GB2065324A - Optical fibres - Google Patents

Abstract

The protective resin covering of a vitreous optical fibre 56 consists of a buffer coat 57 of a readily deformable soft resin, applied directly to the fibre surface, a capping layer 58 of a resin which provides the buffer coat with a surface of greater hardness and lower friction, and an outer jacket 59 of a hard, abrasion-resistant resin extruded over the capping layer and in intimate contact therewith. In a specific example the buffer coat 56, capping layer 58 and outer jacket 59 are respectively formed of a silicone resin, "Kynar", and "Hytrel" (Registered Trade Marks). The buffer coat and capping layer are applied to the fibre in liquid form and then cured, in-line immediately after the fibre is drawn, and the outer jacket may be applied either in-line also, or in a subsequent operation. <IMAGE>

Description

SPECIFICATION Resin-covered optical fibres and their manufacture This invention relates to optical fibre waveguides composed of vitreous material, or comprising at least a core of vitreous material along which radiation is propagated in use, and provided with a protective covering of synthetic resinous material. The invention is more particularly concerned with so-called "buffered" optical fibres, and also relates to the manufacture of the buffered optical fibres described, and to optical cables incorporating the buffered fibres.

One form of protective covering which it has been proposed to apply to optical fibres formed of glass or vitreous silica consists of a buffer coat of a relatively soft resin, applied directly to the vitreous fibre surface, and an outer jacket of a harder resin which is extruded over the buffer coat. This type of covering is particularly effective in protecting the vitreous fibre from surface damage and from deformation, in particular microbending, which would cause optical losses in operation of the fibre. In addition, the presence of a buffer coat increases the mechanical strength of the jacketed fibre, as compared with the strength of a fibre having a protective coat of hard resin alone, while the hard jacket facilitates handling of the buffered fibre in cable manufacturing processes.Another advantage is that it is possible to employ, for both the buffer coat and the jacket, resins which are capable of withstanding high temperatures such as those employed, for example, for vulcanising cable insulation.

However, in the manufacture of a buffered and jacketed optical fibre of the form described above, difficulties arise in handling the fibre with a soft buffer coat for carrying out the extrusion of the jacket over it, due to the easily deformable and sticky nature of the buffer resin. It is an object of the present invention to provide an improved form of buffered optical fibre which will possess the desirable properties referred to above, and in the manufacture of which the above-mentioned difficulty can be overcome.

According to the invention, in a buffered optical fibre consisting of a vitreous optical fibre with a protective resin covering, the said covering is formed of three components which consist of, firstly, a buffer coat composed of a relatively soft resin, as hereinafter defined, applied directly to the vitreous fibre surface, secondly a capping layer composed of a resin having greater hardness and lower friction than the buffer coat resin possesses, applied directly over the buffer coat, and thirdly an outer jacket composed of an abrasion resistant resin, extruded over the capping layer and in intimate contact therewith.

The optical fibre may constitute an optical waveguide of either the monomode or multimode type, and is suitably composed of vitreous silica with one or more dopants for modifying the refractive index of the silica, the dopant being distributed either so as to give a fibre having a graded refractive index profile, or to form a core and cladding of different indices so that the fibre has a step refractive index profile. Alternatively the vitreous fibre may constitute the radiationpropagating core of a waveguide of the type consisting of a vitreous core and a cladding of synthetic resin: in this case the cladding may be constituted by the buffer coat of the protective covering.

The phrase "relatively soft resin", as used above with reference to the buffer coat, is to be understood to mean that the resin is readily deformable and thus capable of protecting the optical fibre from deformation due to externally applied pressure, suitable materials for the buffer coat being silicone resins, The thickness of the buffer coat is not critical, and is typically from 30 to 60 microns. Suitable materials for the capping layer include polyvinyl acetal resins, for example polyvinyl butyral or polyvinyl formal resin, and fluorinated polymers such as polyvinylidene fluoride or a polyvinylidenepolytetrafluoroethylene copolymer.The function of the capping layer is to provide the buffer coat with a hard, low friction surface, in order to facilitate the extrusion of the outer jacket over the buffer coat: therefore the capping layer can be considerably thinner than the buffer coat, a single coat about 5 microns thick being sufficient, although several thin coats may be applied if desired. The extruded outer jacket is suitably formed of, for example, polyethylene, or amorphous nylon, or a melt-processable polyester.

The jacket may be of any thickness required to give a coated optical fibre of a desired overall diameter, typically from 0.3 to 1.5 millimetres, or larger if required for a particular cable construction.

