Note: Descriptions are shown in the official language in which they were submitted.
2~3
This invention relates to the coating of fibres and
has application to the coating of optical fibres which are .
used as dielectric optical waveguides.
Optical fibres are fibres of light transmitting :
material which can transmît light ~y multiple internal
reflection in the fibre. Tfie fi~res have very small diameters,
typically of the order of 10.0 microns, and are accordingly very
fragile. They are coated m order to protect them from axial
distortion leading to increa~ed attenuation fiy radiation,. to
protect them from abrasion, and atmospheric attac~.and so
maintain their pristine strength, and to provide additional
strength. In one known technique ~or coating optical fibres
an optical fi~re is sandwic~ed ~etween two tapes and the.tape.s -
~
are. adhered along the junction of the~r edges. The fihre'is ~ .
held loosel~ within t~e adhered tapes which'constltute a
separate sleeve, so that t~ere.'is a clearance. ~etween th~'~ihre
and the sle.eve.
In another known te.c~nlque'an.opt~cal ~ e is coatedwith a layer of extrudahle'pol~ymer h.y direct:estrusIon Q~ the
: 20 polymer using an extruder wit~'a~cross-$ead die~ The'aoating
produced by this technique'is in contact with and adhered to
the fibre 80 that the ~ibre i8 not loose'w.ithin the'coating.
The ~ibre and the.polymer coating ~ave di~ferent
.
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`- lU~263
coefficients of thermal expansion so that~ as the palymer
sets, they contract at different rates, This can lead to
bending of the fibre which can cause radiation of the light.
This will increase the attenuation of signals which are
transmitted along the fibre and is therefore undesirable.
According to one aspect of the present invention
there is provided an optical fibre including a tubular sleeve
of polymer which loosely envelops the fibre, the sleeve
having its molecules orientated to lie longitudinally with
respect to the axis of the fibre.
The fibre may be an optical fibre for use as a
dielectric optical waveguide. The polymer may be polypropylene.
According to another aspect of the present invention there
is provided a fibre having a tubular sleeve which loosely
envelops it, said sleeve having a relatively large elastic
modulus in its longitudinal direction and a relatively small
elastic modulus in a transverse plane.
By loosely envelops is meant that there is at least
a small distinct clearance between the surface of the fibre
and the internal periphery of the sleeve, that is to say, the
sleeve envelops the fibre making a loose fit therewith. The
fibre is not stuck to the sleeve so that it can be moved
axially within the sleeve ................................
, ~ .
-- 3 --
The diameter of the sleeve is, however, larger than the fibre
diameter so that in the event of a slight longitudinal con-
tract,ion of the sleeve relative to the fibre the buckling of
the f:ibre will be of sufficiently large radius of curvature
to prevent appreciable increase in attenuation. ,~
By polymer we have in mind synthetic extrudable
polymers of which polypropylene is a preferred example and
the term polymer is intended to include any material whose'
molecules can be orientated to lie longitudinally with respect ,
10 to the axis of the sleeve of the polymer which is produced ~ ~'
by the process to be described hereinafter. ; '~
The fibre may be an optical fibre for use as a
dielectric optical waveguide. The polymer may be polypropylene. ,,.,r
According to a further aspect of the present invention there
is provided a method of sheathing an optical fibre which
comprises forming by extrusion a sleeve of polymer, feeding
the fibre into the sleeYe as the sleeve'is extruded, the' '
internal diameter of thé sleeve being substantially greater
than the diameter of the fibre, reducing by a drawing process
20 the internal diameter of the sleeve to such an extent that it ~,
: . . .
- loosely envelops the'fibre a~d annealing the'loosely `
enveloping sleeve.
' The method has p~rticular application to the
coating of optical fibres. During the drawing process
of the sleeve the molecules of the polymer tend to ..~.........
.
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. .
