Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the manufacture of optical fiber
preforms, with microwave-plasma activated deposition, and to the
manufacture of fibers from such preforms.
Currently several different techniques are known to
manufacture silica based (high silica) optical fibers, such as modified
chemical vapour deposition (MCVD) or (internal vapour phase oxydation
process), high temperature CVD outside vapour phase oxydation (OVP0),
vapour phase axial deposition (VAD), microwave plasma activated CVD and
radio frequency (RF) plasma activated CVD.
The basic chemical reaction is the same for all those
techniques:
SiC14 + 2 ~ SiO2 + 2C12
Depending upon the deposition conditions, the form of the
product, SiO2, varies continuously from a particulate to a transparent
glassy form with an opaque porous form between these. A soot deposit
implies the particulate and the opaque porous deposits.
The ideal fiber manufacturing technique should fulfill the
following requirements:
1) High deposition rate.
2) Precise control of dopants concentrations and
their distribution.
3) Minimum impurity contamination.
Currently none of the above techniques fulfill all of these
requirements. The highest deposition rate has been achieved in the OVP0
and the VAD techniques. A soot is generated in a hydrogen-oxygen burner
(originally called the flame hydrolysis technique) followed by sintering
the soot deposit into a glass form. A high level of water contamination
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is the drawback of these processes. An attempt to deposit the soot in a
hydrogen free atmosphere using a RF plasma torch has not been successful
to date due to technical difficulties in controlling the dopant
concentration and the deposition temperature.
Internal processes such as MCVD and MW plasma CVD have
minimum OH contamination. However, the deposition rate is limited by the
requirement of the glass form deposition or the simultaneous fusion from
the soot to the glass form.
The present invention overcomes most, if not all, of the
present disadvantages of the various methods, as related above, and will
provide the above related requirements. Broadly, with the present
invention, optical fiber preforms are manufactured by passing oxygen and
vapours containing one or more additives through a fused silica or other
type of glass tube, the tube being reciprocated relative to a plasma
forming cavity which creates a plasma discharge inside the tube.
Particulate silica (soot) is formed and layers of soot are deposited on
the inner wall of the tube. The chemical reaction is initiated by
microwave plasma. The layers contain one or more additives, or dopants,
and/or flux, and the soot deposit is finally sintered or fused. Sintering
is followed by collapsing to a solid preform. The preform may thereafter
be drawn to a fiber.
The invention will be readily understood by the following
description, in conjunction with the accompanying drawings which is a
diagrammatic illustration of one form of apparatus for carrying out a
method of the invention.
As shown in the drawing, a glass tube 10, for example of
fused silica, is mounted for reciprocal movement, relative to a microwave
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cavity 11. The cavity is connected to a microwave generator 12 by a
waveguide 13 and plasma is generated, in the tube, as indicated at 14.
Reactant gases and vapours and oxygen are fed, to the tube, as indicated
by arrow 15 and residual gases absstracted from the tube, via a liquid
nitrogen trap 16, by pump 17. The microwave generator 12 includes control
means, not shown for controllably varying the power to the plasma to
generate desired temperatures in the tube.
Conveniently, the tube is supported in rotatably mounted
chucks 18 for reciprocating motion on a carriageway 19. Reciprocation
of the tube and chuck assembly can be by any convenient way, for example a
threaded lead screw and reversable motor, a pneumatic jack, or a hydraulic
jack. Other ways of rotatably and reciprocatably mounting the tube can be
used.
In the example, the gases and vapours are provided by
bubbling oxygen through one or more liquids. Thus, as seen in the
drawing, reservoirs 20, 21 and 22 are provided. Oxygen is supplied, via
pipe 23. From the pipe 23, pipes 24, 25 and 26 feed into the reservoirs
20, 21 and 22 respectively. The oxygen bubbles through the liquids and
picks up vapours, the oxygen and vapours passing out through pipes 27, 28
and 29, to a collecting chamber 30. Oxygen is fed directly to the
collecting chamber 30 via pipe 31 and argon can be added, from cylinder 32
and pipe 33 to assist in maintaining the plasma discharge.
