U.S. patent application number 09/284680 was filed with the patent office on 2002-03-14 for method of reducing break sources in drawn fibers by active oxidation of contaminants in a reducing atmoshere.
Invention is credited to DICKINSON JR., JAMES E., GLAESEMANN, G. SCOTT, SNIPES, JAMES A., TAO, TINGHONG, WISSUCHEK JR., DONALD J..
Application Number | 20020029591 09/284680 |
Document ID | / |
Family ID | 21849177 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020029591 |
Kind Code |
A1 |
DICKINSON JR., JAMES E. ; et
al. |
March 14, 2002 |
METHOD OF REDUCING BREAK SOURCES IN DRAWN FIBERS BY ACTIVE
OXIDATION OF CONTAMINANTS IN A REDUCING ATMOSHERE
Abstract
A class preform (30) for making optical waveguides has surface
impurities such as silicon carbide or silicon nitride. The preform
is drawn in a furnace (12) that is supplied with oxygen via a
conduit (40). The oxygen causes the impurities to oxidize and not
effect the strength of the fiber.
Inventors: |
DICKINSON JR., JAMES E.;
(CORNING, NY) ; GLAESEMANN, G. SCOTT; (CORNING,
NY) ; SNIPES, JAMES A.; (WILMINGTON, NC) ;
TAO, TINGHONG; (BIG FLATS, NY) ; WISSUCHEK JR.,
DONALD J.; (HORSEHEADS, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
21849177 |
Appl. No.: |
09/284680 |
Filed: |
April 15, 1999 |
PCT Filed: |
October 3, 1997 |
PCT NO: |
PCT/US97/18039 |
Current U.S.
Class: |
65/379 ; 65/385;
65/424; 65/489 |
Current CPC
Class: |
Y02P 40/57 20151101;
C03B 2205/82 20130101; C03B 37/027 20130101; C03B 2205/16 20130101;
C03B 2205/64 20130101; C03B 2205/80 20130101; C03B 37/029
20130101 |
Class at
Publication: |
65/379 ; 65/424;
65/385; 65/489 |
International
Class: |
C03B 037/027 |
Claims
What is claimed is:
1. A method of producing a fiber in a drawing device having a
refractory, oxide component in a drawing portion, comprising the
steps of: disposing a blank having a refractory contaminant in the
drawing portion; providing an environment in the drawing portion,
wherein said environment causes active oxidation of the contaminant
into gaseous reaction products; and drawing a fiber from the blank
in the environment.
2. The method of claim 1, wherein the refractory, oxide component
comprises a refractory, oxide muffle.
3. The method of claim 2, wherein the refractory, oxide muffle
includes zirconia.
4. The method of claim 1, wherein the contaminant includes a
silicon compound.
5. The method of claim 4, wherein the silicon compound is at least
one member from a group comprised of silicon carbide and silicon
nitride.
6. The method of claim 1, wherein the step of providing the
environment comprises providing a purge gas containing a reducing
gas.
7. The method of claim 6, wherein the reducing gas comprises carbon
monoxide.
8. The method of claim 6, wherein the purge gas containing a
reducing gas comprises helium and carbon monoxide.
9. The method of claim 1, wherein the fiber includes silicon.
10. The method of claim 1, wherein the fiber is an optical
waveguide fiber.
11. The method of claim 10, wherein the contaminant includes a
silicon compound.
12. The method of claim 11, wherein the refractory, oxide component
comprises a zirconia muffle.
13. The method of claim 11, wherein the step of providing the
environment comprises providing carbon monoxide.
14. A method of removing a refractory, contaminant break source of
a fiber drawn from a blank disposed in a drawing portion of a
drawing device, the drawing portion having a refractory, oxide
component, comprising the step of: providing an environment in the
drawing portion, wherein said environment causes oxidation of the
contaminant break source into gaseous reaction products.
15. The method of claim 14, wherein the refractory, oxide component
comprises a refractory, oxide muffle.
16. The method of claim 15, wherein the refractory, oxide muffle
includes zirconia.
17. The method of claim 14, wherein the contaminant break source
includes a silicon compound.
18. The method of claim 17, wherein the silicon compound is at
least one member from a group comprised of silicon carbide and
silicon nitride.
19. The method of claim 14, wherein the step of providing the
environment comprises providing a purge gas containing a reducing
gas.
20. The method of claim 19, wherein the reducing gas comprises
carbon monoxide.
