JP3378287B2 - heating furnace - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heating furnace which has a uniform temperature in the circumferential direction (entire circumferential direction) of a core tube and is useful for manufacturing optical fibers.

[0002]

2. Description of the Related Art In manufacturing an optical fiber, at various stages, for example, a glass particulate deposit (soot) is sintered to be transparent, or a transparent large diameter glass rod (preform) is stretched. When using a heating furnace with a multi-cylinder structure in which concentric cylindrical heating elements are concentrically arranged around the outer periphery of the core tube (muffle) when making the fiber into a fiber by drawing a fiber base material into a fiber, The desired heating is being performed.

The heating furnace having such a conventional structure is shown in FIGS. In this heating furnace, the furnace core tube 1 has substantially the same thickness over the entire length in the upper and lower parts, and is supported by the supporting members 2 and 3 at the upper and lower end faces 1a and 1b.
In the cylindrical heating element 4 located on the outer periphery of the core tube 1, for example, electrodes 5 and 6 for energization are provided at the two locations. Then, at the time of heating, the electrodes 5, 6 are energized, and the heat from the heating element 4 is supplied to (inserted into) the inside of the core tube 1 through the core tube 1 and the outer periphery of the various objects to be heated. And reaches the point where the entire circumference of the object to be heated is heated uniformly.

[0004]

However, in the heating furnace having the above-mentioned conventional structure, when a final product such as an optical fiber is obtained, the desired quality is often not obtained. Therefore, the inventors of the present invention have made extensive studies on this point, and have come to the following conclusions.

That is, in the above heating furnace, the object to be heated is
Although it should be uniformly heated in the circumferential direction, in reality, it is not uniformly heated, and the cross-sectional shape of the molded body after heating is not a perfect circle due to the uneven heating. It was discovered that the product had a non-circular shape and was causing a deterioration in product quality.

For example, in the case of a single-mode optical fiber, if the cross-sectional shape is elliptical due to the non-circular shape of the core and the clad, the degeneracy of the two orthogonal polarization modes, which should be degenerate, should be resolved, and the degeneracy should be different. As a result, it behaves as a mode, and (1) polarization dependence of the propagation constant occurs, and even if an optical fiber type component is manufactured using this optical fiber, polarization independence cannot be maintained. (2) Further, the propagation velocities between the two modes are different, and when an optical pulse is incident on this optical fiber, the waveform pulse is distorted in the optical pulse.

Regarding the cause of such uneven heating,
As a result of a more specific pursuit, in the case of the above-described conventional heating furnace, the vicinity of the electrodes 5 and 6 of the heating element 4 is likely to be thermally dissimilar to other parts, and as a result, the temperature distribution becomes uneven in the circumferential direction. It turns out that it tends to be uniform. In particular, in a heating furnace having a structure in which a cooling medium or the like is supplied to the electrodes 5 and 6 to suppress the temperature rise of the electrodes 5 and 6, this tendency becomes remarkable.

[0008] Such nonuniform heating temperature was actually measured in the heating furnace having the above-mentioned conventional structure.
It was like. That is, when the temperature distribution in the orbiting direction in the high temperature region, which is the heat treatment region in the core tube 1, is measured, an elliptical measured temperature distribution line L _t1 as shown in the figure is obtained, and the side where the electrode is located ( The temperature is low on both the left and right sides in the figure, and the 1600 ° C. line L _t2 and the 1650 ° C. line L _t3
Although the temperature drops to about the middle of the temperature range, the temperature is high on the side without electrodes (upper and lower sides in the figure), 1700 ° C.
It can be seen that the temperature about halfway between the line L _t4 and the 1650 ° C. line L _t3 is maintained, and a temperature difference of about 50 ° C. occurs due to the influence of the electrodes.

The present invention has been made in view of the above conventional circumstances, and provides a heating furnace in which the temperature is made uniform in the circumferential direction of the core tube by changing the wall thickness shape of the core tube. It is something to do.

[0010]

According to the present invention, there is provided a heating furnace having a multi-cylinder structure in which cylindrical heating elements are concentrically arranged on the outer periphery of the core tube, and the wall thickness of the core tube is reduced. In the heating furnace, the core tube is thickened in the high temperature region and at least one end thereof is thinned.

[0011]

[Function] Since the high temperature region of the core tube is thickened, the heat accumulation effect and the good heat conduction effect of the thickened part can ensure uniform heating in the core tube and the thinned thickness of the core tube. By supporting the core tube in part, heat dissipation can be effectively prevented. Moreover, the presence of this portion provides an ideal temperature distribution in the vertical direction of the core tube.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a heating furnace for producing an optical fiber according to the present invention. In the figure, 1
1 is a core tube, 12 and 13 are upper and lower end surfaces 1 of the core tube 11.
A support member for supporting the core tube 11 by contacting a and 1b, 14 is a cylindrical heating element located on the outer periphery of the core tube 11, and 15 and 16 are energizations installed at two locations of the heating element 14. The basic structure of the electrode is substantially the same as that of the heating furnace of FIG. 4 described above, but as shown in FIG. 1, in the core tube 11, a heated object such as an optical fiber preform is heated. The high temperature region 11c, which is a region (heat treatment region) where the original heating temperature is maintained, is formed thick, and the upper and lower end portions 11d and 11e are formed thin.

