Fiber fundamentals — single-mode vs multi-mode

A fiber is a hair-thin glass strand that pipes light from one end to the other. Networking fiber is classified by the diameter of the core and the wavelength of light it's designed for. Get those two right and the rest is detail.

Single-mode vs multi-mode

| | Single-mode (SMF) | Multi-mode (MMF) | |---|---|---| | Core diameter | ~9 µm | 50 µm (modern) or 62.5 µm (legacy) | | Light source | Laser | LED or VCSEL | | Color of jacket | Yellow | Aqua, magenta, orange depending on grade | | Distance | Tens to hundreds of km | Tens to hundreds of meters | | Cost | Cheaper fiber, more expensive optics | More expensive fiber, cheaper optics | | Where | Outside plant, DWDM, long DC runs | Inside one building, between racks |

Single-mode has a tiny core that only allows one "mode" (path) for the light, eliminating modal dispersion and letting the signal travel for tens of km. Multi-mode has a fat core that allows many paths — cheaper to manufacture and easy for an LED to launch into, but those paths arrive at slightly different times, smearing the pulse and limiting distance.

Multi-mode grades (OM)

| Grade | Core | Color | 10 GbE | 40/100 GbE (over parallel fibers) | |---|---|---|---|---| | OM1 | 62.5/125 µm | Orange | 33 m | not supported | | OM2 | 50/125 µm | Orange | 82 m | not supported | | OM3 | 50/125 µm | Aqua | 300 m | 100 m (40G) / 100 m (100G short reach) | | OM4 | 50/125 µm | Aqua / violet | 400 m | 150 m / 150 m | | OM5 | 50/125 µm | Lime green | 400 m | 150 m + wideband for SWDM |

OM3 and OM4 dominate modern data centers. OM1/OM2 are legacy — you'll find them in older buildings but no one designs with them today.

Single-mode grades (OS)

| Grade | Loss at 1310 nm | Loss at 1550 nm | Typical use | |---|---|---|---| | OS1 | 1.0 dB/km | not specified | Inside-plant, tight-buffered | | OS2 | 0.4 dB/km | 0.4 dB/km | Outside-plant, long-haul |

For anything outdoor or longer than a few hundred meters: OS2. For DWDM and ultra-long-haul: still OS2 (DWDM rides on standard SMF).

Wavelengths you'll meet

| Wavelength | What it is | |---|---| | 850 nm | Multi-mode, short-reach (SR optics) | | 1310 nm | Single-mode, medium-reach (LR optics) | | 1550 nm | Single-mode, long-reach (ZR / ZR+ / coherent) | | 1530–1565 nm | DWDM C-band — many wavelengths multiplexed here | | 1565–1625 nm | DWDM L-band — extended capacity | | 1310 / 1550 nm "bidi" | One fiber, two wavelengths going opposite directions |

Loss budget — the engineer's reality check

You don't get to use a fiber link just because both endpoints have an optic with the right form factor. You need the received power to be above the optic's receiver sensitivity but below its saturation. Every km of fiber, every connector, and every splice subtracts dB:

| Loss source | Typical | |---|---| | OS2 fiber at 1550 nm | 0.25 dB/km | | OM4 at 850 nm | 3 dB/km | | Mated connector pair | 0.3–0.75 dB | | Fusion splice | <0.1 dB | | Mechanical splice | 0.3 dB |

If a 10GBASE-LR optic has +0.5 dBm Tx and -14.4 dBm Rx sensitivity, you have a 14.9 dB budget. Lose 7 km of fiber (1.75 dB), four connectors (2 dB), and one splice (0.1 dB) and you're well inside budget. Lose 40 km, eight connectors, and three splices and you're out — bring in a higher-reach optic or add an amplifier.

What to remember

• SMF for distance; MMF for cheap optics in a building.
• OM3/OM4 for new MMF, OS2 for new SMF.
• Wavelength tells you the optic class: 850 / 1310 / 1550 nm.
• Plan a loss budget before you order optics — don't trust "it should reach."