Note: Descriptions are shown in the official language in which they were submitted.
66~
mis inVention relates to optical fiber and optical fiber prefonm
manufacture by a vaPor reaction deposition process.
According to this invention there is provided a method of optical
fiber solid rod preform manufacture wherein a vapor reaction deposition process
is used to form a localized deposit upon the end face of a rod or disc and
wherein, by means of relative movement, the position of the localized deposit
is scanned over the end face of said rod or disc and at the same time the
reaction is progressively changed so as to cause the deposit building up on
the end face to have a radially graded refracti~e index profile.
In one aspect, the invention provides a method of manufacturing an
optical fiber solid rod preform comprising the steps of:
forming a localized deposit upon the end faoe of a rod by a vapor
reaction deposition pro oess;
scanning the position of the localized deposit over the end face of
said rod by means of relative movement; and
progressively changing the reaction at the same time so as to cause
the deposit building up on the end face to have a radially graded refractive
index profile.
m ere follows a description of the manufacture of optical fiber pre-
forms and optical fibers by methods embodying this invention in preferred forms.The description refers to the accompanying drawings in which:
Figures 1, 2 and 3 depict schematic diagrams illustrating three
methods of manufacturing optical fiber preform, and
Figure 4 depicts a schematic diagram illustrating how the methods of
Figures 1, Z and 3 can be used to produce optical fiber on a continuous basis.
J. Irven - 1
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~3~66~ 2 -
Detailed Description of the Invention
For the purpose of this specification, the
term vapor reaction deposition process is to be T
understood as including, inter alia, deposition by
the flame hydrolysis and by the radio frequency excited
plasma flame process~
The first metnod o optical fiber preform
manufacture to be described involves deposition by
flame hydrolysis. Referring to Figure 1, a burner 10
- 10 is provided with a hydrogen supply and an oxygen
supply. The oxygen supply is passed through vapor
entrainment means (not shown) which enables a part
of the gas flow to be passed through a selection of
different liquids to entrain their vapors for
transport to the burner. Typically these liquids
include silicon tetrachloride to react in the flame
to produce silica, and germanium tetrachloride,
phosphorus oxychloride, and boron chloride to react
in the flame so as to produce oxides that dope
the silica and thus modify its refractive index. With
different entrainment apparatus hydrides may be
substituted for one or more of the halides. The
flame 11 issuing from the burner is directed against
a substrate rod (ox disc) 13 upon the face of which
the deposit o the flame hydrolysis collects. The
rod 13 is rotated about its axis and at the same
time the burner is scanned from side to side of the
end face of the rod along a line that passes through
its axis. In this way it is arranged that the deposit
¦ 30 builds up over the whole face. It will be appreciated
that if the deposition rate were constant, and the
scan was in a straight line and at uniform speed the
deposit would build up at a faster rate towards the
center~ This is undesirable and in order to achieve
a uniform deposit at least one of these three
parameters must be modfied. A feature of this
invention is the modification of the flame hydrolysis
.
. ~. Irven - 1~
~3~6~0 (~evision)
reaction in synchronism with the scan in order to
vary the reaction product in order to provide a
graded index profile. This is achieved by varying
the relative proportions of the gas flows through
the individual liquids providiny the vapor reagents
for for~ing the deposit. Thus in the portion of a
scan proceeding ~rom the center towards the periphery
the relative proportions of the precursors of the
index increasing dopants, such as germania and
phosphorus pentoxide, are progressively reduced. It
is therefore generally found convenient to superimpose
the cyclic changes of flow rates necessary to achieve
a uniform coverage of the end face of the rod 13 upon
the cyclic changes of flow rates necessary to achieve
the required index grading. According to a preferred
construction of preform the index grading does not
extend to the extreme periphery but instead there is
grown a surface region of su~stantially constant
refractive index from within which the refractive
-index increases smoothly in an approximately parabolic
manner to a maximum ~alue at the center.
