Single-Mode vs Multimode Fiber: Which Module Do You Need?
Core size, light source and reach class — how to tell single-mode and multimode apart, match the right module, and the jacket colors that give it away.
The core difference
Single-mode fiber (SMF) has a 9 µm core and carries light from a precision laser, typically at 1310 nm or 1550 nm. Multimode fiber (MMF) has a 50 µm or 62.5 µm core and carries light from a cheaper VCSEL, typically at 850 nm. The result: single-mode reaches tens to hundreds of kilometers; multimode is limited to hundreds of meters at 10G — but multimode optics and cabling cost less for those short runs.
Distance rules of thumb
Match the module family to the cable plant and the distance you actually need — not the distance you might need someday:
| Scenario | Fiber | Module family | Typical reach |
|---|---|---|---|
| Rack-to-rack, same room | MMF (OM3/OM4) | 1000BASE-SX / 10GBASE-SR | up to 300–550 m |
| Inside one building or campus | MMF or SMF | SX/LX or mode-conditioned LX | 100 m – 10 km |
| Metro, ISP aggregation | SMF (G.652) | 1000BASE-LX/EX/ZX, 10GBASE-LR/ER | 10–40 km |
| Long-haul, cross-city | SMF (G.652) | 1000BASE-ZX, 10GBASE-ZR, BiDi long reach | 40–120 km |
Can you mix fiber types and modules?
No — a multimode module on single-mode cable (or the reverse) will not link, and a laser source rated for single-mode can overload a multimode receiver. The one sanctioned crossover is 1000BASE-LX over multimode fiber, which requires a mode-conditioning patch cord to launch the beam correctly. If your plant mixes both fiber types between the same two endpoints, the clean fix is a BiDi pair over the single-mode strand or a media conversion at one end.
How to tell them apart on sight
Jacket color is the field shortcut: yellow means single-mode, orange means OM1/OM2 multimode, and aqua (or lime for OM5) means laser-optimized OM3/OM4/OM5 multimode. Connector counts are another clue — BiDi modules take one simplex LC/SC, while dual-fiber modules take a duplex LC. When in doubt, check the printing on the cable itself: fiber class is printed along the jacket.
What we stock
We supply both plants: multimode SX/SR modules and the full single-mode range — LX/EX/ZX at 1.25G, LR/ER/ZR at 10G, plus BiDi pairs that run over a single SMF strand. Share your link distance and switch model and we will confirm the right module and coding before you order.