10~8Z~;~
line up longitudinally thereof. The sleeve of polymer is
normally fed through an oven as it is drawn to assist in the
lining up of the molecules in the longitudinal direction. The
longitudinal orientation of the molecules gives the sleeve
relative strength in a longitudinal plane and relative weakness
in a transverse plane. The combination of increased longitudinal
strength and decreased transverse strength serves to greatly
improve the sleeve's ability to protect the fibre from axial
distortions. The strength in the longitudinal plane is useful
in improving the overall strength and so protecting the fibre
during the cable making process. The weakness in the transverse
plane is useful in stripping the sleeve from the ends of a fibre `~
to enable the fibres to be joined.
The sleeve loosely envelops the optical fibre and ~--
micro-bending problems do not occur in the present method. Also
it is relatively simple to feed the fibre into the sleeve of
polymer since the sleeve has a much greater diameter than the
fibre before it is drawn down.
The sleeve may be heated in an oven as its diameter is
reduced. The sleeve may be passed through a cooling bath after
it is extruded and before its diameter is reduced.
The coated fibre may be passed through a further oven
after the sleeve has been drawn. This is to substantially
eliminate long term shrinkage of the sleeve after extrusion and
drawing.
According to another aspect of the present invention
there is provided apparatus for coating a fibre which comprises
an extrusion device for extruding a sleeve of polymer, means for
feeding the fibre into the sleeve as the sleeve is formed, the
sleeve having a diameter substantially greater than that of the
fibre, and drawing means for reducing the internal diameter of
the sleeve to produce a sleeve which loosely envelops the fibre.
The extrusion device may include a screw-extruder which
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, . . .
108~Z~3
is arranged to feed a cross-head die. The screw-extruder may
have a heated feed tube which is connected to the cross-head
die by a clamp, said clamp being thermally connected to a heater.
The cross-head die may have a through bore one end of which
con~titutes the outlet for extruding polymer, a first tubular
portion which extends into the bore from its other end to a
position adjacent the outlet, said first tubular portion having
a tapered end portion adjacent said outlet, a second tubular ~-
portion mounted coaxially within the first tubular portion, and
a third tubular portion mounted coaxially with the first and
second tubular portions, said third tubular portion extending
into the second tubular portion adjacent the other end of the
die, out through the tapered end of the first tubular portion
and terminating in the vicinity of the outlet of the die, the
arrangement being such that in use a fibre passing through the
third portion is thermally insulated from the heated polymer in
the die over a substantial portion of its length of travel
through the die by the arrangement of tubular portions.
The apparatus may include cleaning means disposed
upstream of the extrusion device for cleaning the fibre before
it is coated. The cleaning means may include a pair of
juxtaposed felt pads between which the fibre is arranged to
pass and a blowing ring for blowing cold air onto the fibre.
The apparatus may include a take-off drum from which
fibre can be fed to the extrusion device and tensioning means
for maintaining the fibre under a substantially constant tension.
The tensioning means may comprise first and second horizontally
spaced pulleys and a floating pulley arranged to sit on the
fibre so that it forms a U-shape intermediate the pulleys, and
two vertically spaced light sensitive detectors located such
that when the fibre is at its correct tension one is above and
the other is below the floating pulley, each light sensitive
detector being illuminated by a light source and being connected
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~` 10882~3
to an elcctrical control for drive means of the take-oEf drum
whereby if the floating pulley rises to interrupt light passing
to the upper detector the speed of rotation of ~he drum is
increased and if the floating pulley falls to interrupt light
passing to the lower detector the speed o~ rotation of the drum
is decreased.
The apparatus may include a cooling bath disposed
immediately downstream from the extrusion device. The cooling
bath may be rectangular and contain a longitudînally extend;ng
guide tube through which the coated fibre is arranged to pass,
the bath having inlet and outlet apertures through whic~ the
coated fibre can pass. A plate may be located in parallel
spaced relationship with the wall of the bath having said inlet
aperture, the plate having an aperture corresponding in size -
and position to the inlet aperture, the size of said apertures
being such that in use, as coated fibre enters the bath, water
can flow out of the bath through said inlet aperture but is
prevented by surface tension from flowing through the aperture
in said plate.
