Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Back~3round of the Invention
- ~ The pr~sent invention relate~ to optical waveguide
filaments, and more particularly to an improved ~ethod of
forming bl~nks fr~m which such filame~ts are drawn~
Optical waveguides, w~ich are the most promising
medi~n for use in optical communication ~ys~em~ operatirlg in
the visible or near visiblP spectra, normally con`sist o an
op~ical ~ilament ha~ving a transparen~ core surrollnded by a
transparen~ cladding material having a refractive index
lower ~han thal: of ~he eore.
The stringent optical requi rements placed on ~he
transmisslon medium to be em~loyed in optical commNnlcations
systems has negated the use of convention~l gla~s ~iber,
~l28739
op~ics, since attenuation therein due ~o both scattering and
impurity absorption is much too high. Thus, unique methods
had to be developed ~or preparing very high puriky glasses
in filamentary form. Certain glass making processes, particu-
larly vapor deposition processe~, have been co~monly employed
in the formation of optical wa~eguide blanks. In one such
process, the source material vapor is directed into a heated
tube wherein it reacts to orm a material which is deposited
in successive layers. The combination of deposited glass
and tube is collapsed to form a draw blank which can be
later heated and drawn into an optical waveguide fila~ent.
In order to obtain uniform deposition along the length
of the substrate tube, a serial deposition process has been
employed. Th~at is, reactants are fed into the end o~ the
tube, but deposition occurs only in a narrow section of the
tube which is heated by a flame. The flame moves up and
down the tube to move the reaction and thus the region of
glass deposition serially along the tube.
One of the limitations of such a process is a comparatively
low ef~ecti~e mass deposition rate. To increase the deposition
rate it appears to be necessary to incxease the inside
diameter of the substrate tube to provide a greater collec~ion
surface area. However, since heat is supplied from the
outside of the tube, a larger tube diameter results in a
lower vapor temperature at the axis of the tube. Moreover,
the flow profile across the tube is such that maximum flow
occurs axially within the tube. As tube diameter increases,
a smaller portion of the reactant ~apor flows in that region
of the tub~ adjacent the wall where reaction temperature is
higher and where the resultant soot~ reaction products are
more readily collected on the heated region of the tube.
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I~ is therefore an object of the present invention to
improve the deposition efficiency of a process whereby a
reactant vapor ~lows into and reactq withln a he~ted t~be
to form a layer therein.
Summary of the In~ention
Briefly, the present invention relates to a method and
apparatus for manufacturing a preform which is intended to
be subsequently drawn into an optical filament. This
method is of the type that includes the steps of flowing a
lû vapor mixture including at least one compound, glass- forming pre-
cursor, which is a gas ~ ich when heated forms glass particles, together with
an oxidizing medi~n, through a hollaw, cylindrical substrate, and heating the
substrate and contained ~apor mixture with a heat source
that moves relative to the substrate in a longitudinal
direction, wherPby a moving hot zone is established within
the substrate, such that a suspension of particulate,
oxidic reaction product material is produced within the
hot zone. The particulàte material travels downstream
where at least a portion thereof co~es to rest on the
inner surface of the substrate where it is fused to form
a continuous glassy deposit. The ~mprovement of the pres-
ent invention comprises confining the flow of the vapor
mixture to an annul~r channel adjacent the subs~rate sur-
face in the hot zone whereby-the deposition efficiency of
the vapor mixture reaction is increased.
In accordance with a preferred embodiment of the
present in~ention, a gas condu~ting baffle tube is disposed
in one end of the cylindrical substrate, one end of the
baff~e tube terminating adjaciPnt the hot zone. Means is
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~L128~3~
provided for moving ~he tube longitudinally with respect to
the substrate in synchronism with the movement oE the
heating means whic~ generates the moving hot zone, Gas
emanating from the baf~le tube ~orms a gaseous mandrel ~n
the hot zone which confines the vapor mixture to an ann~lar
channel adjacent the substrate surface.
Brief Description o~ the ~rawings
Figure 1 is a schematic representation of a prior art
apparatus for depositing a glass layer within a tube.
Figure 2 shows a.section of the tube of Figure 1
depicting obser~ed conditions during processing.
Figure 3 is a schematic representation of an apparatus
suitable for practice of the deposition process in accordance
with the present invention.
Figures 4 and 5 are cross-sectional vie~s of the
apparatus of the present in~ention depicting conditions
oc.curring during processing.
Figure 6 shows the end of a modified baffle tube that
can be employed in the apparatus o the present invention.
