What is single-mode fiber?
Single-mode fiber (SMF) is optical fiber with a very small core, typically about 9 microns across, surrounded by 125-micron cladding (the same cladding size as multimode). The core is so narrow that light travels down it in only one propagation mode: essentially a single path straight along the fiber instead of many paths bouncing at different angles.
Because all the light takes the same path, a pulse doesn't spread out the way it does in multimode fiber (modal dispersion). That's what lets single-mode links run for kilometers, and much farther with the right optics, at high data rates.
Single mode doesn't mean single wavelength
"Mode" refers to the path light takes, not its color. Single-mode systems commonly use lasers at 1310 nm and 1550 nm, and one strand can carry many wavelengths at once. Wavelength-division multiplexing (WDM) runs many separate channels, each on its own wavelength, over one fiber, and bidirectional (BiDi) optics send and receive on two different wavelengths over a single strand.
Single-mode vs. multimode
| Single-mode | Multimode | |
|---|---|---|
| Core size | About 9 microns | 50 or 62.5 microns |
| Light source | Lasers, commonly 1310 and 1550 nm | LEDs or VCSELs, commonly 850 nm |
| Distance | Kilometers and beyond | Up to a few hundred meters at high speeds |
| Typical jacket color | Yellow | Orange (OM1/OM2) or aqua (OM3/OM4) |
| Typical use | ISP and telco links, FTTH, links between buildings | Inside buildings and data centers |
Jacket colors are a convention, not a guarantee, so check the printed legend (for example OS2 for single-mode or OM4 for multimode) before patching.
Working with single-mode
- Single-mode and multimode don't mix. Patching a multimode cord into a single-mode link, or using the wrong type of transceiver, causes heavy loss or no link at all.
- Never look into a fiber end or a transceiver port. Single-mode lasers operate at infrared wavelengths you can't see, and they can damage your eyes.
- Keep connector end faces clean and respect the cable's bend radius; dirt and tight bends both add loss.
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.2 lists single-mode and multimode as the two optical cable types. The pattern to remember: single-mode has the smaller core, uses lasers and goes the longest distances; multimode has a larger core and is meant for shorter runs inside buildings.