U.S. patent application number 09/423237 was filed with the patent office on 2001-12-06 for method of production of porous glass base material for optical fiber.
Invention is credited to FUKUSHIMA, NAOYUKI, HIRANO, NOBUYUKI, KABAYA, TAKAO, MATSUI, MASAHIKO, SUGIYAMA, TAKASHI.
Application Number | 20010047666 09/423237 |
Document ID | / |
Family ID | 12716732 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010047666 |
Kind Code |
A1 |
SUGIYAMA, TAKASHI ; et
al. |
December 6, 2001 |
METHOD OF PRODUCTION OF POROUS GLASS BASE MATERIAL FOR OPTICAL
FIBER
Abstract
In a method of manufacturing a porous glass preform for an
optical fiber which preform is formed as a deposit of fine glass
particles by using a burner, a method to prevent the contamination
of any glass particles having failed to be properly deposited so
that the generation of voids may be minimized in a transparent
glass preform which is produced by heating the porous glass
preform. An inert gas is caused to flow through the burner at a
rate of at least 25 m/s before fine glass particles are formed by
the hydrolysis and/or oxidation of a glass material in a flame
produced by the burner supplied with a mixture of a gas of the
glass material and a gas for combustion and are deposited on a
rotating starting member. It is desirable to elevate the pressure
of the inert gas above the atmospheric pressure by a device
connected to the burner by a pipeline, and cause it to flow rapidly
through the burner.
Inventors: |
SUGIYAMA, TAKASHI;
(YOKOHAMA-SHI, JP) ; MATSUI, MASAHIKO;
(YOKOHAMA-SHI, JP) ; HIRANO, NOBUYUKI;
(YOKOHAMA-SHI, JP) ; FUKUSHIMA, NAOYUKI;
(YOKOHAMA-SHI, JP) ; KABAYA, TAKAO; (YOKOHAMA-SHI,
JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Family ID: |
12716732 |
Appl. No.: |
09/423237 |
Filed: |
November 5, 1999 |
PCT Filed: |
December 10, 1998 |
PCT NO: |
PCT/JP98/05600 |
Current U.S.
Class: |
65/27 ; 65/413;
65/414; 65/421 |
Current CPC
Class: |
C03B 37/0144 20130101;
C03B 37/0142 20130101; C03B 2207/20 20130101; C03B 2207/06
20130101; C03B 2207/50 20130101; C03B 2207/22 20130101; C03B
2207/46 20130101 |
Class at
Publication: |
65/27 ; 65/413;
65/414; 65/421 |
International
Class: |
C03B 037/018 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 1998 |
JP |
10-45346 |
Claims
1. A method of manufacturing a porous glass preform for an optical
fiber by depositing on the periphery of a rotating starting member,
fine glass particles formed by the hydrolysis and/or oxidation of a
glass material in a flame produced by at least one burner supplied
with a mixed gas containing a gas of the glass material and a gas
for combustion, wherein an inert gas is caused to flow through said
burner at a rate of at least 25 m/s before said depositing is
started.
2. A method as defined in claim 1, wherein said inert gas has a
pressure elevated above the atmospheric pressure.
3. A method as defined in claim 2, wherein said pressure is
elevated by a pressurizer connected to said burner.
4. A method as defined in claim 1, wherein said rate is from 25 to
50 m/s.
5. A method as defined in claim 1, wherein said burner is mounted
in a muffle having an exhaust pipe, and the pressure of said
exhaust pipe is reduced by at least about 0.1 kPa.
Description
TECHNICAL FIELD
[0001] This invention relates to a method of manufacturing a porous
glass preform for an optical fiber, and more particularly to an
improvement in the method of manufacturing a porous glass preform
by depositing fine glass particles formed by hydrolysis in a flame
produced by a burner.
