Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF MAKING AN OPTICAL FIBER PREFORM
BACKGROUND OF THE INVENTION
The present invention relates to methods for making
optical fiber preform of both single mode and multimode
design using a plasma outside vapor deposition process.
The prior art teaches various approaches for
fabricating silica glass starter tubes, and for making
optical fiber preforms. Starter tubes can be formed by
heating silica and extruding it through an aperture.
Both starter tubes and optical fiber preforms can be made
by depositing doped or undoped silica onto a target using
one of several techniques such as modified chemical vapor
deposition (MCVD), vapor axial deposition (VAD), outside
vapor deposition (OVD). Each of these methods starts
with providing a rotating target, typically shaped in the
form of a tube or a solid rod, and formed from glass,
ceramic or one of several other materials. In certain
cases, the rod or tube becomes an integral part of the
preform but, in other cases, the rod will be removed. A
heat source, such as a gas burner or a plasma source is
positioned beneath, above or laterally, across the
rotating target. The heat source will provide the
required energy for the glass-forming reactions to form
glass particles. Depending upon the nature of the
process, these deposited glass particles are ready for
the next processing, drying and sintering steps such as
VAD or OVD processes. If it is an MCVD process, these
particles will be fused into vitreous quartz by the same
heat source.
When the target is mounted horizontally, the heat
source travels along the length of the target to ensure
uniform deposition. If the target is a tube, the glass
forming particles and materials may be deposited either
on the inside surface of the tube, in which case the
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outer diameter remains constant, or on the outside of the
tube, in which case the outer diameter grows.
When the target is mounted vertically, it rotates
around its vertical axis, and with burners located either
vertically above or laterally across, grows in both
radial and axial directions. This results in a
substantially cylindrical product whose diameter and
length increase as deposition continues.
UPS 3,737,292 to Keck et al. discloses a method of
l0 forming optical fibers. Multiple layers with
predetermined index of refraction are formed by flame
hydrolysis and deposited on the outside wall of a
starting rad or member. After these layers of glass are
coated on the rod the resulting hollow cylinder is heated
and collapsed to form fibers.
USP 4,224,046 to Izawa et al. teaches a method for
manufacturing an optical fiber preform. Two gaseous
glass forming materials, oxygen, hydrogen and argon are
jetted upwards in a burner towards a vertically mounted,
rotating cylindrical start member. Soot-like glass
particles are formed by flame hydrolysis and deposited. on
the lower end of the start member. The start member is
gradually withdrawn upwards to maintain a constant
spacing between the its growing end. and the burner. Upon
completion of the deposition, the resulting soot-like
glass preform is then dried and sintered to form a
transparent glass preform.
UPS 4,217,027 to MacChesney et al..teaches the
fabrication of preforms by what is usually referred to as
the Modified Chemical Vapor Deposition (MCVD) process.
In this process, a vapor stream consisting of chlorides
or hydrides of silicon and germanium with oxygen is
directed to the inside of a glass tube. The chemical
reactions among these chemicals, which are preferentially
induced by a traversing hot zone, will under proper
conditions result in the formation of glass on the inner
wall of the tube. The particular matter deposited on the
tube is fused with each passage of the hot zone.
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USP 4,412,853 to Partus discloses an MCVD process to
form an optical fiber preform starter tube. The process
starts with a horizontally mounted, rotating tubular
target formed from glass and having a preselected
composition and optical characteristics. A vapor stream
is fed through the tubular target as a heat source
positioned beneath the tubular target, traverses along
the latter~s length. This causes reaction products of
the vapor stream to be deposited on, and fuse to, the
interior surface of the tubular target. The deposited
material has the same index of refraction as the tubular
target, but a different composition. This reference also
suggests that one may achieve the same effect by an
outside vapor-phase oxidation process or an outside
vapor-phase axial deposition process, but does not
explicitly teach how this can be done.
USP 4,741,747 to Geittner et al. is directed to the
Plasma Chemical Vapor Deposition (PCVD) method of
fabricating optical fibers. In this PCVD method glass
layers are deposited on the inner wall of a glass tube by
heating the tube to a temperature between 1100° and 1300°
C, before passing the reactive gas mixture at a pressure
between 1 and 30 hPa, and mo~,ring a plasma back and forth
inside the glass tube. After the glass layers are
deposited, this glass tube is collapsed to produce a
solid preform. Optical fibers can be drawn from this
pref orm .
