Note: Descriptions are shown in the official language in which they were submitted.
Furnace for heating highly pure quartz preform
for optical fiber
The present invention relates to a furnace for
heating a quartz preform for the fabrication of an optical
fiber. More particularly, the invention relates to a furnace
for heating a porous glass preform comprising quartz glass
soot for the purpose of dehydration, addition of dopants and
sintering to produce a highly pure quartz glass preform for
the fabrication of an optical fiber.
The heating furnace of the present invention can
prevent contamination of the preform with impurities and has
good durability.
To produce a glass preform for an optical fiber by
the VAD method or the OVD method, it is necessary to dehydrate
a glass soot preform and then to increase the density and to
sinter the dehydrated soot preform. In some cases, fluorine,
which is a dopant for adjusting the refractive index of the
glass, is added in the dehydration step and/or ~he sintering
step, or between the dehydration step and the sintering step.
For dehydration, sintering and the addition of fluorine, a
heating furnace equipped with a muffle tube is used.
The conventional muffle tube is made of alumina (cf.
Japanese Patent Publication No. 40096/1982 and U.S. Patent No.
4,338,111) or quartz ylass (cf. Japanese Patent Publication
Nos. 58299/1983 and 42136/1983).
With the muffle tube made of alumina, impurities
such as alkali are liberated from the surface of the muffle
tube so that the produced preform tends to be crystallized.
~,:
The muffle tube made of quartz glass includes impurities such
as copper or water and so the produced glass preform provides
an optical fiber having an increased optical absorbance. The
muffle tube itself has unsatisfactory heat resistance.
To overcome the above problems, carbon is proposed
as a material for the muffle tube used in a heating furnace,
see (cf. WO88/06145, U.S. Patent Application Serial No.
07/274,995 filed on October 6, 1988 and EP-Al-0 302 121).
A conventional heating furnace will be described in
detail hereinbelow.
An object of the present invention is to provide a
muffle tube for use in a furnace for heating a glass preform
for the fabrication of an optical fiber, which muffle tube can
be used in a wide temperature range and has a long life.
In accordance with one aspect of the invention there
is provided a heating furnace for heating a porous preform
made of fine particles of highly pure quartz glass for an
optical fiber, which furnace comprises a cylindrical furnace
body, a heater installed in said furnace body and a muffle
tube installed inside said heater to separate a heating
atmosphere from said heater, wherein said muffle tube is made
of highly pure carbon and coated with a gas impermeable
carbon.
The present invention will be described hereinbelow
in detail with the aid of the accompanying drawings, in which:
FIG. 1 schematically shows a cross section of a
conventional heating furnace; and
FIGs. 2 to 5 schematically show various embodiments
of the heating furnace of the present invention.
Herein, the term "heating" is intended to mean any
treatment of the preform at a high temperature, for example,
dehydration of the preform, addition of a dopant to the
preform and sintering of the preform.
The term "highly pure carbon" means carbon having a
total ash content of not larger than 50 ppm, preferably not
larger than 20 ppm.
In the heating furnace of the present invention, a
~ . ., ,.~
gas impermeable carbon coating is provided on both the outer
and inner surfaces of the muffle tube, although the coatlng
may be provided on one of the outer and inner surfaces.
The gas impermeable carbon coating has a gas
permeability of not more than 1 x 10-4 cm2/sec. (for nitrogen
gas). The thickness of the carbon coating is not critical.
To insure impermeability, the thickness is preferably not
smaller than 1 ~m.
The gas impermeable carbon coating can be formed
from pyrolytic carbon or vitreous carbon. These forms of
carbon can form the highly pure coating.
The gas impermeable carbon coating may be formed on
the surface of the muffle tube by any conventional method.
For example, the pyrolytic carbon can be formed by heating a
hydrocarbon such as methane and acetylene at a temperature of,
for example, 1000'C.
In the ~uffle tube to be used according to the
present invention, since the muffle body and the coated layer
are both made of carbon, their thermal expansion
characteristics can be made close so that the coated carbon
does nct peel off or crack. In addition, the muffle body and
the coated carbon are not corroded with chlorine gas even at
high temperatures, or not deformed at a high temperature of
1500C or higher.
One of the common problems of the carbon made muffle
tube is wear due to oxidation of the carbon at a temperature
of 400C or higher. The muffle tube of the present invention
may suffer from such wearing. However, this problem can be
solved by various measures as explained in examples set out
below. Since the muffle tube of the present invention is
highly resistant to oxidation, it has little limitation on the
operation conditions and long life.
A conventional heating furnace is shown in Fig. 1.
