WO1991003751A1

WO1991003751A1 - Optical fibre fusion splicing

Info

Publication number: WO1991003751A1
Application number: PCT/GB1990/001390
Authority: WO
Inventors: David John Clift; Stephen Robert Mallinson
Original assignee: British Telecommunications Public Limited Company
Priority date: 1989-09-11
Filing date: 1990-09-07
Publication date: 1991-03-21
Also published as: GB8920473D0; AU6335490A

Abstract

A fusion splicing method for optical fibre ends in which the fibre ends are heated to a temperature that is sufficient to enable sticking contact with one another but insufficient to permit substantial flow. The fibres (1' and 5') are then advanced towards one another, and beyond an initial contact point, so that any end angles on the fibres induce lateral deflection of the fibre(s) to bring the end faces into substantially planar contact. The heating is then increased to fuse the fibre joint, and a further heating and pulling stage follows to remove any bend at the joint.

Description

OPTICAL FIBRE FUSION SPLICING

This invention relates to fusion splicing of optical fibres.

In optical communications the radiation used is not necessarily in the visible region of the electromagnetic spectrum, and so the words 'optical' and 'light' when used in this specification are not to be interpreted as implying any limitation to the visible spectrum. For example, the wavelengths preferred for transmission through silica optical fibres are in the infra red region because the loss minima of silica fibres occurs at 1.3 and 1.5 microns.

Single mode optical fibre presently available has very low loss characteristics, typically less than 0.3dB/Km, and thus the losses over fibre networks are principally dependent on the jointing or splicing between fibre links, or between fibre links and fibre tails on components. Fusion splicing is the most commonly used method of jointing, and can produce low loss joints, that is below O.ldB loss, but the success rate for achieving acceptable splices is low, generally 0.5dB loss being regarded as the maximum acceptable splice loss.

Two major causes of high splice losses are initial lateral misalignment of the fibres, and fibre end angles (that is to say the end face of at least one of the fibres not being normal to the longitudinal direction of that fibre).

In fusion splicing, some means such as an electric arc is used to heat the ends of adjacent fibres that are to be joined together, the arc melting the fibre ends so that the two molten ends can be abutted together, adhere to one another and become permanently fused on cooling.- Although the principle is simple, the equipment required may be complex in order to ensure, as far as possible, accurate lateral positioning and fibre movement control during the fusion process. Alignment is particularly critical with single mode fibre which has a central, comparatively small (e.g. 9 micron diameter) core along which light is propagated.

A typical alignment procedure comprises mounting the fibre ends that are to be joined in chucks on a splicing machine, viewing the fibre ends through a microscope, and adjusting the positions of the chucks. During the fusion process, the fibre ends become sufficiently molten that surface tension tends to pull the cladding of the fibres into exact alignment. Unfortunately, however, the flow of the cladding tends to deform the cores (the ends of the cores becoming asymmetrically located in the cladding). When the ends of a fibre have end angles, then, at the initial moment of contact of the fibre ends, only parts of the ends are in contact, and the gap created by the angled end or ends becomes filled by cladding flowing into the gap. However, in this instance, the flow of cladding also tends to drag the core with it; with the result that, although the fibre is smoothly joined, the cores are deformed, again resulting in loss. Thus, it is the present practice for fibre ends that are of greater than 2^* off normal to be rejected for splicing.

One technique that has been used for improving splices is to taper the fused splice, for example as reported in 'Low-loss joints between dissimilar fibres by tapering fusion splices', in Electronics Letters, 13 March 1986, Vol. 22 No. 6 pp 318-319. The tapering of a splice between single mode fibres results in a tapering of the core, and a reduction in the core diameter increases the mode field spot size. With an increased mode field spot size, light becomes guided by the cladding in the region of the splice and then subsequently returns to being guided by the core as the spot size reduces again on the far side of the tapered splice. Because the light is cladding guided at the splice, misalignment of the cores, whether caused by lateral offset or end angles, is less critical. However, with cladding guiding, the splice is more susceptible to breakage (since the fibre cross-section is small), and also loss is introduced by materials used to encapsulate the splice. The latter problem may be overcome by using low refractive index encapsulation materials, but since this restriction limits the choice of materials severely, there is usually some other performance penalty to pay.

The aim of the invention is to provide low loss fusion splices with less stringent selection criteria for the fibre ends.

The present invention provides a method of fusion splicing optical fibre ends, the method comprising the steps of heating the fibre ends to a first temperature that is sufficient to cause surface melting but insufficient to permit substantial flow of fibre material, advancing the fibre ends towards one another thereby ensuring the fibre ends adhere to one another at an initial contact point, advancing the fibre ends beyond the initial contact point so that any end angles on the fibres induce lateral fibre deflection to bring the fibre end faces into substantial contact, and subsequently heating the fibre ends to a second temperature ard pulling the joint formed between the fibre ends to remove any bend at the joint, the second temperature being higher than the first temperature. Advantageously, the distance that the fibre ends are advanced beyond the initial contact point is substantially equal to the distance through which the fibre joint is subsequently pulled.

