Note: Descriptions are shown in the official language in which they were submitted.
Backgro~md of the Invention
Field of the Invention
This invention relates to a me~hod of form;ng, by the
flame hydrolysis technique, high optical purity blanks fr~m
which high quality optical waveguides, lenses, prisms and
the like can be made. This invention is particularly appli-
cable to optical waveguides whi.ch m~st be formed fro~
extremely pure materials.
Optical waveguides, which are the most promising medium
for transmission of signals around 1~15 ~z, normally consist
of an optical fiber having a transparent core surrounded by
transparent cladding material having a refractive index
lower than that of the core.
., .
.
... . .. ..
.
1100~01
The stringent optical requirements placed on the trans-
mission medium to be employed in optical communication
systems has negated the use of conventional glass fiber
optics, since attenuation therein due to both scattering and
impurity absorption is much too high. Thus, unique methods
had to be developed for preparing very high purity glasses
in fiber optic form. Glass preparation techniques which
have shown much promise are based on the flame hydrolysis
process which employs vapor phase reaction of high purity
vapors. This approach to the formation of low loss optical
waveguides is based on methods described in U.S. Patents
Nos. 2,272,342 and 2,326,0~9 issued to J. F. Hyde and M. E.
Nordberg, respectively. The flame hydrolysis technique has
been employed to prepare single mode waveguides and multi-
mode waveguides of both the step-index and graded-index
type. Various methods employing the flame hydrolysis technique
for forming glass optical waveguide fibers are taught in
U.S. Patents Nos. 3,737,292; 3,823,995 and 3,884,550.
The usefulness of glass optical waveguides in optical
transmission systems depends upon the attainment of very low
loss transmission over the entire wavelength range of about
700-110~ nm. This can be achieved by reducing attenuation
due to optical scattering and absorption to a level which
approaches the minimum theoretically attainable level.
Waveguides in which at least 80% of the scattering loss can
be accounted for by intrinsic glass scattering have been
made by the aforementioned flame hydrolysis technique.
However, due to the presence of residual water produced by
this technique, absorption losses betweèn 700 nm and 1100 nm
have been excessively large. By residual water in glass is
meant that the glass contains a high level of OH, H2 and
11~ QO~ 1
H2O. One explanation of residual water may be found in U.S.
Patent No. 3,531,271 to W. H. Dumbaugh, Jr. The maximum
attenuation in the aforementioned wavelength range that is
attributable to residual water occurs at about 950 nm. The
remaining portion of the attenuation at 950 nm, which is due
to factors such as intrinsic material scattering, amounts to
about 3 dB/km. For example, a glass optical waveguide
having an attenuation less than 6 dB/km at 800 nm may have
an attenuation greater than 100 dB/km at 950 nm due to the
presence of water therein. To be useful in optical communi-
cation systems, optical waveguide attenuation is preferably
less than 10 dB/km at the wavelength of light being propa-
gated therein. In order to achieve such low attenuation
over the entire range between 700 nm and 1100 nm, a glass
waveguide fiber must be rendered substantially water-free,
i.e., the amount of residual water within the fiber must be
reduced to a level of less than 10 ppm.
Description of the Prior Art
Since it is impossible to reduce the water content to
acceptable levels after flame hydrolysis-produced soot has
been consolidated to form a solid glass coating, the water
must be removed before or during the consolidation process.
Heretofore, various methods were employed to reduce the
water content in optical waveguides produced by flame
hydrolysis. Such disadvantages as long processing times,
equipment problems and incomplete water removal were
encountered.
One prior ~rt method that has been very effective in
reducing the water contenc in fused silica produced by the
flame hydrolysis process is disclosed in U.S. Patent No.
--3--
3,933,454. In accordance with that patent a soot preform
produced by the flame hydrolysis process is consolidated by
inserting it into a consolidation furnace wherein the soot
preform is heated to a temperature within the consolidation
temperature range for a time sufficient to cause the soot
particles to fuse and form a dense glass layer. The soot
preform is simultaneously subjected to a stream of a sub-
stantially dry chlorine containing gas which flows through
the furnace. The chlorine permeates the interstices of the
soot preform during the consolidation thereof and replaces
hydroxyl ions by chlorine ions, thereby resulting in a glass
article that is substantially water-free. However, prior to
making contact with the soot preform, the chlorine containing
gas can react with the walls of the consolidation furnace to
produce volatile compounds such as iron chlorides which can
then contaminate the preform. Thus, while the resultant
glass article exhibits very little excess attenuation at 950
nm due to water absorption, the overall attenuation thereof
across the entire visible spectrum is increased due to
impurities transported by the drying gas.
