Single-mode fibre has a core of about 9 µm and carries one mode of light, usually at 1310 or 1550 nm, over distances of kilometres. Multimode fibre has a 50 or 62.5 µm core and carries many modes, usually at 850 nm, over tens to a few hundred metres. Multimode optics generally cost less; single-mode cable gives far more reach and room for future speeds.
Single mode vs multimode: what physically differs
Single-mode and multimode fibre share the same 125 µm glass cladding, so they fit the same connector ferrules and look identical without a label. The difference is the core. A single mode fiber optic cable has a core of about 9 µm, written 9/125, and guides light along one path. Multimode fibre has a 50 µm core (50/125) or, in legacy OM1, a 62.5 µm core (62.5/125), and guides light along many paths at once.
Those paths have slightly different lengths, so a pulse spreads as it travels. This modal dispersion is what limits multimode reach at high data rates. Single-mode fibre avoids it, so its reach is set mainly by attenuation and chromatic dispersion and runs to tens of kilometres.
The large multimode core also accepts light from cheaper sources. Current multimode optics use vertical-cavity surface-emitting lasers (VCSELs) at 850 nm. Single-mode optics use edge-emitting lasers at 1310 nm or 1550 nm that must be aligned precisely to the small core.
| Fibre | Core/cladding | Grades | Main wavelengths | Jacket colour (TIA-598) |
|---|---|---|---|---|
| Single-mode | 9/125 µm | OS1a, OS2 | 1310 nm, 1550 nm | Yellow |
| Multimode | 62.5/125 µm | OM1 | 850 nm, 1300 nm | Orange |
| Multimode | 50/125 µm | OM2 | 850 nm, 1300 nm | Orange |
| Multimode, laser-optimised | 50/125 µm | OM3, OM4 | 850 nm | Aqua (OM4 also violet in some markets) |
| Multimode, wideband | 50/125 µm | OM5 | 850 to 953 nm | Lime green |
OS2 and OM grades compared
Grades describe cabled performance. ISO/IEC 11801 defines the single-mode categories OS1a and OS2, with maximum cabled attenuation of 1.0 dB/km and 0.4 dB/km respectively. The older OS1 category was usually tight-buffered indoor cable. OS2 is usually built from low water peak fibre to ITU-T G.652.D, and most single-mode fibre made today meets that specification. Bend-insensitive fibre to ITU-T G.657.A1 or A2 is common in patch cords and tight routing, and it splices and mates with G.652.D.
Multimode grades are set by modal bandwidth at 850 nm. That figure decides how far a given Ethernet speed will run. The reaches below are the IEEE 802.3 figures for two common short-reach interfaces.
| Grade | Core | Modal bandwidth at 850 nm | 10GBASE-SR reach | 100GBASE-SR4 reach |
|---|---|---|---|---|
| OM1 | 62.5 µm | 200 MHz·km | 33 m | Not specified |
| OM2 | 50 µm | 500 MHz·km | 82 m | Not specified |
| OM3 | 50 µm | 2000 MHz·km | 300 m | 70 m |
| OM4 | 50 µm | 4700 MHz·km | 400 m | 100 m |
| OM5 | 50 µm | 4700 MHz·km, also specified at 953 nm | 400 m | 100 m |
OM5 adds a bandwidth specification up to 953 nm so that shortwave wavelength division multiplexing optics can run several wavelengths on one fibre. For most new multimode links OM4 is the practical default. Our comparison of OM3, OM4 and OM5 covers that choice in more detail.
When each fibre type is used
Multimode suits short links with many ports, where a lower optic price is multiplied across hundreds of connections.
- Data centre rows and halls: server, top-of-rack and leaf links, and MPO trunks for 40G and 100G parallel optics, typically under 100 m on OM4.
- Enterprise buildings: horizontal and riser links inside one building.
- Equipment and control links: short runs inside machines, control rooms and broadcast racks where installed equipment has multimode ports.
Single-mode suits distance, and plant that has to outlive several generations of transceivers.
- Backbones: campus and building-to-building links, metro, access and long-haul networks.
- Broadcast camera links: SMPTE hybrid camera cable carries two single-mode fibres.
- Defence and field links running for kilometres over tactical cable.
