GB2026718A - Cabling Element for Optical Fibers - Google Patents

Abstract

The invention relates to a cabling element for optical fibers. This element includes two ribbed strip members 3 which form longitudinal channels 2. Optical fibers 1 are loosely arranged within said channels 2. An optical fiber cable is obtained by piling up at least two cabling elements, the pile being embedded within a cladding 11. Reinforcing elongated members or wires 6 may be embedded within the cabling elements, which may be made of a thermoplastic polymer. The piled elements may in cross section form a sector of a circle (Fig. 4 not shown). <IMAGE>

Description

SPECIFICATION Cabling Element for Optical Fibers The invention relates to a new supporting structure for optical fibers which can be used alone or for making cables containing a great number of fibers, for example, communication cables.

The brittleness of optical fibers with respect to tensile or torsional stresses makes it difficult and costly to manufacture optical cables having suitable transmitting features and which can be easily connected.

Various structures have been proposed originating from conventional cable constructions by introducing in said structures carrier or pulling elements in an attempt to limit or to suppress the mechanical stresses onto the fibers and drastic deformations.

Some manufacturers have suggested strip type structures wherein the light fibers are maintained between plastic films of various compositions.

Those films are then either wound around a carrier core, or wound on each other in order to form a cable structure, in order to cancel the mechanical stresses onto the internal or external parts of the obtained cable when the latter is wound. With this type of cable it has been stated that in practice too important stresses are exerted onto the fibres which are bound to the strips which maintain same.

One also knows other structures which are complex and difficult to manufacture, and wherein a cylindrical central core comprises along its generating lines elongated grooves or cavities wherein the optical fibers are arranged.

None of the approaches of the prior art allows to simultaneously solve the problems resulting from the intrinsic brittleness of the fibers with respect to mechanical and/or thermal stresses and from the difficulties for inter-connecting optical fiber cables or elements of cables.

In fact, the main parameters to be taken into account for implementing an optical fiber cable, for example for high density communication links, are the following ones:- -mechanically, the optical fibers presently available show a high ultimate tensile strength, but substantially no elongation. Therefore, the structure of a cable comprising a plurality of fibers must be designed in such a way that no mechanical stress, i.e. drawing, bending, torsion stress is applied onto the fibers. In addition, micro-bends and micro-breakages are liable to appear onto fibres submitted to some type of mechanical stress.It is well known that the aforementioned phenomena result in a very sharp increase of the attenuation of the optical signals transmitted by the fibers; -thermally, it is also known that coating of fibers, which is often recommended for ensuring a mechanical protection, is an extremely difficult operation as it submits the fibers to mechanical stresses, and mainly to thermal stresses liable to damage the fiber as regards its mechanical and light transmission characteristics.In order to avoid those difficulties, it is accordingly suitable to provide a cable structure allowing this coating operation to be suppressed and accordingly preventing the fibbers to be submitted to important stresses during such coating of the cable; -as regards connecting the fibers with one another or with emitting or receiving equipments, it is also known that depending upon the nature and the size of the optical fibers connection systems are required which must be very accurate and are difficult to make an implement. It is known that it is less difficult to connect fiber arrays under the form of superimposed sheets rather than of successive cylindrical layers.

An object of the invention is to provide a novel cabling element and optical fiber cable solving the above problems.

Another object of the invention is to provide a new type of optical fiber cable having a small size for a given number of fibers and showing an improved durability and feasability.

The invention provides a cabling element for optical fibers comprising two elongated longitudinally ribbed strips and associated by their facing protruding portions in order to form parallel longitudinal channels, an optical fiber loosely arranged inside each channel. The invention also relates to a cable obtained by piling such elements.

In order that the invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawings in which: Figure 1 is a sectional view of a first embodiment of a cabling element ofthe invention, Figure 2 is a sectional view of a second embodiment of a cabling element according to the invention, Figure 3 is a sectional view of a first embodiment of a cable according to the invention, and Figure 4 is a sectional view of a second embodiment of a cable according to the invention.