SummarY of the Invention
It is therefore an object of the present invention to
provide an effective and economical method of removing
residual water from a flame hydrolysis-deposited glass soot
preform during the consolidation process. A further object
is to provide a method of forming optical waveguides having
extremely low concentrations of water and contaminants.
Briefly, the present in~ention relates to an improved
method of forming a glass article by the flame hydrolysis
process. This process conventionally comprises the steps of
--4--
11~1Q001
depositing on a starting member a coating of flame hydrolysis-
produced glass soot to form a soot preform and consolidating
the soot preform to form a dense glass layer. The consolida-
tion step conventionally comprises subjecting the soot
preform to a temperature in the consolidation temperature
range, for a time sufficient to permit the soot particles to
fuse and consolidate, thereby forming a dense glass layer
which is free from particle boundaries. It is also conven-
tional to subject the preform to an atmosphere such as
helium, oxygen, argon, neon or mixtures of these gases, or
even to a reduced pressure for the purpose of removing gases
from the preform interstices during consolidation to thereby
reduce the number of seeds in the resultant glass article.
In connection with the fusing of glass soot particles formed
by flame hydrolysis, this process is some~imes referred to
as sintering even though no particle boundaries remain.
In accordance with the present invention, the step of
consolidating is characterized in that it comprises dis-
posing the preform in a furnace wherein it is sub~ected to
a temperature within the consolidation temperature range for
a time fiufficient to cause the soot particles to fuse and
form a dense glass layer. Simu~taneously, a stream of an
stmosphere containing a drying agent is flowed through the
interstices of the porous soot preform, that portion of the
stream which contacts the furnace being prevented from
thereafter contacting the preform.
In accordance with a preferred embodiment of this
invention the starting member is removed prior to the con-
solidation step. During the consolidation process, the
stream which contains the drying agent is flowed into the
aperture formed by removal of the starting member. The
_5_
stream thereafter flows outwardly from the center of the
preform through the interstices therein to the outer surface
thereof.
Brief Description of the Drawings
Figures 1 and 2 illustrate the application of first and
second coatings of glass soot to a starting member.
Figure 3 is a graph which shows the attenuation curves
of a plurality of optical waveguide fibers.
Figure 4 is a schematic representation of a consolida-
tion furnace and consolidation atmosphere system.
Detailed Description of the Invention
It is to be noted that the drawings are illustrativeand symbolic of the present invention and there is no inten-
tion to indicate the scale or relative proportions of the
elements shown therein. For the purposes of simplicity, the
present invention will be substantially described in connec-
tion with the ~ormation of a low loss optical waveguide
although this invention is not intended to be limited thereto.
Referring to Figure 1, a coating 10 of glass soot is
applied to a substantially cylindrical starting member such
as a tube or rod 12 by means of flame hydrolysis burner 14.
Fuel gas and oxygen or air are supplied to burner 14 from a
source not shown. This mixture is burned to produce flame
16 which is emitted from the burner. The vapor of a hydro-
lyzable compound is introduced into flame 16, and the gas-
vapor mixture i~ hydrolyzed within the flame to form a glass
soot that lea~es flame 16 in a stream 18 which is directed
toward starting member 12. The flame hydrolysis method of
forming a coating of glass soot is described in greater
~1~ 000 1
detail in the aforementioned U.S. Patents Nos. 3,737,292;
3,823,995 and 3,884,550. Starting member 12 is supported by
means of support portion 20 and is rotated and translated as
indicated by the arrows adjacent thereto in Figure 1 for
uniform deposition of the soot. It is to be understood that
an elongated ribbon burner, not shown, that provides a long
stream of soot could be used in place of the substantially
concentric burner illustrated in Fi~ure 1 whereby starting
member 12 would only have to be rotated. Further, a plurality
of burners 14 could be employed in a row to similarly require
only rotation.
To form a step-index optical wa~eguide, a second coat-
ing 26 of glass soot may be applied over the outside peri-
pheral surface of first coating 10 as shown in Figure 2. To
form a gradient index fiber, a plurality of layers of glass
soot are appiied to the starting member, each layer having a
prog~essively lower index of refraction as taught in U.S.