Conveniently, the distance that the fibre ends are advanced beyond the initial contact point is in the range of from 5 to 20 microns.

Preferably, after the end faces have been brought into contact, the fibre ends are heated to a temperature intermediate to the first and second temperatures.

In a preferred embodiment, the heating is by an electric arc. Advantageously, an electric current of 7 to 11mA is applied to the electric arc to heat the fibre ends to said first temperature, and the fibre ends are advanced towards one another at a speed of 200«m per second. Preferably, an electric current of 13 to 17mA is applied to the electric arc to heat the fibre ends to said second temperature, and an electric current of 10 to 13mA is applied to the electric arc to heat the fibre ends to said intermediate temperature. The joint may be pulled at 10μm per second.

Preferably, the fibre ends are advanced beyond the initial contact point after a predetermined time interval following contact of the fibre ends at the initial contact point. The predetermined time interval may be 2 seconds.

The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of two monomode fibres prepared for splicing, one fibre having an end angle;

Figure 2 is a schematic diagram of the fibres of Figure 1 joined by a prior art fusion splicing method;

Figure 3 is a schematic diagram of two monomode fibres mounted for splicing in a fusion splicing machine; Figure 4 is a schematic diagram of the fibres of Figure 3 at the end of the tacking stage in a procec⁵"^,~e according to the present invention; and

Figure 5 is a schematic diagram of the fibres af .r completion of the pulling stage of a procedure according to the present invention.

Referring to the drawings, Figure 1 shows a first single mode fibre 1 having a core 2, cladding 3 and an end face 4 normal to the longitudinal axis of the fibre. A second fibre 5, having a core 6 and cladding 7, has an end face 8 at an angle (exaggerated in the drawing) which may typically deviate by up to 8^* from the plane normal to the longitudinal axis of the fibre.

In a fusion splicing programme of the type used prior to the present invention, the ends of the fibres 1 and 5 would be heated in an arc discharge; typically, for an electrode spacing of 2.3 mm in a splicer such as an Ericsson FSU850 fusion splicer, at a current of the order of 10 milliamps prior to contact of the fibre end faces 4 and 8. Upon contact, the current would be increased to the order of 15 or 16 milliamps. Usually, the first (10mA) heating stage starts with the fibres 1 and 5 separated by a gap of about 50 microns, and heating to the higher level starts as soon as the end faces 4 and 8 come into contact, the then relatively molten ends being squashed together for a further distance of travel of about 5 microns. The heating is then maintained at a slightly lower level (12mA) for a period of about two seconds. Subsequently, the arc is switched off, and the fused splice cools. The result is as shown in Figure 2 for fibre ends of the type shown in Figure 1. On contact of the leading edges of the fibres there is a gap 9 (see Figure 1) which is filled by flow of molten cladding material. This flow of molten cladding material drags the cores 2 and 6 with it, so that the cores are angularly offset, thereby resulting in high loss due to phase mismatch.

The principle of the present invention is to use a lower heating current and a push-pull technique in which the two ends to be spliced are pushed against one another to achieve substantially planar sticking contact, and then subsequently pulled back. Owing to the use of a lower heating current, only the surfaces of the fibres melt, and flow into any gap between the fibres is inhibited. The process will now be described in more detail with reference to Figures 3 to 5.

Two fibres 1' and 5' to be spliced together are located in the chucks (not shown) of a fusion splicer, as schematically shown in Figure 3. In test procedures, the ends of the fibres 1' and 5' are selected for their relatively poor quality, having significant end angles so that one or both end faces provided an included end angle in the range of 2^* to 8".

Initially the fibres 1' and 5* are separated by an arbitrary distance; but, for the start of the splicing procedure, they are brought into contact with one another to establish the contact point datum. At this stage, the fibre ends will meet as shown in Figure 1 (in the case of only one poor end angle). The fibre ends are aligned by the usual viewing techniques on fusion splicing machines, and the chucks are then separated to starting positions which may be, for example, a separation of about 80 microns. From the starting positions, the ends are then moved towards one another by movement of one or both chucks; and, at a separation of 50 microns, a low arc current is turned on. Using the previously mentioned Ericsson machine with an electrode separation of 2.3mm, an arc current of 7 to 11 mA is used. It has been found satisfactory to move the fibres 1' and 5' towards one another at a rate of 200 microns per second.

The movement is programmed so that the fibre ends first touch (at the previously establis^hed contact point datum), and are then pushed in on each -her by a distance of 5 to 20 microns. At this stage, the fibres 1' and 5' have molten surfaces that adhere to one another, but the underlying fibres are not molten and so are unyielding. Thus, as the fibres 1' and 5' are pushed together beyond the contact point datum, there is little or no bulging or deformation of the fibre material. Instead, at least one of the fibres 1' and 5' deflects, so that the fibre end faces come into substantially planar contact, with the fibres at an angle as shown in Figure 3. At this stage, the fibres 1' and 5' are essentially tacked together by the contact of the molten surfaces.