T~e drawing means may comprise a first pair of rotatable
belts, said belts being spaced apart a distance corresponding
substantially to the diameter of the extruded sleeve, and a
second pair of rotatable belts located downstream from said
first pair, said second pair of belts being spaced apart a
distance substantially less than the spacing of said first pair.
The apparatus may have an oven intermediate the first
and second pair of belts, the sleeve being arranged to pass
through the oven as its diameter is reduced. The apparatus
may have a further oven located downstream from the second
pair o~ belts. The further oven may have a first relatively
high temperature zone, and a second relatively low temperature
zone. The further oven may have air jets which are so
arranged that air can be blown therethrough onto the sleeve
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as it leaves the further oven.
The apparatus may include a capstan located downstream
from said further oven, the capstan comprising a pair of pulleys
one of which is arranged to be driven at a speed slightly less
than the speed of rotation of the second pair of belts to allow
for shrinkage of the sleeve after it leaves the further oven.
The invention will be described now by way of example
only with particular reference to the accompanying drawings.
In the drawings:
Figure 1 is a schematic representation of an apparatus
for coating an optical fibre;
Figure 2 is a side sectional elevation of a cross-
head die and cooling bath of the apparatus of Figure l;
Figure 3 is a section on the line III - III of Figure 2;
Figure 4 is a side sectional elevation on the line ~V -
IV of Figure 2 showing the screw-extruder of the apparatus of
Figure l;
Figure 5 is a plan view of a heated clamp used with
the extruder of Figure 4;
Figure 6 is an end elevation of the heated clamp, and
Figure 7 is a plan view of a device used in the appara-
tus of Figure 1.
Apparatus for coating a glass fibre has a drum 10 on
which glass fibre to be coated is wound. The drum 10 is
rotatable by means of a motor (not shown~ in order to unwind
fibre 11 from the drum lQ.
First and second horizontally spaced pulleys 12 and
14 are spaced horizontally from the drum 10. Fibre 11 unwinding
from the drum 10 passes to the pulley 12 and is made to travel
along a U-shaped path between the two pulleys 12 and 14 by means
of a floating pulley 16 which sits on the fibre at the bottom of
the U-shape.
Two photocells 15, 17 are spaced in a vertical plane
~U88Zbi3
which is spaced laterally from the vertical plane containing
the pulley 16. One photocell 15 is located at a position
higher than the pulley 16 and the other photocell 17 at a
position lower than the pulley 16. Each photocell is illumi~
nated by a respective light source (not shown), the light
sources being contained in a vertical plane and positioned such
that the plane containing the pulley 16 is disposed intermediate
the plane containing the photocells and the plane containing
the light sources. Each photocell is connected to a circuit
for controlling the speed of the motor which drives the drum 10.
The arrangement is such that if the pulley 16 rises and inter-
rupts light transmitted towards the phot~cell lsthe speed of
the motor is increased. Alternatively if the pulley 16 falls
and interrupts light transmitted towards the photocell 17 the
speed of the motor is decreased. This arrangement ensures that
the pulley 16 remains at a substantially constant vertical
position and maintains a subætantially constant tension in the
fibre.
From the pulley 14 the fibre 11 passes toward a cross-
head die 18 which is connected to a screw-extruder 1~. The axis
of the cross-head die 18 lies on a horizontal tangent from the
pulley 14. A fibre cleaning arrangement, which consists of à
pair of juxtaposed felt pads 21 and 22 and an annular blowing
ring 24, is disposed intermediate the pulley 14 and the cross-
head die 18. The blowing ring 24 has a number of nozzles which
open at its inner peripheral sur~ace. The nozzles are connected
to a source of air pressure and are arranged so that air can be
blown radially inwardly from the nozzles toward the centre of ;
the ring 24.
Fibre travelling from the pulley 14 passes between the
pads 21 and 22 and through the central aperture in the ring 24
before entering the cross-head die 18. The felt pads 21 and 22
are closely spaced so that they contact the fibre as it passes
: ~: ' ' . ' ' ' - ; '
10~t~263
therebetween. The felt pads are provided to remove any dirt
or similar small particles from the surface of the fibre. The
blowing ring blows air onto the fibre to blow off any remaining
dust or dirt.
