GB2175410A - Optical fibre fusion splicing - Google Patents

Abstract

A method of fusion splicing plastics coated optical fibres that is distinguished from the conventional method in that the plastics coating (11) in contact with the glass fibre (10) is not removed until after cleaving, and that thereafter the region of bared fibre that will remain after the cleaved ends have been fusion spliced in not allowed to touch any solid surface until a reinstatement plastics coating has been applied. <IMAGE>

Description

SPECIFICATION Optical fibre fusion splicing This invention relates to the preparation of optical fibre fusion splices, and in particular to a method of preparation which provides a splice with sufficient strength to be suitable for use with reinstatement plastics coatings around the splice.

The splicing of optical fibres is an important aspect in the manufacture and installation of both landline and submerged optical fibre systems. In many landline applications the splices are housed in splice boxes which protect the splices themselves from being subjected to any strain in service. The prime requirememt for such a splice is thus the achieving of a low optical loss coupling, and the strength of the resulting splice is relatively unimportant.

According to conventional practice a fusion splice for this kind of application involves stripping the plastics coating or coatings from the ends of two glass (silica) fibres that are to be spliced, cleaving the bared fibres to produce planar end faces normal to their respective fibre axes, fusion splicing together the prepared ends, and finally applying some form of protective coating to cover the remaining exposed region of bare fibre.

Such a technique may also be suitable for use in submarine cable applications where the splices are located in special cable joint housings. However, for efficiency in the manufacturing and laying of submarine cable it is desirable that the cable shall be produced in uniform profile lengths equal to the distance between consecutive regenerators (repeaters).

Fibre production methods in current use do not produce fibre in lengths as long as this, and so splices are required at intervals along a length of fibre which can be cabled without producing a local bulge in the cable profile.

For this purpose a splice is required that can be provided with reinstatement plastics coating over the region of the splice to provide substantially the same diameter as that of the fibre with its parent coatings. More particulariy however, the strength of the fibre in the reinstatement region should approach closely to that of the fibre elsewhere because in subsequent cabling and laying, and in service, the splices will be liable to be subjected to just the same strains as other regions of the fibre within the cable. In those applications therefore not only is low optical loss an important criterion for a satisfactory splice, but so is high strength.

The present invention is concerned with the achieving of high strength, and the preferred method of splicing is distinguished from conventional fusion splicing in that the primary plastics coating on fibres to be spliced is not removed until after cleaving, and that thereafter the region of bared fibre that will remain after the cleaved ends have been fusion spliced is not allowed to touch anything solid until the reinstatement has been applied.

According to one aspect of the present invention there is provided a method of fusion splicing the ends of a pair of glass optical fibres provided with at least one plastics coating wherein each fibre is cleaved at a point near one end of that fibre, the cleaved ends are fusion spliced, and a reinstatement plastics coating is applied to cover a region of bared glass fibre extending in both directions from the fusion spliced cleaved ends, which method is characterised in that the cleaving is performed at a point where at least the plastics coating directly in contact with the glass remains intact, and that at no time are the sides of said regions of bared glass fibre contacted by any solid surface other than that of its original coating and that of its reinstatement coating.

The invention also provides a method of fusion splicing the ends of a pair of glass optical fibres provided with at least one protective plastics coating on the glass, wherein cleaved ends are prepared for fusion without first removing the plastics coating in contact with the glass from the glass where cleaving is effected.

Typically, though not essentially, the glass of the fibres possesses a waveguiding structure within the glass, and the plastic coatings are protective.

There follows a description of the formation of optical fibre fusion splices according to methods embodying the invention in preferred forms. The description refers to the accompanying drawings in which Figs. 1 and 2 depict schematic representations of successive stages in a method of forming a fusion splice.

The first method to be described is for fusion splicing 125 microns diameter silica optical fibre 10 that possesses a waveguiding structure within the silica and is provided with a silicone primary plastics coating 11 to a diameter of about 270 microns, and a nylon secondary plastics coating 12 to a diameter of 850 microns.