USP 5,522,007 to Drouart et al. teaches the use of
plasma deposition to build up an optical fiber preform
3o having high hydroxyl ion concentration. In this
reference, hydroxyl ions are deliberately entrained in a
plasma generating gas by passing the gas through a water
tank before it is introduced into one end of a plasma
torch having an induction coil. The plasma torch
projects molten silica particles mixed with hydroxyl ions
onto a rotating substrate preform. This results in a
preform having an average hydroxyl ion concentration
lying in the range to 50-100 ppm deposited onto the
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target preform. According to Drouart et al., this
technique results in optical fibers having an attenuation
of 0.32 dB/km and 0.195 db/km at 1310 nm and 1550 nm,
respectively.
In addition to requiring multiple processing steps
to fabricate preforms, some other disadvantages of the
above processes are that:
1. the MCVD and PCVD processes are slower
processes because of their low deposition rate;
2. the preform size is limited by the size of the
deposition tube for MCVD and PCVD process; and
3. the OVD and VAD processes are based on flame
hydrolysis which generates excessive amounts of water and
requires additional drying and sintering steps to produce
high quality optical fiber preforms.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method
for producing an optical fiber preform having low
hydroxyl content at low cost by reducing the number of
steps entailed in its manufacture, while increasing the
size of a preform and increasing the rate of deposition.
This and other objects are achieved by the present
inventive method for forming an optical fiber preform.
In one aspect of the present invention, a plasma
source is placed in proximity to a starter rod formed
from a primary material. The starter rod is held
horizontally at both ends and is arranged to rotate about
its longitudinal axis. The plasma source is used to
deposit silica doped with a known first doping
concentration. The doped silica is deposited along the
length of the starter rod until the latter grows to a
desired diameter. The complex comprising the starter rod
and the doped silica is then drawn down and a thinned
section is extracted for use as a secondary rod. The
secondary rod has a center formed from .the primary
material, and an outer layer formed from the doped
silica. Additional silica, having the same doping
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concentration, is deposited atop this secondary rod until
it, too, reaches a desired diameter, and then is drawn
down and a section extracted. The steps of depositing
drawing down, extracting and depositing may be repeated a
number of times. The result of this activity is a doped
silica rod having a center formed from the primary
material with a first diameter, and an annular layer
formed from the doped silica with a second outer
diameter.
The doped silica rod is subject to further
processing. Specifically, the plasma source is used to
deposit an outer layer of doped silica atop the doped
silica rod and the resulting structure may°-then be drawn
down and a thinned section extracted, as before. The
dopant used in forming the outer layer may be selected to
either increase, or decrease, the index of refraction of
the silica.
If the dopant concentration is varied as the outer
layer is being deposited, the outer layer is a graded
layer. In such case, typically, the dopant concentration
is varied from a maximum, beginning concentration level
when the outer layer is first being deposited, to a
minimum, end concentration level when deposition of the
outer layer is almost complete.
If the dopant concentration is not varied as the
outer layer is being deposited, the outer layer is a
stepped layer. In such case, typically, a second dopant
concentration, different from the first.dopant
concentration, is used throughout the deposition of the
outer layer.
In yet another aspect of the present invention, the
complex comprising the doped silica rod and the outer
layer is subjected to further processing. The plasma
source is used to deposit a cladding layer atop the outer
layer. If the outer layer was graded, the cladding layer
may be formed from silica doped with the same dopant and
same minimum, end concentration level. Alternatively,
the cladding layer can be formed from pure silica, or
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even silica doped with some other dopant and at a third
dopant concentration. If desired, the cladding layer may
also have a graded doping.
In yet another aspect of the present invention, the
complex comprising the doped silica rod, the outer layer
and the cladding layer, is provided with a jacket. The
jacket can be added by either further plasma deposition,
or, alternatively, by providing a jacketing material over
this complex and then applying heat to collapse the
jacketing material into a finished preform.
