U.S. patent application number 09/974929 was filed with the patent office on 2002-04-18 for apparatus for fabricating soot preform for optical fiber.
Invention is credited to Hirasawa, Hideo, Inoue, Dai, Ogino, Go, Otosaka, Tetsuya, Shimada, Tadakatsu.
Application Number | 20020043079 09/974929 |
Document ID | / |
Family ID | 18796321 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020043079 |
Kind Code |
A1 |
Inoue, Dai ; et al. |
April 18, 2002 |
Apparatus for fabricating soot preform for optical fiber
Abstract
An apparatus for fabricating a soot preform for an optical
fiber. The soot preform is fabricated by depositing glass particles
on a starting rod capable of being rotated and pulled up. The
apparatus comprises elements as follows. A reaction chamber is used
for depositing the glass particles on the starting rod. An upper
room is located above the reaction chamber for receiving the soot
preform formed in the upper portion of the reaction chamber. At
least one core burner is installed in the reaction chamber. A
gas-supplying inlet is located in the top part of the sidewall of
the reaction chamber closest to burner(s), and a gas-exhausting
outlet is located in the top part of another sidewall opposite to
the gas-supplying inlet. In addition, at least one cladding burner
is installed in the reaction chamber. Thus, the exhausting
efficiency for the stray glass particles is increased and the
bubbles and impurities in the resulting preform are reduced such
that the optical property in the lengthwise direction is
stable.
Inventors: |
Inoue, Dai; (Annaka-Shi,
JP) ; Ogino, Go; (Annaka-Shi, JP) ; Otosaka,
Tetsuya; (Annaka-shi, JP) ; Shimada, Tadakatsu;
(Annaka-Shi, JP) ; Hirasawa, Hideo; (Annaka-Shi,
JP) |
Correspondence
Address: |
J.C. Patents, Inc.
Suite 250
4 Venture
Irvine
CA
92618
US
|
Family ID: |
18796321 |
Appl. No.: |
09/974929 |
Filed: |
October 10, 2001 |
Current U.S.
Class: |
65/17.4 ; 65/414;
65/415; 65/531; 65/532 |
Current CPC
Class: |
C03B 37/01406 20130101;
C03B 2207/54 20130101 |
Class at
Publication: |
65/17.4 ; 65/531;
65/532; 65/414; 65/415 |
International
Class: |
C03B 037/018 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 18, 2000 |
JP |
2000-317555 |
Claims
What is claimed:
1. An apparatus for manufacturing a soot preform for an optical
fiber by depositing glass particles generated through a flame
hydrolysis reaction of raw material gases onto a starting rod being
rotated and pulled up, the apparatus comprising: a reaction chamber
in which said glass particles are deposited over the starting rod
to thereby render the starting rod into a soot preform; an upper
room located on top of said reaction chamber, for housing the soot
preform being pulled up; at least one core deposition burner
disposed to open in the reaction chamber; a horizontally extending
slit made in that wall of the reaction chamber which is closest to
said core deposition burner, at a location slightly underneath a
ceiling of said reaction chamber, said slit being adapted to pass
gas into said reaction chamber; and a gas exit made in that wall of
the reaction chamber which is opposed to the wall having said
slit.
2. The apparatus of claim 1, further comprising at least one clad
deposition burner.
3. The apparatus of claim 1, wherein a horizontal length of said
slit is at least 75% of the width of said reaction chamber as
measured in parallel with said slit.
4. The apparatus of claim 1, wherein said gas exit is substantially
rectangular, and the distance between a top side of the gas exit
and the ceiling of the reaction chamber is 50 mm or less.
5. The apparatus of claim 1, wherein the horizontal length of said
gas exit is at least 75% of the width of said reaction chamber as
measured in parallel with said slit.
6. A method of manufacturing a soot preform for an optical fiber
using the apparatus of claim 1, wherein a velocity of the gases
passing through said slit is set between 3 m/sec and 20 m/sec.
7. A method of claim 6, wherein passing of a gas through said slit
is caused by forced exhaustion of gas through said gas exit, and
the gas passed through said slit is a prepared gas.
8. The method of claim 7, wherein said prepared gas is an
atmospheric air passed through a dust-tight filter.
9. The method of claim 7, wherein said prepared gas is air in a
clean room of class 10000 or better.
10. The apparatus of claim 1, wherein said upper room is
substantially cylindrical.
11. A method of manufacturing a soot preform for an optical fiber
using the apparatus of claim 10, wherein a downward gas flow is
maintained to flow from the upper part of said upper room toward
the reaction chamber at a velocity of 0.05 m/sec or greater.
12. The apparatus of claim 1, wherein the floor of said reaction
chamber is formed with a raised floor having a height higher than
the core deposition position, and the raised floor is formed at the
foot of that wall of the reaction chamber which has the gas
exit.
13. The apparatus of claim 1, wherein said reaction chamber is
divided by a horizontal partition into an upper reaction chamber
having said slit and said gas exit and a lower reaction chamber,
and a connect hole is made in the bottom of said upper reaction
chamber for communicating the upper and lower reaction chambers
with each other, and said lower reaction chamber has substantially
no exhaust hole except this connect hole.