Patent No. 3,823,995.
In accordance with well known practice the refractive
index of coating 26 is made lower than that of coating 10 by
changing the composition of the soot being produced in
flame 16. This can be accomplished by changing the concen-
tration or type of dopant material being introduced into the
flame, or by omitting the dopant material. Support member
20 is again rotated and translated to provide a uniform
deposi~ion of coating 26S the composite structure inc,uding
first coating lO and second coating 26 constituting an
optical waveguide preform 30.
Since glass starting member 12 is ultimately removed,
the material of member 12 need only be such as to have a
composition and coefficient of expansion compatible with
1 10 QOO ~
the material of layer 10. A suitable material may be a
normally produced glass ha~ing a composition similar to that
of the layer 10 material although it does not need the high
purity thereof. It may be normally produced glass having
ordinary or even an excessive level of impurity or entrapped
gas that would otherwise render it unsuitable for effective
light propagation. The starting member may also be formed
of graphite, low expansion glass or the like.
In the manufacture of optical waveguides, the materials
of the core and cladding of the waveguide should be produced
from a glass having minimum light attenuation characteristics,
and although any optical quality glass may be used, fused
silica is a particularly suitable glass. For structural and
other practical considerations, it is desirable for the core
and cladding glasses to have similar physical characteristics.
Since the core glass must have a higher index of refraction
than the cladding for proper operation, the core glass may
desirably be formed of the same type o glass used for the
cladding and doped with a small amount of some other material
to slightly increase the refractive index thereof. For
example, if pure fused silica is used as the cladding glass,
the core glass can consist of fused silica doped with a
material to increase its refractive index.
There are many suitable materials that can satisfac-
torily be used as a dopant alone or in combination with each
other. These include, but are not limited to, titanium
oxide, tantalum oxide, tin oxide, niobium oxide, zirconium
oxide, aluminum oxide, lanthanum oxide, pho~phorus oxide and
germanium oxide. Optical waveguides can also be made by
forming the core from one or more of the aforementioned
dopant oxides, the cladding being made from one or more
-8-
materials having a lower refractive index. For example, a
core made of pure germanium oxide may be surrounded by a
cladding layer of ~used silica and germanium oxide~
The flame hydroIysis technique results in the formation
of glasses having extremely low losses due to scattering and
impurity adsorption. Optical waveguides made by this
technique have exhibited total losses as low as 1.1 dB/km at
1,060 nm. However, even when optical waveguides are formed
-~ of glasses having such high op~ical quality, ligh~ attenua-
tion at certain regions of the wavelength spectrum may be so
great as to preclude the use of such waveguides for the
propagation of light'in those regions. For example, an
optical waveguide having a core of 81 w~./O SiO2, 16 wt.%
GeO2 and 3 wt.% B2O3 and a cladding of 86 wt.% SiO2 and L4
wt.% B2O3 was made by the ~lame hydrolysis process, no
attempt being made to remove water therefrom. A~tenuatior.
curve 36 for this fiber is illustrated in Figuré 3. Water
was responsible for such an excessive attenuation in the
700-1100 nm region that the waveguide was useless for the
propagation of optical signals at most wavelengths within
that region. At 950 nm the attenuation was about 80 d~/km.
Various oxides from which such glass optical waveguides are
formed, especially SiO2, have a great affinity for water.
However, after such glass waveguides are completely ~ormed,
the inner, light propagating portion thereof is inaccessible
to water~ The tendency of these glasses to absorb water is
not detrimental to water-~ree glass optical waveguides ater
they are formed since most o the light energy is propagated
in and around the core, and the presence o water on the
outer surface has a negligible ~ffect on the propagation o~
- ~uch energy. However, in the ormation of optical waveguides
_g_
1100~0~
by flame hydrolysis, residual water, which is produced by
the flame, appears throughout those portions of the wave-
guide that have been produced by flame hydrolysis. Also,
water is readily adsorbed by the soot during handling in air
prior to the consolidation process because of the extremely
high porosity thereof.
The starting member is removed from the soot preform so
that a gas conducting tube can be affixed to an end of the
preform. This can be accomplished by merely securing the
preform while the handle is pulled therefrom. Preform 30 is
then suspended from tubular support 50 as shown in Figure 4.
