Endface inspection and cleaning: IEC 61300-3-35 grading and methods

A single-mode core is 9 µm across. A 5 µm dust particle sitting on it blocks a good part of the light, and when the connector is mated the particle is pressed into the glass and scratches both endfaces — the fault then travels to every connector the damaged one touches, including the transceiver's port. Contamination is the most frequent fault of a fibre plant and the only one that is completely preventable. This page describes what contamination does, the IEC 61300-3-35 zones and criteria that turn "looks clean" into pass/fail, the inspection equipment and technique including MPO and transceiver ports, the contamination and damage types and how to recognize them, the cleaning methods and when each is right, and the inspect-clean-inspect procedure.

What contamination does

Contaminant	Effect
Dust particle on the core	0.5–3 dB loss, reflectance rises to −30 … −40 dB
Oil film (a fingerprint)	0.2–1 dB, interferometric noise on high-rate links
Particle between the ferrules	an air gap: loss plus a −14 … −25 dB reflection; at mating the particle scratches and pits both endfaces permanently
Dried liquid ring	loss and reflectance; typical after wet cleaning without a dry step
Contaminant at high power	burned into the glass above about +20 dBm; a fibre fuse is possible at watt-level power (Safety & handling)
Transfer	a dirty cord contaminates every port it enters — panel adapters and the module receptacle; the fault migrates through the plant

The consequences on the link: flapping, CRC errors, low Rx power, reflectance-driven laser noise (Plant faults, Reflections & return loss).

Zones and criteria of IEC 61300-3-35

The standard divides the endface into concentric zones and sets, for each connector type, how many scratches and defects of which size are allowed in each. Inspection instruments apply the tables automatically; the numbers below give the shape of the criteria (edition 2).

Zone	Diameter, single-mode	Diameter, multimode	What it is
A — core	0–25 µm	0–65 µm	the light path; strictest
B — cladding	25–115 µm	65–115 µm	close to the light; defects here scatter and migrate
C — adhesive	115–135 µm	115–135 µm	the epoxy ring; no requirements
D — contact	135–250 µm	135–250 µm	the physical contact area; large defects prevent proper mating

Connector type	Zone A	Zone B	Zone D
Single-mode UPC (return loss ≥ 45 dB)	no scratches, no defects	scratches ≤ 3 µm wide: any number; wider: none; defects < 2 µm: any number, 2–5 µm: up to 5, > 5 µm: none	scratches: any; defects ≥ 10 µm: none
Single-mode APC	up to 4 fine scratches (≤ 3 µm); no defects	as UPC	as UPC
Multimode PC	scratches ≤ 3 µm: any number; wider: none; defects < 5 µm: up to 4, ≥ 5 µm: none	scratches ≤ 5 µm: any; wider: none; defects < 2 µm: any, 2–5 µm: up to 5, > 5 µm: none	as above
MPO/MTP	the same criteria applied to every fibre of the array; one failing fibre fails the connector	—	—

Inspection equipment and technique

Tool	Use	Notes
Handheld video probe, 200×/400×	patch cords through 2.5 mm (SC/FC/ST) and 1.25 mm (LC/MU) tips; panel ports and receptacles through bulkhead tips; MPO through pan-and-scan or auto-scanning tips	the camera keeps the eye away from the beam — the only acceptable scope on possibly live fibre
Auto-analysis probe	grades against IEC 61300-3-35, stores images	acceptance reports, consistent judgement between technicians
Bench microscope	lab and factory	never on a live fibre through an eyepiece
Visual fault locator	not an inspection tool, but shows a broken or heavily bent fibre in a cord	red light, Class 2/3R — do not look into it

Technique: confirm the fibre is dark with a power meter; remove the cap only at inspection time; centre and focus on the core; inspect both sides of every connection — the cord and the port, because the port is the forgotten half; on transceivers use the 1.25 mm bulkhead tip for LC receptacles (SFP, QSFP-LR4) and the MPO tip for parallel optics receptacles (SR4, DR4) — the receptacle holds a ferrule stub or a lens array and is graded like any endface; save the image when the result goes into an acceptance record.