14. The apparatus of claim 13, wherein said connect hole is a
circle in shape having a radius which is 45-55 mm greater than the
radius of that part of the soot preform, which is concentrically
passing through said connect hole.
15. The apparatus of claim 13, wherein said connect hole consists
of a void in the shape of a semicircle of which the center
substantially coincides with a rotating axis of the starting rod
and which is cut from a circle by a diameter parallel to said slit,
and a void in the shape of a rectangle whose one side is the chord
of this semicircle, its subtend lying in that sidewall having the
slit, said connect hole being distanced from the sidewalls by at
least 50 mm except the side wall having the slit, and being wide
enough to make a gap of at least 20 mm around the soot preform.
16. The apparatus of claim 14, wherein a core deposition burner is
installed at the lower reaction chamber and a clad deposition
burner is installed at the upper reaction chamber.
17. The apparatus of claim 14, wherein a center core deposition
burner and a side core deposition burner are installed at the lower
reaction chamber and a clad deposition burner is installed at the
upper reaction chamber.
18. The apparatus of claim 16, further comprising a core heating
burner installed at the lower reaction chamber.
19. The apparatus of claim 16, further comprising another clad
deposition burner installed at the lower reaction chamber, adapted
to function as a core heating burner as well, and disposed to
deposit clad soot substantially in the upper reaction chamber.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority benefit of Japanese
application ser. no. 2000-317555, filed on Oct. 18, 2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates in general to an apparatus for
fabricating a soot preform for an optical fiber, and more
specifically relates to an apparatus for stably fabricating a
high-quality soot preform for an optical fiber by vapor-phase axial
deposition.
[0004] 2. Description of Related Art
[0005] Vapor-phase axial deposition (VAD) is a well-known process
for fabricating a fiber preform nowadays. A starting rod is
installed on a shaft capable of being rotated and shifted
vertically into and out of a reaction chamber. Glass particles
generated by a core burner and a cladding burner in the reaction
chamber are deposited on the front end of the starting rod, thereby
a porous soot preform (soot preform, hereinafter) consisting of a
core and a cladding layer for an optical fiber is fabricated.
[0006] In general, the deposition efficiency for depositing the
glass particles on the starting rod is not 100%, a lot of stray
glass particles, which are not adhered or deposited on the starting
rod, occur during the fabrication. Most of the stray glass
particles are exhausted from an exhausting pipe of the reaction
chamber along with other gases that should be exhausted. However, a
portion of the stray glass particles are adhered to a ceiling and
sidewalls of the reaction chamber during exhaustion of stray glass
particles and other gases.
[0007] In general, the glass particles are generated by injecting
chlorides serving as a source gas (such as SiCl.sub.4) into an
oxyhydrogen flame to thereby effect a flame hydrolysis process. The
gases, such as water vapor and hydrochloric acid that are generated
in this process and ought to be exhausted as well as the stray
glass particles, which are not adhered and accumulated on the
starting rod, are at high temperatures and tend to enter an upper
room assembled on top of the reaction chamber.
[0008] Because the temperature of the upper room is not as high as
the temperature of the reaction chamber, water vapor entering the
upper room is condensed on the inner wall of the upper room and the
hydrochloric acid is absorbed by it. Therefore, the upper room is
eroded if the upper room is made of metals. Even if the upper room
is made of erosion-resistant materials, the apparatus is difficult
to clean up after the preform fabricating process is finished.
Furthermore, the stray glass particles adhere to the hydrochloric
acid-wet inner walls of the reaction room, resulting in that the
apparatus is still more difficult to clean up.
[0009] Additionally, the stray glass particles, which first failed
to adhere on the preform but later deposit on the surface of an
off-the-frame portion of the preform, grow finely on the soot
preform like trees. Then, in the subsequent glassification process,
the tree-like protrusions are formed on the surface of the soot
preform, causing difficulty in measuring the distribution of
refraction index.
[0010] In general, for avoiding foregoing issues, a downward gas
flow from the top of the upper room to the reaction chamber is
used, thereby preventing the stray glass particles from adhering
and accumulating on the walls of the upper room to a degree, but
cannot this could not prevent the particles from adhering and
accumulating to the ceiling and the walls of the reaction
chamber.
[0011] In the conventional methods, in the post stage of the
fabrication of the soot preform, flakes of the glass particles
adhered or accumulated on the inner walls of the reaction chamber
get detached from the walls and fell, stirring up the
glassparticles, some of which fell on the soot preform, causing
that bubbles and impurities are formed in the glassified soot
preform.
[0012] Recently, because the fiber demand is increased and its cost
is requested to be reduced, it is very important to enlarge the
optical fiber preform. Naturally, material supply must be increased
for enlarging the fiber preform. Once the material supply is
increased, the amount of the stray glass particles increase even if
the deposition efficiency does not changed. Therefore, in the
conventional methods, the frequency of the falling in masses of the
stray glass particles from the inner walls of the reaction chamber
increased.
[0013] In order to enhance the exhausting efficiency of the stray
glass particles, a method was proposed to increase the
gas-supplying amount and the gas-exhausting amount. However, this
method causes the gas flow in the reaction chamber to be more
turbulent, and the flame of the core burner, whose gas flow rate is
relative low, is seriously disturbed. As a result, the distribution
of refraction index of the soot preform in the length-wise
direction becomes uneven.
