Fiber Optic Cable Types: 7 Key Single-Mode vs Multimode

There are 7 standard types of fibre optic cable: 2 single-mode (OS1, OS2) and 5 multimode (OM1 through OM5). Each type is designed for a different balance of distance, bandwidth, and cost. The key to choosing the right type of fibre optic cable — and the main difference between them — is the core diameter and the number of light paths (modes) it supports.

Here is what we cover: what a mode actually means, each of the 7 cable types explained, a head-to-head comparison table, how to choose the right type, and common mistakes when identifying fibre types by sight.

What Is a Mode in Fibre Optics?

A mode is simply a path that light can take down the fibre. The number of modes a fibre supports depends on the core diameter and the wavelength of light.

Picture a long, straight motorway. Single-mode fibre is a single-lane road: there is only one path, so every car stays in lane and arrives at the same time. Multimode fibre is a multi-lane motorway: cars spread across many lanes, take slightly different routes, and arrive at different times. That difference in arrival time — modal dispersion — limits how far and how fast multimode fibre can transmit.

In the physics: light enters the fibre at different angles. Each angle corresponds to a different mode. A narrow core (9 µm) allows only one angle — one mode. A wide core (50 or 62.5 µm) allows many angles — many modes. The larger the core, the more modes it supports, and the shorter the useful distance becomes.

OS1 and OS2: The Two Single-Mode Types

Single-mode fibre uses a 9 µm core and a laser source to send light straight down the centre. With only one mode, there is no modal dispersion, and the distance limit is set by pure signal loss (attenuation) rather than pulse spreading.

OS1 Single-Mode (Tight Buffer)

OS1 is the indoor single-mode type. It uses tight-buffered construction — the 900 µm buffer coating is bonded directly to the 250 µm coated fibre. This makes OS1 cables stiffer, tougher, and easier to terminate with connectors for indoor runs.

• Core / cladding: 9 / 125 µm
• Max attenuation: 1.0 dB/km at 1310/1550 nm
• Max distance: approximately 2 km
• Jacket colour: Yellow
• Primary use: Indoor risers, campus backbones, patch cables
• Standard: ITU-T G.652.A/B

OS1 was the original single-mode standard. It works well inside buildings where cable bends are tighter and physical robustness matters more than ultra-low loss.

OS2 Single-Mode (Loose Tube)

OS2 is the outdoor and long-haul single-mode type. It uses loose-tube construction — the coated fibres sit loosely inside semi-rigid plastic tubes filled with water-blocking gel. The gel prevents moisture from reaching the glass, and the loose arrangement lets the fibre expand and contract with temperature without stress.

• Core / cladding: 9 / 125 µm
• Max attenuation: 0.4 dB/km at 1310 nm, 0.25 dB/km at 1550 nm
• Max distance: 100 km or more with appropriate transceivers
• Jacket colour: Yellow (indoor cables), black (outdoor cables)
• Primary use: Submarine cables, intercity links, FTTH, long-haul telecom
• Standard: ITU-T G.652.C/D

OS2 has significantly lower attenuation than OS1 because the fibre itself is purer. The loose-tube design handles temperature swings better. OS2 is the default single-mode type for new installations worldwide.

OM1 to OM5: Five Generations of Multimode

Multimode fibre uses a larger core (50 or 62.5 µm) and supports multiple light paths. Each generation from OM1 to OM5 increased bandwidth and distance by improving the glass quality and optimising the fibre for better light sources.

OM1

OM1 is the original 62.5 µm multimode type, introduced in the 1980s and optimised for LED light sources.

• Core / cladding: 62.5 / 125 µm
• Effective modal bandwidth: 200 MHz·km at 850 nm
• Max 10GbE distance: 33 metres
• Max 1GbE distance: 300 metres
• Jacket colour: Orange
• Primary use: Legacy networks, older building backbones

OM1 is now largely obsolete for new installations. Its larger core and low bandwidth make it unsuitable for modern high-speed networks.

OM2

OM2 uses a 50 µm core instead of 62.5 µm, which gives it higher bandwidth. It was still designed for LED sources.

• Core / cladding: 50 / 125 µm
• Effective modal bandwidth: 500 MHz·km at 850 nm
• Max 10GbE distance: 82 metres
• Max 1GbE distance: 600 metres
• Jacket colour: Orange
• Primary use: Legacy data centres, shorter campus runs

Like OM1, OM2 is being phased out in favour of laser-optimised types. You will still find it in older installations, but new cabling projects rarely specify OM2.

OM3 (Laser-Optimised)

OM3 was the first multimode type designed specifically for VCSEL (vertical-cavity surface-emitting laser) sources rather than LEDs. VCSELs couple light into the fibre more efficiently and allow higher data rates.

