Fiber Optic Connector Types: LC, SC, FC, ST, MPO and Very Small Form Factor

Scope

This article is a reference to the connector types used on optical fiber, what distinguishes them, and where each is used. A connector has two independent parts: the body, which sets how it latches, how densely it packs and what it mates with, and the ferrule end-face polish, which sets the back-reflection. This article covers the bodies and the practical rules for choosing and mixing them; the polish geometries are set out in PC, UPC and APC connectors, and keeping any connector working in Fiber Connector Inspection and Cleaning.

How a connector works

The fiber is fixed in the bore of a precision ferrule, usually zirconia ceramic, and the end is polished. Two connectors mate in an adapter whose split sleeve aligns the two ferrules by their outside diameters, and a spring in each connector presses the end-faces into physical contact. The loss of the joint is then set by how well the two fiber cores line up (the ferrule and bore tolerances and the core's concentricity), by the gap and angle between the end-faces, and above all by cleanliness. The mode field diameter of single-mode fiber, about 10 μm, is why its ferrules and alignment are held to sub-micrometre tolerances, while multimode connectors are more forgiving.

Single-fiber connectors

LC displaced SC as the default because its 1.25 mm ferrule halves the footprint, fitting two fibers in the space of one SC; a duplex LC is the optical port on most pluggable transceivers (see pluggable optics).

SC remains common where density matters less than robustness and ease of handling, and in access networks, where SC/APC is the usual connector at the customer premises.

FC is slower to connect because of its threaded nut, but it holds its alignment under vibration and repeated mating, and its key fixes the connector's rotation. That key is why FC/APC is the usual connector for polarization-maintaining fiber, where the key marks the slow axis, and for laboratory patch cords. FC keys come in narrow and wide variants, and an angled FC connector mates at the right angle only in an adapter with the matching key width.

Multi-fiber connectors: MPO

The MPO (multi-fiber push-on) connector holds a row of fibers, or several rows, in a single rectangular MT ferrule. Common counts are 8, 12, 16 and 24 fibers; 12-fiber and 16-fiber rows are the most common, and 8-fiber cables use the outer positions of a 12-fiber ferrule for four-lane parallel optics such as SR4 and DR4 (see Transceiver Names). Alignment comes from two steel guide pins on one connector entering holes in the other, so every MPO pair has one pinned (male) and one unpinned (female) side. MTP is a brand of MPO connector with tighter tolerances and is intermateable with it.

Polarity. A parallel link needs each transmitter to reach the matching receiver, which depends on how the fibers are ordered through the trunk, the adapters and the cords. Cabling standards define three polarity methods, A, B and C, which differ in whether the trunk keeps, reverses or pair-swaps the fiber order and in which adapter type (key-up to key-down, or key-up to key-up) is used. Mixing components designed for different methods is a common cause of a link that shows light on every fiber and still does not work.

Polish. Multimode MPO connectors are usually flat-polished; single-mode MPO connectors are usually angle-polished (APC), because the many fibers of a flat multi-fiber face cannot all be relied on to reach physical contact.

Very small form factor connectors

Transceivers at 400 Gb/s and 800 Gb/s often carry several duplex lanes in one module, and breaking them out to separate destinations needs more duplex connectors than LC can fit across the module's face. The very small form factor (VSFF) connectors, including SN, MDC and CS, use 1.25 mm ferrules in narrower bodies so that two or more duplex connectors fit where one duplex LC would. They are not intermateable with each other or with LC, so a link must use the same type throughout or convert through a patch cord.

Color codes

The connector body or boot color follows a widely used convention:

The color is a convention, not a guarantee; the fiber type printed on the cable jacket is the reliable indication (see Types of Optical Fiber for the jacket colors).

Loss and reflection

A good single-mode connector pair has an insertion loss of a few tenths of a dB or less, and cabling standards budget up to 0.75 dB per mated pair. The return loss depends on the polish: 50 dB or better for UPC and 60 dB or better for APC. How to measure both is in Measuring Insertion Loss and Return Loss.

Choosing a connector

• Data center duplex links: LC, or VSFF where the transceiver uses it.
• Parallel optics and high-count trunks: MPO, with the polarity method chosen once and kept throughout.
• FTTH and PON: SC/APC, the convention in most access networks.
• Laboratory work and PM fiber: FC/APC, keyed.
• Legacy multimode plant: whatever is installed, often ST or SC, with hybrid patch cords to reach modern equipment.

Mixing connector types

Different bodies connect only through a hybrid patch cord or, for some pairs, a hybrid adapter; each extra joint adds loss and a place for dirt. Polish types must never be mixed: an APC connector mated to a UPC or PC connector leaves an air gap at the core, with high loss and strong reflection, and can damage both end-faces. Fiber types must match as well; a multimode connector on single-mode fiber, or the reverse, joins cores of very different size.

References: IEC 61754 series, Fibre optic interconnecting devices and passive components: Fibre optic connector interfaces; TIA-604 series, Fiber Optic Connector Intermateability Standards (FOCIS); ANSI/TIA-568.3-D, Optical Fiber Cabling and Components Standard.