21. The method of claim 14, wherein the step of providing the
environment comprises providing helium and carbon monoxide.
22. The method of claim 14, wherein the fiber includes silicon.
23. The method of claim 14, wherein the fiber is an optical
waveguide fiber.
24. The method of claim 23, wherein the contaminant break source
includes a silicon compound.
25. The method of claim 24, wherein refractory, oxide comprises a
zirconia furnace muffle.
26. The method of claim 24, wherein the step of providing the
environment comprises providing a purge gas containing carbon
monoxide.
27. A method of removing an oxidizable, refractory contaminant from
a blank disposed in a fiber drawing device having a refractory,
oxide component in a drawing portion, comprising the step of:
providing an environment in the drawing portion wherein said
environment promotes active oxidation of the contaminant into
gaseous reaction products and inhibits passive oxidation of the
contaminant into a non-gaseous passivation layer.
28. The method of claim 27, wherein the refractory, oxide component
comprises a refractory, oxide muffle.
29. The method of claim 28, wherein the refractory, oxide muffle
includes zirconia.
30. The method of claim 27, wherein the contaminant includes a
silicon compound.
31. The method of claim 30, wherein the silicon compound is at
least one member from a group comprised of silicon carbide and
silicon nitride.
32. The method of claim 27, wherein the step of providing the
environment comprises providing a purge gas containing a reducing
gas.
33. The method of claim 32, wherein the reducing gas comprises
carbon monoxide.
34. The method of claim 32, wherein the step of providing the
environment comprises providing helium and carbon monoxide.
35. The method of claim 27, wherein the fiber includes silicon.
36. The method of claim 27, wherein the fiber is an optical
waveguide fiber.
37. The method of claim 36, wherein the contaminant includes a
silicon compound.
38. The method of claim 37, wherein the drawing device is comprised
of a furnace having a zirconia muffle.
39. The method of claim 37, wherein the step of providing the
environment includes providing carbon monoxide.
40. An apparatus for producing an optical fiber, comprising: a
fiber drawing furnace, a reducing gas supply device, said reducing
gas supply device connected to said furnace, wherein said reducing
gas supply device supplies a reducing gas to said furnace.
41. The apparatus of claim 40, wherein the fiber drawing furnace
comprises a refractory, oxide muffle.
42. The apparatus of claim 41, wherein the refractory, oxide muffle
includes zirconia.
43. The apparatus of claim 40, wherein said reducing gas supply
device supplies a purge gas to said furnace.
44. The apparatus of claim 40, wherein the reducing gas includes
carbon monoxide.
45. The apparatus of claim 43, wherein the purge gas includes
helium.
46. A method of drawing a fiber from a blank in a drawing device
having a refractory, oxide component in a drawing portion,
comprising the steps of: inhibiting oxidation of an oxidizable,
refractory contaminant on the blank into a solid passivation layer;
promoting oxidation of the contaminant into gaseous reaction
products; and drawing the fiber from the blank.
47. The method of claim 53, wherein the contaminant includes a
silicon compound.
48. The method of claim 54, wherein the silicon compound is at
least one member from a group comprising of silicon carbide and
silicon nitride.
49. The method of claim 53, wherein the blank includes silicon.
50. The method of claim 53, wherein the fiber is an optical
waveguide fiber.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method and apparatus for
drawing a fiber from a blank, more particularly, a method and
apparatus for drawing an optical waveguide fiber from a
silica-containing blank.
[0003] 2. Description of the Related Art
[0004] Optical waveguide fibers (optical fibers) are a transmission
medium used in optical communication systems. Optical fibers are
typically made by well known methods that involve forming blanks
from which the fibers are to be drawn, storing the blanks in
holding ovens, and drawing fibers from the blanks in draw furnaces.
Strength is an important characteristic as optical fibers.
Particulate contaminants on the fiber surface often weaken the
fiber and cause flaw initiation and fiber failure under tensile
loading. Some optical fibers, particularly those drawn in zirconia
(ZrO.sub.2) muffle furnaces, break under low stress due to such
contaminants.
SUMMARY OF THE INVENTION
[0005] An object of the present invention is to improve the
strength of fibers.
[0006] Another object of the invention is to remove break sources
that cause fibers to break at low stress.
[0007] Additional objects and advantages of the invention will be
set forth in part in the description which follows, and in part
will be obvious from the description, or may be learned by practice
of the invention. The objects and advantages of the invention will
be realized and attained by means of the elements and combinations
particularly pointed out in the appended claims.
