U.S. patent application number 10/842759 was filed with the patent office on 2004-10-21 for method and apparatus for fabricating silica based glass.
Invention is credited to Dabby, Franklin W..
Application Number | 20040206129 10/842759 |
Document ID | / |
Family ID | 25053591 |
Filed Date | 2004-10-21 |
United States Patent
Application |
20040206129 |
Kind Code |
A1 |
Dabby, Franklin W. |
October 21, 2004 |
Method and apparatus for fabricating silica based glass
Abstract
A method for fabricating fluorine doped, silica based glass and
fiber by depositing a layer of high purity silica soot on a core
rod while rotating the core rod places the rod of pure fused silica
or doped fused silica and silica soot in a furnace having a lining
of Al.sub.2O.sub.3, elevates the temperature in a fluorine rich
atmosphere to establish the proper differential in the indices of
refraction between the core start rod and the deposited layer, and
heats the resulting rod at consolidation temperatures in an
atmosphere of helium to form a preform. The apparatus includes a
furnace having a lining of high purity alumina that is resistant to
chemical etching and change normally due to the fluorine and
chlorine.
Inventors: |
Dabby, Franklin W.; (Los
Angeles, CA) |
Correspondence
Address: |
HOGAN & HARTSON L.L.P.
500 S. GRAND AVENUE
SUITE 1900
LOS ANGELES
CA
90071-2611
US
|
Family ID: |
25053591 |
Appl. No.: |
10/842759 |
Filed: |
May 10, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10842759 |
May 10, 2004 |
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10235099 |
Sep 5, 2002 |
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10235099 |
Sep 5, 2002 |
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08758906 |
Dec 2, 1996 |
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6474107 |
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Current U.S.
Class: |
65/422 |
Current CPC
Class: |
C03B 2201/12 20130101;
C03B 37/0146 20130101 |
Class at
Publication: |
065/422 |
International
Class: |
C03B 037/075 |
Claims
1-15. Cancelled
16. A method of making a silica-based glass in a structure having
an interior and an interior surface, comprising the steps of: a.
depositing silica soot onto a workpiece; b. drying the silica soot;
and c. sintering the silica soot in the interior of the structure,
the interior surface of the structure comprising aluminum oxide,
and the aluminum oxide having a concentration of at least about
99.97% facing the soot.
17. The method of claim 16 wherein the surface of aluminum oxide
has a potassium content of less than one part per million, a
calcium content of no more than approximately one part per million,
a silicon content of no more than approximately twenty-eight parts
per million, and a magnesium content of no more than approximately
two hundred and fifty parts per million.
18. The method of claim 16, substantially all of the structure
being a muffle, and the aluminum oxide having a visual
transmittance of 96%.
19. The method of claim 16 wherein the sintering step is performed
in the presence of helium.
20. The method of claim 16 wherein the drying step is performed in
the presence of chlorine.
21. The method of claim 16 wherein the drying and sintering steps
are performed in the presence of chlorine and helium.
22. The method of claim 16 wherein the workpiece is a start rod.
Description
PRIORITY APPLICATION
[0001] This application is a divisional of application Ser. No.
10/235,099, filed Sep. 5, 2002, which a continuation of Ser. No.
08/758,906, filed Dec. 2, 1996 now U.S. Pat. No. 6,474,107, the
contents of which are hereby incorporated by reference into this
present application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the art of high purity
glass and glass optical fiber for transmitting light wave signals,
and more particularly to the art of fabricating fused silica
optical waveguides and glass preforms.
[0004] 2. Description of the Prior Art
[0005] In the past, optical fibers of fused silica, SiO.sub.2, have
been formed from preforms in which the relationship of the
relatively higher index of refraction of the core to a relatively
lower index of refraction of the cladding is predetermined so that
when the preform is drawn into a fiber, the fiber will conduct
light at predetermined wavelengths, either in single mode or in
multi-mode form. The performance of fused silica optical fibers is
determined by the amount of attenuation or loss, and by dispersion.
Attenuation is presently considered to be the result of absorption
and scattering, both of which may be the result of irregularities
or imperfections in the formation of the fiber, or of its preform.
Dispersion is the result of changes in the refractive indices with
wavelength.