As illustrated in Figure 1, bare optical fibers 1, i.e. fibers which are only coated with a usual thin pre-coating having a thickness of a few microns, but without any protective coating, are arranged inside elongated channels or cavities 2 which are obtained by superimposing transversely onduiated strips 3 with mutually facing grooves portions, said strips being soldered each other by their contact lines or zones 4.

In a preferred embodiment of the invention, those strips are made of polyester or polypropylene having a high tensile strength and compression strength. Said strips can then be seam welded before being used. In another embodiment, said strips can be made from low thickness aluminum or stainless steel sheets and include a special coating which allows them to be thermowelded.

In any case, the nature of the trniisversely ondulated strips or films is such that friction coefficient relatively to optical fibers is of a low level. A lubricant, for example silicone oil, can be applied to cavities 2. The preferred diameter of bare optical fibers is in the range 100--150 microns and the preferred pitch of transversal channels or ondulations of said strips is in the range 0,3-1 mm and as a consequence fibers 1 are loosely received within the cavities 2 and can freely move owing to the low level of the friction coefficient of said strips. In practice, contrarily to the illustrations, the fibers are not exactly positioned in the center of the channels 2 but may bear against the channel's walls.The outer spaces formed by the strip ondulations are filled with an extruded or not special plastic material 5, in which elongated reinforcing pulling elements 6, are embedded, for instance wires of the product sold under the trademark Kevlar, or glass fibers.

The structure thus obtained may be covered on both faces or on a single face thereof with synthetic or metallic strips 7 which are welded or bonded to the underlying material, acting simultaneously as a mechanical protection means and as a barrier to prevent moisture from entering said structure.

Said structure has a thickness preferably be between 0.8-2.5 mm and the width will depend on the determined number of optical fibers to be selected. Said width will preferably be between 8-25 mm, and in a preferred embodiment, the number of parallel optical fibers will be limited to 10.

Figure 2 shows another embodiment of the base element of the invention, wherein longitudinal channels 2 were optical fibers are received, are obtained by welding or bonding the contact surfaces 9 of two strips or elongated ribbed strip elements 8 made of extruded special plastic material, for instance, high density polyethylene, in which are embedded pulling elements, for instance the product called Kevlar or glass fibers. Each strip element has at least a longitudinally extending groove opening in said contact surface of the strip element, each groove being delimitated laterally by a pair of protruding portions or projections lying in the common plane of said contact surface. Two strip elements are bonded together with said grooves in facing relationship so as to define at least a said longitudinal channel.

On the external or outer face of each ribbed elongated element 8 a complex ribbon 7 may be stuck, which is comprised, for example, of an aluminum foil, at least one of its faces being coated with a thermoplastic material, for example, a ethylene copolymer. Said complex ribbon both acts to prevent moisture from entering and to improve the mechanical strength of the assembly.

It will be appreciated that the strip cabling elements, partly illustrated in Figures 1 and 2, are so called as they can be assembled into larger cables, and may constitute cables which can be used by themselves. Said cables can be easily made and are easy to interconnect. During the manufacturing process, fibers are provided with length higher than that of the final cable. Each fiber takes a sinuous or sinusoidal shape inside its own channel.

Figure 3 shows an embodiment of a cable obtained by stacking or piling cabling elements of the invention, each including, for example, 10 optical fibers.

In this preferred embodiment, base structures 10, as shown in Figures 1 and 2, which contain optical fibers 1, are stacked or superimposed, bonded to each other with a strong adhesive of proper resiliency, and embedded within a cylindrical first packing cladding or sheath 11, said sheath 11 being surrounded with a second sheath 12 made, for example, of polyethylene, the internal face of which may advantageously include a coated aluminum complex 13 acting as a sealing barrier.

During the single step manufacturing process, said structure is progressively subjected to torsion stresses acting around its longitudinal axis in such a way that mechanical stresses during winding on a reel or unwinding when laying are eliminated.

The selected twisting pitch will be chosen about half or less the bending radius of the completed cable.