Two platinum wires, of which only wire 52 is shown, protrude
through preform 30 on opposite sides of aperture 54 and are
affixed to support 50 just above flange 56. The end of gas
conducting tube 58 protrudes from tubular support 50 and
into the adjacent end of preform 30. The preform is con-
solidated by gradually inserting it into consolidation
furnace 60. It is preferred that the preform be subjected
to gradient consolidation, a technique taught in t-ne afore-
mentioned U.S.-Patent No. 3,933,454 wh~reby the bottom tip
of the preform begins to consolidate firs~, the consolidation
continuing up the preform until it reaches that end thereof
adjacent to tubular support 50.
In accordance with the present iDvention an optical
waveguide preform is dried by subjecting the preform to a
high purity drying agent during the c~nsolidation process.
The purity of the drying agent is m~intained by preventing
it from contacting the refractory walls of the consolidation
furnace prior to coming into contact ~ith the preform. The
preferred method of maintaining the p~ity of the drying
~gent involves flowing a stream of an atmosphere containing
- ' - 10: ' -
~ lO ~Q~lthe drying agent into the center of the preform and through
the porous preform walls to the outside surface thereof.
The resultant gases are flushed a~-ay from the blank by a gas
such as helium, oxygen, argon, neon or mixtures thereof.
Thus, the drying agent is unable to transport impurities
from the furnace muffle to the preform. This method there-
fore results in an optical waveguide fiber that is essentially
water free, thereby exhibiting low loss at 950 nm, and which
exhibits low loss at other wavelengths as well. Examples of
drying agents which may be employed are C12, SiC14, GeC14,
BC13, HCl, POC13, PC13, TiC14 and AlC13. Compounds of the
other halogens such as bromine and iodine sho~-ld also be
effective. The particular drying agent to be employed is
im~aterial insofar as the drying process is concerned.
However, such preform properties as refractive index, thermal
coefficient of expansion and the like should be considered
in the selection of the drying agent. In one embodiment of
the type illustrated in Figure 4 the drying agent was delivered
to the center of the soot preform by a system comprising
only glass, Teflon~and a minimum 2mount of stainless steel,
and thus, the drying ag~t was maintained substantially free
from contgmination prior to contacting the preform.
The consolidation temperature depends upon the com-
position of the glass soot and is in the range of 1250-
1700C. for high silica content soot. It is also time
dependent, consolidation at 1250C. requiring a very long
time. The preferred consolidation temperature for high
- silica content soot is between 1350C. and 1450C. Other
glasses can be consolidated at lower temperatures, pure
germania, for example, consoLidating at about 900C.
$ ~P,~
11~00~1
Referring again to Fi~ure 4 the vertical sidewalls of
furnace 60 are broken to illustrate that the relative depth
thereof is greater than that shown. In this figure flow
regulators are schematically represented by the letter "R"
within a circle and flowmeters by the letter "F" within a
rectangle. Sources ~2 and 64 of oxygen and helium, respec-
tively, are connected to orifices 66 in the bottom of fur-
nace 60. Undulated arrows 68 represent the flow of the
flushing gas from the orifices. Sources 72 and 74 of helium
and oxygen, respectively, are connected to containers of
SiC14 and GeC14, respectively, so that helium, oxygen, SiC14
and ~eC14 are present in line 76. Additional helium is
coupled to line 76 by line 78.
The consolidation atmosphere system of Fi~ure 4 is
- merely representative of a number of systems which may be
employed to provide the consolidation furnace and preCorm
with appropriate gas and vapor mixtures. The flushing gas
could be caused to flow from top to bottom of furnace 60.
The system illustrated, whereby flushing gas flows into the
2Q bottom of furnace 60, is preferred since gas naturally tends
to flow upwardly through the furnzce. Also, many different
arrangements may be employed to provide the desired drying
gas mixture, and the present invention is not limited to the
arrangement illustrated in Figure 4. It is only necessary
to provide tube 58, and ultimately preform 30, with the
desired drying gas mixture, the particular means employed to
achie~e this mixture being immaterial.
As indicated by arrow 80, preform 30 is inserted do~-
wardly into furnace 34. The rate of insertion is preferably
3~ low enough to permit the tip of the preform to consolidate
first, the consolidation process then continuing up the
ll~QO~
preform until it reaches that end of the preform adjacent to
tubular support 50. The maximum furnace temperature, which
is preferably between 1350C and 1450C for high silica
content soot, must be adequate to fuse the particles of
glass soot and thereby consolidate the soot preform into a
dense glass body in which no particle boundaries exist.
