Fibre optic splicing: how it works and what determines joint quality

What is fibre optic splicing

Fibre optic splicing, professionally also referred to by the English term fusion splicing (fusion splicing) is a process in which two ends of an optical fibre are permanently joined using an electric arc. This melts the fibre ends and fuses them so that light passes through the joint with almost no scattering. Precision is critical: the core of a single-mode fibre, which carries the signal, is only about nine micrometres thick, i.e. A fraction of the thickness of a human hair. Even the slightest inaccuracy during splicing results in signal loss.

Mechanical splices also exist; they hold fibres together without fusion and can be used for quick temporary repairs. However, they are unsuitable for permanent infrastructure. They have higher and less stable attenuation, and over time the splice properties may deteriorate due to temperature changes or vibrations. Therefore, backbone routes, drop connections and professional installations rely almost exclusively on fusion splicing.

How a splice is made, step by step

The actual splice takes only a few seconds, but the entire preparation and inspection process is considerably longer:

1. Fibre preparation - removal of the protective sheath and cleaning of the fibre from residual gel and dirt.
2. Cleaving - precise fibre cleaving with a cleaver to ensure the cut is perpendicular to the fibre axis.
3. Cleave inspection - The splicer automatically evaluates the angle and quality of the splice before fusion.
4. The splice itself - Fibres are precisely aligned by core using a motorised mechanism, not just by outer cladding, and then fused with an electric arc.
5. Joint inspection - After splicing, the device estimates the resulting attenuation and displays a magnified image of the splice point.
6. Splice protection - The splice is protected with a heat-shrink sleeve and placed in a splice cassette.

The entire process of a single splice takes an experienced technician just a few minutes, including preparation. However, with multi-fibre cables where dozens of fibres need to be fused simultaneously, the execution time logically multiplies, therefore, for larger projects, sufficient time reserves are planned to ensure the work is carried out carefully and without haste that could compromise splice quality.

When fibre optic splicing is essential

Splicing is the fundamental building block of virtually every optical route:

• splicing individual cable sections to form a longer route
• Fibre introduction into the building and termination in the distribution cabinet or optical socket.
• Repair of a severed cable following excavation or other damage.
• Installation of patch panels and data cabinets
• implementation FTTH connections at the end-user's premises

In all these cases the same rule applies, cables are supplied from the factory in limited lengths and multiple sections must always be joined along the route, fibres terminated in a distribution cabinet or new branches connected to an existing network. A fusion splicer is therefore standard equipment for every technician working with optical networks, whether implementing a new route or carrying out maintenance work.

Factors affecting splice quality and attenuation in dB

The quality of the splice is assessed based on attenuation, i.e., how much signal is lost at the joint. It is expressed in decibels (dB) and its level is influenced by a number of factors:

• cleanliness of fibres and tools - Dust or grease on the fibre end increases attenuation and may damage the splicer electrodes.
• cleave quality - An angled or slightly chipped cut will always be visible in the resulting splice.
• alignment and condition of the splicer electrodes - worn electrodes produce an uneven arc
• fibre type - single-mode (SM) and multi-mode (MM) fibres have different requirements for splicing
• technician experience - minor inaccuracies in preparation are calculated to tenths of a dB

For a properly executed splice, the attenuation is typically in the range of 0.05 to 0.1 dB. Higher values usually indicate an issue with fibre preparation or equipment alignment, and it is advisable to re-splice such a joint.

How splice quality is verified

After each splice, the fusion splicer displays an estimated loss, but this is only a reference calculation based on the splice image. The actual quality of the entire route, including all splices, connectors and any cable bends, will be shown only by OTDR testing. We cover this in detail in a separate article about OTDR optical measurements.

Clients should be aware that these two steps, splicing and measurement, go hand in hand. A splice without subsequent testing is merely an assumption that the joint functions as planned, whereas the measurement report provides objective proof of the actual quality of the entire route. For larger projects, we therefore recommend combining splicing with OTDR testing and requesting the resulting report together with the project documentation.