Note: Descriptions are shown in the official language in which they were submitted.
OPTICAL CABLE INCORPORATING A COMPOSITE
CIRCUMFERENTIALLY RIGID, FLEXIBLE TUBE
This lnvention relates to optlcal cables for the
transmission of the ultra-violet, visible and infra-red
regions of the electromagnetic spectrum, which regions,
for convenience, will hereinafter all be included in the
generic texm "light" and sspecially, but not
exclusively, to optical cables for use in the
communications field adapted for transmission of light
( ha~ing a wavelength within the range 0.8 to 2.1
micrometres.
It is an object of the present invention to
provide an improved optical cable which is especially,
but not exclusively, suitable for use as an aerial
optical cable.
According to the invention, the improved optlcal
cable comprlses a substantially clrcumferentlall~ rigid,
flexible tube which is of composite form and comprises a
plurolity of elongate flexible bodies helically laid up
together, each of which bodies is of a cross-section
approximating to a sector of an annulus and comprises
extruded electrically insulatlng plastics material and a
multipliclty of longitudlnally stressed elongate
flexible non-metallic reinforcing elements each
substantiolly encapsulated in sald plastics m~terial,
the:multlplicity of non-metalllc relnforclng elements
~25 being substantially evenly distributed throughout the
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cross-sectlonal area of the body; an outer protectlve
sheath of non-metallic materlal surrounding the
circumferentially rigid tube; and, housed in the bore
of and movable relative to the circumferentially rigid
tube, at least one flexlble optical guide.
Since the or each flexible optical guide ls
movahle relatlve to the circumferentially rlgld tube in
which it is housed, limited relative movement between
the or each flexible optical gulde and the
circumferentially rigid tube can take place when the
improved optical cable vibrates, oscillates or ls
otherwise flexed as may, for example, occur when the
optical cable, being employed as an aerlal cable, is
freely suspended from spaced supports in long length~
and is subjected to winds.
The elongate bodies of the circumferentially rlgid
tube preferably extend helically of the tube with a long
. length of lay, e.g. a lay length lying in the range 50
to 300mm.
~ Preferably, at any position along the length of
the optical cable the proportion of non-metallic
reinforcing elements in the composition of each elongate
body of the circumferentially rigid tube lies in the
range 60 to 70~ by volume.
25~ :: By way o~ example, the circum~erentlally rigid
tube may have an overall dlameter lylng in the range
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3.
10 to 20mm and an internal diameter lying in the range 5
to 10mm and may comprise eight elongate bodies which
extend helically of the tube wlth a lay length lying in
the range 50 to 300mm.
The longitudinally stressed, elongate flexible
non-metallic reinforcing elements of each elongate body
preferably are made of an aromatic polyamide;
alternative non-metallic materials of which they ma~ be
made include non-optical glass and carhon.
The electrically insulating plastics material in
which the non-metallic reinforcing elements are
encapsulated preferabl~ is a thermoplastics material,
such as polypropylene or nylon but, in some
circumstances, it may be a thermosetting plastics
material, such as a polyester or epoxy resin or
polyurethane.
Other electrically insulatin~ plastics materials
ln which the non-metallic reinforcing elements may be
encapsulated are thermotropic li~uid crystalline
polymers such as wholly aromatic polyesters,
aromatic-allphatic polyesters, aromatic polyaæomethines,
aromatic polyester-carbonates and wholly or non-wholly
aromatic polyester amides.
In one aspect o~ the lnvention, the or each
fle~ible optical guide may comprlse a separat~ optical
fibre, an optical bundle as hereinafter defined, or an
optical fibre ribbon structure.
4.
By the expresslon "optlcal bundle" is meant a
group of optical fibres or a group of ~ibres includlng
one or more optlcal flbres and one or more non-optlcal
reinforcing flbres or other reinforcing elongate
elements.
In another aspect of the lnvention, the
circumferentially rigld tube may tightly surround an
optical cable element comprising an assembly of
separately formed tubes of plastlcs materlal, ln at
least one of which at least one separate optlcal fibre,
optical bundle, optical fibre ribbon structure or other
flexible optical guide ls loosely housed. In a
preferred embodiment of this aspect of the invention,
the optical cable element comprises a plurality of
separately formed plastics tubes helically wound around
a central flexible elongate member of non-metallic
material,~e.g. an aromatic polyamide.
In all cases where the or each flexible optical
guide of the improved optical cable is an optical fibre
ribbon structure, preferably the optical fibre ribbon
structure is of a construction in accordance with our
Britlsh Patent No: 2141558B and comprises a plurality of
optical:fibres:and at least one flexible elongate
relnforclng element of substantially resllient material
arranged slde by side and embedded in an elongate body
of:plastlcs material, wherein the or each resllient
5.
reinforcing element is set in such a form that the
ribbon structure follows a path of smoothly curved
undulations whose axes of curvature lle transverse to
the longitudinal axis of the rlbbon s~ruc~ure, the
S arrangement being such that, when the undulating ribbon
structure is sub~ected to a tenslle force, the ribbon
structure straightens in a lengthwise direction against
the action of the undulating resilient reinforcing
( element or elements thereby reducing the tensile force
applied to the optical fibres and, when the tensile
force is removed, the xibbon structure returns towards
its original undulating form.
The outer protective sheath of the improved
optical cable may be of any suitable plastics material,
polyethylene being preferred.
The invention is further illustrated by a
description, by way of example, of one preferred form of
the improved optical cable with reference to the
- accompanying drawing which shows a transverse
cross-sectional view, drawn on an enlarged scale, of the
preferred form of optical cable.
The preferred form of optical cable shown in the
drawing comprlses a substantially circumferentially
rigid, flexible tube 5 of composite form which comprises
eight elongate flexible bodies 6, each of a
cross-section approximating to a sector o~ an annulus,
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6.
helically laid-up together. Each of the elongate bodles
6 comprises extruded electrically lnsulatlng
thermoplastics material and a multiplicity of
longitudinally stressed elongate flexible reinforcing
5 elements of an aromatic polyamide each substantially
encapsulated in said thermoplastics materlal, the
multiplicity of reinforcing elements being substantially
evenly distributed throughout the cross-sectional area
( of the body and the proportlon of reinforcing elements
10 in the body lying in the range 60 to 70% by volume. The
circumferentially rigid tube 5 is lined with a
longitudinally applied, transversely folded paper tape
4. An optical fibre rlbbon structure 1 is loosely
housed in the bore of the circumferentially rigid tube 5
lS and the tube ls surrounded by a protective sheath 7 of
plastics material.
The optical fibre ribbon structure 1 may comprise
c a plurality of optical fibres and at least one flexible
elongate reinforcing element of substantially resilient
20 material arranged side by side and embedded in an
elongate body of plastics material, the or each
resilient reinforcing element being se~ in such a form
that the ribbon structure follows a path of smoothly
curved undulatlons whose axes o~ curvature lie
25 transverse to the longitudinal axls of the ribbon
structure, the arrangement being such that, when the
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7.
undulating ribbon structure is sub~ected to a tenslle
force, the rlbbon structure straightens ln a lengthwise
direction agalnst the actlon of the undulating resillent
reinforcing element or elements théreby reduclng the
tensile force applied to the optical fibres and, when
the tensile force is removed, the ribbon structure
returns towards its original undulating form.
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