Glass manufacturing – Fiber making apparatus – With measuring – controlling – sensing – timing – inspecting,...
Reexamination Certificate
2000-04-07
2003-04-08
Hoffman, John (Department: 1731)
Glass manufacturing
Fiber making apparatus
With measuring, controlling, sensing, timing, inspecting,...
C065S530000, C037S003000
Reexamination Certificate
active
06543257
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a porous optical fiber preform dehydration and vitrification apparatus for dehydrating and vitrifying a porous optical fiber preform.
More specifically, the present invention relates to a porous optical fiber preform dehydration and vitrification apparatus for improving the air-tightness of an upper portion of a furnace tube for introducing and elevating and lowering the porous optical fiber preform.
Still more specifically, the present invention relates to a porous optical fiber preform dehydration and vitrification apparatus having a structure capable of shortening a dehydration and vitrification time and having a controlling means. Further, the present invention relates to a method of heat treatment of a porous optical fiber preform for adequate control of temperature by using such a dehydration and vitrification apparatus.
Still more specifically, the present invention relates to a porous optical fiber preform dehydration and vitrification apparatus free from damage to the furnace tube and capable of being extended in service life even if used for a large-sized porous optical fiber preform.
Still more specifically, the present invention relates to a porous optical fiber preform dehydration and vitrification apparatus combining all of the above.
2. Description of the Related Art
A single mode optical fiber having for example a diameter of a core of 10 &mgr;m and a diameter of a cladding formed on an outer circumference of the core of 125 &mgr;m is produced by drawing an optical fiber preform in a heated state.
Such an optical fiber preform is produced by dehydrating and vitrifying a porous optical fiber preform produced by a vapor axial deposition (VAD) method, an outer vapor deposition (OVD) method, or the like.
When dehydrating and vitrifying such a porous optical fiber preform produced by the VAD process, the OVD process. etc., use is made of for example a porous optical fiber preform dehydration and vitrification apparatus illustrated in FIG.
1
.
A furnace tube
2
is provided in the dehydration and vitrification apparatus, and a heating furnace
8
is provided around the periphery of the furnace tube
2
. By gradually lowering the porous optical fiber preform
1
downward in the furnace tube
2
from above and passing it through a high temperature portion of the heating furnace
8
having a heater
7
accommodated in a furnace body
6
disposed at the outer circumference of the furnace tube
2
, the porous optical fiber preform
1
is first dehydrated. At the time of this dehydration, helium (He) gas, a chlorine-containing gas, or the like is passed inside the furnace tube
2
. Also, the temperature in the furnace tube
2
is made for example about 1100 to 1300° C.
After this dehydration ends, the porous optical fiber preform
1
is pulled up once from the interior of the upper portion of the furnace tube
2
of the dehydration and vitrification apparatus, the temperature in the furnace tube
2
is raised to for example 1500 to 1600° C., and the dehydrated porous optical fiber preform
1
is lowered into the furnace tube
2
again from above the furnace tube
2
to heat the porous optical fiber preform
1
by the heater
7
and vitrify it to form an optical fiber preform. At the time of this vitrification, He gas may be passed through the furnace tube
2
or a gas the same as that at the dehydration may be passed.
After the dehydration, the porous optical fiber preform
1
is sometimes doped by adding a dopant for forming a refractive index distribution. This doping is carried out by feeding an atmospheric gas (mainly He gas) and a doping gas (gas containing either of fluorine, boron, or chlorine) into the furnace tube
2
and controlling the temperature in the furnace tube
2
to for example about 1000 to 1300° C. (this temperature differs according to the doping gas).
Further, the doping in the dehydration and vitrification apparatus may be carried out simultaneously with the vitrification. In this case, the doping is carried out by incorporating the doping gas into the atmospheric gas in an ordinary vitrification furnace tube
2
.
As disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 6-59438, there has also been proposed a method of dehydration and vitrification by arranging a plurality of heaters (heat sources) in a longitudinal direction of the furnace tube at the outer circumference of the furnace tube in place of movement of the porous optical fiber preform
1
at the time of such heat treatment, sequentially switching the plurality of heaters along the longitudinal direction of the porous optical fiber preform, and therefore sequentially heating the heaters to predetermined temperatures without moving the porous optical fiber preform.
Further, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-110456, there also has been known a method of dehydrating and vitrifying a porous optical fiber preform formed by the VAD process or the OVD process, drawing this, depositing fine glass particles synthesized in a burner flame on the surface using this as a target to prepare a porous optical fiber preform (such a porous optical fiber preform being referred to as a “porous optical fiber preform with target” in the present specification), where when no dopant changing the refractive index distribution in advance is contained in the porous layer, the dehydration and vitrification are performed by heating the porous optical fiber preform with target all at once by a plurality of heat sources.
According to the two above heat treatment methods using the plurality of heaters, since it is not necessary to lower the porous optical fiber preform, there is the advantage that the treatment time can be shortened compared with the method of heat treatment by a dehydration and vitrification apparatus using one heater.
However, the method of heat treatment using a plurality of heaters suffers from the problem that it is hard to apply this method to dehydration and vitrification of a porous optical fiber preform containing a dopant forming a refractive index distribution in advance (this will be referred to as a “porous optical fiber preform without target” in the present specification). The reason for this will be explained. In dehydration, the dopant contained in the porous optical fiber preform in advance reacts, so it is difficult to make the concentration of chlorine gas uniform in the longitudinal direction of the porous optical fiber preform and, as a result, it is difficult to make the refractive index distribution of the porous optical fiber preform uniform in the longitudinal direction of the porous optical fiber preform. Further, it is also difficult to make the temperature in the furnace uniform in the longitudinal direction. Therefore, even if the concentration of chlorine gas could be made uniform in the longitudinal direction, it would be difficult to make the refractive index distribution of the porous optical fiber preform uniform in the longitudinal direction of the porous optical fiber preform.
Even in the case of the “porous optical fiber preform with target” in which a dopant changing the refractive index is not contained in the porous layer at the surface, there was a problem that the dehydration temperature could only be raised to a temperature of the same extent as that for the “porous optical fiber preform without target”, for example about 1300° C. at most. This is because due to the treatment at a constant temperature for a long time, for example about 2 to 6 hours, if the entire porous optical fiber preform is raised to a high temperature, firing proceeds from the surface of the porous optical fiber preform and dehydration becomes difficult.
Further, where doping the porous optical fiber preform, there was a problem that the distribution of the dopant of the porous optical fiber preform after the heat treatment did not become uniform in the longitudinal direction. As the cause of this, inadequate temperature hysteresis of the porous opti
Koaizawa Hisashi
Orita Nobuaki
Sakamoto Yoshiyuki
Sugiyama Satoshi
Takeda Junichi
Hoffman John
The Furukawa Electric Co. Ltd.
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