1 is the temperature during the normal traverse, and t2 is the temperature during the standby process.

That is, when the upper end portion of the soot preform approaches the heating area of the heating furnace, the heating temperature of the soot preform is raised or lowered.
The temperature increase and the temperature decrease may be performed gradually, and may be performed in a step-like manner. Either way, with future temperature control,
Almost the same effects as in the case of the above traverse speed control can be obtained.

Further, according to the present invention, it is also possible to provide a manufacturing method in which the above-described control for adjusting the traverse speed and the control for adjusting the heating temperature on the heating furnace side are appropriately combined.

[0023]

As is apparent from the above description, according to the method of manufacturing the optical fiber preform according to the present invention, the traverse speed control is performed on at least the upper end portion of the soot preform which has been apt to be insufficiently heated. Sufficient heating is performed by controlling the heating temperature of the heating furnace and the heating furnace. As a result, good dehydration is performed over the entire length of the soot preform.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view illustrating each step in a method for manufacturing an optical fiber preform according to the present invention.

2 is a graph showing transmission characteristic curves of the optical fiber using the preform according to the present invention and the optical fiber using the preform according to the conventional method of FIG. 1;

FIG. 3 is a schematic explanatory view illustrating each step in a method for manufacturing another optical fiber preform according to the present invention.

FIG. 4 is a schematic explanatory view showing a dehydrating step of an optical fiber preform.

FIG. 5 is a schematic explanatory view showing a relationship between a heating furnace and an upper end portion of a soot preform in a dehydrating step of FIG.

[Explanation of symbols]

 Reference Signs List 10 heating section 20 soot preform 20a upper end portion T1 lower traverse step T2 standby step T3 upper traverse step T1a deceleration adjustment step T3a acceleration adjustment step T1b temperature increase adjustment step T3b temperature decrease adjustment step

Claims (2)

[Claims] In the step of traversing and dehydrating a soot preform suspended in a heating furnace, when at least an upper end portion of the soot preform approaches a heating area of the heating furnace, the soot preform is traversed. A method for manufacturing a preform for an optical fiber, wherein the speed is adjusted and controlled. 2. In the step of traversing and dewatering a soot preform suspended in a heating furnace, when at least an upper end portion of the soot preform approaches a heating area of the heating furnace, a heater temperature of the heating furnace is reduced. A method for producing a preform for an optical fiber, comprising: