U.S. patent application number 10/420374 was filed with the patent office on 2004-03-18 for furnace for drawing optical fiber preform to make optical fiber and method for drawing optical fiber using the same.
This patent application is currently assigned to LG Cable Ltd.. Invention is credited to Bae, Sang-Joon, Jang, Myung-Ho, Kim, Chul-Min, Kwon, Young-Il, Lee, Joon-Keun.
Application Number | 20040050112 10/420374 |
Document ID | / |
Family ID | 31987294 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040050112 |
Kind Code |
A1 |
Bae, Sang-Joon ; et
al. |
March 18, 2004 |
Furnace for drawing optical fiber preform to make optical fiber and
method for drawing optical fiber using the same
Abstract
A furnace for drawing an optical fiber includes a body having an
upper and lower openings for supplying a preform and discharging a
drawn optical fiber, a heating unit installed in the body for
receiving and melting the preform, an atmosphere blocking tube
installed to the lower opening for discharging the drawn optical
fiber and blocking the optical fiber from the atmosphere, an upper
introduction port formed at an upper portion of the body for
introducing an inert gas toward the preform and partially
discharged outside through a gap between the preform and the upper
opening, a central and lower introduction ports formed at central
and lower portions for introducing an inert gas into the body, a
first flow guiding means for guiding the inert gas introduced
through the central introduction port upward and then flowed down
along a surface of the preform, and a second flow guiding means for
guiding the inert gas introduced through the lower introduction
port upward and then discharged outside through the atmosphere
blocking tube along a surface of the drawn optical fiber.
Inventors: |
Bae, Sang-Joon; (Ansan-Si,
KR) ; Kwon, Young-Il; (Seoul, KR) ; Lee,
Joon-Keun; (Seoul, KR) ; Kim, Chul-Min;
(Suwon-si, KR) ; Jang, Myung-Ho;
(Gyeongsangbuk-do, KR) |
Correspondence
Address: |
Eric D. Cohen
Welsh & Katz, Ltd.
22nd Floor
120 South Riverside Plaza
Chicago
IL
60606
US
|
Assignee: |
LG Cable Ltd.
Seoul
KR
|
Family ID: |
31987294 |
Appl. No.: |
10/420374 |
Filed: |
April 22, 2003 |
Current U.S.
Class: |
65/424 ;
65/533 |
Current CPC
Class: |
C03B 37/029 20130101;
C03B 2205/98 20130101; C03B 2205/81 20130101; C03B 2205/83
20130101; C03B 2205/90 20130101; C03B 2205/96 20130101; C03B
2203/36 20130101 |
Class at
Publication: |
065/424 ;
065/533 |
International
Class: |
C03B 037/018 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2002 |
KR |
2002-52300 |
Claims
What is claimed is:
1. A furnace for drawing an optical fiber comprising: a furnace
body having an upper opening for supplying a preform and a lower
opening for discharging an optical fiber drawn from the preform;
heating means installed in the furnace body, the heating means
receiving and melting the preform therein in order to draw the
preform; an atmosphere blocking tube installed at the lower opening
for ensuring the passage of the drawn optical fiber therein, the
atmosphere blocking tube protecting the drawn optical fiber from
the external atmosphere; an upper introduction port formed at an
upper portion of the furnace body for introducing an inert gas
toward the preform, the inert gas being partially discharged
outside through a gap between the preform and the upper opening; a
central introduction port formed at a central portion of the
furnace body for introducing an inert gas into the furnace body; a
lower introduction port formed at a lower portion of the furnace
body for introducing an inert gas into the furnace body; a first
flow guiding means for guiding the inert gas introduced through the
central introduction port upward along the melting area of the
heating means so that the inert gas collides with the inert gas
introduced through the upper introduction port and then flows
toward a lower portion of the furnace body along the surface of the
preform; and a second flow guiding means for guiding the inert gas
introduced through the lower introduction port upward along the
melting area of the heating means so that the inert gas collides
with the inert gas introduced through the central introduction port
and is then discharged outside through the atmosphere blocking tube
along the surface of the drawn optical fiber.
2. A furnace for drawing an optical fiber according to claim 1,
wherein the heating means include: a first graphite core tube
installed spaced apart from the wall of the furnace body in order
to form the melting area therein for receiving and melting the
preform; and a second graphite core tube installed, spaced apart
from the wall of the furnace body, at a lower portion of the first
graphite core tube.
3. A furnace for drawing an optical fiber according to claim 2,
wherein the first graphite core tube heats at a temperature of
2,100.about.2,300.degree. C. and the second graphite core tube
heats at a temperature of 1,600.about.1,800.degree. C.
4. A furnace for drawing an optical fiber according to claim 2,
wherein the first graphite core tube heats at a temperature of
2,100.about.2,300.degree. C. and the second graphite core tube
receives a heat from the first graphite core tube and then
transfers the heat to the preform.
5. A furnace for drawing an optical fiber according to claim 2,
wherein the first flow guiding means include: a passage between the
first graphite core tube and the wall of the furnace near the
central introduction port; a support flange extended from the wall
of the furnace body for supporting the second graphite core tube;
and a passage formed between the first and second graphite core
tubes in which an upper end of the second graphite core tube having
a diameter smaller than the first graphite core tube is partially
inserted into the first graphite core tube.
6. A furnace for drawing an optical fiber according to claim 5,
wherein the passage between the first and second graphite core
tubes is formed near a neck-down zone of the preform.
7. A furnace for drawing an optical fiber according to claim 2,
wherein the second flow guiding means includes: a passage formed
between the second graphite core tube and the wall of the furnace
body near the lower introduction port; and a passage formed between
the atmosphere blocking tube and the second graphite core tube.
8. A furnace for drawing an optical fiber according to claim 1,
wherein a flow rate of the inert gas at the central introduction
port is 3.about.4 times as large as a flow rate of the inert gas at
the upper introduction port, and wherein a flow rate of the inert
gas at the lower introduction port is 2.about.3 times as large as a
flow rate of the inert gas at the upper introduction port.
9. A furnace for drawing an optical fiber according to claim 1,
further comprising a ring-shaped spraying sleeve having a plurality
of nozzles for supplying the inert gas introduced through the upper
introduction port evenly to the preform.
10. A furnace for drawing an optical fiber according to claim 1,
wherein the atmosphere blocking tube is as long as the drawn
optical fiber has a temperature of about 1,200.about.1,300.degree.
C. at the exit of the atmosphere blocking tube.
11. A furnace for drawing an optical fiber according to claim 10,
wherein the length of the atmosphere blocking tube is in the range
of 2.0.about.2.5 m.
12. A furnace for drawing an optical fiber according to claim 10,
wherein the upper end of the atmosphere blocking tube is positioned
200.about.250 mm below a neck-down zone of the preform.
13. A furnace for drawing an optical fiber comprising: a furnace
body having an upper opening for supplying a preform and a lower
opening for discharging an optical fiber drawn from the preform; a
first core tube installed spaced apart from the wall of the furnace
body to form a first passage between the first core tube and the
wall of the furnace body, the first core tube forming a melting
area therein to heat and melt the preform; a second core tube
installed spaced apart from the wall of the furnace body to form a
second passage between the second core tube and the wall of the
furnace body, the second core tube having a diameter smaller than
the first core tube so as to be partially inserted into a lower end
of the first core tube to thereby form a third passage between the
first and second core tubes; an upper introduction port formed at
an upper portion of the furnace body for introducing an inert gas
toward the preform, the inert gas being partially discharged
outside through a gap between the preform and the upper opening; a
central introduction port communicated with the first passage
formed between the first core tube and the wall of the furnace body
for introducing an inert gas into the first passage; a lower
introduction port communicated with the second passage formed
between the second core tube and the wall of the furnace body for
introducing an inert gas into the second passage; a support flange
interposed between the first and second passage to divide the first
and second passages; and an atmosphere blocking tube installed at
the lower opening for forming a fourth passage in which the inert
gas introduced through the second passage flows between the second
core tube and the atmosphere blocking tube, the atmosphere blocking
tube ensuring the passage of the drawn optical fiber therein to
protect the drawn optical fiber from the external atmosphere.
14. A furnace for drawing an optical fiber according to claim 13,
wherein the third passage is formed near a neck-down zone of the
preform.
15. A furnace for drawing an optical fiber according to claim 13,
wherein a flow rate of the inert gas at the central introduction
port is 3.about.4 times as large as a flow rate of the inert gas at
the upper introduction port, and wherein a flow rate of the inert
gas at the lower introduction port is 2.about.3 times as large as a
flow rate of the inert gas at the upper introduction port.
16. A furnace for drawing an optical fiber according to claim 13,
further comprising a ring-shaped spraying sleeve having a plurality
of nozzles for supplying the inert gas, introduced through the
upper introduction port, evenly to the preform.
17. A furnace for drawing an optical fiber according to claim 13,
wherein the atmosphere blocking tube is as long as the drawn
optical fiber has a temperature of about 1,200.about.1,300.degree.
C. at the exit of the atmosphere blocking tube.
18. A furnace for drawing an optical fiber according to claim 17,
wherein the length of the atmosphere blocking tube is in the range
of 2.0.about.2.5 m.
19. A furnace for drawing an optical fiber according to claim 17,
wherein the upper end of the atmosphere blocking tube is positioned
200.about.250 mm below a neck-down zone of the preform.
20. A method for drawing an optical fiber by melting a preform in a
furnace which includes a furnace body having an upper opening for
supplying a preform, a lower opening for discharging an optical
fiber drawn from the preform, and upper, central and lower
introduction ports for introducing an inert gas; heating means
installed in the furnace body to receive and melt the preform in a
melting area therein; and an atmosphere blocking tube installed at
the lower opening to ensure the passage of the drawn optical fiber
therein, wherein the method comprises the steps of: (a) supplying a
preform into the melting area through the upper opening; (b)
melting the preform by heating the preform with the heating means;
(c) drawing an optical fiber from a melted portion of the preform
at its lower end; (d) introducing an inert gas through the upper
introduction port so that the inert gas is partially discharged
outside through a gap between the preform and the upper opening;
(e) introducing an inert gas through the central introduction port
so that the inert gas flows upward in the melting area and collides
with the flow of inert gas introduced through the upper
introduction port, and then flows downward along a surface of the
preform; and (f) introducing an inert gas through the lower
introduction port so that the inert gas flows upward along the
melting area and then collides with the inert gas introduced
through the central introduction port, and is then discharged
outside through the atmosphere blocking tube along the surface of
the drawn optical fiber.
21. A method of drawing an optical fiber according to claim 18,
wherein the step (e) includes the steps of: supplying an inert gas
through a passage formed between a wall of the furnace body and a
first cylindrical core tube installed in the furnace body; and
guiding the inert gas, having passed through the passage between
the wall of the furnace body and the first core tube, to an upper
portion of the melting area through a passage between an inner side
of the first core tube and an outer side of an upper portion of a
second core tube installed in the furnace body wherein an upper end
of the second core tube is partially inserted into a lower end of
the first core tube.
22. A method of drawing an optical fiber according to claim 20,
wherein the step (f) includes the steps of: supplying an inert gas
through a passage formed between a wall of the furnace body and the
second core tube; and guiding the inert gas, having passed through
the passage between the wall of the furnace body and the second
core tube, to an upper portion of the melting area through a
passage formed between the atmosphere blocking tube and the second
core tube.
23. A method of drawing an optical fiber according to claim 20,
wherein a flow rate of the inert gas at the central introduction
port is 3.about.4 times as large as a flow rate of the inert gas at
the upper introduction port, and wherein a flow rate of the inert
gas at the lower introduction port is 2.about.3 times as large as a
flow rate of the inert gas at the upper introduction port
24. A method of drawing an optical fiber according to claim 20,
wherein the step (c) further includes the step of adjusting a
temperature of the drawn optical fiber to be maintained to about
1,200.about.1,300.degree. C. while the drawn optical fiber moves
out of the atmosphere blocking tube.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a fiber-drawing furnace for
melting and drawing an optical fiber preform to make an optical
fiber and its drawing method, and more particularly to a
fiber-drawing furnace for drawing an optical fiber having low
Polarization Mode Dispersion (PMD) by decreasing diameter
variations and non-circularity of the optical fiber and a method
for drawing an optical fiber using the furnace.
[0003] 2. Description of the Related Art
[0004] Generally, an optical fiber is manufactured by drawing a
transparent glass-sintered material, called an optical fiber
preform, in a furnace at a high temperature. As well known in the
art, a resistance furnace and an induction furnace are commonly
used in drawing the optical fiber preform.
[0005] The furnace has a graphite core tube positioned vertically
in a work place for receiving a preform supplied through an upper
opening thereof. The graphite core tube is heated by resistance
heating or induction heating and acts as a heating body. The
graphite core tube is commonly heated up to
2,100.about.2,300.degree. C. in order to melt the preform and draw
an optical fiber from the preform. Under such a high temperature,
the graphite core tube may be easily oxidized. Thus, the core tube
should be maintained in a non-oxidizing circumstance using helium,
nitrogen, argon and the like for the prevention of oxidization.
[0006] In the furnace, SiO.sub.2 and C particles separated from the
graphite core tube and the transparent glass preform react to
thereby create SiC particles. If the silicon carbides or carbon
particles adhere to the preform, a tensile strength at the adhered
position is considerably reduced, which causes break of the optical
fiber during a drawing process. In addition, a melt vapor generated
from the preform can adhere to an inner wall of the graphite core
tube and damage it.
[0007] To prevent such problems, an inert gas is relatively
strongly blown into the furnace. The flow of an inert gas has a
profound influence on the optical fiber formation. In other words,
the inert gas introduced into the furnace creates a turbulence
flow, particularly strongly at a neck-down zone where the optical
fiber preform is melt and reduces its section abruptly. Such a flow
of the gas becomes irregular in the circumferential direction of
the preform. Therefore, a diameter of the preform near the
neck-down zone varies irregularly, and a drawn optical fiber thus
has an irregular diameter with an increase in the
non-circularity.
[0008] An optical fiber drawn from the preform is cooled while
moving down below a drawing device. At this time, a boundary layer
of the inert gas formed on a surface of the optical fiber gives an
influence on the cooling of the optical fiber. The inert gas
introduced into the furnace cannot form a uniform laminar flow on
the surface of the drawn optical fiber. This makes the optical
fiber cooled irregularly in the circumferential direction, so
increases variations in the diameter of the optical fiber.
[0009] As well known in the art, if a perfect circular symmetry of
the optical fiber is not achieved, it may give rise to a
birefringence during a signal transmission through the optical
fiber, resulting in Polarization Mode Dispersion (PMD) which
adversely affects high-speed optical transmission
characteristics.
[0010] In order to provide a uniform laminar flow on an optical
fiber preform, there is proposed a structure in which an upper end
of the core tube for passage of the preform is sealed and an inert
gas flows from an upper portion to a lower portion. German Patent
DE-A 37 31 347 discloses a sealing structure in which a diaphragm
ensures gastightness at the upper end of the core tube. This
gastightness is achieved by the diaphragm which is provided with
nozzles for injecting combustible gas that is ignited. However,
this gastight device is too complex to install and also shows
difficulty in accurately guiding the preform to a center of the
furnace.
[0011] To decrease the diameter variations, there is also suggested
a method for slowly cooling a drawn optical fiber until the optical
fiber moves out of the furnace. In this slow cooling process, it is
important to prevent a turbulent flow of the inert gas and maintain
a uniform laminar flow in order to perform the uniform cooling.
U.S. Pat. No. 5,637,130 issued to Nagayama et al. discloses a
technique in which a cylindrical member is formed at a lower
portion inside a furnace in order to cool the drawn optical fiber
slowly and control a flow of the inert gas. However, sufficient
cooling and uniform laminar flow cannot be obtained only by means
of the cylindrical member. Particularly, impurity particles
generated in the furnace cannot be fully discharged due to the
cylindrical member, but stick to the preform or the optical fiber,
causing frequent break.
SUMMARY OF THE INVENTION
[0012] The present invention is designed to overcome the above
problems of the prior art, and an object of the invention is to
provide a furnace for drawing an optical fiber which may not only
provide a uniform gas flow of an inert gas inside the furnace with
a simple structure but also fully discharge impurities generated in
the furnace outward in order to improve reliability of a drawn
optical fiber, and a method for drawing an optical fiber using the
furnace.
[0013] According to the present invention, an inner space of the
furnace is isolated from the atmosphere due to the gas injected
through an upper introduction port, and impurity particles
generated in the furnace is carried upward and discharged outside
due to an inert gas injected through a central introduction port.
In addition, an inert gas injected through a lower introduction
port formed at a lower end of the furnace isolates a drawn optical
fiber from the atmosphere and gives a uniform laminar flow on a
surface of the drawn optical fiber.
[0014] According to the present invention, the drawn optical fiber
is slowly cooled while moving through an atmosphere blocking tube
and has reduced diameter variations because of the uniform laminar
flow formed on its surface. For this reason, a length of the
atmosphere blocking tube is set depending on a drawing speed in a
preferred embodiment of the present invention.
[0015] In an aspect of the present invention, there is provided a
furnace for drawing an optical fiber which includes a furnace body
having an upper opening for supplying a preform and a lower opening
for discharging an optical fiber drawn from the preform; heating
means installed in the furnace body, the heating means receiving
and melting the preform therein in order to draw the preform; an
atmosphere blocking tube installed at the lower opening for
ensuring the passage of the drawn optical fiber therein, the
atmosphere blocking tube protecting the drawn optical fiber from
the external atmosphere; an upper introduction port formed at an
upper portion of the furnace body for introducing an inert gas
toward the preform, the inert gas being partially discharged
outside through a gap between the preform and the upper opening; a
central introduction port formed at a central portion of the
furnace body for introducing an inert gas into the furnace body; a
lower introduction port formed at a lower portion of the furnace
body for introducing an inert gas into the furnace body; a first
flow guiding means for guiding the inert gas introduced through the
central introduction port upward along the melting area of the
heating means so that the inert gas collides with the inert gas
introduced through the upper introduction port and then flows
toward a lower portion of the furnace body along the surface of the
preform; and a second flow guiding means for guiding the inert gas
introduced through the lower introduction port upward along the
melting area of the heating means so that the inert gas collides
with the inert gas introduced through the central introduction port
and is then discharged outside through the atmosphere blocking tube
along the surface of the drawn optical fiber.
[0016] Preferably, the heating means includes a first graphite core
tube installed spaced apart from the wall of the furnace body in
order to form the melting area therein for receiving and melting
the preform; and a second graphite core tube installed, spaced
apart from the wall of the furnace body, at a lower portion of the
first graphite core tube.
[0017] More preferabley, the first graphite core tube heats at a
temperature of 2,100.about.2,300.degree. C., and the second
graphite core tube heats at a temperature of
1,600.about.1,800.degree. C. In addition, the second core tube may
indirectly irradiate heat transferred from the first graphite core
tube, not heating by oneself.
[0018] At this time, the first flow guiding means include a passage
between the first graphite core tube and the wall of the furnace
near the central introduction port; a support flange extended from
the wall of the furnace body for supporting the second graphite
core tube; and a passage formed between the first and second
graphite core tubes in which an upper end of the second graphite
core tube having a diameter smaller than the first graphite core
tube is partially inserted into the first graphite core tube.
[0019] In addition, the second flow guiding means includes a
passage formed between the second graphite core tube and the wall
of the furnace body near the lower introduction port; and a passage
formed between the atmosphere blocking tube and the second graphite
core tube.
[0020] According to a preferred embodiment of the present
invention, a flow rate of the inert gas at the central introduction
port is 3.about.4 times as large as a flow rate of the inert gas at
the upper introduction port, and a flow rate of the inert gas at
the lower introduction port is 2.about.3 times as large as a flow
rate of the inert gas at the upper introduction port.
[0021] Preferably, the atmosphere blocking tube is as long as the
drawn optical fiber has a temperature of about
1,200.about.1,300.degree. C. at the exit of the atmosphere blocking
tube, and more preferably the length of the atmosphere blocking
tube is in the range of 2.0.about.2.5 m.
[0022] According to another aspect of the invention, there is
provided a furnace for drawing an optical fiber which includes a
furnace body having an upper opening for supplying a preform and a
lower opening for discharging an optical fiber drawn from the
preform; a first core tube installed spaced apart from the wall of
the furnace body to form a first passage between the first core
tube and the wall of the furnace body, the first core tube forming
a melting area therein to heat and melt the preform; a second core
tube installed spaced apart from the wall of the furnace body to
form a second passage between the second core tube and the wall of
the furnace body, the second core tube having a diameter smaller
than the first core tube so as to be partially inserted into a
lower end of the first core tube to thereby form a third passage
between the first and second core tubes; an upper introduction port
formed at an upper portion of the furnace body for introducing an
inert gas toward the preform, the inert gas being partially
discharged outside through a gap between the preform and the upper
opening; a central introduction port communicated with the first
passage formed between the first core tube and the wall of the
furnace body for introducing an inert gas into the first passage; a
lower introduction port communicated with the second passage formed
between the second core tube and the wall of the furnace body for
introducing an inert gas into the second passage; a support flange
interposed between the first and second passage to divide the first
and second passages; and an atmosphere blocking tube installed at
the lower opening for forming a fourth passage in which the inert
gas introduced through the second passage flows between the second
core tube and the atmosphere blocking tube, the atmosphere blocking
tube ensuring the passage of the drawn optical fiber therein to
protect the drawn optical fiber from the external atmosphere.
[0023] According to still another aspect of the invention, there is
also provided a method for drawing an optical fiber by melting a
preform in a furnace which includes a furnace body having an upper
opening for supplying a preform, a lower opening for discharging an
optical fiber drawn from the preform, and upper, central and lower
introduction ports for introducing an inert gas; heating means
installed in the furnace body to receive and melt the preform in a
melting area therein; and an atmosphere blocking tube installed at
the lower opening to ensure the passage of the drawn optical fiber
therein, wherein the method includes the steps of (a) supplying a
preform into the melting area through the upper opening; (b)
melting the preform by heating the preform with the heating means;
(c) drawing an optical fiber from a melted portion of the preform
at its lower end; (d) introducing an inert gas through the upper
introduction port so that the inert gas is partially discharged
outside through a gap between the preform and the upper opening;
(e) introducing an inert gas through the central introduction port
so that the inert gas flows upward in the melting area and collides
with the flow of inert gas introduced through the upper
introduction port, and then flows downward along a surface of the
preform; and (f) introducing an inert gas through the lower
introduction port so that the inert gas flows upward along the
melting area and then collides with the inert gas introduced
through the central introduction port, and is then discharged
outside through the atmosphere blocking tube along the surface of
the drawn optical fiber.
[0024] Preferably, the step (e) includes the steps of supplying an
inert gas through a passage formed between a wall of the furnace
body and a first cylindrical core tube installed in the furnace
body; and guiding the inert gas, having passed through the passage
between the wall of the furnace body and the first core tube, to an
upper portion of the melting area through a passage between an
inner side of the first core tube and an outer side of an upper
portion of a second core tube installed in the furnace body wherein
an upper end of the second core tube is partially inserted into a
lower end of the first core tube.
[0025] Also preferably, the step (f) includes the steps of
supplying an inert gas through a passage formed between a wall of
the furnace body and the second core tube; and guiding the inert
gas, having passed through the passage between the wall of the
furnace body and the second core tube, to an upper portion of the
melting area through a passage formed between the atmosphere
blocking tube and the second core tube.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] These and other features, aspects, and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings,
in which like components are referred to by like reference
numerals. In the drawings:
[0027] FIG. 1 is a side sectional view showing a configuration of
an optical fiber drawing furnace according to a preferred
embodiment of the present invention;
[0028] FIG. 2 is a sectional view taken along a II-II line of FIG.
1;
[0029] FIG. 3 is a sectional view taken along a III-III line of
FIG. 1;
[0030] FIGS. 4A to 4C show simulation results of an inert gas flow
in the optical fiber drawing furnace according to a preferred
embodiment of the present invention, in which FIG. 4A shows a flow
at a U area, FIG. 4B shows a flow at an M area, and FIG. 4C shows a
flow at an L area; and
[0031] FIGS. 5A to 5C show simulation results of flow velocity
distributions of the inert gas in the optical fiber drawing furnace
according to a preferred embodiment of the present invention, in
which FIG. 5A shows a distribution at a U area, FIG. 5B shows a
distribution at an M area, and FIG. 5C shows a distribution at an L
area.
DETAILED DESCRIPTION OF THE INVENTION
[0032] Hereinafter, preferred embodiments of the present invention
will be described in detail with reference to the accompanying
drawings.
[0033] FIG. 1 shows a schematic configuration of a furnace for
drawing an optical fiber according to a preferred embodiment of the
present invention. Referring to FIG. 1, the optical fiber drawing
furnace preferably has a cylindrical furnace body 10, generally
made of stainless steel. An upper opening 11 is formed at a top end
of the furnace body 10 for supplying a preform 100 into the
furnace, and a lower opening 13 is formed at a bottom of the
furnace body 10 through which an optical fiber 100' drawn from the
preform 100 can pass.
[0034] In the furnace body 10, a heating unit is provided for
receiving the supplied preform 100 in a heating area and then
heating the preform 100 so as to be drawn into an optical fiber.
Preferably, the heating unit includes first and second cylindrical
graphite core tubes 20 and 30 fixed in the furnace body 10. The
first and second graphite core tubes 20 and 30 receive the optical
fiber preform 100 supplied from above by a supplying means (not
shown). The graphite core tubes 20 and 30 heat the inner space of
the furnace up to 2,100.about.2,300.degree. C. by resistance of
electricity supplied from a power resource (not shown) so as to
melt the optical fiber preform 100. Preferably, the first graphite
core tube 20 of the present invention heats at a temperature of
2,100.about.2,300.degree. C., while the second graphite core tube
30 heats at a temperature of 1,600.about.1,800.degree. C. In
addition, the second graphite core tube 20 may indirectly irradiate
the heat transferred from the first graphite core tube 20, not
heating by oneself. As another example, the core tubes 20 and 30
can be heated by induction of coils (not shown) respectively
installed in a space between the furnace body 10 and the core tubes
20 and 30.
