Optical Fiber Splicing
Optical Fiber Splicing: A permanent or semi-permanent connection between two individual optical fibers is known as Optical Fiber Splice, and the process of joining two fibers is called Optical Fiber Splicing.
A fiber splice is a permanent joint formed between two optical fibers. Splicing is required
- when the length of the system span is more than the manufactured cable length.
- when the cable is broken and needs to be repaired.
In order to achieve a low-loss splice, it is essential for the fiber ends (to be joined) to be smooth, flat, and perpendicular to the core axes. This is normally achieved using a cleaving tool (a blade of hard metal or diamond). The technique is called ‘scribe and break‘ or ‘score and break‘. It involves scoring the fiber under tension with a cleaving tool, as shown in figure below. This generates a crack in the fiber surface that propagates in the transverse direction and a flat fiber end is produced.
Types of Optical Fiber Splicing
The primary objective of splicing is to establish transmission continuity in the fiber-optic link. This can be done in two ways, namely, through
- Fusion splices
- Mechanical splices
A good quality permanent joint may be obtained by fusion or welding the prepared fiber ends. A widely used heating source for fusion is the electric arc. The set-up for arc fusion is shown in the figure below. Herein, the prepared fiber ends are placed in a precision alignment jig. The alignment is done with the help of an inspection microscope (not shown). After the initial setting, a short arc discharge is applied to ‘fire polish’ the fiber ends. This removes any defects due to imperfect cleaving. In the final step, the two ends are pressed together and fused with a stronger arc, thus producing a fusion splice. A possible drawback of such a splicing mechanism is that the heat produced by the welding arc may weaken the fiber in the vicinity of the splice.
There are several mechanical techniques for splicing fibers. These normally use appropriate fixtures for aligning the fibers and holding them together. A popular technique, known as the snug tube splice, uses glass or ceramic capillary with an inner diameter just large enough to accommodate the optical fibers, as shown in the figure below. The prepared fiber ends are gently inserted into the capillary and a transparent adhesive (e.g., epoxy resin) is injected through a transverse hole. The adhesive ensures both mechanical bonding and index-matching. A stable low-loss splice may be obtained in this way but it poses stringent limits on the capillary diameters.
A slightly different technique uses an oversized metallic capillary of square cross section, as shown in figure below. The capillary is first filled with the transparent adhesive, after which the prepared fiber ends are inserted into it. The two fiber ends are forced against one of the four inner corners of the capillary.
V-groove Optical Fiber Splicing
Other techniques of mechanical splicing normally employ V-grooves for securing optical fibers. The simplest technique uses an open V-groove, into which the prepared fiber ends are placed as shown in the figure below. The splice is accomplished with the aid of epoxy resin.
It is also possible to obtain a suitable groove by placing two precision pins (of appropriate diameter) close to each other. The fibers may then be placed in the cusp as shown in the figure below. A transparent adhesive ensures bonding as well as index-matching, and a flat spring on the top applies pressure ensuring that fibers remain in their positions. Such a groove is called a spring groove.
Elastomeric Optical Fiber Splicing
There is yet another technique that utilizes the principle to realize what is known as an elastomeric splice, shown in the figure below. In this method. the prepared fiber ends are sandwiched between two elastomeric internal parts, one of which contains a V-groove. An outer sleeve holds these two parts compressed so that the fibers are held tightly in alignment. Index-matching gel is employed to improve its performance. Originally, the technique was developed for coupling multimode fibers, but it can also be used for single-mode fibers as well as fibers with different core diameters.
Splicing with most of these techniques, if properly carried out, results in splice loss of about 0.1 dB for multimode fibers. Some of these can also be used for splicing single-mode fibers.
For ribbon cables containing linear arrays of fibers, the following technique has been used. In this method. shown in the figure below, the fiber ends are individually prepared, and then placed in a grooved substrate. Adhesive is then used for bonding and index-matching. A cover plate retains the fibers in their position and also maintains mechanical stability.