Splicing and termination: fusion, mechanical, pigtails, field connectors

Every permanent joint in a fibre plant is a splice, and every place a module or patch cord plugs in is a termination. Done well they cost 0.03 dB and last decades; done badly they are the reflective, lossy, intermittent events that fill OTDR traces. This page describes the fusion-splicing process step by step, splicer types and their maintenance, mass splicing of ribbons, mechanical splices, the four ways to put a connector on a fibre, the acceptance criteria for each, and the defects to recognise.

Fusion splicing, step by step

Step	What	Detail
1. Prepare	strip cable, expose tubes, clean gel with wipes/gel remover; secure cable in tray	1.5–2 m of fibre working length in a closure
2. Strip the fibre	remove 250 µm (or 900 µm) coating over 30–40 mm with a precision stripper	no nicks in the cladding — a nicked fibre breaks at the splice later
3. Clean	wipe bare fibre with lint-free wipe and 99 % isopropyl alcohol, one direction	dust on the fibre = bubbles and loss
4. Cleave	precision cleaver: flat perpendicular endface, angle ≤ 0.5° (≤ 1° acceptable)	most splice loss comes from bad cleaves; replace blade positions as specified
5. Load	place fibres in V-grooves without touching endfaces; close clamps	keep endfaces away from anything
6. Align	splicer aligns cores (core-alignment) or claddings (V-groove), checks cleave angles and endface quality	reject and re-cleave if angle > 1° or endface chipped
7. Pre-fuse and fuse	arc cleans then melts endfaces; fibres pushed together (overlap ~10 µm)	arc power calibrated for altitude/humidity
8. Estimate loss	splicer image-processing estimate (±0.02–0.05 dB)	not a measurement — confirm with OTDR later
9. Proof test	200 g tension pull inside splicer	catches weak splices
10. Protect	heat-shrink splice sleeve (40 or 60 mm) with steel/ceramic rod, shrink in oven	do not touch until cooled
11. Store	lay in splice tray with bend radius ≥ 30 mm, secure sleeve, route fibres without crossing sharply	label per splice diagram

Splicer types

Type	Alignment	Typical SMF loss	Speed	Use
Core alignment	cameras find the cores and move fibres in X/Y	0.02 dB typ., ≤ 0.05	7–15 s splice, 20–30 s heat	backbone, DWDM, anything critical
Active cladding alignment	aligns claddings; relies on core concentricity	0.05 dB typ.	similar	FTTH, access, indoor
Fixed V-groove (passive)	fibres sit in grooves, no motors	0.05–0.1 dB	fast	drop cables, low cost
Ribbon (mass) splicer	4/8/12 fibres at once, cladding alignment	0.05–0.15 dB per fibre	12 fibres in ~15 s	high-count ribbon cables, data-centre trunks
Handheld / drop splicer	compact, battery	0.05–0.1	—	pole and wall work

Splicer maintenance: electrodes (replace after ~1 000–3 000 arcs or when loss creeps up), arc calibration at each site (altitude, temperature, humidity), V-groove cleaning with a cotton swab and alcohol, cleaver blade rotation and height adjustment, mirror/lens cleaning.

Splicing dissimilar fibres

Pair	Approach
G.652.D ↔ G.657.A	standard program; ≤ 0.05 dB
G.652 ↔ G.657.B / G.655 / DSF	splicer's dissimilar-fibre or MFD-matching program (longer arc diffuses the core); expect 0.1–0.3 dB; OTDR shows a gainer one way — average both directions
SMF ↔ SMF of different vintage (old high-PMD fibre)	normal splice; loss fine, PMD stays
MMF OM3 ↔ OM4	fine; OM1 ↔ OM2+ avoid
Bend-insensitive drop fibre to pigtail	standard; verify the splicer detected the fibre type

Types and their parameters: Fibre types.

Mechanical splices

Item	Value
Principle	cleaved fibres pushed together in an alignment element with index-matching gel
Loss	0.2–0.5 dB
Reflectance	−35 … −45 dB (gel dries out over years → worse)
Time	1–2 min, no power needed
Use	emergency restoration, temporary links, places without a splicer
Rule	replace with fusion during permanent repair (Maintenance & restoration)

Putting a connector on a fibre

Method	Loss	Reflectance	Time	Notes
Pigtail fusion splice (factory connector + splice)	0.1–0.3 (connector) + 0.03 (splice)	−55 (UPC) / −65 (APC)	3–5 min	the standard for panels and closures
Splice-on connector (SOC)	0.1–0.2 dB	−50 … −60	3 min	short fibre stub fused in the connector body; good for drops and repairs
Mechanical field-installable connector	0.3–0.5 dB	−40 … −50	2 min	cleaved fibre meets a factory stub in gel; FTTH indoor, temporary
Epoxy and polish (factory)	0.1–0.2	−55 / −65	—	pre-terminated cords and cassettes; field polishing is obsolete
Pre-terminated cable assembly	as factory connectors	—	none in field	MPO trunks, plug-and-play; needs exact lengths

Connector geometry and grades: Connectors & fibre types.

Acceptance criteria

Joint	Loss (SMF)	Loss (MMF)	Reflectance	Verification
Fusion splice	≤ 0.1 dB (typ. ≤ 0.05); redo above 0.3	≤ 0.3	< −60 dB (none visible)	bidirectional OTDR average (Testing)
Ribbon splice	≤ 0.15 dB per fibre	—	—	OTDR
Mechanical splice	≤ 0.5 dB	≤ 0.5	≤ −40 dB	OTDR (reflective event)
Pigtail / connector pair	≤ 0.5 dB design, ≤ 0.3 typical	≤ 0.75	UPC ≤ −50, APC ≤ −60	LSPM, inspection scope
Field connector	≤ 0.5 dB	—	≤ −45	LSPM + scope

Defects and their look

Defect	Splicer image / OTDR	Cause	Fix
Bubble	bright spot at the joint; high loss	dirt or bad cleave	re-cleave, re-clean
Fat / thin splice	bulge or neck at the joint	wrong overlap/arc power	recalibrate arc, check push distance
Core offset	cores misaligned; 0.2+ dB	cladding alignment on eccentric fibre; dirty V-groove	clean groove, use core alignment
Bad cleave angle	tilted endface; splicer warns	worn blade, fibre not seated	rotate/replace blade
Dust in the arc	black specks, high loss	dirty environment	clean, shield the splicer
Micro-crack at coating edge	breaks in proof test or later	stripper nick	replace stripper blades, check technique
Reflective splice	spike on OTDR	mechanical splice, air gap, or fibre not fused (arc failed)	redo as fusion
Wet splice	gel or water in the tray	closure ingress	dry, reseal (Plant faults)

Tools for the job

Fusion splicer with cleaver and stripper, fibre shears, gel remover, 99 % IPA and lint-free wipes, splice sleeves, sleeve oven (in splicer), cable jacket stripper and ring cutter, buffer tube cutter, aramid scissors, splice trays and closures, VFL, OTDR for verification, inspection scope for connectors, sharps container, safety glasses (Safety & handling).

In CodingBox

A splice never shows in the module memory — but its loss shows in the module's Rx power. Recording Rx at commissioning and after every re-splice (with CodingBox on the bench or the switch in service) turns "we re-spliced closure 14" into a number: the link passport before and after (DDM in the app).

Inspecting and cleaning the connectors this page produces — zones, criteria, methods: Endface inspection & cleaning; why mechanical splices and open UPC ends matter for the link — reflectance and return loss: Reflections & return loss.