Mechanical splices

    The mechanical splice performs a similar function to the fusion splice except that the fibers are held together by mechanical means rather than by a welding technique. Physically, they often look very similar to splice protectors.

Advantages and disadvantages
There are several advantages. They do not require any power supplies. Indeed, many designs require no tools at all beyond a stripper and cleaver, so the mechanical splice can be used in situations that may be considered hostile to many fusion splicers. Mechanical splices are often re-usable and can be fitted in less than a couple of minutes, which makes them ideal for temporary connections. The disadvantage is that they cause a loss, called the insertion loss, of about 0.1–0.3 dB per connection which is significantly higher than a good fusion splice. This would suggest the use of a fusion splice as the first choice in situations where losses are critical. Mechanical splices can be used to connect either singlemode or multimode fibers.

Cost

On cost grounds alone, the choice between a fusion splicer and mechanical splices depends on the number of splices to be undertaken. If we already have a fusion splicer, the cost of each fusion splice is negligible but for the cost of a reasonably good fusion splicer we could purchase a thousand or more mechanical splices. It is also possible to hire fusion splicers and other fiber optic equipment.

How they work

In essence, it is very easy. The fiber must be stripped, cleaned and cleaved. They must then be aligned and then held in position either by epoxy resin or by mechanical clips. There are only three basic designs.

Vee-groove

This was the obvious choice since it worked so well in positioning the fibers in the fusion splicer. See Figure 11.1. Most mechanical splices are designed around the vee-groove. They consist of a base plate into which the vee-groove has been cut, ground or molded.

The prepared fibers are placed in the groove and their ends are brought into contact. Some index matching gel is used to bridge the gap between the two ends to prevent gap loss and to reduce Fresnel reflection. A gripping mechanism then holds the fibers in position and provides mechanical protection for the fiber. As an alternative to the index matching gel, an index matching epoxy can be used. This performs the same index matching task as the gel but also holds the fiber in position. It is usually cured by UV light.

Bent tube — Figure 11.2

If a length of fiber is pushed into a tube which is curved, the springiness of the fiber will force it to follow the outside of the curve. Now, if the tube is of square cross-section, the fiber will follow the far corner. This is very similar to a vee groove since the fiber is now positioned by a vee-shaped wall of the tube. This is called a bent tube design. A small spot of index matching gel is added before the fibers are inserted. In some designs, a bent tube with a circular cross section is used but the principle is just the same.

Precision tube

This type is very simple. A hole, very slightly larger than the fiber diameter isformed through a piece of ceramic or other material. When a piece of bare fiber is inserted from each end, the two fibers are inevitably aligned when they meet. The insertion losses are higher than the other types due to tolerances in the holediameter.

Specifications

The specifications will give information about several things.

Cladding and buffer diameter

The cladding diameter will usually be 125 μm for most fibers. The buffer diameter is likely to be either 250 μm or 900 μm.

Insertion loss

This is the loss caused by the device when it is installed in the system. Typical values are around 0.2 dB.

Return loss

This is the proportion of the incoming light that is reflected back along the fiber. Usually between –40 dB and –60 dB. The low loss is the result of adding the index matching gel to reduce Fresnel reflection.

Fiber retention

How much tension can be applied to the completed splice before the splice fails? The failure mode may be obvious and catastrophic as in the fiber actually becoming disconnected or the ends of the fiber being pulled apart very slightly causing enormous gap loss but no visible damage. Typical values are around 4 N but some are much more rugged with figures up to 180 N when the correct external protection is applied.

A practical guide to fitting a typical mechanical splice

All mechanical splices come with an instruction sheet. It is essential that thetime is taken to read it as each splice has a slightly different fixing method. As mentioned earlier, some are permanent fixings using epoxy resin and some are designed to be re-usable. It is not a good idea to pump in epoxy resin first, then settle down to read the instructions!

• Strip, clean and cleave the fiber leaving about 12 mm of primary buffer removed.
• If re-using the splice (if this is possible) clean with isopropyl alcohol and a piece of lint free cloth and use a syringe to inject a small bead of index matching gel into the center of the splice.
• Release the small clip at one end of the splice and insert the fiber until it comes to a stop. Operate the clip to lock the fiber in position.
• Release the clip at the other end and insert fiber until it comes up against the fiber already loaded. Operate the clip to lock that fiber also.
• Test the operation and if not satisfactory, the clips can be released to allow the fiber positions to be optimized.

Kevin M Contreras H
CI 18.255.631
CRF

http://www.kiet.edu/ensite/downloads/Introduction%20to%20Fiber%20Optics%20-%20John%20Crisp.pdf