As soot preform 30 enters furnace 60 the drying gas
passes through tube 58 into preform aperture 54 from which
it passes into and through the interstices of the preform as
indicated by arrows 82. Successful drying of the soot
preform has been achieved by employing well known drying
agents such as chlorine gas and SiC14. However, since the
inner portion of a soot preform that is to be formed into an
optical waveguide fiber contains a dopant to increase the
refractive index thereof, the application of such conventional-
drying agents to the preform aperture where the dopant
concentration is greatest causes leaching of the dopant from
the preform. This results in a decrease in the refractive
index of the glass at the center of the resultant optical
waveguide. Although the resultant fî~er functions as an
optical waveguide, certain properties thereof may be adversely
affected, especially in the case of ~aded index fibers.
Therefore, the drying gas preferably contains a component
that will, upon reaction in the pref~m, produce that dopant
oxide, the concentration of which te~s to be reduced by the
aforementioned leaching action. The Fequired amount of the
compensating component depends upon ~2rious factors including
the concentration of the dopant oxid~ at the center of the
soot preform. It has also been found that excessive amounts
o the compensating component in th~ d~ying gas can cause
the formation of a thin layer of a g~ass rich in the dopant
'13-
11~0001
oxide at the inner surface of the hollow preform. This cancause breakage due to unbalanced stresses in the resulting
consolidated blank. Moreover, even if a fiber can be drawn
from such a blan~, the refractive index at the center thereof
may be excessively high due to the high concentration of
dopant oxide at the fiber center. Thus, for each different
waveguide composition the amount of the compensating c~mponent
in the drying gas will have to be determined, determination
by empirical means having been found to be satisfactory.
The excess attenuation at 950 nm for fibers drawn from
untreated fibers is typically 50-100 dB/km or more. Curve
38 of Figure 3 illusèrates the attenuation vs. wavelength
curve for one of the best fibers produced by this method.
It has been observed that treatment in accordance with the
method of the present invention also decreases the attenuation
of cor.solidated soot blanks by nearly ~ dB/km at each of the
standard measurement wavelengths of 630, 800, 820 and 1060
nm. The improvement over the prior art can be seen by
comparing curve 38 of Figure 3 with curve 34, which illus-
trates the loss characteristics of a waveguide fiber formed
from a soot preform dried in accordance with the teachings
of ~.S. Patent ~o. 3,933,454. The drying process of the
present invention appears to either decrease the light
scattering property of the glass or remove metallic impuri-
ties by forming volatile compounds th~reof which are then
flushed from the soot preform. Thus, in addition to pro-
viding substantial drying of a soot p~eform, the method of
the present invention also decreases ~ttenuation at wave-
lengths which are unaffected by water.
Excessive amounts of SiC14 and ~æC14 tend to increase
the overall attenuation of the fiber, probably either by
-14-
llO~Q~l
contaminating the preform or by changing the oxidation state
of impurities always in a blank. Excessive amounts of
oxygen (insufficient helium) cause the blank to be seedy.
Insufficient oxygen causes the attenuation of the fiber to
increase because of oxidation state changes of the impurities.
The method of the present invention broadly involves
flowing a gaseous drying agent through the interstices of a
soot glass preform while simultaneously preventing that
drying agent from contacting the walls of the furnace in
which the preform is being consolidated. In the embodiment
of Fi~ure 4 this is accomplished by flowing the drying agent
into the top of the preform aperture and flushing it away
with a counter-current flow of flushing gas supplied to tke
bottom of the furnace. The flushing gas could be supplied
to the top of the consolidation furnace and thus flow in the
same direction as the drying agent, means being provided at
the bottom of the furnace for removing both drying and
flushing gases. In another variation of the method of this
in~ention, a laminar flow of drying agent is provided at the
bottom of the furnace. As the drying gas encounters the
soot preform, that gas flowing between the preform and the
furnace wall acts as a buffer to prevent the gas which has
contacted the wall from entering the preform. In accordance
with a modification of this last mentioned embodiment, a
"guard flow" of inert gas is introduced at the bottom of the
furnace near the vertical wall, this "guard flow" tending to
increase the shielding of the preform from impurities in the
furnace refractories.
