Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the subject matter of
my co-pending Canadian application Serial No. 201,346 filed
May 31, 1974, and entitled "OPTICAL FIBER CABLE AND
MANUFACTURE THEREOF".
The present invention concerns a composite, uni-
tary element for the transmission of modulated light signals,
e.g., in telecommunication cables, and which comprises an
optical fiber.
As is known, optical fibers are fibers made of
glass or of plastic synthetic material, of a very small dia-
meter, of the order of 0.1 - 0.01 mm., and are constituted
by a tubular core and by a sleeve or coating whose refractive
index is smaller than that of the core, for example, a re-
fractive index of 1.50 - 1.52 for the sleeve and 1.56 - 1.64
for the core.
Because of the differences between the refractive
indexes of the materials respectively constituting the core
and the sleeve, light entering from one end of the fiber ~s
totally reflected inside the fiber itself and may be trans-
mitted along the axis of the latter, even if it is curvi-
linear, as far as the other end of the fiber. By adopting
particular types of highly transparent glass, it has been
found that the initial impulse is transmitted to the terminal
end of the fiber with relatively little attenuation.
Optical fibers of this type can be of interest
also for use as elements intended for the transmission of
signals in telecommunication cables. The use of such
fibers involves, however, some problems deriving mainly
from the typical physical and mechanical characteristics of
the fibers themselves.
In fact, it is to be taken into account that said
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fibers, which are extremely thin, can satisfactorily with-
stand tension stresses, but such fibers have a low ultimate
elongation and are, therefore, brittle. It follows that,
for the employment of such fibers in telecommunication
cables, in which they are to be stranded together in a unit,
it is first of all necessary to solve the problems of
arranging the fibers in an orderly manner and of reducing
the deformations and stresses which may act on the fibers
themselves.
It is evident that the optical fiber can be in-
corporated, with an appropriate support, in the cable
according to several methods, such as, for example, by means
of a stranded or lapped winding. During the winding pro-
cess, the fiber is subjected to stresses, not easily fore-
seen, along different planes.
Moreover, even when the cable is finished, further
stresses, also acting on planes which cannot be predicted
with certainty, can take place during the operations employed
in laying or transporting the cable itself.
It is therefore understandable that the use of an
optical fiber involves the problem of providing a protection
~ for the same against the various stresses acting along
; different planes.
The present invention has, as one object, the pro-
vision of a composite, unitary element comprising an opti-
cal fiber, intended for the transmission of signals in
telecommunication cables, and having a structure which over-
comes the problems mentioned hereinbefore.
Accordingly, the principal object of the present
invention is a composite, unitary element for the trans-
mission of signals in telecommunication cables which com-
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prises, in a synthetic, thermoplastic resin material, an optical fiber and at
least three metallic filaments, each lying at equal distance from said fiber
and in a plane of its own passing through the optical fiber, each plane being
inclined at substantially equal angles with respect to the next adjacent planes.
In accordance with this invention here is provided a composite optical
fiber element comprising a light-transmitting optical fiber and at least three
continuous, reenforcing filaments embedded in and surrounded by a synthetic
thermoplastic resin, said filaments being made of a material having a modulus
of elasticity at least equal to the modulus of elasticity of said fiber, being
spaced from and around said fiber, being substantially parallel to said fiber
and respectively lying substantially in planes parallel to and intersecting
said fiber and inclined with respect to each other at substantially equal angles,
said filaments stiffening said element and absorbing the greatest part of the
stresses when said element is subjected to bending whereby said filaments
substantially reduce the stress which would otherwise be applied to said optical
fiber with bending of said element.
In the preferred embodiments, the composite, unitary element comprises
three metallic filaments arranged around the optical fiber at the same distance
from it, said filaments being in three separate planes which intersect the
fiber and which are inclined at 120 with respect to one another or comprises
four metallic elements at the same distance from the optical fiber and which
lie in two separate planes which interest the fiber and which are orthogonal
with respect to each other.
Said preferred embodiments are particularly advantageous for maintain-
ing the integrity of the optical fiber. In fact, the fiber, for its whole
length and therearound, is protected by the filaments which, along with the
fiber, are contained in a synthetic thermoplastic material.
The efficiency of said protection is comparable with that which would
be obtained, for example, by providing a continuous jacket around the fiber which
is able to absorb the stresses acting in all possible planes. In fact, in
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practice, the symmetrical arrangement of the three metallic filaments at planes
arranged at 120 to each other or of the four metallic filaments in orthogonal
planes, provides, in any stress plane, at least one filament resistant to
tension and two filaments resistant to compression, or two resistant to tension
and one to compression, such filaments having physical characteristics which
enable them to oppose
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the stresses and prevent deformation of the optical fiber.
For these reasons, the composite, unitary element
having the hereinbefore described structure is particularly
suitable to be used as an element for the transmission of
signals in a telecommunication cable. Accordingly, a
further object of the present invention is a telecommunica-
tion cable comprising at least one composite, unitary element
as described hereinbefore.
The present invention will be better understood
from the following detailed description of preferred embodi-
ments thereof, which description should be considered in con-
junction with the accompanying drawings, in which:
Fig. 1 is an enlarged, cross-sectional view of
a preferred embodiment of a composite element of
the invention;
Fig. 2 is an enlarged, cross-sectional view of
a further embodiment of a composite element of
the invention;
Fig. 3 is a cross-sectional view of several
composite elements of the invention grouped to-
; gether; and
Fig. 4 is a perspective view of a telecommunication
cable comprising the composite element.
The composite element 11 illustrated in Figs. 1
and 2 constitutes an element for transmitting modulated
light signals which form part of telecommunication cables
according to the present invention.
