Note: Descriptions are shown in the official language in which they were submitted.
Furnace for producina hiah puritY quartz glass preform
The present invention relates to a furnace for
producing a high purity quartz glass preform for an optical
fiber. More particularly, the present invention relates to a
heating furnace for the thermal treatment such as dehydration,
doping or sintering of a porous glass preform consisting of
fine particles of quartz glass in order to obtain a high
purity quartz glass preform for fabricating the optical fiber.
In a heating furnace for producing a glass preform --
for use in the fabrication of an optical fiber, use of a
quartz glass tube used as a muffle tube has been proposed in,
for example, Japanese Patent Kokoku Publication Nos.
42136/1983 and 58299/1983 and Japanese Patent Kokai
Publication No. 86049/1985. However, the quartz glass tube
has a serious disadvantage that it tends to deform at high
temperatures. In practice, when the heating furnace is
operated at a temperature higher than 1500C, the quartz glass
tube deforms so that the furnace cannot be used again, unless
a pressure difference between the outside and the inside of
the muffle tube and a means for supporting the muffle tube are
both accurately controlled. Further, when the furnace is used
at a temperature higher than 1150C for a long time, the
quartz glass tube is devitrified or crystallized. Since the
thermal coefficient of expansion of the glass layer is
different from that of the devitrification layer, the muffle
tube is destroyed by the resulting strain.
It has been found that a carbon tube is suitable as
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a muffle tube which overcomes the above problems (see, for
example, PCT International Publication No. WO 88/06145). The
carbon tube not only has an excellent heat resistance since it
is stable at a temperature higher than 2000C, but is also
easily purified to a high purity level with an ash content
less than 20 ppm. In addition, such a muffle tube does not
react with a reactive gas (for example, C12, CC14, SiF4, SF6 and
CC12F2) which is useful for the thermal treatment of the glass
preform for the optical fiber. The carbon tube can be
fabricated precisely so that it may be made in large
quantities on an assembly line to reduce production costs.
Further, in order to improve gas tightness of the carbon made
tube, it is possible to coat the outer surface thereof with a
SiC layer or a further carbon layer whereby the glass preform
for the optical fiber having an excellent quality can be
produced.
A detailed description of the prior art is given
hereinbelow.
According to one aspect of the invention there is
provided a heating furnace comprising a furnace body, a
cylindrical zone heater in said furnace body, a muffle tube
installed through said furnace body for thermally treating a
porous preform made of high purity quartz glass by moving the
preform vertically therethrough, and a partition means in the
portion of the muffle tube projecting above the furnace body
to divide an interior space of the muffle tube into an upper
space and a lower space.
The present invention will be described in detail
hereinbelow with the aid of the accompanying drawings, in
which:
Fig. 1 schematically shows a cross-sectional view of
a conventional heating furnace;
Fig. 2 schematically shows an apparatus for
measuring the amount of air inflow into a muffle tube;
Fig. 3 shows the results of the air inflow
measurement using the apparatus shown in Fig. 2;
Fig. 4 schematically shows a cross-sectional view of
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another conventional heating furnace;
Fig. 5 schematically shows a heating apparatus to
indicate the length of the apparatus comprising the heating
furnace as shown in Fig. 4;
Figs. 6 to 8 schematically show detailed cross-
sectional views of the partition conventionally used in the
conventional heating furnace as shown in Fig. 4;
Fig. 9 schematically shows a cross-sectional view of
one embodiment of the heating furnace according to the present
invention;
Fig. 10 schematically shows a detailed plane view of
the partition means which is used in the heating furnace
according to the present invention;
Fig. 11 shows a detailed side view of the partition
means used in the heating furnace according to the present
invention;
Fig. 12 schematically shows a cross-sectional view
of the heating furnace equipped with the partition means
according to the present invention; and
Fig. 13 (appearing on the same sheet of drawings as
Figure 5) schematically shows a heating apparatus comprising
the heating furnace according to the present invention to
indicate the length of the apparatus.
Fig. 1 shows one example of a conventional heating
furnace wherein the thermal treatment of a glass soot preform
1 is carried out with a cylindrical zone heater. A carbon
heater 4 and a muffle tube 3 are provided in a furnace body 5.
This heating furnace has an inlet 6 for introducing nitrogen
gas for purging the furnace body interior, an inlet 7 for
introducing an atmosphere gas to the muffle tube, and a
supporting rod 2 for the preform 1 which is placed inside the
heating furnace. The muffle tube 3 consists of an upper
member 34, a middle member 35 and a lower member 36. At least
the middle member 35 is made of carbon, on the surface of
which a SiC or carbon coating may be provided.