Figure 2 shows another embodiment of the invention of a multioptical fiber cable obtained from the special base structures comprised of precabled sectors, which are assembled together in a further step. In said embodiment, the different strip members of a base structure have decreasing transversal dimensions, each strip member having a substantially trapezoidal shape, the laterial faces of the piled strip members extending in common radially extending planes so as to define a portion of a cylinder in the formsof an angular sector as shown in Figure 4.

It should be noted that Figure 4 only illustrates an example of the dimensions, number, shape and agencement of the base structures, as well as the dimensions, number and shape of said sectors or precabled arrays without departing from the spirit and scope of the invention.

In Figure 4, base structures still include longitudinally extending parallel channels 2, wherein are loosely placed optical fibers 1, as well as reinforcing pulling elements 6. Each base structure is made integral with the preceding one and with the following one, in the converging stacking arrangement through either special lateral ribs 1 5 or resilient adhesive, or both of them. All base structures belonging to the same sector are maintained together by a plastic material ribbon 1 6 made, for instance, of polyester, which is wound when assembling and preforming said sector shaped base structures.

Several sectors 1 7 are then cabled together and bound with a peripheral ribbon on a machine, known of those skilled in the art, allowing sectors to be positioned while preventing optical fibers to be subjected to mechanical stresses.

The core of the cable so obtained is then covered with a thermoplastic sheath 1 8 made, for example, of polyethylene, which can be associated with a metallic shield 1 9 made, for example, of aluminum, according to a known method. Said sheath may include pulling elements and/or may be covered with means to mechanically protect said core and/or to prevent moisture from entering said core, according to well known engineering technics.

The configuration illustrated in Figure 4, shown only as an example, provides a cable including 60 optical fibers, spread in 6 sectors of 10 fiber arrays. Of course, the external shape of the cable so obtained will be cylindrical and the diameter thereof will preferably be about 20-25 mm.

Depending on the requirements and without departing from the spirit and scope of the invention, metallic circuits including single, pair or quad conductors, could replace one or several optical fibers within one or several channels in the structures described in Figures 1 to 4. Metallic conductors could also act as said elongated pulling means.

When a metallic conductor is substituted to an optical fiber, this may be covered with a thermoplastic material, for example, polyethylene.

When two or four metallic conductors are substituted to an optical fiber, those a"re first covered with an insulating material or twisted together in order to form suceessive torsion pitches between said two or four conductors.

Each of all metallic conductors placed inside the structure can be used to feed repeaters along a transmission line, or to transmit electric signals.

The foregoing description presents preferred embodiments of this invention. Modifications and changes may appear to those skilled in the art, which will come within the scope and spirit of the appending claims.

Claims (10)

Claims

1. A cabling element for optical fibers comprising two elongated longitudinally ribbed strip members, each strip member having at least a longitudinally extending groove defined iaterally by a pair of longitudinally extending projections, said strip members being paired and interconnected along their facing projections, whereby said grooves arranged in facing relationship define a longitudinal channel, in which an optical fiber is loosely received.

2. A cabling element according to claim 1, wherein each strip member has a series of parallel longitudinally grooves, whereby said grooves arranged in facing relationship define parallel longitudinal channels in each of which an optical fiber is loosely received.

3. A cabling element according to claim 1 or 2, wherein said strip member is formed of a transversely ondulated ribbon.

4. A cabling element according to claim 3, wherein said ribbons are externaliy coated on one side with a material which fills the ribs.

5. A cabling element according to any of claims 1 to 4, wherein said elongated longitudinally ribbed strip members are obtained by an extrusion process.

6. A cabling element according to any of claims 1 to 5, further including elongated reinforcing members for improving the tensile strength of the cabling element.

7. An optical fiber cable, which includes a pile of at least two cabling elements according to claims 1 to 5, said pile being embedded within a cladding.

8. A cable according to claim 7, wherein said piled elements have a decreasing dimension and constitute a sector structure, a plurality of said sectors being assembled within said cladding.

9. A cable according to claim 7 or to claim 8, wherein said piles are twisted before being introduced within said cladding.

10. A cabling element such as substantially disclosed in reference with Figures 1 , 2 and 3.