[0014] Furthermore, if the radius of the soot preform is large, the
gap between the soot preform and the wall of the upper room changes
drastically as the soot preform is gradually moved into the upper
room, especially which the tapered top of the preform passes the
boundary between the upper room and the reaction chamber.
Therefore, at the boundary between the reaction chamber and the
upper room, i.e., the entrance of the reaction chamber, the gas
flow from the upper room to the reaction chamber is very fast after
the trunk of the soot preform is moved into the upper room.
[0015] The gas flow, adjusted as of the beginning of the
fabrication to properly prevent the gases and the stray glass
particles from entering the upper room, becomes so strong when the
preform starts entering the upper room that it disturbs the core
deposition.
[0016] For solving the foregoing problems, Japanese Laid Open
9-118537 and 11-343135 provide apparatuses capable of effectively
exhausting the stray glass particles that are not properly
deposited on the soot preform.
[0017] According to Japanese Laid Open 11-343135, it comprises two
reaction chambers: one is for depositing core and the other is for
depositing cladding. Each of the reaction chambers is equipped with
an exhausting damper capable of adjusting exhausting pressure
respectively. The exhausting pressure for the separated cladding
reaction chamber is set higher than that used in the conventional
reaction chamber so that even when the exhausting amount from the
cladding reaction chamber is increased, the flame for depositing
the core is not disturbed. Therefore, the soot preform can be
fabricated stably.
[0018] However, because the reaction chamber is divided into two
separated reaction chambers, the control of the exhausting pressure
for each reaction chamber becomes difficult. In addition, the glass
particles generated during the core deposition tend to accumulate
on the lower side face of the partition, leading to the same
drawbacks described above.
[0019] According to Japanese Laid Open 11-343135, gas is introduced
through a whole sidewall behind a burner in the reaction chamber
and a gas-exhausting outlet is installed in a sidewall opposite to
the sidewall through which the gas is introduced. Furthermore, a
flow-guide wall having numerous gas blowout holes is provided to
each of the two sidewalls, between which the soot preform poses,
thereby the amount of the glass particles adhered or accumulated on
the inner walls of the reaction chamber is reduced.
[0020] However in an apparatus as this, the gas flow around the
core burner becomes faster and more turbulent, thereby the core
deposition is disturbed and an even distribution of refraction
index in the lengthwise direction cannot be obtained.
[0021] In addition, according to FIG. 1 of Japanese Laid Open
11-343135, for preventing the air from entering the upper room
through gaps between an upper cap and a driving shaft, a seal gas
is introduced into the upper room from its top. However, it could
not efficiently prevent the exhausting gases and the stray glass
particles from entering the upper room.
SUMMARY OF THE INVENTION
[0022] According to the foregoing description, an object of this
invention is to provide an apparatus for fabricating a soot preform
for an optical fiber by the VAD process. The apparatus can increase
an exhausting efficiency of the stray glass particles and reduce
bubbles and impurities in the resulting preform. Accordingly, using
the apparatus, a soot preform for an optical fiber having stable
optical characteristics along the lengthwise of the soot preform is
obtained.
[0023] According to the object mentioned above, the invention
provides an apparatus for fabricating a soot preform for an optical
fiber. A soot preform is fabricated by depositing glass particles
on a starting rod capable of being rotated and pulled up, wherein
the glass particles are generated from source gasses by a flame
hydrolysis reaction. The apparatus comprises subject elements as
follows. A reaction chamber is used for depositing the glass
particles on the starting rod by the flame hydrolysis reaction. An
upper room is located above the reaction chamber for housing the
pulled up soot preform formed in the reaction chamber by
deposition. At least one core burner is installed in the reaction
chamber. A gas-supplying inlet in the form of a horizontal slit is
made in that sidewall of the reaction chamber which is nearest the
burner(s), in the vicinities of the ceiling of the reaction
chamber, and a gas-exhausting outlet is located at another sidewall
of the reaction chamber opposite to the gas-supplying inlet. In
addition, at least one cladding burner is installed in the reaction
chamber.
[0024] According to preferred embodiments of the invention, the
length of the gas-supplying inlet is at least 75% of the width of
that sidewall of the reaction chamber in which it is made. And, the
gas-exhausting outlet is substantially rectangular, and the
distance between a top end of the gas-exhausting outlet and the
ceiling of the reaction chamber is within 50 mm. The horizontal
length of the gas-exhausting outlet is at least 75% of the width of
that sidewall of the reaction chamber in which it is made. In the
apparatus of claim 1, a gas-supplying flow velocity at the
gas-supplying inlet is preferably between 3 m/sec and 20 m/sec. In
the apparatus of claim 1, the supply of air from the gas-supplying
inlet is effected by a pressure difference across the gas-supplying
inlet, and the air passing through the gas-supplying inlet is
prepared air.
[0025] The prepared air means an external air passed through a
filter, or air in a clean room of class 10000 or better.
Additionally, the upper room is substantially cylindrical, and it
is preferred to use a gas flow descending from the upper part of
the upper room to the reaction chamber, and a velocity of the gas
flow from the top of the upper room down into the reaction chamber
is preferably above 0.05 m/sec at the lower end of the upper
room.
