GB2258319A

GB2258319A - Optical fibre cable

Info

Publication number: GB2258319A
Application number: GB9116402A
Authority: GB
Inventors: Andrew Thomas Summers; Gavin Morland; Rachel Willetts
Original assignee: Northern Telecom Europe Ltd
Current assignee: Nortel Networks Optical Components Ltd
Priority date: 1991-07-30
Filing date: 1991-07-30
Publication date: 1993-02-03
Anticipated expiration: 2011-07-30
Also published as: GB9116402D0; GB2258319B

Abstract

In an optical fibre aerial cable the transmission element comprises one or more fibre ribbons 15 accommodated in a rectangular cross-section channel 14 disposed within a tubular strength member 11 e.g. glass reinforced polyester. The height of the channel and the overfeed of the ribbon are such that the minimum bend radius of an undulating path followed by the ribbon is constrained to be greater than a predetermined minimum. 13 is a plastics channel member and 12 is a plastics sheath. <IMAGE>

Description

OPTICAL FIBRE CABLER This invention relates to optical fibre cables, and in particular to cables in which the transmission element comprises an optical fibre ribbon. The invention further relates to a method of making such cables.

A recent development in optical fibre cable technology has been the introduction of cables incorporating a transmission ribbon element in which a number of fibres are arranged in a parallel side-by-side configuration. To minimise the risk of straining or breaking the fibres during cabling and installation operations it is customary to provide an elongate tube or channel in the cable and within which the ribbon fibre is disposed. An excess length of fibre ribbon is provided so that tensile strain introduced by stretching or bending of the cable is avoided. Typically the ribbon element follows a generally sinusoidal configuration within the channel to accommodate this excess length.

Two significant problems have been experienced with this technique. Firstly it has been found that the ribbon does not always follow a uniform path but 'bunches' at various regions along the cable length. This causes excessive local bending of the ribbon and impairs the optical properties of the cable. Secondly it has been found, in some cable designs, that the ribbon can twist within the channel again causing the above-mentioned bending problem.

The object of the invention is to minimise or to overcome this disadvantage.

According to the invention there is provided an optical fibre aerial cable, including a longitudinal dielectric strength member having a longitudinal channel or duct therein of generally rectangular cross-section, and an optical fibre ribbon element disposed within the channel, there being an excess length of ribbon relative to the cable length, and wherein the height of the channel is less than the width of the ribbon whereby to prevent twisting of the ribbon and is such that the minimum bend radius of an undulating path followed by the ribbon along the channel is constrained to be greater than a predetermined minimum.

According to the invention there is further provided an optical fibre aerial cable, including a transmission package, a longitudinal dielectric strength member of annular cross section and within which the transmission package is secured, and a dielectric sheath, wherein said transmission package comprises a dielectric member having a longitudinal channel therein of generally rectangular cross-section and within which a plurality of optical fibre ribbon elements are disposed in a parallel stacked configuration, there being an excess length of ribbon relative to the cable length, wherein the height of the channel is such as to prevent twisting of the ribbons and is such that the wavelength of an undulating path common to all said ribbons is constrained to be greater than a predetermined minimum whereby to limit the bending radius to which the ribbons are subjected.

The cable construction is of particular application to self-supporting aerial cables suitable for installation on the support towers or pylons of an electric transmission line.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a cross-sectional view of an optical fibre ribbon cable; Fig. 2 is a longitudinal section of the cable of Fig. 1; Fig. 3 illustrates the relationship between excess fibre ribbon length and wavelength for the cable of Figs. 1 and 2; and Fig. 4 illustrates the relationship between wavelength and radius of curvature of the fibre ribbon Referring to Figs 1 and 2 of the drawings, the cable includes a tubular dielectric strength member 11 enclosed in a plastics sheath 12. Typically the strength member 11 is formed from a glass reinforced polyester.

The bore of the strength member 11 accommodates a plastics channel member 13 having a longitudinal tube or channel 14 of generally rectangular cross-section. Within the channel 14 one or more optical fibre ribbon elements 15 are accommodated. The channel member 13 may be formed by extrusion, the ribbon fibre elements 15 being fed into the channel 14 as it is formed. The ribbon fibre elements are fed at a rate slightly greater than the extrusion rate so as to ensure a predetermined excess fibre length.