[0008] As explained more fully below, it has been determined that
breaking optical fibers contain silicon carbide (SiC) and silicon
nitride (Si.sub.4N.sub.4) refractory contaminants that cause the
fibers to fail at low stress. The present invention improves the
strength of fibers by removing the contaminants through active
oxidation during the fiber-drawing process.
[0009] To achieve the objects and in accordance with the purpose of
the invention, as broadly described herein, the invention provides
an improved method of producing a fiber in a drawing device having
a refractory, oxide component in a drawing portion, comprising the
steps of disposing a blank having a refractory contaminant in the
drawing portion, providing an environment in the drawing portion
that causes active oxidation of the refractory contaminant, and
drawing a fiber from the blank in the environment.
[0010] The invention also provides an improved apparatus for
producing a fiber, comprising a drawing portion that has a
refractory, oxide component and that heats a blank having a
refractory contaminant, a supply device that supplies gas to the
drawing portion to provide an environment in the drawing portion
that causes active oxidation of the refractory contaminant, and a
device for drawing a fiber from the blank in the environment.
[0011] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive of the invention, as
claimed.
[0012] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate an embodiment
of the invention and together with the description, serve to
explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a sectional view of a preferred embodiment of a
draw furnace according to the present invention.
[0014] FIG. 2 is a sectional view of a holding oven.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0015] Reference will now be made in detail to the presently
preferred embodiment of the invention, an example of which is
illustrated in the accompanying drawings. Wherever possible, the
same reference numbers will be used throughout the drawings to
refer to the same or like parts.
[0016] It has been discovered, in connection with the present
invention, that the fibers drawn in conventional zirconia muffle
furnaces that break under low stress contain silicon carbide and
silicon nitride, which are non-oxide, refractory contaminants.
These contaminants are in the size range typical of airborne
particles (less than 5 .mu.m) and attach to the surface of the
blank before or during the drawing process, thus producing a draw
trough on the surface of the fiber.
[0017] It has also been discovered that each contaminant has an
adhered passivation layer of amorphous silica (SiO.sub.2) formed
thereon, at least in part, due to the environment in a conventional
zirconia muffle furnace. The passivation layer is a solid reaction
product of passive oxidation. The passive oxidation mechanisms for
silicon carbide and silicon nitride are represented by the
following formulas:
(2)SiC+(3)O.sub.2.fwdarw.(2)SiO.sub.2(s )+(2)CO(g)
Si.sub.2N.sub.4+(3)O.sub.2.fwdarw.(3)SiO.sub.2(s)+2N.sub.2(g).
[0018] A conventional zirconia muffle furnace has sufficient
oxygen, which is provided by ambient air leaking into the furnace,
to form passivation layers on the contaminants through passive
oxidation.
[0019] It has been further discovered, in connection with the
present invention, that these passivation-layered contaminants act
as low-stress break sources for the optical fibers.
[0020] The draw process of the present invention has been designed
to remove these contaminants through active oxidation. The active
oxidation mechanism produces a gaseous reaction product and thus
corrode the silicon carbide and silicon nitride contaminants. The
active oxidation mechanisms for silicon carbide and silicon nitride
are represented by the following formulas:
SiC(s)+O.sub.2(g).fwdarw.SiO(g)+CO(g)
Si.sub.3N.sub.4(s)+(3/2)O.sub.2(g).fwdarw.3SiO(g)+2N.sub.2(g).
[0021] Thus, by promoting active oxidation, the contaminants can be
removed by corrosion. For example, graphite muffle furnaces produce
fibers that do not contain passivation-layered contaminants
because, it is believed, these furnaces promote active
oxidation.
[0022] The oxygen concentration and the temperature of the
environment determine whether the passive or active oxidation
mechanism will predominate. For example, at a given temperature,
the passive oxidation mechanism predominates for silicon carbide
and silicon nitride when P.sub.O2>P.sub.CO+Si), and the active
oxidation mechanism predominates when P.sub.CO+SiO>P.sub.O2.
[0023] Accordingly, the present invention preferably promotes
active oxidation by providing a low-oxygen environment in a drawing
portion of a draw furnace. In a preferred mode, a low-oxygen
environment is provided by introducing a reducing gas into the
drawing portion that will react with oxygen to reduce the oxygen
concentration. The reducing gas can be any gas that reacts readily
with oxygen to reduce oxygen concentration and thereby create a
benign gas, i.e., a gas that will not react with the blank or
fiber.