Since the conventional heating furnace is
constituted as shown in Fig. l, an amount of air around the
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furnace (an atmosphere in the operation room) flows in the
muffle tube when the glass preform enters or leaves the tube.
Fig. 2 schematically shows an apparatus which is
used in measuring the amount of air flowing into the muffle
tube. This apparatus is comprised of a muffle tube 101, an
inlet 102 for purging gas, a gas sampling tube 103, a device
104 for measuring the oxygen concentration and a pump 105. A
zone heater (not shown) is provided around the muffle tube
101. The inner diameter of the muffle tube 101 is 150 mm, and
the front end of the gas sampling tube 103 is fixed at a point
lm below from the upper edge of the muffle tube.
The results are shown in Fig. 3. These results
indicate that air flows into the muffle tube, and such air
inflow cannot be prevented by an increase of the amount of
purging nitrogen gas.
Inflow of air will cause various problems.
Firstly, the interior space of the muffle tube is contaminated
by dust in the air. The dust is comprised of SiO2, Al203,
Fe203, and the like. Among them, Al203 will cause
devitrification of the preform, and Fe203 will cause an
increase in the transmission loss of the optical fiber.
Secondly, the inner surface of the carbon muffle tube is
oxidized. During oxidation of the sintered body of carbon, it
is known that tar or pitch which is used as a binder is
oxidized first. Therefore, the remaining graphite particles
are dropped or splashed and float in the furnace. Since these
particles adhere to the surface of the sintered glass preform,
the optical fiber fabricated from the glass preform has many
areas of low strength. As a natural consequence, the lifetime
of the carbon made muffle tube is extremely shortened.
The first measure to prevent such oxidation of the
muffle tube is to reduce the temperature to 400C or lower at
which the carbon is not oxidized during the insertion and
removal of the glass preform. However, at such a low
temperature, the operation rate of the furnace is greatly
decreased. In addition, once the muffle tube is exposed to
the air, a considerable amount of oxygen and moisture in the
, . .
air is adsorbed by the muffle tube since the carbon muffle
tube is porous. The oxidation cannot be prevented completely.
As the second measure, a method is described in PCT
International Publication No. WO 88/06145. The method
comprises first placing the porous glass preform in a front
chamber on the top of the muffle tube and inserting the glass
preform into the muffle tube after replacing the atmosphere in
the front chamber with an inert gas. The muffle tube equipped
with a front chamber is shown in Fig. 4.
The carbon heater 4 and the carbon muffle tube 3 are
provided in the furnace body 5 as shown in Fig. 4. The
heating furnace is comprised of an inlet 6 for introducing a
purging nitrogen to the furnace body, an inlet 7 for
introducing an atmosphere gas to the muffle tube, supporting
rod 2 for the glass preform, a front chamber 11, an exit 14
for exhausting the gas from the front chamber, and a partition
16. The glass preform 1 is inserted into the heating furnace.
The insertion of the porous glass preform into the
heating furnace shown in Fig. 4 is carried out as follows:
(1) A porous glass preform 1 is attached to a
rotatable and vertically movable chuck by the supporting rod
2.
(2) An upper cover of the front chamber 11 is
opened, and the porous preform 1 is lowered into the front
chamber 11.
(3) The upper cover is closed, and the interior
space of the front chamber is purged with an inert gas (e.g.
nitrogen or helium).
(4) The partition 16 which separates the front
chamber 11 from the heating atmosphere is opened, and the
porous preform 1 is introduced in the heating atmosphere which
has been kept at a temperature at which the preform should be
thermally treated.
(5) The partition 16 is closed.
The removal of the glass preform from the heating
furnace is carried out as follows:
(1) The partition 16 is opened.
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(2) The preform 1 which has been thermally treated
is pulled up from the heating atmosphere to the front chamber
11. In this step, the temperature of the heating atmosphere
is not necessarily lowered.
(3) The partition 16 is closed.
(4) The upper cover of the front chamber 11 is
opened, and the preform is removed from the chamber 11.
Though the above furnace is superior in preventing
oxidation of the muffle tube, the overall length of the
heating apparatus is too long and the structure of the
partition 16 is complicated.
Fig. 5 shows an example of the heating apparatus for
thermal treatment of a porous glass preform having an overall
length of 800 mm and a seed rod length of 200 mm. In this
case, the length from the lower end of the muffle tube to the
lower end of the chuck is 6760 mm and the overall length of
the apparatus reaches nearly 8000 mm by taking into account
the space needed for operation.