Descri tion of the Preferred Embodiment
P ~
Figures 1 and 2 show a prior art system comprising a
substrate tube 10 having handle tube 8 affixed to.the
upstream end thereo and exhaust tube 12 affixed to the
downstream end thereof. Tubes 8 and 12 are chucked in a
conventional glass tusning lathe (not shown~, and the
combination i5 rotated as indicated by the arrowD The
~andle tube, w~ich may be omitted, is an lnexpensive glass
tube ha~ing the same diameter as the substrate tube, and it
does not form a part of the resultant optieal wa~eguidP. A
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~873~
hot zone 14 is caused ~o traverse tube 10 by moving heating
means 16 as schematically depicted by arrows 18a and 18~.
Heating means 16 can consis~ o~ any suitable source o heat
such as a plurality o burners encircl~ng tube 10. Reac~ants
are introduced into tube 10 via inlet tube 20, which i5
connected to a plurality of sources of gases and vapors. In
Figure 1, flow meters are represented by a circle having the
letter "F" therein. A source 22 of oxygen is connected by
flow meter 24 to inlet tube 20 and by flow meters 26, 28 and
30 to reservoirs 32, 34 and 36, respectively. A sour~e 38
of boron trifluoride is connected to tube 20 by a flow meter
40. Reservoirs 32, 34 and 36 contain normally liquid
reactant materials which are introduced into tube 10 by
bubbling oxygen or other suitable carrier gas therethrough.
Exiting material is exhausted through exhaust tube 12. Not
shown is an arrangement of mixing valves and shutoff valves
which may be utilized to meter flows and to make other
necessary adjustments in composition.
Burner 16 initially moves at a low rate of speed rela-
tive to tube 10 in the direction of arrow 18bS the same
direction as the reactant flow. The reactants react in hot
zone 14 to produce soot,-i.e., a powdery suspension of
particulate o~idic material, which is carried downstream to
region 42 of tube 10 by moving gas. In general, between
twenty and seventy percent of reaction product produced in
the vapor str~am removed from the substrate surface and
results in deposited soot having the desired glass composi-
tion.
It is noted that essentially no soot is for~ed in
region 46 of tube 10 upstream rom hot zone 14. As burner
16 continues to move in the direction of arrow 18b, hot zone
14 moves downstream so tha~ a part of soot buildup 44 extends
into the hot zone and is consolidated thereby to form a
unitary, homogeneous glassy layer 48. Such process parameters
as temperatures, flow rates, reactants and t~e like can be
found in the publications J. B. MacC~esne~ et al., Proceedings
of the IEEE, 1280 (1974) and W. G. French et al., Applied
Optics, 15 (1976~. Reference is also made to the text V~
Deposition Edited by C. F. Powell et al. John Wil~y and
Sons, Inc. (1966).
When burner 16 reaches the end of tube 10 adjacent to
exhaust tube 12, the temperature of the flame is reduced and
the burner returns in the direction of arrow 18a ~o the
input end of tube 10. T~erea~ter, additional layers of
glassy material are deposited ~ithin tube 10 in the manner
described above. A~ter suitable layers have been deposited
to serve as the cladding andlor core material of the resultant
optical wa~eguide filament, the temperature of the glass is
increased to about 2200G ~or hig~ silica content glass to
cause tube lO to collapse. This can be accomplished by
reducing the rate of traverse o the hot zone. The resultant
draw blank is then d~a~n in accordance ~ith well known
tec~niques to form an optical waveguide filament having the
desired diameter.
To optimize the process from the standpoint of reaction,
high temperatures are utilized. -For the usual silica based
system~ temperatures at the substrate wall are generally
maintained betweèn about 1400 and 1900C at the moving
position co~responding with the hot ~one. Indicated tem-
peratures axe those measured by a radlation pyrometer
focused at the outer tube surface.
~Zt~73~
It îs co~monly known that one of the ~actors whlch
limits deposition rate is the rate of sintering deposited
soot to form a transparen~ glass layer. For a given com-
position of glass to be deposited, there is a ma~imum layer
thickness of glass that can be sintered using the optimum
combination of hot zone width, peak temperature of the hot
zone and burner traverse rate. If the thickness o the
sintered glass layer can be kept to the maximum value for
different tube diameters, deposition rate increases propor-
tionately with tube inside diameter because of increased
surfaee area. However, because of the nature of 10w dynamics
of the reactant vapor stream and soot particle dynamics, the
percentage vf soot produced which deposits in the substrate
tube decreases with increased tube diameter, thereby causing
an effecti~e decrease of deposition rate.
