Glass manufacturing – Fiber making apparatus – With measuring – controlling – sensing – timing – inspecting,...
Reexamination Certificate
1999-02-05
2001-02-27
Vincent, Sean (Department: 1731)
Glass manufacturing
Fiber making apparatus
With measuring, controlling, sensing, timing, inspecting,...
C065S379000, C065S476000, C065S477000, C065S537000
Reexamination Certificate
active
06192715
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a furnace for forming an optical fiber which heats, melts, and draws an optical fiber preform to produce an optical fiber (optical fiber drawing furnace), more particularly an optical fiber drawing furnace provided with a means for preventing entry of ambient air from the outside of the optical fiber drawing furnace at a bottom thereof.
2. Description of the Related Art
There is strong demand for the improvement of the productivity of optical fibers due to the rapid increase in applications of optical fibers. To satisfy this demand, attempts are being made to increase the diameter of the optical fiber preform to be drawn and to raise the drawing speed. The high speed drawing of an optical fiber preform having a large diameter, however, causes the pressure in an inner space of an optical fiber drawing furnace to vary more due to the vibration of the moving optical fiber preform having the large diameter which in turn causes variations in the drawing speed of the optical fiber and variations in the diameter of the drawn optical fiber. Sometimes, the pressure in the inner space of the optical fiber drawing furnace becomes lower than the atmospheric pressure (pressure outside optical fiber drawing furnace). If the pressure in the inner space of the optical fiber drawing furnace is lower than that of the atmosphere, air may invade the inner space to contact the surface of the optical fiber preform, the surface of the drawn optical fiber, and the inner surface of a furnace tube arranged in the optical fiber drawing furnace. The contact of the surfaces of the optical fiber preform and the optical fiber by air results in deterioration of the quality of the drawn optical fiber and weakens the mechanical strength of the drawn fiber. The contact of the inner surface of the furnace tube with air shortens the life of the tube. Accordingly, a variety of improvements have been experimented with.
One approach for overcoming the above problem is to increase the amount of inert gas which is introduced and filled in the inner space of the optical fiber drawing furnace. The increase of the amount of inert gas introduced into the inner space, however, lowers the temperature of the optical fiber preform and makes it necessary to raise the heating temperature of the heater to maintain a suitable temperature for drawing the optical fiber in the inner space. Since the furnace tube is usually made of carbon, the raise of the heating temperature may accelerate the deterioration of the furnace tube. The rapid deterioration of the furnace tube causes the rapid generation of carbon duct which in turn reduces the quality and strength of the optical fiber. The rapid deterioration of the furnace tube, i.e., the insufficient life of the furnace tube, therefore makes it necessary to frequently replace the furnace tube, resulting in troublesome maintenance and increased production costs.
Other countermeasures have been devised as well.
FIG. 1
is a sectional view of an optical fiber drawing furnace
101
of the related art.
The optical fiber drawing furnace
101
includes a furnace body
104
, a heater
103
for heating and melting an optical fiber preform
105
introduced into an inner space
108
of the furnace body
104
from the top of the furnace body
104
, a furnace tube
102
arranged at an inner wall of the heater
103
, a gas introducing portion
107
for introducing inert gas such as argon (Ar) gas or helium (He) gas into the inner space
108
from the lower portion of the furnace body
104
, and a bottom cover
109
. The bottom cover
109
has a hole (aperture) through which an optical fiber
106
is drawn to the outside of the furnace body
104
.
The furnace tube
102
is made of, for example, carbon and is arranged in the inner wall of the furnace body
104
at a position at which the heater
103
is provided to prevent direct contact between the heater
103
and the optical fiber preform
105
.
In the optical fiber drawing furnace
101
, the optical fiber preform
105
is introduced into the inner space
108
of the furnace body
104
, heated and melted by heat from the heater
103
, and pulled down at a predetermined tension to form the optical fiber
106
. The optical fiber
106
is extracted through the hole formed in the bottom cover
109
.
Usually, the optical fiber preform
105
is heated and melted at around 2000° C. Thus, if the optical fiber
106
is formed in a normal atmosphere including oxygen, the carbon furnace tube
102
may be oxidized and damaged and dust may occur. The dust may lower the characteristics of the drawn optical fiber
106
. The oxidization of the furnace tube
102
results in a shorter life of the furnace tube
102
and ends up raising the production costs of the optical fiber
106
.
Therefore, the gas introduction portion
107
is provided at the lower portion of the furnace body
104
to introduce an inert gas such as argon (Ar) gas, nitrogen (Ni) gas, or helium (He) gas into the inner space
108
of the furnace body
104
so as to prevent entry of ambient gas, such as air, through the hole in the bottom cover
109
. The pressure in the inner space
108
should be higher than the pressure of the ambient gas to prevent entry of ambient gas from the hole of the bottom cover
109
. The inert gas introduced into the inner space
108
heads mainly to the top of the furnace body
104
and partially to the outside of the optical fiber drawing furnace through the hole of the bottom cover
109
.
The hole in the bottom cover
109
must be a predetermined diameter so that the drawing optical fiber
106
can pass through it at a high speed without contacting the same. Accordingly, it is impossible to completely prevent entry of ambient gas through the hole of the bottom cover
109
. To improve the extent to which entry of ambient gas is prevented, a higher pressure state of the inner space
108
and a larger amount of the introduction of the inert gas through the gas introduction portion
107
are necessary, but these lower the temperature in the inner space
108
and waste the inner gas, as discussed above.
Another related art will be described referring to FIG.
2
.
An optical fiber drawing furnace
111
illustrated in
FIG. 2
includes a furnace body
114
, a heater
113
, a furnace tube
112
, a bottom cover
119
arranged at a bottom of the furnace body
114
and having a hole thorough which a drawing optical fiber
116
passes, and a gas introduction portion
117
provided at a lower portion of the furnace body
114
and immediately above the bottom cover
119
. These structures are substantially identical to those in FIG.
1
.
The optical fiber drawing furnace
111
illustrated in
FIG. 2
further includes an additional gas introduction portion
120
at an upper portion of the furnace body
114
through which an optical fiber preform
115
is introduced into an inner space
118
. Inert gas introduced through the additional gas introduction portion
120
prevents entry of the ambient gas to the inner space
118
from the top at which the optical fiber preform
115
is introduced into the inner space
118
.
The inert gas is introduced into the inner space
118
through the gas introduction portion
117
to maintain a positive pressure state of the inner space with respect to the outside of the furnace
11
so as to prevent entry of the ambient gas into the inner space
118
through the hole of the bottom cover
119
.
The optical fiber drawing furnace
111
suffers from the disadvantage of the entry of the ambient gas into the inner space
118
of the furnace body
114
through the hole in the bottom cover
119
due to the same reasons as to those described above with reference to FIG.
1
. Thus, the optical fiber drawing furnace
111
still suffers from the disadvantages of the short life of the optical fiber drawing furnace
111
, the low characteristics of the optical fiber
116
, and the increased production cost of the optical fiber
116
.
Japanese Examined Patent Publication (Kok
Koaizawa Hisashi
Kumada Tetsuya
Orita Nobuaki
Sakata Yoshiyuki
Arent Fox Kintner & Plotkin & Kahn, PLLC
The Furukawa Electric Co. Ltd.
Vincent Sean
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