BACKGROUND ART
[0002] One of the known methods of manufacturing a porous glass
preform for an optical fiber is the vapor phase axial deposition
(VAD) method. In this method, as shown in FIG. 2, a mixture of a
gas for combustion and a gas of a glass material is jetted out
through a burner used for forming glass particles, hereafter simply
`burner` 2 (or 2') to produce a flame 3 (or 3') in which the glass
material is hydrolyzed or oxidized to form fine glass particles,
and while the glass particles are deposited on the lower end of a
rotating starting member 6, to form a deposited porous body, the
starting member 6 is moved relative to the burner 2 (or 2') with
the growth of the porous body, whereby a porous glass preform 1 is
obtained. Although the method shown in FIG. 2 employs two burners,
a method using only one burner, or more than two is alternatively
possible. The porous glass preform 1 is heated in an electric
furnace to form a transparent glass preform and it is drawn into an
optical fiber.
[0003] In the conventional VAD method the burner 2 (or 2') is
provided with, for example, SiCl.sub.4 as the glass material, and a
fuel gas such as a hydrogen or hydrocarbon gas, and a gas assisting
combustion, such as oxygen or air, as the gases for combustion.
Fine glass particles (SiO.sub.2) are formed by reaction of the
following formula (I):
SiCl.sub.4+2H.sub.2O.fwdarw.SiO.sub.2+4HCl (I)
[0004] Not all of the glass particles that are formed, however, are
deposited as the preform 1. Some of the glass particles float in
the muffle 4 and attach to its inner wall to form a layer of glass
particles thereon. If this layer grows to some extent in thickness,
glass particles are likely to fall off the muffle wall, attach to
the surface of the porous glass preform 1 and form a gap therein.
In this case, the voids may be formed when the preform is heated
into transparent glass.
[0005] As a method for solving this problem, the following method
has been proposed in Unexamined Published Japanese Patent
Applications Nos. 162642/1987 and 123831/1988. A heater and an
outlet to a muffle for gas are added, and the gas heated to high
temperature is forced to flow around the burner and the porous
glass preform along the inner wall of the muffle; thereby the gas
is prevented from remaining near the inner wall of the muffle;
glass particles are prevented from attaching to the muffle; the gas
is caused to flow in the muffle so as to retain the glass particles
from floating in the muffle. The burner for synthesizing glass
particles also has a problem with respect to the attachment and
mixture of glass particles. Namely, when the mixture of the
combustion gas and the glass material is jetted from the tip of the
burner for synthesizing glass particles, a part of the gas mixture
is likely to scatter around the burner and attach to the vicinity
of its outlet as glass particles. The glass particles are also
likely to even enter the burner as a result of their entrainment by
the gas surrounding it. Moreover, even if the floating of glass
particles may be restrained during the synthesizing of glass
particles, by the method proposed in the above mentioned patent
applications, it is still likely that after the manufacturing of a
preform is stopped, the glass particles may enter the burner during
the cooling of the preform.
[0006] If the glass particles which have attached to the burner, or
entered it as described above are left as they are, they are likely
to leave the burner and attach to the surface of a preform during
the subsequent preform manufacturing. In this case also, the
particles attach in a manner different from new particles produced
in a flame and deposited on the preform, and are likely to form
voids when it is heated into transparent glass. Moreover, the
attached glass particles spoil the burner if they form transparent
glass in the burner under the heat of the gas for combustion.
Therefore, it is necessary to clean the burner after manufacturing
of each preform by removing, by suction or other means, the glass
particles which have attached or entered therein.
DISCLOSURE OF THE INVENTION
[0007] Although the methods proposed in the above mentioned patent
applications have been somewhat effective for preventing the glass
particles from attaching to the inner wall of the muffle and for
restraining the flotation of glass particles, the formation of
voids in a preform by the glass particles attaching to it is still
an outstanding problem.
[0008] The inventors have further studied this problem.
[0009] The glass particles are likely to remain not only in the
muffle, but also in the burner even after it is cleaned as
mentioned above, and attach to a preform after starting of the
synthesizing and become the cause of the voids. Namely, the
attached glass particles may fall off the muffle wall or the outlet
end of the burner when manufacturing of a preform is stopped, or
may not be sucked from the burner completely even by very careful
cleaning, and may enter it. Some of the falling glass particles
sometimes enter deep into the burner, for example, near its joint
to a pipeline, and hence their removal by suction is very
difficult. The creation of a sufficiently large pressure difference
for removing any foreign matter from such a deep region in the
burner by suction is very likely to result in the destruction of
its glass wall having a thickness of, say, only 1 mm near its
outlet end. The use of a new burner for manufacturing each preform
is a very costly solution to such a problem.