The rod 13 is slowly withdrawn ~rom the burner
10 at a rate matching the build up of the deposit on
its end ace so as to maintain constant the distance
between the burner and the surface upon which the
flame 11 impinges.
It would be preferable to arrange the deposition
conditions so that the deposit vitrifies as it is
deposited (hereinafter referred to as direct
~o vitrification), ra~her than to collect the deposit
in a non-vitreous particulate form requiring subsequent
vitrification. It is found, however,- that with this
particular deposition method this is not possible
when it is desired to incorporate volatile dopant oxides
such as germania, phosphorus pentoxide or boric oxide
into the depositO The volatility o~ these dopants
is so great that with ~his deposition method it is not
J Irven - 14
; (~evision)
~3~6b~ 4
possible to leave a significant proportion of these
dopants in the deposit. To make a preform doped
with these dopants the deposit is first collected
at a tempera-ture in which it forms a dense particulate
deposit, and then the deposit is subsequently sintered i
at a higher temperature just high enough to vitrify
it.
The subsequent vitrification of the particulate
deposit may be performed by causing the rod 13 and
the deposit on its end face to be withdrawn from the
burner through a furnace (not shown). It is of
course not necessary for the vitrification to be
performed concurrently with the deposition and, if
desired, the vitrification may be performed as an
entirely separate independent subseqeunt process step.
. Alternatively~ a cyclic operation may be employed
in which, when a predetérmined thickness o particulate
deposit has been accumulated, the supply of vapors
to the burner may be temporarily halted and the
1ame adjusted to raise its temperature sufficiently
to enable it to vitrify the accumulated deposit
before restarting the deposition process.
A further alternative method of vitrifying a
deposit which it is impractical to collect by direct
vitrification involves a form of concurrent deposition
and vitriication in which the deposit is formed as a
particulate deposit in a localized zone traversed
across the end face of the rod ~or the material that
has accumulated on that end face), while the particulate
material thereby deposited is vitrified in a second
localized zone also traversed over the end face (or
the material ~ at has accumulated thereon). One
possible arrangement for perEorming this operation
is depicted in Figure 2. This difEers from the arrange- ¦
ment of Figure 1 only in the provision of a second
burner 20 that is scanned with the first. This
second burner is just supplied with hydrogen and
,
. .
. Irven - 14
(Revision)
` ~3~6~0 5 _ I
oxygen without any of the additional vapors, and is
adjusted to provide a hot enough flame 21 to melt
the deposit previously left by the passage of burner
10 and its flame 11. For this purpose the scanning is
arranged so that flame 21 follows in the path of
flame 11. At the end of each traverse the relative
position of the two burners is reversed ready for the
traverse back in the opposite direction.
In a further modification which is not
illustrated the second burner surrounds the first in a
concentric arrangement.
In the foregoing description deposition by
- flame hydrolysis has been exemplified. An alternative
deposition process that can be used is radio frequency
excited plasma flame deposition. A feature of the
flame hydrolysis is that, since hydrogen and oxygen
are both present in the reaction, the resulting
product is liable to be contaminated with OH groups
which give rise to attenuation bands that may extend
into the region of the spectrum for which the fiber
has been designed. With a plasma flame deposition
pxocess it is possible to choose a reaction from which
hydrogen and hydrogen containing compounds have been
excluded so as to preclude the formation of OEI groups
which might otherwise become incorporated into the
deposit. Figure 3 depicts an arrangement using a
xadio frequency excited plasma torch. This differs
from the arrangement-of Figure 1 only in the replacement
of the flame hydrolysis burner with a plasma torch 30
This torch has a concentric arrangement in which a
radio frequency (typically in the range 3 to 27 MHz)
excited oxygen plasma jet issues from the inner
duct while materials in vapor form to form the deposit
are directed into the plasma flame from the surrounding
annular duct. These materials are converted into
oxides by the oxygen of the plasma or by oxygen gas
in which the vapoxs are entrained. Conveniently these
. .