[0006] For optical fibers to be effective, the attenuation must be
minimized. The elimination of attenuation, or loss, has been
achieved in the past for the most part by eliminating the
impurities in the fused silica, except for the dopant or dopants
necessary to control the differential in the indices of refraction
between the core and the cladding. Even for the dopant or dopants,
a high degree of purity has been sought to achieve sharp fiber
profiles and performance.
[0007] It was known early that one of the impurities in silica that
absorbed light, and thus caused higher attenuation or loss, was the
hydroxyl ion (OH.sup.-1). Various methods and processes for
reducing the hydroxyl ion were taught, among them the reduction of
hydroxyl ions in the silica in the preform's formation stage by
fluorination. See, for example, U.S. Pat. No. 4,579,571 to J. W.
Hicks, Jr., which well sets forth the benefits of expelling the
hydroxyl ions from the preforms formed by flame hydrolysis by
drying the preforms by adding fluorine not only as a drying agent,
but also to reduce the refractive index of the cladding.
[0008] Fluorination of the silica can be accomplished by depositing
silica soot onto a start rod, and placing the start rod with the
deposited silica soot in a space or zone, sometimes called a
muffle, of a furnace which is made rich in fluorine. Sometimes, the
pressure in the space, zone or area of the muffle in which the
silica preform is sintered, is raised and the temperature is
elevated so that the fluorine in the atmosphere is coerced into the
interstices of the silica to expel the hydroxyl ions and to dope
the silica with the fluorine. In all of these methods, one of the
objects, and often the primary object is the reduction of the
refractive index of the cladding glass after sintering and
secondarily reduction of the hydroxyl ions by fluorination.
[0009] One of the ever present problems in fluorination is the very
high reactivity of fluorine with almost all materials. Attempts
have been made in the past to use various materials for the
structure of the furnace in which fluorination takes place. Most
workers in the art have returned to using fused silica for the
composition of that part of the furnace structure that faces the
interior of the muffle space or zone.
[0010] Silica, however, dissipates in the face of the very active
fluorine in the fluorinating processes. The silica structure,
consequently, has to be replaced after only a few uses of the
muffle or muffle tube, or alternatively the muffle tube becomes a
part of the preform. These unhappy and expensive results are
described, for example, in Abe, U.S. Pat. No. 4,643,751. The
problems are exacerbated when such a silica wall surrounding the
reaction zone is broached, and the highly reactive hydrogen
fluoride (HF) escapes. In an effort to reduce the corrosion due to
fluorine, chemical forms of fluorine that are inert at room
temperatures, such as, for example sulfur hexafluoride are used to
supply the fluorine. Reactive fluorine results, however, from the
high temperature dissociation of sulfur hexafluoride (SF.sub.6),
and forms hydrogen fluoride from the interaction of the fluorine
with the hydrogen from the OH.sup.-1 ions imbedded in the soot.
Abe, cited above, made the silica muffle tube become part of the
preform.
[0011] Efforts have been made in the past to realize a substitute
for the silica muffle tube in the drying, fluorinating and
sintering furnace. For example, one attempt was made to substitute
alumina as the composition for the furnace wall in the muffle.
However, it was reported that the fluorine so reacted with the
alumina muffle that aluminum and possibly other impurities were
imparted into the silica preforms. See, for example, Berkey, U.S.
Pat. No. 4,629,485, where such an attempt to substitute alumina
resulted in a thick devitrified surface layer on the preform that
rendered the preform useless.
[0012] Aluminum oxide, however, if it could be used, would last
substantially longer than silica in the highly reactive environment
of fluorine, and consequently it remains a highly sought objective
to make at least the interior surface of the muffle of alumina.
However, it is an objective to have an alumina surfaced muffle in
the drying, fluorinating and sintering furnace where the glass
optical preform and/or fiber resulting from its use does not have
impurities from the alumina go into the preform or fiber. Further,
it is highly desired in any use of a substitute for the silica
composition, that no devitrified surface layer forms on the
preforms that would render the preform useless.