As an example, reservoir 20 contains silicon tetrachloride
in liquid form, reservoir 21 contains phosphorus oxychloride in liquid
form and reservoir 22 contains germanium tetrachloride in liquid form. It
should be appreciated that not all the reservoirs need be provided, or if
provided, used. For example, depending on the particular fiber structure
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desired, only reservoirs 20 and 21 need be used, or provided, for
germanium doped silica deposition. For phosphorus as a dopant or other
additives, only reservoirs 20 and 22 need be used, or provided. For a
preform with germanium doping of the silicon, with phosphorus also added,
all three reservoirs are used. Other materials, for example boron, for
use as dopants or other additives, can be provided by providing suitable
liquids in the reservoirs.
A control valve 35 is provided in each of the pipes 27, 28
and 29, plus a monitor 36 which monitors the amount of dopant or additive
in the vapour. The monitors 36 provide signals which are ~ed back to the
valves 35 to control the flow through the pipes and maintain the desired
admixture of gas and vapours in the collecting chamber 30. The mixed
gases and vapours feed via pipe 37 to the tube 10.
A typical process is as follows. The flow of oxygen to the
reservoir 20, and to reservoir 22, for germanium doping, is started and
the desired concentrations set at the monitors 36. The pump 17 extracts
the gases and vapours to maintain a pressure within the tube at a desired
value. A control valve 38 and pressure gauge 39 are provided. A typical
range is between 10 and 30 Torr, although up to 200 Torr has been used.
Difficulties in maintaining a stable plasma discharge can occur outside
this range. To assist in maintaining the plasma discharge, argon can be
added. Once the flow rates have settled, the plasma is initiated, the
tube 10 rotated and the tube reciprocated through the cavity 11.
Upon striking the plasma, heat is generated within the tube
10. A typical frequency for the generator is 2.45 GHz and the amount of
power supplied is controlled to be that sufficient to produce a
temperature at the inner surface of the tube below about 500~C.
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Particulate material, or soot, is formed and deposits on the inner surface
of the tube as a result of the dissociation and oxydation reaction
initiated by the plasma discharge.
After the desired number of traverses of the tube, and the
deposition of the desired number of layers of soot, the soot deposit is
sintered or fused. One way of doing this is to heat the tube with a
hydrogen-oxygen burner. A burner is indicated in the drawing, in dotted
outline at 41. The tube can be traversed, or the burner, while the tube
is rotated. The temperature of sintering is about 1500-1700~C. After
sin~ering the temperature is increased to collapsing temperature to
collapse the tube under surface tension to a solid preform, at about
2100~C. Finally, for production of a fiber, the preform can be positioned
in a drawing furnace and the fiber drawn.
Compared with the modified chemical vapour deposition ~MCVD)
process, which is the most usually used process at present, the method of
the present invention has various advantages. Thus, the deposition rate
is higher because soot thickness per pass is not limited by the
simultaneous fusion requirement of the MCVD process. As a corollary, the
number of passes per unit of time can be higher. Effectively therefore, a
larger volume of deposit can be produced in a given time period. Energy
to initiate the chemical reaction is efficiently coupled to the chemicals
through the tube wall and the deposited soot layers. Similar soot
accumulation is difficult to obtain in the MCVD process because the heat
required to initiate the reaction, approximately 1400~C causes premature
sintering which can result in the formation of bubbles in the deposited
layers. The index profile is more accurate and consistent because (a) the
depletion of the volatile dopants, e.g. germanium and/or phosphorus, at
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the deposition tempteratures used in MCVD process does not occur at the
soot deposition temperature of the present invention process and (b) the
deposition can consist of many relatively thin layers, glving a better
stepwise radial approximation of the ideal profile. The control of the
invention process is much easier than that of MCVD, for example
temperature control. Because of the low deposition temperature,
distortion of the tube 10 is minimal. Thin wall tubes, of fused silica,
or less costly tubes such as are sold under the trade name Vycor, can be
used to reduce costs.
Compared with the OVPO and MW plasma CVD process, OH
contamination is minimal due to the hydrogen free environment and a higher
deposition rate is obtainable.
The higher deposition rate occurs because it is possible to
form soot faster than it can be fused or sintered. A large advantage of
the present invention is that all of the deposit can be formed as a soot,
at high deposition rates, and then the much slower fusing or sintering
step is carried out on the total deposit. Thus the slower step occurs
only once for the production of the preform, instead of controlling the
deposition rate of every layer as previously occurred.
With the present invention it is possible to make a much
bigger preform starting with a larger diameter substrate tube of thin wall
structure, with a large core material deposition. After collapsing to a
solid rod, the rod can be inserted into a further tube which is a fairly
close fit on the rod. The whole is then put in a drawing furnace and
drawn, the outer tube collapsing on to the rod as the fiber is drawn. For
example, a preform can be produced which will produce over 10 Km of fiber.
