What is the difference between Single-Mode Fiber (SMF) and Multimode Fiber (MMF)?

Question

Accepted Answer

SMF has a tiny core and a laser sending one straight beam for long distances; MMF has a larger core and a cheaper LED sending light at multiple angles, good for shorter runs.

* SMF: tiny core, laser, one straight path, long haul. MMF: larger core, LED, many light paths, shorter runs. *

Characteristic
Single-Mode Fiber (SMF)
Multimode Fiber (MMF)

Core size
Very small (~9 microns)
Larger (50/62.5 microns)

Light source
Expensive lasers
Less expensive LEDs

Light path
Single straight path
Multiple paths at different angles

Distance
Long-distance (up to 100 km+)
Shorter distance (up to 550 m at 10 Gbps)

Cost
More expensive
Less expensive

Use case
Long-haul, submarine cables
Enterprise/building backbones

Go deeper:

Single-mode vs multimode fiber — FS.com — core sizes, light sources, distance, cost, and typical uses side by side.

Single-mode optical fiber — Wikipedia — why a ~9 µm core carries a single mode for long distances.

Multi-mode optical fiber — Wikipedia — the larger-core counterpart used for shorter, lower-cost runs.

Characteristic	Single-Mode Fiber (SMF)	Multimode Fiber (MMF)
Core size	Very small (~9 microns)	Larger (50/62.5 microns)
Light source	Expensive lasers	Less expensive LEDs
Light path	Single straight path	Multiple paths at different angles
Distance	Long-distance (up to 100 km+)	Shorter distance (up to 550 m at 10 Gbps)
Cost	More expensive	Less expensive
Use case	Long-haul, submarine cables	Enterprise/building backbones