Single-mode optical fiber is normally how many microns in diameter?

Single-mode optical fiber is normally how many microns in diameter?

• A. 6
• B. 12
• C. 9
• D. 15

Answer: (C)

Single-mode fiber is designed so that light travels in only one path, which requires a very small core. In standard telecom fibers, the core diameter is about 9 microns, with a cladding of around 125 microns. That small core size is chosen so that, at the typical operating wavelengths (roughly 1.3 to 1.55 micrometers), only the fundamental mode can propagate. If the core were significantly larger, multiple transverse modes would fit inside and travel together, creating modal dispersion and reducing performance. Core sizes around 9 microns provide true single-mode operation, while much larger cores would become multimode. Smaller cores would make coupling and fabrication impractical.

Single-mode fiber is designed so that light travels in only one path, which requires a very small core. In standard telecom fibers, the core diameter is about 9 microns, with a cladding of around 125 microns. That small core size is chosen so that, at the typical operating wavelengths (roughly 1.3 to 1.55 micrometers), only the fundamental mode can propagate. If the core were significantly larger, multiple transverse modes would fit inside and travel together, creating modal dispersion and reducing performance. Core sizes around 9 microns provide true single-mode operation, while much larger cores would become multimode. Smaller cores would make coupling and fabrication impractical.