The cross-head die 18 has a main body portion 25 with
an inlet port 26 which communicates radially with a bore 27
which extends axially through the body portion 25. The bore 27
has a cylindrical first portion 28, a generally cylindrical
second portion 29 extending coaxially from the first portion 28
and having a diameter larger than that of the first portion, and
a cylindrical third portion 30 coaxial with the first and second
portion. The cylindrical third bore portion 30 constitutes an
outlet from the cross-head die 18. The inlet port 26 is clamped ~-
to the screw-extruder 19 in a manner to be described later. The
screw-extruder contains polypropylene which is to be extruded ^~
through the outlet constituted by the cylindrical third bore
portion 30.
The wall of the cylindrical first bore portion 28
supports a thermal insulating assembly 35 for preventing
undesirable heating of the fibre as it passes through the cross-
; head die 18. The assembly 35 comprises an outer generally
tubular member 36 which is supported by the wall of the cylind-
rical first bore portion 28. The tubular member 36 extends
from a position external of the die 18, through the bore portion
28 and into the bore portion 29. The end 38 of the tubular
member 36 within the bore portion 29 is tapered and defines a
cylindrical bore 39 the diameter of which is substantially
less than that of the diameter of the main part of the tubular
member 36. The bore 39 terminates adjacent the cylindrical
third bore portion 30. The other end 40 of the tubular mem~er
36 has a radially inwardly extending annular flange 41 and
adjacent the flange 41 the circumferential wall of the tubular
member 36 has a radially outwardly extending port 42.
-- 10 --
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A second intermediate tubular member 44 is mounted
coaxially with the tubular member 36. The second tubular
member 44 is supported towards one end 45 thereof by the annular
flange 41 and at its other end 46 by a triangular spacer 47
which is carried within the tubular member 36. The second
tubular member 44 has a radially, outwardly extending port 48
adjacent its end 45.
A third inner tubular member 50 is mounted coaxially
with the fir~st and second tubular members 36, 44. The third
member 50 extends through the second member 44, through the
bore 39, and terminates within the third bore portion 30 of
the bore 26. ~
Fibre travelling from the cleaning arrangement passes ~ -:
through the inner tubular member 50. Air from a suitable air ~
supply is passed in through the port 48, circulates between :
the tubular members 36, 44 and 50 and passes out through the
port 42. This arrangement effectively insulates the interior
of the tubular member 50 from heated polypropylene within the
die 18 over a substantial portion of the length of the tubular
member 50. This serves to prevent undesirable heating of the
fibre as it passes through the tubular member 50.
As the fibre 11 leaves the tubular member 50 poly-
propylene is extruded in the form of a sleeve 51 around the
fibre between the exterior o the third tubular member 50 and
the wall of the cylindrical third bore portion 30. The internal
diameter of the extruded sleeve 51 is substantially greater than
the external diameter of the fibre.
A cooling bath 55 is disposed downstream from t~e
cross-head die 18. The cooling bath 55 comprises a small tank
of rectangular cross section. The opposite end walls of the
tank each have an aperture 57, 58, the axis of each aperture
being coaxial with the axis of the cylindrical portion 30. A
longitudinally extending guide tu~e 60 is mounted within the
1()t~32~a;3
tank so that its axis is coaxial with the axis of the apertures
57, 58. The guide tube 60 has a pair of diametrically opposite,
radially extending ports 61, 62 at its axial-mid point. A plate
64 is mounted in closely spaced relationship with the end wall
65 of the tank closest to the cross-head die 18. The plate 64
has a circular aperture 67 which is coaxial with and has a size
substantially the same as the aperture 57 in the wall 65. A
drain passage 69 is disposed vertically beneath the plate 64.
The tank 55 is mounted on a pair of parallel guide
rails 7~ (.only one shown). The tank 55 is slidable along the :
guide rails toward or away from.the die 18. In normal operation ~`
the tank 55 is disposed relati.vely close to the outlet of the
die 18.
