Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Thiq invention relates to optical cableq for the
transmi~sion of the ultraviolet, visible and infra-red
regions of the electromagnetic spectrum, which regions for
convenience, will hereinafter all be included in the generic
term "light", and e~pecially, but not ex-clusively, to optical
cables for use in the communication field adapted for trans-
mis-qlon of light having a wavelength within the range o.8
to 1.3 micrometreq.
For the purpo~e of transmitting light in an optical
transmission ~yQtem it ha3 been proposed to employ optical
guides in the form of cableq including one or more than one
optical fibre. Each optical fibre ls usually of ~ubqtantially
circular cro~s-section but, in qome circumstances, it may be
Or clrcular cro~s-section.
Optical cable~ with which the invention i~ concerned
include cable~ comprising one or more than one optical fibre
of glass or other tranqparent material whose refractive index
is arranged to decrea~e over at lea~t a part of the radius of
the fibre in a direction towards the outer surface of the
fibre and cables including one or more than ons optical
fibre o~ composite form c~mprising a core of transparent mater-
ial clad throughout its length with another tran~parent
material of lower refractive index which, by total internal
refraction of light being tranqmltted along th~ fibre, con~ines
at least a maJor proportion of the light withi~ the core.
A compo~ite optlcal fibre iq generally, but not necessarily,
made up of two glasses of different refractive indiceq, the
glass forming the core having a higher refra^tive i~dex than
the glass forming the claddlng, the refract~Je index of the
glass of the core may gradually decrease in a directior towardq
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the outer ourface of the core over at least a part of the dis-
tance between the central axis Or the core and its outer
surface. In an alternative form of composite fibre the core
may be made of a transparent liquid having a higher refractive
index than that of the cladding.
me present invention provides an optical cable
which by virtue of its con~truction reduce~ the strain that
would otherwise be imparted to the optical fibres Or the cable
if the cable should be stres~ed in such a way as to bend to
sub~ect each or any of the optlcal fibres to a tensile force.
According to the invention the optical cable com-
prises a plurality of separately formed tubes of rubber or
plastics material in each Or some or all of which is loosely
housed at least one optical guide compri~ing at least one
optlcal fibre; at least one 3eparately formed flexible elongate
~iller having at least a circumferentially continuou~ layer
of compre3sible material; and, surrounding the aosembly of
tubes and flexible filler or fillers, an outer protective
oheath, the compressible material of the or each ~lexible
filler being substantially softer than the materlal of the
tubes and the radial thicknes~ and compressibility of the
compressible material being such that, when the cable is
stressed in such a way as to ~ub~ect an optical fibre or
optical fibre~ to a tensile force, a tube or tubes can move
transversely of the cable to compress the compreo~ible
material of the filler or of at least one c~ the filler~ to
ouch an extent as to reduce the strain that weuld otherwise be
imparted to the optical fibre or fibreo loo~ely hou~ed therein.
The or each optical guide may be an optical bundle
ao hereinafter defined or it may be a separate opt~cal ribre.
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By the expression "optical bundle" as used in the
specification and claims forming part thereof is meant
a group Or optical fibres or a group of fibreQ ~ncluding at
least one optical fibre and at least one non-optical reinforc-
ing fibre or other reinforcing elongate member- Each
optical fibre of the optical bundle may be used independ-
ently a~ a ~eparate light guide, each with it~ own modulated
light source and detector, or a plurality of optical fibres
Or a bundle may be u3ed together as a single light guide, with
a single light source.
By virtue of being housed loosely in a tube, limited
relative movement between the or each optical bundle and/or the
or each ~eparate optical fibre and the tube in which it is
loosely housed can take place when the cable iQ fle~ed. The
tubes themselve~ may be loo~ely housed in the ~heath thereby
permitting limited relative movement between the tubes when
the optical cable is fle~ed.
Preferably, the flexible filler or each o~ ~ome or
all of the flexlble fillers is made wholly Or compre~ible
material but, in some circumstances, it may comprise a core
of substantially non-compre~sible material and an outer peri-
pheral layer of compre~ible material or a core o~ CompreQs-
ible material and an outer peripheral layer of ~ubstantially
non-compresQible material.
The compre~sibls material of the flller or fillers
may be any material that will be compressed to the required
e~tent when a fibre-containing tube or tubeQ moves or move
transver~ely with respect of the optical cable, but preferably
it comprises cellular plastics material or cellular rubber.