At the end of the inward, pushing stroke, the chucks are held stationary for about two seconds. The arc current is then increased to about 10 to 13 mA to create additional melting to fuse the fibre ends together in the tacked position. Since the ends of the fibres 1* and 5' have been tacked together with the full surfaces of the fibre ends substantially in contact, there is no gap for cladding to flow into and induce deformation of the cores. It should also be noted that this increased temperature stage is at a lower temperature than the second stage of the prior art process, and also takes place after the fibres 1' and 5' are stationary, so that the more molten fibre is not pushed and deformed.

After two seconds (or a different time if a different current is used) the current is again increased (to a range of 13 to 17 mA), and the chucks are moved apart in an outward stroke by approxima- y the same distance as the fibre ends were moved inwards beyond the contact point datum. The chucks may be moved apart at any suitable speed, but it has been found that a speed of 10 microns per second for the relatively small distance of travel required is satisfactory. During this final stage, there is again no gap between the fibre ends, and the temperature reached is sufficient for surface tension effects to bring about final alignment in the claddings 3' and 7'. At this stage, there may be a tendency for flow of the cladding material to cause some core movement but this may be offset (or counteracted) to some extent by the longitudinal drag element induced by pulling. In any event, there is no deterioration in lateral alignment criteria with respect to the prior art.

Figure 5 shows schematically the fibre splice region at the end of the outward pulling stroke. During the pulling stroke, the kink in the spliced fibre is straightened, with the result that the fibres 1' and 5' and the cores 2' and 6' are both aligned and straight.

The distance through which the fibre ends are pushed together beyond the contact point datum depends upon the end angle(s) to be spliced. For example, small end angles of about 2' need only about 4 to 5 microns of additional travel to make the ends abut as shown in Figure 4, whereas larger end angles require a greater travel distance. It has been found that end angles of up to 8^* can be satisfactorily spliced using the method of the invention with an additional travel distance of about 20 microns. If it is desired to cope with a range of end angles, the travel distance is set to correspond to that of the largest end angle to be spliced.

When smaller end angles are subjected to the additional travel the fibres will either bend elastically or slide back in the chucks. There may be end angles on one or both of the fibre ends. In some instances, if both fibre ends have angles the angles may align in a complementary way, so that planar contact is achieved without substantial flexing or deviation of the fibres. In such a case, the fibres are treated as if the ends had no end angles. At other times, the end angles may both tend to leave a gap at the same place. Preferably the maximum included angle between fibre ends is 8^*.

Claims

1. A method of fusion splicing optical fibre ends, the method comprising the steps of heating the fibre ends to a first temperature that is sufficient to cause surface melting but insufficient to permit substantial flow of fibre material, advancing the fibre ends towards one another thereby ensuring the fibre ends adhere to one another at an initial contact point, advancing the fibre ends beyond the initial contact point so that any end angles on the fibres induce lateral fibre deflection to bring the fibre end faces into substantial contact, and subsequently heating the fibre ends to a second temperature and pulling the joint formed between the fibre ends to remove any bend at the joint, the second temperature being higher than the first temperature.

2. A method as claimed in claim 1, wherein the distance that the fibre ends are advanced beyond the initial contact point is substantially equal to the distance through which the fibre joint is subsequently pulled.

3. A method as claimed in claim 1 or claim 2, wherein the distance that the fibre ends are advanced beyond the initial contact point is in the range of from 5 to 20 microns.

4. A method as claimed in any one of claims 1 to 3, wherein, after the end faces have been brought into contact, the fibre ends are heated to a temperature intermediate to the first and second temperatures.

5. A method as claimed in any one of claims 1 to 4, wherein the heating is by an electric arc.

6. A method as claimed in claim 5, wherein an electric current of 7 to 11mA is applied to the electric arc to heat the fibre ends to said first temperature.

7. A method as claimed in claim 6, wherein the fibre ends are advanced towards one another at a speed of 200MIΪI per second.

8. A method as claimed in any one of claims 5 to 7, wherein an electric current of 13 to 17mA is applied to the electric arc to heat the fibre ends to said second temperature.

9. A method as claimed in any one of claims 5 to 8, wherein an electric current of 10 to 13mA is applied to the electric arc to heat the fibre ends to said intermediate temperature.

10. A method as claimed in any one of claims 5 to 9, wherein the joint is pulled at 10μm per second.

11. A method as claimed in claim 4, or in any one of claims 5 to 10 when appendent to claim 4, wherein the fibre ends are advanced beyond the initial contact point after a predetermined time interval following contact of the fibre ends at the initial contact point.

12. A method as claimed in claim 11, wherein said predetermined time is 2 seconds.

13. A fusion splicing method substantially as hereinbefore described with reference to Figures 3 to 5 of the accompanying drawings.

14. An optical fibre splice whenever made according to the method of any one of claims 1 to 13.

15. An optical fibre splice substantially as hereinbefore described with reference to, and illustrated by, Figure 4 or Figure 5 of the accompanying drawings.