The invention will be further described with reference
3~ to specific embodiments thereof which are set forth in the
following examples. In these examples, which pertain to the
-15-
110~
manufacture of optical waveguides, the inside diameter ofthe furnace muffle is 3 1/4 inches and the length thereof is
50 inches. In all examples a flushing gas mixture comprising
20 l/min helium and 500 ml/min oxygen was supplied to the
bottom of the furnace as shown in Figure 4.
Example 1
A tubular starting member of fused quartz, approximately
0. 6 cm in diameter and about 50 cm long is secured to a
handle. Liquid SiC14, liquid GeC14 and BC13 are maintained
at 35C, 47C and 20C in first, second and third containers,
respectively. Dry oxygen is bubbled through the first
container at 2000 cc per minute and through the second
container at 200 cc per minute. BC13 is metered out of the
third container at 60 cc per minute. The resultant vapors
entrained within the oxygen are combined and passed through
a gas-oxygen flame where the vapor is hydrolyzed to form a
steady stream of particles having a composition of 16% by
weight GeO2, 3% by weight B203 and 81% by weight SiO2. The
stream is directed to the starting member and a soot coating
comprising particles of this composition is applied up to
about 2.5 cm in diame~er. A second coating of 86 wt.% SiO2
and 14 wt.% B2O3 is then applied over the first soot coating
by terminating the flow of oxygen to the liquid GeC14 and
adjusting the flow of BC13 out of the third container to 300
cc per minute while maintaining the flow of oxygen through
the first container at 2000 cc per minute. This cladding
soot is applied until an outside diameter of approximately 5
cm is attained. The starting member is pulled from the soot
preform, thereby leaving a soot preform weighing 450 g and
having a diameter of 5 cm and a length of 50 cm. The drying
-16-
1100~1
gas tube 58 of Figure 4 is inserted into the preform aperture
which has a diameter of about 0.6 cm. Platinum wire is
employed to attach ,he upper end of the preform to a tubular
support. - `
The gases and vapors constituting the drying gas flow
into the preform aperture at the following rates: 30
ml/min oxygen, 3 ml/min SiC14 vapor, 3.7 ml/min GeC14 vapor
and 1500 ml/min helium. This mixture is obtained by main-
taining the SiC14 and GeC14 at 25C and by bubbling helium
through the SiC14 at 6 ml/min and oxygen through the GeC14
at 30 ml/min and flowing helium at a rate of 1.5 l/min.
through bypass line 78. At 25C the vapor pressure of the
SiC14 is about 240 Torr so that the helium picks up about 3
ml/min of SiC14 vapor, and the vapor pressure of the GeC14
is about 85 Torr so that the oxygen picks up about 3.7
ml/min of GeC14 vapor.
As the drying gas mixture flows into the preform aper-
ture, the preform is lowered into the furnace at about 0.5
- cm per min, the maximum furnace temperature being about
1380C.
The preform is completely consolidated in about 90 min.
The resultant dense glass body is withdrawn from the furnace
and cooled. The resultant structure is drawn at a temperature
of about 1800C to collapse the central hole and decrease
the outside diameter thereof. Drawing is continued until
the final waveguide diameter of 125 ~m is achieved, the core
diameter being about 62 ~m. Waveguide attenuation at standard
measurement wavelengths of 630, 800, 8~0, 900 and 1060 nm is
8.7, 3.4, 3.2, 2.7 and 1.6 dB/km, respecti~ely. The excess
absorption due to water at 950 nm is estimated to be about
17 dB/km. In this example and in some of the examples set
1 100 Q~ 1
forth in Table I below the attenuation at 950 nm is estimated
by measuring the attenuation at 820 nm and 900 nm. The
estimated attenuation at 950 nm is then determined by the
equation
A950 = 33(Agoo~A820 ~ l. )
There is a small spike in the refractive index profile at
the center of the fiber due to the formation of a small
excess of GeO2 at the center of the fiber during the drying
process.
The specific drying agent employed in Example 1 is
effective in the drying of optical waveguides due to the
occurrence of several chemical reactions. First, the SiC14
and GeCl4 react with oxygen to form chlorine according to
the equations:
SiC14 + 2 ~ SiO2 + 2 C12
GeC14 ~ 2 ~ GeO2 + 2 C12
The chlorine formed in these reactions removes hydroxyl
groups from the glass according to the reactions:
2~SiOH + C12 ~SiOSi- + 2 HCl + 12 2
where ~SiOH denotes that the silicon atom is connected to
three other parts of the glass network.
