Fibre-optic cable types — single-mode versus multi-mode (OS2, OM1-OM5) per ISO/IEC 11801

The [guide on line current differential protection (87L) over a fibre-optic channel](/guides/protection/lijndifferentieelbeveiliging-87l-glasvezelkanaal) and the guide on IEC 61850 and GOOSE messages both assume a fibre-optic connection is available, without going into which type of fibre is suitable for that. This article covers the distinction between single-mode and multi-mode fibre-optic cable per ISO/IEC 11801, and why choosing wrongly — or accidentally mixing the two types — results in a non-working or severely underperforming connection.

Single-mode: small core, effectively unlimited reach

Single-mode fibre (SMF), classified as OS1 (indoor) or OS2 (with low water-peak attenuation, suitable for both indoor and outdoor use), has a very small core diameter of approximately 9 µm. That small core lets light travel through the fibre via essentially only one propagation path (mode), so that modal dispersion — a light pulse spreading out because different propagation paths have different travel times — is almost entirely eliminated. The result is very low attenuation (typically around 0.4 dB/km at both 1310 nm and 1550 nm wavelength) and a bandwidth-distance product that is not the limiting factor for practical link lengths (up to tens of kilometres without amplification). Single-mode is therefore the standard choice for long-distance links, such as the fibre-optic channels covered in the guide on 87L line differential protection between two stations.

Multi-mode: larger core, bandwidth-distance product as the limiting factor

Multi-mode fibre (MMF) has a considerably larger core (62.5 µm for the older OM1, 50 µm for OM2 through OM5), so light travels through the fibre via multiple propagation paths at once. This does cause modal dispersion, which limits the usable bandwidth-distance product — the higher the desired bit rate, the shorter the maximum cable length. ISO/IEC 11801 and the associated IEC 60793-2-10 fibre specifications distinguish five multi-mode classes with progressively higher performance:

Class	Core	Typical application (indicative)
OM1	62.5/125 µm	LED-driven, short-distance links up to a few hundred metres at lower bit rates
OM2	50/125 µm	similar reach to OM1, higher bandwidth at 850 nm
OM3	50/125 µm, laser-optimised	10 Gbit/s up to roughly 300 m
OM4	50/125 µm, laser-optimised	10 Gbit/s up to roughly 400 m, 40/100 Gbit/s over shorter distances with parallel optics
OM5 (wideband multimode)	50/125 µm	supports multiple wavelengths simultaneously (SWDM) for higher combined bandwidth over fewer fibres

Note: the table gives indicative, commonly used values; the exact guaranteed distance-bit-rate combination for a specific fibre follows from the cable manufacturer's data and the transceiver used, not from one universal figure per class.

Colour coding as a quick, but not foolproof, identification

In practice fibre types are often visually distinguished by the colour of the cable jacket or connector housing: single-mode (OS1/OS2) typically yellow, OM1/OM2 typically orange, OM3 typically aqua/turquoise, OM4 typically violet (or also aqua, depending on manufacturer) and OM5 typically lime green. This colour coding is a useful, quick identification aid on the shop floor, but does not replace the need to check the actual cable printing or documentation — colour conventions are widespread but not universally mandatory, and a wrongly assumed colour can lead to a wrong-fibre-type error.

Why mixing single-mode and multi-mode is a common practical mistake

The core diameters of single-mode (≈9 µm) and multi-mode (50 or 62.5 µm) differ so much that connecting a single-mode patch cable to multi-mode equipment (or vice versa) does not merely underperform subtly, but typically results in a severely attenuated or completely non-working connection: light coupled from a larger multi-mode core into a much smaller single-mode core loses most of its power at the transition, and a single-mode laser transmitter connected to multi-mode fibre overdrives and disturbs the receiver at the other end due to the abundance of propagation paths suddenly available there. This makes accidentally swapping patch cables — for example during a rushed repair or extension — one of the most common causes of an inexplicably poorly performing or completely failed fibre-optic channel.

Practical choice: distance and application are decisive

For the applications covered on this platform, the rule of thumb is typically:

Single-mode: long-distance links between stations, such as for 87L line current differential protection or teleprotection (85), where distance can run to tens of kilometres.
Multi-mode: short, intra-station links, such as process-bus or station-bus cabling between IEDs, or between an IED and an Ethernet switch within the same building (see the guide on IEC 61850 and GOOSE messages), where distance typically stays under a few hundred metres and multi-mode transceivers are cheaper than their single-mode counterparts.

Common mistakes

Using a single-mode patch cable on multi-mode equipment (or vice versa) — this typically yields no connection or a severely attenuated one, not merely a small performance loss.
Relying solely on cable-jacket colour without checking the printing or documentation, since colour conventions can differ by manufacturer.
Using multi-mode fibre for a long-distance link because it happens to already be present, without checking whether the bandwidth-distance product is adequate for the required bit rate and length.
Treating multi-mode classes (OM1-OM5) as interchangeable — a connection that works correctly with OM4 at a given distance and bit rate can already fall out of specification with OM1 or OM2 at the same distance.