Datacenter · Compare
Single-mode vs multimode fiber: which to spec for a data center
Single-mode holds kilometers at any rate; multimode is cheaper and short. On new high-speed builds, lean single-mode.
Short answer
Pick single-mode (OS2) for any link that has to outlast future rate steps, and pick multimode (OM4) only for short links where the rate is settled and cheap optics matter most. The single biggest deciding factor is the refresh horizon versus reach: single-mode reach stays in kilometers no matter the rate, while multimode reach shrinks every time the speed climbs, so a multimode link that fits at 100G can fall short at 400G and force a re-pull in a live room. On new high-speed builds, the design center has moved to single-mode because the old optic-price premium has nearly collapsed.
Single-mode fiber vs Multimode fiber: side by side
| Factor | Single-mode fiber | Multimode fiber |
|---|---|---|
| Core diameter | ~9 microns, carries one mode (no modal dispersion) | ~50 microns (62.5 on legacy OM1), carries many modes |
| Reach by rate | Kilometers at 10G, 100G, 400G, 800G alike | Tens to ~100 m, shrinks as rate rises (~100 m at 100G, ~50 m at 400G VR4) |
| Light source / optic | 1310/1550 nm laser (LR/FR/DR); higher cost, falling fast | 850 nm VCSEL (SR); lower cost at short reach |
| Upfront cost | Glass is cheap; optic premium now small, near parity at high rates | Lowest system cost at low rates and short reach |
| Grade to spec | OS2 (low-water-peak, ~0.4 dB/km); OS1 legacy indoor | OM4 (aqua, ~4700 MHz-km at 850 nm); OM3, OM5 wideband |
| Attenuation / loss budget | Very low (well under 0.5 dB/km); connectors dominate the budget | Higher (~3 to 3.5 dB/km at 850 nm); fiber eats budget as run grows |
| Jacket color | Yellow | Aqua (OM3/OM4), violet (some OM4), lime green (OM5) |
| Best use | Cross-hall, building, and any run that must survive rate steps | Short in-row / row-to-row where rate is settled, existing OM4 plant |
| Future-proofing | Reach does not move with rate; no re-pull at next speed step | Reach risk each refresh; re-pull is the most expensive cable |
Which should you pick?
Choose Single-mode fiber when
- The link crosses a hall or building, where nothing else reaches
- The plant must last fifteen to twenty-five years through rate steps you cannot predict
- You are building a new high-speed or AI fabric at 400G/800G and want reach insurance
- You want one media that avoids a re-pull when 800G and beyond land
Choose Multimode fiber when
- The link is short (in-row, row-to-row) and inside multimode reach
- The rate is settled and cheap SR optics drive the system cost down
- You are extending or patching existing OM4/OM3 plant at its built rate
- Cost per port matters more than long-term reach headroom
Bottom line
It depends on link distance, data rate, and how long the plant has to live. The textbook split still holds on physics: multimode (OM4) for short links because SR optics were cheap, single-mode (OS2) for long links because it is the only thing that reaches. What has changed is the economics. Single-mode optics have dropped from several times the price of multimode toward a small premium, so the crossover keeps sliding toward single-mode. If the link is short, cheap optics matter most, and the rate is settled, multimode still makes sense. If the plant has to survive rate steps you cannot predict, single-mode is the lower-risk pull. Cost the whole link over the refresh horizon, not the optic on day one, because the cheapest fiber to install is sometimes the most expensive to own.
FAQ
What is the difference between single-mode and multimode fiber?
Single-mode fiber has a roughly 9 micron core that carries one light path for kilometers on a laser. Multimode has a roughly 50 micron core that carries many paths over shorter distances on a cheaper 850 nm VCSEL, with modal dispersion limiting reach. Single-mode goes far, multimode is cheap and short, and the price gap between them is closing.
How far can multimode fiber run at 400G?
Multimode reach shrinks as the rate climbs. On OM4, a 400G SR4.2 optic reaches roughly 100 m and a shorter VR4 around 50 m, against a few hundred meters at 10G. Single-mode holds kilometers at the same rate. Always confirm the supported distance against the specific transceiver datasheet and the applicable IEEE 400G specification.
Why are data centers moving to single-mode fiber?
Multimode reach falls every time the rate steps up, while single-mode reach stays in kilometers and single-mode optic prices have collapsed toward parity as silicon photonics scaled. On a high-speed build that must last through several refreshes, single-mode avoids the re-pull a shrinking multimode link forces at the next rate step, and a re-pull in a live room is the most expensive cable you will ever install.