[0035] According to this embodiment, the first graphite core tube
20 is spaced from a wall of the furnace body 10 in order to ensure
a space for an inert gas to flow as described below. At this time,
an upper end of the first core tube 20 is sealed with the furnace
body 10, and a lower end is opened so that a gas in the space flows
downward. The second graphite core tube 30 installed below the
first graphite core tube 20 is also fixed to a support flange 12
extended from the furnace body 10 by using a fixing flange 12a so
as to keep a space between the second graphite core tube 30 and a
wall of the furnace body 10. A lower end of the second graphite
core tube 30 is not contacted with a bottom of the furnace body 10
but spaced a predetermined distance from the bottom. Thus, the gas
introduced through the space between the second core tube 30 and
the wall of the furnace body 10 flows downward and then changes its
flowing direction upward.
[0036] In this embodiment, the furnace body has a cylindrical
shape. However, an auxiliary tube integrally connected to the
furnace body can be further provided at a lower end of the furnace
body, and this modification should be understood to be included in
the scope of the invention.
[0037] The furnace body 10 also has an upper introduction port 40,
a central introduction port 50 and a lower introduction port 60 for
blowing inert gases into the furnace body 10. The upper
introduction port 40 is formed inwardly toward a surface of the
preform 100 from the outside of the furnace body 10. Preferably,
the upper introduction port 40 is formed in the tangential
direction of the furnace as shown in FIG. 2, and a ring-shaped
spraying sleeve 32 having a plurality of nozzles 31 is mounted to
an upper portion of the furnace in order to supply the introduced
gas evenly to the preform.
[0038] According to this embodiment, there is provided a first flow
guiding means which guides the inert gas introduced through the
central introduction port 50 upward along the melting area of the
heating unit so that the inert gas collides with another inert gas
introduced through the upper introduction port 40, and then flows
down toward a lower portion of the furnace along a surface of the
preform 100. The first flow guiding means may be formed by a
passage between the first graphite core tube 20 and the furnace
body 10 and a passage between the first and second graphite core
tubes 20 and 30.
[0039] More specifically, the central introduction port 50 is
communicated with the passage formed between the first graphite
core tube 20 and the wall of the furnace body 10. In addition, the
inside diameter of the first graphite core tube 20 is larger than
the outer diameter of the second graphite core tube 30 so that the
upper end of the second graphite core tube 30 may be inserted into
the lower end of the first graphite core tube 20 by a predetermined
length. Thus, there can be also formed a passage between the first
and second core tubes 20 and 30 through which the inert gas may
pass. Here, the support flange 12 blocks a space between the second
core tube 30 and the wall of the furnace body 10, so the inert gas
introduced through the central introduction port 50 flows down
through the passage formed between the first core tube 20 and the
wall of the furnace body 10 and then changes its flowing direction
due to the support flange 12, and then flows upward to the upper
portion of the melting area in the furnace along the passage
between the first and second core tubes 20 and 30. Preferably, it
is designed to ensure that the inert gas introduced through the
central introduction port 50 flows upward near the neck-down zone
of the preform 100.
[0040] On the other hand, the furnace of the present invention also
includes a second flow guiding means which guides the inert gas
introduced through the lower introduction port 60 upward along the
melting area of the heating unit so that the inert gas collides
with another inert gas introduced through the central introduction
port 50 and is then discharged outside through an atmosphere
blocking tube 70 along a surface of the drawn optical fiber 100'.
Preferably, the second flow guiding means is realized by a passage
between the second core tube 30 and the furnace body 10 as well as
the atmosphere blocking tube 70.
[0041] More specifically, the lower introduction port 60 is
communicated with the passage formed between the second core tube
30 and the wall of the furnace body 10. FIG. 3 is a sectional view
taken along the III-III line of FIG. 1, which shows a section of
the furnace near the lower introduction port 60.
[0042] The atmosphere blocking tube 70 of the present invention is
mounted to the lower opening 13 of the furnace body 10. The
atmosphere blocking tube 70 protects the drawn optical fiber from
the atmosphere to be cooled as slow as the clad of the drawn
optical fiber has an enough viscosity to minimize the differential
stress in the circumferential direction of the optical fiber, and
guides the inert gas into a laminar flow.
[0043] The atmosphere blocking tube 70 is made of quartz or
graphite so as to endure high temperature in the furnace. The outer
diameter of the atmosphere blocking tube 70 is selected in a range
of 15.about.50 mm depending on the flow rate of the inert gas. In
this embodiment, a length of the atmosphere blocking tube 70 is
adjustable according to a drawing speed of the optical fiber.
Preferably, the atmosphere blocking tube 70 is as long as the drawn
optical fiber has a temperature of about 1,200.about.1,300.degree.
C. at the exit of the atmosphere blocking tube 70. For example, if
the drawing speed of the optical fiber is 25 m/sec, the atmosphere
blocking tube 70 has a length of 1.5.about.2.5 mm, more preferably
2.0.about.2.5 m.
[0044] According to the present invention, a top end of the
atmosphere blocking tube 70 is positioned within about
200.about.250 mm from the neck-down zone of the preform 100 so that
the preform 100 is drawn into an optical fiber having a reduced
section and then advances into the atmosphere blocking tube 70.
