Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2000-08-29
2003-09-16
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S413000, C065S421000, C065S379000, C065S027000
Reexamination Certificate
active
06619074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for production of a porous optical fiber preform, more particularly relates to an apparatus and method for production of a porous optical fiber preform for preventing cracking and maintaining a uniform quality of the porous optical fiber preform when producing a porous optical fiber preform by the VAD method.
2. Description of the Related Art
Optical fibers are being used in various fields such as optical communications. Optical fibers are, briefly, produced by the following method. First, a porous optical fiber preform is formed. Next, the porous optical fiber preform is heated and vitrified. Then, the vitrified optical fiber preform is heated and drawn to form a single mode optical fiber comprised of for example a core of a diameter of 10 &mgr;m and a cladding of a diameter of 125 &mgr;m formed at the circumference of the core. The thus formed optical fiber is then covered by a resin.
The VAD method is mainly used for producing the porous optical fiber preform used for producing the optical fiber in this way. A porous optical fiber preform is produced by the VAD method as shown, for example, in
FIG. 1
illustrating the interior of a reaction vessel
11
, by charging silicon tetrachloride (SiCl
4
) gas into oxyhydrogen flames
3
emitted using a core forming multitube burner
1
and a cladding forming multitube burner
2
arranged in a reaction portion
11
C of the reaction vessel
11
so as to generate fine particles of glass, that is, silicon dioxide (SiO
2
), by a flame hydrolysis reaction, making the fine particles of glass deposit on a rotating seed rod
4
, and thereby form a fine glass particle deposit
5
around the seed rod
4
. The rotating seed rod
4
is pulled up so that the fine glass particle deposit
5
is formed in its longitudinal direction and thereby obtain a porous optical fiber preform. At this time, if charging a small amount of fine particles of germanium chloride (GeCl
2
) etc. into the core forming multitube burner
1
along with the silicon tetrachloride as a dopant for raising the refractive index of the core portion from the cladding portion, fine particles of germanium oxide (GeO
2
) etc. are simultaneously produced and a spread of germanium dioxide can be created in the radial direction of the fine glass particle deposit
5
. Note that reference numeral
10
shows a feed port,
13
a main exhaust port, and
12
a secondary exhaust port.
In the above method, the fine particles of glass are deposited on the seed rod
4
in a horizontal stream of gas
6
from the burner
1
and
2
side to the secondary exhaust port
12
side while rotating and pulling up the seed rod
4
. The not deposited excess fine particles of glass are carried by the horizontal stream of gas
6
introduced from the feed port
10
through the reaction portion
11
C to the main exhaust port
13
where they are exhausted to the outside of the reaction vessel
11
. By exhausting the excess fine particles of glass not deposited on the seed rod
4
in this way, it is possible to prevent the excess fine particles of glass not deposited on the seed rod
4
from depositing on the reaction vessel
11
and then peeling off from the reaction vessel
11
and depositing on the surface of the fine glass particle deposit
5
and thereby causing bubbles in the fine glass particle deposit
5
after vitrification.
As the horizontal stream of gas
6
, air or an inert gas such as argon gas is used. The horizontal stream of gas
6
is usually introduced in an ordinary temperature state.
When forming a fine glass particle deposit by the VAD method, however, the soot growth rate, that is, the distance by which the seed rod (deposit of fine particles of glass) is pulled up per unit time, must be made constant in order to prevent fluctuations in the cutoff wavelength, to obtain a uniform shape of the fine glass particle deposit, etc. That is, if the flames can be kept steady without flickering, the rate of growth of the fine glass particle deposit becomes stable and the distribution of the dopant for raising the refractive index of the core portion, for example, the germanium, in the longitudinal direction of the fine glass particle deposit becomes constant so the final optical fiber ends up with less variations in characteristics.
In practice, however, disturbances in the flow of gas in the reaction vessel make the flames flicker and therefore it is difficult to make the growth rate in the longitudinal direction of the fine glass particle deposit constant. Therefore, various measures have been devised to prevent flickering of the flames.
As one example, to prevent the flow of horizontal stream of gas
6
from being disturbed at the portion where the fine glass particle deposit
5
is formed on the seed rod
4
, the method has been proposed of stabilizing the flow of the horizontal stream of gas
6
by making the horizontal stream of gas
6
forcibly flow by a blower (see Japanese Unexamined Patent Publication (Kokai) No. 1-242431).
As another example, the method has been proposed of providing a baffle plate surrounding the main exhaust port
13
so that the horizontal stream of gas is exhausted smoothly from the main exhaust port
13
(see Japanese Unexamined Patent Publication (Kokai) No. 2-2836321).
These methods however have had the following problems.
The first problem is that the horizontal stream of gas
6
strikes the fine glass particle deposit while it is being formed. The horizontal stream of gas
6
introduced at an ordinary temperature causes the surface temperature of the fine glass particle deposit
5
to fall, so the density of the fine glass particle deposit
5
falls and cracks form In the fine glass particle deposit
5
.
The second problem is that the horizontal stream of gas
6
strikes the flames of the burners
1
and
2
. The flames of the burners
1
and
2
are made to flicker and therefore the quality of the fine glass particle deposit
5
being formed, in other words the quality of the porous optical fiber preform, fluctuates in the longitudinal direction.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus for the production of a porous optical fiber preform which prevents the horizontal stream of gas from the feed port to the main exhaust port from disturbing the shapes of the flames carrying fine particles of glass and prevents it from contacting the portion where the fine glass particle deposit is being formed.
Another object of the present invention is to provide an apparatus for the production of a porous optical fiber preform which prevents the surface temperature of the fine glass particle deposit from falling and thereby prevents cracks in the fine glass particle deposit.
Still another object of the present invention is to provide an apparatus for the production of a porous optical fiber preform which prevents the flames carrying the fine particles of glass from the burners from flickering and thereby prevents fluctuations in the quality of the fine glass particle deposit (porous optical fiber preform) in the longitudinal direction.
Still another object of the present invention is to provide an apparatus for the production of a porous optical fiber preform which enables fine particles of glass emitted from the burners to efficiently deposit on the seed rod and thereby efficiently form a fine glass particle deposit (porous optical fiber preform).
Still another object of the present invention is to provide a method for production of the above porous optical fiber preform.
According to a first aspect of the present invention, there is provided an apparatus for the production of a porous optical fiber preform having a core portion and a cladding portion formed at the circumference of the core portion, the apparatus for the production of a porous optical fiber preform provided with a reaction vessel having a reaction portion, a feed port for introducing a stream of gas into the reaction portion, and a main exhaust port, facin
Kanao Akihiro
Kohmura Yukio
Mikami Toshihiro
Wada Hiroyuki
Colaianni Michael
The Furukawa Electric Co. Ltd.
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