Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
METHOD OF FABRICATING OPTICAL FIBER PREFORMS
Technical Field
This invention relates to the Axial Vapor-phase
Deposition (AVD) method of fabricating optical fiber
preforms.
Background of the Invention
In the typical prior art method of fabricating
optical fiber preforms by the AVD process (alternatively
referred to as Vapor-phase Axial Deposition [VAD]), a
porous soot for~ is grown while being pulled in an upward
axial direction. See, for example, the paper by T. Izawa
et al. entitled "Material and Processes for Fiber Preform
Fabrication-Vapor-Phase Axial Deposition" published in the
October 1980 issue of the Proceedings of the IEEE,
Vol. 68, No. 10, pp. 1184-1187. As illustrated in this
article, the Elames are directed upward and, hence, the
soot is deposited in an upward direction. While this is
consistent with the direction of convection flow, due to
the hot gasses produced by the torch, it is opposite to the
downward pull of gravity. Thus, two of the para~eters
controlling the efficiency with which soot is deposited are
tending to operate in opposite directions. For a
discussion of the effect of gravity on this process see
"Influence of Gravity on Chemical Vapor Deposition
Processes" by G. Wahl Prog. Astronaut Aeronut 52 lMater.
Sci. Space Appl. Space Processes,) 451-~82, 1977.
In addition to the opposing influences of
convection flow and gravity on the deposition efficiency,
the convection flow carries "fluff" (i.e., random density
particles) upward toward the growing soot form and deposits
it about the outer surface of the rotating form. This can
have an adverse effect upon the refractive index profile of
the resulting preform made from the soot form.
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Summary of the Invention
The various disadvantages and limitations in the
prior art AVD method of fabricating soot Eorms for con-
solidation into optical fiber preforms are mitigated, in
accordance with the present invention, by the downward
deposition of the precursor materials. While the effects
of gravity and convection flow still tend to operate in
opposite directions, the natural tendency of the heated
gas to flow upward can be minimized by focussing the gas
flow onto the growing soot form. However, it has been
found that enough of the convection 10w away from the
downward directed gas stream remains so as to minimize
the accumulation of fluff.
In accordance with an aspect of -the invention
there is provided a method of forming a glass soot form
suitable for production of optical ~ibers, which comprises
forming a stream of precursor materials by flowing flame-
producing reactants through an outer portion of a torch
and by flowing soot-forming raw materials throug~ at least
one portion of the torch located internally of t~e flame
producing reactants, focusing said stream of precursor
materials by flo~ing flame-producing reactants so as to
form a converging flame and flowing said soot-forming raw
materials through the converging Elame so as to produce a
collimated stream of precursor materials; directing said
stream of precursor materials onto a support member or a
soot form, and forming from said stream of precursor
materials a soot which is deposited so as to produce said
soot form.
3Q These and other advantages of the invention are
described hereinbelow in connection with the following
figures.
Brief Description of the Drawings
FIG. 1 shows an arrangement for fabricating soot
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forms in accordance with the present invention;
FIG. 2 shows the effect of convection upon a
downward directed flame produced by a ~orch of uniform
diameter;
FIG. 3 shows an arrangement for simultaneously
depositing core and cladding layers; and
FIGS. 4 and 5 illustrate the use of tapered
adapters for focussing the gas flow from a conventional
torch.
Detailed Description
Referring to the drawings, FIG. 1 shows an
arrangement 10 for fabricating soot forms employing the
Downward Axial Vapor-phase Deposition (DAVD) method in
accordance with the present invention. The form is grown
on a silica starting member 11 which is rotated about its
vertical axis by a motor 12 which is connected to member
11 by means of a shaft 9. A second motor 13 causes the
starting member to move in a downward direction as the
soot form grows, so as to maintain the growing surface at
a fixed location relative to the focal point of the flame.
~aw materials, such as SiC14, GeC14, POCL3,
oxygen and hydrogen, are fed into the base chamber 15 of
torch 14, which produces fine glass particles by the flame
hydrolysis reaction. The particles are, initially,
deposited onto the end of starting member 11. As the soot
form grows, the glass particles are deposited onto the
upper surface of the downward drawn, axially growing form.
If one attempts to practice the DAYD process
employing a conventional torch 20 of uniform cross section,
3Q the situation depicted in FIG. 2 is produced. In this
case, the convention effect is so pronounced as to cause
the flame 21 to bend upward and completely away from the
starting member 11. As a result, deposition is erratic
and totally unsatisfactory if at all. To avoid this, the
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gas flow must be focussed in the manner produced, for
example, by the tapered torch disclosed in United States
Patent No. 4,368,063 which issued to H.M. Presby on January
11, 1983. When the torch is tapered, as shown in FIG. 1,
the flame configuration is subs~antially independent of
orientation and, hence, the torch can be directed downward
at any angle ~ to the vertical where 0~90 degrees. A
further advantage of the use of the tapered torch is that
it provides a means for controlling the diameter of the
soot form. The smaller the torch diameter at the output
end, the smaller the diameter of the resulting form. As an
example, a 3/4" diameter form was grown in accordance with
the invention using a 3/8" diameter torch. The resulting
form is considerably smaller than the typical 2" to 3"
diameter forms produced by the upward AVD process using
conventional torches. In additionr the form grew with a
flat upper surface, was of uniform diameter, and free of
fluff.
The use of a focussing torch further permits the
simultaneous deposition of one or more cladding layers
using additional torches. FIG. 3 shows a soot form 35
comprising a core region 31 being deposited by a first
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torch 32, and a single cladding region 33 being deposited
by a second torch 34. The latter can be directed
perpendicular to the vertical (i.e., ~=90 degrees).
Experience has shown that the precise location of the focal
point is not critical. FIG. 3 also shows the well
controlled manner in which the soot form grows with clean
vertical lines and a flat upper surface. Additional
torches can be similarly employed to simultaneously deposit
additional cladding layers.
After the soot form is fabricated, it is
consolidated by heating to form the optical fiber preform.
The fiber is then drawn from the preform.
As noted in connection with FIGS. 1 and 3, the
res~ ting soot forms have well defined upper and side
boundaries. This is the result of the focussing action of
the tapered torch. The latter generates a converging gas
flow whose focal point is advantageously located near the
center of the upper surface of the growing form. This
tends to produce a well defined temperature gradient across
the upper surface of the form and a well defined cutoff
temperature below which deposition does not occur. This,
plus the fact that the convection flow carries the
nondeposited particles away from the growing soot form,
accounts for the well defined boundaries.
A further advantage of a focussed flame is that
the cladding flame operates substantially independently of
the core producing flame thus permitting their simultaneous
use. The simultaneous deposition of a cladding layer is
not normally practical with the upward AVD process.
FIGS. 4 and 5 illustrate the use of tapered
adapters for focussing the gas flow from a conventional
torch. The tapered tip 41, illustrated in FIG. 4, is a
single tapered tube which fits over the end of the
torch ~0. In the embodiment of FIG. 5, the tapered
focussing tip 43 comprises a plurality of concentric
cylindrical sections 44, 45 and 46 which serve to preserve
the separate flow of the constituent materials. Tip 43 can
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be made in the manner described in the above mentioned
United States Patent number 4,368,063.
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