Single-mode vs Multimode Fiber

Fiber optic cables carry light through a glass core. Which glass, and how the light travels through it, decides everything else — how far the signal reaches, which transceivers work, what jacket color to look for, and what your data-center vendor is going to insist on.

There are two categories worth memorizing.

Single-mode (SM)

Single-mode fiber uses a very small core — 9 µm — that only lets a single mode of light travel down it. That single, coherent path is what makes long distances possible without pulse dispersion muddying the signal.

• Standard: OS1, OS2
• Core / cladding: 9 / 125 µm
• Wavelength: 1310 nm and 1550 nm
• Reach: 10 km and beyond, depending on the transceiver
• Jacket color: Yellow
• When to use: long-distance links, campus and metro backbones, CWDM/DWDM, anything where transceiver reach exceeds a couple hundred meters

Multimode (MM)

Multimode fiber uses a larger core — 50 µm (OM3/OM4/OM5) or 62.5 µm (OM1/OM2) — which lets multiple modes of light travel simultaneously. That makes for cheaper transceivers (VCSELs instead of lasers) but limits reach because those modes eventually smear.

• Standard: OM1, OM2, OM3, OM4, OM5
• Core / cladding: 50 or 62.5 / 125 µm
• Wavelength: 850 nm and 1300 nm
• Reach: 300 m (OM3) to 550 m (OM4) at 10G-SR; less at higher speeds
• Jacket color: OM1/OM2 orange, OM3/OM4 aqua or magenta, OM5 lime green
• When to use: short-reach data-center links, in-building runs, structured cabling inside cabinets and between adjacent racks

The 30-second decision

• Is your link over 300 m or crossing buildings? Use OS2 single-mode.
• Is it short-reach 10G/40G/100G inside a rack or between adjacent racks? Use OM3.
• Same as above but the runs are longer, and you want headroom for 100G? Use OM4.

Everything downstream — connector type, polish, jacket, length — is easier once this call is locked in.