Optical fiber drawing furnace with a zig zag heating element

Details Optical fiber drawing furnace with a zig zag heating element Optical fiber drawing furnace with a zig zag heating element

1996-07-03

2003-04-15

Hoffman, John (Department: 1731)

Glass manufacturing

Fiber making apparatus

With drawing means

C065S374150, C219S541000, C219S553000

Reexamination Certificate

active

06546760

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber drawing furnace which can draw an optical fiber with small non-circularity.
2. Description of the Related Art
An optical fiber is obtained by heating a perform for optical fiber in an optical fiber drawing furnace to melt and by drawing the molten preform for optical fiber from the lower end thereof. More concretely, as disclosed in Japanese Patent Application Laid-open No. 8233/1988, the optical fiber drawn from the lower end portion of the optical fiber preform is instantly applied a resin protecting layer, such as ultraviolet ray curing resin on the outer periphery by a resin applicator. Furthermore, by passing through a resin curing apparatus, the resin protection layer is cared. Thereafter, the optical fiber is taken up by a take-up machine. On the other hand, by a diameter measuring device provided between the drawing furnace and the resin applicator, a diameter of the optical fiber is measured. Then, a drawing speed of the optical fiber to be drawn from the optical fiber preform is adjusted so that the diameter of the drawn optical fiber is held constant.
On the other hand, when the non-circularity of the optical fiber is large, a diameter of the hole of the optical fiber connector ferrule for setting the optical fiber has to be made greater. When the hole diameter for setting the optical fiber is made greater, an offset between an axis of the hole and an axis of the optical fiber becomes greater to possible cause large connection loss. Similarly, upon mutual connection by mating a pair of optical fibers utilizing a V-shaped groove formed in a positioning block, radius of the optical fiber may fluctuate at the portion contacting with the V-shaped groove. Therefore, the non-circularity of the optical fiber should be a cause of offset of axis upon connection or connection loss.
In order to derive the non-circularity, the diameters of the optical fiber are measured at a plurality of positions in the circumferential direction. With taking the largest diameter among the measured diameters as “longer diameter” and the smallest diameter as “shorter diameter”. Then, the non-circularity can be expressed by (longer diameter-shorter diameter)/average diameter. In general, assuming that the maximum diameter of the objective circularity is D
1
and the minimum diameter is D
2
, the non-circularity &egr; is expressed by the following equation.
&egr;={2×(D
1
−D
2
)/(D
1
+D
2
)}×100(%)
Conventionally, in order to reduce the non-circularity to zero percent as close as possible, the lower end of the optical fiber preform is matched with the center of the furnace to uniformly heat the optical fiber preform along the circumferential direction thereof. On the other hand, as disclosed in Japanese Patent Application Laid-open No. 96042/1989, there has been proposed a technology for solving a problem of temperature fluctuation by rotating a muffle tube. Also, as disclosed in Japanese Patent Application Laid-open No. 227837/1994, there has been a proposal for reducing non-circularity by restricting temperature fluctuation along the circumferential direction of the heater by extending a pair of electrode connecting portions opposing with an angle of 180 degrees interval to the outer peripheral well of the furnace body and connecting electrodes to the tip ends of the electrode connecting portions.
One example of sectional structure of such conventional drawing furnace is shown in FIG.
9
and the external view of the heater of the conventional drawing furnace is illustrated in FIG.
10
. As shown, in a center portion of a furnace body
102
, a cylindrical muffle tube
103
is assembled. Also, between the center portion of the furnace body
102
and the muffle tube
103
, a heater
104
of cylindrical configuration as a whole is diagnosed. Surrounding the heater
104
, a heat insulator
101
is set. By the heater
104
, a lower end portion of the optical fiber preform (not shown) supplied into the muffle tube
103
is heated for melting. Then, the molten portion of the optical fiber preform is drawn from the lower end of the furnace body
102
as an optical fiber.
The heater
104
has a heating portion
105
meandering in vertical direction and formed in to a cylindrical configuration as a whole, and a set of two electrode connecting portions
106
and
107
mutually offset with an angle of 180 degrees interval in circumferential direction of the heating portion
105
. These one set of the electrode connecting portions
106
and
107
are extended from the upper end of the heating portion
105
to radially project outwardly. To these one set of the electrode connecting portions
106
and
107
, one set of electrodes
108
and
109
lead outside of the furnace body
102
are connected via connecting members
110
. These electrodes
108
and
109
are connected to a single phase AC power source
113
via a transformer
111
and a power control unit
112
. A current supplied from the single phase AC power source
113
via the power control unit
112
passes through the heating portion
105
via the connecting member
110
and one of the electrode connecting portion
106
from one of the electrode
100
and flows to the connecting member
110
and the other electrode
109
from the other electrode connecting portion
107
, or flows in opposite direction to heat the heating portion
105
In the recent years, associating with lowering of cost of the optical fiber, the optical fiber preform is becoming into greater diameter and drawing speed is becoming higher. On the other hand, the drawing furnace per se inherently cause certain non uniformity of the temperature in the circumferential direction due to presence of electrodes, cooling water flow and so forth, for example. Non-uniformity of temperature distribution in the circumferential direction becomes more significant is greater diameter of the optical fiber preform than that smaller diameter to cause more significant temperature fluctuation within the optical fiber preform. Furthermore, when the optical fiber is drawn at higher speed than that in the prior art, a period to pass a neck down portion becomes shorter where the optical fiber preform is molten and reduced in diameter. This results in difficulty of reduction of the temperature fluctuation to increase tendency of increasing of non-circularity.
On the other hand, in case of the method, in which the muffle tube is rotated as disclosed in Japanese Patent Application Laid-open No. 96042/1989, the temperature fluctuation of the optical fiber preform is advantageously reduced to permit reduction of the non-circularity of the optical fiber. However, on the other hand, by rotation of the muffle tube, flow of a gas within the furnace is disturbed to make fluctuation of the diameter of the optical fiber greater. As a result, upon connection by means of an optical connector or by mutually melting the connecting ends, connection loss can be increased.
Furthermore, the method to extend a pair of electrode connecting portions to the outer periphery of the furnace body as disclosed in Japanese Patent Application Laid-open No. 227837/1994, is advantageous in that the temperature fluctuation in the circumferential direction of the heater can be improved in comparison with the prior art. However, there still present a difference of temperature between the atmosphere in the vicinity of the electrode connecting portions opposing with an angle of 180 degrees interval and the atmosphere in the direction perpendicular to the opposing direction of the electrode connecting portions to possible make cross section of the obtained optical fiber somewhat elliptic.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical fiber drawing furnace which makes it possible to draw an optical fiber with smaller non-circularity.
According to the first aspect of the present invention, an optical fiber drawing furnace comprises:
a muffle tube, in whi

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