The core tube 11 is made of carbon or the like, and the high temperature region 11c, which is a thickened portion, has a specific thickness of 10% or more, preferably 20% of the outer diameter of the core tube 11. The above is recommended. This value is obtained, for example, from the graph shown in FIG. That is, in the core tube 11 having an outer diameter of about 72 mm, the wall thickness of the high temperature region 11c was changed like the horizontal axis by changing the inner diameter of the core tube 11 (up to 5 to 20 mm). The temperature difference in the orbiting direction inside is as shown on the vertical axis.
The wall thickness of 1c is about 10% of the outer diameter of the core tube 11 (about 7 m
m) gradually decreases from the thickness of about 20%
This is because it can be seen that it becomes considerably small in a portion having a thickness of about (about 15 mm).

The reason why such a temperature difference becomes small is that when the wall thickness of the high temperature region 11c is large, the amount of heat stored in the wall thickness portion increases, and a slight heat fluctuation on the side of the heating element 14 (the heating element concerned). When 14 itself is also formed of carbon or the like, it is difficult to uniformly manufacture the thickness of the cylindrical shape in the up-down direction and the circumferential direction, and some heat fluctuation is unavoidable. The heat from is effectively taken in, and at that time, the temperature on the electrodes 15 and 16 side is slightly low, and even if it is heated non-uniformly, the heat taken in from the high temperature part without electrodes has good thermal conductivity in the material. Since it can smoothly go around to the low temperature part corresponding to the electrode side,
It is considered that more uniform heating can be secured in the circulating direction in the core tube 11.

Further, thin-walled portions 1 at both upper and lower ends of the core tube 11
Basically, the thicknesses of 1d and 11e may be such that the core tube 11 can be retained, and a thickness of about 1.5 to 3 mm is desirable. If this part is thinned, its end face 11
When the core tube 11 is supported by a and 11b, the contact area is small, and it is possible to effectively prevent conduction (escape) of heat through the support members 12 and 13. Further, due to the existence of the thin portions 11d and 11e, the heat amount is small in the unnecessary heat portion, and as described above, a large amount of heat is stored in the high temperature region 11c which is the portion where the original heat treatment is performed. An ideal core tube temperature distribution can also be obtained.

In the heating furnace having the structure as shown in FIG. 1, the dimensions of each part are as follows: inner diameter of heating element 80 mm, length (height) of heating element 90 mm, outer diameter of core tube 72 mm, high temperature region of core tube. Using a wall thickness of 15 mm (core tube inner diameter of the high temperature region part 57 mm = 72 mm-15 mm),
When the optical fiber preform was heated and drawn, it was possible to obtain an optical fiber in which the non-circular outer diameter of the preform was about 2%, but was reduced to about 0.3% or less.

FIG. 3 shows a furnace core tube which is another embodiment of the heating furnace for producing an optical fiber according to the present invention. In this furnace core tube 11 ', a high temperature region 11'c and The upper and lower thin portions 11'd and 11'e are separately formed and assembled, and the same operation and effect as in the case of FIG. 1 can be obtained.

In each of the above embodiments, the core tubes 11, 1 are used.
The thin portions 11d, 11e, 11'd, and
11'e was provided, but the core tubes 11, 11 '
When supporting itself only at the lower end or suspending only at the upper end, it is possible to provide a thin portion only on one side. Further, even when the supporting members 12 and 13 are in contact with each other, they may be in contact with the entire peripheral surface of the end face or may be in partial contact. Further, it may be supported by contacting in a dot or line shape with a contact area as small as possible from the outer circumference to the side surface of the tube.

[0019]

As is apparent from the above description, according to the heating furnace of the present invention, a heating furnace having a multi-cylinder structure in which cylindrical heating elements are concentrically arranged on the outer periphery of the core tube,
Since the wall thickness of the core tube is increased in the high temperature region of the core tube and at least one end side thereof is thinned, the core has a heat storage action and a good heat conduction action in the wall portion of the high temperature region. The uniform heating inside the tube can be ensured, and one end of the thinned core tube effectively prevents the dissipation of heat conduction and supports the core tube itself and the vertical direction (axial direction) of the core tube.
It is possible to obtain an ideal temperature distribution at.

By providing such a heating furnace, it becomes possible to perform heat treatment with high accuracy in transparency, drawing, and drawing in the production of optical fibers, etc., and it is possible to obtain a final product having desired quality characteristics. it can.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view showing an embodiment of a heating furnace according to the present invention.

FIG. 2 is a graph showing the relationship between the wall thickness of the high temperature region and the temperature difference in the circulating direction in the furnace core tube of the heating furnace according to the present invention.

FIG. 3 is a vertical cross-sectional view showing a core tube which is another embodiment of the heating furnace according to the present invention.

FIG. 4 is a schematic vertical sectional view showing a conventional heating furnace.

5 is a cross-sectional view taken along the line VV of FIG.

FIG. 6 is an explanatory diagram showing measured temperature distribution lines in a core tube of a conventional heating furnace.

[Explanation of symbols]

11, 11 'Reactor tube 11a, 11'a End surface 11b, 11'b End surface 11c, 11'c Thickened portion (high temperature region) 11d, 11'd Thin portion 11e, 11'e Thin portion 12 Support Member 13 Supporting member 14 Heating element 15 Electrode 16 Electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-148725 (JP, A) Actual development 61-185999 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) F27D 11/02 C03B 19/00 C03B 37/027 G02B 6/00 356

Claims (2)

(57) [Claims] 1. A heating furnace having a multi-cylinder structure in which cylindrical heating elements are concentrically arranged on the outer periphery of a core tube, wherein the wall thickness of the core tube is increased in a high temperature region of the core tube. , A heating furnace characterized by thinning at least one end side thereof. 2. The heating furnace according to claim 1, wherein the thickness of the high temperature region in which the thickness of the core tube is increased is 10% or more of the outer diameter of the core tube.