In the manufacture of a buffered optical fibre in accordance with the invention, the buffer coat and the capping layer are each applied to the fibre in liquid form, and the coated fibre is heated after each application to cure the resin coating, this procedure preferably being carried out in-line during the fibre drawing operation.Thus a preferred process for the continuous production of a said buffered optical fibre includes the steps of drawing a fibre vertically downwards from a heatsoftened vitreous body, such as a preform rod, of the desired composition and structure of the optical fibre, passing the drawn fibre directly, without first ailowing it to come into contact with any solid surface, through a liquid resin bath for the application of a first resin coating, and through means for adjusting the thickness of the resin coating thus applied to the fibre, then passing the coated fibre successively through heating means for curing the said first resin coating to form the buffer coat, means for applying a second liquid resin coating, means for adjusting the thickness of the second resin coating, and heating means for curing the second resin coating to form the capping layer.The coated fibre may then pass directly to an extruder by means of which the outer jacket is applied thereto, or alternatively the coated fibre may be wound on a drum, the jacket being extruded over it in a subsequent operation. If desired, several coats of the resin forming the capping layer may be applied, by passing the buffer-coated fibre through a series of the appropriate resin coating and curing means.

It will be understood that the "liquid resin" through which the fibre is passed for the application of the buffer coat and the capping layer respectively may in each case be either a resin monomer or polymer in the liquid state, or a solution or suspension of the resin in a suitable liquid medium.

A specific form of buffered optical fibre in accordance with the invention, and the method which we have employed for its manufacture, will now be described in the following Example 1.

EXAMPLE 1 The optical fibre of the example is composed of silica with a dopant in a concentration varying radially across the fibre to give a graded refractive index profile, and has a diameter of 120 microns.

The fibre has a protective covering consisting of a buffer coat composed of silicone resin, suitably "Sylgard 184" (Registered Trade Mark), 30 microns in thickness, a capping layer approximately 5 microns thick, composed of a polyvinylidene fluoride resin sold under the Registered Trade Mark "Kynar", and an outer jacket formed of a polyester resin sold under the Registered Trade Mark "Hytrel", of a suitable thickness to give the required overall diameter of the covered fibre, depending upon the construction of the cable in which the fibre is to be incorporated. Buffered optical fibres of this form are shown in detail in Figure 5 of the accompanying drawings (to be described below), the silica fibre, buffer coat, capping layer and outer jacket being indicated respectively by the reference numerals 56, 57, 58, and 59.

In the manufacture of the buffered optical fibre of the example, the silica fibre is drawn from a rod preform produced in known manner by chemical vapour deposition of the doped silica in the bore of a silica support tube, and subsequent collapse of the tube. The drawing of the fibre is carried out by feeding the preform rod into a vertically disposed tubular furnace element maintained at a temperature of 20000 C, in a flowing atmosphere of argon, and drawing the fibre vertically downwards from the lower end of the rod.The drawn fibre is immediately, without coming into contact with any surface which might cause damage or contamination, passed vertically down through a bath of the liquid silicone resin, mixed with 10% of its weight of a suitable hardener, contained in a vessel which has a die located centrally in the base, the die having a conical taper, suitably of 120 inclusive angle, and an exit aperture of appropriate diameter for controlling the amount of liquid resin retained on the fibre to give the required thickness of buffer coat after curing. The coated fibre is then passed through a vertical oven 0.5 metre long, maintained at a maximum temperature of 5000 C, to cure the resin. If necessary the fibre may then be passed through a second oven one metre long with a maximum temperature of 1 500 C, to "polish" the cure.

The buffer coated fibre is then passed round a guide pulley to change its direction of travel from vertical to horizontal, and is coated with a solution of "Kynar" powder in acetone. The resin solution is applied to the fibre by passing the fibre over a pulley which has a peripheral V-shaped groove into which the resin solution is drip-fed continuously, so that the fibre is coated with the solution without coming into contact with the pulley surface. The excess liquid is removed by passing the fibre through a die to leave the required thin coating, and evaporation of the solvent and curing of the resin are effected by passing the fibre through an oven one metre long, at a temperature of 25O0C.

A suitable rate of travel of the fibre through the whole coating and curing system, with the oven lengths and temperatures as stated above, is one metre per second.

The fibre is subsequently passed through an extruder by means of which it is covered with a tightly fitting tubular jacket of "Hytrel".

It will be appreciated that the hard capping layer of "Kynar" makes it possible to store the fibre, if desired, for a length of time before the outer jacket is applied, and facilitates the handling of the fibre during the extrusion process. The combined covering of the buffer coat, capping layer, and jacket provides excellent protection for the vitreous fibre against damage and deformation, even under severe conditions of manipulation and use, and imparts high mechanical strength to the assembly of fibre and covering. Furthermore, all the resins employed in the buffered fibre of the example are capable of withstanding high temperatures for a sufficient length of time to enable a cable incorporating the fibre to be subjected to a vulcanising process, without the fibre covering suffering any damage.