During plasma deposition, a dry plasma gas having a
low hydroxyl concentration is used to form the plasma. A
dry quartz source gas comprising SiCl" or other similar
source gases having low hydroxyl concentration, and a
dopant source gas such as GeCl" which is sometimes co-
doped with POC13 or PC15 are introduced in proximity to
the plasma. This causes the material to be converted to
silica (Si02), or silica doped with germanium oxide (GeOz)
and or phosphorous pentoxide (P205) and deposited onto the
target and fused into vitreous quartz in one simple step.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of
the present invention can be seen in the drawings in
which:
Fig. 1 shows an apparatus used to perform plasma
deposition;
Fig. 2 shows a partial side view of a plasmatron
used in the apparatus of Fig. 1;
Fig. 3 shows a top view of a plasmatron similar to
that shown in Fig. 2;
Fig. 4 shows a f low pattern of the plasma within the
plasmatron of Fig. 3;
Fig. 5 shows an optical fiber preform made in
accordance with the method of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
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Fig. 1 shows an apparatus 20 used for plasma outside
vapor deposition. The apparatus comprises a chamber 22
Which is sealed so as to prevent impurities from being
introduced into the final product.
Within the chamber 22 is a lathe 24, such as that
available from Heathway Ltd. or Litton Engineering Lab.
The lathe 24 has a headstock 25 and a tailstock 26. The
headstock 25 and the tailstock 26 are provided with a
pair of opposing rotating spindle chucks 28 which hold
l0 the ends of an elongated target 30 having a substantially
cylindrical outer wall. The spindle chucks.28 rotate
target 30, as indicated by arrow A1. A movable carriage
32 movably mounted to the lathe 24 is arranged to travel
in either direction along the target, as indicated by
double headed arrow A2.
A plasma source, shown generally as 40, is supported
by carriage 32. Carriage 32 thus moves plasma source 40
along the length of the target 30. This results in the
deposition of material on top of the target 30 to form an
optical fiber preform. The spindle chucks 28 rotate the
target 30 to ensure that material is uniformly deposited
by the plasma source 40 around the target so as to form a
tubular member 34 having nearly perfectly.cylindrical
outer walls.
In the preferred embodiment, the plasma source 40
positioned on the carriage 32 moves in both directions
along a substantial portion of the length of the target
30. This allows the plasma source 40 to travel along
this portion of the target 30 and deposit materials
3o therealong.
Instead of moving the plasma source 40 along the
length of the target, the target 30 may be moved while
the plasma source 40 remains stationary. This can be
reali2ed by having the headstock 25 and the tailstock 26
of the lathe move the target in a reciprocating fashion
so that all relevant portions of the target are brought
directly 'above the plasma source 40.
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As another alternative, a plurality of plasma
sources may be~~spaced apart along the length of the
target. This allows for reduced movement of either the
headstock 25 and tailstock 26 of the lathe 24, or the
carriage 32 to which the plasma sources are attached,
depending on which of the two is configured to move. In
the extreme case where a great number of plasma sources
are provided all along the length of the target, no
movement of either the carriage 32 or the headstock 25
and tailstock 26 of the lathe 24 is needed.
In the preferred embodiment, the plasma source 40 is
a plasmatron torch having a dry plasma gas introduced
into it through a first gas line 42 and a-source gas
introduced into it through a second gas line 44.
The plasma gas is substantially comprised of
nitrogen and oxygen in an appropriate, predetermined
proportion. Air may serve as the plasma gas. In such
case, filtered air first passes through a first dryer 46
to remove moisture before entering the first gas line 42.
This ensures that the hydroxyl concentration of the
plasma gas is low, on the order of 2.0 ppm, or less. The
total volume of gas being delivered will be regulated by
a mass flow controller (MFC) 80 or by a flowmeter, as an
alternative.
The source gas comprises a source.chemical such as
SiCl4, and at least one carrier gas, such as oxygen OZ or
nitrogen Nz. The carrier gases enter the~_second dryer 48
to remove moisture. This ensures that the hydroxyl
concentration of the source gas is also very low, on the
order of 0.5 ppm. After the carrier gases are dried,
they proceed to a MFC 81 before entering a bubbler 50 to
pick up the source chemical. Depending upon the
characteristics of the MFC, it is also possible to use it
downstream of the bubbler. The gas stream comprising
carrier gases laden with the source chemical then
proceeds to the second gas line 44. Optionally, by
opening valve 52, a dopant gas may be introduced into the
gas stream before it reaches the plasmatron torch.
