A fiber optic connector is the plug on the end of a fiber cable that holds the glass fiber in a precise ferrule and aligns it with a matching port, patch panel adapter or other cable so light can pass with little loss. Connectors come in several common types, such as LC, SC and MPO, and with different end-face polishes. On each connection the fiber type and polish need to match, and the connector must fit the port or adapter.
At a glance
- A connector holds the fiber in a ceramic or metal ferrule and latches it into a port or adapter.
- LC is the usual type on modern optics; SC, ST, FC and MPO are also common, depending on the equipment and era.
- End faces are polished either UPC (usually blue on single-mode) or APC (angled, usually green); the two should not be mated.
- Connectors are simplex (one fiber) or duplex (a transmit and receive pair).
- A wrong connector, polish or fiber type, or a dirty end face, is a common cause of failed turn-ups on new links.
What problem it solves
Fiber cores are microscopic; a single-mode core is about 9 µm across. Joining two fibers so light passes from one core into the other needs precise alignment and a clean, well-polished end face. Splicing does this permanently; connectors do it in a way that can be unplugged and re-patched. Connectors are what let a carrier hand off a circuit to your equipment, a data center patch your cage to a carrier, and an IT team move links between switches.
How it works
The fiber is fixed inside a ferrule, a small precision cylinder whose end face is polished. When two connectors meet in an adapter (or a connector plugs into an optical transceiver), the ferrules butt together and a spring keeps their end faces in contact.
Connector types
| Type | Ferrule | Latching | Typical use |
|---|---|---|---|
| LC | 1.25 mm | Push-pull with latch | The usual connector on SFP-family and many QSFP optics; dense patch panels |
| SC | 2.5 mm | Push-pull, square body | Carrier and building equipment, patch panels, passive optical networks |
| ST | 2.5 mm | Bayonet twist-lock | Older building and campus multimode cabling |
| FC | 2.5 mm | Threaded screw-on | Test equipment and some older carrier and telecom gear |
| MPO / MTP | Multi-fiber (often 12 or 24 fibers) | Push-pull | Parallel optics and high-density trunks in data centers; MTP is one manufacturer’s brand of MPO-compatible connector |
| MT-RJ | Two-fiber | RJ-style latch | Legacy duplex connector on older equipment |
| E2000 | 2.5 mm | Push-pull with spring shutter | Often met in carrier and European installations |
Polish. UPC (ultra physical contact) end faces are polished with a slight dome and are typically blue on single-mode, while multimode connector colors vary by grade and maker. APC (angled physical contact) end faces are polished at about 8 degrees so reflected light leaves the core, and are typically green. APC is common on passive optical networks and video-over-fiber, and less common on Ethernet optics. Mating APC to UPC leaves an air gap at an angle, causing high loss and possibly damaging both end faces.
Simplex and duplex. Most Ethernet optics use two fibers (one each direction) in a duplex LC. BiDi optics send and receive on a single fiber at different wavelengths and use a simplex connector. MPO carries many fibers in one connector for parallel optics.
Fiber type. The same connector style is made for both single-mode and multimode fiber. An LC that fits does not mean the fiber underneath matches.
When it matters for buyers
- Circuit orders. Carriers typically ask for, or state, the handoff, for example “SMF, LC/UPC” or “SMF, SC/APC.” Confirm it matches your equipment’s optic and patch cords before the install date.
- Colocation cross-connects. In a colocation facility, the cross-connect is delivered to a patch panel in your cage or rack with a specific connector type. Order patch cords from that panel to your equipment to match.
- Building fit-out. Specify connector type and polish at every patch panel in the cabling contract, and ask for loss test results per connector.
- Mixed estates. Hybrid patch cords (for example SC to LC) solve connector mismatches, but not fiber-type or polish mismatches.
For help ordering and managing cross-connects and carrier handoffs in data centers, see our interconnection solution page.
Questions to ask vendors
- What connector type and polish will the handoff or cross-connect be (for example LC/UPC or SC/APC)?
- Is it single-mode or multimode, and simplex or duplex?
- Where exactly is the demarcation point: a patch panel, the provider’s device, or our port?
- Who supplies the patch cord between the demarcation point and our equipment?
- Will installers inspect and clean connectors, and provide loss results for each link?
- For MPO links: what fiber count, polarity method and pinning (male or female) are used?
How it differs from an optical transceiver
A connector is a passive mechanical part: it joins fibers so light can pass. An optical transceiver is an active module in a switch or router port that converts electrical signals to light and back. The transceiver has a connector receptacle (often LC or MPO), so ordering a link means matching both: the right optic for the speed, distance and fiber type, and the right connector and polish on the cable.