Note: Descriptions are shown in the official language in which they were submitted.
13l332o
OPTICAL CABLE
This invention relates to optical cable.
Optical cables have certain common elements.
These include at least one optical fiber for transmission
purposes, means for protecting the fiber from damage, and a
jacket which provides the outer layer of the cable.
In some cable structures, optical fibers are
housed in grooves formed in the outer surface of a central
support member, the grooves extendinq around the member
either helically in one direction or alternately, in each
direction around the member.
In other cable structures, an optical fiber or
fibers is housed within a plastic tube located coaxially of
the cable. These tubes are normally provided for the sole
purpose of forming a passage for the fibers and any
protection to prevent crushing of the cable and thus of the
fibers is provided by a compression resistant shield which
surrounds the fiber carrying tube.
Conventionally, protective tubes are formed from
plastics which provide an inadequate tensile strength to
protect the optical fibers against tensile loadings. Hence,
in conventional cables some other method of providing the
necessary tensile strength is required such as steel fila-
ments extending longitudinally of the cable and lying
exteriorly of the protective tubes. It is normal to provide
the steel filaments in a jacket surrounding a protective
tube or tubes. A steel sheath around a core of protective
tubes may also provide the required tensile strength.
In a proposed structure related to a cable, an
optical fiber has a reinforced plastic coating surrounding
it. This is described in a paper entitled "New Applications
of Pultrusion Technology RP Covered Optical Fiber" by K.
Fuse and Y. Shirasaka and read before the 4Oth Annual
Conference in January 1985 of Reinforced Plastics/Composites
Institute, The Society of the Plastics Industry Inc. As
described in that paper, an optical fiber is surrounded by a
buffer material and then enclosed within a tube of re-
inforced plastics by a manufacturing process referred to as
pultrusion. In this process, reinforcing fibers are coated
1313320
with a resin and the coated fibers and the pre-buffered
optical fiber are passed through a die with the optical
fiber located centrally so that the resin on the reinforcing
fibers merges to form the plastic coating.
According to the present invention an optical
cable is provided having a plurality of optical fibers, a
cable jacket and a tubular tensile strength member surround-
ing the plurality of optical fibers as a group, the tubular
tensile strength member comprising a plurality of tensile
filaments with gaps between adjacent tensile filaments
filled by a rigid material holding the tensile filaments in
their relative positions in which the tensile filaments
extend side-by-side longitudinally of the cable, the tensile
filaments occupying more volume of the tubular tensile
strength member than is occupied by the rigid material, and
the cable jacket extruded onto and contacting the tubular
tensile strength member and with the tubular tensile
strength member having an inner diameter greater than the
combined diameters of the optical fibers of the group
whereby each optical fiber is radially movable within the
tubular tensile strength member.
With the structure according to the invention, the
optical fibers are loosely contained within the tubular
tensile strength member so as to enable relative longi-
tudinal movement of the optical fibers and tube duringflexing or bending of the cable while axial tension is not
placed upon the fibers by the surface of the tubular tensile
strength member.
In addition, the tubular tensile strength member
in the construction of the invention acts as a protective
tube for the optical fibers and has mechanical properties
which are superior to those offered by a conventional
protective tube.
In a cable of the present invention, each longi-
tudinally extending tensile filament in the tubular tensilestrength member surrounding the optical fibers is a tensile
strength element. Hence, because the tensile filaments are
densely packed side-by-side, a tubular tensile strength
3 1313320
member of relatively small diameter in the cable of the
invention may have a tensile strength comparable to and
possibly exceeding the tensile strength of an optical cable
of much larger diameter having a protective tube for optical
fibers and steel strength elements lying outside the tube,
e.g. within the jacket. While an elastomeric jacket is
provided around a tubular tensile strength member in the
cable of the present invention, it follows that no tensile
strength elements are required either within the jacket or
in any other location outside the tubular tensile strength
member. In a practical example of inventive cable which may
dispense with the use of a jacket and steel sheath, the
cable may consist of a plurality of optical fibers loosely
contained within a tubular tensile strength member having an
outside diameter as small as 4.10 mm and an inside diameter
of 1.70 mm.
