Single-mode fiber (SMF) is optical fiber with a very small glass core, around 9 µm across inside a 125 µm cladding, that carries light along essentially one path. Because the light does not spread across many paths as it travels, single-mode fiber can carry high speeds over much longer distances than multimode fiber. It is the standard medium for carrier circuits, metro and long-haul networks, and long backbone runs between buildings.
At a glance
- A very small core (about 9 µm) carries one mode of light, which keeps the signal clean over long distances.
- Grades you will see are OS1 and OS2, performance categories defined mainly by maximum loss; OS2 is specified with lower loss.
- Most carrier fiber handoffs, dark fiber and wavelength services are delivered on single-mode fiber.
- Common operating wavelengths are in the 1310 nm and 1550 nm regions; jackets and patch cords are usually yellow.
- It needs single-mode optics; plugging in multimode optics or patch cords is a common cause of failed turn-ups.
What problem it solves
Copper and multimode fiber both run out of reach. Multimode fiber’s larger core lets light travel along many paths that arrive at slightly different times, which smears the signal and limits how far a given speed can go. Single-mode fiber’s tiny core removes most of that effect, so the main limits become loss along the path and the power of the optics.
For a buyer, that means existing single-mode fiber can often carry a 1 Gbps circuit today and 10, 100 Gbps or more later without pulling new cable. Before reusing it, check two things separately: that the measured end-to-end loss, including connectors and splices (and their reflectance), fits the new interface’s power budget, and that the fiber type and distance stay within the interface’s specified reach and dispersion limits (chromatic dispersion and, at higher speeds, polarization-mode dispersion). Changing the optics works only if the path passes both checks. That is why carriers run single-mode in their networks and why many building owners and IT teams now specify it for riser and campus backbones even where multimode would reach.
How it works
Light from a laser in an optical transceiver is launched into the core. The cladding around the core has a slightly lower refractive index, which keeps the light inside the core as it travels. Because the core is so small, only one mode (one path) of light propagates.
Wavelengths. Single-mode systems commonly operate around 1310 nm and 1550 nm, where loss in glass is low. The 1550 nm region is also where dense wavelength division multiplexing (DWDM) places many channels on one fiber pair, which is how carriers sell wavelength services. Passive optical networks such as GPON also run over single-mode fiber, using different wavelengths in each direction.
Grades. OS1 and OS2 are the two single-mode performance categories you will see in specifications. They are defined in cabling standards chiefly by maximum attenuation, with OS2 specified for lower loss. OS1 is commonly deployed as tight-buffered indoor cable and OS2 as loose-tube outside-plant cable, but those are common deployment patterns rather than the definition. Both use the same optics and connectors.
Reach. How far a link runs depends on the optic and the loss budget: the fiber itself, every splice, every fiber optic connector and every patch panel along the way. A short building link may cross several patch panels; a metro link may cross dozens of splices. Carriers use amplifiers to go much farther than unamplified optics can.
Terminations. Single-mode cable ends in connectors such as LC or SC, with either a UPC (blue) or APC (green) polish. The polish must match on both sides of every mated connection, and the connector style must fit the adapter or port.
When it matters for buyers
- Ordering a circuit. Carriers commonly hand off on single-mode fiber. Your order should state the handoff, for example “SMF, LC/UPC, 10GBASE-LR,” and who supplies the optic in your router or switch.
- Colocation cross-connects. Data centers typically run single-mode between your cage and a carrier or cloud on-ramp. Confirm fiber type and connector with the facility before ordering patch cords and optics.
- Building fit-out. For a new office or riser backbone, single-mode is the safer long-term choice for runs between floors and buildings; ask your cabling contractor to quote it alongside multimode.
- Leasing dark fiber. Dark fiber is almost always single-mode. You supply the optics, so you need the route’s measured loss to pick them.
- Troubleshooting a turn-up. A yellow patch cord on one side and aqua on the other, or a green connector mated to a blue one, is often the whole problem.
For help planning backbone cabling in a new or refreshed office, see our wired and wireless LAN infrastructure solution page.
Questions to ask vendors
- What fiber type, connector and polish will the handoff be (for example SMF, LC/UPC)?
- What optic or interface type do you expect at our end (for example 1000BASE-LX or 10GBASE-LR), and who supplies it?
- Is the cabling OS1 or OS2, and can you provide test results (loss measurements, and OTDR traces for longer runs)?
- How many patch panels, splices and connectors are on the path between our equipment and the demarcation point?
- For dark fiber: what is the measured end-to-end loss, at which wavelengths, and is it guaranteed in the contract?
- Will spare strands be terminated and tested, and who maintains them?
How it differs from multimode fiber
Multimode fiber has a much larger core (50 or 62.5 µm), which lets light travel along many paths. That makes multimode optics simpler and historically cheaper, but limits reach: high speeds over multimode typically run tens to a few hundred meters, depending on speed and optics. Single-mode handles long distances and is what carriers typically deliver fiber circuits on. The two are not interchangeable: the cable, patch cords and optics generally need to be the same type.