This is helpful to maintain a uniform boundary layer of the gas
near a surface of the optical fiber. If the atmosphere blocking
tube 70 is too close to the neck-down zone of the preform, the flow
of the inert gas becomes unstable. If too far from the neck-down
zone, the atmosphere blocking tube 70 cannot bring forth a desired
result.
[0045] With such a configuration, the inert gas introduced through
the lower introduction port 60 is guided to an upper portion of the
heating area in the furnace through the passage formed between the
second core tube 30 and the wall of the furnace body 10 and the
passage formed between the second core tube 30 and the atmosphere
blocking tube 70.
[0046] In another embodiment not shown in the drawings, an
insulating material can be installed between the furnace body 10
and the first and second graphite core tubes 20 and 30. In this
case, there should be formed passages for the inert gas, of
course.
[0047] Now, a method of drawing an optical fiber from a preform
with use of the furnace of the invention constructed as above is
explained in detail.
[0048] At first, a common supplying device (not shown) supplies a
preform 100 into the furnace through the upper opening 11 of the
furnace body 10. After that, an electric current is applied to the
first and second graphite core tubes 20 and 30 from a power source
(not shown) to heat the core tubes 20 and 30. Then, the preform 100
is heated and melted and an optical fiber 100' is drawn from the
bottom end of the melted preform 100. The drawn optical fiber 100'
is moved downward through the atmosphere blocking tube 70, and then
coated with polymer on its surface. Then, the coated optical fiber
is cured and taken up regularly around a bobbin.
[0049] During the above process, inert gases such as helium,
nitrogen and argon are introduced through the upper, central and
lower introduction ports 40, 50 and 60 so as to establish an inert
gas atmosphere inside the furnace. A flow rate of the introduced
inert gas is preferably adjusted depending on positions of the
ports. Preferably, a flow rate of the inert gas at the central
introduction port 50 is 3.about.4 times as large as that of the
inert gas at the upper introduction port 40, and a flow rate of the
inert gas at the lower introduction port 60 is 2.about.3 times as
large as that of the inert gas at the upper introduction port
40.
[0050] At this time, the flow of the inert gas in the furnace is as
indicated by arrows in FIG. 1. In other words, the inert gas
introduced from the upper introduction port 40 is sprayed toward a
surface of the preform and at the same time discharged outside
through a gap between the upper opening 11 and the preform 100
together with the inert gas introduced through the central
introduction port 50 and flowed upward, as described below. Thus,
the inert gas introduced through the upper introduction port 40
blocks the inflow of the atmosphere into the furnace through the
upper opening 11 and discharges minute particles from the furnace
outside.
[0051] The inert gas introduced through the central introduction
port 50 flows through the passage between the first core tube 20
and the wall of the furnace body 10, then changes its flowing
direction due to the support flange 12 and the second core tube 30,
and then flows upward through the passage between the first and
second core tubes 20 and 30. The introduced inert gas carries
impurity particles generated in the furnace while flowing upward
along the inner wall of the first core tube 20. Then, the impurity
particles are discharged outside through the upper opening 11 along
with the inert gas introduced through the upper introduction port
40. The inert gas introduced through the central introduction port
50 partially changes its direction near the upper introduction port
40 and then flows downward along the surface of the preform
100.
[0052] According to the present invention, since the inert gas
introduced into the furnace through the central introduction port
50 does not directly contact with the surface of the preform 100,
an irregular turbulence flow is not generated. In other words,
while flowing upward in the furnace and then flowing downward along
the surface of the preform 100, the inert gas forms a uniform
laminar flow near the surface of the preform 100.
[0053] The inert gas introduced through the lower introduction port
60 flows along the passage between the second core tube 30 and the
wall of the furnace body 10 and then into the furnace through a gap
between a lower end of the second core tube 30 and a bottom of the
furnace body 10. The inert gas introduced through the lower
introduction port 60 is not directly injected to the drawn optical
fiber 100' but flow upward through the passage between the
atmosphere blocking tube 70 and the second core tube 30 and then
discharged outside through the blocking tube 70 together with the
gas which is introduced through the central introduction port 50
and then descends along the surface of the preform 100. This inert
gas prevents the inflow of the air from the outside at the lower
portion of the furnace and also forms a uniform laminar flow on the
surface of the drawn optical fiber 100' which passes through the
atmosphere blocking tube 70.
[0054] The flow distribution of the inert gas in the furnace
according to the present invention is well shown in FIGS. 4A to 4C.
In the figures, FIG. 4A shows a flow distribution of the inert gas
at an upper portion U of the furnace, FIG. 4B shows a flow
distribution of the inert gas at a central portion M of the
furnace, and FIG. 4C shows a flow distribution at a lower portion L
of the furnace. These figures are simulation results in case an
argon gas of about 45.degree. C. is introduced into the furnace
through the upper introduction port 40 at a flow rate of
1.351.times.10.sup.-4 kg/sec, through the central introduction port
50 at a flow rate of 4.866.times.10.sup.-4 kg/sec, and through the
lower introduction port 60 at a flow rate of 4.055.times.10.sup.-4
kg/sec, respectively. At this time, the preform and the furnace
body is spaced apart at the upper opening 11 by a distance of 1.5
mm. As shown in FIG. 4A, the inert gas in the furnace forms an air
flow which is discharged outside through the opening 11.