SUMMARY
[0013] In brief, in accordance with one aspect of the present
invention, a silica soot is deposited on a start rod or core rod,
which is then lowered into a muffle or zone of a furnace in which
the atmosphere is maintained with highly reactive fluorine. The
muffle or zone in the furnace has an interior facing surface which
is composed of a high purity aluminum oxide. The purity is of the
order of one or less parts per million of sodium, calcium,
potassium, iron and titanium, with an alumina (Al.sub.2O.sub.3)
content of 99.97%. The temperature is raised to a level suitable
for drying and removing water from the soot deposited core rod. The
core rod having the soot formed on it may be dried separately in a
fluorine or chlorine atmosphere maintained in the muffle or zone.
The fluorine may continue to dope into the soot cladding. The
temperature in the muffle or zone is raised again to a
predetermined level so that the fluorine is doped into the
deposited soot. The temperature is raised yet again to another
level for sintering and consolidating the core rod with soot
preform into a glass.
[0014] These and other novel aspects of the invention, together
with other aspects thereof, can be better understood by the
following description of the preferred embodiments, which are
designed to be read in conjunction and together with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a side elevational, partial cross-sectional view
of a start rod showing a depositing of silica soot;
[0016] FIG. 2 is a cross-sectional view of the start rod of FIG. 1
taken along line 2-2 showing a depositing of silica soot;
[0017] FIG. 3 is a side elevational, partial cross-sectional view
of a silica and silica soot preform in a furnace;
[0018] FIG. 4 is a side elevational, partial cross-sectional view
of the silica and silica soot preform of FIG. 3 in a second
position within the furnace;
[0019] FIG. 5 is a schematic diagram of a valved gas flow for the
furnace of FIGS. 3 and 4; and,
[0020] FIG. 6 is a block diagram showing the preferred embodiment
of the method of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] Very thin communications grade optical fiber is made often
from a preform having a core and a cladding corresponding in ratios
and geometry to those of the ultimate glass fiber desired to be
drawn from the preform. The composition of the core and cladding
must be such that there is a lower index of refraction in the
cladding than in the core in order to guide the light waves
propagating through the waveguide. A preform can be made by
building up first a start rod and forming it into a glass that will
eventually become the core of the fiber. A cladding layer can then
be built up on the core rod or start rod by deposition
techniques.
[0022] A preform 22 is made starting with a cylindrical core rod or
start rod 10, reference being had initially to FIGS. 1 and 2 of the
accompanying drawings in which reference numerals used will
correspond to like reference numerals in this specification. The
core rod 10 is a pure fused silica (SiO.sub.2) glass. The start rod
or core rod 10 can be formed using much of the same techniques as
used in making the cladding, without the insertion of fluorine to
lower the index of refraction, which will be described in greater
detail herein, or by other methods that are known. Other methods
may call for a carefully doped fused silica.
[0023] The cladding material 12, in the preferred embodiment
starting as a pure fused silica soot or doped fused silica soot, is
deposited on the core rod 10 by directing from a nozzle 14 a stream
16 of the soot material 12. If it is desired to have germanium
oxide or other material as the dopant for the cladding, the soot
material will be a mixture of a soot of fused silica and amounts of
a soot of the dopant so that the glass resulting after fluorination
will have an index of refraction which will be lower than the index
of refraction of the core material 10. The higher index of
refraction of the core material 10 from that of the cladding
material 12 results in an effective light waveguide. A method of
making a fluorinated preform is described in U.S. Pat. No.
5,364,430 to Sarkar and U.S. Pat. No. 4,629,485 to Berkey.
[0024] The soot 12 is evenly distributed and placed on the core rod
10 by rotating the core rod 10 in the direction shown by the arrow
18. In addition, the core rod 10 is translated reciprocally along
its axis in the directions of the arrow 20, to expose the core rod
10 evenly along the length of the preform 22 to the stream 16 of
the soot.
[0025] The stream 16 of soot 12 is directed within a closed
chamber, not shown, from a burner and nozzle 14 representatively
depicted. The burner is comprised of a flame generator and a
continuous supply of the gases that react to form the soot
composition to be directed out of the burner along with the flame
towards the core rod 10 in the stream 16. The temperature at the
surface of the core rod 10 is elevated to from approximately 900 to
1350 degrees Centigrade to receive and hold the soot 12. More
details and alternative methods of forming the cladding soot 12 on
the start rod 10 can be seen from U.S. Pat. No. 5,364,430.