[0024] Presently, carbon monoxide (CO) is the preferred reducing
gas. The following experiments illustrate the effect of carbon
monoxide on the environment in the drawing portion of a zirconia
muffle furnace.
[0025] Initially, the oxygen concentration was measured while
flowing commercially pure helium (He) through the muffle at varying
rates. The results are shown in Table 1.
1 TABLE 1 Helium Flow (S.L.P.M.) % Oxygen 0.0 21.80 0.8 21.70 2.25
15.00 3.50 2.85 4.50 1.91 5.35 1.55
[0026] Next, the oxygen concentration was measured while flowing a
gas consisting of commercially pure helium and 10% carbon monoxide
through the muffle at varying rates. The results are shown in Table
2.
2 TABLE 2 Helium and Carbon Monoxide Flow (S.L.P.M.) % Oxygen 3.19
0.367 4.4 0.0845 4.9 less than 0.00001 5.6 less than 0.00001 14.3
less than 0.00001
[0027] As shown by Tables 1 and 2, use of carbon monoxide as a
reducing gas effectively reduces the oxygen concentration of the
environment in the muffle.
[0028] The reduced oxygen environment improves the strength of
fibers drawn in a zirconia muffle furnace, as shown by the
following experiments.
[0029] Waveguide blanks were contaminated with a high concentration
of silicon carbide contaminants. The mean particle size was 6.74
microns and the maximum size was 25 microns. Contaminants were
deposited to achieve a coverage density of greater than 20 per
square centimeter of blank surface.
[0030] The seeded blanks were drawn into fiber in a conventional
zirconia muffle furnace. Under conventional operating conditions
(helium purge gas only), the fiber was difficult to wrap after
drawing, yielding lengths of only 200 to 400 meters between breaks.
Strength testing of approximately 2 kilometers of fiber produced an
average of approximately two low strength breaks per meter. Break
source analysis confirmed that the breaks during wrapping and
strength testing were due to silicon carbide contaminants blanketed
with a layer of amorphous silica.
[0031] Next, carbon monoxide was added to the helium purge gas in
the zirconia muffle furnace to reduce the oxygen concentration in
accordance with the equation: 2CO+O.sub.2.fwdarw.2CO.sub.2. When
the blanks were loaded into a furnace environment containing an
appropriate amount of carbon monoxide in addition to the helium
purge gas, the draw performance improved dramatically, yielding
lengths of up to 65 kilometers between breaks. Strength testing of
more than 200 kilometers of fiber and associated analysis of break
ends showed that there were no low strength breaks due to silicon
carbide.
[0032] Finally, the drawing process was commenced using only helium
as the purge gas and carbon monoxide was added to the helium midway
through the drawing process. Draw performance improved instantly
and dramatically, with the fiber changing from being unwrappable
(without carbon monoxide) to yielding wrappable lengths of greater
than 100 kilometers (with carbon monoxide).
[0033] Similar testing with blanks contaminated with silicon
nitride contaminants yielded similar results.
[0034] As shown by these experiments, the reduced oxygen
environment created by the addition of carbon monoxide creates a
passive to active oxidation transition. The contaminants corrode
away due to active oxidation and do not form break sources.
[0035] A preferred embodiment of a draw furnace according to the
present invention is shown in FIG. 1 and is designated generally by
the reference numeral 10. In accordance with the invention, draw
furnace 10 includes a drawing portion that has a refractory, oxide
component and that heats a blank having a refractory contaminant to
a fiber drawing temperature, and a supply device that supplies gas
to the drawing portion to provide an environment in the drawing
portion that causes active oxidation of the refractory
contaminant.
[0036] As shown herein, the drawing portion 12 includes a zirconia
muffle 14, which is a refractory, oxide component. The zirconia
muffle distributes heat generated by a heating coil 16 that has
passed through insulation 18. In the present invention, the
integrity of the environment in the drawing portion has been
improved by providing a high temperature ceramic glue (CERAMABOND
#503, Armco Products) that forms a gas-tight seal between beaker
top 20 and upper muffle extension 22, and a flat, closed-cell
silicone gasket 24 (Material No. 7204, Groendyk Mfg. Co.) that
forms a gas-tight seal between lower muffle extension 26 and Elmer
tube 27.
[0037] A blank support rod 28 holds blank 30 in drawing portion 12.
