Abstract:
A method of forming low loss splices between single-mode optical fiber ends involves taking prepared fiber ends and aligning the one with the other by inserting them into opposite ends of the bore of a ferrule, and fusing the fiber ends together by applying thermal energy thereto by means of an aperture in the wall of the ferrule. Splices prepared in this way have losses of less than 0.5 dB. The use of a precisely dimensioned ceramic ferrule enables the use of splicing apparatus which does not incorporate means for 3-dimensional micromanipulation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of forming a splice between two single-mode optical fibre ends and to splices so formed. 
     2. Description of Related Art 
     There are many situations where it is necessary to form low loss joints between optical fibre ends. Such situations occur for example anywhere in a telecommunications network where lengths of optical fibre cable have to be joined together or to line equipment such as exchanges, distribution points, etc. In addition to such joints having to be low loss, that is generally no more than 0.5 dB per joint, preferably no more than 0.2 dB, it is necessary that the joints be of low cost. These two requirements dictate that joints only be made demountable where this is a positive requirement. Non demountable joints are referred to as splices and are formed by fusing or gluing the aligned fibre ends together. The difficulty in making low loss fibre splices is that very precise alignment is required, particularly with single-mode fibres, which typically have an overall diameter of about 125 μm but a light carrying core diameter of only about 8 μm. In order to achieve the desired accuracy of alignment, splicing machines are equipped with micromanipulators capable of providing precise positional adjustment in three dimensions. The resultant bulk of the machines severely limits their use in confined spaces. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome some of the disadvantages of known splicing techniques and in particular seeks to avoid the need actively to adjust the position of the fibre ends in three-dimensions prior to splicing. 
     According to a first aspect the present invention provides a method of forming a low-loss splice between two single-mode optical fibre ends, the method comprising the steps of: 
     i) taking prepared fibre ends and aligning them one with the other by inserting them into opposite ends of the bore of a ferrule; and 
     ii) fusing together the fibre ends by applying thermal energy thereto by means of an aperture in the wall of the ferrule. 
     Surprisingly we have discovered that it is possible to make low loss splices, that is those exhibiting an optical loss of less than 0.5 dB, typically about 0.2 dB, from single-mode fibre without ensuring precise fibre end alignment in three dimensions prior to fusing the ends together. While the ferrules used in our invention are accurately formed with fine tolerances, it is still very surprising that such low losses can be achieved with single-mode fibres given the magnitude of the misalignment which can be expected in view of the tolerance on fibre diameter and ferrule bore dimensions, particularly in view of the apparent need for precise alignment in 3-dimensions as exemplified by the almost universal current use of 3-dimensional micromanipulation to achieve the accurate alignment prior to fusion of the fibre ends. 
     Previously it had been assumed that this type of splicing technique is only suitable for splicing graded-index fibre, that is fibre with relatively large core diameters, typically 50 μm diameter, which support several guided modes. Such fibre is also known as multi-mode fibre. Despite the fact that essentially the same technique was proposed by the Japanese for use with multi-mode fibres, it was not appreciated that the technique could be made to work with single mode fibres and produce low loss, i.e. less than 0.5 dB, splices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 shows diagrammatically side and end elevations of a ferrule for use in the method according to the invention; 
     FIGS. 2 to 5 show diagrammatically various steps in the inventive method; 
     FIG. 6 shows diagrammatically a completed splice after potting; 
     FIG. 7 shows diagrammatically a planar ferrule structure for use with a ribbon cable; 
     FIGS. 8a, 8b and 8c show diagrammatically ferrules with clustered multiple bores; 
     FIG. 9 shows diagrammatically an embodiment of splicing equipment according to the invention; 
     FIG. 10 illustrates a heat shrink over the enclosure of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     At the heart of the invention is a ferrule having a very accurately dimensioned bore as shown in FIG. 1. Typically the ferrule 1 would be 8 to 15 mm in length, circular in cross-section with about a 2 mm outside diameter 2 and 126 μm internal diameter 3. The tolerance on the internal diameter is no more than about 0.5 μm. The bore has a polished finish and is straight to within 2 μm. We have discovered that using such a ferrule to ensure alignment between the two fibre ends it is possible routinely to form low loss splices without the need for micromanipulation in three dimensions. 