[0026] Moreover, a raised bottom portion higher than the core
deposition level is formed in a bottom of the reaction chamber
adjacent that sidewall having the gas-exhausting outlet, whereby
the core deposition becomes more stable. The reaction chamber is
divided by a horizontal partition into a lower reaction chamber and
an upper reaction chamber, the latter having the gas-supplying
inlet and the gas-exhausting outlet, and a connect hole is formed
in the partition for communicating the upper and lower reaction
chambers with each other. There are no gas-exhausting outlets
formed in the lower reaction chamber except the connect hole for
connecting the lower and the upper reaction chambers.
[0027] The connect hole is substantially circular concentric with a
rotary shaft and having a radius about 50 mm greater than that of
the part of the soot preform passing the hole, or alternatively the
connect hole consists of a half of this circle as cut parallel to
the wall having the gas-supplying inlet and a rectangle whose one
side is the chord of said half circle, its subtend lying in that
sidewall having the gas-supply inlet, wherein the connect hole is
apart from sidewalls by at least 50 mm except the sidewall having
the gas-supplying inlet.
[0028] The apparatus further comprises a core burner in the lower
reaction chamber and a cladding burner in the upper reaction
chamber. In addition, a core-heating burner can be further
installed in the lower reaction chamber. Alternatively, in addition
to the core burner, a cladding burner can be installed in the lower
reaction chamber capable of performing a function of a core heating
burner as well, wherein a cladding deposition by this additional
cladding burner is essentially performed in the upper reaction
chamber. Furthermore, instead of one core burner, a first core
burner for depositing a center core and a second core burner for
depositing a side core may be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter which is
regarded as the invention, the objects and features of the
invention and further objects, features and advantages thereof will
be better understood from the following description taken in
connection with the accompanying drawings in which:
[0030] FIG. 1 schematically illustrates a cross-sectional view of
the apparatus according to the first embodiment of the
invention;
[0031] FIG. 2 schematically illustrates a cross-sectional view of
the apparatus according to the second embodiment of the
invention;
[0032] FIG. 3 schematically illustrates a cross-sectional view of
the apparatus according to the third embodiment of the
invention;
[0033] FIG. 4 schematically illustrates a cross-sectional view of
the apparatus according to the fourth embodiment of the
invention;
[0034] FIG. 5 shows a side view of a sidewall having a
gas-supplying inlet thereon;
[0035] FIG. 6 shows a side view of another sidewall having a
gas-exhausting outlet thereon;
[0036] FIGS. 7A and 7B show different examples for a connect hole
formed in a bottom of an upper reaction chamber of the apparatus;
and
[0037] FIG. 8 shows a cross-sectional view of a conventional
apparatus without a separating wall for dividing the reaction
chamber into an upper and a lower reaction chambers and a raised
bottom portion of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0038] The apparatus for fabricating a soot preform for an optical
fiber is described in detail as follows with reference to the
drawings.
[0039] FIGS. 1.about.4 schematically illustrate front
cross-sectional views of the apparatus for fabricating the optical
fiber preform according to embodiments of the invention. FIGS. 5
and 6 schematically illustrate side views of the apparatus, in
which FIG. 5 shows a sidewall having burners formed nearby and FIG.
6 shows another sidewall opposite to the sidewall in FIG. 5. In
addition, FIGS. 7A and 7B show different examples for a connect
hole formed in a bottom of an upper reaction chamber of the
apparatus. Furthermore, FIGS. 1, 2 and 4 show cross-sectional views
along a line A-A in FIG. 7A, and FIG. 3 shows a cross-sectional
view along a line B-B in FIG. 7B. FIG. 8 shows a cross-sectional
view of a conventional apparatus without a separating wall for
dividing the reaction chamber into an upper and a lower reaction
chambers and a raised bottom portion of the invention.
[0040] As shown in FIGS. 1.about.4, the apparatus of the invention
comprises a gas-supplying inlet 1 and a gas-exhausting outlet 2.
The gas-supplying inlet 1, in a slit shape, is installed near a
ceiling of the reaction chamber and on a sidewall having burners
formed nearby. The exhausting outlet 2 is installed on another
sidewall opposite to the sidewall having the gas-supplying inlet 1
formed thereon. When a gas, such as the air, is introduced from the
gas-supplying inlet 1 toward the exhausting outlet 2, a gas flow
from the gas-supplying inlet 1 to the exhausting outlet 2 is formed
near the ceiling of the reaction chamber.
[0041] Accordingly, among glass particles generated by flames of
the burners, most of stray glass particles that do not form the
soot preform 3 rise and are exhausted outside the reaction chamber
by the gas flow above. Therefore it can significantly reduce the
probability that the stray glass particles are adhered, and
accumulated on the inner walls of the reaction chamber and then
fall from the walls.
[0042] The greater the horizontal lengths of the gas-supplying
inlet 1 and the gas-exhausting outlet 2, the better for forming a
regular gas flow near the ceiling of the reaction chamber.
Preferably, the horizontal lengths of the gas-supplying inlet 1 and
the gas-exhausting outlet 2 are larger than 75% of the width of the
reaction chamber. Similarly, like the gas-supplying inlet 1, it is
preferable that the distance between a top end of the
gas-exhausting outlet 2 and the ceiling of the reaction chamber
within 50 mm.
