The big three plus one
LC — 1.25 mm ferrule, small duplex latch; dominates transceivers (SFP/SFP+/QSFP) and high-density panels. SC — 2.5 mm ferrule, push-pull square body; the FTTH standard at the customer end. ST — 2.5 mm bayonet twist-lock; legacy but still everywhere in older plants. MPO/MTP — a 12/24-fiber array connector for parallel optics (40/100/400G) and pre-terminated trunks; polarity methods A/B/C from TIA-568.3-D must be planned, not improvised.
UPC vs APC
The polish geometry controls return loss (reflection back to the laser). UPC (blue) domes the endface: insertion loss slightly lower, return loss ≈ −50 dB. APC (green) angles it 8°, sending reflections into the cladding: return loss ≈ −65 dB, mandatory in PON, RF-over-glass and any analog system. The iron rule: never mate APC to UPC — the angled and flat faces meet at a point, creating a huge gap loss and permanent damage.
Performance numbers to carry in your head
A good factory connector pair inserts ≤0.3 dB; the standards allow 0.75 dB per mated pair in budgets. Repeatability depends entirely on cleanliness — the subject of its own lesson.
Care of connectors
Cap every unmated connector and every empty adapter. Support cords so no tension reaches the ferrule. And replace, don't fight, a connector with a cracked or chipped ferrule — reworking a bad one costs more than a new pigtail.