The heating furnace of this type comprises a cylindrical
furnace body 5, and a muffle tube 3 which is inserted through
the furnace body. A heater 4 is installed inside the furnace
body. The furnace body 5 has an inlet 6 for an inert gas, and
the muffle tube 3 has an inlet 7 for an atmospheric gas (e.g.,
Cl2, SiF4, He, etc.). The muffle tube 3 consists of an upper
part 34, a middle part 35 and a lower part 36.
When the furnace is used, a porous soot preform 1 is
supported in the muffle tube by means of a supporting rod 2
and is heated.
The muffle tube disclosed in WO88/06145, U.S. Patent
Application Ser. No. 07/274,995 filed on October 6, 1988 and
EP-Al-0 302 121 is characterized in that at least the inner
layer consists of highly pure carbon. Examples of the
disclosed designs of the muffle tube wall are as follows:
1. A silicon carbide or quartz wall having a highly
pure carbon coating on the inner surface.
2. A highly pure carbon wall having a silicon
carbide coating on the outer surface.
3. A wall consisting of an outer layer of silicon
carbide and an inner layer of highly pure carbon.
However, each of these constructions has the
following drawbacks:
1. In the first design, the carbon coating tends to
be peeled off or cracked because of difference in the
coefficients of thermal expansion between the silicon carbide
or quartz and the highly pure carbon, or weak bonding of the
carbon coating to the silicon carbide or quartz wall. Since
the quartz wall is softened and deformed at a temperature of
1500C or higher, it is impossible to maintain the bonding
between the quartz wall and the carbon coating. Since the
silicon carbide wall is corroded with chlorine gas (Cl2) at a
temperature of 900~C or higher, the life of the muffle tube is
greatly shortened by treatment with the chlorine gas when the
carbon coating is peeled off or cracked.
2. In the second design, since the highly pure
carbon generally has gas permeability, a part of the
atmospheric gas in the muffle tube reaches the silicon carbide
layer. When chlorine gas kept at a temperature of 900~C or
higher is used, silicon atoms are removed from the silicon
carbide layer to leave a carbon layer. Since the carbon layer
which is formed through the removal of silicon atoms from the
silicon carbide layer has a smaller density than a usual
carbon layer, gasses can easily pass through the layer at high
temperatures, whereby the glass preform is contaminated with
impurities present outside the muffle tube.
3. The third design has the same problems as those
of the second design. In addition, since the silicon carbide
layer is not a coated material but made of a sintered
material, it becomes brittle when it is corroded with the
chlorine gas and its life is consid~rably shortened.
As explained above, with a muffle tube made of
conventional material, the preform must be heated at a limited
temperature in a limited atmosphere. In addition, the muffle
tube life is short.
Fig. 2 schematically shows a cross section of a
first embodiment of the heating furnace according to the
present invention. The heating furnace of Fig. 2 is comprised
of a cylindrical furnace body 5 and a muffle tube 3 which i5
installed inside the furnace body 5. Further, a heater 4 is
provided between the furnace body 5 and the muffle tube 3.
The furnace body 5 has an inlet 6 for an inert gas, and the
muffle tube 3 has an inlet 7 for an atmospheric gas (e.g. Cl2,
SiF4, He, etc.).
With the heating furnace of the present invention, a
porous preform 1 attached to a supporting rod 2 is inserted in
the muffle tube and heated.
Preferably, the muffle tube consists of three parts,
namely an upper part 34, a middle part 35 and a lower part 36
in view of economy and ease of production. When the muffle
tube is separated into three parts, the middle part 35 which
is more quickly worn than the upper and lower parts 34 and 36
can be changed while leaving the upper and the lower parts
unchanged.
The differences between this heating furnace from
35 that of Fig. 1 is that all of the three parts 34, 35 and 36
are made of highly pure carbon and coated with a gas
impermeable carbon coating. In addition, the furnace body is
,~
longer than that of the conventional furnace body so that the
furnace body can cover that part of the outer wall of the
muffle tube which is heated at a temperature of 400C or
higher during the heating operation.
Fig. 3 schematically shows a cross section of a
second embodiment of the heating furnace according to the
present invention. In this embodiment, the muffle tube is
coated with a gas impermeable coating and has an inner tube 8
made of highly pure carbon inside the middle part 35.
Preferably, the inner tube 8 is inserted in the muffle tube 3
without leaving a gap between the inner tube and the muffle
tube. Preferably, the inner tube has an outer diameter about
1 mm smaller than an inner diameter of the muffle tube,
whereby the inner tube is easily inserted in the muffle tube.
Such diameter difference is sufficient to prevent the
oxidation of the muffle tube. In this case, the inner tube 8
may be coated with the gas impermeable carbon.