[0026] The burner inside the nozzles 14 produces a flame to raise
the temperature of the soot mixture to oxidation temperature.
Further details of such a deposition arrangement may be found in
the art, such as for example, U.S. Pat. No. 3,826,560, U.S. Pat.
No. 4,148,621, and U.S. Pat. No. 4,173,305. A suitable nozzle may
be see in U.S. Pat. No. 3,957,474. Burners are described in U.S.
Pat. No. 3,565,345 and U.S. Pat. No. 4,165,223.
[0027] FIG. 2, a cross-section of the preform of FIG. 1 taken along
line 2-2 of FIG. 1, shows the relative diameter difference for the
core 10 and the cladding soot 12 being deposited onto the core or
start rod 10 by the soot material being directed towards the core
rod 10 through the soot stream 16 from nozzle 14, while the core
rod core 10 is rotated in the direction of arrow 18 and is
translated along the axial direction defined by the core 10.
[0028] As better seen in FIGS. 3 and 4 of the accompanying
drawings, the preform 22 having the cladding in the form of soot 12
is then prepared for drying, fluorination and sintering. The
preform 22 comprised of the soot cladding 12 and the core 10 is
placed in a muffle tube or chamber 24 of a furnace 26. Drying can
be accomplished by elevating the temperature in the furnace 26 to
approximately 1150 to 1250 degrees Centigrade and making the
atmosphere within the furnace 26 hydrophilic by making it rich in a
gas, such as chlorine, that forces water or hydroxyl ions
(OH.sup.-1) out of the soot 12.
[0029] The drying gas is introduced into the muffle tube 24 of the
furnace 26 through inlet port 28. Often, chlorine is used for such
drying. However, fluorine can eliminate hydroxyl ions from the soot
12 but also can become a dopant in the soot 12 to impart an index
of refraction to define the cladding of the fiber. In the case of
fluorine doped silica, the index of refraction is lowered to result
in an index different from that of the core 10, where the core 10
is pure fused silica.
[0030] The gas flows are controlled and fed into the inlet port 28
from the gas reservoirs 40, as shown schematically in FIG. 5 of the
drawings. An oxygen reservoir 42 has the flow of its oxygen into
the inlet port 28 controlled by mass flow control valve 44.
Similarly, the flow of helium from the helium reservoir 44 is
controlled by mass flow control valve 48, which valves the flow
into inlet port 28. In like manner, the flow of chlorine from the
chlorine reservoir 52 is controlled by mass flow control valve 54.
The flow of fluorine from the sulfur hexafluoride (SF.sub.6)
reservoir 56 is controlled by mass flow control valve 58.
[0031] For drying, the temperature is elevated over a twenty minute
period to a temperature of from 1000 to 1150 degrees Centigrade. An
oxygen flow of 140 standard cubic centimeters per minute ("scc/m"),
a helium flow of 7000 scc/m and a chlorine flow of 350 scc/m are
maintained. The preform 22 is kept in the muffle tube 24 for a
period of approximately thirty minutes at 1150 degrees Centigrade,
and withdrawn from the hot zone 34 thereafter slowly during a ten
minute time period.
[0032] The muffle tube 24 and the preform 22 are purged by dry
helium and dry oxygen being injected into the muffle tube 24
through the inlet port 28. The process of purging is described in
greater detail in U.S. Pat. No. 5,318,611. The injection of helium
is controlled at a rate of 6.0 standard liters per minute ("sl/m"),
and the injection of oxygen is controlled at a rate of 400 scc/m
for a period of approximately sixty minutes. The preform 22 and
muffle tube 24 are then purged by increasing the flow of helium to
7.0 sl/m for approximately thirty minutes. The chlorine flow at a
reduced level may be maintained to scavenge any residual moisture
in the helium or oxygen.
[0033] The gas is expelled from the muffle tube 24 through the
outlet port 30. The pressure within the muffle tube 24 is
controlled by the pressure control chamber 32 at the upper end of
the muffle tube 24. The preform 22 is then removed from the hot
zone 34 in the muffle tube 24 while the tube 24 is purged and
cleaned by a flow of helium at a rate of 7000 scc/m for a period of
approximately seventy (70) minutes. The temperature of the hot zone
34 is maintained at approximately 1450 degrees Centigrade during
this purging period.