[0043] Additionally, a preferred velocity at the gas-supplying
inlet 1 is above 3 m/sec so that the stray glass particles can be
efficiently exhausted from the reaction chamber by means of the gas
flow near the ceiling of the reaction chamber. Because this gas
flow directly hits upon the soot preform, the soot preform is
vibrated if the gas-supplying velocity exceeds 20 m/sec. This
vibration causes the distribution of the refraction index of the
soot preform along its axis to be non-uniform. Therefore, it is
better to restrict the gas-supplying velocity under 20 m/sec.
[0044] The gas introduced from the gas-supplying inlet 1 can be
air, or gas having no influence on the reaction, for example an
inert gas such as nitrogen (N.sub.2), helium (He) and argon (Ar).
Considering the cost, air is the best choice.
[0045] In an apparatus of this kind, the pressure of the reaction
chamber is set to a negative pressure with respect to the external,
thereby toxic gases, such as the hydrochloric acid, generated
during the preform fabricating process do not leak from tiny gaps
of the apparatus. For example, this can be done by installing an
outward blower connected to the gas-exhausting outlet 2. As a
result, the external air is caused to flow into the reaction
chamber to form the gas flow.
[0046] Furthermore, the gas-supplying velocity can be adjusted to a
desired value by means of properly setting the pressure difference
between the external and the internal of the reaction chamber.
However, tiny impurities also enter the reaction chamber along with
air, and then adhere on the soot preform 3, causing the preform
loss or impurities formed on the soot preform. For avoiding such
issues, the supplied gas has to be prepared properly.
[0047] According to a preferred embodiment of the invention, for
solving such problems, a dust-tight filter 4 is mounted on the
gas-supplying inlet 1 for avoiding tiny impurities from flowing
into the reaction chamber. For example, a commercial HEPA filter
can be used for the dust-tight filter 4. Moreover, according to
another preferred embodiment, the apparatus can be set in a proper
clean room for reducing the impurities.
[0048] The formed soot preform 3 is gradually pulled up into an
upper room 5, which is of a cylindrical shape for example.
According to the conventional apparatus disclosed in Japan Laid
Open 11-343135, a seal gas is introduced from the top part of the
upper room. In this, the introduction of the seal gas into the
upper room is for the purpose of preventing the atmospheric air
from entering the reaction chamber, then is no need to vary the
flow rate of the seal gas corresponding to the cross section of the
upper room.
[0049] However, as the soot preform is scaled up, the diameter of
the cylindrical upper room has to be enlarged. As a result,
high-temperature gases generated in the reaction chamber and the
stray glass particles enter the upper room carried by the updraft
flow and then are cooled, and water vapor is condensed on the inner
walls of the upper room. The generated hydrochloric acid is
absorbed by the condensed water, causing it hard to clean upper
room.
[0050] Therefore, the present invention increases the gas flow
introduced from the top of the upper room 5 for avoiding the water
vapor from condensing in the upper room. Namely, from the beginning
of the fabricating process for the soot preform to the end, the
velocity of the gas flow at the bottom of the upper room 5 is
maintained above 0.05 m/sec for avoiding the foregoing issues. The
velocity of this gas flow at the bottom of the upper room 5 is
determined by the cross-sectional areas of the upper room 5 and of
the soot preform 3 in the upper room 5.
[0051] According to the invention, unlike the conventional
apparatus, the gas introduced from the top of the upper room 5 is
not used for avoiding the atmospheric gas from entering the
reaction chamber. The gas introduced from the top of the upper room
5 can be atmospheric air after being dried and compressed.
Aternatively, gas having no influence on the reaction, for example,
inert gases such as nitrogen (N.sub.2), helium (He) or argon (Ar)
can be also used. Considering the cost, air is the best choice.
[0052] According to the present invention, a relatively fast flow
is formed in the upper part of the reaction chamber, flowing from
the gas-supplying inlet 1 to the gas-exhausting outlet 2, and as a
result, a circulating flow is generated underneath this one-way
flow. This circulating flow can be a new cause for disturbing the
core deposition. This circulating flow is created in the following
manner: first a downward flow is formed falling along the wall
underneath the gas-exhausting outlet 2, and then it turns and moves
on the floor toward the wall having the gas-supplying inlet 1, and
then the flow rises along this wall, and hence a circulatory flow
is formed.
[0053] This flow on the floor directly collides with the flame of
the core burner 11 whereby a stable deposition of core soot is
prevented. In order to avoid this, a preferred embodiment of the
invention forms a raised floor 6 on the gas exhaustion side at a
level higher than the core deposition level, as shown in FIGS. 1-4.
On account of this, the flow on the floor moves at a level higher
than the core deposition level, so that this circulating flow will
not interfere with the core deposition, and a stable deposition of
core soot is possible.
[0054] However, the gas flow in the reaction chamber is not a
stable flow. Sometimes, a strong downward gust along the sidewalls
flanking the burners suddenly occurs, which disturbs the core
deposition such that the distribution of the refraction index has
local variations.