In use the tank 55 is filled with water to a depth
such that the guide tube 60 is fully submerged. The fibre and
its sleeve coating pass into the tank through the apertures 67
and 57 and into the guide tube 60 and then out through the ~ .
aperture 58. Water flows in through the ports 61, 62 along the
guide tu~e around the polypropylene sleeve and thereby cools it.
It will be noted that the tube 60 guides the sleeve without ~
actually contacting the sleeve since there is water between the
sleeve and the tube 60. The water can flow out through the
aperture 57 around the external ~urface of the polypropylene
sleeve and down into the drain passage 69. The diameter of the
apertures 67 and 57 are so selected that when the flow rate of
water into the tank is adjusted correctly water can flow out
of the tank through.the aperture 57 but is prevented from flowing
through the aperture 67 by the surface tension of the water.
The water flowing out through the aperture 57 falls through the
space between the plate 6~ and the end wall 65 into the drain
passage 69.
A first pair of belts 75, 76 are spaced horizontally
from the cooling bath 55. The belt 75 extends around rollers 78,
Z~;3
79 and 80 and the belt 76 extends around rollers 81, 82 and 83.
The spacing between adjacent portions of the belts 75 and 76 i8
substantially equal to the outer diameter of the sleeve which
lea~es the cooling bath 55. Each of the rollers 78 and 81 is :~
rotatable by suitable drive means in order to drive the belts
75 and 76.
A first oven 90 is disposed downstream from the belts
75, 76. The first oven 9Q is generally cylindrical and has a
central cylindrical bore 91 the axis of which is coaxial with
the bore in the cross-head die 18. ~he coated fibre which is
driven from the first set of belts 75, 76 passes along the bore
91 in the oven 90.
A second pair ox belts 94, 95 are disposed downstream
from the first oven 90. The belt 94 extends around rollers 9~, :
97 and 98 and the belt 95 extends around rollers 99, 100 and 101.
The rollers 96 and 99 are rotatable by suitable drive means in
order to drive the belts 94 and 95. The spacing between adjacent :
portions of the belts 94 and 95 is considerably less than the
spacing between the belts.75 and 76. A mechanism 104 is provided
adjacent the rollers 96 and 99 for moving the coated fibre
passing from the oven 90 towards the belts 94, 95 from side to
side. This mechanism includes two rollers 106, 107 which are ``
arranged with their axes vertical. The rollers are actuated to
move perpendicular to the axis of the belts by a rotatable cam
108 driven from the drive means of the rollers 96 and 99. The
cam 108 contacts and is arranged to move axially, a shaft 102
which can push a mounting 109 of the rollers lQ6, 107 against ~.
the bias of springs la5.
A second oven 110 is disposed downstream from the
belts ~4, 95. The second oven is generally cylindrical and
has a cylindrical bore 111 the axis of which is coaxial with the
bore of the oven 90. The ov~n llQ has two heating zones. One
zone at the end of the oven closest to the belts 94, 95 can be
- 13 -
10~8X~;3
heated to a higher temperature than the other zone towards the
end of the oven remote from the belts 94, 95. An annular member
wit~ a number of nozzles 112 is provided at the end of the oven
110 remote from the belts 94, 95. The nozzles are connected to
a source of air so that the sleeve can be cooled as it leaves
the oven 110 by blowing cold air onto it.
A capstan 115 is mounted downstream from the oven 110.
The capstan comprises a pair of juxtaposed pulleys 116, 117
between which the coated fibre is arranged to pass. One of the
pulleys is rotatably driven. A take-up drum 118 for receiving
the coated fibre is mounted downstream from the capstan 115.
