GB2185127A - Fusion splicing optical fibres - Google Patents

Abstract

Optionally dissimilar, coated optical fibres 1,6 are fused together, said fibres 1,6 having their ends cleaved with end angles of less than one degree between the axis of the optical fibre and the direction normal to the cleaved end of the core, being clamped between two electrodes 12,13 in a fusion splicing machine 7 and their cores aligned, possibly by means of power monitoring 9, the aligned fibre ends being fused together by means of an electric arc acting for 0.2-0.4 seconds whilst simultaneously producing a relative longitudinal movement of the fibres towards one another. At least one of the fibres may be monomode and may have a D-shaped cross-section. <IMAGE>

Description

SPECIFICATION Method of splicing optical fibres This invention relates to a method of splicing optical fibres, such fibres comprising essentially a radiation transmitting core surrounded by a cladding of lower refractive index.

The splice loss of optical fibres is partially determined by the core-misalignment of the two optical fibres which are spliced. Core-misalignment can be reduced to an acceptable level when splicing conventional fibres (i.e. circular) by the use of, for example, V-grooves. However, optical fibres are now being used as sensors to detect and/or measure for example, temperature, pressure, electric fields, and these special sensing fibres have a geometry which is often dissimilarto conventional fibres (i.e.

non-circular). If conventional fibres are required to be joined to special fibres, or special fibres to special fibres, then standard jointing techniques may not be feasible, due to the dissimilar or non-circularfibre geometries as the result may be core misalignment.

Some sensor applications additionally have the requirement that polarization of the light should be preserved. The polarization requirement is one with which certain jointing and/or splicing techniques do not comply.

U.K. Patent Specification No. 1570032 discloses a method of thermally fusing optical fibres over a relatively long period oftime but this means that splice losses commonly occur due to distortion of the fibres at or nearthe fibre joint and/or core interfaces. Also disclosed in the prior art is a method of partially fusing a joint and later completing the fusion. This technique, known as prefusing, suffers from the disadvantagethatthe core interfaces of both thefibre ends are permanently distorted, thereby resulting in relatively high splice losses.

According to the present invention a method of splicing coated optical fibres comprises the steps of: (i) cleaving the fibre ends with end angles of less than 1 degree, and, (ii) placing the cleaved fibre ends in the clamps of a fusion splicing machine, and, (iii) aligning the cores at the ends of the fibres between two electrodes of the fusion splicing machine, and (iv) fusing the fibre ends together in between 0.2 and 0.4 seconds, and, (v) during the fusing time producing a relative longitudinal movement of the fibres towards one another so asto minimizethesplice loss.

The fused splice junction may be coated with an epoxy, such as a UV curable epoxy, while clamped in the fusion splicing machine. The coated junction may have a protective sleeving such as a heat-shrink sleeving which is sl id over the fused epoxy-coated joint, the sleeve then being shrunk by the action of heat, such a:ine use of a hot air gun.

The fibre cores may be aligned more accurately by means of power monitoring. A laser source, such as a 1.3 ism wavelength semiconductor laser, may be connected to the end of the first fibre (preferably but not necessarily conventional) opposite to the proposed joint end and a power meter head connected to the end ofthe second fibre opposite to the proposed joint end. The cores of both first and second fibres are aligned by maximizing the power transmitted to the meter head through both joint end faces of the optical fibres by moving the fibres ends in at least one ofthree orthogonal directions. During the fusing time the fibres may undergo a relative longitudinal movement, of between 2 and 3 Fm, towards one another so as to minimize the splice loss.The fusion time is preferably between 0.25 and 0.35 seconds and ideally is 0.30 seconds. The fusion current is preferably between 10 and 14 milliAmps and ideally is 12.0 milliAmps.

The electrodes may be of any shape or size suitable to allow the electric discharge to fuse the optical fibres together. In particularthe electrodes may be manufactured from a circular metal rodwith the discharge end ground orfilled into a sharp point.

The metal is preferably tungsten.

Either or both optical fibres may be monomode and have a core diameter of between 8 and 10 Fm having a coating with a D cross section with a diameter of approximately 125 Fm and a minimum radial dimension of approximately 100 iim.

Alternatively either or both optical fibres may be rectangular or square in cross section, or indeed any other cross section as desired. Cladding mode strippers may be used on a or the non-conventional fibre if the primary coating is not mode stripping.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which Figure 1 shows symboiicallythe apparatus used for power monitoring, Figure2showsthe positioning of the first and second optical fibres between the two electrodes, Figure 3shows the completed fusion splice, and Figure 4shows a cross section through the special fibre in this case of D-section, looking in the direction of the arrows A-A on Figure 2.

Referring to Figure 1 one end 2 of afirstoptical fibre 1 of conventional, i.e. circular, cross-section is coupled to a 1.3 ,u m wavelength semiconductor laser 3. The other end 4 of the firstfibre 1 and the end 5 of the second special fibre 6 are cleaned and then cleaved with a special cleaving tool (not shown) such thatthefibre ends are substantiallysquare andfiat with end-angles of less than 1 degree. Low loss splices, i.e. of less than 0.6 dB splice loss will be difficult to achieve if the end-angle is much above 1 degree. The cleaved fibre ends 4a, 5a, are clamped in the fusion splicing machine 7, and the end 8 of the special fibre 6 is interfaced to a power meter head 9.