[0034] The preform 22 is then sintered, reference being had to FIG.
4. The temperature within the furnace 26 is elevated over a period
of forty minutes by energizing the heating coils 36, to a third or
consolidation level of 1450 degrees Centigrade. Sulfur hexafluoride
dissociates in the hot zone 34 into reactive fluorine gas. The flow
rate of sulfur hexafluoride into the muffle tube 24 is maintained
at a rate of 135 scc/m. The preform 22 having the cladding in the
form of soot 12 is lowered slowly through the muffle tube 24
through the hot zone 34 within the muffle tube 24 at a rate of 0.2
cm/minute. The preform 22 is held within the hot zone 34 for 180
minutes while the flow rate of helium is maintained at 7000 scc/m.
The helium flow will remove the chlorine or other drying gas from
the preform 22.
[0035] The structure of the muffle tube 24 in the preferred
embodiment is made from aluminum oxide (Al.sub.2O.sub.3) in the
present invention. The aluminum oxide walls must be highly pure. It
is considered that a purity of 99.97% aluminum oxide will provide a
glass muffle tube wall 24. The aluminum oxide wall should have a
bulk density of approximately 3.99 grams/cubic centimeter and a
water absorption of virtually zero. The aluminum oxide wall should
have a service temperature of up to approximately 1800 degrees
Centigrade. A high total and inline visual transmittance would be a
good indicator of the purity desired for the muffle tube wall 24 of
the preferred embodiment. It is believed that a visual
transmittance of 96% will be satisfactory. The aluminum oxide wall
of the composition preferred for this invention should have less
than one part per million ("ppm") each of sodium, of potassium, of
iron and of titanium. A low content of silicon, for example
approximately 28 ppm, and of calcium, for example one ppm, is
preferred. Magnesium at a level of approximately 250 ppm is also
preferred.
[0036] An aluminum oxide muffle tube will be amorphus and not
polycrystaline. Such a composition for the muffle tube 24 will
provide a higher softening point than a silica tube. The higher the
softening point of the alumina allows for a greater pressure of
fluorine to be maintained in the muffle tube 24 than can be
achieved in a silica muffle tube. At 1450 degrees Centigrade, a
silica muffle tube will balloon under a pressure only slightly
higher than atmospheric or surrounding pressures. The higher the
fluorine pressure, the greater the infusion of the fluorine into
the cladding soot 12, and the more the refractive index of the
cladding being doped by the fluorine in the alumina muffle tube 24
of the present invention is lowered.
[0037] The muffle tube may have the aluminum oxide applied to the
interior of the muffle tube so as to form an Al.sub.2O.sub.3
surface to face the highly active fluorine.
[0038] In operation, as seen better in the schematic block diagram
of FIG. 6 of the accompanying drawings, first a core rod or a start
rod 10 is formed. A silica soot initially is deposited or formed 70
on a mandrel to make a silica soot core. The silica soot core is
then dried 68 at the same temperatures, and with the same gases and
gas flow rates within the same time periods as set forth herein
above for drying silica soot 12 on the core rod 10.
[0039] This silica soot core then is sintered 72 into a glass core
rod 10. In this s1nter1ng step 72, the temperature is elevated over
a period of forty minutes to a consolidation level, or
approximately 1450 degrees Centigrade. The gas used to flow through
the muffle tube 24 does not include fluorine, which may dope the
resulting glass. Instead, chlorine and helium is used. The chlorine
flow should be maintained at the same flow rate of 135 scc/m. The
silica soot core is then lowered slowly through the muffle tube 24
through the hot zone 34 within the muffle tube 24 at a rate of 0.2
cm/minute. The silica soot core is held within the hot zone 34 for
180 minutes while the flow rate of helium is maintained at 7000
scc/m. The helium flow will remove the chlorine or other drying gas
from the preform 22.