In accordance with the present i~vention means is
provided for confining the flow of reactants to an annular
channel adjacent the wall of the substrate tube in the hot
zone. As show~ in Figure 3 a portion of gas conducting tube
50 extends into that end o~ substrate or bait tube 52 into
which the reactants a~e introd~ced. That portion of tube 50
within tube 52 terminates just prior to the hot zone 54
created by mo~ing heat sou~ce 56. Tube 50 is mechanically
coupled b~ means represented by dashed line 58 to burner 56
to ensure that tube 50 is maintained the proper distance
upstream of the hot zone 54. Alternatively, the heat source
and gas feed tube may b~ kept stationary, and the rotating
substrate tube may ~e tra~ersed. ~he input end of tube 52
is connected to tube S0 by a collapsible member 60, a rotating
seal 62 being disposed between member 60 and tube 52. As
shown in Figure 4, which is a cross-sectional view of the
Z8739
hot zone and adjacent regions of tube 52, gas emanating ~r~m
tube 50 provides an effective mandrel or barrier to the
reactants ~lowing in the direction of the arrows between
tubes 50 a~d 52, thereby con~ining those reactants to an
annular channel adjacent the wall o~ tube 52 in hot zone 54.
For some distance downstream from hot zone 54, gas from tube
50 conti~ues to act as a barrier to soot formed in the hot
zone, thereby enhancing the probability that such soot will
deposit on the wall of tube 52 as shown at 44'. Dashed line
66 of Figure 5 represents the boundary between the gas
emanating from tube 54 and the reactant vapor flowing in the
hot zone 54.
The gas supplied to the hot zone by tube 50 may be any
gas that does not detrimentally affect the resultant optical
waveguide preform. Oxygen is p~eferred since it meets this
requiremen~ and is reIatively inexpensive. Other gases such
as argon, heIium, nitrogen and the like may also be employed
As shown in Figure 4, the end of tube 50 is separated
fr~m the center of the hot ZQne by a distance ~ which must
be great e~ough to prevent the deposition of soot on tube
50. The distance x will vary depending upon such parameters
as the width of the burner and the temperature of the hot
- zone. The following findings were made for a deposition
system wherein the outPr dia~eters of tubes 50 and 52 were
20 and 38 mm, espectiveIy, and the wall thicknesses thereof
were 1.6 and 2 mm, respecti~ely. The burner ace orifices
were located within a 45 mm diameter circle. In this
syst~m it was found that soot will deposit on tube 50 if the
distance x is about 13 mm. Mixing of the reactant vapor
stream with`the gas flow through~the ba~fle tube increases
with the longitudina~ distance from the baffle tube. The
3~
advantage derived by restricting reactant vapor to an annular
region close to the wall of tube 52 may be obtained wlth a
distance x up to about 15 cm. Best resul~s are obtained
when the distance x is within the range of 25-75 mm.
The size and shape of tube 5~ should be such that a
substantially laminar flow exists in the hot zone and in the
region ~mmediately downstream therefrom~ Any turbulence
which is introduced by tube 50 tends to pic~ up soot particles
and carry them downstream to the exhaust tube.
In the prior art deposition process described in
conjunction with Figures 1 and 2, deposition efficiency
falls with an increase in tube diameter. An increase in
deposition rate with increased ~ube diameter can be obtained
by increasing tu~e dia~eter to about 30 mm. For tubes
having diameters greater than 30 mm, deposition efficiency
falls at a faster rate so that further increase in deposition
rate is difficult to obtain. However, with the use of a
ba~1e tube, the reactant vapor stream is restricted to a
fixed distance fr~m the ins~de surace of the bait tube that
~0 produces opti~um deposition efficiency irrespective o bait
tube di~meter. The maximNm size of the outer tube is Limi~ed
by su~h consideratio~s as that size tube for which ~he inner
hole can be closed to form an optical wa~eguide preform.
The wall thickne ses of the bai~ tube and the baffle tube
are usually maintained reIativeIy small, i.e., a few milli-
meters in thickness.
~ cylindrically shaped baffle tube such as that illus-
trated in Figures 3 and 4 has been found to be easily con-
structed and to function satisfactorily to supply a~mandrel
of gas to the hot region of the bait tube without introducing
an undue ~mount of turbulence. O~her shapes such as that
_g_
~2~739
shown in Figure 6 could also be ~mployed to perform this
function. The direction of ~as ~low from tube 70 is s~own
by arrow 72.
