Single-Mode vs Multimode Fiber: The First Decision in Optics Selection

The physical difference

Single-mode fiber has a core diameter of about 9 micrometres. Multimode fiber has a core of 50 or 62.5 micrometres. A larger core lets multiple light paths — modes — travel through the fiber simultaneously. That is convenient for coupling light in, and it is exactly why multimode loses signal faster: the different paths arrive at slightly different times, smearing the pulse.

The practical consequences

| | Multimode (OM3/OM4) | Single-mode (OS2) | |---|---|---| | Core diameter | 50 µm | ~9 µm | | Light source | VCSEL (850 nm) | Laser (1310 / 1550 nm) | | Typical 10G reach | Up to 300 m | Up to 10 km (LR) | | Typical 100G reach | 70–100 m (SR4) | 2–10 km (CWDM4 / LR4) | | Transceiver cost | Lower | Higher | | Cable cost | Higher | Lower | | Typical deployment | Within a rack row, within a hall | Cross-row, cross-hall, campus, DCI |

Why short-distance links still use multimode

Because the transceiver, not the cable, is the recurring cost. Multimode optics use inexpensive 850 nm VCSELs. As long as your distance fits within the SR reach class, multimode delivers the lowest total cost per link even though the cable itself costs more up front.

Where the crossover happens

At 100G and above, the multimode reach advantage narrows quickly — SR4 covers 70 m on OM3 and 100 m on OM4, which may not span a full hall. Once a run exceeds a few hundred metres, single-mode becomes the only realistic option, and the cost difference per transceiver stops mattering against the cost of re-cabling.

A second factor pushes the same direction: parallel single-mode options such as PSM4 and CWDM4 let you reach 500 m–2 km over single-mode with the same density as multimode parallel optics.

The decision rule

1. If the link is under 100 m and you already have OM3/OM4 — stay multimode.
2. If the link is 100 m–10 km — go single-mode.
3. If you are cabling a new build above 100G — favour single-mode structured cabling even if today's optics are multimode-class, because it protects the next two upgrade cycles.

Do not mix them

Multimode to single-mode requires a media converter; you cannot simply join them. And the modules at each end must match in wavelength — an SR module at one end and an LR module at the other will not bring the link up.

Canjin Technology supplies both multimode and single-mode optics across SFP, SFP+, SFP28, QSFP+ and QSFP28 in every standard reach class.