Fiber optic cable transmits data as pulses of light. Two fundamentally different physical designs serve different distance requirements.

Multi-Mode Fiber (MMF)

MMF has a core diameter of 50 micrometres -- large enough for multiple light paths (modes) to travel simultaneously. The light source is a VCSEL (Vertical-Cavity Surface-Emitting Laser) -- a cheap, low-power laser that can be modulated at high speed without precision cooling or alignment.

The multiple modes that propagate through MMF arrive at slightly different times, causing modal dispersion -- a smearing of the signal over distance. This limits MMF to shorter reaches. The larger core and cheaper light source make it the right choice for short distances where cost matters.

MMF comes in four grades relevant to AI clusters:

OM3 supports up to 100m at 100G. The oldest modern grade; still found in legacy installations. Not recommended for new AI cluster deployments.

OM4 supports up to 150m at 100G through 400G. This is the current standard for AI cluster intra-rack and intra-row cabling. OM4 with MPO-16 connectors handles 800G at 100m. Most SuperPOD deployments use OM4 for all leaf-to-spine connections within the same switch row.

OM5 adds a second wavelength (953 nm alongside the standard 850 nm). With WDM (wavelength division multiplexing), OM5 can achieve 400G over 150m using only four fiber strands instead of eight -- useful for congested cabling trays. OM5 is backward compatible with OM4 transceivers.

Single-Mode Fiber (SMF)

SMF has a core diameter of 8-10 micrometres -- narrow enough to allow only a single light path. The light source is a precision edge-emitting laser at 1310 nm or 1550 nm wavelength. Because there is only one mode, there is no modal dispersion. SMF can carry signals for kilometres with only gradual attenuation.

The narrow core requires extremely precise connectors and alignment. The laser source is more expensive to manufacture and operate than a VCSEL. SMF infrastructure costs more per link, but the distance capability is fundamentally different.

SMF transceiver types define the reach:

DR4/DR8 (Data center Reach): up to 500m. Uses MPO connectors. This is the standard choice for long cross-row runs in large data halls, or for connecting switch rows that are separated by a significant distance within the same building.

FR4 (Far Reach): up to 2 km. Uses LC duplex connectors. Cross-building connections within a campus.

LR4 (Long Reach): up to 10 km. LC duplex. Inter-building over dark fibre.

ZR (eXtended Reach): 80 km+. Uses coherent DWDM technology. For data centre interconnects, not AI cluster internal fabric.

Choosing fiber for an AI cluster

The decision tree follows distance:

Within a rack (1-5m): copper DAC or AEC -- no fiber at all. Within a row (5-30m): OM4 MMF with AOC or SR4/SR8 pluggable. Cross-row within the same hall (30-100m): OM4 MMF SR4/SR8 pluggable. Cross-row in a large hall (100-500m): SMF DR4/DR8 pluggable. Cross-building within campus: SMF FR4. Between buildings or DCs: SMF LR4 or ZR/DWDM.

For a standard DGX BasePOD with ToR (top-of-rack) leaf placement, all leaf-to-spine runs stay within the same switch row at distances under 30m. OM4 MMF with OSFP SR4 pluggable transceivers is the standard choice. The server-to-leaf connections within the same rack use passive DAC copper.