Why are single-mode fibers typically used for long-distance high-bandwidth links?

• A. They require higher-cost electronics to generate the light and support higher bandwidth over long distances
• B. They are cheaper to install than multimode fibers
• C. They support lower bandwidth than multimode
• D. They only work in LAN environments

Answer: (A)

This question tests understanding that the advantage of single-mode fiber for long-distance, high-bandwidth links comes from how light travels inside it. In a single-mode fiber, light propagates in essentially one path, so there’s far less modal dispersion. Pulses don’t spread out as they travel, which means signals stay sharper and can carry more data over longer distances. The fiber also typically has lower attenuation at the wavelengths used (around 1300–1550 nm), so the signal can travel farther before needing regeneration. To make the most of these benefits, the system relies on precise laser transmitters and sensitive receivers, which are more expensive than the cheaper light sources used with multimode fiber. That combination—minimal dispersion and lower loss—explains why single-mode fibers are preferred for long-haul, high-bandwidth links.