Typically the channel 14 is filled with a water blocking gel compound having a yield stress lower than the stress imparted by the ribbon fibres during movement. Typically the yield stress is 30 to 80 Pa. This ensures that the ribbon fibres are free to move within the channel 14 without encountering significant resistance.

The height of the channel 14 is less than the width of a fibre ribbon 15 so as to prevent twisting of the ribbon elements within the channel. Further, the height of the channel 15 is defined in correspondence with the percentage excess ribbon fibre length whereby the wavelength of the undulating ribbon path is determined.

The channel member 13 may be formed by an extrusion process, the ribbon 15 and gel filler being fed into the channel 14 during the extrusion process. The strength member 11 and the sheath 12 are then formed by further extrusion processes, These extrusions may be carried out in tandem.

The relationship between excess ribbon length and wavelength for an excess or overfeed of 0 to 0.6% is illustrated in Fig. 3, for channel depths between 0.8 mm and 1.5 mm. Typically the overfeed will be from 0.2 to 0.6% and the wavelength will be from 40 to 100 um.

When determining the height of the channel due regard must be given to the minimum allowable bend radius of the ribbon fibre. The relationship between path wavelength and radius of curvature for channel height between 0.8 mm and 1.5 mm is illustrated in Fig. 4. In designing a cable the two graphs of Figs. 3 and 4 are taken together so as to calculate the required cable dimensions.

Within the channel, the amplitude a of the ribbon waveform is given by the expression

where e is the percentage overfeed and N is the wavelength of the undulating paths. Consideration must also be given to the minimum allowable bend radius Rmin which, for a wavelength 1 and amplitude a is given by the expression.

Thus for given values of Rmin and the percentage overfeed e, a suitable value for the amplitude a and hence the channel depth can be determined. It will be appreciated that where the channel contains a number n of ribbon fibres of thickness t the channel depth is increased by n t to accommodate the ribbons.

It will be appreciated that a single set of extrusion apparatus may be employed for a variety of cables, the channel dimensions being adjusted according to the ribbon count, percentage overfeed and the desired minimum bend radius.

Claims

1. An optical fibre aerial cable, including a longitudinal dielectric strength member having a longitudinal channel or duct therein of generally rectangular cross-section, and an optical fibre ribbon element disposed within the channel, there being an excess length of ribbon relative to the cable length, and wherein the height of the channel is less than the width of the ribbon whereby to prevent twisting of the ribbon and is such that the minimum bend radius of an undulating path followed by the ribbon along the channel is constrained to be greater than a predetermined minimum.

2. An optical fibre aerial cable, including a transmission package, a longitudinal dielectric strength member of annular cross section and within which the transmission package is secured, and a dielectric sheath, wherein said transmission package comprises a dielectric member having a longitudinal channel therein of generally rectangular cross-section and within which a plurality of optical fibre ribbon elements are disposed in a parallel stacked configuration, there being an excess length of ribbon relative to the cable length, wherein the height of the channel is such as to prevent twisting of the ribbons and is such that the wavelength of an undulating path common to all said ribbons is constrained to be greater than a predetermined minimum whereby to limit the bending radius to which the ribbons are subjected.

3. An optical fibre cable as claimed in claim 1 or 2, wherein said channel incorporates a water blocking gel compound.

4. An optical fibre cable as claimed in claim 1, 2 or 3, wherein the excess ribon length is from 0.2 to 0.68 of the cable length.

5. An optical fibre cable as claimed in claim 1, 2, 3 or 4, wherein the channel depth is from 0.8 to 1.5um.

6. An optical fibre cable as claimed in any one of claims 1 to 5, wherein the wavelength of the undulating path is from 40 to 100um.

7. An optical fibre aerial cable substantially as described herein with reference to and as shown in Figs. 1 and 2 of the accompanying drawings.

8. A method of making an optical fibre aerial cable substantially as described herein with reference to and as shown in the accompanying drawings.