JPH04325427A - Production of optical fiber preform - Google Patents

Abstract

PURPOSE:To produce the optical fiber preform at the time of drawing a first preform rod into a second preform rod having a smaller diameter than the former by controlling the power supplied to a drawing furnace so that the rod is drawn always with a fixed tension irrespective of the variations in the outer diameter of the first rod and thermal conduction of the furnace itself. CONSTITUTION:A first preform rod 1 is passed through a drawing furnace 3 and drawn into a second preform rod 2 having a smaller diameter than the former to produce the optical fiber preform. In this process, the outer diameter of the first rod 1 is measured over its entire length, and the power to be supplied to the furnace 3 is controlled in accordance with the outer diameter of the first rod 1 or in accordance with the tension changed by the penetrating position of the first rod 1 passed through the furnace 3. Consequently, the tension applied on the first rod 1 is fixed, and the optical fiber is produced.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention provides a first base material rod,
When drawing a second base material rod with a smaller diameter, the stretching tension is always constant, regardless of changes in the outer diameter of the first base material rod or fluctuations due to heat transfer in the furnace itself. This invention relates to a method for manufacturing an optical fiber preform in which the power input to a drawing furnace is controlled.

0002

BACKGROUND OF THE INVENTION Conventionally, in order to draw a first base material rod (for example, a glass rod) into a second base material rod having a smaller diameter, it has been introduced into a drawing furnace (such as an electric furnace). The base material is softened by heating, and in this state it is stretched.

[0003] At this time, power is generally applied to the drawing furnace at a constant value over the entire length of the first base material rod, that is, from the start to the end of drawing.

0004

[Problem to be Solved by the Invention] However, according to research conducted by the present inventors, it has been found that the above method has a problem in that it is not possible to stretch with a constant stretching tension when considering the actual properties of the base material and the properties of the furnace itself. It was done. In other words, the actual base material does not always have a uniform outer diameter over its entire longitudinal length, and there are subtle variations, making it impossible to stretch with a constant stretching tension and adapt to changes in the outer diameter. It was discovered that a controlled power control is necessary.

Furthermore, even if the outer diameter of the first base material rod is uniform over its entire length, the stretching tension gradually decreases immediately after the stretching starts due to the characteristics of the furnace itself (such as properties such as thermal conductivity). This tendency was observed, and it was found that the method of constantly inputting a constant electric power does not allow stretching at a constant stretching tension.

The present invention has been made in view of such conventional circumstances.

[Means for Solving the Problems] The present invention is characterized in that a first base material rod is passed through a drawing furnace and stretched to a second base material rod having a smaller diameter to form an optical fiber. In the method for manufacturing a base material, the outer diameter of the first base material rod is measured over the entire length, and the outer diameter of the first base material rod is measured depending on the outer diameter of the first base material rod passed through the drawing furnace, or An optical fiber preform that maintains a constant drawing tension of the first preform rod by controlling input power to the drawing furnace in accordance with the drawing tension that changes depending on the entry position of the first preform rod passed through. It is in the manufacturing method.

[0007]

[Function] By controlling the input power in accordance with the outer diameter of the first base material rod and the entry position in the furnace, the input power can be reduced depending on the entry position, that is, the time from the start of stretching. In addition, with respect to the outer diameter, if the input power is increased when the outer diameter is large and the input power is decreased when the outer diameter is small, the amount of heat supplied to each unit rod member will be equalized, and the rod will be uniformly heated. Softening is performed and the production of an optical fiber preform with a constant drawing tension is realized.

[0008]

Embodiment FIG. 1 shows an embodiment of the method for manufacturing an optical fiber preform according to the present invention.

In the figure, 1 is a first base material rod, and 2 is a first base material rod that is passed through a drawing furnace 3 and
This is a second preform rod extending to a smaller diameter than the preform rod 1, and the desired optical fiber preform is obtained from this second preform rod.

The drawing furnace 3 is an electric furnace, and includes an internal cylindrical muffle 4 and a heating element 5 such as an electric heater that heats the first base material rod 1 through the muffle 4.

In this drawing furnace 3, the first base material rod 1 is sent out at a predetermined speed V1 from its initial outer diameter D, and is stretched while being softened by heating in the drawing furnace 3. The second base material rod 2 having an outer diameter (diameter) d0 is taken off at a predetermined speed V2.

[0012] At this time, ideally, the first base material rod 1 must be always stretched with a constant stretching tension in view of the transmission characteristics of the obtained optical fiber. For example, if there is variation in the outer diameter over the entire length of the first base material rod 1, a constant drawing tension cannot be obtained by applying constant power to the drawing furnace 3, that is, the heating element 5.

Therefore, in the present invention, an outer diameter measuring device such as a laser method (
(not shown) is used to measure the outer diameter over the entire length of the first base material rod 1, and the outer diameter data and position data (
For example, position data from the tip of the rod) is stored in a storage unit such as a computer.

On the other hand, the entry position of the first base material rod 1 sent into the drawing furnace 3 (for example, the entry distance of the rod tip into the furnace, or the elapsed time from the start of drawing) is determined by the rod delivery device. Measurements can be made using a pulse generator attached to a drive mechanism (not shown), a contact sensor, an optical sensor, etc. that detects the passage of the rod tip.