Further, since the partition 16 should be adaptable
to both cases where the supporting rod is and is not through
the partition, a total of three members should be used, two of
which are split members each having a partly cut off portion
for the supporting rod, and one of which is a member to close
an aperture through which the rod is passed.
The procedure for operating the partition 16 is
described with reference to Figs. 6 to 8.
Fig. 6 shows a cross-sectional view of the partition
in detail when the glass preform 1 is disposed in the front
chamber 11. Fig. 7 shows the glass preform 1 being inserted
into the heating atmosphere of the muffle tube through the
partition 16 after the atmosphere in the front chamber is
replaced with an inert gas and the partition 16 is opened.
Fig. 8 shows the glass preform being thermally treated.
To carry out the above three operations, three
members, that is, the covering member 72 for covering an
aperture and two split members 71 which are operated with two
sliding rods 73 are used.
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Problems arise as follows:
tl) The partition 16 consists of three members so
that it takes a considerable time to close or open the
partition;
(2) The structure of the partition 16 is rather
complicated. In particular, it is difficult to confirm the
perfect closing of the split members. Then, there is a great
possibility to open the front chamber to the air with an
insufficient sealing; and
(3) It takes a long time to replace the interior
atmosphere of the front chamber with the inert gas since the
volume of the partition portion increases due to the
complicated structure of the partition.
The present invention will be hereinafter described
with reference to the remaining drawings.
Fig. 9 shows one example of an embodiment of the
heating furnace according to the present invention. A heating
furnace comprises a carbon heater 4 in a furnace body 5 and a
muffle tube 3 through the furnace body. The muffle tube is
comprised of a middle member 35 and a lower member 36 each
made of high purity carbon coated with SiC or carbon. An
upper member 33 and an upper cover 37 are provided, each made
of quartz. A porous glass preform 1 is inserted into the
heating furnace which is further comprised of an inlet 6 for
introducing nitrogen as a purging gas to the furnace body, an
inlet 7 for introducing an atmosphere gas to the muffle tube,
a supporting rod 2 for the glass preform, an outlet 21 for
exhausting the atmosphere gas from the muffle tube, an inlet
22 for introducing nitrogen gas for replacement of the upper
space of the muffle tube and a partition means 23 made of
quartz, having a small aperture 24 for passing a gas. The
partition means 23 is arranged such that it can be opened or
closed from the outside by a quartz rod 26. When the
partition means 23 is closed, the aperture 24 may be omitted
if the gas can flow from the lower space of the muffle tube to
the upper space of the muffle tube through any narrow gap or
if any outlet for exhausting the gas is provided in the muffle
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tube below the partition.
Such a partition means 23 with no aperture is
described with reference to Figs. 10 and 11 which show in
detail the structure of the partition means 23, and Fig. 12
which shows one embodiment of the heating furnace having such
a partition means.
The partition means in Figs. 10 and 11 is comprised
of a partition 23, an upper flange 41 which is connected to
the upper member 33 of the muffle tube 3 and a lower flange 43
which is connected to the middle members 34 and 35 and with
lower member 36 of the muffle tube. Both the flanges sandwich
the box member 41. When the glass preform 1 is inserted into
the muffle tube through the partition means, the preform is
passed through the hole 44 in the partition 23.
The partition 23 may be opened or closed from the
outside, by the operating rod 26. The partition 23 is guided
by the rails 45 so that it does not deviate from the direction
along which it should be moved. As understood from Fig. 11,
the operating rod has a step so that the diameter thereof is
changed in the intermediate portion of the rod. The tip
portion 46 of the operating rod has, for example, a hook
member such that the tip portion can be engaged with a
projection on the edge portion of the partition. Thus, when
the partition 23 is closed, the operating rod 26 is inserted
into the box member 41 so that the step of the operating rod
is engaged with projection 47 (the right hand side surface of
the projection in Fig. 11) and then the partition 23 is
pushed. On the contrary, when the partition is opened, the
tip portion 46 of the operating rod is engaged with the
projection 47 (the left hand side surface of the projection in
Fig. 11) so that the partition can be pulled.
In the embodiment as shown in Fig. 12, the
engagement between the projection 47 of the partition and the
tip portion 46 having the hook means can be released by
rotating the rod 26. Therefore, after the release, the
operating rod 26 is moved back and rotated again so that the
further engagement can be established between the hook means
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and the opposite side of the projection 47. In such a case
the step of the operating rod can be omitted.
