Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2003-05-28
2004-11-02
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S540000
Reexamination Certificate
active
06810692
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a furnace and a method for drawing an optical fiber from heated and molten preform.
BACKGROUND ART
Various techniques have been known relating to a furnace for drawing an optical fiber from heated preform formed with silica glass as a main component, and they are described in Japanese Patent No. 2,542,679, Japanese Patent Laid-Open No. 147969/1993 and Japanese Patent Laid-Open No. 2832/1997. Since the figures and terminology in these prior arts differ partially from those used in this invention, descriptions will be made below where they are interpreted according to the figures and terminology used in this invention in order to clarify the difference between the prior arts and this invention.
A main part of the furnace for drawing an optical fiber disclosed in Japanese Patent No. 2,542,679 is shown in
FIG. 8
, in which numeral
21
denotes a preform,
21
a
denotes an optical fiber,
22
denotes an inner tube,
22
a
denotes a gas blowing inlet,
23
denotes an outer tube,
24
denotes a gas supplying inlet,
24
a
denotes a gas passage,
25
denotes a dummy rod,
25
a
denotes a connecting part,
26
denotes a retainer,
27
denotes a seal piston,
28
denotes a muffle tube, and
29
denotes a heater.
In the furnace for drawing an optical fiber, the dummy rod
25
and the preform
21
are arranged inside the muffle tube
28
and the inner tube
22
arranged to be connected to an upper end of the muffle tube
28
, while they are connected via the connecting part
25
a
and are descended together. The vicinity of a lower end of the preform
21
is melted through heating by the heater
29
arranged outside the muffle tube
28
, and the optical fiber
21
a
is drawn downward from the lower end of the preform
21
. The inner tube
22
arranged to be connected to the upper end of the muffle tube
28
is to contain the long preform
21
on starting the drawing.
The gas passage
24
a
is formed between the inner tube
22
and the outer tube
23
arranged outside the same, and an inert gas is supplied from the gas supplying inlet
24
to the gas passage
24
a
, so as to blow the inert gas into the inside of the inner tube
22
from the numerous gas blowing inlets
22
a
provided circumferentially and in the height direction on the wall surface of the inner tube
22
. The inert gas is flowed inside the inner tube
22
and the muffle tube
28
to prevent oxidation deterioration of the muffle tube and the like, and when the temperature distribution of the inert gas by heating and the flow of the inert as are not uniform, the fluctuation of the diameter of the optical fiber drawn from the preform is liable to occur.
Therefore, in this example of the furnace for drawing an optical fiber, the seal piston
27
connected to the dummy rod
25
via the retainer
26
and moved with the dummy rod
25
is provided at the dummy rod
25
arranged on the upper part of the preform
21
. In the beginning of drawing, the dummy rod
25
and the seal piston
27
are in the upper part because the preform is long. With proceeding the drawing, the preform
21
is shortened from the lower end, and is descended, and thus, the dummy rod
25
and the seal piston
27
are also descended.
In this case, if the seal piston
27
were not present, the space between the dummy rod
25
and the inner tube
22
would gradually increase, but because the seal piston
27
is present, the volume of space above the preform
21
is substantially constant. Therefore, it has been said that turbulence of the stream of the inert gas in the space between the preform
21
and the seal piston
27
occurs only scarcely due to the provision of the seal piston
27
.
In the furnace using the seal piston for drawing an optical fiber, when the preform has a length of 1.5 m or more, the seal piston necessarily has an proportionate length, and the weight thereof becomes also heavy. Because a supporting member for supporting them at the upper part must withstand the weight of the preform and the seal piston, the supporting member becomes also necessarily large. Because the seal piston must resist to high temperature, a heat resistant material, such as carbon, quartz and the like, is necessarily used for the seal piston, and it becomes costly when it is of large scale.
Furthermore, because the seal piston moves as sliding on the inner wall surface of the inner tube, dust is liable to come from the sliding part, which may adversely affect the strength of the drawn optical fiber.
Furthermore, as the seal piston descents, the numerous gas blowing inlets provided in the wall surface of the inner tube are sealed one by one from the upper part with the seal piston, a precise controller for continuously controlling the gas flow rate is necessary to maintain the constant flow rate of the stream of the inert gas.
A furnace for drawing an optical fiber disclosed in Japanese Patent Laid-Open No. 147969/1993 will then be described. A main part of the furnace for drawing an optical fiber is shown in FIG.
9
. In
FIG. 9
, the same references as in
FIG. 8
show the same components. Numeral
30
denotes a separating plate,
30
a
denotes a gap,
30
b
denotes pores,
31
a
denotes an upper space,
31
b
denotes a lower space, and
32
denotes an upper lid. The furnace for drawing an optical fiber shown in
FIG. 9
is different from that shown in
FIG. 8
in the following points. There is no member corresponding to the seal piston in
FIG. 8
, and an upper end of an inner tube
22
is closed with the upper lid
32
except for the part through which a dummy rod
25
penetrates.
In the furnace for drawing an optical fiber, a preform
21
is arranged inside a muffle tube
28
and the inner tube
22
connected to an upper end thereof, and the preform
21
is supported by hanging by the dummy rod
25
through a connecting part
25
a
. The vicinity of the lower end of the preform
21
is heated and melted by a heater
29
arranged outside the muffle tube
28
, and an optical fiber
21
a
is drawn downward.
An outer tube
23
is arranged concentrically outside the inner tube
22
, and an inert gas is blown into the inside of the inner tube via a gas blowing inlet
22
a
through a gas passage
24
a
formed with the outer tube
23
and the inner tube
22
. The inert gas is introduced to the gas passage
24
a
from a gas supplying inlet
24
. The inert gas blown into the inner tube
22
descends through the space between (the inner tube
22
and the muffle tube
28
) and (the preform
21
or the dummy rod
25
), then it is exhausted through the vicinity of the optical fiber
21
a.
The space inside the inner tube
22
is separated by the separating plate
30
comprising a quartz plate or the like into an upper part and a lower part, and the inert gas flows from the upper space
31
a
of the separating plate
30
to the lower space
31
b
through the gap
30
a
between the separating plate
30
and the inner tube
22
or the pores
30
b
provided in the separating plate
30
. Because the lower space
31
b
is of a relatively high temperature and turbulence of the gas stream is decreased owing to the presence of the separating plate
30
, it has been said that an optical fiber having a small fluctuation in diameter can be drawn even in the case of a large preform.
However, in the case of this furnace for drawing an optical fiber, when the preform
21
becomes small with proceeding the drawing, the upper space
31
a
becomes large. On the other hand, the temperature near the upper end of the preform
21
is increased by heating the vicinity of the lower end of the preform
21
, and said temperature becomes higher when the preform becomes small. Because of the presence of the separating plate
30
comprising a quartz plate, the temperature is somewhat decreased above the separating plate, but it becomes 550° C. or more near the lower end of the upper space
31
a
. At this time, the temperature near the upper end of the upper space
31
a
is about 200° C. to form a considerable difference in temperature between the upper pa
Kuwahara Kazuya
Nagayama Katsuya
Taru Toshiki
Tsuchiya Ichiro
Hoffmann John
Sumitomo Electric Industries Ltd.
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