Fibre optic splicing: fusion and mechanical

What fibre optic splicing is

Fibre optic splicing joins two bare fibres so that their cores line up and light crosses the joint. Unlike a connector, a splice is not meant to be opened again. Splicing is how long cable runs are joined, how damaged cables are repaired, and how connectors are added to outdoor cable by joining factory-terminated pigtails to the cable fibres. In North America the same work is usually called fiber splicing.

Loss at a splice comes from anything that stops the two cores meeting exactly: a lateral offset, an angle between the fibres, a poor cleave, dirt, or a difference in core size or mode field diameter between the two fibres. Single-mode fibre, with a core about 9 µm across, is far less tolerant of offset than 50 µm multimode fibre. Fibres of different families, such as single-mode and multimode, or 50 µm and 62.5 µm multimode, should not be spliced together.

Fusion splicing vs mechanical splice

The two methods differ in how the fibres are held together and in the equipment they need.

A fusion splice is essentially one continuous piece of glass, so it has very low loss and negligible back reflection, and it is stable over temperature and time. The splicer is the main cost. A mechanical splice needs cheaper tools and no power, but it has higher loss and higher reflectance than a fusion splice, which matters for analogue video and other reflection-sensitive single-mode systems. Both are covered by the same TIA-568 allowance in a loss budget, but fusion is the normal choice for permanent networks.

Ribbon cable can be mass fusion spliced, joining a whole ribbon, commonly 12 fibres, in one operation on a ribbon splicer.

The steps of a fusion splice

1. Prepare the cable: remove the jacket, strength members and buffer tube, clean off any gel, and slide a heat-shrink protection sleeve onto one fibre. It cannot be fitted after the splice is made.
2. Strip: remove the 250 µm or 900 µm coating to expose the 125 µm glass, using a precision stripper.
3. Clean: wipe the bare glass with lint-free tissue and isopropyl alcohol. Any residue left on the fibre spoils the cleave and the splice.
4. Cleave: score and break the fibre with a precision cleaver to give a flat end face at right angles to the fibre axis. Do not touch or clean the fibre after cleaving.
5. Align: place both fibres in the splicer. A core-alignment splicer uses cameras to line up the cores; a cladding-alignment splicer lines up the outer glass in fixed V-grooves. The splicer checks the cleave angle and rejects poor ends.
6. Fuse: the splicer cleans the ends with a short arc, pushes them together and fuses them with a main arc, then displays an estimated loss.
7. Protect: slide the sleeve over the bare splice and shrink it in the splicer's heater. The sleeve's steel rod restores the strength the bare joint lacks.
8. Test: store the splice in its tray and measure it, normally with an OTDR, because the splicer's figure is an estimate.

Splice trays, closures and pigtail splicing

Every splice sits in a splice tray, which holds the protection sleeves in slots and routes the spare fibre in loops that keep within its minimum bend radius. The slack lets a technician resplice later. Trays are mounted in an enclosure suited to the location:

• Outdoor splice closures: sealed dome or inline closures in manholes, on poles or buried, joining lengths of outside plant cable or branching it.
• Rack-mount splice enclosures and patch panels: indoor housings in which incoming cable fibres are spliced to pigtails whose connectors appear on adaptors at the front.
• Optical distribution frames: high-count versions of the same idea in telecom rooms and exchanges.

Pigtail splicing is the usual way to terminate loose tube outdoor cable, which is hard to fit with connectors directly. A factory-made fibre pigtail with a polished connector on one end is fusion spliced to each cable fibre in the panel. The connector quality is then set in the factory, and the field work is limited to the splice.

Splice loss allowances and OTDR testing

TIA-568 allows a maximum of 0.3 dB per splice, fusion or mechanical, and 0.75 dB per mated connector pair. Designers use these allowances to build a link loss budget before the cable is installed, and the same figures give the pass or fail limit after installation. A well-made fusion splice usually measures far below 0.3 dB, but the budget should still use the allowance. Our fibre loss budget guide shows the calculation, including room for future repair splices.

An optical time domain reflectometer (OTDR) sends pulses into the fibre and measures the light scattered back, so it shows the position and loss of every splice, connector and bend along the link. Two points matter when reading splice loss:

• Test in both directions: when the two fibres have slightly different mode field diameters or backscatter, a splice can look like a gain from one end and a larger loss from the other. The true splice loss is the average of the two readings.
• Use launch and receive fibres: lengths of fibre at each end let the OTDR measure the first and last connectors, which it cannot see on its own.

A light source and power meter test then confirms the total end-to-end loss of the link.

When pre-terminated or field-installable products avoid splicing

Splicing remains normal for long outside plant routes, repairs and cables whose final length is unknown before pulling. Inside buildings and data centres, many links can avoid it.

• Pre-terminated trunks: MPO trunk cables arrive with connectors fitted and tested, often with a pulling grip over the ends. Installation is a matter of routing and plugging in, with no splicer, no splicing time and no splice loss in the channel.
• Pre-terminated breakouts and harnesses: custom fibre optic assemblies built to a drawing can replace a spliced panel for a known route, such as a rack-to-rack link or a cable into a machine or vehicle.
• Field-installable connectors: connectors with a factory-polished fibre stub are fitted on site with a built-in mechanical splice or a fusion splice-on method. They suit small jobs but still add a joint inside the connector.

Pre-terminated cable needs an accurate route length, room to pull the connectors through ducts and containment, and somewhere to store slack. Where those are known, it is usually faster to install and easier to test.

What to specify if you choose pre-terminated assemblies

1. Fibre type and grade (for example OS2, OM3 or OM4) and fibre count.
2. Connector and polish at each end, and for MPO the fibre count, pin gender and polarity.
3. Measured route length plus slack, length tolerance, and breakout leg lengths and stagger.
4. Pulling grip or protective end cover, and the largest duct or opening the ends must pass through.
5. Cable construction, jacket material, armour if needed and the fire rating for the country of installation.
6. Maximum insertion loss per connector and the test report required with delivery.
7. Labelling at each end.

Radix Fibrotech builds pre-terminated trunks, breakouts and assemblies to drawing in Pune. Send the details above with a request for a quote and ask for the loss values that apply to your build.