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In the preferred embodiment, the source chemical is
SiCl9. This chemical is. chosen for its re~ective
properties in a plasma. Specifically, the SiCl, serves as
a source of Si to form Si02 which is deposited on the
target 30. The dopant can be a fluorine dopant gas in
the form of SiF4 or SiF6. Fluorine dopants will lower the
index of refraction and also change the viscosity of the
quartz. In addition, fluorine dopants result in
increased design flexibility for optical fiber preforms.
As is well known, however, if one wishes to increase the
index of refraction, Ge02 or other equivalent substance
may be used as the dopant.
In the preferred embodiment, the source chemical for
GeOz is GeCl4. This chemical is chosen for its purity
because of its having similar physical and chemical
properties SiCl9. The delivery of the GeCl4 will be
similar to SiCl,. The carrier gas from the dryer 48, can
be split to another branch where it will be regulated by
a MFC 82, before proceeding to a bubbler 83 to pick up
the source chemical GeCl,. Similar to the control of
chemical SiCl4, the MFC can also be located downstream of
the bubbler. This gas stream can feed into the gas line
44 and form a mixture before entering the plasmatron
torch. It is also possible to directly introduce the
GeCl4 gas stream by a separate line 84 to the plasmatron
torch. One advantage of using the separated delivery
lines is to minimize the competing chemical reactions
between GeCl, and SiCl,. Other source chemicals that can
be used for doping instead of germanium oxide (GeOz) or
co-doping with germanium oxide are materials such as
POC13, PC15, and other similar index increasing dopants
such as Aluminum and Titanium containing chemicals.
Fig. 2 shows a cutaway side view of the plasmatron
torch 40 positioned below the target 30. The plasmatron
torch 40 comprises a substantially tubular torch housing
50 formed from quartz. The housing has a diameter of 60
mm and a height of 220 mm. However, diameters ranging
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from 40-80 mm and heights between 180-400 mm may also be
used.
A copper induction coil 52 is provided around the
upper portion of the housing 50. The coil 52 comprises a
plurality of windings 54 having a diameter of
approximately 72 mm and spaced apart from each other by 6
mm. A gap between the housing and the coil can be
between 2-10 mm. The uppermost portion of the coil 52,
as indicated by uppermost winding 54', is separated from
the outer surface of the tubular member 34 by a spacing
designated by L, which is on the order of 30-55 mm.
As the quartz glass is deposited, its outer diameter
increases. However, the spacing L is maintained by
adjusting the height of a support stand 56 on which the
plasma torch 40 is placed. Support stand 56, in turn, is
mounted to carriage 32, and moves laterally therewith.
Initially, the support stand 56 is set at a predetermined
height, and this height is reduced as the diameter of the
deposited material increases during deposition. This
maintains a predetermined distance between the plasma
torch 40 and the deposited material. An optical or other
sensor mounted on the carriage 32 and connected to a
controller may be used to gauge the distance of the
radially growing tubular member 34 from the carriage, and
adjust the height of the support stand 56, accordingly.
On either side of the uppermost portion of the
housing 50 is a plasma stabilizer bar 58. Each
stabilizer bar is formed from quartz and comprises a U-
shaped gutter extending laterally from the rim of the
housing 50. The stabilizer bars 58 have a diameter of 60
mm and extend 20 mm on diametrically opposite sides of
the housing rim, although diameters in the range of 40-80
mm and lengths of 15-40 mm may also be used. When the
plasmatron torch 40 is in use, the stabilizer bars 58 are
aligned parallely to the target. This arrangement helps
spread the reactive source chemicals being deposited onto
the growing tubular member 34.
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A pair of injection ports 60 connect the second gas
line 44 carrying the source chemicals to the plasmatron
torch 40. The injection ports 60 enter the housing at
substantially the same height along the housing 50, at a
point between the uppermost windings 54~ of the coil 52
and the stabilizer bars 58. The injection ports comprise
quartz tubing having a diameter of 5 mm, although tubing
diameters on the order of 3-10 mm may be used with the
plasmatron torch 40 of the present invention. In the
preferred embodiment, a pair of injection ports 60 enter
the housing 50 at the same height and are.positioned
diametrically across from each other. Instead of just
two such ports, however, three or even more ports,
symmetrically arranged, may be provided. In Fig. 2, the
two injection ports 60 are shown to be directly beneath
the stabilizer bars. This, however, is not an absolute
necessity, and the injection ports 60 may be angularly
offset from the stabilizer bars 58, in a top view of the
plasmatron torch, as shown in Fig. 3.
