What is the primary difference between SMF and MMF?

• A. Single-mode fiber uses copper conductors, while multi-mode uses glass fiber.
• B. SMF is used for long reach with a narrow core, while MMF is used for short reach with a wider core.
• C. SMF is more expensive than MMF in all cases.
• D. Single-mode fiber is used for long reach with a narrow core, while multi-mode fiber is used for short reach with a wider core.

Answer: (D)

The key idea is how light travels inside the fiber and how that affects distance and bandwidth. Single-mode fiber has a very narrow core, so light propagates along a single path. This minimizes modal dispersion, allowing signals to travel long distances with high data rates when paired with the right wavelengths. Multimode fiber has a wider core, which lets many light paths exist. Those paths have different lengths and speeds, causing modal dispersion that limits reach and speed, making MMF better suited for shorter distances with cheaper components. In practical terms, SMF is used for long reach with a narrow core, while MMF is used for short reach with a wider core.