We terminate fibre optic cable two ways - with connectors that can mate two fibres to create a temporary joint and/or connect the fibre to a piece of network gear or with splices which create a permanent joint between the two fibres. These terminations must be of the right style, installed in a manner that makes them have little light loss and protected against dirt or damage in use.
No area of fibre optics has been given greater attention than termination. Manufacturers have come up with over 80 styles of connectors and and about a dozen ways to install them. There are two types of splices and many ways of implementing the splice. Fortunately only a few types are used most applications.
Different connectors and splice termination procedures are used for singlemode and multimode connectors, so make sure you know what the fibre will be before you specify connectors or splices!
We'll start our section on termination by considering connectors. Since fibre optic technology was introduced in the late 70s, numerous connector styles have been developed. Each new design was meant to offer better performance (less light loss and back reflection), easier and/or termination and lower cost. Of course, the marketplace determines which connectors are ultimately successful.
Connector and splice loss
Connector and splice loss is caused by a number of factors. Loss is minimised when the two fibre cores are identical and perfectly aligned, the connectors or splices are properly finished and no dirt is present. Only the light that is coupled into the receiving fibre's core will propagate, so all the rest of the light becomes the connector or splice loss.
End gaps cause two problems, insertion loss and return loss. The emerging cone of light from the connector will spill over the core of the receiving fibre and be lost. In addition, the air gap between the fibres causes a reflection when the light encounters the change n refractive index from the glass fibre to the air in the gap. This reflection (called fresnel reflection) amounts to about 5% in typical flat polished connectors, and means that no connector with an air gap can have less than 0.3 dB loss. This reflection is also referred to as back reflection or optical return loss, which can be a problem in laser based systems. Connectors use a number of polishing techniques to insure physical contact of the fibre ends to minimise back reflection. On mechanical splices, it is possible to reduce back reflection by using non-perpendicular cleaves, which cause back reflections to be absorbed in the cladding of the fibre.
The end finish of the fibre must be properly polished to minimise loss. A rough surface will scatter light and dirt can scatter and absorb light. Since the optical fibre is so small, typical airborne dirt can be a major source of loss. Whenever connectors are not terminated, they should be covered to protect the end of the ferrule from dirt. One should never touch the end of the ferrule, since the oils on one's skin causes the fibre to attract dirt. Before connection and testing, it is advisable to clean connectors with lint-free wipes moistened with isopropyl alcohol.
Two sources of loss are directional: numerical aperture (NA) and core diameter. Differences in these two will create connections that have different losses depending on the direction of light propagation. Light from a fibre with a larger NA will be more sensitive to angularity and end gap, so transmission from a fibre of larger NA to one of smaller NA will be higher loss than the reverse. Likewise, light from a larger fibre will have high loss coupled to a fibre of smaller diameter, while one can couple a small diameter fibre to a large diameter fibre with minimal loss, since it is much less sensitive to end gap or lateral offset.
These fibre mismatches occur for two reasons. The occasional need to interconnect two dissimilar fibres and production variances in fibres of the same nominal dimensions. With two multimode fibres in usage today and two others which have been used occasionally in the past and several types of singlemode fibre in use, it is possible to sometimes have to connect dissimilar fibres or use systems designed for one fibre on another. Some system manufacturers provide guidelines on using various fibres, some don't. If you connect a smaller fibre to a larger one, the coupling losses will be minimal, often only the fresnel loss (about 0.3 dB). But connecting larger fibres to smaller ones results in substantial losses, not only due to the smaller cores size, but also the smaller NA of most small core fibres.