Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVEMENTS IN OR RELATINC TO OPTICAL CABLE ~:LE~ITS.
Light, which term includes the ultra-violet,
visible and infra-red regions of the electromagnetic spectrum,
is transmitted in an optical transmission system by means of
optical guides in the form of cables including one or more
than one optical fibre.
One form of opticzl cable which is especially, but
not exclusively, suitable for use in the communications field
for transmission of light having a wavelength within the
range of 0.8 to 1.9 micrometres includes a plurality of
optical cable elements each comprising a separately formed
tube of rubber or plastics material in which is or are
loosely housed at least one separate optical fibre and/or at
least one optical bundle comprising a group of optical fibres
or a group of fibres including at least one optical fibre and
at least one non-optical reinforcing fibre or other
reinforcing elongate member.
Optical cable elements as above described will
hereinafter be referred to as "of the kind specified".
When optical cable elements of the kind specified
are used in a wet and/or sub-zero environment, the
possibility of entry of water into the space bounded by the
tube of an optical cable element exists, either by permeation
through the material of the tube itself or by flow into an
open end of the tube. Subsequent freezing of this water can
lead to ice crystals pressing upon the or each fibre and
consequential raising of the optical attentuation of any
signal being transmitted along the fibre; the very presence
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of water in contact with an optical fibre can also weaken the
fibre mechanically. With a view to reducing substantially
the risk that water will enter the space bounded by the tube
of an optical cable element of the Xind specified, it has
been proposed to fill that part of the space not occupied by
an optical fibre(s) and/or optical bundle(s) with a water-
impermeable medium of a grease-like nature.
The present invention provides an improved method
of manufacturing an optical cable element of the kind
specified having a water-impermeable medium of a jelly-like
nature as hereinafter defined, filling the space in the tube
of the optical cable element not occupied by the optical
fibre(s) and/or optical bundle(s).
According to the invention, the method comprises
causing at least one separate optical fibre and/or at least
one optical bundle to travel in a rectilinear direction in
the direction of its length under a controlled tension;
applying to the advancing optical fibre and/or optical bundle
immediately upstream of extrusion apparatus a water-
impermeabl0 medium in a liquid or semi-liquid state in such a
way and under such a pressure that the advancing optical
fibre and/or optical bundle with water-impermeable medium
therearound passes into the extrusion apparatus; extruding a
tube of pol~meric material around the advancing optical fibre
and/or optical bundle and the surrounding water-impermeable
medium in such a way that the internal diameter of the tube
is substantially greater than the overall diameter of the
optical fibre and/or optical bundle; drawing down the
extruded tube of polymericmaterial to reduce its internal
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diameter to such an extent that the tube loosely envelops the
optical fibre and/or optical bundle with the space within the
tube not occupied by the optical fibre and/or optical bundle
substantially filled with water-impermeable medium of a
jelly-like nature and heating the extruded tube as it is
being drawn down; and annealing the drawn-down tube to reduce
the risk of subsequent substantial shrinkage of the tube.
By a "water-impermeable medium of a jelly-like nature"
is meant a medium which, at the normal operating temperature of a
cable of which the optical cable element forms a component part,
will deform in an elastic manner and which, if subjected to
sufficient pressure, will flow.
Preferably, water-impermeable medium in a liquid state
is applied to the advancing optical fibre and/or optical bundle
immediately upstream of the extrusion apparatus by causing the
advancing optical fibre and/or optical bundle to pass into a
chamber which is connected to the upstream end of the extrusion
apparatus and which contains water-impermeable medium in a liquid
state and at a substantially constant pressure head. The chamber
is preferably of substantially elongate form and, over a part of
its length remote from the extrusion apparatus, is open at the top
so that water-impermeable medium being introduced into the chamber
can flow out of the chamber into a reservoir positioned beneath
the chamber. Water-impermeable medium in a liquid state is
preferably introduced, from a storage tank in which water-
impermeable medium is maintained at a substantially constant
temperature by heating means, into the chamber through at least
one port in its wall, rate of flow of water-impermeable
medium from the storage tank into the chamber being manually or
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automatically controlled. Preferably, the storage tank, chamber
and reservoir positioned beneath the chamber form part of a
circulatory system, water-impermeable medium collected in the
reservoir from the chamber being pumped back into the storage
tank. The reservoir ma~ be heated to maintain the water-
impermeable medium in a liquid state.
To facilitate initial threading of an optical fibre and
/or optical bundle through the chamber and into the extrusion
apparatus, preferably a substantially rigid capillary tube
extends through the chamber and into the e~trusion apparatus and
is longitudinally movable with respect to the chamber and
extrusion apparatus. Initially, the optical fibre and/or optical
bundle is threaded through the capillary tube and through the
extrusion apparatus. When manufacture of an optical cable element
of the kind specified has commenced and water-impermeable medium
in a liquid state is being introduced at the required rate and
pressure into the chamber, the capillary tube is ~radually
withdrawn from the extrusion apparatus and chamber in an
upstream direction so that water-impermeable medium is applied
directly to the optical fibre and/or optical bundle being drawn
through the chamber.