• Core / cladding: 50 / 125 µm
• Effective modal bandwidth: 2000 MHz·km at 850 nm
• Max 10GbE distance: 300 metres
• Max 40/100GbE distance: 100 metres
• Jacket colour: Aqua (cyan)
• Primary use: Data centres, high-speed local networks

OM3 was a breakthrough when it arrived around 2002. The jump from 500 to 2000 MHz·km was enough to support 10 Gigabit Ethernet at practical data-centre distances.

OM4

OM4 is an improved version of OM3 with roughly double the bandwidth. It uses the same 50 µm laser-optimised design but with tighter tolerances that reduce differential mode delay.

• Core / cladding: 50 / 125 µm
• Effective modal bandwidth: 4700 MHz·km at 850 nm
• Max 10GbE distance: 550 metres
• Max 40/100GbE distance: 150 metres
• Max 400GbE distance: 100 metres
• Jacket colour: Aqua (cyan)
• Primary use: High-speed data centres, enterprise networks

OM4 is the current standard for new multimode data-centre installations.

OM5 (Wideband)

OM5 is the newest multimode standard, ratified in 2016. It uses the same 50 µm core as OM3 and OM4 but is optimised for a wider wavelength range: 850–950 nm instead of just 850 nm. This allows shortwave wavelength division multiplexing (SWDM) — sending multiple colours of light down the same fibre to multiply capacity.

• Core / cladding: 50 / 125 µm
• Effective modal bandwidth: 4700 MHz·km at 850 nm, plus 2470 MHz·km at 880, 910, and 940 nm
• Max 40/100GbE distance (SWDM4): 150 metres using 4 wavelengths
• Jacket colour: Violet (light purple)
• Primary use: Next-gen data centres, SWDM applications

OM5 is designed to support 40 and 100 Gigabit Ethernet over a single pair of fibres by using four different laser colours. This is useful in data centres where fibre counts are limited.

All 7 Fibre Types at a Glance

Attenuation figures are approximate maxima. Actual values depend on fibre quality and manufacturer.

How to Choose the Right Fibre Optic Cable Type

The choice comes down to three questions.

How far does the signal need to go? Over 2 km requires single-mode (OS2). Under 300 m works with any multimode type, but use OM4 for future-proofing. Between 300 m and 550 m OM4 handles 10GbE; beyond that, switch to OS2.

What data rate do you need? 1 Gigabit Ethernet works on any multimode type. 10 GbE requires OM3 or better for distances over 82 m. 40 or 100 GbE needs OM4 or OM5. For 400 GbE, single-mode is the practical choice.

Are you building new or replacing? For new installations, use OS2 single-mode for any run over 100 m and OM4 for short data-centre runs. OM1 and OM2 are legacy types — avoid them for new builds.

The trend across the industry is clear: single-mode fibre is taking over. OS2 now costs less per metre than OM4 because it is produced in much higher volumes. The transceivers are more expensive, but for new installations, single-mode is increasingly the standard even for shorter runs.

A Common Misconception: All Yellow Fibre Is the Same

Many people assume that any yellow-jacketed fibre is "single-mode" and leave it at that. But OS1 and OS2 have different performance limits. Using OS1 outdoors is a mistake — its tight-buffered construction can crack in freezing temperatures and its 1.0 dB/km attenuation limits reach. Using OS2 indoors works technically but is more expensive and harder to terminate than OS1.

Similarly, not all aqua fibre is OM4. OM3 and OM4 share the same jacket colour. The only way to tell them apart is to read the printing on the cable jacket, which specifies the type and bandwidth rating. If the cable says "OM3 50/125 2000 MHz·km," it is OM3. "OM4 50/125 4700 MHz·km" is OM4.

The industry standard TIA-598-C defines the colour code, but manufacturers sometimes deviate. Always check the printed spec, not just the colour.

External resources

• Wikipedia: Optical Fiber — Types — comprehensive reference on OS and OM types, dimensions, and performance specifications
• Cable Matters: Fiber Optic Cable Types — Complete Guide — practical guide to cable types, jacket colours, and connector identification
• Corning: Optical Fiber Product List — manufacturer specifications for SMF-28 Ultra (OS2) and ClearCurve (OM4/OM5) fibres

For the physics that makes fibre optics possible, see our guide on total internal reflection. For a broader introduction, read what is optical fiber. For how the glass is made and structured, see what is fiber optic cable made of.

Recommended Products

• Fibre Optic Termination Kit on Amazon — includes tools for stripping, cleaving, and connectorising all OS and OM fibre types
• LC to LC OS2 Patch Cable on Amazon — single-mode duplex patch cable in yellow jacket for FTTH and telecom use
• OM4 LC to LC Patch Cable on Amazon — aqua multimode patch cable for high-speed data centre connections