GB2134842A

GB2134842A - Manufacturing optical cable with an optical fibre disposed loosely within a helical protective package

Info

Publication number: GB2134842A
Application number: GB08331255A
Authority: GB
Inventors: Norman Charles Fielder; Terence Alexander Ketteringham; Raymond Sidney Ashpole
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1982-12-22
Filing date: 1983-11-23
Publication date: 1984-08-22
Also published as: GB2134842B; GB8331255D0

Abstract

Hot thermoplastic package material is extruded through a cooling zone such as a water bath (3) from a die (5), with one or more interior channels (12) into which the optical fibres (10) are continuously fed into, via small-bore tubes (8a) which extend through the die and developing package up to a location in the water bath at which the package is cooled sufficiently to avoid adhesion to the fibres, and the package is continuously twisted by a torsional gripping means at a point downstream of the water bath to permanently deform the thermoplastic section of the package and form said channels into said helical configuration. The package may be twisted in a torsionally oscillating sense (in which case flexible small- bore tubes may be used) to form the channels into a cylindrical sinusoidal configuration. Contact of the fibres with the innermost channel sides is avoided, either by feeding the fibres into the channel at a faster rate, or drawing into the channel at a higher rate by means of a further torsional gripping twisting means, acting with greater amplitude downstream of means which induces the helical deformation. <IMAGE>

Description

SPECIFICATION Optical cable The present invention relates to processes for the production of optical cables, that is to say cables incorporating one or more optical fibres within a flexible protective cover, commonly called a package, and which will hereinafter be referred to as such, and to the cables produced by such processes.

An object of the present invention is to provide an essentially continuous process for the manufacture of optical cable incorporating one or more optical fibres loosely held within the cable package in a helical configuration about the cable axis.

Such cable has the advantage that when it is bent virtually no compression or extension of the optical fibre(s) occurs, since the or each optical fibre is approximately evenly distributed about the neutral axis of the cable so that tension on a section of optical fibre on the outside of the bend is relieved by slack in an adjacent section of the fibre on the inside of the bend. The shorter the pitch of the helically wound fibre(s), the smaller the bend radius that can be accommodated in this manner. However the pitch must not be so short that the minimum allowable radius of curvature of the fibre(s) is approached, or that friction between the fibre(s) and package prevents the package from sliding over the fibre(s) when the cable is bent. In practice a relatively shallow pitch has been found to represent the best compromise (typically, for a cable of 10 mm diameter, 0.5 m).

In a commonly employed refinement of this cable construction, the fibre pitch is periodically reversed at intervals as short as half a turn, so that the or each fibre has approximately a sinusoidal configuration projected onto a cylindrical surface coaxial with the package. This construction allows the cable to be fed to a takeup spool without having to be untwisted. A further advantage is that it allows torsional strains to be accommodated, since a twist in a given direction will respectively wind up and unwind oppositely pitched adjacent channel sections or vice versa.

The term "helical" when applied hereinbelow to optical fibres and cables is to be understood to include not only helices wound with a continuous sense, but also helices wound with a periodically reversing sense with a period as short as half a turn (i.e. a cylindrical sinusoidal configuration) and other configurations sufficiently similar to these to confer strain relieving properties onto an optical fibre within an appropriate optical cable.

Hitherto, according to one method of manufacture, helical configuration optical cables have been manufactured by providing a package formed with helical grooves. In such a method it is obviously necessary to provide some means for ensuring that the fibres do not hang out of the grooves during manufacture, and furthérmore it is necessary to extrude some type of protective sheath over the package/fibre assembly to provide environmental protection in use.

According to the present invention, in a method of manufacturing optical cable comprising one or more optical fibres loosely held in a package in a helical configuration about the package axis, hot thermoplastic package material is continuously extruded from a die member and conveyed to a cooling zone from which the developing package emerges in a set but flexible condition, one or more interior channels parallel to but displaced from the package axis are continuously formed within the package and at least one optical fibre is continuously conveyed into a said channel by a delivery tube which extends within the channel downline through the cooling zone at least to a point at which the package material is sufficiently cool to prevent any appreciable contamination of the optical fibre surface or adhesion of the optical fibre to the channel wall, and the hot section of the developing package is twisted by first torsional gripping means rotating about its axis to form said channel or channels into said helical configuration.

The or each channel is preferably formed by an extrusion over a second tube forming part of the die member. Preferably at least the downstream end of the or each delivery tube, that is to say the end from which the respective optical fibre is arranged to emerge, is flexible, allowing the direction of rotation of the torsional gripping means to be periodically reversed, and is of sufficient length to ensure that the emerging fibre is prevented from making contact with part of the channel wall which has not had time to set, and is therefore unduly soft and sticky. The delivery tube may be made of stainless steel and may be of the type used in hypodermic syringes for example.

Alternatively the delivery tube may be in the form of a small diameter closely wound helical spring, to give increased flexibility.

The cooling zone is preferably located intermediate the die member and torsional gripping means, and is conveniently provided by a liquid (e.g. water) bath through which the package assembly is fed.