Furthermore, the characteristics of the drawing furnace 3 itself (for example, variations in drawing tension due to thermal conduction of the rods in the furnace) are also measured in advance. This change in characteristics was as shown in FIG. 2 in the case of the drawing furnace tested by the present inventors (in the figure, 6 is the drawing tension pattern). In other words, it can be seen that the stretching tension of the furnace tends to gradually decrease as it increases toward the entry position of the first base material rod 1 (same as the elapsed time after the start of stretching).

Therefore, as shown in FIG.
It is assumed that the input power is applied approximately according to the power control pattern 7 shown in the figure, corresponding to the entry position of the first base material rod 1.

However, in actual operation of the drawing furnace 3, since there is some variation in the outer diameter of the first base material rod 1 in the overall length as described above, the first base material rod 1 The outer diameter data and its position data are compared with the above-mentioned entrance position data into the furnace, and the part with the larger outer diameter is determined to correspond to the outer diameter of the first base material rod 1, that is, the part with the larger outer diameter is the optimum heating part. When the power control pattern 7 is reached, a somewhat larger amount of power is input using the power control pattern 7 as the base, and when the outer diameter becomes thinner, the power control pattern 7 is used as the basis, but the power control pattern 7 is applied as a base. , the input power is controlled so as to reduce the input power.

[0018] As a result, the stretching tension of the first base material rod 1 is kept almost constant without being affected by its outer diameter or the characteristics of the furnace, and ideal stretching can be obtained. Due to this uniform stretching tension, the obtained optical fiber is free from distortion and has extremely good transmission characteristics.

Incidentally, if the control value of the input power due to the change in the outer diameter of the base material rod is calculated mathematically, it is as follows. First of all,
Between the outer diameter D of the first base material rod 1 and the stretching tension F,
Assuming that the following relationship holds true, the electric power input to the drawing furnace 3 is controlled so that the drawing tension F is constant. (D/d)2 ・η F

(1) However, in the same formula, d: outer diameter of the second base material rod 2, η: viscosity of the base material rod (silica glass rod) at a certain temperature.

[0020] Therefore, in order for the stretching tension F to be constant, it is sufficient that the following relationship holds true. η=A(d/D)2

(2) However, in the same formula, A: constant.

On the other hand, the following relationship holds between the viscosity of the rod (glass) and the temperature. η=Bexp(E/RT)

(3) However, in the same formula, B: constant, E: quantity specific to the substance, R: gas constant, and T: absolute temperature.

Furthermore, the following relationship holds between the temperature of the drawing furnace 3 and the input power P. T=CP1/4

(4) However, in the same formula, C: constant.

Therefore, the following equation is derived from the above equations (2) to (4). (d/D)2 = C1 exp (C2 P-1/4
)
(5) However, in the same formula, C1 and C2 are constants.

In other words, for changes in the outer diameter of the base material rod,
While the control is performed using the power P determined by this equation (5), the characteristics of the furnace itself may be taken into consideration by the power control shown in FIG. 3 described above.

[0025]

As described above, according to the optical fiber manufacturing method according to the present invention, the following excellent effects can be obtained.

(1) First, the outer diameter of the first base material rod is measured over the entire length, and the power input to the drawing furnace is adjusted according to the outer diameter of the first base material rod passed through the drawing furnace. If controlled, the stretching tension of the first base material rod can be kept constant regardless of variations in the outer diameter.

(2) Furthermore, if the input power of the drawing furnace is controlled in accordance with the drawing tension that changes depending on the entry position of the first base material rod passed through the drawing furnace, the drawing furnace can be used regardless of the characteristics of the furnace. The stretching tension of the first base material rod can be kept constant.

(3) Furthermore, by combining the above two factors, regardless of the variation in the outer diameter of the first base material rod and the characteristics of the furnace, the drawing tension of the first base material rod can be made almost ideal. It can be kept constant in shape.

[Brief explanation of drawings]

FIG. 1 is a schematic explanatory diagram showing an embodiment of an optical fiber manufacturing method according to the present invention.

FIG. 2 is a graph showing the relationship between the drawing tension and the entry position of the base material rod in the drawing furnace.

FIG. 3 is a graph showing the relationship between input power and the entry position of the base material rod in the drawing furnace.

[Explanation of symbols]

Claims (3)

[Claims] Claim 1: Passing a first base material rod through a drawing furnace;
In a method of manufacturing an optical fiber preform by stretching a second preform rod having a smaller diameter, the outer diameter of the first preform rod is measured over the entire length of the first preform rod, and the first preform rod is passed through the drawing furnace. A method for manufacturing an optical fiber preform, comprising controlling the input power of the drawing furnace according to the outer diameter of the first preform rod to keep the drawing tension of the first preform rod constant. 2. Passing the first base material rod through a drawing furnace;
In a method of manufacturing an optical fiber preform by stretching a second preform rod having a smaller diameter, the first preform rod is passed through the drawing furnace and the first preform rod is passed through the drawing furnace, and the drawing tension varies depending on the entry position of the first preform rod. A method for manufacturing an optical fiber preform, which comprises controlling input power to a drawing furnace to maintain a constant drawing tension of the first preform rod. 3. Passing the first base material rod through a drawing furnace;
In a method for producing an optical fiber preform by stretching a second preform rod having a smaller diameter, the first preform rod may be adapted to a drawing tension that varies depending on the entry position of the first preform rod passed through the drawing furnace; The outer diameter of the first base material rod is measured over the entire length, and the power input to the drawing furnace is controlled according to the outer diameter of the first base material rod passed through the drawing furnace. A method for manufacturing an optical fiber preform, characterized by keeping the stretching tension of the preform rod constant.