To illustrate the improvement in deposition rate and
eficiency, a deposition sy~tem was opera~ed both with and
without a baffle tube 50 therein, all other process parameters
remaining unchanged. Apparatus similar to that shown in
Figure l was employed to supply the reactant stream; however,
only one reser~oir 32 was employed. Oxygen was flowed
through reservoir or bub~ler 32 containing SiC14 maintained
at 35C to provide a flow of about 2.5 g/m SiC14. The 10w
rate of the BC13 was 92 sccm, and the 10w o ox~gen through
flow meter 24 was 2.4 slm. The bait tube was a borosilicate
glass tube having an outer diameter of 38 mm and a 2 mm wall
thickness. A borosilicate glass having a composition of
about 14 wt.% B203 and 86 wt.% SiO2 was deposited. From the
flow rates of SiC14 and B~13, the rate of o~ide production
was calculated to be Q.85 g/min SiO2 and 0.29 g/min B203.
The deposition rate was 0.251 glmin and the deposition
efficiency was 26.2% w~en no baffle tube was employed. The
system was then modiied by adding a fused silica baffle
tube having an outside di~meter of 2Q mm and a wall thick-
ness of 1 6 ~m. The end of the ba~fle tube was separated
from the center of the hot zone by a distance of 50 mm. By
employing the baffle tube, the deposition rate increased
from 0.251 ~o 0.451 glmin and the efficiency increased from
26.2 to 43.~%.
TabIe I illustrates the effect of changing ~arious of
the process parameters on deposition rate and efEiciency.
-10 -
~ l~Z8739
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--11
2~3739
In Examples 1 through 6 of this Table the balt tubes consis~ed
of 38 mm OD borosilicate tubes having a 2 mm wall ~hickness
and the baffle tubes consisted of 20 mrn OD fused ~ilica
tubes having a 1.6 mm wall thicknPss. In the course of
these experiments, a plu~ality of layers o~ glass were
deposited within the bait tube in the manner described
above. Af~er 10 to 30 layers were deposited, the bait tubes
~ere broken, and the thicknPss of each of the layers was
m~asured under a microscope. The deposition rate was calculated
from the layer thickness, and the deposition efficiency was
defined as the deposition rate in glmin di~ided by the total
mass flow o~ soot entering the tube, assuming a 100% conver-
sion to oxides. The best results obtained werP a deposition
rate of 0.691 g/min, at 40.3% efficiency.
A tube of c~mmercial grade borosilicate glass
havi~g a 38 mm outside di~meter and a 2 mm wall thickness is
cleaned by sequential ~mmersion in hydrofluoric acid, deionized
water and alcohol. This bait tube, which is about 120 cm
long, is attached to a 90 cm length of exhaust tube having a
65 mm outside diameter on one end and a 60 cm handle tube of
the same size as the bait tube on the other end. This
combination is inserted into a lathe such ~hat the tubes are
rota~ably supported. T~e fre end of the handle tube is
provided with a rotatable seal through which a 1~0 cm long
section of fused silica baffle tube having a 20 mm outside
diameter and a 1.6 mm wall thickness is inserted. The
baffle ~ube is supported at two dif~erent points along its
length on a support which moves along with ~he burner. The
burner tra~erses a 100 cm length of the bait ~u~e at a rate
~Q
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~lZ873g
of 25 cm/min. The burner is adjusted to provide a deposition
temperature of 1800C at the outer surface of the bait tube.
After the burner reaches the end of its traverse during
which a layer of glass is deposited, it returns to its
starting point at a rate of 100 cm/min.
Oxygen flows into the ba~fle tube at the rate of 2.5
slm. Three reservoirs are provided containing SiC14, GeC14
; and POC13, respectively, these reservoirs being maintained
at a temperature of 32C. Oxygen flows through the first
and third reservoirs at the rates of 0.3 lpm and 0.56 lpm,
respectively thereby delivering constant amounts of SiC14
and POC13 to the bait tube during the en~ire deposition
process. The rate at which oxygen is supplied to the second
container increases linearly from 0 to 0.7 lpm so that
during the first pass of the burner along the bait tube, no
GeC14 is supplied to the bait tube, but the amount thereof
is linearly increased during the remaining 49 passas of the
burner. BC13 is supplied to the bait tube at the constant
rate of 15 sccm, and bypass oxygen is supplied thereto at
the rate of 2.4 slm.
After about 3 hours and 20 minutes, the time required
for 50 burner passes, the rate of burner movement is decreased
to 2.5 cm/min and the temperature increases to about 2200C
at the outer surface of the bait tube. This causes the
collapse of the bait tube into an optical waveguide preform
having a solid cross-section. The usable length of this
preform is about 84 cm.
; 30
- 13 -
39
. The resulting preform or blank is then heated to
a temperature at which the materials thereof have a low
enough viscosity for drawing (approximately 2000C). This
structure is then drawn to form about 25 km o optical
waveguide filament having an outside diameter o about
110 ~m.
: 30
