The IEC 61300-3-35 in Section 5.1 defines the 'contact zone' for single fiber connectors as a diameter between 130µm to 250µm for both multi-mode and single-mode fibers. If there are any residues in the contact zone, the residues will transfer and cross-contaminate both connectors when they are mated together or plugged into an optical transceiver. The dark ring stain that is visible on both of the connectors (photo, right) is the original residue transferred from the contact zone of Connector 2 on to Connector 1. Additional Discussion: Here's an important cleaning tip: NEVER assume the any connectors or patch cords will be 100% clean when you take a cable assembly out of the bag and uncap it. Investing in a quality inspection ferrule scope and having some optical-grade cleaning wipes on hand during an install will give your network optimal performance while saving you time and frustration caused by cross-contamination.
As you can see in the photos (right) if an operator only cleans one end of a mated connector pair (connector 1) and then mates it with an uncleaned connector (connector 2), both end-faces end up contaminated. Residues will transfer from the contaminated end-face to the clean end-face. You can tell when this happens because you will usually see a coffee ring stain on both connector end faces. This is called 'cross-contamination.'
Dust also will transfer from contaminated end-faces to clean end-faces. When this happens, usually the dust particles will start to break apart and even smaller particles will spread across both ferrule end-faces. Another common problem that happens with dust is that it creates pits and scratches on both ferrule end faces. The only solution to these permanent defects is to remove the damaged connector and splice a new one in place. Can you spell, 'wasted time'? In effect, contamination migrates. Always clean both ends of the mated pair, plus any tools, light sources, OTDRs, and other probes you might insert into a port or place over a jumper.