[0010] Under these circumstances, it is a subject of this invention
to provide a method which can prevent fine glass particles from
attaching to a burner, or entering it, and thereby avoid the
formation of voids in a transparent glass preform.
[0011] This subject is essentially attained by starting the
deposition of fine particles of glass after causing an inert gas to
flow at a rate of at least 25 m/s through a burner for producing
those particles.
[0012] The inert gas preferably has a pressure elevated above the
atmospheric pressure. The pressure of the inert gas is preferably
elevated by a pressurizer connected to the burner. The inert gas is
preferably caused to flow at a rate of 25 to 50 m/s. When the inert
gas is made to flow in the burner for manufacturing glass particles
the pressure is preferably reduced by at least about 0.1 kPa in an
exhaust pipe extending from a muffle in which the burner is
mounted.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic sectional view illustrating the method
of this invention; and
[0014] FIG. 2 is a schematic sectional view illustrating the known
method.
[0015] In the drawings, 1 is a porous glass preform, 2 and 2' are
each a burner for producing fine particles of glass, 3 and 3' are
each a flame, 4 is a muffle, 5 is an exhaust pipe, 6 is a starting
material, 7 is a pressurizer, 8 is a pipeline, and 9 is a
valve.
BEST MODE FOR CARRYING OUT THE INVENTION
[0016] According to this invention, an inert gas is introduced into
a burner for producing fine glass particles so as to flow at a rate
of at least 25 m/s therethrough before the deposition of fine
particles of glass starts to form a porous glass preform. Glass
particles which are attached to the burner or have entered it and
any other foreign matters can easily be removed from the burner,
and discharged from the muffle through an exhaust pipe by the inert
gas flowing out of the burner. Even such particles or foreign
matter entering the burner during or after its cleaning can be
removed easily if the inert gas is introduced immediately before
the formation of a preform is started.
[0017] In this invention, the flow rate of the inert gas in the
burner for producing fine glass particles is 25 m/s or more, and
particularly preferably from 25 to 50 m/s. At a rate below 25 m/s,
the gas may fail to remove the entered glass particles thoroughly
from the burner, while at a rate above 50 m/s, it is likely to
exert so heavy a load on the joint between the burner 2 and a
pipeline 8 (FIG. 1) or the burner 2 itself to cause it to be
broken. As a method for flowing the inert gas at a rate of at least
25 m/s, an inert gas having an elevated pressure is flowed, and its
flow rate may be adjusted depending on its pressure.
[0018] The inert gas may be of any kind if it does not have any
adverse effect on the burner, and if it does not contain any
foreign matter, such as a metal powder, or dust, that may remain in
a porous glass preform and eventually in an optical fiber and
increase its transmission loss. Examples of the inert gas are
nitrogen, argon and helium.
[0019] The invention will now be described more specifically with
reference to FIG. 1. In this invention, the method for synthesizing
the porous glass preform itself is the same as the conventional
method. A gas of a glass material and if required, a mixed gas
containing a dopant gas, a fuel gas, a gas for assisting combustion
and an inert gas are supplied to burners 2 and 2' for synthesizing
glass particles in a muffle 4. The glass material is hydrolyzed
and/or oxidized in flames 3 and 3' produced by the burners 2 and 2'
to form fine glass particles, and those particles are deposited on
a starting member 6, to form a porous glass preform 1. The gas in
the muffle 4 is exhausted through an exhaust pipe 5.
[0020] According to this invention, before starting the
synthesizing of the porous glass preform 1, an inert gas of which
pressure is elevated by a pressurizer so as to flow at a rate of at
least 25 m/s in the burner, is introduced at a stretch into the
burner through a pipe line and a valve, and thereby the inner part
of the burner is cleaned. Although the pressurizer 7, pipeline 8
and valve 9 are shown only for the burner 2, the burner 2' is also
provided with a pressurizer, a pipeline and a valve, and is
likewise supplied with an inert gas to clean it.
