The Fiber Distributed Data Interface (FDDI) uses:

A Local Area Network standard's physical layer specifies a matched pair: an optical fiber's core diameter, and the type of light source driving it. Multimode fiber has a wide core that efficiently gathers the wider-angle, incoherent light an LED emits, whereas single-mode fiber has a narrow core matched to the narrow, coherent beam of a laser diode (an ILD). The fiber-source pairing a standard adopts follows from that trade-off: a wide-core, LED-driven link is inexpensive and safe over shorter distances at modest speed, while a narrow-core, laser-driven link supports much higher bandwidth over longer distances at higher cost and with laser-safety handling.

FDDI is a 100 Mbps token-passing dual counter-rotating ring standardized to span up to 200 km and connect as many as 500 stations, including direct connections to end-user workstations. At that speed and station count, the extra bandwidth-distance capability of a laser-driven, narrow-core link is not needed, while running a laser source into a workstation's own port raises real eye-safety concerns. FDDI's Physical Layer Medium Dependent (PMD) specification accordingly adopted the wide-core, LED-driven combination as its standard medium -- that is, multimode fiber and LEDs.

• single mode fibers and LEDs: pairs a narrow core with a wide-angle, incoherent LED beam, a mismatch that couples light into the fiber poorly, so it is not the medium FDDI's PMD specifies.

• single mode fibers and ILDs: an efficient, genuinely usable pairing (used on longer, higher-bandwidth telecom links), but it adds laser cost and safety handling that FDDI's 100 Mbps, workstation-connected design does not need.

• multimode fibers and ILDs: pairs a wide core with a laser's narrow, coherent beam, which couples efficiently into a suitably sized multimode core, adding the laser source's cost without needing the fiber's extra width.

• multimode fibers and LEDs: the wide core and the LED's wide-angle beam couple efficiently, and the LED source is inexpensive and safe for a workstation-connected LAN -- this is the pairing FDDI's PMD specification uses.

Hence FDDI uses multimode fibers and LEDs.