[0055] In order to avoid this, according to a more preferred
embodiment of the invention, as shown in FIGS. 1, 2, and 4, a
partition 7 is provided to divide the reaction chamber into an
upper reaction chamber 8 and a lower reaction chamber 9, and a
connect hole 10 is formed to communicate the two chambers 8, 9 with
each other at a location where the core of the porous preform 3 is
disposed; also the lower reaction chamber 9 is made to have
essentially no outlet except this connect hole 10. The connect hole
10 shared by the reaction chambers 8, 9 has a cross section smaller
than that of the upper reaction chamber 8, and this partition 7
effectively performs the same function as the raised floor 6 as
well.
[0056] By adopting this construction, the circulatory flow and the
sudden downward gusts along the walls will not reach the lower
reaction chamber 9, whereby the lower reaction chamber 9 will have
an essentially stagnant atmosphere so that the flame of the core
deposition burner 11 will not be disturbed by the air flows.
[0057] The partition 7 dividing the two reaction chambers 8, 9 is
so disposed that the core deposition level comes within the lower
reaction chamber 9. The gases and the stray glass particles
generated by the flames of the burners provided in the lower
reaction chamber 9 flow into the upper reaction chamber 8 by way of
the connect hole 10, and are exhausted from the gas exhaustion
outlet 2.
[0058] According to a still more preferred embodiment of the
invention, the position and the size of the connect hole 10 are
specified. In order to bring about a stagnant atmosphere in the
lower reaction chamber 9, the connect hole 10 must not be too
large. Especially, if those gaps made around the soot preform 3 in
the connect hole 10 on the sides of the exhaustion gas outlet port
2 and the sidewalls adjacent to the wall containing the exhaustion
gas outlet port 2 are too wide, a satisfactory effect cannot be
attained.
[0059] Preferred examples of the connect hole 10 are shown in FIG.
7A and FIG. 7B. FIG. 7A shows the bottom of the upper chamber 8 of
FIG. 1, 2 or 4, wherein the connect hole 10 is circular. This
circular connect hole 10 is preferably of a radius about 50 mm
greater than the radius of that part of the soot preform 3 passing
this hole. The radius of the connect hole 10 can be the range of
45-55 mm greater than the radius of the passing preform.
[0060] FIG. 7B shows the horizontal bottom of the upper reaction
chamber 8 of FIG. 3, and this connect hole 10 consists of the void
of a semicircle cut in half from the above defined circle
orthogonally to the line B-B and the void of a rectangle whose one
side is the chord of this semicircle, its subtend lying in that
sidewall having the gas-supplying inlet, wherein the connect hole
10 is distanced from the sidewalls by at least 50 mm except the
side wall having the gas-supplying inlet, and is wide enough to
make at-least-20 mm gap all around the soot preform.
[0061] By virtue of this construction, the downward flow is
prevented from entering the lower reaction chamber 9, and the
atmosphere in the lower reaction chamber is rendered stagnant, and,
therefore, the flames of the core burners are not disturbed, and a
good result is obtained.
[0062] On the contrary, if the connect hole 10 is too small, such
that the gap formed around the soot preform 3, which is
concentrically disposed through the connect hole 10, is narrower
than 20 mm, then the gases and the stray glass particles generated
by the flames of the burners provided at the lower reaction chamber
tend to fail to flow into the upper reaction chamber, so that the
stray glass particles adhere to and grow on the ceiling (the lower
face of the partition 7) and the inner edge of the connect hole 10,
and they may be detached and float until some of them fall on the
soot preform to eventually become impurities and bubbles.
Furthermore, if the connect hole 10 is too small, the upper
reaction chamber 8 and the lower reaction chamber 9 are so much
isolated from each other that there occurs difference in the
pressure of the two chambers 8, 9 whereby a current of air is
generated across the narrow gap of the connect hole, which disturbs
the flames of the burners installed at the lower reaction chamber.
Therefore, in a preferred embodiment of the invention, the gap
around the starting rod in the connect hole 10 is 20 mm or
greater.
[0063] The manufacturing apparatus of the invention which has been
described hereinabove has a rectangular cross section when cut by a
horizontal plane, but the invention is not restricted to this but
may be an apparatus that has circular horizontal cross section or
the like.
[0064] It is also possible to provide a gas-supplying inlet in the
lower reaction chamber so long as it does not cause a disturbance
in the flames of the core deposition burners. Incidentally, the
soot preform thus made is, next, dehydrated and then heated to
glassify to become an optical fiber preform; then the preform is
elongated, if necessary, to have a diameter suitable for fiber
drawing, and this elongated preform is eventually drawn to be an
optical fiber. In some production process, the preform made by the
method described above does not have enough cladding portion. In
such case, the preform is elongated to proper size and is added
cladding portion at the subsequent process, then drawn to be an
optical fiber. Embodiments
[0065] <<First Embodiment>>
[0066] The manufacture apparatus as shown in FIG. 1 is designed to
manufacture porous soot preforms by VAD method, and it comprises: a
reaction furnace including a cylindrical upper room 5 having a
radius of 130 mm provided on top of a substantially cubic reaction
chamber, which has sides of about 500 mm, and being so designed
that the center of the ceiling of the reaction chamber is
coincidental with the center line of the cylindrical upper room 5;
and a rotary shaft (not shown), whose rotation axis is coincidental
with the central axis of the upper room 5, and which is capable of
freely moving vertically. As shown schematically in FIG. 1, a
gas-supplying inlet 1, a slit of 480 mm in length and 15 mm in
width, is made in a vertical wall having burners 13 equipped
thereon, at a location 5 mm beneath a ceiling of the reaction
chamber, and a gas-exhausting outlet 2, 480 mm in length and 200 mm
in width, is made in a wall opposite the gas-supplying inlet 1, at
a location 5 mm beneath the ceiling.