In the tubular tensile strength member of the
inventive optical cable, the filaments occupy a greater
volume of the tubular strength member than the rigid
material, there is preferably at least 70% of the tube
volume in the form of tensile filaments with the rigid
material in the interstices between the filaments providing
the remainder of the volume of the ~ubular tensile strength
member. In one particularly practical example, the tensile
filaments occupy approximately 80% by volume of the tubular
tensile strength member and the rigid material occupies
approximately 20% by volume.
The rigid material in the tubular tensile strength
member is preferably a thermosetting material such as a
polyester or epoxy resin and the tensile filaments are
preferably glass filaments, but alternatively may be, for
instance, high strength aramid fibers such as "Kevlar"
(trade mark).
In a preferred arrangement, the inside of the
tubular tensile strength member unoccupied by the optical
fibers is filled with a water bloc-king medium. This water
blocking medium may be a viscous water blocking medium or is
preferably a thixotropic water blocking medium.
1313320
one embodiment of the invention will now be
described, by way of example, with reference to the
accompanying drawings, in which:-
Figure 1 is an isometric view of part of a cable
according to the invention;
Figure 2 is a cross-sectional view taken along the
axis of the cable and on a larger scale than Figure 1;
Figure 3 is a cross-sectional view of the cable
taken along line III-III in Figure 2;
Figure 4 is a diagrammatic side elevational view
of apparatus according to the invention for making the cable
of Figures 1 and 2; and
Figure 5 is a view similar to Figure 3 of a part
of the apparatus on a larger scale.
As shown in Figure 1, an optical cable 10 has a
plurality of optical fibers 14 housed within a tubular
tensile strength member 16. The strength member 16 is
surrounded by a dielectric jacket 18 which may be, for
instance, a polyethylene based material.
As can be seen from Figures 1 and 2, the optical
fibers 14 have diameters substantially less than the inside
diameter of the tubular tensile strength member. As a
group, the combined diameters of the fibers are less than
the inside diameter of the strength member 16 whereby the
fibers are loosely contained and radially movable within the
tubular tensile strength member even though there may be ten
or more fibers in the cable.
The cable construction of this embodiment does not
require a cable sheath or shield to protect the optical
fibers as the tubular tensile strength member 16 is capable
of withstanding substantial tensile loads with insignificant
strain. For the same reason, no tensile strength element
outside the tubular tensile strength member 16, such as
steel filaments in the jacket are required. In the par-
ticular construction shown in Figure 1, the cable is capableof being subjected to a tensile load of 600 lbs. While the
strength member 16 satisfactorily protects the fibers from
such loading. The outside diameter of the cable of the
1;~3~ZO
embodiment is 6.5 mm and the tubular tensile strength member
has an outside diameter of 4.1 mm and an inside diameter of
1.7 mm.
The tubular tensile strength member 16 comprises a
plurality of side-by-side and closely packed tensile glass
filaments 20 which extend longitudinally of the strength
member and are embedded within a continuous phase solidified
rigid carrier material 22 which occupy gaps between the
filaments 20. The glass filaments 20 occupy at least 70~
and preferably 80% by volume of the strength member 16 with
the remainder of the volume of the strength member occupied
by the rigid carrier material 22. This rigid carrier
material is a polystyrene or polyester based resin. As can
be seen from Figure 3, the tensile glass filaments 20 lie in
close side-by-side positions while extending longitudinally
of the tubular tensile strength member 16. The strength
member is capable of withstanding up to 600 lbs tensile
load, as has been indicated, and with a minimum strain which
prevents tensile loads acting directly upon the fibers
themselves. Because the strength member provides the
tensile strength of the cable, it is unnecessary to provide
the cable with a metal sheath for tensile purposes or to
provide tensile strength elements such as steel filaments
outside the strength member. Hence, the strength member 16
is simply surrounded by the jacket 18 which is provided
solely to protect the tubular tensile strength member from
outside environmental conditions.