An O-ring 32 forms a seal between rod 28 and sealing member 34,
which is formed of metallic foil or the like. Sealing member 34
connects to end cap 36, which itself is connected to annular member
38.
[0038] As shown herein, the supply device includes pipe 40 that
extends through annular member 38. Pipe 40 is connected to gas
supply 42 and supplies gas from gas supply 42 to the drawing
portion 12, thereby providing an environment in drawing portion 12
that causes active oxidation of the refractory contaminant and
inhibits passive oxidation.
[0039] Pipe 40 preferably flows gas through muffle 14 at a constant
flow rate of 2 to 5 standard liters per minute. The flow rate can
be altered based on factors such as the flow rate needed to
maintain control of fiber attributes.
[0040] Preferably, the gas supply 42 supplies a purge gas
containing a reducing gas that reacts with oxygen to lower the
oxygen concentration of the environment of the drawing portion.
More preferably, the purge gas consists of helium and carbon
monoxide. Carbon monoxide reacts with oxygen to produce carbon
dioxide (CO.sub.2), thus reducing the oxygen concentration in the
environment.
[0041] When using the preferred purge gas, the gas supply 42 can
be, for example, a reservoir of both helium and carbon monoxide or
separate reservoirs of helium and carbon monoxide, the outputs of
which are combined before or as they enter the draw furnace. In
view of the toxic nature of carbon monoxide, however, it may be
preferable to use an external furnace that produces carbon monoxide
by reaction and, therefore, renders unnecessary a reservoir of
carbon monoxide.
[0042] FIG. 1 diagrammatically illustrates such an external furnace
70. The external furnace 70 includes a reactive material 72 that
reacts with at least a gas of a non-toxic gas mixture (provided by
unillustrated gas reservoir(s)) to produce carbon monoxide. The
reactive material 72 can be a porous carbon or graphite material
(such as a carbon honeycomb substrate manufactured by Corning
Incorporated, e.g., part no. K2225) through which the non-toxic gas
mixture can be passed.
[0043] The non-toxic gas mixture preferably contains helium and a
reactive gas. The reactive gas, which can be, for example, carbon
dioxide or oxygen, will react with the carbon material 72 to
produce carbon monoxide. The desired amount of carbon monoxide
(preferably about 2% by volume) can be produced by manipulating the
reactive gas concentration and the reaction temperature (the
external furnace 70 preferably operates at atmospheric
pressure).
[0044] When the reactive gas is carbon dioxide, the equilibrium
reaction is:
CO.sub.2+C=2CO.
[0045] This reaction proceeds to near completion (more than 95%,
conversion) at 1000.degree. C. and atmospheric pressure.
[0046] When the reactive gas is oxygen, two competing reactions
occur:
O.sub.2+C=CO.sub.2
O.sub.2+2C=2CO
[0047] The reaction producing carbon monoxide is favored at high
temperatures and low oxygen pressures. At 1000.degree. C. and
atmospheric pressure (P.sub.O2<0.05), thermodynamic equilibrium
predicts that the CO:CO.sub.2 ratio should be greater than 100:1.
This ratio may be decreased if gas flow rates are fast enough to
cause an incomplete reaction. However, the typical flow rate for a
zirconia draw furnace (4.5 standard liters per minute) is slow
enough to ensure that the reaction is not kinetically limited. This
is true when either carbon dioxide or oxygen is the reactive
gas.
[0048] Since the preferred non-toxic gas mixtures will have to be
heated to produce the desired amount of carbon monoxide, the
external furnace 70 will preferably include a heating device. The
heating device can include a muffle 74 that distributes heat
generated by a heating coil 76 to heat the gas to a preferred
temperature of 1000.degree. C. The muffle 74 may be made with
alumina, but can be any material that will withstand relatively
high temperatures and will not react with gas flowing through the
external furnace 70.
[0049] Accordingly, the external furnace 70 can provide a purge gas
containing carbon monoxide without the risks inherent in
maintaining a reservoir of carbon monoxide.
[0050] The purge gas preferably contains only as much carbon
monoxide as is necessary to provide an oxygen concentration that
promotes active oxidation. The amount of carbon monoxide required
can be theoretically determined by, for example, calculating the
amount of carbon monoxide required to cause P.sub.O2 (after
introducing carbon monoxide) to be greater than P.sub.O2 (before
introducing carbon monoxide). Present zirconia muffle furnaces
require approximately 2 to 5% carbon monoxide in the purge gas to
meet this requirement. Also, the necessary amount of carbon
monoxide can be determined by measuring the oxygen concentration in
the drawing portion and adjusting the amount of carbon monoxide
until the appropriate oxygen concentration is achieved. It is
presently contemplated that a delta-F electrolyte detector can be
used to measure the oxygen concentration in the drawing
portion.