[0067] Also a partition 7 having a circular hole of a radius about
50 m greater than the radius of that part of the soot preform
passing this hole is provided extending horizontally from the wall
having the gas-supplying inlet 1, at a level 150 mm from the bottom
of the reaction chamber, and this partition 7 divides the reaction
chamber into an upper chamber 8 and a lower chamber 9. Further, a
raised floor 6 is formed adjacent the wall having the
gas-exhausting outlet 2 in the bottom of the reaction chamber, and
this raised floor 6 is flush with the partition 7. One core
deposition burner 11 and one core heating burner 12 are installed
at the lower reaction chamber 9, and two clad deposition burners 13
are installed at the upper chamber 8.
[0068] The level at which the soot for core is deposited on one end
of the starting rod or growing soot preform is set at 100 mm from
the bottom of the lower reaction chamber 9. From the start till the
end of the soot deposition operation, air at a rate of 300 l/min
was introduced from the top of the upper room 5 to maintain a
downward air current of 0.09 m/sec or higher. An HEPA filter as a
dust-tight filter 4 is provided at the air-supplying inlet 1,
whereby the external air supplied into the chamber 8 through the
inlet 1 is freed from dusts that would result in impurities in the
preform. During the operation, the gas drawing pressure at the
gas-exhausting port was regulated such that the gas flow velocity
at the gas-supplying inlet 1 is maintained at 5 m/sec.
[0069] The core deposition burner 11 was supplied with 450 ml/min
of SiCl4 and 25 ml/min of GeCl4 as raw material gases. The two clad
deposition burners 13 were supplied with SiCl4 as a raw material
gas at the rates of 1.0 l/min and 2.5 l/min, respectively. All of
the deposition burners as well as the core heating burner 12 were
supplied with hydrogen H2 as the combustion gas and oxygen O2 as
the oxidizing gas.
[0070] A soot preform for single mode optical fiber was made in the
above-specified apparatus. The soot preform had an overall diameter
of 200 mm and a core diameter of 40 mm, and the deposition rate was
450 g/hr. Throughout the manufacturing operation, the amounts of
the stray glass particles that adhered and accumulated on the
ceilings and the walls of the upper room 5 and the reaction
chambers were reduced compared with the conventional method and the
glass particles did not fall from their surfaces. The porous soot
preform was glassified and then subjected to a measurement for
refraction index distribution, and it was found that the
distribution was uniform along the length of the preform and it had
excellent optical properties.
[0071] <<Second Embodiment>>
[0072] In a second embodiment, an apparatus of FIG. 2 was placed
inside a clean room of class 10000 and no dust-tight filter 4 was
mounted across the gas-supplying inlet 1.
[0073] In regard to burners, installed at the lower reaction
chamber 9 are one core deposition burner 11 and one clad deposition
burner 14, which latter is disposed near the burner 11 and is
substantially effective as a core heating burner too, and then one
clad deposition burner 13 is installed at the upper reaction
chamber 8. The position of the clad deposition burner 14 installed
at the lower reaction chamber 9 was adjusted such that the
deposition of the glass particles on the clad takes place inside
the upper reaction chamber. Incidentally, the apparatus of FIG. 2
of the second embodiment is the same as that of the first
embodiment shown in FIG. 1 except that the former lacks a
dust-tight filter and has fewer burners, of which one is for both
clad deposition and core heating.
[0074] Except that the core deposition burner 12 of FIG. 1 is
absent in the second embodiment, the apparatus of FIG. 2 was
operated under the same gas supply conditions as the first
embodiment, to manufacture a soot preform for single mode optical
fiber, and as a result a soot preform having an overall diameter of
200 mm and a core diameter of 40 was obtained. The deposition rate
was 450 g/hr. Throughout the manufacturing operation, the amounts
of the stray glass particles that adhered and accumulated on the
ceilings and the walls of the upper room 5 and the reaction
chambers were reduced compared with the conventional method and the
glass particles did not fall from their surfaces. This porous soot
preform was glassified and then subjected to a measurement for
refraction index distribution, and it was found that the
distribution was uniform along the length of the preform and it had
excellent optical properties.
[0075] <<Third Embodiment>>
[0076] The apparatus shown in FIG. 3 was installed in a clean room
of class 10000, and a dust-tight filter 4 of the first embodiment
was not provided across an gas-supplying inlet 1 of this third
embodiment. As schematically shown in FIG. 7B, the bottom of an
upper reaction chamber 8 was shaped such that a connect hole 10
made in a horizontal partition 7 consists of the void of a
semicircle of a radius of 100 mm, whose center substantially
coincides with the rotating axis of a starting rod, and which is
cut from a circle by a diameter orthogonal to a line B-B, and of
the void of a rectangle whose one side is the chord of said
semicircle, its subtend lying in that sidewall having the
gas-supplying inlet 1. One center core deposition burner 15 and one
side core deposition burner 16 were installed at a lower reaction
chamber 9, and one clad deposition burner 13 was installed at an
upper reaction chamber 8. Incidentally, the apparatus of FIG. 3 of
this third embodiment is the same as that of the first embodiment
shown in FIG. 1 except that the former lacks a dust-tight filter
and has a connect hole of a different configuration and has fewer
burners, of which one is for center core deposition and another for
side core deposition.