In order to prepare an end of such a fibre for cleaving the secondary nylon coating 12 is stripped back from a point 13 located a predetermined distance 'd', typically about 35mm, from the end of the fibre using a mechanical stripping tool based upon a design of wire stripping tool. Next a much shorter length 1, typically about 1cm, of the exposed primary coating 11 is stripped from the end of the fibre using a loop of nylon line. This is in order to expose a portion of bare fibre 10 long enough to be secured by a clamp 14 of apparatus used for cleaving. This apparatus includes a second clamp 15 which is arranged to clamp the secondary coating near the end 13 of that coating. When the fibre has been secured in these two clamps they are biassed apart under predetermined tension, typically of about 2 Newtons (--200 grams weight).An anvil 16 with a 60mm radius is located under the tensioned fibre at a height just sufficient to cause the fibre to follow the curvature of the anvil at the point where it is to be cleaved, and a diamond faced cutting tool 17 is gently lowered on to the primary coating 11 of the fibre 10 above the anvil. The cutting tool is designed to apply a load of about 0.02 Newtons (--3 grams weight) to the fibre which is sufficient to penetrate the primary coating and cause the fibre to cleave when the cutting edge reaches the glass, whereupon the two clamps spring apart. Cleaving is arranged to occur a set distance 's', typically about 9mm, from the end 13 of the secondary coating.

Next a short length of primary coating 11 needs to be stripped from the cleaved end so that none of this coating material shall be close enough to the fusion splice to contaminate it as it is being made. This primary coating needs to be removed in a manner that does not involve any solid surface coming into contact with the bare fibre thus detracting from the strength of that fibre. A convenient way of removing this coating is to dissolve it in pure concentrated sulphuric acid by dipping the cleaved end of the fibre into the acid to a predetermined depth 'h', typically about 3 to 4mm. The end is then repeatedly rinsed with deionised water to remove all traces of acid.

Excess water can be removed after each rinse by lightly flicking the secondary coating of the fibre near the end 13.

When two fibre ends have been prepared in this way they are mounted in clamps 20 (Fig.

2) of fusion splicing apparatus. Each of these clamps includes a pad 21 which bears against the secondary coating and a second pad 22 which bears against the primary coating. The distance s-h of exposed primary coating is made just long enough to safely accommodate the pad 22 without any risk of the bared portion of the fibre from coming into contact with any part of the clamp.

The two clamps 20 are moveable independently of each other, and are micromanipulated to bring the axes of the two fibres into exact alignment with the cleaved end faces normally less than 1 microns apart. The fibres are then moved apart axiallyso as to remove them from the zone where they risk becoming contaminated by the striking of an electric arc (not shown). The direction of movement is then reversed and they are moved axially back together to meet and fuse in the arc.

After the arc has been extinguished, the two fibres, now joined together by their end, are located in a Vee-groove in a polytetrafluoroethylene platen (not shown) for reinstatement of the primary coating on the exposed region of bare fibre. The length of the Vee groove is less than 2s, but substantially more than 2h, so that the fibres are located in the groove by their primary claddings without risk of the region of bare fibre coming into contact with the platen. A drop of freshly prepared silicone resin is then applied to the region of bare fibre which flow by capillary action along to the shoulders of the parent primary coating 11. The resin is cured in situ by energising a heater embedded in the platen.

Finally, after reinstatement of the primary coating, the assembly is removed from the platen and placed in an injection mould (not shown) for receiving reinstatement secondary coating of nylon which contends between the two shoulders 13 and has substantially the same profile as the parent secondary coating 12.

Using this method splices have been made on a routine basis that have been capable of withstanding dynamic strains in the range from 2 to 4%; whereas when making splices with the same apparatus, but with the sole difference of stripping back more primary coating so that bare fibre is cleaved and the pads 22 bear on bare fibre, the resulting splices are significantly less reliable and typically break at strains of not more than 1%.

These strain figures are quoted for splices that have not been provided with reinstatement plastics coatings.

Minor modifications have been made when adapting the above described method of fusion splicing silicone primary coated fibre to the fusion splicing of acrylate coated fibre.

Typically, though not necessarily, such fibre is not provided with a secondary plastics coating. However the main differences in procedure result from the difference in the properties of silicone and acrylate resins. A particular difference lies in the fact that whereas silicone resin primary coatings are satisfactorily removed by dissolving them in pure concentrated sulphuric acid at room temperature, we have found that acrylate coatings are typically resistant to this treatment and require the acid to be at a considerably elevated temperature, typically above 120 C and usually about 1 80 C.

A different procedure is therefore preferred for stripping the acrylate coating in a manner that avoids any requirement to use hot concentrated acid. A wire-stripping type tool (not shown) is employed to make two circumferential cuts in the acrylate coating to a depth insufficient to reach the underlying glass. This tool is one that has been specifically designed for stripping optical fibres provided with thin plastics coatings. One cut is at distance '1' from the end of fibre before cleaving while the other is at a distance 'h' from the end of the fibre after cleaving. This second cut is however made before cleaving.