The interior space of the partition means is purged
via inlets 22 which are used to introduce an inert gas on the
side of the surface. Inlets 22 are located near the upper
flange 42 of the partition means and in the sliding portion 48
between the box member 41 and the sliding rod 26. The sliding
portion 48 is arranged such that the operating rod 26 can be
slid into the box member 41 and the sliding portion is sealed
by an O-ring or may be an injection syringe structure with
nitrogen purging.
In the case where the hole 44 is covered by the
partition 23, the atmosphere gas is exhausted through the
outlet 49 when the atmosphere gas is supplied into the middle
and lower members of the muffle tube through the inlet 7. The
difference between the embodiments shown in Figs. 9 and 10 is
that the purging gas in the middle and lower members of the
muffle tube is exhausted to the upper member of the muffle
tube 33 in the former or through the outlet 49 in the latter
when the partition is closed. Otherwise there is no
substantial difference between these two embodiments.
A heater 25 can be provided. It can heat the glass
preform 1 at 100 to 800C when the preform is present in the
interior space of the muffle tube above the partition means
23. Such a heater may be a resistance heater or an infrared
heating lamp. The glass preform 1 should be completely
accommodated within the upper space of the muffle tube. The
position where the partition means is located between the
upper and lower spaces is not critical and also is not limited
to the embodiment shown in Fig. 12. Generally, the partition
means is located in the upper space above the furnace body.
When the porous glass preform is inserted into the
muffle tube of the heating furnace as shown in Fig. 12, the
partition means is operated as follows:
(1) The porous glass preform 1 is fixed to a chuck
via the supporting rod 2.
(2) After the upper cover 37 of the muffle tube 3
is opened, the preform is inserted in the upper member 33 of
the muffle tube. In this insertion step, the partition 23 is
closed and the purging nitrogen gas is supplied to the lower
muffle tube 3 from the inlet 7 and is exhausted through the
outlet 49 to the outside. Therefore, the middle and lower
members 34, 35 and 36 of the muffle tube is in nitrogen
atmosphere and, even when the upper cover 37 is opened, the
air may not flow in the middle and lower members of the muffle
tube. In the embodiment as shown in Fig. 9, the atmosphere
gas is exhausted through the aperture 24 into the upper member
33 of the muffle tube.
(3) The upper cover 37 is closed and nitrogen gas
is supplied through the inlets 22 disposed in the upper
portion of the muffle tube, the upper portion of the partition
means and the sliding portion for replacement with nitrogen of
the interior atmosphere of the upper portion of the muffle
tube. During such a replacement, it is preferred to heat the
preform 1 by means of the heater 25 so that any gas which has
been adsorbed by the porous glass preform is advantageously
released.
(4) The partition 23 is opened and the glass
preform is lowered to a position where the thermal treatment
is initiated, for example, a position where the lower end of
the porous glass preform meets the lower end of the heater 4.
The nitrogen supply through the inlets 22 for introducing the
purging gas is stopped, the heater 25 is turned off, and the
thermal treatment of the porous preform is started.
The preform is removed from the heating furnace as
follows:
(1) After finishing the thermal treatment, the
preform is drawn up to the level above the partition 23.
Nitrogen gas is introduced through the inlet 7 and exhausted
through the outlets 21 and 49 to completely replace the
interior atmosphere of the muffle tube with nitrogen and to
form a nitrogen atmosphere in the muffle tube.
t2) The partition 23 is closed. In this step, the
amount of nitrogen gas may be reduced. However, nitrogen gas
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should be continuously supplied such than any gas outside the
muffle tube is not introduced through the outlet 49 into the
inside of the middle and the lower members of the muffle tube.
(3) The upper cover 37 is opened and the preform 1
is removed from the muffle tube.
The present invention will be hereinafter explained
by the following examples.
Example 1
A heating apparatus comprising the heating furnace
according to the present invention as shown in Fig. 9 was
built, in which a porous glass preform having a total length
of 800 mm and the seed rod having the length of 200 mm can be
suitably treated. The required minimum length for the heating
apparatus from the lower end of the muffle tube to the lower
end of the chuck is 5560 mm as shown in Fig. 13. The total
length of the heating apparatus containing the length
necessary for the furnace and the length necessary for
operation was 6800 mm, which is shorter by about 1.2 m than
that of the apparatus shown in Fig. 5 in which the same glass
preform can be treated.
Example 2
A heating furnace as shown in Fig. 9 was used. The
porous glass material was disposed in the upper member 33 of
the muffle tube and the upper cover 37 was closed. Nitrogen
gas was introduced into the upper member at a flow rate of 10
1/min. for ten minutes and nitrogen gas was supplied into the
lower and the middle members of the muffle tube at a flow rate
of 10 l/min. for ten minutes. During this procedure, the
partition 23 was closed. The muffle tube comprising the
middle and the lower members 34, 35 and 36 is made of high
purity carbon, the outer surfaces of which were coated with a
gas impermeable carbon layer or a SiC layer.
