A permanent joint formed between two individual optical fibers in the field or factory is known as a fiber splice.
For long distance communication it is necessary to join two fibers. In order to get the maximum splicing efficiency the fiber should satisfy following criteria:
a) They should be perfectly aligned.
b) They should have the same geometric characteristics i.e. same core diameter.
c) They should have same optical characteristics i.e. ∆, NA should be same.
Splices may be divided into two broad categories depending upon the splicing technique utilized:
Fusion splicing or welding
I. Fusion splicing
Fusion splicing is accomplished by applying localized heating (e.g. by a flame or an electric arc) at the interface between two butted prealigned fiber ends causing them to soften and fuse.
The fusion splicing of single fibers involves the heating of the two prepared fiber ends to their fusing point with the application of sufficient axial pressure between the two optical fibers. It is therefore essential that the stripped fiber ends are adequately positioned and aligned in order to achieve good continuity of the transmission medium at the junction point.
Hence the fibers are usually positioned and clamped with the aid of an inspection microscope.
- Perfusions method for accurately splicing optical fibers:
The protective plastic that causes the glass cladding is stripped from each fiber end, which is then cleaned with a special tool, producing a smooth flat end.
Fibers to be joined are placed in opposite ends of the fusion set alignment with these ends slightly separated, called as initial setting.
A short duration arc is used to soften and for rounding the fiber ends. This is called as prefusion.
After prefusion the fiber ends are joined together in the alignment heat. A long duration arc is used to soften both fiber ends and surface tension causes the softened glass to flow together in a uniform seamless joint.
Fusion splicing provides high quality joint.
It has low loss.
It has small size of splice.
The heat used for fusion weakens the fiber in the vicinity of the splice.
Tensile strength is 30% of uncoated fiber which is low.
II. Mechanical Splices:
In mechanical splicing the broken fiber edges are aligned and are locked in position with the help of various positioning devices and optical cement.
A number of mechanical techniques for splicing individual optical fibers have been developed namely:
- Snug tube splice
- Loose tube splice
- Sleeve splice
- V groove splice
- Elastomeric splice
- Precision pin splice
They are easily implemented in the field.
They require no tooling.
Size is not as small as fusion splice.
Precision is required.
1. Snug tube splice:
The method involves the use of an accurately produced rigid alignment tube into which the prepared fiber ends are permanently bonded. This snug tube may utilize a glass or ceramic capillary with an inner diameter just large enough to accept the optical fibers.
Transparent adhesive (e.g. epoxy resin) is injected through a transverse bore in the capillary to give mechanical sealing and index matching of the splice.
Average insertion losses lows as 0.1dB have been obtained.
2. Loose tube splice:
This splicing technique avoids the critical tolerance requirements.
This loose tube splice uses an over signed square section metal tube which easily accepts the prepared fiber ends.
Transparent adhesive is first inserted into the tube followed by the fibers.
The splice is self aligning when the fibers are curved in the same plane forcing the fiber ends simultaneously into the same corner of the tube.
Mean splice insertion loss of 0.073 dB have been achieved.
3. V-groove splice:
It utilizes a V-groove into which two prepared fiber ends are pressed.
The two fiber can slide in the groove until they touch.
The splice is made permanent by securing the fibers in V-groove with epoxy resin.