The #1 cause of failures
Industry surveys agree: contaminated connector endfaces cause more optical failures than every other cause combined. A single 1 µm dust particle on a 9 µm single-mode core can block a large share of the light, raise reflection, and — under high power — burn permanently into both mated faces.
The IEC 61300-3-35 workflow
The standard defines endface zones — core, cladding, adhesive, contact — with pass/fail criteria for scratches and defects in each. The professional loop is fixed: inspect → clean → inspect → connect. Use a video inspection scope (never a bare-eye loupe on possibly-live fiber). If the first inspection passes, connect; if not, clean and re-inspect until it passes or the connector is condemned.
Cleaning methods that work
Dry first: one push of a click-type (cassette) cleaner is correct for routine dust. Wet-to-dry for stubborn residue: a lint-free wipe with a small zone dampened by fiber-grade solvent, dragging the ferrule from wet to dry in one stroke. Clean both sides of a connection — including inside adapters with the proper stick — and always finish with an inspection. Canned air can push particles around and leave propellant; it never substitutes for wiping.
Habits of professionals
Caps stay on until the moment of mating. One wipe zone is used once. The scope travels in the same case as the splicer. And "it looks clean" is never said about anything that was not inspected.