[0051] A conventional drawing mechanism (not shown) can be used to
draw a fiber from the blank in the environment in the drawing
portion. A slow drawing speed is better for ablating contaminants,
but the particular drawing speed chosen can also depend on other
factors such as the furnace type and the product type.
[0052] A holding oven has been designed to prove the efficiency of
the above-described process. This holding oven and its use in
conjunction with a drawing furnace are disclosed and claimed in a
U.S. Application by J. E. Dickinson, D. J. Wissuchek, J. A. Snipes,
J. L. Dunn, B. W. Reding, and G. S. Glaesemann and entitled
Apparatus and Method for Inhibiting Passive Oxidation of a
Contaminant in a Blank Used for Drawing an Optical Waveguide Fiber
(Attorney docket no. A-8614), filed concurrently herewith, the
disclosure of which is hereby incorporated by reference.
[0053] A passivation layer formed on a contaminant before a blank
enters the draw furnace may inhibit corrosion of the contaminant by
active oxidation in the drawing process. The passivation layer
hinders the reaction by creating a diffusive barrier for oxidation
reactants and products. For example, the reaction rate for the
corrosion of silicon carbide and silicon nitride is governed by the
rate of diffusion of carbon monoxide or nitrogen through the
passivation layer.
[0054] Thus, for blanks having a contaminant with a passivation
layer, the draw process must supply sufficient time under active
oxidation conditions to ablate the contaminant with its passivation
layer. If the passivation layer is sufficiently thick, the drawing
process may not fully remove the contaminant or may remove it so
slowly that the process is not practical.
[0055] The improved holding oven inhibits passive oxidation of
contaminants and prevents the formation of a passivation layer. An
embodiment of the improved holding oven is shown in FIG. 2 and is
designated generally by reference numeral 50. Holding oven 50 is a
conventional holding oven that has been modified to provide an
environment that inhibits passive oxidation of contaminants.
Holding oven 50 includes a compartment for storing a blank, and a
supply device that supplies gas to the compartment that provides an
environment in the compartment that inhibits passive oxidation of a
refractory contaminant of the blank.
[0056] As shown herein, compartment 52 for storing blank 30
includes muffle 54 that is centered by centering ring 56 and top
seal 58. The top of compartment 52 is covered by top seal 58 and
cover 60. A handle 62 extends through cover 60 to hold blank 30.
Heaters and insulation 64 maintain the compartment 52 at an
appropriate temperature, preferably about 950.degree. C.
[0057] In the form shown, the supply device includes a pipe 66 that
extends into compartment 52 through top seal 58. Pipe 66 is
connected to a gas reservoir 68 and supplies the gas from reservoir
68 to compartment 52, thereby creating an environment that inhibits
passive oxidation of the contaminant.
[0058] The gas in reservoir 68 preferably is commercially pure
argon (Ar), which has an oxygen concentration of less than 0.1 part
per million (ppm). Argon provides a clean environment by preventing
other impurities from getting onto the blank. Also, argon weighs
more than air and, therefore, will remain in an uncovered
compartment, other benign gases can be selected, such as
commercially pure nitrogen (N.sub.2), which has an oxygen
concentration of approximately 80 ppm.
[0059] The argon gas is preferably flowed through the compartment
at a constant flow rate of 0.5 to 1.0 standard liters per
minute.
[0060] It will be apparent to those skilled in the art that various
modifications and variations can be made in the method and
apparatus of the present invention without departing from the scope
or spirit of the invention. For example, although a preferred
embodiment has been described with reference to the drawing of
optical waveguide fibers from silica-containing blanks, certain
aspects of the invention may be applied to the drawing of fibers of
other suitable materials. As a further example, although the
invention has been described with reference to silicon carbide and
silicon nitride contaminants, the invention may be used for other
oxidizable, refractory contaminants, such as tungsten carbide.
[0061] Other embodiments of invention will be apparent to those
skilled in the art from consideration of the specification and
practice of the invention disclosed herein. It is intended that the
specification and examples be considered as exemplary only, with a
true scope and spirit of the invention being indicated by the
following claims.
* * * * *