Most connectors are available as epoxy polish versions. For simplicity, the following method is based on terminating simplex ruggedised multimode fibre with an ST connector, as both are very common in the marketplace.
Termination: step-by-step
- Slide the connector boot and the crimp ferrule onto the cable
- Strip the outer jacket back by the dimension specified by the connector manufacturer
- Cut the Kevlar to the length specified by the manufacturer (usually between 10 – 12 mm)
- Strip the secondary coat from the cable, again to the dimensions specified. Do this in 5 mm steps to avoid breaking the protruding fibre
- Remove the primary coat in the same way as the secondary coat
- Clean the exposed fibre with isopropyl alcohol until you hear a clean squeak sound, to ensure that all the primary coat is removed and the fibre is absolutely clean
- Check that the connector bore is clear – this can be done by either holding the connector up to the light, or by threading a piece of fibre through
- Mix and load epoxy into a syringe, then load the connector ferrule with the epoxy glue. As a small blob appears on the front face of the connector ferrule, gently ease the syringe plunger back, but be careful not to suck the blob of epoxy back into the connector body. It is essential for the polishing process that a small bead of epoxy is present on the front face of the connector
- Carefully thread the fibre into the connector until it protrudes from the front, and you feel the secondary coat butt against the rear of the ceramic ferrule
- Fan the Kevlar out so that it resembles the end of a chimney sweep's broom and covers uniformly the rear of the connector (see diagram, over)
- Slide the ferrule over the Kevlar, and crimp into position
- Apply a protecting sleeve to the ceramic ferrule to encapsulate the protruding fibre while the connector is curing
- Place the connector in a curing oven for the time and at the temperature specified by the epoxy manufacturer
- Once the cure cycle is complete, gently remove the fibre protector, taking care not to break the protruding fibre
- Carefully scribe the protruding fibre, with a cleave tool, as close to the bead of epoxy as possible. Then gently push the top of the fibre until it cracks along the scribed line. Dispose of the broken fibre in a sharps bin
- Next comes the polishing process. There are several grades of lapping film and methods available, the method used is personal choice, the epoxy supplier can recommend a suitable grade of film
- Generally you should have three types of film: a coarse (9-15 microns); medium (3-5 microns); and fine (0·5-1 microns). Excess force during any of the polishing stages could result in pitting, scratches or cracks on the fibre. Some people lubricate their g polishing mats but this is personal preference, just ensure that the polishing surface is flat and clean
- First, remove the protruding stub of fibre – hold the connector vertically with the ceramic ferrule at the top and gently, with the coarse paper, polish the protruding fibre using only the weight of the paper to polish
- Once the stub is removed, use the coarse paper on a rubber mat to remove most of the epoxy pip – place the connector into a polishing puck and scribe a figure of eight pattern on the polishing paper, applying no pressure to the connector. Continue to do this until only a thin film of epoxy is left on the front face
- Transfer onto the medium grade paper (which again should be on a rubber mat) and describe the same figure of eight pattern until the last of the epoxy is removed
- Now move on to the final stage polish using the fine grade lapping film, this is to ensure a good polished finish on the connector. Again, use the figure of eight patterns and the fine film on a rubber mat. After tracing the pattern around eight times, inspect using a microscope
- If there is any evidence of pitting or scratching then try and repolish on the fine grade to remove them, failing that, you are unfortunately going to have to reterminate
- Finally, test using a light source and power meter.
Fusion splicing
There are other methods for attaching connectors to fibre cables. An increasingly common method is to use connectors that have been factory preterminated and tested onto a fibre tail of one or two metres in length.
This connector and tail is then fusion spliced onto the main fibre cable to effect termination. This method guarantees factory quality connectorisation and fibre polishing, but it does add a small additional loss due to the fibre splice, which must be taken into account in loss budget calculations.
To terminate a cable using preterminated connectors, the technique is quite straightforward. You will need a fusion splicer, a piece of machinery that will generally take two fibre cable ends that have been correctly stripped and cleaved using a specialist cleaving tool. The machine will accurately align the two fibres, flash clean them using an electric arc and then fuse the fibres together using a slightly more powerful arc.
This method produces extremely low losses. Some machines will even predict the loss across the joint before fusing the fibres, allowing the installer to fine-tune the alignment in order to achieve minimum loss.
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Termination of fibre cables
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Electrical and Mechanical Contractor
Postscript
Merrick Barton is a specialist in KRONE's fibre team.