Turning now to Figures 4 to 6 the screw-extruder has a
hopper 120 into which is fed granulated plastic material which
in this case is polypropylene. Polypropylene is fed from the
hopper 120 via a baffle 123 into a horizontally extending feed
tube 124. The feed tube 124 has an axially extending Archime
dean screw which is shown diagramatically at 125. The
Archimedean screw 125 is driven via a shaft 126. The feed tube ~ -
124 has four regions. A first region 127 is water cooled; the
subsequent three regions are heated by three electrical heating
elements 128, 129, 130. The polypropylene melts as it passes
through the regions heated by the elements 128, 129, 130.
At the end of the feed tube 124 remote from the
hopper 120 there i~ provided a breaker plate 132 which serves
to smooth out the helical flow of polypropylene created by the
Archimedean screw and also provides a back pressure for the screwu
This end of the feed tube 124 is attached to the inlet port
26 of the cross-head die 18. The joint between the feed tube
124 and the inlet port 26 is made by means of a heated clamp 134.
Referring to Figures 5 and 6 the clamp 134 consists of
two similar members 136, 137 which are connected together at
one side by a hinge arrangement 13g. Each member 136, 137 has
a semi-circular surface 141, 142, the semi-circular surface
-- 1()8~Z~3
being so arranged that when the two members 136 and 137 are
placed in juxtaposition the surfaces define a circular aperture
for the junction between the tube 124 and the port 26. The
surfaces 141, 142 are recessed at 143 to provide means for
engaging flanges on the end of the tube 124 and on the end of
the port 26. The two members 136 and 137 can be clamped
together by means of a nut 144 and screw 145. Each member 136,
137 has a cartridge heater 147, 148.
The heated clamp ring 134 is provided to prevent
excessive heat loss which can occur in the vicinity of the
clamp ring and which can cause a low temperature point between
the feed tube 124 and the inlet port 26. If such a low
temperature point were to occur the result could be a fluc-
tuation in the dimension of the sleeve 51 extruded from the
cross-head die 18.
Prior to use of the heated clamp ring the temperature
in the cross-head die itself was 205C and the temperature in
the vicinity of the clamp ring 200C plus or minus 10C. With
the heated clamp ring the temperature in the cross-head die is
20QC and the temperature in the vicinity of the clamp ring
195C.
In use glass fibre is fed from the drum 10 through the
cleaning arrangement consisting of the felt pads 21, 22 and
the blowing ring 24 to the cross-head die 18. As the fibre
leaves the cross-head die polypropylene is extruded from the
cross-head die around the fibre in the form of a sleeve. The
internal diameter of the sleeve is of the order of 0.5m.m. to
lm.m. and the external diameter of the order of 2m.m. The
internal diameter of the sleeve of polypropylene is t~us sub-
stantially greater than the diamter of the fibre which typicallyhas a diameter of 125 microns. The sleeve of polypropylene is
then cooled as it passes through the cooling ~ath 55. The
sleeve and fi~re are pulled through the oven 90 by means of the
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belts 94, 95 which are driven at a speed substantially greater ,
than the speed of the belts 75, 76. Typically the belts 94,
95 are driven at a speed of the order of 10 times greater than
the speed of the belts 75, 76. The effect of this is to reduce
the cross sectional dimensions of the sleeve as it passes
through the oven 90. At this stage the polypropylene is in a
semi-viscous state. The oven 90 is operated at a temperature
of approximately 120C - 130C. After being drawn down the
outer diameter of the sleeve is of the order of 600 microns and
the internal diameter is in the range of 160 microns to 300
microns. Thus, after drawing down the sleeve of polypropylene
has an internal diameter which is of the order of twice that of
the fibre and is such that the sleeve loosely envelops and is
not adhered to the fibre. The fibre can be moved longitudinally
within the sleeve but the diameter of the sleeve is sufficiently
small to substantially inhibit buckling of the fibre within the
sleeve.
During extrusion of the sleeve of polypropylene from
the die its molecules are slightly orientated so that they tend
to line up in a direction of the longitudinal axis of the
sleeve. The molecules become highly orientated when the sleeve
passes through the oven 90. Because of the orientation of its
molecules the sleeve 150 which is produced has a relative
strength in a longitudinal plane and relative weakness in a
transverse plane. The strength in the longitudinal plane greatly
assists in mechanical isolation of the fibre from axial distor-
tions and gives strength to the fibre. The weakness in the
tran~verse plane is of assistance when the sleeve has to be
stripped from the ends of the fibre to allow it to be joined to
another fibre.