     Suitable ceramic ferrules are manufactured in Japan and sold by Sancera, Adamant and Kyocera. The conventional use of such ferrules is for fibre alignment and retention of optical fibres in fibre submounts. Ferrules are available with bore diameters of 125, 126 and 127 μm, with a maximum tolerance of ±0.5 μm. Although the specified tolerance on the 125 μm O.D. of single-mode fibre is ±2 μm, it is typically manufactured with a 125 μm O.D. with a tolerance of ±0.5 μm. The ferrule size for a given fibre should be chosen such that the fibre fits the bore without significant lateral movement of the fibre in the bore being possible. Since one of the aims of the invention is to get away from a need to provide micromanipulator stacks for 3-dimensional movement, it is intended that lateral alignment of the unfused fibre ends when fully within the ferrule should be largely and preferably wholely unaffected by modest movement of the fibre ends outside the ferrule. 
     A lateral slot 1 mm wide and about 1.1 mm deep is machined, using conventional ceramic machining techniques, in the ferrule to enable heat to be applied to the fibre ends to effect fusion thereof within the ferrule. 
     As shown in FIGS. 2 and 3, the prepared fibre ends 4, 4&#39; are fed into opposite ends of the ferrule&#39;s bore. A temporary stop 5 is introduced through an aperture 6 in the sidewall of the ferrule and protrudes into the bore sufficiently far to act as a stop for both fibre ends. Preferably, as shown, the aperture 6 is so placed that the stop can be inserted about mid-way along the length of the ferrule. Preferably the stop 5 is provided with a flange 7 which engages with the walls of the aperture 6 to ensure that the position of the fibre ends abutted from against the stop is known relative to the rest of the ferrule. 
     The next method step is shown in FIGS. 4 and 5 and involves withdrawing the temporary stop 5 from the ferrule, driving the two fibre ends together, positioning electrodes 9,9&#39; on either side of the ferrule, striking an arc between the electrodes 9,9&#39; and fusing the fibre ends together. Fusing should be effected at a temperature high enough to enable surface-tension effects substantially to remove any lateral offset between the already well aligned fibre ends. With proper dimensional control, particularly of the stop blade 8, the two fibre ends can be brought together by a predetermined amount of axial movement, preferably supplied by a piezoelectric translator. 
     After fusing the fibres together, the electrodes are withdrawn and the splice potted in the ferrule using an epoxy or silicone resin 10. Finally, the ferrule and adjacent fibre portions are encased in a thermally shrunk-to-fit protective sheath 11 (see FIG. 10). 
     Although the invention has so far been described solely with reference to a single pair of fibre ends being spliced within a simple ferrule having just a single bore, clearly multiple bores could be provided to enable multiple splices to be produced in a single structure. The multiple bores could be arranged side-by-side to produce a planar structure suitable for use with ribbon cable, as shown in FIG. 7. Alternatively the multiple bores could be arranged in a cluster, as shown in FIGS. 8a-8c. Clearly, since the fibers are spliced while they are in the ferrule, provision must be made to enable the application of thermal energy to the fibres to effect fusion. In the clusters shown in FIGS. 8a-8c the ferrule bores are each located at a corner of the cluster, there being sufficient space between adjacent bores to enable a pair of electrodes to be introduced into the vicinity of the fibres to effect fusion. Of course alternative sources of thermal energy, such as lasers or flame, could be used in place of an electric arc to effect fibre fusion. 
     As a further variant, the ferrules could be made of a material other than a ceramic, although ceramics are preferred, provided a similar level of accuracy on bore dimensions is maintained. Potentially the ferrules could be made of a dimensionally stable thermoplastic or a metal. Of course it is important that the ferrule does not distort during the fibre fusion process so as to affect alignment. 
     An example of a compact fusion splicer suitable for use according to the invention is shown in FIG. 9. The fusion splicer comprises a baseplate 20 on which is mounted a piezoelectric translater, not shown, to which are connected arms 21, 21&#39; at the ends of which are fibre clamps 22, 22&#39;. Between and aligned with the fibre clamps is a ferrule clamp 23 which holds the ferrule firmly and precisely in place during the splicing operation. Adjacent the ferrule clamp 23 is the fibre stop assembly 24 which is pivotally mounted about a pair of supports 25 to enable it to be swung into engagement with the aperture 6 in ferrule 1. Also adjacent the ferrule clamp 23 but on the opposite side to the stop assembly 24 is the electrode assembly 26 pivotally mounted on supports 27,27&#39;. An electrical umbilical, 28, connects the piezoelectric translator and the electrodes to the control electronics and the power supply which are housed remotely. 
     Optical fibre splices made according to the method of the invention routinely exhibit losses of 0.3 dB and have exhibited losses as low as 0.2 dB, although the realisation of such low losses is dependent on the prepared fibre ends having flat end faces orthogonal to the fibre axis, consequently care should be taken when cleaving the fibre ends in preparation for splicing.