Cooling can be provided to cool the tube 10 and this will
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permit an increased deposition rate. Cooling can be by air of other gas,
passed along the outside of the tube, as indicated by arrows 40.
Some typical parameters or data for producing a germanium
doped preform are as follows:- flow rate of silicon tetrachloride about
80cc/min; phosphorus oxychloride flow rate from about 1.5cc/min to about
15cc/min; oxygen flow rate about 55cc/min. The flow rate of germanium
tetrachloride is varied, normally at a constant rate, for the whole
deposition time, for example from zero to about 10cc/min, to give a graded
index which increases with distance from the inner wall of the tube. The
flow rate can also be varied in accordance with a predetermined value to
give a non-linear graded index.
The particular flow rates are not critcal or specific to the
present invention, being variable as in conventional vapour deposition
techniques on an inner surface of the tube.
The deposition is carried out at a temperature below about
500~C, the deposit then being sintered or fused at a temperature between
about 1500-1700~C~ These temperature are not critical.
A specific example of a process, in accordance with the
present invention, is as follows.
The substrate tube 10 is mounted at each end and connections
made to the supply pipe 37 and exhaust pump 17. The tube is also
supported for reciprocal movement through the microwave cavity 11. The
tube 10 is first purged, as by passing oxygen and/or argon through the
tube. The desired flow rates are then set for oxygen via pipe 31, and for
the oxygen bubbling through reservoirs 20, and 21. Once the gas flows
have been stabilized, the plasma is struck. Argon may be added via pipe
33. Ini~ially the oxygen flow rate via pipe 31 is at about 55cc/min; the
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flow rate of silicon tetrachloride, via pipe 27, and about 80 cc/min; and
the flow rate of phosphorus oxychloride about 1.5cc/min. The flow rate of
germanium tetrachloride via pipe 29 is zero at the start of the deposition
process. The tube is reciprocated back and forth through the microwave
cavity, the temperature within the tube 10 being below 500~C but above
200VC for best deposition rate and stable plasma. The flow of germanium
tetrachloride is slowly increased, normally at a constant rate, for the
whole deposition time, to a maximum of about 10 cc/min, giving a
constantly increasing refractive index. The tube is traversed for
approximately 1000 complete passes through the cavity building up a
deposit thickness of about 300 microns.
The plasma is then shut off and the cavity removed from
around the tube. The burner 41 is traversed along the tube slowly while
the tube is rotated, the flow of gases and vapours continuing. The burner
heats the tube to about 1500-C, at the external surface, and as the burner
traverses the deposit on the tube wall fuses.
The length of the deposition time, that is the thickness of
the soot deposit, which can be carried out apparently varies with the
particular dopant or additive used. Thus, for germanium, it has been
found that deposition times of up to about half an hour are satisfactory
but at deposition thicknesses produced by longer deposition times, loss of
germanium occurs during sintering to the extent that useful levels of
germanium doping do not seem to occur. This is not the case with other
dopants and additives and therefore the deposition time - thickness of
deposit - can be varied in accordance with the particular dopant or
additive.
Deposition has been continued for up to about two hours with
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phosphorus as additive or dopant, while periods of up to half an hour have
been used with germanium. Other materials used are silicon tetrafluorlde
and boron trichloride to provide dopant and/or additives.
A further feature or characteristic of the present invention
is the ability to cause the deposited material to shrink away from the
support or substrate tube during sintering. By suitable pretreatment of
the tube inner surface prior to commencing the deposition, as by a
fluorine based, or fluorine containing, etchant the deposit will shrink
from the inner surface while without such treatment the deposit can be
caused to fuse to the tube inner surface. Thus, at will, it can be
obtained that either the deposit fuses to the tube, tube and fused layer
being then collapsed to form a preform or the deposit shrinks away from
the tube on fusing, the fused layer then being removed from the tube and
collapsed to a preform.
In either case, the collapsed preform can be positioned
inside a tube of suitable form, for example of fused silica or other
glass, the final structure then being capable of being drawn to an optical
fiber.
As a further feature it is possible to fuse the deposited
material in steps. Thus the deposited soot material can be fused after
some predetermined deposition time, deposition continued for a further
period and then the further deposited soot material fused. This can be
repeated as desired.