It will be appreciated that the water-impermeable medium
must be a compound that will be fluid and o~ a low viscosity at
the temperature at which the polymeric material is extruded around
the advancing optical fibre and/or optical bundle, that is to say
at a temperature in the range 70 - 80 C, in order that the
water-impermeable medium can flow into the upstream end of the
extrusion apparatus under the combined action of gravity and
longitudinal movement of the advancing optical fibre and/or
optical bundle. In addition, preferably the water-impermeable
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medium will remain in a jelly-like condition within a
temperatue range of -40 to +70 C. A preferred water-
impermeable medi~n comprises a compound containing a major
proportion of technical white oil and a minor proportion of a
thermoplastics rubber. Preferably, the compound contains:~
100 parts technical white oil having a viscosity of 14
c/s at 40 C.
3 to 5 parts Shell Elexar thermoplastics rubber 8431Z
Drawing down of the extruded tube of polymerlc material
causes the molecules of the polymeric material to line up
longitudinally and such longitudinal orientation of the
molecules is facilitatedd by heating the extruded tube as it is
being drawn down, for instance by causing it to pass through an
eD. The longitudinal orientation of the molecules substantially
increases the strength o~ the tube in a longitudinal direction So
that the tube affords good protection for the loosely housed
optical fibre and/or optical bundle when a plurality of optical
cable elements of the kind specifled are being assembled together
to make an optical cable or when the optical cable is belng
installed. Longitudinal orientation of the molecules also reduces
the co-efficient of thermal expansion of the polymerlc material to
bring it substantially closer to that of the material of the
optical fibre or fibres.
After the tube of polymeric material has been extruded
and before its diameter is reduced by drawing down, the tube may
be cooled by passing it through a cooling bath of water at a
temperature within the range 70 - 75 C to maintain the water-
impermeable medium in the tube in a liquid state.
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Preferably, to ensure that the water-impermeable medium
in the advancing tube is maintained in the desired liquid state
throughout the manufacturing process, at any position along the
line at which the tube is exposed to the atmosphere, the tube is
heated. Preferably the optical fibre and/or optical bundle travels
at a speed substantially the same as the final speed of the tube.
Since the optical fibre and/or optical bundle i5 drawn into the
tube by frictional forces where the tube is wound several times
around the take-up drum, substantially similar speeds will be
achieved if the take-up drum is chosen to be of sufficiently large
diameter.
By way of example, ~ust after extrusion of the tube of
polymeric material, the tube may be moving at a speed of 4 m/min;
after it has been drawn down the tube may be moving at 20 m/min;
and after annealing of the tube it may be moving at 16 m/min. The
advancing optical fibre and/or optical bundle will be moving at 16
m/min throughout the manufacturing process. Since the bulk of the
water-impermeable medium in the tube moves at the speed of the
tube, thP optical fibre and/or optical bundle loosel~ housed in
the tube i5 subjected to considerable shear forces and these must
be maintained at a low value by use of a compound of low
viscosity, e.g. 20 c/s, at the line temperature.
Preferably the tube is drawn from the extruder and
through the cooling and heating means by at least two endless belt
haul-off devices which determine the linear speeds of the tube and
hence draw it down to the desired cross-sectional size. The
control circuit of the drive means for the first endless belt
haul-off device may be linked with the output of a diameter gauge,
e.g. an optical diameter gauge, to give automatic control of the
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speed of the extruder screw. ~ third endless belt haul-off device
is preferably employed downstream of the final cooling bath for
controlling the degree of contraction resulting from the annealing
process.
One or each, but not the last, of the endless belt haul-
off devices may be heated to assist in ensuring that the water-
impermeable medium in ~he advancing tube is maintained in the
desired liquid state throughout the manufacturing process.
~ he invention also includes apparatus for use in
the manufacture of an optical cable element of the kind specified
by the method as hereinbefore described.
The invention further includes an optical cable element
of the kind specified when manufactured by the method as herein-
before described.
~ he invention is further illustrated by a description,
by way of example, of preferred apparatus for applying a water-
impermeable medium in a liquid state to an advancing optical fibre
in the manufacture of an optical cable element of the kind
specified, with reference to the accompanying diagrammatic drawing
which shows a sectional side view of the apparatus.
Referring to the drawing, the apparatus comprises
extrusion apparatus 1 to the upstream end of which i5 connected an
elongate tubular chamber 2 which, over a part 3 of its length
remote from the extrusion apparatus, is open at the top.
~onnected to a port 4 in the wall of the elongate chamber 2, is an
electrically heated storage tank 5 in which water-impermeable
medium i5 maintained in a liquid state.
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The water-impermeable medium comprises a compound
containing:-
100 parts technical white oil WOT 14 ex Burmah Castrol
4 parts Shell Elexar thermoplastics rubber 8431Z.
The rate of flow water-impermeable medium in the storage tank 5 is
controlled manually by a stopper 6 of polytetrafuoroethylene which
can be inserted into the port 4 by means of a manually controlled
threaded shaft 7.
Positioned beneath the part 3 of the elongate chamber 2 open at
the top, is a reservoir 8 which is connected through a pump 9
and via a pipe 10 back into the top of the storage tank 5.
Slidably mounted in the elongate chamber 2 is a stainless steel
capillary tube 11 for use in initially threading an optical fibre
F through the chamber, when the chamber is filled with water-
impermeable medium, and into and through the extrusion apparatus.
It will be appreciated that since the elongate chamber 2 is open at
the top along a substantial part 3 oE its length the water-
impermeable medium being fed into the chamber through the port 4
will be maintained at a substantially constant pressure head since
excess water-impermeable medium will :Elow out of the chamber and
into the reservoir 8.
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