A pair of plasma gas inlets 62 connect the first gas
line 42 carrying the plasma gases to the plasmatron torch
40. The plasma gas inlets 62 enter the housing at
substantially the same height, proximate to the base of
the housing. These inlets 62 comprise. stainless steel
tubing having a diameter of 5 mm, although a range of
diameters may suffice for this purpose.
The plasmatron torch 40 is also provided with a
coolant inlet 64 and outlet 66. During use, a coolant,
such as water, passes through the inlet 64, circulates
within the outer wall of the housing 50, and exits
through the outlet 66. The coolant inlet and outlet are
formed from stainless steel and have a diameter of 5 mm.
As with the plasma gas inlet and the injection port, this
diameter may also vary.
The plasma gas inlets 62, the coolant inlet 64 and
the coolant outlet-66 are all formed in a stainless steel
chamber 68. The chamber 68 is a stainless steel square
block 80 mm on a side, and having a height of
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approximately 40 mm. The chamber 68 is mounted onto the
support stand 56 which, in turn, is mounted on the
carriage 32 for movement along the target 30.
A high frequency generator (not shown) is
electrically connected to the coil 52, powering the
latter with a variable power output up to 80 kW at a
frequency of approximately 5.0 MHz. In the preferred
embodiment, the generator is Model No. T-80-3MC from
Lepel Corporation. This generator is driven with a 60
Hz, 3-phase 460 V power supply to energize the plasmatron
torch 40. As an alternative, a Model No. IG 60/5000
generator is available from Fritz Huttinger Electronic
GmbH of Germany.
Fig. 4 depicts the plasma jet.70::formed within the
plasmatron torch 40 when the dry plasma gas is fed
through the inlets 62 and converted into.a plasma. The
plasma jet 70 is substantially symmetric about the
torch's longitudinal axis A'. The position of the
injection ports 60 is such that the source chemicals are
introduced into the plasma just above a point V where the
vertical velocity of said plasma is zero. This provides
the needed structure of hydrodynamic and thermal flow of
the source chemical jet into the border layers to realize
efficient deposition onto the growing tubular member 34.
And while the preferred embodiment has the injection
ports entering laterally into the housing, this is not an
absolute requirement. Instead, the source.;gases may
introduced into the center of the plasma jet 70 by a
water cooled probe extending along the~1"ongitudinal axis
A' of the plasmatron torch 40.
Fig. 5 illustrates a well-known.procedure which can
performed with a lathe 124, such as Model No. PFH842XXLS
Precision Quartz and Glass Working Lathe, manufactured by
Heathway. The headstock 125 and tailstock 126 of the
lathe 124 can move longitudinally relative to one
another. This allows for easy loading and unloading of a
finished workpiece 130 of length L3 which has been
deposited atop an initial target. More significantly, it
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also allows one to draw down a portion of a workpiece
into a secondary rod of a reduced diameter comparable to
that of the original target. This is accomplished by
keeping the headstock 125 stationary and moving the
tailstock 126 away from the headstock 125 while the
plasma source 140 is moved in a direction opposite to
that of tailstock 126. Alternatively, this can also be
accomplished by placing a plasma source 140, or other
heat source, at one end of the workpiece 130 to soften
it. Then the headstock 125 and tailstock 126 are moved
in the same direction, but with different speeds by
distances L5, L4, respectively, to the positions shown in
phantom 125', 126'. The result is a thin, secondary rod
132, which can (but need not) have the same diameter as
the original target. As is known to those skilled in the
art, the secondary rod has the same cross-sectional
composition as the workpiece from which it ~s derived, as
so has a center whose consistency along is substantially
similar to that of the original target;~ar~d outer layer
substantially similar to the materials deposited atop the
target during the formation of the workpiece.
