What is the core size of a singlemode fiber?

• A. 5 mm
• B. 9 microns
• C. 50 microns
• D. 63.5 microns

Answer: (B)

The key idea is that singlemode fiber uses a very small core so only one light path can propagate. For standard telecom fiber, that core diameter is about nine micrometers. This small size keeps the V-number below the cutoff for higher-order modes at typical operating wavelengths (around 1.3 to 1.55 micrometers), so only the fundamental mode travels. Larger cores, like 50 or 63.5 micrometers, support multiple modes and are used for multimode fiber; a core as large as several millimeters would not confine light effectively in ordinary fibers. So the practical, widely used singlemode core size is around 9 microns.