Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS AND ME TROD FOR DRAWING WAVEGUIDE FIBERS
FIEhD OF THE INVENTION
The present invention relates to a method and
apparatus for drawing waveguide fibers. More
particularly, the present invention relates to a furnace
that significantly reduces point defect losses in fibers
generated during the draw process.
BACKGROUND OF THE INVENTION
Relatively high temperature heat sources are required
for drawing high strength, low loss fibers from a high
silica-content fiber preform or blank. The two
predominant heat sources that have been utilized for
drawing such fibers are zirconia and graphite furnaces.
Fiber draw furnaces generally operate at temperatures
greater than about 1900°C, typically as high as about
2050°C.
A zirconia induction furnace conventionally includes
a housing in which there is a centrally disposed tubular,
yttria-stabilized zirconia susceptor surrounded by a
cylindrical quartz beaker containing granular zirconia
insulating material. An induction coil surrounding the
insulating material provides an alternating
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electromagnetic field when energized. The field couples
to the susceptor and raises the temperature of the
susceptor to form a hot zone. An end portion of glass
optical fiber preform is lowered into the hot zone to melt
the end portion and a fiber is drawn from this melted end
portion.
One disadvantage associated with zirconia induction
furnaces is that extended use and thermomechanical
stresses cause cracks in the muffle and susceptor. This
cracking causes zirconia particles to migrate from the
inner surface of the furnace onto the preform and/or fiber
being drawn from the preform resulting in substantially
weakened fiber and unacceptable product losses.
Graphite induction furnaces typically have a graphite
muffle that is less susceptible to cracking, but graphite
furnaces suffer from the disadvantage that the graphite
muffle oxidizes at high drawing temperatures. It has been
suggested that drawing a waveguide fiber in a graphite
furnace must be performed in an inert protective
atmosphere to prevent oxidation of the furnace muffle.
Oxidation occurs when gasses from ambient atmosphere react
with the solid carbon muffle at high temperatures
according to the following reactions:
( 1 ) C + OZ -~ COZ
(2) C + COZ -~ 2C0.
A typical onset temperature for reaction (1) for a
graphite grade used in a draw furnace is about 700°C.
Reaction (2) becomes significant above 900°C. These
reactions of the furnace muffle with oxygen and carbon
dioxide cause the furnace muffle to be consumed,
especially at elevated fiber drawing temperatures.
The graphite muffle material is a composite of
graphite grains bonded together by a carbon binder matrix.
It is believed that the binder material is more
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susceptible to oxidation than the graphite grains.
Therefore, when the composite of the two materials is
exposed to oxygen at temperatures above the oxidation
onset temperatures, the matrix binder material
preferentially oxidizes. The graphite grains, having no
binder left to hold them place, are then free to fall away
from the composite structure. It is believed that this
mechanism causes graphite particulate to migrate from the
muffle wall to the fiber preform and/or fiber during
drawing.
Graphite particulate that becomes incorporated into
the fiber during drawing causes unacceptable product
losses due to point defects. Point defects manifest
themselves as sharp attenuation increases in the signal
transmitted through the fiber. Point defect product
losses due to graphite particulate from a draw furnace
losses can be greater than about S~, which is an
unacceptably high loss. Graphite particulate that has
adhered to the fiber during the draw~process also
contributes to fiber breaks.
As mentioned above, it has been suggested that
oxidation of the graphite furnace muffle may be overcome
by drawing in an inert, protective gas atmosphere. The
outer surface of a graphite muffle may be insulated by
enclosing the muffle in a housing and flowing inert gas
between the housing and the outer wall of the muffle.
However, it is difficult to eliminate all oxygen from the
furnace muffle, especially the inner surface of the muffle
which is exposed to oxygen from ambient air that may leak
into the furnace during loading and unloading waveguide
fiber preforms. In addition, oxygen is believed to be
present in the furnace due to the difficulty in
eliminating oxidants from the furnace. For example, the
upper region of the muffle is susceptible to oxidation
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from the oxygen-containing porous soot section of an
optical fiber blank that dwells in the furnace muffle
during loading of the blank in the furnace. It is
believed that oxygen present in the porous region of the
blank oxidizes the muffle, producing graphite particulate.
In view of the above considerations, it would be
desirable to provide a graphite fiber draw furnace muffle
that does not generate graphite particulate, and thus
significantly reduces point defect losses in the fiber.
SUi~MARY OF INVENTION
Accordingly, the present invention generally provides
an apparatus for heating a glass waveguide fiber preform
to a temperature sufficient to draw a fiber therefrom
comprising a generally tubular graphite m~~rfle including
an inner surface having a coating of high purity silicon
carbide on the inner surface of the muffle. The coating
preferably has a thickness of at least about 2 mils and
contains less than about 900 parts per billion impurities.
In another aspect, the invention provides a method
for producing a waveguide fiber in a draw furnace
including a generally tubular graphite muffle having an
inner surface. The method includes the steps of providing
a high purity silicon carbon coating on the inner surface
of the graphite muffle. The method further includes
disposing a waveguide,fiber preform in the furnace muffle,
heating the furnace to a temperature sufficient to draw
fiber from the preform, and drawing fiber from the blank.