~xam~les 2-9
Gas flow rates and optical waveguide attenuation values
for Examples 2-9 are set forth in Table I. Each of these
examples employs the same type of optical waveguide as
Example 1 except Examples 5 and 6 in which graded index
fibers are formed. The fibers in these two examples have
the same cladding and axial compositions as the waveguide of
-18-
Example 1. However, in Examples 5 and 6 the amount of GeO2
gradually decreases between the fiber axis and the cladding.
Also, Examples 5 and 6 employ the same soot preform consoli-
dation process as described in Example 1 except for the
drying gas mixture. Example 11 of Table I refers to an
untreated fiber.
-19-
ll~
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U~ O
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O E~ ~ --I ~ ~ ~ O X_I ~ ~-1 ~ ~ ~
O n~ O hC ~ O~15 0 ~ OLl O S~ O
O O O ~ 14 ~ F ~ ~ h
O ~
C~ ~ er *
O~ O U~~ ~ o C~ o o
XV ~ t~S 3 E~
1 3 _I
~ Ln
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C ~ ~ ~
~ o ~ O ~ O
~ o
~ . . . . . . . . . ~1
~ l ~;r er ' ~ ~ ~r o ~ o
H ,~ O . . . I . . . ,_1
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~ I C ~ ` V
~ a~ C ~ e ~ ~ ~ 3 G C
_I ~1 ~ ~ S~
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O S: ~ ~ ~ u~u~ u~ In In U~ ~1
~4 ~ ~ ~ ,1 ~ ~ ~ ,~ ~ ~1 0
o~
~ e
C ~ C ~
.,, .,, .,. ~, ~
~ e ~ ~ V
~ ~r ~
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' . S ~ I ~ ~ ~ C
O O O O ~ C SJ
C C
C C C ~ o
Q ~ e ~ ~ ~ c
Q3 ~ ~ ~ ~ ,,
~ ~ ~ ~ ~ I ~ e
.~ ~ R ~ ~ Q
U~ U~ In ~ U~
o~ O o
r~
a)l
~ ~ ~r u~ ~ o _~
--20--
Q~l
It is to be noted that attenuation curve 38 of Fi~ure
3 pertains to the optical waveguide formed in accordance
with Example 6.
As indicated hereinabove a balance of SiCl4 and GeC14
is required in the drying and consolidation of germania
doped optical waveguides to prevent distortion of the refrac-
tive index profiles thereof. An excess of SiC14, for example,
is believed to cause leaching of GeO2 from the soot preform
according to the reactions:
SiC14 + 2 ' SiO2 + C12
C12 + GeO2 (glass) ) GeOC12 or GeC14 or
other volatile germa-
nium products
or
SiC14 + GeO2 (glass) ~ SiO2 (glass) + GeC14
On the other hand, excessive amounts of GeC14 cause a
deposition of a thin layer (50-100 ~m thick) of a glass rich
in GeO2. This thin layer of glass has a higher expansion
than the bulk of the blank. On cooling cf ~he bLank, this
layer goes into tension ~nd often causes blank breakage
because of unbalanced stresses. For example, several blanks
were dried during consolidation by applying to the preform
aperture a gas-vapor mixture obtained by bubbling about 8
ml/min oxygen through GeC14 at 25C and mixing the resultant
oxygen-vapor mixture with 1.5 l/min helium. Most of the
resuitant optical waveguide blanks broke because of the high
stress introduced therein during the dryin~ process.
Without special care, such breakage approaches 80%. If the
blank is not cooled between the consolidation process and
the process of drawing the resultant blank into a fiber,
- -21-
1 l~QQ~ i
breakage is not a problem, but this technique is very
inconvenient.
The amount of water remaining in the consolidated blank
is a function of the amount of water initially present in
the soot preform. This variable can be eliminated by con-
- trolling the humidity of the atmosphere to which the soot
preform is subjected from the time of soot deposition until
the consolidation is completed. Then, by varying the amount
of chlorine or other drying agent present in the atmosphere
flowing rom tube 58 to preform aperture 54, the amount of
water removal can be controlled. Also, by controlling the
amount of dopant in such atmosphere, the adverse effect of
the drying process on the refractive index profile of the
resultant optical waveguide fiber can be minimized.
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