The tensile glass filaments 20 extend substan-
tially longitudinally of the tubular tensile strength member
16 so as to resist any extension of the cable caused by
tensile loading such as may occur during bending or twisting
of the cable as it is being installed or after installation.
For instance, twenty-four tensile glass filaments are
provided with the filaments closely positioned together.
~ach filament comprises a plurality of strands or rovings of
glass fibers.
The passage in the center of the tubular tensile
strength member 16 is filled by the optical fibers and a
6 13~33ZO
thixotropic water blocking medium which fills any spaces not
occupied by the fibers.
The cable is made by the in-line apparatus shown
in Figures 4 and 5. Such an apparatus and a method for
manufacturing cable of the present invention is the subject
of co-pending Application Serial No. 511,890. As may be
seen, the apparatus comprises a reservoir 24 holding a bath
26 of the polystyrene or polyester based resin. Downstream
along a passline for the groups of tensile glass filaments
20 is disposed a strength member forming means 2~. This
forming means comprises a tubular guide means in the form of
a stainless steel tube 30 which extends along the passline
of the glass fibers and has a polished outer surface.
Surrounding a downstream end portion of the tube 30 is a
heating means 32 which comprises a housing 34 shrouding
heating elements 36 which may be electrical. As can be seen
from Figure 4 the housing 34 has an inner cylindrical
surface 38 which is polished and surrounds the downstream
end portion of tube 30 while being spaced from it to define
a tubular space 40 between the heating element and the tube
30.
Guide means is provided for holding the tensile
glass filaments 20 in laterally spaced relationship as they
pass through the reservoir 24, for disposing these filaments
in spaced apart positions around an arc concentric with the
tube 30 and also for causing convergence of the coated
tensile filaments towards an upstream inlet end 42 of the
tubular space 40 to bring the tensile filaments into close
relationship as they enter the space. This guide means
comprises a plurality of side-by-side guide pulleys 44, one
pulley for each of the tensile filaments. In Figure 4 only
one of the guide pulleys 44 is shown as the guide pulleys 44
for all filaments are in alignment in that Figure. From
guide pulleys 44 to pulleys 50 lying downstream, the paths
of all tensile filaments are in alignment, i.e. around
pulleys 46 and 48 so that one only of each of these pulleys
and of pulley 50 are shown in Figure 4. The guide means
also comprises a circular guide plate 52 through which the
1313320
tube 30 passes at an upstream end portion of the tube. The
guide plate 52 has a plurality of guide holes 54, i.e. one
for each of the tensile filaments and these holes are spaced
apart around a pitch circle coinciding with the axis of the
tube 30 in equally spaced positions around that axis. The
guide means also comprises a leadin~ chamfered edge 59 of
the housing 34 (see Figure 5) for smoothly contacting the
coated tensile filaments as they move into the tubular space
40.
The apparatus also comprises a means for intro-
ducing the water blocking thixotropic medium into the
tubular tensile strength member 16. This means comprises an
applicator 56 which comprises a housing 57 mounted at the
upstream end of the tube 30. The housing 57 defines
passageways 58 from an inlet 60 to an outlet 62 of the
housing to enable the thixotropic medium to be pumped
through the inlet 60 from a source not shown, through the
passages and out of the housing into the inlet of the tube
30. A pump 64 (see Figure 5) is provided for pressurizing
the thixotropic medium so that it is forced along the tube
30. The pump 64 is adjustable in speed to alter the
pressure for a reason to be discussed below. At an upstream
side of the housing 57 there is provided a concentric inlet
tube 66 for admittance of the optical fibers 14 to enable
the fibers to be fed into the tubular tensile strength
member 16 during its formation, as will now be described.
In use of the apparatus shown in Figures 4 and 5,
and as described in co-pending Application Serial No.
511,890, the groups of tensile filaments 20 are mounted
respectively upon individual reels 68 upstream of the
reservoir 24. Also at the upstream end of the apparatus are
disposed a plurality of spools 70, each spool wound with one
of the optical fibers 14. The tensile glass filaments 20
are fed around their respective pulleys 44, 46, 48 and 50.