- Large data centres running 400G and 800G optics such as DR4, which use parallel single-mode fibre over MPO connectors.
The last point is a real shift. Many operators now install single-mode MPO trunk cables inside the data hall even for short links, because the fastest optics usually appear in single-mode versions first.
Cost: optics matter more than cable
The fibre itself is rarely the deciding cost. Single-mode glass is made in very large volumes for telecom, and per metre it generally costs less than laser-optimised OM3 or OM4, which carry a premium for bandwidth grading. Connectors and assembly labour are similar for both.
The difference sits in the transceivers. Multimode optics use VCSELs and relaxed alignment against a 50 µm core, so they have historically cost less per port than single-mode optics. The gap has narrowed at 100G and above, but on a project with hundreds of ports it still adds up.
A fair comparison counts the whole link over its life:
- Cable, connectors and termination for each link.
- Optics for both ends, multiplied by the number of ports.
- The number of transceiver upgrades the cable must carry before it is replaced.
- Whether the reach of the next speed step will still fit the installed grade and its connector loss.
If a multimode link is close to its reach or loss limit today, the next speed step may not fit. Check the numbers with the fibre loss budget calculator before committing to a grade.
Mixing fibre types
Never mate single-mode to multimode in a working link. Light leaving a 50 µm core cannot be coupled efficiently into a 9 µm core, so loss in that direction is very high. Mating 62.5 µm to 50 µm multimode also adds loss where light passes from the larger core into the smaller one.
There is one recognised exception. 1000BASE-LX optics, which use a single-mode laser, can run over existing multimode plant with a mode-conditioning patch cord at each end. The cord launches light off-centre into the multimode core to avoid a bandwidth problem called differential mode delay.
Connector colours give a quick check in the rack: blue for single-mode UPC, green for single-mode APC, beige for OM1, and aqua for OM3 and OM4. Colours are a convention, so read the legend printed on the cable jacket as well.
What to specify for a patch cord or assembly
- Fibre type and grade, for example OS2 with G.657.A1 fibre, or OM4.
- Connector type and polish at each end, such as LC/UPC to SC/APC, or MPO with pin gender.
- Fibre count, and polarity for duplex or MPO assemblies.
- Length and length tolerance.
- Cable construction: diameter, jacket material such as LSZH or PVC, and any armour.
- Labelling, and the test data you need with delivery.
Radix makes single-mode and multimode fibre optic patch cords and assemblies to the customer's drawing. Send the details above with a request for a quote, and ask for the insertion loss values that apply to your build.
Questions buyers ask
Can I use a single-mode transceiver on multimode fibre?
In general, no. The optic is designed for a 9 µm core and the link will not meet its specification. The exception is 1000BASE-LX type optics, which IEEE 802.3 allows over multimode when a mode-conditioning patch cord is fitted at each end.
Is OS2 the same as G.652.D?
Not exactly. OS2 is a cabled performance category in ISO/IEC 11801, with a maximum attenuation of 0.4 dB/km. G.652.D is an ITU-T specification for the fibre itself. Most OS2 cable is made with G.652.D or compatible G.657.A fibre.
Should a new data centre use single-mode or multimode?
It depends on link lengths and the speed roadmap. OM4 remains common for short server and switch links where optic cost per port matters. Many large operators now install single-mode in new builds because 400G and 800G optics usually reach the market in single-mode versions first, and the cable then carries several upgrades.
How far can multimode fibre carry 10 Gigabit Ethernet?
IEEE 802.3 gives 10GBASE-SR reaches of 33 m on OM1, 82 m on OM2, 300 m on OM3 and 400 m on OM4. Those figures assume the channel stays within its loss budget, so the number and quality of connectors also matter.
Why is single-mode cable yellow?
TIA-598 sets jacket colours for premises cable: yellow for single-mode, orange for OM1 and OM2, aqua for OM3 and OM4, and lime green for OM5. Outdoor cable is usually black whatever the fibre type, so check the printed legend.
Can bend-insensitive fibre be spliced to standard single-mode?
Yes. G.657.A1 and A2 fibres are designed to be compatible with G.652.D in splicing and connection. That is why they are widely used in patch cords and indoor cables that connect to standard outside plant.