[0021] Here, "an inert gas is introduced at a stretch" mentioned
above means a method in which the valve 9 of the pipeline 8, the
pressure of which is elevated, is opened in an instant and is kept
open for about five seconds to allow the inert gas to flow and then
the valve 9 is closed. More preferably, after the valve 9 is opened
once, the valve 9 is shut and the pressure is elevated in the
pipeline, and then the valve 9 is opened in an instant again and
kept open for about five seconds to allow the inert gas to flow;
thus these processes are repeated several times. The number of
times for which it is repeated depends on the degree of
contamination of the inner part of the burner.
[0022] In this case, the pressure of the exhaust pipe 5 is
preferably reduced by about 0.1 kPa or more. This is because the
glass particles which have flowed out of the burners 2, 2' for
synthesizing the glass particles are prevented from floating in the
muffle, and according to the inert gas flow, the foreign matters
are exhausted to the exhaust pipe from the neighborhood of the
inner wall of the muffle, and thereby the burners 2, 2' and the
inner wall of the muffle 4 are cleaned more effectively.
[0023] Although FIG. 1 shows the mode in which two burners 2 and 2'
are used for forming a porous glass preform 1, the method of this
invention can also be carried out by using only one burner, or more
than two burners.
[0024] Although the foregoing description has been based on the VAD
method, this invention is equally applicable to any other method of
forming a porous glass preform from glass particles produced by a
burner, such as the outside vapor phasedeposition (OVD) method, to
provide equally satisfactory results in the cleaning of the
burner.
EXAMPLE
[0025] As an example, the porous glass preform was manufactured
using equipment having the construction shown in FIG. 1 according
to this invention. Each of the burners 2, 2' had a diameter of 50
mm and a length of 500 mm. Before starting the synthesizing, the
pressure of nitrogen gas as an inert gas was elevated to 6
kg/cm.sup.2 (or 588,399 Pa) by the pressurizer 7 and then, the
valve 9 was opened in an instant and was kept open for about five
seconds, and was shut. Then, the operations for elevating the
pressure for about 20 seconds and opening the valve 9 for about 5
seconds at a stretch were repeated three times. When the valves 9
were opened, the inert gas flowed rapidly into the burners 2, 2'
through the pipelines 8. It flowed at a rate of 25 m/s through the
burners 2, 2'. The pressure of the exhaust pipe 5 was reduced by
0.1 kPa.
[0026] The burner 2 was supplied with SiCl.sub.4 at a rate of 0.2
liter per minute, GeCl.sub.4 at a rate of 0.1 liter per minute,
hydrogen at a rate of 20 liters per minute, oxygen at a rate of 30
liters per minute and argon at a rate of 10 liters per minute,
while the burner 2' was supplied with SiCl.sub.4 at a rate of three
liters per minute, hydrogen at a rate of 70 liters per minute,
oxygen at a rate of 70 liters per minute and argon at a rate of 20
liters per minute.
[0027] Thus, there were produced 10 porous glass preforms 1 each
having a diameter of 150 mm and a length of 800 mm. Each preform
was heated in an electric furnace to form a transparent glass
preform. The number of voids found in these transparent glass
preforms was on the average only 0.2 per piece of them.
[0028] For comparative purposes, 10 porous glass preforms each
having the same size as stated in the above example were
manufactured using equipment having the construction shown in FIG.
2, in the same manner with the example except that no inert gas
having an elevated pressure was flowed through the burners 2, 2'
for synthesizing glass particles before starting the formation of
each preform. The porous glass preform thus obtained were heated
under the same condition as in the above example so as to become
transparent glass preforms, in which as many as 3.5 voids per piece
on the average were found.
[0029] From the result of the above-mentioned example and
comparative example, it is confirmed that this invention can form a
transparent glass preform having a very small number of voids as
compared with the product of the conventional method.
[0030] As is obvious from the foregoing, this invention makes it
possible to prevent the contamination of any porous glass preform
by glass particles attaching to, or remaining in the burners and
thereby reduce greatly the voids that may eventually be formed in
any transparent glass preform for an optical fiber.
[0031] This invention requires only a simple apparatus and a simple
operation, and can be carried out even immediately before the
formation of a porous glass preform is started. The cleaning of the
burners prolongs their life and enables them to be used repeatedly
for making many preforms. Therefore, this invention can very
effectively reduce the cost of manufacturing porous glass preforms
for optical fibers.
* * * * *