Fig. 4 schematically shows a cross section of a
third embodiment of the heating furnace according to the
present invention. In this embodiment, at least a middle
portion of the muffle tube which is heated to 400~C or higher
has a closed double wall structure an outer wall of which is
composed of a part of the upper part 34, the middle part 35
and a part of the lower part 36 and an inner wall of which is
composed of an upper inner wall 37 and a lower inner wall 38.
The outer and inner walls define a closed space which
communicates outside through an inlet 9 for the insertion of
an inert gas.
The inner walls 37 and 38 are made of highly pure
carbon and may be coated with the gas impermeable carbon.
Fig. 5 schematically shows a cross section of a
fourth embodiment of the heating furnace according to the
present invention. This furnace is a modification of the
furnace of Fig. 2 and has a front chamber 11. In addition to
all the elements of the heating furnace of Fig. 2, this
heating furnace comprises the front chamber 11, an outlet 14
for a front chamber gas, an inlet 15 for a gas for purging the
gas in the front chamber, and a partition 16.
The front chamber is preferably made of a heat
resistant material which liberates no impurities, such as
quartz glass, SiC, Si3N4, BN, and the like. In the heating
furnace of Fig. 5, the muffle tube 3 may be replaced with that
of Fig. 3 or Fig. 4.
Since the heating furnace of Fig. 2 does not use
silicon carbide which is corroded with chlorine gas at a
temperature of 900C or higher or quartz glass which is
softened at a temperature of 1500C or higher, it is stable
and has a long life. In addition, since the highly pure
carbon material is coated with the gas impermeable carbon,
impurities or water do not diffuse from the outside of the
muffle tube into the inside of the muffle tube. Therefore,
the highly pure quartz glass preform which is produced with
the heating furnace of the present invention can provide an
optical fiber having low light transmission loss.
When a corrosive gas such as the chlorine gas is
used as the atmospheric gas in the muffle tube, the gas does
not diffuse outside the muffle tube so that the furnace body
is not corroded with the corrosive gas.
To prevent the wearing of the muffle tube through
oxidation, the preform is inserted in or removed from the
muffle tube at a muffle tube temperature of 400C or lower.
During insertion and removal of the preform, the conventional
muffle tube absorbs a considerable amount of oxygen or water
since the highly pure carbon muffle tube is porous. Then, it
takes a long time to replace the interior atmosphere of the
muffle tube with the inert gas after insertion of the preform.
In some cases, it is impossible to completely replace the
interior atmosphere with the inert gas. In the present
invention, since the carbon muffle tube is coated with a gas
impermeable carbon, only a slight amount of oxygen or water is
absorbed by the muffle tube so that the time required for
replacing the interior atmosphere with the inert gas can be
shortened and the interior atmosphere can be completely
replaced with the inert gas.
....
\
When a slight amount of oxygen or water is absorbed
by the preform, since the coating is uniformly oxidized, no
powder is generated while the highly pure carbon muffle tube
having no coating generates powder through oxidation.
Accordingly, the quartz glass preform produced with the
heating furnace of the present invention provides an optical
fiber having fewer weak parts.
When the gas impermeable coatings are formed on both
surfaces of the muffle tube, the outer coating keeps gas
impermeability after the inner coating is worn out through
oxidation. Since oxygen or water remaining in the interior
space of the muffle tube reacts with the carbon of the muffle
tube body when oxygen or water passes therethrough, the outer
carbon coating is not worn or is only slightly worn.
Therefore, the muffle tube of the present invention has a very
long life and is stable and can be used under various heating
conditions.
In the heating furnace of Fig. 3, the inner tube 8
protects the carbon coating on the inner surface of the muffle
tube even when a slight amount of oxygen or water is liberated
from the gas absorbed by the porous soot preform. That is,
since the oxygen or water in the interior space of the muffle
tube reacts with the carbon of the inner tube 8, oxygen or
water does not reach the inner surface of the muffle tube.
In the heating furnace of Fig. 4, an inert gas is
introduced in the space between the outer and inner walls and
the pressure in the space becomes positive. Since the inner
walls 37 and 38 are made of carbon and a sintered carbon
material produced by an isotropic molding has a gas
permeability of about 101 cm2/sec, the introduced inert gas
passes through the pores of the carbon and flows into the
interior space of the muffle tube. Since the atmosphere near
the inner surface of the muffle tube is always rich in the
inert gas, the inner surface of the muffle tube is not or is
only slightly oxidized with the air which flows in during
insertion and removal of the preform. As a result, the quartz
glass preform which is produced with the heating furnace of
.¢
i~ h
the present invention provides an optical fiber having fewer
weak parts.