The torsional gripping means may comprise a pair of opposed spring loaded grip-rollers and may either oscillate torsionally to produce a cylindrical sinusoidal optical fibre configuration or may rotate continuously about the cable axis. Second torsional gripping means may grip the package downline of the first torsional gripping means but upline of pulling means arranged to pull the completed package assembly. In the former case the second torsional gripping means may torsionally oscillate in phase with, but with a larger amplitude of oscillation than, the first torsional gripping means and in the latter case may rotate slightly faster than the first gripping means.In each case the channels are temporarily twisted (and therefore extended) to a slightly greater extent than the fibres within them, and since the package is elastic at this stage the package and channels untwist slightly after passing through the second gripping means and allow the optical fibres to slacken off within the channels. As a result the fibres are not held tautly against the radially innermost surface of their associated channels, but with suitable control of the degree of overtwist introduced by the second gripping means may be arranged to lie on the neutral axes of their associated channels. As a result the fibres in the completed cable are virtually unstrained (and therefore free of microbends) and may slide freely to accommodate bends in the cable.Alternatively the optical fibres may be fed at a suitably greater rate than the speed of travel of the package, for achieving a similar effect to that produced by overtwist. In order to further facilitate movement of the fibres within the respective channels, a solid lubricant such as talc may be coated onto the optical fibre surfaces.

The die member may incorporate means for introducing at least one strengthening member, such as a central reinforcing wire or cable of steel, aluminium or impregnated aromatic polyamide, into the package. The package material is preferably of high density polypropylene, but other extrudable thermoplastic materials may be used.

Two embodiments of the invention will now be described by reference to Figures 1 and 2 of the accompanying drawing, of which: Figure 1 is a schematic elevation, partly in section, of apparatus for producing optical cable by one method in accordance with the invention, and Figure 2 is a schematic sketch perspective view of apparatus for producing optical cable by another method in accordance with the invention Similar parts are designated by like reference numerals.

Referring to Figure 1, polypropylene at a temperature of approxiamtely 2000C is extruded from an extruder 1 to form a package 2 of approximately 10 mm diameter which is drawn through a water bath 3 by a conventional tractor 4.

The extruder 1 has an extruder head comprising a conventional die component 5 and a pierced point component 6. Molten polypropylene is forced through the annular gap between components 5 and 6 as shown by the arrows and simultaneously a central reinforcing cable 7 is drawn through a central hole in the point component 6.The point component is provided with a plurality of tubes 8 of approximately 2 mm external diameter (only two of which are shown) symmetrically disposed about the central hole, through which thin flexible steel delivery tubes 8a extend into the developing core to a position 9 in the water bath 3 at which the temperature of the core is approximately 800 C. Coated optical fibres 10 of 250 ym diameter are positively fed through the flexible tubes 8a from a reel 11 and emerge at position 9 into channels 12 formed in the package by the tubes 8. The channels 12 and all points downline of position 9 are at temperatures below 800 C, which has been found to be sufficiently cool to prevent contamination or adhesion to the channel walls of the optical fibre surfaces.The tubes 8a can be slid in and out so as to terminate at the most suitable position in the water bath in response to the particular extrusion rate, extrusion temperature and ambient conditions obtaining.

Downline of the water bath 3 torsional gripping means 19 comprising a pair of opposed springloaded grip rollers 13 mounted on an annular gear 14 grip the cooled cable. The torsional gripping means 19 is torsionally oscillated by a pinion drive from a reversible motor 15 and continuously periodically twists the entire section of cable between the die component 5 and rollers 13 as shown by the curved arrow A. Annular seals 16 in the walls of the water bath provide the necessary torsional freedom of movement for the core assembly. The section of the package lying within the water bath is too cool for significant plasticsdeformation to occur, but a significant permanent periodically reversing twist is imparted to the hot section 17 of the package emerging from the die component 5.The projecting ends of the flexible tubes 8a follow the oscillations of the package but make contact only with the cool sections of the channels 12 (which in this Figure are shown straight for the sake of clarity but which are in fact formed in a cylindrical sinusoidal configuration). The twist imparted to the channels 12 implies that the optical fibres 7 need to be fed at a rate slightly greater than the rate of feed of the package, particularly if the fibres 10 are to lie on the neutral axes of the channels 12. This is achieved by providing a stepped portion 27 on the reel 1 1 of slightly reduced (with respect to the neutral axis of the reinforcing cable) diameter (shown greatly exaggerated for the sake of clarity) over which the cable 7 runs and drives the reel.

The speed of rotation of the reel 11 is measured by a tachometer 18 and used to control the rate of reversal of a motor 15 and hence the pitch of the sinusoidally disposed channels 12.

Figure 2 illustrates an alternative method of "overfeeding" the optical fibres into the cable.