When the power meter head 9 is in operation it is requiredthatonlythe light passing through the optical fibre cores 10,11 be monitored. lflighttravels through the fibre coatings 15,16 then the true optimum fibre end position may not be found. This problem may be soived by coating or immersing the second fibre coating 16 with or in a mode stripping substance 17 with a refractive index su bstantialiy the same as or slightly higherthan the refractive index of the second fibre coating 16. The mode stripping substance 17 is in the form of a solid (such as glass) or a liquid (such as oil). The mode stripping substance 17 prevents the propagation of light throughthefibrecoating 16.

Referring now to Figures 2 and 3, the cleaved fibre ends 4a,5a are aligned such that the respective fibre cores 10,11 are substantially coaxial and such that the respective ends 4a,5a are approximately equally spaced from the tips of the tungsten electrodes 12,13. This aligning may be aided by the use of a microscope and/or a mechanism capable of small accurate movements. The cores 10,11 are then aligned accurately by moving the fibres' ends 4a,5a in at least one of three orthogonal directions with respect to each other. The optimum position of the fibres cores 10,11 occurs when the power transmitted from the laser 3, through the fibre ends 4a,5a and into the power meter head 9 is at a maximum. Afusion of the fibre ends 4a,5a and their surrounding areas 14 is made in approximately 0.3 seconds by using a fusion current of approximately 12.0 milliAmps.Thefibres 1,6 are fused by means of the electric current, in the form of a discharge, bridging the gap between the tips of the electrodes 12,13. The electrode gap should be between 1.5 and 2.5mm. A sufficiently short time interval is used so as to achieve the minimum movement, due to surface tension, of the at least partially molten fibre ends.

During the fusion time relative longitudinal movement of the fibre ends towards each other is produced so as to minimize the splice loss. The relative movement, which can be obtained by feeding one or both fibres, being betwen 2 and 3 > m.

Figure 3 shows the conventional optical fibre 1 fused to the special optical fibre 6 in at least part of the region 14with the fibre cores 10,11 forming a low loss splice joint. The region 14 is characterized in that the optical fibre coatings of the respective fibres have melted, and have at least partially diffused into one another and cooled sufficientlyforthem to become rigid. Thereby the two optical fibres are permanentlyjoined with a continuous core and a smooth coating surrounding the joint.

Figure 4 shows the fibres 1,6 aligned between the tungsten electrodes 12,13 priorto being fused. The electrodes 12,13 although shown in the drawings as conical or pyramidal may be of any shape or size which will allow an electric dischargeto fuse the fibres 1,6 together as herein described.

Once the splice has cooled sufficiently such that the optical fibres 1,6, are permanentlyjointed, then with both fibres 1,6, still being clamped in the fusion splicing machine 7,the splice junction 14 is coated with an epoxy, (not shown) such as a UV curable epoxy, so as to increase the tensile strength ofthe joint. The joint is then packaged with a protective sleevin'g (not shown) which is slid over at least the coatedjunction. Such a protective sleeving could be a heat-shrinksleeving which is shrunk by means of a hot air gun. Although this example describes the fusing of a conventional fibre 1 to a special fibre 6 in which power monitoring is used, the power monitoring may not be necessary when joining equivalentfibres (e.g. conventional fibre to another conventional fibre of the same size, a special fibre to another special fibre of the same size and cross sectional orientation). Usually though when fusing special fibres, power monitoring will be used.

Throughout this specification the end angle refers to the angle between the axis ofthe optical fibre core and the normal direction to the cleaved end ofthe optical fibre core.

Claims (12)

1. A method of splicing two coated optical fibres by cleaving the fibre ends such that the angle between the axis of the optical fibre core and the direction normal to the cleaved end ofthe core is less than one degree, placing the cleaved fibre ends in the clamps ofa fusion splicing machine, aligning the cores atthe ends of thefibres between two electrodes ofsaid fusion splicing machine and fusing the fibre ends together for a time between 0.2 and 0.4 seconds whilst simultaneously producing a relative longitudinal movementofthefibrestowards one another, so as to minimise the splice loss.

2. A method of splicing coated optical fibres claimed in Claim 1 wherein thefused splice junction is coated with an epoxy while clamped in the fusion splicing machine.

3. A method of splicing coated optical fibres as claimed in Claim 2 wherin the epoxy is UV curable.

4. A method of splicing coated optical fibres as claimed in Claim 2 or 3 wherein the coated function hasaprotectivesleeving slid overthefused epoxy coated joint.

5. A method of splicing coated optical fibres as claimed in Claim 4 wherein the protective sleeving is shrunk by the action of heat.

6. A method of splicing coated optical fibres as claimed in any preceding claim wherein the fibre cores are aligned by means of power monitoring.

7. A method of splicing coated optical fibres as claimed in Claim 6wherein a laser source is connected to the end of a first one ofthe fibres opposite to the proposed end and a power meter head is connected to the end of the second fibre opposite to the proposed joint end; the cores ofthe first and second fibres are aligned by maximising powertransmifted to the meter head, through both joint end faces of the optical fibres by moving the fibre ends in at least one of three orthogonal directions.

8. A method of splicing coated optical fibres as claimed in any preceding claim wherein the electrodes comprise circular metal rods each having the discharge end formed into a sharp point.

9. A method of splicing coated optical fibres as claimed in any preceding claim wherein the electrodes are formed of tungsten.

10. A method ofsplicing coated optical fibres as claimed in any preceding claim wherein at least one ofthe optical fibres is monomode.

11. A method as claimed in Claim 10 wherein at least one of the optical fibres has an approximately D-shaped cross section.

12. A method substantially as hereinbefore described with reference to Figures 1 to 4 ofthe accompanying drawings.