When the two cuts have been made the end of the fibre is dipped into a suitable solvent, such as dichloromethane, to cause the acrylate to swell and become loosened from the underlying fibre. The fibre end is first dipped into the solvent to a depth just sufficient to cover the first cut at a distance '1' from the end. Evaporation of the dichloromethane is conveniently inhibited by floating a layer of water on its surface. After immersion in dichloromethane for about 30 seconds the length of acrylate coating beyond the first cut can readily be pulled off with rubber faced tweezers.

Following removal of this end section of the acrylate coating, the fibre end is once again dipped into the dichloromethane, but to a greater depth than before. This time the fibre is dipped in to a depth where the liquid has risen above the point where cleaving is to occur, but not quite as far as the second cut.

The purpose of this immersion is to soften the acrylate coating to facilitate the cleaving operation. After about 30 seconds the fibre is removed and immediately mounted in the cleaving apparatus. The cleaving procedure is itself unchanged, and is performed before there has been time for the acrylate coating to have become restored to its original hardness.

The fibre is next removed from the cleaving apparatus and the end dipped for a third time into dichloromethane to a depth up to or just above the second cut, the one that is located a distance 'h' from the cleaved end. Tweezers are again used to remove the loosened portion of cladding, and then the end is dipped into pure concentrated sulphuric acid to a depth just insufficient to reach the acrylate coating above the position of the second cut.

It is found that the unheated acid will remove any residual traces of acrylate left after the stripping operation, and for this purpose the fibre end is typically left in the acid for about 10 minutes.

The rinsing procedure, and the procedures for employing an arc to effect a splice between two acrylate fibres prepared in this way, are unchanged. However when providing an acrylic reinstatement coating using the platen, it is preferred to effect curing of the resin through the agency of ultraviolet light.

Although both specific embodiments of the invention have involved the splicing of glass optical fibres possessing a waveguiding structure within the glass, but it is to be understood that this invention is also appiicable to plastics-clad-glass fibre structures where the waveguiding structure is provided by the glass/plastics interface. In such structures the glass, typically being made of silica, forms the optical core of the fibre while the plastics coating in contact with the glass is of lower refractive index and forms the optical cladding.

Claims (9)

1. A method of fusion splicing the ends of a pair of glass optical fibres provided with at least one plastics coating wherein each fibre is cleaved at a point near one end of that fibre, the cleaved ends are fusion spliced, and a reinstatement plastics coating is applied to cover a region of bared glass fibre extending in both directions from the fusion spliced cleaved ends, which method is characterised in that the cleaving is performed at a point where at least the plastics coating directly in contact with the glass remains intact, and that at no time are the sides of said regions of bared glass fibre contacted by any solid surface other than that of its original coating and that of its reinstatement coating.

2. A method as claimed in claim 1 wherein after the two fibres have been cleaved, but before they have been fused together, a portion of the coating is removed from each by dissolution in concentrated sulphuric acid to provided said region of bared glass.

3. A method as claimed in claim 2 wherein the plastics coating directly in contact with the fibres is a silicone resin and the concentrated sulphuric acid is unheated.

4. A method as claimed in claim 2 wherein the plastics coating directly in contact with the fibres is an acrylate resin and the concentrated sulphuric acid is heated above room temperature.

5. A method as claimed in claim 1 wherein the plastics coating directly in contact with the fibres is an acrylate resin, and wherein after the two fibres have been cleaved, but before they have been fused together, a portion of the acrylate coating is removed from each by mechanical stripping having first loosened the portion of coating by immersion in a solvent that causes it to swell.

6. A method of fusion splicing substantially as hereinbefore described with reference to the accompanying drawings.

7. A method of fusion splicing the ends of a pair of glass optical fibres provided with at least one protective plastics coating on the glass, wherein cleaved ends are prepared for fusion without first removing the plastics coating in contact with the glass from the glass where cleaving is effected.

8. A method as claimed in any preceding claim wherein the glass of the fibres possesses an optical waveguiding structure within the glass.

9. A length of glass optical fibre possessing a waveguide structure within the glass and which has been provided with at least one protective plastics coating, which fibre is composed of two or more sections which have been united by the fusion splicing method claimed in any preceding claim.