[0077] The center core deposition burner 15 was supplied with 100
ml/min of SiCl4 and 35 ml/min of GeCl4 as raw material gases. The
side core deposition burner 16 was supplied with 500 ml/min of
SiCl4 and 60 ml/min of GeCl4 as raw material gases. The clad
deposition burner 13 was supplied with 2.5 l/min of SiCl4 as a raw
material gas. Further, each deposition burner was supplied with
hydrogen H2 as the combustion gas and oxygen O2 as the oxidizing
gas.
[0078] A soot preform for dispersion shifted single mode optical
fiber was made in the above-specified apparatus. This soot preform
had an overall diameter of 180 mm, and an outer diameter of a side
core measured at the connect hole was 120 mm, and the deposition
rate was 300 g/hr. Throughout the manufacturing operation, the
amounts of the stray glass particles that adhered and accumulated
on the ceilings and the walls of the upper room 5 and the reaction
chambers were reduced compared with the conventional method and the
glass particles did not fall from their surfaces. The porous soot
preform was glassified and then subjected to a measurement for
refraction index distribution, and it was found that the
distribution was uniform along the length of the preform and it had
excellent optical properties.
[0079] <<Fourth Embodiment>>
[0080] An apparatus of this fourth embodiment shown in FIG. 4 is
the same as that of the second embodiment shown in FIG. 2 except
that the former has a connect hole of a different configuration and
has only one burner.
[0081] The connect hole 10 of a horizontal partition 7 was made in
a shape of a circle of a radius of 100 mm whose center coincides
with the rotation axis of the starting rod. One core deposition
burner 11 was installed at a lower reaction chamber 9 and no burner
was provided at an upper reaction chamber. The core deposition
burner 11 was supplied with 1.6 l/min of SiCl4 and 150 ml/min of
GeCl4 as raw material gases, and also with hydrogen H2 as the
combustion gas and O2 as the oxidizing gas.
[0082] A soot preform for multimode optical fiber was made in the
above-specified apparatus, and the soot preform had an overall
diameter of 140 mm, and the deposition rate was 140 g/hr.
Throughout the manufacturing operation, the amounts of the stray
glass particles that adhered and accumulated on the ceilings and
the walls of the upper room 5 and the reaction chambers were
reduced compared with the conventional method and the glass
particles did not fall from their surfaces. The porous soot preform
was glassified and then subjected to a measurement for refraction
index distribution, and it was found that the distribution was
uniform along the length of the preform and it had excellent
optical properties.
[0083] <<Comparative Embodiment>>
[0084] An apparatus of this comparative embodiment shown in FIG. 8
is the same as that of the second embodiment shown in FIG. 2 except
that the former does not have a gas-supplying inlet 1 and a raised
floor 6 formed adjacent that wall of the reaction chamber which has
the gas-exhausting outlet, and that in the comparative embodiment
no partition 7 to divide the reaction chamber into the upper and
lower reaction chambers 8, 9 was provided. Other conditions were
set same as in the second embodiment, and a soot preform for single
mode optical fiber was made in the above-specified prior art
apparatus. It was observed that large amounts of stray glass
particles adhered and accumulated on the ceiling of the reaction
chamber and some of these fell from the ceiling in masses during
the manufacturing operation. The resulting porous soot preform was
glassified and found to have numerous impurities and bubbles in
it.
[0085] In an apparatus for manufacturing soot preform as
constructed according to the present invention, the stray glass
particles that did not adhere and accumulate on the target body are
carried by the gas flow passing underneath the ceiling of the
reaction chamber and promptly brought outside the reaction chamber.
Especially, the stray glass particles and water vapor, which
condenses on the wall of the upper room and makes cleaning
operation difficult, are effectively prevented from entering the
upper room, and thus cleaning operation after the manufacturing
operation becomes considerably easier.
[0086] Also, since the adhesion and accumulation of the stray glass
particles on the soot preform are restricted, it becomes possible
to manufacture soot preforms of higher quality with high
dependability. Furthermore, since the flame of the core deposition
burner is not disturbed by the circulatory flow in the reaction
chamber or the strong and occasional downward gusts along the walls
of the reaction chamber, it is possible to obtain a glass preform
in which the refraction index distribution is uniform along its
length.
[0087] Moreover, through the use of a manufacturing apparatus of
the present invention, it is also possible to obtain soot preforms
for producing high quality optical fibers having dependable optical
properties, such as a single-mode optical fiber having a stepped
refraction index distribution, a dispersion-shifted optical fiber
having a center core and a side core, and a multi-mode optical
fiber having a parabolic refraction index distribution.
[0088] While the present invention has been described with a
preferred embodiment, this description is not intended to limit our
invention. Various modifications of the embodiment will be apparent
to those skilled in the art. It is therefore contemplated that the
appended claims will cover any such modifications or embodiments as
fall within the true scope of the invention.
* * * * *