Quick answer: Single mode fiber, often written SMF, has a core of about 9 micrometres that carries one propagation path. At 1310 nm it loses roughly 0.35 dB per kilometre, and at 1550 nm about 0.2 dB, which allows links of several kilometres without repeaters. Connectors and splices each cost a fraction of a decibel, so the budget is spent on terminations as much as on distance. SMF is the default for anything beyond a building.
The decision between single mode and multimode is made once, when the cable is pulled, because the optics at each end must match the fiber. Single mode costs more per transceiver and less per kilometre, and it does not become obsolete when the speed increases.
Why the core size matters
One path, no modal dispersion
A 9 micrometre core supports essentially one mode at 1310 and 1550 nm, so all the light arrives together. Multimode’s 50 micrometre core carries many paths that arrive at slightly different times, which limits reach long before the loss does.
Wavelength choices
1310 nm has zero dispersion and moderate loss; 1550 nm has the lowest loss and is used with erbium-doped amplifiers on long haul routes. Short links inside a campus usually use 1310 nm optics because they are cheaper.
Budget arithmetic
| Item | Loss | Notes |
|---|
| Fiber at 1310 nm | 0.35 dB/km | Typical installed value |
| Fiber at 1550 nm | 0.2 dB/km | Long haul |
| Connector pair | 0.3 to 0.5 dB | UPC or APC |
| Fusion splice | 0.1 dB | Better than a connector |
| Typical 10G module budget | About 6 dB | Depends on the optic |
Terminations and testing
SMF is normally terminated with LC or SC connectors, in UPC for 1310 nm links and APC when return loss matters, such as PON or RF over glass. Mixing polish types damages both ferrules and produces a high loss, so label the patch panels.
Test with a light source and power meter for end-to-end loss, and use an OTDR when you need to locate a fault. Cleaning is not optional: a single speck of dust on a 9 micrometre core can add several decibels, and the damage is permanent once the connector is scratched.
When SMF is the right choice
Between buildings, along a site boundary, or anywhere the distance exceeds a few hundred metres. It is also the right choice when the link speed may rise later, because single mode optics exist for every rate from 1G to 400G and the cable does not need replacing.
Inside a data hall, multimode remains cheaper because short-reach optics are inexpensive. For the transition, the single mode patch cables page covers cordage, the fiber connector types reference covers polish and housings, and the multimode vs single mode fiber comparison sets the two side by side.
Practical scenarios
- Two buildings 800 m apart: SMF with 1310 nm optics, fusion spliced at each end to a patch panel.
- Rural link of 5 km: SMF at 1550 nm, and check the optic’s budget before assuming the distance is fine.
- Garden office 120 m away: SMF is often cheaper than copper plus surge protection and extenders.
- Industrial site with electrical noise: fiber removes the earth loop entirely, which copper cannot.
FAQ
Can single mode fiber be used for short links?
Yes. The optics are more expensive than multimode, but the cable is often comparable in price. Many sites standardise on single mode to simplify spares, at the cost of paying more per transceiver.
What is the difference between UPC and APC?
UPC has a flat, polished ferrule and a return loss around 50 dB; APC is polished at an 8 degree angle and reaches 60 dB or better. APC connectors are green, and mating them with UPC causes a gap and high loss.
How far can SMF go without a repeater?
Tens of kilometres with standard optics, limited by the optical budget rather than the fiber. A 6 dB budget at 0.35 dB per kilometre allows roughly 15 km before connectors and splices are counted.
