Fiber Optic Connector Types: LC, SC, ST, FC and MPO

LC is the small-form-factor connector defined by IEC 61754-20. It latches like an RJ-45 plug, and because it is small a duplex LC pair fits where a single older connector used to, which is why it became the transceiver interface and why an equipment room is mostly LC-to-LC cords.

SC, defined by IEC 61754-4, is a square-bodied push-pull connector. It is easy to work with in a crowded enclosure because it needs no latch release, and it remains common on carrier handoffs, on media converters and on older switch plant. An SC-to-LC cord is the ordinary bridge between an existing SC panel and current LC optics.

ST is a bayonet connector — push and twist — that dominated early multimode plant and is still found in industrial and campus installations. FC is a threaded connector, once standard in test and metrology because the screw coupling holds alignment under vibration. Neither is something to specify for new work, but both are things to identify correctly when extending or repairing an existing run.

MPO, defined by IEC 61754-7, is a multi-fiber connector: instead of one fiber in a round ferrule, it holds a row of fibers in a rectangular one. CommScope describes the standard MPO12 as housing 12 fibers in a single row on 250 µm centerlines, numbered one to twelve from left to right. In North America TIA-604-5, also known as FOCIS-5, is the standard that supports physical compatibility between MPO connectors.

MPO exists because parallel optics need multiple fibers per link. A 40 or 100 Gigabit transceiver that transmits on four lanes needs eight fibers — four transmit, four receive — and CommScope notes that such applications are more precisely called MPO12/8, because the connector holds 12 positions and the application uses 8. The center four positions are typically filled with a fiber stub to keep dust out.

MPO was originally specified in 12- and 24-fiber versions, with 16- and 32-fiber versions added later. The practical pattern in a data center is an MPO trunk between panels, broken out at each end into duplex LC for the equipment — which is why an MPO-to-4×LC harness is one of the most common assemblies in the rack.

Polish type is the first thing to match. UPC (ultra physical contact) is the blue-bodied default for most multimode and much single-mode work; APC (angled physical contact) is green-bodied and uses an angled endface to send back-reflections into the cladding, which matters on single-mode links carrying analog or long-haul services. UPC and APC do not mate: joining them damages the endfaces and produces a link that either fails or degrades quietly.

Polarity is the second, and it is where MPO installations go wrong. Because an MPO carries a row of fibers, the trunk, the adapters and the breakout harnesses must all follow the same polarity method end to end. Mixing methods across a channel produces transmit-to-transmit pairs at one end, and the fault looks like a dead link rather than a wiring error.

The third is cleanliness. A fiber endface is a contact interface, and a single particle on a ferrule is enough to raise insertion loss past the link budget or to pit both endfaces when they are mated. Inspect and clean before every mating; it is the cheapest step in the whole installation and the one most often skipped.