Particularly, as well known from FIG. 4B, a uniform laminar flow is
formed along the surface of the preform 100 near the neck-down
zone, and this laminar flow is maintained even inside the
atmosphere blocking tube 70 shown in FIG. 4C. Thus, while passing
through the atmosphere blocking tube 70, the optical fiber 100'
drawn from the preform 100 may be slowly cooled with a gas boundary
layer on its surface kept uniformly. Consequently, the drawn
optical fiber 100' is cooled at a uniform speed in its
circumferential direction, so the optical fiber may have a uniform
stress distribution in the circumferential direction.
[0055] In the above simulation, a flow velocity distribution of the
inert gas introduced into the furnace at each point is well shown
in FIGS. 5A to 5C. In these figures, FIG. 5A shows a flow velocity
distribution at the upper portion U, FIG. 5B shows a flow velocity
distribution at the central portion M, and FIG. 5C shows a flow
velocity distribution at the lower portion L.
[0056] As shown in FIG. 5A, the inert gas introduced through the
upper introduction port 40 flows up to the upper portion of the
furnace together with the inert gas introduced through the central
introduction port 50, and is then discharged through the upper
opening 11 at a high speed. This shows that the inside of the
furnace is protected from the atmosphere and also impurity
particles in the furnace can be discharged well outside. As shown
in FIG. 5B, the inert gas introduced through the central
introduction port 50 flows up to the upper portion of the furnace
and then changes its direction so as to flow down at a high speed
along the surface of the preform with forming a uniform laminar
flow on the surface of the preform. Furthermore, the flow is, as
shown in FIG. 5C, kept to the inside of the atmosphere blocking
tube 70 together with the inert gas introduced through the lower
introduction port 60, forming a rapid laminar gas flow on the
surface of the drawn optical fiber 100' and blocking the inflow of
air.
[0057] In the present invention, a length of the atmosphere
blocking tube 70 is suitably selected in order to ensure slow
cooling of the drawn optical fiber 100'. Preferably, the length of
the atmosphere blocking tube 70 is set so that the optical fiber
discharged out of the atmosphere blocking tube 70 has a temperature
of about 1,200.about.1,300.degree. C.
EXPERIMENTAL EXAMPLE
[0058] An optical fiber preform is drawn using the fiber-drawing
furnace according to the present invention. The furnace has a
cylindrical shape of which an inside diameter is 110 mm and the
length is 450 mm. An argon gas is introduced into the furnace at
the flow rate of 5 liter/min through the upper introduction port
40, at the flow rate of 20 liter/min through the central
introduction port 50, and at the flow rate of 10 liter/min through
the lower introduction port 60, respectively. While the first
graphite core tube 20 heats at 2,100.degree. C. and the second
graphite core tube 30 heats at 1,600.degree. C., preform having a
diameter of 90 mm is melt and an optical fiber is drawn from the
perform at a rate of 10 m/sec, 20 m/sec and 25 m/sec, respectively.
At this time, the atmosphere blocking tube 70 having an inside
diameter of 25 mm, a thickness of 2 mm and a length of 2.0 m is
installed only in the third case. Then, by using a laser
non-contact diameter-measuring device, an outer diameter of the
optical fiber is measured. The measured results are illustrated in
Table 1.
1 TABLE 1 Atmosphere Outer diameter blocking tube Drawing speed
variation Case 1 No 10 m/sec .+-. 0.70 .mu.m Case 2 No 20 m/sec
.+-. 1.50 .mu.m Case 3 Provided 25 m/sec .+-. 0.15 .mu.m
[0059] As well understood in Table 1, in case the atmosphere
blocking tube is not installed, the outer diameter variation is
relatively good when a drawing speed is 10 m/sec and the outer
diameter variation is increased when a drawing speed is 20 m/sec or
above.
[0060] In the third case, the atmosphere blocking tube is adopted
to the furnace of the present invention. In this case, the outer
diameter variation is satisfactory though a drawing speed of the
optical fiber is high as much as 25 m/sec.
[0061] According to the present invention, since the atmosphere
flowing in at the upper portion of the furnace can be blocked by
means of simple structure of the upper opening, it is not needed to
prepare a complex sealing device as used conventionally. In
addition, the inert gas introduced at a central portion of the
furnace is not directly injected to the preform, particularly the
neck-down portion, but providing a stable flow stream. The inert
gas also carries impurity particles, generated in the furnace, in
the upper direction and so that the particles are fully discharged
outside. In addition, the atmosphere blocking tube provided to a
lower portion of the furnace protect the drawn optical fiber from
the atmosphere as well as makes a stable laminar flow around the
optical fiber so that the optical fiber can be slowly cooled to a
temperature at which a viscosity of an outer cladding is
sufficiently reduced. Furthermore, the inert gas introduced through
the lower introduction port blocks inflow of the external air
through the atmosphere blocking tube.
[0062] Such a effect of the present invention enables to draw an
optical fiber with dramatically reduced diameter variations,
restraining Polarization Mode Dispersion (PMD) and therefore
improving a high-speed optical transmission characteristic through
an optical fiber.
[0063] The present invention has been described in detail. However,
it should be understood that the detailed description and specific
examples, while indicating preferred embodiments of the invention,
are given by way of illustration only, since various changes and
modifications within the spirit and scope of the invention will
become apparent to those skilled in the art from this detailed
description.
* * * * *