The presence of the harder capping layer has the additional advantage that it facilitates the stripping off of the outer jacket, as may be required, for example, when making connections between two lengths of optical fibre or cable.

The buffered optical fibres of the invention are suitable for use in any form of cable incorporating one or more optical fibres, with or without electrical conductors, and are particularly advantageous for use in cables which are required to be subjected to vulcanisation treatment and/or are liable to be subjected to arduous conditions in use, for example mine trailing cables.

Some specific forms of optical cable in which the buffered optical fibres of the invention may advantageously be used are shown in the accompanying diagrammatic drawings and are described in the following Examples 2 to 6. In the drawings.

Figures 1 to 4 show, in cross-section and on an enlarged scale, four different forms of mine trailing cable including both electrical conductors and optical fibres, and Figure 5 is a cross-section, on an enlarged scale, of an all-dielectric optical cable.

EXAMPLE 2 The cable shown in Figure 1 includes three electrical conductor elements 1, 2, 3 and an optical transmission element 4 incorporating a further electrical conductor, these four elements being helically twisted around a bundle of wires which constitutes an earth conductor and is disposed along the axial region of the cable. The structure so formed is encased in a sheath 6 of elastomeric material. Each of the elements, 1, 2, 3 consists of a power core 7 composed of a helically twisted assembly of a plurality of helically intertwisted bundles of wires, covered by an inner layer 8 of elastomeric insulating material and an outer layer 9 of braid incorporating wire strands to form an earth screen.The optical transmission element 4 includes four optical fibre units each consisting of a single optical fibre 10 with a resin covering 11 composed of a buffer coat, a capping layer and an outer jacket as described in the above Example 1, or formed of other suitable resins as described above; these buffered optical fibres, which are each of overall diameter 1.6 mm, are helically coiled around a mandrel consisting of a pilot core 1 2 formed of helically twisted bundles of wires, with a covering layer 13 of elastomeric insulating material : the diameter of the mandrel is 6 to 7 mm, and the pitch of the fibre coils is 25 mm. The pilot core 12 is of the same construction as, but of smaller diameter than, the power cores 7.The assembly of buffered optical fibres is covered by a sleeve 14, which may be a winding of tape formed of resin-impregnated textile fabric or of polyester sheet, or may be an extruded tube of elastomeric material, and the sleeve is covered by a further layer 1 5 of elastomeric material, to build up the element to a diameter equal to that of the elements 1, 2 and 3.

In a specific example of a cable of the form shown in Figure 1, the cores 7 and 12 and the earth conductor 5 are all composed of fine tinned copper wires, the insulating layers 8 and 13 consists of ethylene-propylene rubber or chlorosulphonated polyethylene, the build-up layer 1 5 and the sheath 6 consist of polychloroprene, and the braid forming layers 9 is composed of tinned copper wires and nylon filaments, all the elastomeric materials being vulcanised.

EXAMPLE 3 In the cable shown in Figure 2, an optical transmission element 16 is disposed along the axial region of the cable and is surrounded by a helically intertwisted assembly of five electrical conductor elements 1 7 to 21 inclusive, the whole structure being embedded in an elastomeric sheath 22. Each of the elements 17, 18 and 19 consists of a power core 23 covered by a layer 24 of elastomeric insulation, and braid 25 incorporating a metallic earth screen. The element 20 consists of a pilot core 26 covered by a layer 27 of elastomeric insulation and a further build-up layer 28 of elastomeric material. The element 21 consists of an earth core 29 covered by an elastomeric insulating layer 30 and an elastomeric build-up layer 31.The cores of all these elements are composed of wires, for example of tinned copper, and are of similar construction to the conducting cores described above with reference to Figure 1; the insulating layers, build-up layers, braid and sheath may be composed of the same materials as the corresponding layers described in the above specific example with reference to Figure 1.

The optical transmission element 1 6 of Figure 2 includes four buffered optical fibres 32 as described in Example 1, helically coiled around a mandrel 33, which may be formed wholly of elastomeric material or may incorporate a conducting core if desired, and covered by a sleeve 34 formed of resin-impregnated fabric tape, polyester tape, or an extruded tube of elastomeric material. The interstices between the said sleeve and the electrical conductor elements 17 to 21 are filled with suitable packing material 35, which is preferably elastomeric.