Contamination and damage types

Appearance	What it is	Action
Bright specks	airborne dust, particles from dust caps	dry clean
Smear or haze over the core	skin oil from a touched ferrule	wet-dry clean
Ring or crescent	dried liquid — often alcohol residue from a wet clean without a dry step	wet-dry again, less solvent, immediate dry
Fibres and lint	wipes, clothing, cotton swabs	dry click cleaner; stop using that wipe
Gel or grease	cable filling gel from a splice closure	wet-dry with an approved solvent
Plastic shavings	new dust caps shed plastic	always clean after uncapping a new cord
Linear scratches	mating with debris, abrasive wiping	replace the cord if scratches enter zone A; re-polish or re-terminate a pigtail
Dark pits and chips	mating with a hard particle, high power, dropped connector	replace
Crack across the endface or ferrule	mechanical damage	re-terminate
Dark or burned core	high-power damage	replace both mating connectors; inspect the far side

Cleaning methods

Method	For	Notes
Dry click cleaner ("one-push"), 1.25 and 2.5 mm	patch cords and, with the extended tip, panel adapters and receptacles	first choice; 500–800 cleans per tool; push straight in, one click, no rotation
Cassette or reel dry cleaner (lint-free cloth)	patch cord ferrules	one or two straight strokes on a fresh area
Wet-dry: optical-grade solvent on a lint-free wipe, then a dry area	oils, dried residues, gel	a drop, not a soak; the dry pass is mandatory; 99 % isopropanol or engineered cleaning fluid, never 70 % alcohol or acetone
1.25 and 2.5 mm lint-free swabs	panel adapters and receptacles	single use, straight in, one turn, out; dry unless oil is visible
MPO/MTP click cleaner, MPO cassette cleaner	MPO connectors and adapters	dry; wet-dry only for oil; never touch the guide pins; clean the female side too
Optical-grade gas duster	blowing loose debris out of receptacles before swabbing	not for lint or oil; ordinary canned air leaves propellant residue
Not acceptable	cotton buds, breath, shirt, reused wipes, touching the ferrule, wet solvent inside a transceiver receptacle with a lens array	—

Transceiver receptacles: a dry click cleaner with the 1.25 mm bulkhead tip for LC receptacles; a dry MPO cleaner or gas duster for parallel-optics receptacles; solvent only if the module vendor allows it, because SR4/DR4 receptacles contain lens arrays.

The procedure: inspect, clean, inspect

Confirm the fibre is dark with a power meter or by shutting the far-end laser.
Remove dust caps immediately before inspection, not earlier.
Inspect the cord and the port it goes into.
Dirty → dry clean → inspect again. Still dirty → wet-dry → inspect again. Still dirty or scratched in zone A → replace the cord or re-terminate; a damaged port adapter is replaced, a damaged transceiver receptacle usually means a new module.
Mate immediately after a passing inspection; do not touch, breathe on or set down the ferrule.
Cap everything that is unmated, with clean caps stored in a sealed bag.
At acceptance, save the images with the port identifier (Documentation & labelling).

When	Inspect
Every mating	the connector and the port, always
Before every measurement	the reference cords first — a dirty reference cord invalidates the test (Testing & measurement)
After transport or storage	all cords and modules
Quarterly	panel ports that are re-patched often (Maintenance & restoration)
Any link problem	before touching anything else

Standards

Standard	Content
IEC 61300-3-35	visual inspection zones and pass/fail criteria for polished endfaces
IEC TR 62627-01	connector cleaning methods and their evaluation
IPC-8497-1	cleaning methods and contamination assessment for optical assemblies
IEC 60825-2	eye safety when inspecting possibly live fibre (Standards map)

In CodingBox

The bench is where the cleaning habit is tested: a module read in CodingBox from a calibrated source through a reference cord should show the expected Rx power within a few tenths of a decibel. When it reads 1–2 dB low, inspect the module receptacle and the cord before doubting the module — and watch the Rx value recover live while you clean. Record the clean value as the module's baseline (DDM in the app, Connectors & fibre types).