The lathe 124 allows the headstock 125 and tailstock
126 to be moved far enough longitudinally to stretch the
secondary rod to a distance L4, which is substantially
the same as the length L3 of the workpiece from which it
is derived. The secondary rod 132 may be cut from the
workpiece, mounted on the lathe 124 in place of the
workpiece 130, and used as a target for subsequent
deposition with the plasma source 140. Thus, the
original, or first-generation, target is used to create a
first-generation workpiece, from which a secondary rod
can be drawn to be used as a second-generation target.
Deposition atop this second-generation target can will
thereby form a second-generation workpiece, and so on.
This iterative process of plasma deposition on a target
to form a workpiece, stretching one end of she workpiece
to form a reduced-diameter rod, and using this reduced-
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diameter rod as a subsequent target for further
deposition can be repeated an arbitrary number of times.
If the material being deposited atop the target is
unchanged through the iterations, the result of N
iterative steps is an N-th generation rod having a very
small center which is substantially identical in
composition to the original target, and an annular layer
reflective of the materials deposited atop the target.
For instance, if the original target has a diameter D1
and the finished workpiece has a diameter D2 = M x D1,
then the proportion of the original target. material in
the first-generation workpiece is approximately 1/M~. If
a second-generation target of diameter D1 is drawn from
this workpiece and material sufficient to form a second-
generation workpiece of diameter D2 is deposited thereon,
the proportion of the original target material in the
second generation workpiece is approximately 1/M4. Thus,
it can be seen that one may readily form a workpiece
having a predetermined proportion of the original target
material therein by controlling_M during deposition,
along with the total number of iterations.
A method for forming a multimode optical fiber
preform using the aforementioned iterative technique will
now be described. In order to provide a more detailed
explanation, some dimensions. are given. However, it must
be noted that in the actual process, many different value
are possible.
The method begins by providing a first generation
target, horizontally mounted on a lathe, such as that
shown in Fig. 5. The target is preferably formed from
pure silica, in which case it may be purchased from a
commercial vendor, such as Product no. F300, available
from Heraeus Amersil of Georgia. Alternatively, the
first-generation target may be an Nth-generation doped
silica rod formed using the current process. In the
preferred embodiment, the first-generation target has a
length of one meter and a diameter D1 = 6 mm.
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Silica doped with Ge02 is deposited atop the first-
generation target using the plasma source described
above. The dopant concentration for the Ge02 depends on
the desired numerical aperture (NA) of the multimode
optical fiber being produced. For instance, to form a
fiber with a NA of 0.2, the maximum Ge02 dopant
concentration is approximately 10%. And to form a fiber
with a NA of 0.275, the maximum Ge02 dopant concentration
will be approximately 18%.
l0 The dopant concentration may be held at the same
level during deposition, in which case a stepped layer,
is formed. Alternatively, the dopant concentration may
be gradually varied to form a graded layer. This is done
by automatically controlling, by means ~of_a
microprocessor or like, an adjustable flow meter through
which the dopant is introduced. It should be noted that
stepped and graded layers may succeed one another in
subsequent generations of workpieces, and that layers
having different, constant doping concentrations may
succeed one another, as well. Thus, a graded layer may
be deposited on the first-generation target, and a
stepped layer may be deposited atop the second-generation
target formed after drawing down the first-generation
workpiece. Similarly, one may deposit a stepped layer
atop a graded layer, which has been deposited atop an
original first-generation target. Also, a first stepped
layer, having a first dopant concentration, may be
deposited atop a target, and a second stepped layer,
having a second dopant concentration; deposited atop the
next generation target. Additional layers, either graded
or stepped, may be deposited atop any of the above
structure.
In the preferred embodiment, silica doped with 18 %
GeOZ is deposited as a stepped layer atop the 6 mm
diameter first generation target until a workpiece having
a length of one meter and a diameter of D2 = 48 mm is
formed (i.e., M = 8). This resulting first-generation
workpiece has approximately 64 times the cross-sectional
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area of the original first-generation target. The first-
generation workpiece is then drawn down into 64 first-
generation doped silica rods, each having a length of one
meter and a diameter of 6 mm. Each of these doped silica
rods may then be used as a second-generation target.
The second generation target is placed in a lathe
and a second deposition layer is applied to form a
second-generation workpiece having a 48 mm diameter.
This second deposition is carried out with the same,
l0 constant dopant concentration as the first deposition.
