What is the difference between single-mode and multimode fiber?
Show the answer
Say this first
Multimode has a large core (50 or 62.5 microns) that carries many light paths, driven by cheap 850 nm VCSEL optics. The paths arrive at slightly different times (modal dispersion), which limits reach to hundreds of meters. Single-mode has a 9 micron core that carries one path, driven by 1310 or 1550 nm lasers, and reaches 10 to 80 km and beyond. Both have 125 micron cladding. The optic must match the fiber type.
Go deeper
Fiber types
| Type | Core | Jacket color (common) | 10GBASE-SR reach |
|---|---|---|---|
| OM1 | 62.5 micron | Orange | 33 m |
| OM2 | 50 micron | Orange | 82 m |
| OM3 | 50 micron, laser optimized | Aqua | 300 m |
| OM4 | 50 micron, laser optimized | Aqua or violet | 400 m |
| OM5 | 50 micron, wideband | Lime green | 400 m, built for SWDM |
| OS1, OS2 | 9 micron | Yellow | n/a (single-mode optics) |
Typical optics
| Optic | Fiber | Wavelength | Reach |
|---|---|---|---|
| 1000BASE-SX | MMF | 850 nm | 550 m on OM2 50 micron |
| 1000BASE-LX/LH | SMF | 1310 nm | 10 km (5 km in the original IEEE spec) |
| 10GBASE-SR | MMF | 850 nm | 300 m OM3, 400 m OM4 |
| 10GBASE-LR | SMF | 1310 nm | 10 km |
| 10GBASE-ER | SMF | 1550 nm | 40 km |
| 100GBASE-SR4 | MMF, 8 fibers on MPO | 850 nm | 70 m OM3, 100 m OM4 |
| 100GBASE-LR4 | SMF, duplex LC | 4 wavelengths near 1310 nm | 10 km |
How I choose
Multimode for short runs inside a building or row when optics cost dominates. Single-mode for anything between buildings, for long runs, and increasingly everywhere, because one plant then supports every future speed.
They will ask next
What happens if you plug a multimode optic into single-mode fiber?
It may link up on a very short patch, but the large multimode beam couples poorly into the small core, so loss is high and errors follow. It is unsupported. The reverse, single-mode optics on multimode, also suffers; 1000BASE-LX over multimode needs a mode-conditioning patch cord.
What is a BiDi optic?
It sends and receives on different wavelengths over a single fiber strand, so one strand carries both directions. The two ends must be a matched pair (for example one transmits at 1310 nm and the other at 1490 or 1550 nm, depending on the product).
Traps
- Mixing up the core sizes. 9 micron is single-mode; 50 and 62.5 micron are multimode.