Multimode vs Singlemode Fiber
The two fiber families side by side: geometry, light sources, reach, and the cost story as it actually stands — the old rule that single-mode is the expensive option is half-dead, and knowing which half matters is worth real money on a bid. Physical properties below are fiber facts; the reach cells reflect IEEE application specs.
Side-by-side comparison
| Aspect | Single-mode (OS2) | Multimode (OM1–OM5) |
|---|---|---|
| Core diameter | 9 µm nominal | 50 µm (OM2–OM5) or 62.5 µm (OM1 legacy) |
| Cladding | 125 µm | 125 µm (identical — connectors share sizes) |
| Light source | Laser (DFB/FP), 1310/1550 nm | VCSEL at 850 nm (OM3+); LED on legacy OM1/OM2 |
| Typical reach | Tens of km (10 km standard optics, 40–80 km extended) | Hundreds of meters (OM4: 400 m at 10G, 150 m at 40/100G) |
| Cable cost | As cheap as or cheaper than multimode per meter | Same or higher — the old “SM cable is pricey” rule is dead |
| Optics cost | Higher per port (laser transmitters) | Lower per port (VCSELs) — MM’s remaining advantage |
| Jacket color | Yellow | Orange (OM1/OM2), aqua (OM3/OM4), lime (OM5) |
| Bend-insensitive variant | Yes — ITU G.657 (radius down to ~7.5 mm) | Yes — BIMMF in OM3/OM4/OM5 grades |
| When it wins | Between buildings, risers with any distance doubt, anything that may outlive its electronics | Inside a room or data hall where VCSEL optics keep per-port cost down |
The decision in practice
Run the two cost lines: cable cost (a wash, or single-mode ahead) and optics cost (multimode ahead by a per-port margin that shrinks every year). Multimode pays off only when the port count is high and every link is short — a single data hall, a dense equipment room. The moment a run leaves the building, exceeds a few hundred meters, or needs to outlive two generations of electronics, single-mode is the answer: OM1 and OM2 plants are already stranded at 1G, while 1990s single-mode carries 400G today. Check the exact reach numbers on the fiber distance chart and the optic prices behind them via the transceiver chart.
Common questions
Which is better, single-mode or multimode fiber?
Neither — they solve different problems. Single-mode's 9 µm core carries one light mode, so it runs kilometers and never becomes the bottleneck; multimode's 50 µm core accepts cheap VCSEL transmitters, so its per-port optics cost less at short reach. Inside a data hall, multimode still earns its keep; between buildings or anywhere distance is uncertain, single-mode wins outright.
Is single-mode fiber more expensive than multimode?
The cable is not — single-mode bare cable now runs as cheap as or cheaper than multimode per meter, which surprises people carrying a decade-old mental model. What still costs more is the electronics: single-mode transceivers use laser transmitters where multimode uses VCSELs, so the per-port delta is in the optics. As that delta keeps shrinking, new campus designs increasingly default to single-mode everywhere.
Can you mix single-mode and multimode fiber?
Not in one link — the 9 µm and 50 µm cores mismatch so badly that most of the light is lost at the joint, and a multimode optic cannot drive single-mode glass usefully. Connecting the two worlds is done with media converters or a switch carrying one optic of each type, never with a hybrid jumper.