Maintaining the dopant concentration at the same level
throughout the deposition process results in a first-
generation doped silica rod with a center formed from the
original target material and an annular layer which has
substantially the same composition therethrough. This
ensures that the optical properties of the second layer
is substantially the same as that of the first layer
which was deposited on the original target. The second-
generation workpiece is then drawn down into 144 second-
2o generation doped silica rods, each having a length of one
meter and a diameter of 4 mm. Each of these may be used
as a third-generation target. It should be noted here
that the iterative process may continue with the
deposition of additional layers having the same dopant
concentration. At some point, however, a workpiece with
a desired proportion of original target material will be
formed, after which no further iterations are needed.
Indeed, this may even be reached after the first
generation workpiece is formed.
In the preferred embodiment, a graded deposition
layer having an outer diameter of approximately 80 mm is
deposited atop the 4 mm diameter third-generation target.
The dopant concentration starts out at a..maximum value of
18% Ge02 closest to the outer surface of the third-
generation target, and is gradually reduced to a minimum
value approximately o.l% Geo2 at its outermost portion,
where the diameter is about 80 mm. This results in a
third-generation workpiece having a center formed from
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the original target, two layers having substantially the
same optical properties and fairly indistinguishable from
one another, and a third, graded layer.
In the preferred embodiment, the 80 mm diameter
third-generation workpiece is subject to additional
processing to form a primary optical fiber preform.
Specifically, a cladding, or barrier, layer is deposited
atop the third-generation workpiece. The thickness of
the cladding layer depends on the type of finished
optical fiber preform to be made. For a 62.5/125.fiber
preform, the finished primary preform will have a final
diameter of about 93 mm. For a 50/125 fiber preform, the
finished primary preform will have a final diameter of
about 96 mm. The cladding layer is formed by depositing
silica doped at the same concentration of Ge02 as the
minimum doping concentration level used to form the third
layer, i.e., i0% GeOz. This results in a structure having
the original target material at the center, a constantly
doped pair of second layers having the same optical
properties, a graded layer having a dopant concentration
varying from a maximum value to a minimum value, and a
cladding layer comprising silica doped at the minimum
value.
Once the cladding layer is applied, the finished
primary preform must be stretched to form.ahe final
preforms. From a single, 1 meter long 62.5/125 preform
having a diameter of 93 mm diameter, one _can obtain
eight, one-meter long preform pieces, each having an
outer diameter of 32 mm. And from a single, 1 meter long
50/125 preform having a diameter of 96 mm diameter, one
can obtain twelve, one-meter long pieces, each having an
outer diameter of 27 mm.
A jacketing layer may be applied atop the cladding
layer of these preform pieces. The jacketing layer
preferably has the same index of refraction as pure
silica. The jacket may be applied by plasma outside
vapor deposition using pure silica. Alternatively, a
tube or sheet of pure silica, having an appropriate
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diameter or width, may be provided around a preform
piece, and heat applied to fuse the jacket onto the
preform piece to form the final optical fiber preform.
In the preferred embodiment, the final optical preform
has an outer diameter of about 56 mm. This final preform
may then be drawn into approximately 200 Km of fiber
having a diameter of 125 ~cm.
Although, for best performance, a cladding and then
a jacketing layer is applied, it should be noted that one
may dispose of the cladding step and directly apply a
jacketing tube to the third-generation workpiece, once it
has been stretched.
A similar method for making single. mode optical
fiber preform can be achieved by using the following
procedure. The starting target can be a pure silica rod
that can be either a F300 rod purchased from Heraeus or a
pure silica Nth-generation rod fabricated in house.
Multiple fluorine doped silica layers with constant
concentration are deposited on the target until it
reaches a desired diameter. Single mode optical fibers
can be drawn from this preform. There are many different
glass index modifiers such as F, Ge02, P205, Ti02, A1203,
etc., and in the proper combination, they can be used to
make the doped core and/or doped cladding. In the
preferred embodiment, the target is a Nth-generation GeOz
doped rod with pure silica or doped silica cladding
layers deposited on it. The preform is completed when
the desired diameter is reached.
While the present invention has been~aisclosed with
reference to certain preferred embodiments, these should
not be considered to limit the present invention. One
skilled in the art will readily recognize that variations
of these embodiments are possible, each falling within
the scope of the invention, as set forth in the claims
below.
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