Single-mode vs multimode fiber
The two families of optical fiber look the same from the outside and carry light in very different ways. Here is how to tell them apart, what each is for, and what the OM numbers mean.
The short version
| Single-mode | Multimode | |
|---|---|---|
| Core diameter | About 9 µm | 50 µm (OM2 to OM5) or 62.5 µm (OM1) |
| Cladding diameter | 125 µm for both | |
| Wavelengths | 1310 and 1550 nm (also 1490, 1577 and 1625 nm on PON and test) | 850 and 1300 nm |
| Reach | Kilometers to tens of kilometers | Hundreds of meters at most |
| Where it is used | Outside plant, FTTH, carrier and long links, most new campus backbones | Inside buildings and data centers |
| Premises jacket color | Yellow | Orange (OM1, OM2), aqua (OM3, OM4), lime green (OM5) |
All outside plant fiber you splice on a pole or in a pedestal for FTTH is single-mode. Multimode is something you meet inside buildings.
Why the core size matters
A single-mode core is so small that light can only travel down it one way, as a single mode. A multimode core is wide enough for light to take many paths at once, and the paths arrive at slightly different times. That spreading (modal dispersion) blurs fast signals over distance, which is what limits multimode to short runs. Single-mode has no modal dispersion, so it reaches far.
OM1 to OM5
| Grade | Core | Jacket | 1000BASE-SX | 10GBASE-SR | 100GBASE-SR4 |
|---|---|---|---|---|---|
| OM1 | 62.5 µm | Orange | 275 m | 33 m | — |
| OM2 | 50 µm | Orange | 550 m | 82 m | — |
| OM3 | 50 µm, laser-optimized | Aqua | 550 m | 300 m | 70 m |
| OM4 | 50 µm, laser-optimized | Aqua (violet in some regions) | 550 m | 400 m | 100 m |
| OM5 | 50 µm, wideband | Lime green | 550 m | 400 m | 100 m |
OM5 adds performance at wavelengths between 850 and 953 nm for optics that send several wavelengths down one fiber. For ordinary 850 nm optics it behaves like OM4. The exact reach for any optic is on its datasheet.
Never mix them
Joining multimode to single-mode loses most of the light going from the big core into the small one, and a link with a mismatched jumper either fails or works badly. The connectors fit each other, so check the jacket color, the printing on the jumper, and the connector color (blue or green for single-mode, beige or aqua for multimode) before you plug in. Splicing them together is the same mistake made permanent.
Loss limits
TIA-568.3 allows up to 0.5 dB/km for single-mode outside plant cable, 1.0 dB/km for single-mode inside plant cable, 3.0 dB/km for 50 µm multimode at 850 nm (3.5 for OM1 and OM2) and 1.5 dB/km for multimode at 1300 nm. See fiber loss per km for the full table and how to turn it into an acceptance limit.
Which to install
Outside a building, single-mode, without exception. Inside, multimode still makes sense where the optics are already multimode and the runs are short. Many new installs run single-mode everywhere because single-mode optics have become cheap and the fiber never limits a future upgrade.