34 Several important advantages will be appreciated from
the foregoing summary. The principal advantage of the
present invention is significantly reducing point defect
losses in waveguide fibers drawn in a furnace having a
graphite muffle. Additional features and advantages of
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the invention will be set forth in the description which
follows. It is to be understood that both the foregoing
general description and the following detailed description
are exemplary and explanatory and are intended to provide
5 further explanation of the invention as claimed. Various
elements of the accompanying drawing are not intended to
be drawn to scale, but instead are sometimes purposely
distorted for the purposes of illustrating the invention.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a schematic illustration of an exemplary
embodiment of optical fiber draw furnace of the present
invention.
DETAILED DESCRIPTION
Reference will now be made in detail to the present
preferred embodiment of the invention, an example of which
is illustrated in the accompanying drawing.
The present invention includes an apparatus for
heating a waveguide fiber to a temperature sufficient to
draw a fiber therefrom. An exemplary embodiment of the
present invention is shown in Fig. 1 and is designated
generally by reference numeral 10.
As embodied herein and referring to Fig. 1, furnace
10 is comprised of a. generally cylindrical housing 12
having a side wall 14, a top portion 16, and a bottom
portion 18. Top portion 16 has a central opening 22
therein which is vertically aligned with an opening 24 in
bottom portion 18. Insulating material 26 is axially
disposed in housing 12, which may be formed from a
plurality of segments. A generally tubular, graphite
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muffle 28 is centrally located within the insulating
material 26. The muffle 28 and insulating material may be
separated from the bottom portion 18 by a spacer ring 20
having an aperture 21 through which fiber is drawn to
insulate the muffle from the bottom portion. The spacer
ring 20 may be made from silica. An induction coil 30,
which is connected to a power source (not shown),
surrounds the insulating material 26 to provide a heating
source for the furnace 10.
Housing 12, which is water cooled, may be fabricated
of stainless steel or the like. Preferably, housing 12
axially runs the full length of the muffle 26 to fully
enclose the muffle. An inert gas such as argon is flowed
into the housing 12 to prevent oxidation of the outer
surface of the muffle 26.
A waveguide fiber preform 32 (shown in phantom) is
axially inserted into muffle 26 until a first end 34
thereof is position at the "hot zone" located within the
induction coil 30. After hot zone has reached a
temperature sufficient to draw fiber from the preform,
which is preferably above 1900°C, an optical fiber 36 is
drawn from the end portion 34 of the preform 32. In an
important aspect of the invention, the inner surface of
the muffle 28 adjacent the preform 32 has a coating of
high purity silicon carbide thereon to prevent
deterioration of the graphite muffle. The graphite muffle
28 preferably comprises at least two and, more preferably,
three axial segments because it is difficult to coat
sections of the muffle longer than about 40 inches.
The-thickness of the silicon carbide coating is
preferably at least about 2 mils and less than about 100
mils. Coating thinner than about 2 mils does not
adequately prevent graphite particulate from contaminating
fiber drawn from the furnace, and coating thicker than
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about 100 mils is susceptible to microcracking and thermal
shock. The thermal expansion of the SiG coating must be
closely matched to the carbon. binder matrix material which
holds the graphite grains of the muffle together to
prevent delamination of the coating due to thermal
expansion mismatch.
The silicon carbide coating in the inner surface of
the muffle is preferably formed by a chemical vapor
deposition process using a silicon containing gas. Such a
coating may be formed by reacting a silicon containing gas
such as a silane with hydrogen to form SiC, wherein the
silicon and carbon are present in a ratio of about one to
one. The SiC is coated on the inner surface of the
substrate which has been heated above 100t" C. High purity
coatings are preferred on the inner surface of the draw
furnace muffle to prevent contamination of fibers drawn in
the furnace of the present invention. Preferably the
impurity level in the silicon carbide coating is less than
about 900 parts per billion, and more preferably less than
about 200 parts per billion.
Another aspect of the present invention is directed
to a method for producing a waveguide fiber in a draw
furnace including a graphite, generally tubular muffle
having an inner surface. The method comprises the steps
of providing a high purity silicon carbide coating on the
inner surface of the graphite muffle, disposing a
waveguide fiber preform in the muffle, heating the furnace
to a temperature sufficient to form draw fiber from the
preform, and drawing fiber from the preform.
The--furnace is preferably heated to a temperature of
at least about 1900°C, more preferably to at least about
2000°C, to enable the tip of the waveguide preform to
soften and allow fiber to be drawn therefrom. The high
purity silicon carbide is preferably about 99.999 pure,
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PCTIUS98J218'12
and more preferably contains less than about 900 parts per
billion of impurities. The low impurity level is an
important aspect of the present invention because higher
impurity levels may cause optical or mechanical defects in
the fiber produced in the furnace.
Waveguide fibers produced by utilizing the furnace
and method of the present invention exhibit significantly
reduced point defect losses. Fibers drawn in a
conventional graphite muffle draw furnace exhibited
product losses from attenuation due to point defects of
approximately 5~. Fibers produced in a furnace of the
present invention including a generally tubular, graphite
muffle having an inner surface thereof coated with a
silicon carbide layer about 5-8 microns thick exhibited
product losses from attenuation due to point defects of
approximately 0.8~.
It will be apparent to those skilled in the art that
various modifications and variations can he made in the
method and apparatus of the present invention without
departing from the spirit or scope of the invention.
Thus, it is intended that the present invention cover the
modifications and variations of this invention provided
they come within the scope of the appended claims and
their equivalents.