When the preform is inserted in the heating furnace
of Fig. 5, the partition 16 is closed and then the preform l
is temporarily maintained in the front chamber 11. After
replacing the atmosphere in the front chamber with the inert
gas, the partition 16 is opened and the preform is lowered in
the muffle tube 3, whereby the in-flow of air into the muffle
tube is prevented. Therefore, it is not necessary to lower
the muffle tube temperature to 400C when the preform is
inserted in or removed from the muffle tube.
The present invention will be illustrated by the
following Examples.
Example 1
With the heating furnace of Fig. 2, a porous soot
preform which had been produced by the VAD method was
dehydrated, added with fluorine and sintered.
The muffle tube consisted of a body made of highly
pure carbon, all the surfaces of which were coated by
pyrolytic carbon in a thickness of 30 ~m. The treating
conditions were as follows:
Treatment Atmosphere Heater surface ~ Traversing
in furnace tem~erature (C) rate (mm/min.)
Dehydration ~e 98 %, llO0 6
and removal Cl2 2 %
or impurities
F-addition~e 97 %, 1300 6
SiF4 ~ %
Sintering t 1640 6
By using the same muffle tube, 20 transparent glass
preforms were produced.
Each transparent glass preform was bored to form a
tubular cladding part. In the bore, a pure SiO2 glass core rod
was inserted and heated to collapse the cladding part onto the
core rod. Around the collapsed cladding part, glass soot was
..~ .
,~ ''
deposited and sintered to form an outer layer. Then, the
preform was drawn to fabricate a single mode optical fiber,
which had good transmission loss of less than 0.19 dB/km at a
wavelength of 1.55 ~m.
The transparent glass having no outer layer was
drawn to an outer diameter of 125 ~m which is the same outer
diameter as a usual optical fiber and subjected to a tensile
test. More than 90% of the drawn fibers had tensile strength
at break of more than 5.5 kg.
After the production of 20 transparent glass
preforms, the muffle tube was detached and inspected. A part
of the carbon coating on the inner surface at a center portion
of the muffle tube was worn out and the surface of the muffle
tube was partly exposed. However, no carbon powder was
generated. The carbon coating on the outer surface of the
muffle tube was intact.
Exam~le 2
In the same manner as in Example 1 except that the
heating furnace of Fig. 4 was used, 20 transparent glass
preform were produced. The outer walls 34 and 35 and the
inner walls 37 and 38 were made of highly pure carbon, and all
the surfaces of the outer walls were coated with pyrolytic
carbon in a thickness of 30 ~m. Helium gas was introduced
through the inlet 9 at a flow rate of 5 liter/min.
As in Example 1, each of the transparent glass
preforms was drawn to fabricate an optical fiber, which had
good transmission loss of less than 0.19 dB/km at a wavelength
of 1.55 ~m. In the tensile test, more than 90% of the fibers
had tensile strength at break of more than 5.5 kg.
After the production of 20 transparent glass
preforms, the muffle tube was detached and inspected. None of
the outer and inner surfaces of the outer walls was worn. The
appearance of the highly pure carbon on the inner walls was
not changed. No carbon powder was generated.
Comparative Example
The heating furnace of Fig. 1 was used. The muffle
tube was made of highly pure carbon and the outer surface of
the tube was coated with SiC.
The same experiment as in Example 1 was repeated,
and the data for the first ten preforms and those for the
latter ten preforms were separately analyzed.
The transmission loss was less than 0.19 dB/km at a
wavelength of 1.55 ~m for all optical fibers fabricated from
the first ten preforms, while it was more than 0.19 dB/km for
two of the optical fibers fabricated from the latter ten
preforms.
In the tensile test, more than 90% of the optical
fibers fabricated from the first ten preform had tensile
strength at break of more than 5.5 kg, while 70% of the
optical fibers fabricated from the latter ten preform had
tensile strength of more than 5.5 kg.
After the production of the preform, the muffle tube
was detached and inspected. The SiC coating was discolored in
the center portion of the outer surface of the muffle tube.
The discolored part of the SiC coating was analyzed to find
that the SiC was changed to graphite. The highly pure carbon
in the center portion of the inner surface was corroded and
carbon powder was generated on the surface.
Example 3
In the same manner as in Example 1 except that the
heating furnace of Fig. 5 was used, 20 transparent glass
preform were produced.
When the preform was inserted, the front chamber was
purged for 20 minutes with nitrogen gas at a flow rate of 20
liter/min. while keeping the muffle tube temperature at 800C.
In Examples 1 and 2, the muffle tube was kept at 400C when
the preform was inserted in the muffle tube.
The optical fibers fabricated from the produced
preforms were examined in the same manner as in Example 1.
The results were substantially the same as in Example 1.
The center portion of the muffle tube was worn but
no carbon body was exposed.
i. . ., ~, ,