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J. ~rven
(Revision)
s ~L3 ~66~ - 6
vapors may be the same halides and oxyhalides
employed in the arrangement previously described
with reference to Figure 1. In this instance,
however, the resultiny reaction is a direct oxidation
reaction instead of a hydrolysis reaction. Sufficient
energy can be provided by the plasma flame to promote
vitrification directly opposite the burner of material
deposited in non-vitreous form nearer the periphery
of the flame. There is therefore no necessity to
make special provision for a separate subsequent
vitrification step when depositing material incorpor-
ating one or more volatile dopants such as germania.
- It is to be noted that each of the above
described arrangements can be operated to produce
optical fiber on a continuous basisO This is
depicted in Figure 4. A flame hydrolysis burner
or plasma torch 40 ls scanned across the end of an
optical fiber preform 43 so that the deposit produced
by its flame 41 replenishes the material of the
2~ -preform. The preform is rotated about its axis and
withdrawn from the burner or torch 40 by means (not
shown) which is adjusted to withdraw the preform at
a rate matching that at which it builds up so
that a constant distance is maintained between
the burner or torch 40 and the end of the preform
- 43. This same feed means feeds the preform into a
drawing furnace 45 where the tip of the preform
is raised to a temperature at which it can be drawn
into fiber 46. The resulting fiber is collected on a
take-up drum 47~ Normally the fiber will be passed
through a coating bath (not shown) prior to being
wound on the drum in order to provide a coating for
the freshly drawn fiber so as to protect its surface
from degradation by atmospheric attack. Although
the preform is being continuously rotated about its
axis while it is being drawn into fiber, it has been
found unnecessary to rotate the drawn Eiber in
D~nchronism with the preform because continuous shear
,
J. Irven ~
~13~66~ ( Revision)
can be tolerated at the point of drawing.
It is believed that the deposition reactions
described above are at least predominately homogeneous
vapor phase reac-tions involving the nucleation of a
mixed oxide 'soot', with its subsequent deposition
in particulate form followed by sintering or deposition
and simultaneous fusion of the soot to a glassy state.
A further alternative deposition method can be
employed which involves a heterogeneous surface phase
. 10 nucleation mechanism in which glass is grown directly
on the substrate surface~ Typically a heterogeneous
phase reaction can proceed at a lower substrate temper-
ature than that required for forrning a glassy deposit
by an equivalent homogeneous phase reaction. The plasma
flame deposition reaction previously described with-
reference to Figure 3 is believed to be predominately
a homogeneous phase reaction under normal operating
conditions and flow rates, but, by modifying the apparatus
so that the torch, substrate and scanning assembly operate
- in a xeduced pressure environment instead oE at atmos-
pheric pressure, a heterogeneous phase reaction can
be promoted in which the deposit is formed directly as
a glass. Such a reaction is produced for instance by
operation at a pressure in the region of 1 - 50 torr
~5 with a 27 MHz inductive or 'H' plasma of several
. kilowatts power level. The inductive plasma may
alternatively be rep].aced with a higher frequency micro-
wave plasma, such as one operating at 2.45 GHz leaving
the pressure and power level re~uirements substantially
unchanged.
~ eterogeneous surface nucleation can be promoted
at atmospheric pressure by thermally activated reaction,
albeit at relatively slow deposition rates, it is
therefore believed that operation at reduced pressure
3~ is not a necessary condition for obtaining plasma
activated heterogeneous surface nucleation, provided
that the flow rate, reactant concentrationi ionic
and electron temperatures of the plasma discharge
are ohosen to provide a low concentration of activated
reagent specials in the plasma.
"
' ~3v~660 J. Irven - 14 ~
(Revision) ,,
~ I
While I have described above the principles of 1
my invention in connection with specific apparatus
it is to be clearly understood that this description
is made only by way of example and not as a limitation i
to the scope of my invention as set forth in the I
objects thereof and in the accompanying claims.
SG:ggs
20 Feb~Dary 1979
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