The arrangement is generally similar to that shown in Figure 1, except that second torsional gripping means 1 9a is provided and the optical fibres 10 are not positively fed into the channels but are drawn in with the package. This is achieved by initially temporarily gripping the walls of the channels 12 (indicated diagrammatically) against the fibres at a point (not shown) downline of the gripping assembly 1 9a. The torsional gripping means 1 9a torsionally oscillates in phase with, but with a greater oscillates in phase with, but with a greater amplitude than the gripping means 19 in response to the rate of feed of the reinforcing cable 7 as determined by the tachometer 18. This control is indicated diagrammatically by the arrowed lines connected to tachometer 18. The section of package material 2 between gripping means 19 and 1 9a is elastic and therefore partially relaxes its twist after passing downline of the latter. The optical fibres 10 are not of course appreciably elastic and are drawn into the package at a slightly enhanced rate by the oscillations of gripping means 1 9a.

The net result is that the fibres are somewhat slacker downline of the gripping means 1 9a and the degree of extra slack can be controlled by controlling the amplitude of this oscillation to cause them to lie approximately on the neutral axes of the channels 12.

The sinusoidal configuration of the channels 1 2 and flexure of the tubes 8 is indicated by dotted lines in Figure 2 and a similar configuration would be achieved by the apparatus of Figure 1.

In each case the fibres 10 may be passed through a vibrating tray of solid lubricant such as talc before being fed into the package. It will be apparent that in each case the or each torsional gripping means 19, 1 9a may rotate continuously in one direction to achieve a unidirectional helical configuration of the channels 12, the "overfeed" effect in Figure 2 then being achieved by rotating the gripping means 1 9a slightly faster than the gripping means 19.

Claims

Claims

1. A method of manufacturing optical cable comprising one or more optical fibres loosely held in a package in a helical configuration about the package axis, wherein hot thermoplastic package material is continuously extruded from a die member and conveyed to a cooling zone from which the developing package emerges in a set but flexible condition, one or more interior channels parallel to but displaced from the package axis are continuously formed within the package and at least one optical fibre is continuously conveyed into a said channel by a delivery tube which extends within the channel downline through the cooling zone at least to a point at which the package material is sufficiently cool to prevent any appreciable contamination of the optical fibre surface or adhesion of the optical fibre to the channel wall, and the hot section of the developing package is twisted by first torsional gripping means rotating about its axis to form said channel or channels into said helical configuration.

2. A method according to Claim 1 wherein the or each channel is formed by extruding the hot thermoplastic package material over a second tube disposed within the die member, and the or each delivery tube by which an optical fibre is fed into the channel extends through a respective second tube.

3. A method according to Claims 1 or 2 wherein the direction of twist is periodically reversed so that the or each said fibre has a substantially sinusoidal configuration projected on to a cylindrical surface coaxial with the package.

4. A method according to Claim 3 wherein the package is given a degree of overtwist and is subsequently allowed to relax slightly before the extrudate has fully set.

5. A method according to any preceding Claim wherein a solid lubricant is applied to the fibre surface or surfaces prior to the extrusion process.

6. Apparatus for producing optical fibre cable comprising one or more optical fibres loosely held in a package in a helical configuration about the package axis, comprising an extruder head from which the package is continuously extruded and shaped to produce one or more longitudinal channels in the package spaced from the package axis, at least one delivery tube, carried by the extruder head through which an optical fibre is introduceabie into a respective channel, means for twisting the extrudate to form the channel or channels into said helical configuration, and means for cooling the extrudate to a set but flexible condition.

7. Apparatus according to Claim 6 wherein at least the downline end of the or each delivery tube is flexible.

8. Apparatus according to Claim 7 in which the delivery tube is of stainless steel.

9. Apparatus according to Claim 7 wherein the delivery tube is in the form of a small diameter closely wound helical spring.

10. Apparatus according to any one of Claims 6 to 9 in which the longitudinal position of the or each delivery tube relative to the die member is variable.

11. Apparatus according to any one of Claims 6 to 10 wherein the means for cooling the extrudate comprises a liquid bath through which the package is passed.

12. Apparatus according to Claim 1 1 wherein the downline end of the or each delivery tube extends beyond the point of entry of the package into the liquid bath.

13. Apparatus according to any one of Claims 6 to 1 2 wherein the means for forming the channel or channels into said helical configuration comprises torsional gripping means capable of engaging the package downline of the cooling means, and of rotating about the package axis.

14. Apparatus according to Claim 13 including means for producing an oscillatory rotation of the torsional gripping means.

15. Apparatus according to Claims 14 or 15 including second torsional gripping means for gripping the package at a region downline of the first torsional gripping means, but upline of pulling means arranged to pull the complete package assembly, the second torsional gripping means being arranged to oscillate in phase with, but with a larger amplitude of oscillation than, the first torsional gripping means.

17. Apparatus according to any one of Claims 6 to 16 wherein the die member incorporates means for introducing a strengthening member into the package.

18. An optical cable produced by a method according to any one of Claims 1 to 5.

19. A method of manufacturing optical cable carried out substantially as shown in and as hereinbefore described with reference to either Figure 1 or Figure 2 of the accompanying drawing.

20. Apparatus for manufacturing optical cable substantially as shown in and as hereinbefore described with reference to either Figure 1 or Figure 2 of the accompanying drawing.