[0040] As an alternative to making the glass core rod 10 as
described, the core rod 10 may be obtained 72 directly. In either
case, the surface of the core rod 10 is examined for scratches,
dirt or other imperfections which may be cleaned. Cleaning 76, if
necessary, may be accomplished by etching with hydrofluoric acid or
with a flow of fluorine gas. Heating the fluorine gas will increase
the activity of the fluorine gas. A fire polishing, however, is not
indicated because the hydrogen and oxygen may combine to introduce
unwanted OH.sup.-1. Such a cleaning, however, may not be
necessary.
[0041] A deposit of silica soot 12 is evenly distributed 78 over a
predetermined length of the core rod 10, and placed in a furnace 26
having a muffle tube 24 of alumina having a purity of 99.97%
aluminum oxide. The resulting preform 22 is dried 80. During the
drying process, helium and oxygen is made to flow through the
muffle tube 24 at the flow rates for the time periods as described
above.
[0042] The muffle tube 24 is then purged. The hot zone 34 of the
alumina muffle tube 24 is preheated, and the preform is then
sintered 82 by lowering it at a prescribed rate into the preheated
muffle tube 24 while the flow rates for helium, chlorine and
fluorine are maintained as described above. After maintaining the
preform 22 within the muffle tube at the temperature and subjected
to the He, Cl and SF.sub.6 flow rates set forth above, the preform
is cooled down at a predetermined rate to result in a properly
sintered and usable preform.
[0043] The preform then should be examined for imperfections that
can be cleaned, as described. If necessary, the surface of the
preform then may be cleaned, as indicated in block 86. In this
cleaning step 86, the same procedure may be used for cleaning as in
step 76 above. Alternatively, the step 86 may be accomplished by a
firepolishing. This step 86 may not be necessary, and may be
by-passed if the surface does not have imperfections or is
sufficiently clean. The preform may then be drawn 88 into a
waveguide having the ratios of diameters and difference of indices
of refraction as predetermined.
[0044] Alternatively, the preform 22 can be elongated 90 to reduce
the diameters of the core 10 and cladding 12, in preparation for
addition of a further cladding layer. In this alternative process,
after elongation 90, the surface of the preform can be cleaned 92
without introducing OH, by hydrofluoric acid etching. The surface
of the preform may be cleaned with a firepolishing. Since there is
already a cladding of sufficient thickness, the OH that may be
introduced into the surface by a firepolish is far enough away from
the core in which the light travels. The transmitting light,
consequently, does not "see" the surface OH, and no increase in
attenuation due to the OH will occur, all as described by Sarkar,
U.S. Pat. No. 5,364,430. The cleaning step 92 may not be necessary,
and may be by-passed, for the reasons and as described herein above
for step 68.
[0045] A further deposition 94 of silica soot is then applied to
the preform 22. After deposition 94, the preform 22 is dried 96 and
then sintered 98. In this alternative, the sintering step 98 does
not use fluorine, and is performed as set forth for step 70 herein
above. The resulting glass preform then can be drawn 100 into a
fiber or a waveguide.
[0046] In yet another alternative process, the preform 22 is
elongated 104. Again, after examination, the surface of the preform
22 can be cleaned 106 without introducing OH by hydrofluoric acid
etching. If there is already fabricated or built onto the core 10 a
cladding of sufficient thickness so that the OH that may be
introduced into the surface by a firepolish will not be "seen," as
described above in the description for step 92, a cleaning by
firepolish may be used without affecting the attenuation of the
resulting fiber due to OH.
[0047] Another layer of silica soot is then deposited 108 on the
cladding 12. The silica soot preform is then dried 110 in a similar
manner and method as described for adding a further cladding layer
in the steps of schematic block 96. The preform is then sintered
112 by lowering it at a prescribed rate into the preheated muffle
tube 24 of alumina while the flow rates for helium, chlorine and
fluorine are maintained as described for step 82 herein above.
above. After maintaining the preform 22 within the muffle tube at
the temperature and subjected to the He, Cl and SF.sub.6 flow rates
set forth above, the preform can then be drawn 116 into a fiber or
a waveguide.
[0048] The foregoing detailed description of my invention and of
preferred embodiments as to products, compositions and processes,
is illustrative of specific embodiments only. It is to be
understood, however, that additional embodiments may be perceived
by those skilled in the art. The embodiments described herein,
together with those additional embodiments, are considered to be
within the scope of the present invention.
* * * * *