GB2122767A

GB2122767A - Optical fibre cables

Info

Publication number: GB2122767A
Application number: GB08217583A
Authority: GB
Inventors: Robert Dewi Edwards; David Delme Jones
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-06-17
Filing date: 1982-06-17
Publication date: 1984-01-18
Also published as: AU556745B2; GB2122767B; AU1572883A

Abstract

A number of optical fibres (1) are formed into ribbons (2), the fibres following an undulating path in the plane of the ribbon. A number of ribbons are then superimposed to form a stack of square section. Each ribbon may be provided with longitudinal reinforcing elements along its edges and a balancing ribbon, comprising reinforcing elements only, added to the top and bottom of the stack. The resulting assembly may be twisted in a helical configuration about its central axis.

Description

SPECIFICATION Optical fibre cables This invention relates to optical fibre cable constructions and particularly to those having a so-called ribbon configuration.

It is already known to provide an optical fibre cable construction comprising a plurality of optical fibres arranged side-by-side in a ribbon formation occupying a single plane, the fibres following an undulating path in the plane of the ribbon and the ribbon having parallel edges.

It is also known for such a ribbon or ribbons to be wrapped or twisted about a longitudinal reinforcing element with its or their edges in abutting or overlapping relationship.

According to the present invention in its broadest aspect, there is provided an optical fibre cable construction comprising a plurality of protected fibres joined to each other and forming at least part of a ribbon occupying a single plane, the fibres following an undulating path in the plane of the ribbon and the ribbon having parallel edges, characterised in that a plurality of such ribbons are joined in superimposed parallel relationship to form a stack.

The invention will now be described by way of example with reference to Figs. 1 to 6 of the accompanying drawings.

Fig. 1 shows a known construction in which a ribbon 2 contains optical fibre elements 1 which are given an undulating configuration in the plane of the ribbon, the edges of the ribbon being linear and parallel.

In the embodiment of the invention shown in Fig. 2, discrete strength members 3 of, for example, steel or glass-reinforced plastics rod may be included adjacent and parallel to the edges of the ribbon 2. A similar result may be achieved by dispersing carbon or glass fibres or other highmodulus material in the plastics sheath.

By virtue of their parallel edges and planar configuration, it will be apparent that a plurality of ribbons such as those shown in Fig. 1 or 2 may be superimposed to form a stack similar to that shown in Fig. 3 and that this may be twisted about a central axis as shown in Fig. 4.

In the case of a stack of reinforced ribbons of the kind shown in Fig. 2 however, the structure will have a greater resistance to bending in one plane than in the other since the strength members 3 will only lie adjacent two sides of the square. This can be rectified as shown in Fig. 5 by adding balancing ribbons 4 containing no optical fibre elements but a sufficient number of reinforcing elements 5 to complete the square section.

Of importance is the degree of stress relief to be included in the ribbon cable. This is because glass (optical) transmission elements have a life failure mechanism related to residual strain. Glass components under compression or stress-free cannot fail due to crack propagation. Metals used as transmission elements, especially copper and aluminium, yield to a significant degree (approx 10%) before fracture. This means that cables containing glass only or a mixture of metal and glass have to be reinforced to a level consistent with not unduly straining the glass or alternatively the glass has to be decoupled from the strains existing in other parts of the cable.

The reinforcement method means the introduction of higher-modulus strength member material than would be the case with the same weight of cable employing metallic conductors only.

The decoupling techniques in use at present rely on the glass lying loose within a tube or space such that there is a larger length of glass fibre contained within the cable than the overall length of the cable. This means that the cable can be strained to the amount whereby all the slack fibre is taken up before the glass fibre component undergoes strain.

Typically the amount of over feed of fibre is around 2% due to the onset of microbending losses from the helix which the fibre forms when overfed into practical sized tube or slot or space of approximately 1 or 2 mm.

The degree of strain relief may be incorporated into the S Ribbon cable is controlled by the dimension p and q in the graph shown in Fig. 6.

To a first approximation the excess length of fibre incorporated in the cable in comparison with the cable length may be derived as follows: Excess length m -q q

(p2 + q2)+ + -1 q (p2 + q2)+ ie ------- - 1 x 100% q Assuming for the purposes of calculation that fibres can accept strain of 2% and a 50% derating is required for safety ie the cable requires to be fully relieved to 1% before extension strain acts upon the fibre.

Then (p2 + q2)+ 1= -1 -------- -1 100 q

q p-- 7 2p dictates the minimum width of the ribbon cable. q is dictated by the characteristics of the optical fibre. As q decreases optical attenuation increases until in the limit the loss penalty becomes excessive.

Assuming 25 mm as an acceptable value for q then for a 1% strain relieved cable, the minimum cable width will be 2p 2q 7 2 x 25 7 =7mum The 'S pattern' repeats over an interval of 4q. In cable terminology the distance over which the pattern repeats is known as the 'lay length'.

It is believed that the information here given is sufficient to describe and dimension a cable for a specific degree of strain relief. This can be further expanded by the application of basic mechanical analysis formulae to yield minimum permissable bend radius, maximum permissable tensile load and other prime parameters.

In addition to being assembled to form a stack, twisted or otherwise, a ribbon cable construction as shown in Fig. 1 or 2 may be wrapped or twisted about another cable or cable element with its edges in abutting or overlapping relationship.

Claims

1. An optical fibre cable construction comprising a plurality of protected fibres joined to each other and forming at least part of a ribbon occupying a single plane, the fibres following an undulating path in the plane of the ribbon and the ribbon having parallel edges, characterised in that a plurality of such ribbons are joined in superimposed parallel relationship to form a stack.

2. An optical fibre cable construction as claimed in claim 1 in which each ribbon also includes longitudinal reinforcing elements along its edges.

3. An optical fibre cable construction as claimed in claim 2 in which a balancing ribbon is added to either face of the stack parallel to the optical fibre ribbons which balancing ribbon contains a number of reinforcing elements of the same nature as those contained in the optical fibre ribbons, the number of reinforcing elements in each balancing ribbon being such as to form, together with the reinforcing elements along the edges of the optical fibre ribbons, a square section having an equal number of reinforcing elements on each side.

4. An optical fibre cable construction as claimed in claim 3 in which the assembly of optical fibre ribbons and balancing ribbons is twisted in a helical configuration about its central axis.

5. An optical fibre cable construction substantially as described with reference to Figs. 2 to 6 of the accompanying drawings.