EXAMPLE 4 The cable shown in Figure 3 again consists of five elements helically twisted around an axial member 36, whic is suitably formed of a natural or synthetic rubber or may, if desired, be an additional insulated conductor core, the whole structure being embedded in an elastomeric sheath 37. The intertwisted elements consist of three insulated power cores 38, 39, 40, of the same construction as the elements 1 7, 18 and 1 9 in Figure 2, an insulated earth core 41 of the same construction as the element 21 in Figure 2, and an optical transmission element 42 of the same construction as the element 4 in Figure 1, incorporating an insulated pilot core 43 as the mandrel.The materials of the conductor core covering layers and the sheath may be the same as those described with reference to Figure 1, and all the conductor cores are formed of intertwisted assemblies of tinned copper wires.

EXAMPLE 5 The cable shown in Figure 4 comprises an axially disposed earth conductor 44, surrounded by a helically twisted assembly of three electrical conductor elements 45, 46, 47 and an optical transmission element 48. Each of the elements 45, 46 and 47 consists of a power core 49 jacketed with a layer 50 of elastomeric insulation, suitably ethylene-propylene rubber or chlorosulphonated polyethylene, and an outer layer 51 of semiconducting elastomer. The optical transmission element 48 is of similar construction to that of the element 4 of Figure 1, including four buffered optical fibres in accordance with the invention helically twisted around a mandrel in the form of an insulated pilot core 52, but the element has an outer covering layer 53 of semiconducting elastomer.The elements 45, 46, 47 and 48 are partially embedded in a semiconducting cradle 54, in which the earth conductor 44 is also embedded, and the cable is completed with a sheath 55, suitably of polychloroprene. The outer layers 51 and 53 of the elements 45, 46, 47 and 48, and the cradle 54, are suitably composed of carbonloaded rubber, and all the cores 44, 49 and 52 are formed of intertwisted tinned copper wires.

EXAMPLE 6 The cable shown in Figure 5 comprises an assembly of three buffered optical fibres 56 in accordance with the invention, the buffer coat, capping layer and outer jacket being shown at 57, 58 and 59 respectively, and three elongate strength members 60, all helically stranded around a central strength member 61. Each of the strength members 60 and 61 is composed of helically stranded aromatic polyamide yarns impregnated with acrylic resin in such a manner that each individual filament of the yarns is coated with the resin, encased in an extruded amorphous nylon sleeve 62. The buffered optical fibres and the strength members are all of the same overall diameter, suitably 1.5 to 1.7 mm. The whole assembly of buffered fibres and strength members is surrounded by an extruded outer sheath 63 of polyethylene 1.0 mm thick.

Claims (10)

1. A buffered optical fibre consisting of a vitreous optical fibre with a protective synthetic resin covering, wherein the said covering is formed of three components which consist of, firstly, a buffer coat composed of a relatively soft resin, as hereinbefore defined, applied directly to the vitreous fibre surface, secondly a capping layer composed of a resin having greater hardness and lower friction then the buffer coat resin possesses, applied directly over the buffer coat, and thirdly an outer jacket composed of an abrasion resistant resin, extruded over the capping layer and in intimate contact therewith.

2. A buffered optical fibre according to Claim 1, wherein the said buffer coat is composed of a silicone resin.

3. A buffered optical fibre according to Claim 1 or 2, wherein the said capping layer is composed of a polyvinyl acetal resin of a fluorinated polymer.

4. A buffered optical fibre according to Claim 1, 2 or 3, wherein the said outer jacket is composed of polyethylene, or amorphous nylon, or a meltprocessable polyester.

5. A buffered optical fibre according to any preceding Claim, wherein the thickness of the said buffer coat is from 30 to 60 microns, and the thickness of the said capping layer is approximately 5 microns.

6. A method of manufacturing a buffered optical fibre according to any preceding Claim, which includes the steps of drawing a vitreous optical fibre vertically downwards from a heat-softened vitreous body, passing the drawn fibre directly through a liquid resin bath for the application of a first resin coating and through means for adjusting the thickness of the resin coating thus applied to the fibre, then passing the coated fibre successively through heating means for curing the said first resin coating to form the said buffer coat, means for applying a second liquid resin coating, means for adjusting the thickness of the second resin coating, and heating means for curing the second resin coating to form the said capping layer, and subsequently passing the coated fibre through an extruder by means of which the said outer jacket is applied thereto.

7. A buffered optical fibre according to Claim 1, substantially as hereinbefore described in Example 1.

8. A method according to Claim 6, carried out substantially as hereinbefore described in Example 1.

9. An optical cable incorporating one or more buffered optical fibres according to any of the preceding Claims 1 to 5 and 7.

10. An optical cable according to Claim 9, substantially as shown in any one of Figures 1 to 5 of the accompanying drawings, and as hereinbefore described in Examples 2 to 6 respectively.