The element 11 comprises a support 1 formed by a
synthetic thermoplastic material containing, interiorly
thereof, a glass fiber 2 disposed at the center of the
support 1, and at least three metallic filaments 3, 4 and 5
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disposed around the fiber 2 to protect it. The physical
characteristics of the filaments 3-5 are selected in a
manner obvious to those skilled in the art so as to prevent
bending of the element 11 by an amount which will break the
fiber 2. In other words, the filaments 3-5 stiffen the
element 11 and absorb the greatest part of the stresses,
and the stresses on the fiber 2 are practically negligible.
Preferably, the modulus of elasticity of the filaments 3-5,
when they are made of metal, is about 21,000 kilograms per
square millimeter, or about three times the modulus of
elasticity of the optical fiber 2. Preferably, also, the
filamen~s 3-5 can be bent around a smaller radius of curva-
ture than the optical fiber 2 without breaking. Said fila-
ments 3-5 also have a coefficient of thermal expansion sub-
stantially equal to that of the fiber 2 in order to avoid
subjecting the fiber 2 to objectionable deformation because
of the thermal expansion and contraction of the filaments
3-5.
The arrangement and number of the filaments 3-5 in-
side the support 1 may vary, provided that at least three
metallic filaments are used, each filament lying at the same
distance from the fiber 2 and in a plane of its own passing
through the fiber 2, the angles between the planes being
substantially equal. For example, the number of filaments
may be greater, but for ease of manufacture and to keep costs
to a minimum, the number of filaments preferably is not
greater than twelve, an increase above such number not justi-
fying, in improved protection, the increase in manufacturing
difficulties and the expense. In fact, four such filaments,
arranged as described hereinafter, are usually adequate.
By means of said arrangement of the metallic fila-
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ments, such filaments are practically distributed on the
surface of an imaginary cylinder around the fiber 2 and with
a spacing such that they can efficiently withstand the
stresses in each plane.
In particular, in the preferred embodiments, said
filaments are three in number, as shown in Fig. 1, and are
arranged to lie in planes forming angles of 120 with re-
spect to one another, or are four in number (filaments 6, 7,
8 and 9 shown in Fig. 2) and are arranged to lie in two
planes orthogonal to each other.
Considering in greater detail the parts of the
composite element 11, the diameter of the optical fiber 2
is between 0.01 and 0.1 mm., and the diameter of the fila-
ments 3-9 is about the same order of magnitude as that of
the fibers 2. It is preferred to make the filaments 3-9
of steel or of steel alloys with a nickel percentage of 42%. -
The support 1, which is made of synthetic thermo-
plastic resin material, may be made, for example, of a
polyester, a polyamide or a polyolefin, such materials
having appropriate properties. The support 1 can have a
different cross-sectional shape. For example, it may be -~square, rectangular, etc., with a maximum transverse di-
mension of about 1.5 mm.
Obviously, the small dimensions of the support 1
permit, advantageously, the grouping in a limited space of
a large number of composite elements 11, as potential means
intended for the transmission of signals. In this case, -
each support 1 for the optical fibers 2 is formed with a
thermoplastic material which is preferably loaded with
carbon black so as to avoid the possibility that the trans-
mission of light inside one optical fiber will be influenced
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by the light coming out of contiguous fibers.
The composite elements 11 can be associated to-
gether in various ways, for example, by stranding, and can
be contained in a single sheath 10 made of polyethylene or of
another thermoplastic material, as shown in Fig. 3.
As mentioned hereinbefore, the composite element 11
is useful as an element for the transmission of signals in
telecommunication cables. Said element 11, because of its
particular structure, which includes metallic filaments for
protecting the optical fiber, can be joined with others so
as to form a bunch of stranded fibers in a telecommunication
cable and will be subjected to practically negligible
stresses.
In particular, Fig. 4 shows the application of a
plurality of composite elements 11 helically wound around
the core 13 of a telecommunication cable, the core 13 com-
prising a stranded wire rope 12 and a protective covering 14.
Instead of individual elements 11, the group of elements 11
surrounded by the sheath 10, as shown in Fig. 3, may be
wound around the core 13 in the same manner as the individual
elements 11.
The stranded arrangement of the element 11 about
the cable core 13 is advantageous, since said stranding,
having, for example, a pitch greater than 100 mm., permits
the use of fibers with a developed length exceeding the
cable length by only a small amount.
The metallic filaments 3-9 could be replaced by
non-metallic filaments, provided that the physical character-
istics of the material used and the arrangement of said non-
metallic filaments are such as to increase the flexing re-
sistance of the composite element 11. Examples of non-
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metallic materials useful for the filaments 3-9 are glass
and plastics having a modulus of elasticity at least equal
to the modulus of elasticity of the optical fiber 2 and
which will not break with normal bending of the element 11.
In addition, it will be understood that the pro-
tection for the optical fiber 2 can be obtained in a manner
practically equivalent to that described above by using
filaments 3-9 arranged on planes having inclination angles
slightly different from one another, as even under such
conditions it is possible for the filaments to be disposed
so as to be compression-resistant and tension-resistant.
Also, a protection of the fibers equivalent to
that obtained in the examples shown in Figs. 1 or 2 could
be obtained by adopting filaments 3-9 having different dia-
meters. In the latter case, the optical fiber 2 would not
be arranged at the same distance from the filaments, but
instead, would be disposed in the position of more reduced
flexing stress, namely, along the neutral axis of the re-
sistance section determined by the characteristics of the
filaments. For example, the fiber 2 would be located
farther from a smaller diameter filament than from a larger
diameter filament.
Although preferred embodiments of the present
invention have been described and illustrated, it will be
understood by those skilled in the art that various modifi-
cations may be made without departing from the principles
of the invention.
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