As the tensile filaments are passed through the bath 26,
each individual tensile filament becomes coated with the
resin which is at room temperature. The coated tensile
filaments then proceed from the bath around the pulley 50,
8 131~320
and around any additional guiding pulleys which are required
~not shown) to bring the filaments through individual holes
54 in the guide plate 52 and form them into a circular array
surrounding the tube 30. The filaments then are caused to
converge towards each other and towards the tube 30 so as to
guide them into the tubular space 40. As the filaments
enter the tubular space 40, they lie in close relationship
and the tubular space 40 becomes filled with the filaments
and the resin coating material which surrounds them.
The glass filaments and resin are drawn along
their passlines and through the tubular space 40 by a cable
reeler 71 and are caused to be molded within the tubular
space 40 into the solidified tubular tensile strength member
16 by the heating means 32 operating at the required
temperature, in this case approximately 300F, to solidify
the resin before it leaves the space. The completely
solidified tubular tensile strength member thus moves
downstream from within the heater 32.
During the movement of the filaments in the above
described manner along their passlines, the fibers 14 are
passed from the spool 70 through the tube 66 and device 56
and into the entrance of the tube 30 as shown in Figure 5.
The thixotropic water blocking medium is passed into the
passage 58 of the applicator device 56 by the pump 64 so
that it surrounds the fibars 14 and is forced in a down-
stream direction along the tube 30 and, upon leaving this
tube, enters into the solidified tube 16. The flow of the
thixotropic medium draws the optical fibers 14 from their
spools 70 so as to move them into the tubular tensile
strength member 16 as it is being manufactured. Thus the
tubular tensile strength member is completely filled by the
optical fibers and the water blocking medium.
It is desirable that each of the optical fibers
has a greater axial length than the tubular tensile strength
member 16 into which it is being fed so that any bending of
the tubular tensile strength member 16, in use of the
finished cable will merely tend to cause relative axial
movement of the tubular tensile strength member and optical
1313320
fibers in the vicinity of the bend without placing the
optical fibers in tension. To enable the length of each
optical fiber to be greater than that of the tubular tensile
strength member the pressure placed upon the filling medium
is changeable by altering the speed of the pump 64 so that
an increased flow of the medium will draw the optical fibers
from their spools at a greater rate. This drawing action
forces the optical fibers along the tube 30 at a greater
speed than that of the tensile filaments through the space
40, whereby upon the optical fibers and water blocking
medium emerging into the tubular tensile strength member 16
at the downstream end of the guide means 30, the speed of
the optical fibers and of the filling medium is reduced.
This leads to a meandering of the optical fibers within the
oversize passage of the tube 16 as illustrated by Figure 2.
The degree of this meandering may be controlled by the
changing of the speed of the pump 64.
Upon the finished tubular tensile strength member
16 surrounding the optical fibers emerging from the ap-
paratus 28, it then proceeds in in-line fashion through a
cross-head 72 of an extruder (not shown) in which the
strength member is provided with the surrounding jacket 18
to complete the cable 10.
As can be seen from the above embodiment, the
cable structure is relatively simple in construction and
avoids the necessity of using steel reinforcing elements or
a shield surrounding the tubular tensile strength member 16
for protection of the optical fibers during normal tensile
loading conditions. It has been shown that the tubular
tensile strength member 16, because of its structure, is
capable of withstanding significant tensile loads while
protecting the optical fibers. The tubular tensile strength
member 16 has an inside diameter which is far in excess of
that of the combined diameters of the optical fibers so that
each of the fibers is radially movable within the tube. The
optical fibers have axial lengths which are greater than the
axial length of the tubular tensile strength member 16
whereby tensile loads placed upon the tubular tensile
1313320
strength member and finished cable which tend to stretch the
cable will merely tend to straighten the optical fibers, as
described, without placing them into tensile loaded con-
ditions.