After passing between the belts 94, 95 the coated
fibre is then passed through the oven 110. The first zone of
this oven closest to the belts 94, 95 is operated at a tempera-
- 16 -
i3
ture of approximately 250C to 300C and the second zone remote
from the belts 94, 95 is operated at a temperature of 120C to
130C. This arrangement is provided so that the sleeve entering
the oven is quickly heated up in the first zone to a temperature
at ~Ihich it anneals and i~ then maintained at this temperature
in t:he second zone. The oven 110 acts as an annealing oven and
is provided to substantially eliminate long term shrinkage of
the polypropylene sleeve after extrusion. If the sleeve is
not passed through the oven 110, shrinkage of 5% to 10% can
occur and this can produce undesirable bends in the optical
fibre.
Coated fibre is pulled from the annealing oven by
means of the capstan 115 and is then wound onto the take-up
drum 118. The pulleys 116, 117 of the capstan are arranged to
rotate at a speed which is approximately 8% less than the speed
of movement of the belts 94, 95. This difference in speed is -
provided to allow for shrinkage of the sleeve which occurs as ~ -
it passes through the annealing oven. This shrinkage is of the
order of 8%. -
The device 104, which moves the sleeve 51 to and fro
transversely to the axis of the ~eIts 94, 95, is provided to
minimise wear of the belts 94, 95.
The start-up procedure for the process i8 as follows.
The cooling hath 55 which in normal operation i8 relatively
close to the outlet of the cross-head die 18 is moved axially
away from the die. A short length of sleeve is extruded from ,
the die and is fed manually through the cooling ~ath between
the belts 75, 76 through the or;entation oven and fed into the
belts 94, 95. Once this has occurred t~e cooling bath 55 is
moved back to a position close to the outlet of the cross-head
die 18 so that the process can continue normally. The sleeve
is fed through the annealing oven 11~, the capstan 115 and on - -
to the take-up drum 118. Fibre 11 is then fed into the sleeve
,- . . , . - . :
2~i3
and manipulated so that it attaches to the internal surface of
the sleeve. The sleeve, which i8 driven through by the belts
94, 9S then pulls the fibre with it so that it begins to wind
round the take-up drum. Once the coated fibre is on the take-
up drum the friction between the fibre and the sleeve on the
drum ensures that the fibre is pulled through the apparatus
along with the sleeve.
An alternative start-up procedure, which has been
found to be very satisfactory, iæ as follows. The sleeve is
extruded by the same procedure described in the preGeding
paragraph. When the sleeve has been fed through to the take-
up drum 118 fibre 11 i8 fed into the sleeve 51 at a speed
somewhat slower than that of the belts 75, 76. This speed is
controlled by the pay-off drum 1~, which is driven by a servo-
motor, until the end of the fibre is in the region of the
annealing oven 110 (the length of fibre inside the sleeve is
determined by measuring the length of fibre fed into the sleeve~.
The pay-off drum 10 is then rapidly accelerated to a~out the `~
speed of the pulleys of the capstan 115.
The coated fibre produced by the present method consists
of an optical fibre in a loose fitting sleeve, the internal
diameter of the sleeve being slightly greater than the diameter
of the fibre whereby the fibre can be slid longitudinally within
the sleeve. The material of the sleeve has a relatively high
elastic modulus in the direction of the sleeve axis whilst in
a transverse plane it has a relatively low modulus. The
structure is thus resistant to bending but can be compressed
in a direction perpendicular to its longitudinal axis.
We have found that within experimental error coating
the fibre in this manner introduce~ no significa~t additional
loss in the fibre.
As an alternative to a screw-extruder, a ram extruder
may be used.
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It will be appreciated that whilst the above description
relates to the coating of a dielectric optical waveguide the
present method can be used to coat other types of fibre which
require protective coating.
. i
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