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Where, in the or a filler of composite form, an
outer peripheral layer of cellular plastic~ material or
rubber ~urrounds a core, the core may be of ~olid plastic~
material or ~olid rubber bonded to the outer peripheral
layer; alternatively the core may be an-elongate rein~orcing
member of ~uch a material and of 3uch a crosq-sectional area
having regard to the material or materials and cross-~ectional
area Or the optical bundle or bundleq and/or of the ~eparate
optical fibre or fibre~ that the strain otherwise imparted to
the or each optical fibre when the cable is qtressed in such
a way aq to tend to cub~ect the or any optical fibre to a
tensile force iq reduced at lea~t to a substantial extent
by the reinforcing member.
Where the or a filler is of composite form including
a corc of cellular plastics material or cellular rubber,
pre~erably the outer peripheral layer is of ~olid plastics
material or ~olid rubber and i~ bonded to the core.
Where the tubes Or the optical cable are arranged in
one or more than one layer around an elongate central core, this
core may be con3tituted by one or more than one fle~ible
elongate filler having at least a circumferentially continu-
ou~ layer of compre~sible material. Where the elongate core
conqi~t~ of a single rle~ible filler having at lea~t a
circumferentially continuous layer Or compres~ible material,
the transver~e cross-~ection of the flller may be Gf a ~hape
complementary to the ~hape~ of the tubeq immediately overly-
ing the filler. Alternatively, or additionally, flexlble
fillers having at leaqt a circumferentially continuouq layer
of compressible material may be arranged in the layer or at
least one of the layer of tubeq, preferably with the filler~
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alternating with the tubes; the tubes and fillers may be arranged with their
axes substantially parallel to the axis of the cable or they may extend
helically around the elongate central core.
One or more than one of the tubes of the optical cable may have
loosely housed in the tube at least one elongate electric conductor. Alter-
natively or additionally at least one elongate bare or insulated electric
conductor may be assembled with the plurality of tubes, and for example, may
constitute an elongate central core around which the tubes are arranged.
The or each optical bundle and/or separate optical fibre may be of a
leng~ch substantially greater than that of the tube in which it is loosely
housed but preferably the or each bundle and/or separate optical fibre and the
tube are of equal or approximately equal lengths.
The invention is further illustrated by a description, by way of
example, of two preferred forms of optical cable with reference to the
accompanying diagrammatic drawing, in which:-
Figure 1 is a transverse cross-sectional end view of the first form
of optical cable, and
Figure 2 is a transverse cross-sectional end view of the second form
of optical cable.
The optical cable shown in Figure 1 comprises six tubes of extruded
polyethylene which are arranged helically around a central flexible elongate
filler 3 comprising a reinforcing steel wire 4 surrounded by a circumferentially
continuous cushioning layer 5 of cellular plastics material. Loose]y housed in
each tube 2 are two separate optical fibres 1 whose lengths approximate to the
length of the tube in which they are housed. The assembly of filler 3 and
tubes 2 is surrounded by an extruded outer protective sheath 6 of polyethylene
in sucll a way that they are ]oosely housed in the sheath. When the cable is
s~ressed in such a way as to subject an optical fibre or fibres
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1 'tO a tensile force, a tube or tubes 2 can move transversely of
the cable to compres~ the cushioning layer 5 to such an e~tent
as to reduce the strain that would otherwise be ~mparted to the
optical fibre or fibres housed in the tube or tubes.
The optical cable shown in Figure 2 comprisss a
central fle~ible elongate filler 13 around which are helically
lapped three tubes 12 of extruded polyethylene and three
flexible elongate fillers 17, each tube being positioned between
and in contact with two fillers. The filler 13 comprises
a reinforcing steel wire 14 surrounded by a clrcumferentially
continuous cushioning layer 15 of cellular plastics material
and each filler 17 comprise~ a core 18 of 9 olid plastics
material surrounded by a circumferentially continuous cushioning
layer 19 of cellular plastics material. In each tube 12 are
loo~ely housed two optical fibres 11, each of a length approx-
imating to that of the tube in which it is hou~ed. The as~embly
of tubes 12 and fillers 13 and 17 i~ enclosed in an ex~ruded
outer protective sheath 16 of polyethylene. When the cable is
stre~sed in such a way a~ to sub~ect an optical fibre or fibre~
11 to a tensile force, a tube or tubes 12 can move transversely
of tha cable to compre~s the cushionin~ layers 15 and 19 to
such an extent as to reducs the strain that would otherwi~e be
imparted to the optical fibre or fibres housed in the tube or
tubes.