The partition 23 was opened and the glass preform
was inserted in the middle and the lower members of the muffle
tube and thermally treated to obtain a preform for the
production of an optical fiber. The conditions of the
dehydration and the sintering operations were as follows:
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Dehydration operation:
He supply rate 10 1/min.
Cl2 supply rate500 cc/min.
Preform traverse rate8 mm/min.
Temperature 1100 C
Sintering operation:
He supply rate 10 1/min.
Preform traverse rate3 mm/min.
Temperature 1650 C
A glass preform for an optical fiber was produced by
using the so obtained preform as a core material and a
fluorine-doped glass pipe, which was separately produced, as a
cladding. The preform and the glass pipe were integrated in a
resistance heating furnace. The preform, the outside surface
of which was coated with an additional glass layer to adjust
the diameter thereof was drawn to produce a pure silica-core
single mode optical fiber. The transmission loss of the
optical fiber at a light wavelength of 1.55 ~m was found to be
as low as 0.18 dB/km.
Example 3
The thermal treatment of the porous glass preform as
in Example 2 was repeated forty times. The amount of lost
carbon during the series of the treatments was 14 g, which
amount corresponded to an exhaustion of the carbon layer of
35 ~m in thickness due to oxidation in the heated portion.
Such an amount indicates that the carbon made muffle tube can
be used for about two years.
Comparative Exam~le 1
A heating furnace of the prior art as shown in Fig.
4 was used.
The porous glass preform was disposed in the front
chamber 11 and the upper cover of the chamber was closed.
Nitrogen gas was introduced into the front chamber at a flow
rate of 10 l/min. for ten minutes to replace the interior
atmosphere of the front chamber. During this step, the
partition 16 was closed. Then, the partition was opened, the
porous glass preform was lowered into the muffle tube 3 and
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then thermally treated, after closing the partition, to
produce a transparent glass preform for an optical fiber.
When the preform was removed from the heating
furnace, firstly the partition 23 was opened and then it was
closed after the preform was moved into the front chamber.
Finally, the upper cover was opened.
An optical fiber was produced by using the obtained
preform as the core in the same manner as in Example 1. The
transmission loss of the optical fiber at the light wavelength
of 1.55 ~m was measured to be as low as 0.18 dB/km.
Comparative Example 2
The thermal treatment of the porous glass preform as
in Comparative Example 1 was repeated forty times. The amount
of lost carbon during the series of the treatments was 20g,
which amount corresponds to an exhaustion of the carbon layer
of 50 ~m in thickness due to oxidation in the heated portion.
Such an amount indicates that the carbon made muffle tube can
be used for about one year and a half.
The difference in the exhausted amount of carbon
made muffle tube between Example 3 and Comparative Example 2
was due to effects resulting from the following points:
(1) Nitrogen was further supplied to the lower and
the middle members of the muffle tube in Example 3;
(2) The structure of the partition means was more
complicated in Comparative Example 1 so that the volume to be
replaced with nitrogen gas was larger; and
(3) During the replacement with nitrogen gas, the
position at which the porous glass preform was located was
nearer to the heater in Example 3, so that gas desorption from
the porous glass preform was promoted.
Example 4
Example 2 was repeated except that the period for
the nitrogen replacement was increased to twenty minutes and
the glass preform was heated by an infrared lamp of 800 W
during the replacement. Further, in order to heat the preform
uniformly, it was rotated during the replacement. The
exhausted amount of carbon of the muffle tube was 6 g, which
corresponded to an exhaustion of the carbon layer of 5 ~m in
thickness from the surface. Such an amount indicates that the
carbon made muffle tube can be used for about five years.
The effects of the present invention are as follows:
An inflow of the outside atmosphere around the
heating furnace can be prevented, whereby the contamination of
the heating atmosphere in the muffle tube due to the
impurities in the air may be prevented. The devitrification
of the glass preform can be prevented and also the
transparency of the preform can be improved.
In the case where the muffle tube is made of carbon,
the life of the muffle tube may be extended since the carbon
loss due to oxidization is suppressed. The total length of
the heating apparatus of the present invention can be made
shorter.
The operating efficiency of the heating furnace is
improved since the glass preform can be inserted into and
removed from the muffle tube without reducing the temperature
of the muffle tube.
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